VOLUME I HYDROLOGY HYDRAULICS APPENDIX TO CORPS INFORMATION PAPER ON COLORADO RIVER FLOOD DAMAGE EVALUATION PROJECT PHASE I prepared for The U.S. Army Corps of Engineers and The Lower Colorado River Authority by Halff Associates, Inc. In cooperation with David Ford Consulting Engineers Espey Consultants SAM, Inc. July 2002
TABLE OF CONTENTS ACKNOWLEDGEMENTS...iv EXECUTIVE SUMMARY...ES-1 GLOSSARY OF TERMS...GL-1 I. General Documentation...1 A. Overview of the Colorado River Flood Damage Evaluation Project...1 1. Purpose...1 2. Phase I - Identification of Existing Conditions...1 3. Phase II - Detailed Evaluation Of Alternatives (Future Phase)...1 4....2 B. Study Area Description...3 1. Basin Description...3 2. Climatology...5 C. Historical Flood Data...7 1. USGS Stream Gauges...7 2. LCRA Stream Gauges...7 3. Flood History...9 D. FEMA Flood Insurance Study Discharges...11 E. Study Tasks Overview...11 F. Coordination Efforts During Study...12 1. Technical Meetings...12 2. Other Coordination...12 G. Previous Studies...13 H. Limitations of Data and Models Used in Study...13 II. Engineering Analyses Methodology...16 A. General Overview Of Technical Approach (Hydrologic And Hydraulic Analyses)...16 1. Period-of-Record Flow Analysis (Chapter 2)...16 2. Historical Frequency Analysis at Each Gauge (Chapter 1)...16 3. Initial/Preliminary HEC-RAS Hydraulic Model (Chapter 6)...16 4. Initial/Preliminary UNREGULATED Basin-Wide HMS Model (Chapter 4)...16 5. Rainfall Information for HMS Model...17 6. HMS Storm Reproduction (Calibration) PHASE...17 7. HMS Verification Phase (Unregulated Conditions)...18 Halff Associates, Inc. i July 2002
8. HEC-5 Reservoir Operation Model for Regulated Basin Conditions (Chapter 5)...18 9. Final RAS Hydraulic Model(s) for Main Stem...18 10. Convert Flood Profiles to Floodplain Inundation Layers for GIS Mapping...19 III. Mapping and Geographic Information System (GIS) Applications (Chapter 3)...20 A. Data Sources... 20 1. Terrain Data... 20 2. Field Survey Data... 20 B. Hydrology Study Applications... 20 1. Pre-Pro (UT-CRWR)...20 2. HEC-GeoHMS...21 C. Hydraulic Study Applications...21 1. TIN Development...21 2. HEC-GeoRAS...21 3. River Channel Issues...21 4. River Centerline Issues...22 5. Floodplain Delineation Issues...22 IV. Summary Of Findings...23 A. General...23 B. Flood Peak Discharges...23 C. Flood Elevations...23 D. Reason for Changes in Flood Elevations...27 E. Floodplains...28 F. Flood Profiles...28 V. References and Previous Studies...44 Halff Associates, Inc. ii July 2002
TABLE OF CONTENTS (Continued) LIST OF TABLES TABLE ES-1 Summary and Comparison of 100-year Flood Peak Discharges (cfs) TABLE ES-2 Summary and Comparison of 100-year Peak Flood Elevations TABLE I-1 River Miles and Drainage Areas TABLE I-2 Existing Reservoirs TABLE I-3 Average Annual Precipitation TABLE I-4 Stream Gauges TABLE I -5 Historical Flood Data TABLE I-6 Summary of Colorado River Flood Insurance Study Discharges TABLE IV-1 Summary and Comparison of 100-Year Flood Peak Discharges TABLE IV-2 Colorado River Reservoir Summary TABLE IV-3 Summary and Comparison of 100-Year Peak Flood Elevations TABLE IV-4 Vertical Datum Comparison (1929 vs. 1988) FIGURE ES-1 FIGURE IV-1 FIGURE IV-2 FIGURE IV-3 FIGURE IV-4 FIGURE IV-5 FIGURE IV-6 FIGURE IV-7 FIGURE IV-8 FIGURE IV-9 FIGURE IV-10 FIGURE IV-11 FIGURE IV-12 FIGURE IV-13 FIGURE IV-14 LIST OF FIGURES Study Area Map Profiles for the Buchanan HEC-RAS Reach Profiles for the Inks HEC-RAS Reach Profiles for the LBJ HEC-RAS Reach Profiles for the Marble Falls HEC-RAS Reach Profiles for the Travis HEC-RAS Reach Profiles for the Lake Austin HEC-RAS Reach Profiles for the Town Lake HEC-RAS Reach Profiles for the Bastrop HEC-RAS Reach Profiles for the La Grange HEC-RAS Reach Profiles for the Columbus HEC-RAS Reach Profiles for the Garwood HEC-RAS Reach Profiles for the Wharton HEC-RAS Reach Profiles for the Bay City HEC-RAS Reach Profiles for the Matagorda HEC-RAS Reach VOLUME II A-D TECHNICAL SUPPORT DATA Volume II-A, Chapter 1 Flood Frequency Analysis Volume II-A, Chapter 2 Period-of-Record Analysis Volume II-B, Chapter 3 Mapping and GIS Applications Volume II-B, Chapter 4 Hydrology (HEC-HMS) Volume II-B, Chapter 5 Reservoir Operation Modeling (HEC-5) Volume II-C, Chapter 6 Hydraulics (HEC-RAS) Volume II-D, Chapter 7 Digital Data (CD s) CD-1...HEC-HMS Models CD-2...HEC-5 Models CD-3 to 6... HEC-RAS Models CD-7... Inundation Surfaces CD-8...GIS Hydrology and Hydraulics Shape Files CD-9... Coordination Efforts CD-10... Report PDF s Halff Associates, Inc. iii July 2002
ACKNOWLEDGEMENTS The Halff Associates study team wishes to acknowledge the valuable assistance of the various individuals and organizations who have helped in the preparation of this report. We wish to express our gratitude to all those listed below who have contributed their time and effort to this study. Messrs Wes Birdwell, Tom Donaldson, John McLeod, Chris Riley, Bob Huber, Mark Jordan, Rick Diaz and Ms. Melinda Luna, Martina Bluem, Maryann McDonald of the Lower Colorado River Authority have provided invaluable assistance, advice, and practical input, as well as encouragement to the study team. Messrs. Elston Eckhardt, Tom Vogt, Mike Danella, Edward Foo, Mead Sams, Paul Rodman, Jerry McCrory, and Ms. Julie Gibbs, Stacy Gray, and Mr. Robert VanHook (Galveston District) of the Corps of Engineers have also provided significant input and technical review and comments to the study team. Other agencies providing assistance in data collection or technical review include: Mr. Raymond Slade and Will Asquith of the U.S.Geological Survey, Dr. David Maidment of the University of Texas at Austin (CRWR). The employees of Halff Associates who have worked most closely with the project include: Messrs. Troy Lynn Lovell, Russell Killen, Michael Anderson, Erin Atkinson, Andrew Ickert, Joshua Logan, Tim Whitefield, Seth Weaver, and Ms. Emilia Salcido, Ms. Stacie McGahey, Ms. Stephanie DuPree, Ms. Beverly Lavender, and Ms. Vicki Moore. Other key study team members include: Mr. Ron Hula, Mr. Richard Hayes, Mr. Vern Bonner, Dr. David Ford, Mr. Joe Devries and Mr. Randy Grose of David Ford Consulting Engineers; Dr. Bill Espey, Mr. Leo Beard, Mr. Brian Reis, and Ms. Kim Davis of Espey Consultants Inc.; Mr. Keith McNease of Surveying and Mapping, Inc., Mr. David Curtis and Mr. Brian Hoblit of Nexrain Corporation. Halff Associates deeply appreciates the dedicated efforts of all the groups and individuals who have helped in the performance of this study. Without the cooperation and assistance of everyone listed, this massive, complex, and technically sensitive study could not have been completed. Halff Associates, Inc. iv July 2002
EXECUTIVE SUMMARY Lower Colorado River Basin Feasibility Study Phase I Information Report INTRODUCTION AND PURPOSE This hydrologic and hydraulic study is a thorough and in-depth, basin-wide approach for modeling, simulating, and computing frequency-based rainfall, runoff, reservoir elevations, and stream flood elevations along the entire Colorado River corridor. The analytical tools and engineering analyses prepared for this appendix include the most comprehensive and detailed examination of flooding issues in the Colorado River basin to date. The use of extensive detailed topographic mapping along the river corridor, state-of-the art Geographical Information System (GIS) and statistically sound hydrologic modeling tools provide not only baseline conditions flood data, but will support future analysis and decisions related to solutions. In response to the June 1997 flood on the Highland Lakes, the LCRA initiated steps to review flood management of the Colorado River, including a critique of reservoir operations (December 1998) and the initiation of a Corps flood damage evaluation feasibility study. This two-phase flood damage evaluation feasibility study is being developed as a cooperative effort by the Fort Worth District, U.S. Army Corps of Engineers, and the local sponsor, the Lower Colorado River Authority. Phase I will result in a Corps Information Paper, which will include this Hydrology and Hydraulic Appendix. This study included detailed river corridor topographic mapping and flood elevation determinations for 482 river miles, along the main stem of the Colorado River. The 18,300 square mile basin was divided into 290 sub-basins with an average size of approximately 63 square miles. The study team chosen to prepare this Hydrology-Hydraulics Appendix consists of Halff Associates, Inc., David Ford Consulting Engineers, Espey Consultants, Inc., Surveying and Mapping, Inc., and other flood modeling consultants. The study, which started in July 2000, has been closely coordinated between the Corps of Engineers, LCRA, and other agencies and communities. Study findings and results have been reviewed by the Fort Worth District Corps of Engineers, LCRA, peer review within the Study Team, Tulsa District of the Corps, the U.S. Geological Survey, and independent consultants. The stated purpose of this basin-wide feasibility study is to develop and evaluate alternatives for implementing solutions to water resource-related problems within the Lower Colorado River Basin. Specific products to be developed in Phase I of the feasibility study include: 1. An assessment of existing conditions flood damages for the major urbanized areas along the river. This will include detailed, regionally consistent existing conditions models for hydrology, hydraulics, and economic flood damage analyses. Furthermore, floodplain boundary delineations will be incorporated into the LCRA Geographic Information System (GIS) database. Halff Associates, Inc. ES-1 July 2002
2. An inventory of existing conditions environmental resources (wildlife and aquatic habitat, land cover classification, threatened and endangered species) throughout the basin and identification of potential ecosystem restoration areas. 3. An assessment of previously identified cultural resources within the basin. 4. An assessment of recreation development and identification of recreation needs within the basin. This has been prepared to partially fulfill the requirements stated in Item 1 of the above product list. Additionally, this Appendix will: 1. Provide the technical data to assist the U.S. Army Corps of Engineers and the Lower Colorado River Authority in minimizing basin-wide flooding in the Colorado River Watershed. 2. Develop the technical elements to enhance existing and future basin-wide, real-time flood forecasting and operation systems and flood warning programs to alert the public and local officials of imminent flooding. 3. Provide frequency-based flood profiles/elevations developed for application in the Corps Flood Damage Assessment (FDA) Program to estimate expected flood damages along the Colorado River. The next phase (Phase II) of the Corps study will include a detailed analysis of alternatives, and the selection of Recommended Plan(s). These detailed analyses will be conducted by the Corps of Engineers as Interim Feasibility Reports. Congressionally authorized projects emanating from these interim studies will proceed to final design and implementation (upon approval and agreement of sponsorship by a local sponsor). STUDY AREA DESCRIPTION The Colorado River basin contains about 40,000 square miles of total drainage area, beginning in New Mexico and traversing Texas, from west to the southeast, to the Gulf of Mexico. The Lower Colorado River basin encompasses about 18,300 square miles of contributing drainage area, including several areas of major urban development. This lower basin study area includes the watershed from the O.H. Ivie Reservoir downstream through the Highland Lakes to the mouth of the river at the Gulf of Mexico. (See Figure ES-1) This lower portion of the basin contains several major tributaries to the Colorado River, most notably of which are the Llano River, the Pedernales River, the San Saba River, Pecan Bayou, Sandy Creek and Onion Creek. The 18,300 square mile basin was divided into 290 sub-basins with an average size of approximately 63 square miles. There are also several reservoirs within the Lower Colorado River basin. Five of the dams (Buchanan, Inks, Alvin Wirtz, Max Starcke, and Mansfield) are owned by the LCRA, and a sixth (Tom Miller) is leased from the City of Austin. These dams form six reservoirs known as the Highland Lakes: Buchanan, Inks, Lyndon B. Johnson, Marble Falls, Travis, and Austin. These lakes were built in pairs, and within each pair, a smaller lake is just downstream of a larger lake. Lake Buchanan, at the upstream end of the Highland Lakes, is a large water supply lake. The middle lakes - Inks, LBJ, and Marble Falls - are categorized as pass through lakes because they pass water from Lake Buchanan, the Llano River and Sandy Creek into Lake Travis. Halff Associates, Inc. ES-2 July 2002
FIGURE ES-1 Halff Associates, Inc. ES-3 July 2002
Lake Travis is the only reservoir specifically designed for flood control, and Mansfield Dam, is the only one of the six dams, which is governed by an operating plan approved by the Corps of Engineers. This lake, constructed by the Bureau of Reclamation, comes under flood control operations by the Corps of Engineers. Flood releases from the lake are determined by assessing future inflows, current lake elevation, and downstream flows in the river below Mansfield Dam. These releases enter Lake Austin, which is also considered a pass-through lake. STUDY PROCEDURES This study was developed by combining state-of-the-art Geographical Information System (GIS) mapping and hydrologic-hydraulic tools, updated topographic mapping, and significant historical flood records. Major steps included: 1. Detailed flood frequency analyses of historical stream gauge records for both pre- and post-reservoirs conditions, covering seventy years of record (1939-1999); 2. Development of basin-wide hydrologic (rainfall-runoff) models, calibrated to the historical data; 3. Preparation of hydraulic river models of the main stem Colorado River from Matagorda Bay upstream to near San Saba (Red Bluff River Gauge); 4. Detailed reservoir operation modeling of the main stem lakes; and 5. Delineation of the floodplains computed from this set of analyses. MAJOR TASKS HYDROLOGY / HYDRAULICS Colorado River Flood Evaluation Study INITIAL HEC-RAS MODEL STEADY CALIBRATED HEC-HMS TO HISTORICAL EVENTS INITIAL HMS MODEL UNREGULATED HISTORICAL FREQUENCY ANALYSIS (STREAM GAUGES) UNREGULATED PERIOD-OF-RECORD FLOW ANALYSIS (UNREGULATED AND REGULATED) FREQUENCY RESULTS HEC-HMS SYNTHETIC HEC-5 RESERVOIR OPERATION MODEL FINAL RAS UNSTEADY FLOOD PROFILES FLOOD PLAINS Halff Associates, Inc. ES-4 July 2002
The Hydrology-Hydraulics Appendix includes technical chapters describing each of the major study components. Results are presented in the form of tables, graphs, flood profiles, and floodplain delineations. FINDINGS General This hydrologic and hydraulic analysis of the lower Colorado River basin includes 482 river miles of the Colorado River, covers 18,300 square miles of watershed, includes seventy years of historical flood data, and delineates floodplains for eight different flood events (2-year to 500-year floods and the Standard Project Flood). This executive summary contains primarily findings for the 100-year flood (Statistically, a one (1.0) percent chance of being equaled or exceeded in any given year) at key locations along the river corridor. Detailed findings are found in the eight technical appendices. Flood Peak Discharges A summary of 100-year frequency flood peak discharges at selected locations is shown in Table ES-1. In general, the peak discharges computed for this study were lower than the published FEMA flood insurance study values. In some cases, lower peak discharges do not always produce lower flood elevations, due to updated modeling data and techniques. Earlier flood studies utilized steady-state hydraulic models while this study uses unsteady modeling along the Colorado River. The use of updated and detailed topographic mapping along the river corridor, state-of-the art Geographical Information System (GIS) and statistically sound hydrologic modeling tools also are factors in the differences. TABLE ES-1 Summary and Comparison of 100-year Flood Peak Discharges (cfs) Colorado River at Selected Locations Location On the Colorado River Red Bluff Gauge Near San Saba Tom Miller Dam Austin Gauge Upstream of U.S. 183 Below Mouth of Onion Creek Bastrop Gauge at Loop 150 Columbus Gauge at U.S. 90 Wharton Gauge at U.S. 59 (Business) Current Study Computed 100-year Discharge (1) FEMA 100-year Discharge 237,100 N/A 90,100(2) 170,000 (3) 90,300(2) 170,000 (3) 138,300 210,000 (4) 142,000 149,300 135,200 136,000 114,100 139,500 (1) Computed values used to determine flood elevations. See Chapter 4 for additional data. (2) Releases from Mansfield Dam. (3) Value in Published Flood Insurance Study is 170,000 cfs. Values in the effective FEMA models range from 90,000 to 100,000 cfs. (4) Value from Travis County FIS at Travis-Bastrop County Line. Halff Associates, Inc. ES-5 July 2002
100-year Flood Elevations - A summary of 100-year frequency peak flood elevations at selected locations is shown in Table ES-2. Note that the peak flood elevations computed for this study differ from earlier FEMA flood insurance study values. For the computed pool elevations at the upstream face of the dams, this study has equal or lower flood elevations at the upstream face of five dams (Buchanan, LBJ, Inks, Austin, and Town Lake); and higher elevations on two dams (Marble Falls and Travis). In the Austin area the current study elevations are slightly higher. At Bastrop, the estimated flood elevation is lower and at Wharton the estimated flood level is below the earlier studies. Some minor differences in the vertical elevation datum from the previous studies (NGVD29-1929 mean sea level) to the current datum (NAVD88-1988) does occur as noted in Table IV 4 of this Volume and in Volume II B, Chapter 3. TABLE ES-2 Summary and Comparison of 100-year Peak Flood Elevations Colorado River at Selected Locations Location on the Colorado River Current Study Computed 100-year Elevation (Feet NAVD88) FEMA 100-year Elevation (Feet NAVD88) (3) Lake Buchanan (1) 1021.0 1021.2-0.2 Inks Lake (1) 901.7 901.9-0.2 Lake LBJ (1) 828.1 828.1 0.0 Lake Marble Falls (1) 754.3 753.2 +1.1 Lake Travis (1) 722.0 716.2 +5.8 Lake Austin (1) 492.8 493.3-0.5 Town Lake (1) 438.6 439.8-1.2 Austin Gauge Upstream of U.S. 183 437.0 435.3 +1.7 Bastrop Gauge at Loop 150 352.2 353.9-1.7 Columbus Gauge at U.S. 90 190.2 194.1-1.9 Wharton Gauge at U.S. 59 (Business) 102.4 103.3-0.9 Difference Current FEMA (Feet) (2) (1) Flood Elevation computed at upstream face of the dam. Flood elevations on each lake will rise along the river, upstream of the dam. See flood profiles in Section IV and in Volume II, Chapter 6. (2) See Table IV 4 for explanation of vertical datum differences. (3) Current effective FEMA 100-year elevations adjusted to NAVD88. There are several reasons that the 100-year flood elevations differ from earlier studies along the Colorado River and especially on the Highland Lakes: 1. This is the first detailed, comprehensive, basin-wide approach for modeling, simulating, and computing frequency-based rainfall, runoff, reservoir elevations, and flood elevations along the entire river corridor. Halff Associates, Inc. ES-6 July 2002
2. There is an additional 25 years of historical flood and rainfall records that have been collected since the previous flood studies of the mid to late 1970 s. This provides a more comprehensive statistical database for developing flood frequency estimates. 3. The calibration and verification of the flood models used in the study has been enhanced significantly by the additional historical rainfall and flood data and the computational power of large capacity computers. The use of NEXRAD radar and GIS tools in the collection of data, development of computer models, and display of results has provided a greater degree of accuracy in the floodplain delineation and overall flood analysis process. 4. A more realistic assumption of the long-range river flood forecasting abilities of reservoir operators has had an effect on predicted 100-year pool levels. For example, in earlier flood studies to determine FEMA pool elevations on Lake Travis, an unrealistic assumption of a reliable 36-hour forecast time was used. Even with advanced NEXRAD radar and additional rainfall and stream gauges, a 12-hour flood forecast is considered by the LCRA and the Corps as the maximum time that can be safely used in dam gate operations. 5. Within the historical period of record (1930-1999) used in this study, the 1938 flood would have caused Lake Travis to reach approximately the projected 100-year flood pool (722) if the lakes had been in place. This 1938 flood, which was a high volume event, is statistically considered to be approximately the 100-year flood. In addition, the 1936, high volume flood, would have reached an estimated 719 elevation on Lake Travis. 6. As noted above, there are some minor vertical elevation datum differences throughout the study area as shown on Table IV 4, and in Volume II B (Chapter 3). The changes in datum from the previous studies to this study vary from near zero in the lower basin to a maximum of 0.3 feet in the Highland Lakes area. Floodplains - Based on the computed flood elevations from this study, the total 100-year floodplain for the Colorado River, from the mouth to the Red Bluff gauge, is about 449 square miles or 287,000 acres. Since this is the first time much of the river has been studied in detail, there are no comparisons from previous studies. Volume 2 of this appendix contains a complete set of 100- and 500-year floodplain delineations, and computed flood profiles of the 2-, 5-, 10-, 25-, 50-, 100-, 500-year frequency floods and the Standard Project Flood (flood of specific size and magnitude as defined in Corps of Engineers documentation, and generally corresponding to approximately a 500- to 1,000 year frequency). Halff Associates, Inc. ES-7 July 2002
GLOSSARY OF TERMS Antecedent Moisture Condition (AMC) A measure of the degree of wetness of a watershed at the beginning of a storm. Backwater The increase in stage, or elevation of the water surface, on the upstream side of a bridge, culvert, dam, other hydraulic structure, object, or deposit above that which would occur in the absence of the structure, object, or deposit. Base Flood For the FEMA Federal Flood Insurance Program, the base flood is the 100-year flood based on an existing conditions watershed. The regulatory floodplain is that area inundated by the base flood. Basin Drainage of watershed area. Bottomland The low-lying land along a watercourse (usually used in plural). CFS Abbreviation for cubic feet per second, which is a unit of water flow. Cross Section (of a stream or valley) In floodplain studies it is determined by a line approximately perpendicular to the main path of water flow, along which measurements of distance and elevation are taken in order to define channel and floodplain geometry. Can be surveyed in the field or determined from topographic maps. Detention The use of a surface water runoff storage facility to hold (detain) surface water temporarily during and immediately after a runoff event. Discharge As applied to a stream, the rate of flow, or volume of water flowing in a given stream at a given place and within a given period of time, usually quoted in cubic feet per second (cfs) or gallons per minute (gpm). Drainage Area The area draining into a lake, stream, sewer, or drain at a given point. The area may be of different sizes for surface runoff, subsurface flow, and base flow, but generally the surface runoff area is used as the drainage area. Also called catchment area, watershed, and river basin. Drainage Subarea (Subwatershed) Small drainage area used in detailed flood studies. Typically many subareas (subwatersheds) comprise the overall drainage area. Encroachment Fill, levees or structures which obstruct flow in the natural or existing floodplain usually for land reclamation and development reasons. Environmentally Significant Reaches of the floodplain, which contain significant stands of trees and wildlife habitat and which should be preserved in their natural state. Federal Emergency Management Association (FEMA) An independent agency of the federal government, founded in 1979, reporting to the President. FEMA s mission is to reduce loss of life and property and protect our nation s critical infrastructure from all types of hazards through a comprehensive, risk-based, emergency management program of mitigation, preparedness, response and recovery. Halff Associates, Inc. GL-1 July 2002
GLOSSARY OF TERMS (Continued) FEMA Floodplain The area inundated by the base flood, assuming existing channel, bridge, and floodplain conditions. Flood An overflow of water onto land not normally covered by water and that is used or usable by man. Floods have two essential characteristics. The inundation of land is temporary; and the land is adjacent to and inundated by overflow from a river or stream or an ocean, lake, or other body of standing water. Normally, a flood is considered as any temporary rise in a streamflow or stage, but not the ponding of surface water, that results in significant adverse effects in the vicinity. Adverse effects in sewers and local drainage channels include creation of unsanitary conditions or other unfavorable situations by deposition of materials in stream channels during flood recessions and rise of ground water coincident with increased streamflow. Flood Crest The maximum stage of elevation reached by the waters of a flood at a given location. Flood Frequency A means of expressing the probability of flood occurrences as determined from a statistical analysis of representative streamflow, rainfall and runoff records. A 10-year frequency flood would have an average frequency of occurrence in the order of once in 10 years (a 10 percent chance of being equaled or exceeded in any given year). A 50-year frequency flood would have an average frequency of occurrence in the order of once in 50 years (a 2 percent chance of being equaled or exceeded in any given year). A 100-year frequency flood would have an average frequency of occurrence in the order of once in 100 years (a 1 percent chance of being equaled or exceeded in any given year). A 500-year frequency flood would have an average frequency of occurrence in the order of once in 500 years (a 0.2 percent chance of being equaled or exceeded in any given year). Flood of Record The maximum recorded flood discharge or elevation as a given location. Flood Peak The maximum instantaneous discharge of a flood at a given location. It usually occurs at or near the time of the flood crest. Flood Stage The stage or elevation at which overflow of the natural banks of a stream or body of water begins in the reach or area in which the elevation is measured. Floodplain The relatively flat area or low lands adjoining the channel of a river, stream of watercourse or ocean, lake or other body of standing water which has been or may be covered by flood water. Floodplain Information The development of hydrologic and hydraulic data used to produce topographic mapping delineating the floodplain for a particular stream. Floodplain Management The proper management of the stream corridor to minimize flood damage, correct existing flood problems, preserve the natural valley storage characteristics of the stream and optimize the usefulness of this valuable natural resource. Flood Profile A graph showing the relationship of water surface elevation to location, the latter generally expressed as distance above the mouth for a stream of water flowing in an open channel. It is generally drawn to show surface elevation for the peak of a specific flood, but may be prepared for conditions at a given time or stage. Halff Associates, Inc. GL-2 July 2002
GLOSSARY OF TERMS (Continued) Floodway The channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height. Freeboard The vertical distance from the water surface up to the point of overtopping of a channel or other control structure. On a bridge, freeboard is measured vertically from the water surface to the low beam. Frequency An expression or measure of how often a hydrologic event of given size or magnitude should, on an average, be equaled or exceeded. For example, a 50-year frequency flood should be equaled or exceeded in size, on the average, only once in 50 years. Fully Urbanized Conditions In the context of a drainage study, the watershed or drainage area of a stream is considered to be completely developed, i.e. all land is assumed to be functioning in it's ultimate use. Other descriptions include: Fully Developed, 100 Percent Urbanized, Ultimate Development or Land Use, and Maximum Development. Greenbelt Preserves Areas of scenic and environmental significance identified for preservation. These sites may be acquired by either purchase, dedication or gift. Hydrograph A graph showing, for a given point on a stream or for a given point in any drainage system, the discharge, elevation, velocity or other property or water with respect to time. Land Use A land classification which indicates the manner in which a portion of land is being or will be utilized. Mean Sea Level A determination of mean sea level that has been adopted as a standard datum for heights. Elevation in feet and decimals thereof is a measurement vertically above the datum as used in surveys and engineering reports. NEXRAD Refers to the Next Generation Weather Radars installed by the National Weather Service, which use the Doppler principle. Weather radars send out radio waves from an antenna to measure rainfall. NEXRAD electronically converts the reflected radio waves into pictures showing the location and intensity of precipitation. One Hundred Year or 100-Year Flood A flood having an average frequency of occurrence on the order of once in 100 years at a designated location, although a flood of this magnitude may occur in any year and possibly in successive years. The 100-year flood has a 1 percent chance of being equaled or exceeded in any given year. In the past, this flood has been referred to as the Intermediate Regional Flood. 100-Year Floodplain The area inundated by the 100-year flood. Regulated - A term used to describe a river watershed that has dams and reservoirs that affect the hydrology of the stream. Halff Associates, Inc. GL-3 July 2002
GLOSSARY OF TERMS (Continued) Standard Project Flood The flood that may be expected from the most severe combination of meteorological and hydrological conditions that are considered reasonably characteristic of the geographical area in which the drainage basin is located, excluding extremely rare combinations. Peak discharges for these floods are generally about 40% to 60% of the Probable Maximum Floods for the same basins. Such floods, as used by the Corps of Engineers, are intended as practicable expressions of the degree of protection that should be sought in the design of flood control works, the failure of which might be disastrous. Unit Hydrograph A discharge hydrograph resulting from one inch of direct runoff distributed uniformly over the watershed, with the direct runoff generated at a uniform rate during the given storm duration. A watershed may have different unit hydrographs for storms of different durations. Unregulated - A term used to describe a river watershed that does not have significant structures, such as dams and reservoirs, which could affect the hydrology of the stream. USGS United States Geological Survey, Department of the Interior. A federal agency which collects, analyzes, and stores water resources information in addition to preparing and providing many different types of maps. Valley Storage The term used to describe a channel and floodplain s capacity to store some portion of the runoff volume as a flood wave moves downstream. Watershed The area drained by a stream or drainage system. The area contained within a divide above a specified point on a stream. Sources of Definitions United States Department of Agriculture, Soil Conservation Service, SCS National Engineering Handbook, Section 4, Hydrology, (Washington, D.C.: United States Government Printing Office, 1972), pp. 22-1 through 22-10). The G. and C. Merriam Company, Webster s New Collegiate Dictionary, 150 th Edition, (Springfield, Massachusetts, 1981). Halff Associates, Water Resources Department. Walesh, Stuart G., Urban Surface Water Management (New York: Wiley & Sons, 1989). Hoyt, William G., and Walter B. Langbein, Floods, (Princeton, New Jersey: Princeton University Press, 1955). U.S. Army Corps of Engineers, Various Publications. Halff Associates, Inc. GL-4 July 2002
VOLUME I REPORT I. General Documentation A. Overview of the Colorado River Flood Damage Evaluation Project 1. Purpose The purpose of this feasibility study is to develop and evaluate alternatives for implementing solutions to water resource related problems within the Lower Colorado River Basin. This Project is being developed as a cooperative effort by the U.S. Army Corps of Engineers and the local sponsor, the Lower Colorado River Authority. The study will be separated into several distinct parts: 2. Phase I - Identification of Existing Conditions The products resulting from PHASE I will include detailed, regionally consistent existing conditions models for hydrology, hydraulics, and economic flood damage analyses. Furthermore, floodplain boundary delineations will be incorporated into the base Geographic Information Systems (GIS) database provided by the non-federal sponsor. An assessment of existing conditions flood damages for the major urbanized areas along the river; An inventory of existing conditions environmental resources (wildlife and aquatic habitat, land cover classification, threatened and endangered species) throughout the basin and identification of potential ecosystem restoration areas; An assessment of previously identified cultural resources within the basin; An assessment of recreation development and identification of recreation needs. 3. Phase II - Detailed Evaluation Of Alternatives (Future Phase) This future part of the study will include the detailed analysis of alternatives, and selection of Recommended Plan(s). Congressionally authorized projects emanating from these interim studies will proceed to final design and implementation (upon approval and agreement of sponsorship by a non-federal entity). The areas to be studied in detail during this feasibility study will include, but are not limited to: Onion Creek Watershed Shoal Creek Watershed Walnut Creek Watershed Highland Lakes City of Wharton Halff Associates, Inc. 1 July 2002
4. Colorado River Flood Damage Evaluation Project Phase I This hydrologic and hydraulic study is a thorough and in-depth, basin-wide approach for modeling, simulating, and computing frequency-based rainfall, runoff, reservoir elevations, and stream flood elevations along the entire Colorado River corridor. The analytical tools and engineering analyses prepared for this appendix include the most comprehensive and detailed examination of flooding issues in the Colorado River basin to date. The use of extensive detailed topographic mapping along the river corridor, state-of-the art Geographical Information System (GIS) and statistically sound hydrologic modeling tools provide not only baseline conditions flood data, but will support future analysis and decisions related to solutions. In response to the June 1997 flood on the Highland Lakes, the LCRA initiated steps to review flood management of the Colorado River, including a critique of reservoir operations (December 1998) and the initiation of a Corps flood damage evaluation feasibility study. This two-phase flood damage evaluation feasibility study is being developed as a cooperative effort by the Fort Worth District, U.S. Army Corps of Engineers, and the local sponsor, the Lower Colorado River Authority. Phase I will result in a Corps Information Paper, which will include this Hydrology and Hydraulic Appendix. This study included detailed river corridor topographic mapping and flood elevation determinations for 482 river miles, along the main stem of the Colorado River. The 18,300 square mile basin was divided into 290 sub-basins with an average size of approximately 63 square miles. The study team chosen to prepare this Hydrology-Hydraulics Appendix consists of Halff Associates, Inc., David Ford Consulting Engineers, Espey Consultants, Inc., Surveying and Mapping, Inc., and other flood modeling consultants. The study, which started in July 2000, has been closely coordinated between the Corps of Engineers, LCRA, and other agencies and communities. Study findings and results have been reviewed by the Fort Worth District Corps of Engineers, LCRA, peer review within the Study Team, Tulsa District of the Corps, the U.S. Geological Survey, and independent consultants. The stated purpose of this basin-wide feasibility study is to develop and evaluate alternatives for implementing solutions to water resource-related problems within the Lower Colorado River Basin. Specific products to be developed in Phase I of the feasibility study include: An assessment of existing conditions flood damages for the major urbanized areas along the river. This will include detailed, regionally consistent existing conditions models for hydrology, hydraulics, and economic flood damage analyses. Furthermore, floodplain boundary delineations will be incorporated into the LCRA Geographic Information System (GIS) database. An inventory of existing conditions environmental resources (wildlife and aquatic habitat, land cover classification, threatened and endangered species) throughout the basin and identification of potential ecosystem restoration areas. An assessment of previously identified cultural resources within the basin. Halff Associates, Inc. 2 July 2002
An assessment of recreation development and identification of recreation needs within the basin. This has been prepared to partially fulfill the requirements stated in the first item of the above product list. Additionally, this Appendix will: Provide the technical data to assist the U.S. Army Corps of Engineers and the Lower Colorado River Authority in minimizing basin-wide flooding in the Colorado River Watershed. Develop the technical elements to enhance existing and future basin-wide, realtime flood forecasting and operation systems and flood warning programs to alert the public and local officials of imminent flooding. Provide frequency-based flood profiles/elevations developed for application in the Corps Flood Damage Assessment (FDA) Program to estimate expected flood damages along the Colorado River. B. Study Area Description 1. Basin Description The Colorado River watershed extends diagonally northwest to southeast from southeast New Mexico to the Gulf of Mexico near Matagorda, Texas. The basin is bounded on the east by the Brazos River Basin, and on the west by the Guadalupe, Nueces, Lavaca-Navidad and Rio Grande Basins. The length of the watershed is about 595 miles, its maximum width is about 170 miles, and its total drainage area is 42,344 square miles. The width of the extreme upper part of the watershed is about 85 miles. This width increases gradually to about 170 miles near Milburn; then decreases to 30 miles at Austin, maintaining this width to Columbus; below Columbus the width gradually diminishes toward the Gulf of Mexico. The upper portion of the Basin lies in the Great Plains, a flat semiarid region with numerous closed basins, of which 11,403 square miles do not contribute to the Colorado River drainage. From the eastern limits of the Great Plains to the vicinity of Austin, the river traverses the North Central Plains, the topography of which varies from gentle rolling plains to the rough broken terrain of the Edwards Plateau. Leaving the plains area at the Balcones Escarpment above Austin, the river enters the Coastal Plains, an area of rolling hills extending to the vicinity of Columbus, and then enters the flat coastal prairie extending to the Gulf. The Colorado River Basin, has a total contributing drainage area of 30,941 square miles. The general land elevation of the Colorado River Basin decreases gradually from 4,500 feet NGVD in the High Plains sections southeasterly to 2,600 feet NGVD in the Big Spring area. Between Big Spring and the western edge of the Balcones Escarpment above Austin, the elevation decreases southeastwardly from 2,600 to 1,000 feet NGVD. Between the escarpment and the coastline, the land elevations decrease to a foot above sea level near the coast. The Colorado River system consists principally of the main stream and six major tributaries. The six major tributaries are Beals Creak, Concho River, Pecan Bayou, Halff Associates, Inc. 3 July 2002
San Saba River, Llano River, and Pedernales River. All of the tributaries enter the Colorado River above Austin, and all except Pecan Bayou enter from the western bank. Table I-1 lists key locations of the Colorado River system, the river mileage above the Gulf of Mexico, and the contributing drainage area at each location. The contributing area above Marshall Ford Reservoir (Lake Travis), the major flood control structure on the Colorado River main stem, is approximately 27,565 square miles. TABLE I-1 River Miles And Drainage Areas Colorado River Basin LOCATION ON RIVER DRAINAGE AREA IN SQ MI COLORADO RIVER MILE (1) TOTAL CONTRIBUTING Below Concho River 628.9 24,128 12,725 Below Pecan Bayou 513.1 28,163 16,760 Below San Saba River 479.8 31,473 20,070 Below Llano River 400.3 36,712 25,309 Below Pedernales River 354.6 38,763 27,360 At Austin Gauge (08158000) 290.3 39,009 27,606 At Bastrop Gauge 236.6 39,979 28,576 At Columbus Gauge 135.1 41,640 30,237 At Wharton Gauge 66.6 42,003 30,600 At Bay City Gauge 32.5 42,240 30,837 (1) RM from USGS Water Resources Data Texas Water Year 1999 In the Colorado River Basin above Austin, there are five Federal and 13 non-federal reservoirs existing or under construction with an individual capacity greater than 5,000 acre-feet. Of the five Federal reservoirs, O.C. Fisher Lake on the North Concho River and Hords Creek Lake on Hords Creek are the only existing Corps of Engineers projects. Twin Buttes Reservoir on the South and Middle Concho Rivers and Marshall Ford Reservoir (Lake Travis) on the mainstem Colorado River were constructed by the Bureau of Reclamation. Congress has given the Corps of Engineers responsibility for flood control for the abovementioned Bureau of Reclamation reservoirs. Brady Creek Reservoir on Brady Creek was constructed by the Soil Conservation Service (SCS) in cooperation with the City of Brady. Pertinent data for these Federal and non-federal reservoirs are presented in Table I-2. Halff Associates, Inc. 4 July 2002
RESERVOIR STREAM Colorado River Flood Damage Evaluation Project Phase I TABLE I-2 Existing Reservoirs Colorado River Basin (1) RIVER MILE FEDERAL CONTRIBUTING DRAINAGE AREA (SQ MI) TOTAL STORAGE (AC-FT) O.C. Fisher North Concho River 6.6 1,383 396,400 Twin Buttes South Concho River 13.0 3,015 640,600 Middle Concho River 4.0 Hords Creek Hords Creek 27.8 48 25,310 Brady Creek Brady Creek 34.0 508 90,480 Marshall Ford Colorado River 318.0 27,567 1,951,400 NON-FEDERAL J. B. Thomas Colorado River 837.0 987 204,000 Colorado City Morgan Creek 2.5 290 32,000 Champion Creek Champion Creek 0.9 203 42,000 E.V. Spence Colorado River 730.9 4,044 489,000 Oak Creek Oak Creek 20.0 244 39,000 Nasworthy South Concho River 7.6 2,655 14,000 O.H. Ivie Colorado River 615.1 12,647 Brownwood Pecan Bayou 57.1 1,544 150,000 Coleman Jim Ned Creek 52.2 292 40,000 Buchanan Colorado River 413.6 20,685 992,000 Inks Colorado River 409.4 20,724 17,000 Lyndon B. Johnson Colorado River 388.0 25,750 137,000 Marble Falls Colorado River 381.8 25,810 9,000 Lake Austin Colorado River 297.6 27,670 21,000 (1) U.S. Army Corps of Engineers, Reconnaissance Report, Central Colorado River Basin, September 1989. 2. Climatology Climatological conditions over the watershed are generally mild and vary from subtropical along the Gulf Coast to semiarid in the upper headwater regions. The rainfall decreases rather uniformly from the Gulf toward the headwaters. At San Halff Associates, Inc. 5 July 2002
Angelo in the upper Colorado River Basin, the average rainfall is about 20 inches annually. The average yearly rainfall over the Highland Lakes is about 30 inches, while Bay City near the Gulf Coast gets about 44 inches annually. Rainfall in Central Texas also varies greatly from year to year. Even though Austin receives about 32 inches annually on average, the rainfall is less than 26 inches in about one-fourth of the years. The average annual temperatures over the Basin are generally moderate, with the highest at the Gulf and decreasing gradually with the increase in latitude and elevation. Winter temperatures are generally mild, but occasional cold periods of short duration result from the rapid moving of cold high-pressure air masses from the northwest. Snowfall and subfreezing temperatures are rare in the lower section of the Basin near the Gulf, but are experienced occasionally during the winter season in the northerly parts of the Basin. Summer temperatures are high throughout the Basin. The National Oceanic and Atmospheric Administration (NOAA) have a first order recording station at Austin, Texas. Records at this station are shown below. MEANS Rainfall (Annual) 32.69 in. Maximum (1919) Minimum (1954) Temperature (Monthly) 68.0 degrees F. Daily Maximum (July 1954) Daily Minimum (Jan. 1949) Relative Humidity 67 percent EXTREMES 64.68 in. 11.42 in. 109 degrees -2 degrees Mean annual precipitation over the Colorado River Basin ranges from a minimum of about 18 inches in the northwest extremity of the Colorado River Basin (contributing drainage area) to a maximum of about 33 inches at Austin. Table I-3 presents average annual precipitation at rainfall gauges in the Colorado River Basin upstream of Austin. TABLE I-3 Average Annual Precipitation Colorado River Basin STATION YEARS OF RECORD AVERAGE ANNUAL PRECIPITATION Big Spring 39 18.01 inches Colorado City 69 22.19 inches San Angelo WSO AP 41 19.02 inches Ballinger 88 22.73 inches Coleman 90 27.29 inches Llano 90 26.84 inches Fredericksburg 67 28.87 inches Austin WSO AP 57 32.69 inches Halff Associates, Inc. 6 July 2002
C. Historical Flood Data 1. USGS Stream Gauges The observation of Colorado River streamflow began in 1898 when the U.S. Geological Survey (USGS) established a gauge on the Colorado River at Austin. In 1903, NOAA established gauges on the Colorado River at Columbus and Ballinger. For the period 1898-1988, stage and discharge records of varying lengths are available for about 91 streamflow and reservoir gauges in the Colorado River Basin. The primary gauges used in this Central Colorado River Basin study are shown in Table I-4. 2. LCRA Stream Gauges The LCRA operates a hydrometeorological (Hydromet) data collection system of automated precipitation and stage gauges along the Colorado River (See Table I-4). Table I-4 Stream Gauges GAUGE NAME LCRA ID USGS ID Latitude (DMS) Longitude (DMS) Colorado River at Winchell 1199 08138000 312807-990944 Pecan Bayou near Mullin 1390 08143600 313102-984429 San Saba River at Menard 1499 08144500 305509-994708 San Saba River near Brady 1563 08144600 310013-991608 San Saba River at San Saba 1769 08146000 311250-984311 Colorado River near San Saba 1911 08147000 311304-983349 Cherokee Creek near Bend 1929 310156-983439 Lake LBJ at 1431 Bridge 2096 303927-982539 Llano River near Junction 2306 08150000 303013-994405 Johnson Fork near Junction 2313 302538-994047 James River near Mason 2399 303516-991833 Comanche Creek near Mason 2424 304308-991152 Llano River near Mason 2431 08150700 303936-990631 Beaver Creek near Mason 2435 303839-990548 Willow Creek near Mason 2443 304418-990704 Hickory Creek near Castell 2498 304255-984914 San Fernando Creek near Llano 2616 304518-984911 Llano River at Llano 2641 08151500 304504-984011 Little Llano River near Llano 2669 304821-983431 Lake LBJ at 2900 Bridge 2699 303833-982648 Sandy Creek near Willow City 2851 303310-984205 Sandy Creek near Kingsland 2891 08152000 303328-982820 Lake LBJ at Sandy Harbor 2899 303337-982549 Backbone Creek at Marble Falls 2992 303501-981703 Pedernales River near Fredericksburg 3299 08152500 301314-985212 Pedernales River near Johnson City 3385 08153500 301732-982358 Miller Creek near Johnson City 3491 301702-981753 Halff Associates, Inc. 7 July 2002
TABLE I-4 (Continued) GAUGE NAME LCRA ID USGS ID Latitude (DMS) Longitude (DMS) Lake Austin at Davenport Ranch 3990 302055-974748 Bull Creek at Loop 360, Austin 3992 08154500 302219-974706 Barton Creek at Loop 360, Austin 4520 08155300 301439-974808 Town Lake near Longhorn Dam 4543 301457-974313 Colorado River at Austin 4553 08158000 301440-974140 Walnut Creek at Webberville Road, Austin 4561 08158600 301658-973917 Onion Creek at Buda 4595 300512-975055 Onion Creek at Hwy 183, Austin 4598 08159000 301038-974120 Gilleland Creek near Manor 5417 301752-973405 Wilbarger Creek near Elgin 5464 301355-972558 Big Sandy Creek near Elgin 5473 08159170 301556-971941 Colorado River at Bastrop 5499 08159200 300617-971909 Cedar Creek below Bastrop 5523 300211-971850 Colorado River at Smithville 5541 08159500 300043-970944 Colorado River above La Grange 5599 08160500 295445-965349 Buckners Creek near Muldoon 5608 295043-970241 Cummins Creek near Frelsburg 5696 294933-963450 Colorado River at Columbus 6300 08161000 294223-963214 Colorado River near Garwood 6399 293055-962432 Colorado River at Wharton 6499 08162000 291834-960613 San Bernard River at East Bernard 6637 293159-960322 Colorado River near Lane City 6537 291125-960411 Colorado River at Bay City 6599 08162500 285829-960039 Lake LBJ at 1431 Bridge 2096 303927-982539 Llano River near Junction 2306 08150000 303013-994405 Johnson Fork near Junction 2313 302538-994047 James River near Mason 2399 303516-991833 Comanche Creek near Mason 2424 304308-991152 Llano River near Mason 2431 08150700 303936-990631 Beaver Creek near Mason 2435 303839-990548 Willow Creek near Mason 2443 304418-990704 Hickory Creek near Castell 2498 304255-984914 San Fernando Creek near Llano 2616 304518-984911 Llano River at Llano 2641 08151500 304504-984011 Little Llano River near Llano 2669 304821-983431 Lake LBJ at 2900 Bridge 2699 303833-982648 Sandy Creek near Willow City 2851 303310-984205 Sandy Creek near Kingsland 2891 08152000 303328-982820 Lake LBJ at Sandy Harbor 2899 303337-982549 Backbone Creek at Marble Falls 2992 303501-981703 Pedernales River near Fredericksburg 3299 08152500 301314-985212 Pedernales River near Johnson City 3385 08153500 301732-982358 Miller Creek near Johnson City 3491 301702-981753 Halff Associates, Inc. 8 July 2002
TABLE I-4 (Continued) GAUGE NAME LCRA ID USGS ID Latitude (DMS) Longitude (DMS) Lake Austin at Davenport Ranch 3990 302055-974748 Bull Creek at Loop 360, Austin 3992 08154500 302219-974706 Barton Creek at Loop 360, Austin 4520 08155300 301439-974808 Town Lake near Longhorn Dam 4543 301457-974313 Colorado River at Austin 4553 08158000 301440-974140 Walnut Creek at Webberville Road, Austin 4561 08158600 301658-973917 Onion Creek at Buda 4595 300512-975055 Onion Creek at Hwy 183, Austin 4598 08159000 301038-974120 Gilleland Creek near Manor 5417 301752-973405 Wilbarger Creek near Elgin 5464 301355-972558 Big Sandy Creek near Elgin 5473 08159170 301556-971941 Colorado River at Bastrop 5499 08159200 300617-971909 Cedar Creek below Bastrop 5523 300211-971850 Colorado River at Smithville 5541 08159500 300043-970944 Colorado River above La Grange 5599 08160500 295445-965349 Buckners Creek near Muldoon 5608 295043-970241 Cummins Creek near Frelsburg 5696 294933-963450 Colorado River at Columbus 6300 08161000 294223-963214 Colorado River near Garwood 6399 293055-962432 Colorado River at Wharton 6499 08162000 291834-960613 San Bernard River at East Bernard 6637 293159-960322 Colorado River near Lane City 6537 291125-960411 Colorado River at Bay City 6599 08162500 285829-960039 3. Flood History The storms that cause precipitation on the Colorado River Basin are of three general types: (1) thunderstorms, sometimes causing devastating cloudbursts (2) frontal storms, and (3) cyclonic storms originating in the tropics or the western Gulf of Mexico. In addition, the Colorado River crosses the Balcones Escarpment area above Austin in which the physical features of the land exercise some influence on rainfall. Some of the highest rainfall rates experienced in the United States have been recorded in this area. Table I-5 presents historical peak discharges for stream gauges on the Colorado, Llano, and Pedernales Rivers. Halff Associates, Inc. 9 July 2002
TABLE I-5 Historical Flood Data Colorado River Basin COLORADO RIVER NEAR SAN SABA D.A. = 20,111 SQ. MI. DATE OF FLOOD PEAK DISCHARGE (CFS) September 25, 1900 184,000 November 10, 1918 77,100 April 26, 1922 130,000 October 17, 1930 78,900 May 19, 1935 86,000 September 21, 1936 179,000 July 23, 1938 224,000 September 11, 1952 69,000 May 14, 1957 66,200 COLORADO RIVER AT AUSTIN D.A. = 27,835 SQ.MI. DATE OF FLOOD PEAK DISCHARGE (CFS) July 7, 1869 550,000 June 8, 1899 113,000 April 7, 1900 151,000 December 4, 1914 164,000 May 1, 1922 120,000 June 15, 1935 481,000 September 28, 1936 234,000 July 25, 1938 276,000 LLANO RIVER AT LLANO D.A. = 4,197 SQ. MI. DATE OF FLOOD PEAK DISCHARGE (CFS) June 14, 1935 380,000 September 10, 1952 232,000 October 5, 1969 154,000 October 13, 1973 154,000 August 3, 1978 139,000 September 8, 1980 210,000 Halff Associates, Inc. 10 July 2002
PEDERNALES RIVER NEAR JOHNSON CITY D.A. = 900 SQ.MI. DATE OF FLOOD PEAK DISCHARGE (CFS) September 11, 1952 441,000 April 24, 1957 125,000 October 4, 1959 142,000 August 3, 1978 127,000 D. FEMA Flood Insurance Study Discharges See Table I-6 for a summary of various effective (as of 1998) FEMA flood insurance study discharges for the 10-, 50-, 100-, and 500- year floods. TABLE I-6 Summary Of Colorado River Flood Insurance Study Discharges LOCATION FIS DATE 10-YR 50-YR 100-YR 500-YR Burnet County November 16, 1990 At Inks Dam 66,865 126,756 161,410 304,101 At Alvin Wirtz Dam 117,938 259,824 (1) 330,269 481,505 At Max Starke Dam 117,741 268,336 329,033 480,512 Travis County June 5, 1997 Lake Travis 691 (2) 710 (2) 716 (2) 728.5 (2) At Tom Miller Dam 25,000 102,000 170,000 335,000 At Confluence of Onion 50,000 102,000 170,000 335,000 Creek At Travis/Bastrop 75,000 150,000 210,000 350,000 County Boundary Bastrop County @ Loop 150 December 8, 1998 63,386 119,464 149,310 231,296 Colorado County @ US Route 90 January 3, 1990 n/a n/a 136,000 n/a Wharton, TX @ US 59 March 16, 1982 70,000 127,500 (3) 139,500 (3) 247,000 (1) Typo in FIS (2) Stillwater Pool Elevations (Computed at upstream face of dam) (3) Adjusted for overflow loss E. Study Tasks Overview Volume II A-D, Technical Support Data contains very detailed descriptions of each major task of this study, including assumptions, comparisons with previous studies, and results. Following is a list of those major tasks. Halff Associates, Inc. 11 July 2002
Prepare a Period-of-Record Flow Analysis Prepare Historical Frequency Analysis at Each Gauge Prepare An Initial/Preliminary HEC-RAS Hydraulic Prepare initial/preliminary UNREGULATED Basin-wide HEC-HMS Hydrologic Model Rainfall Information for HMS Model HEC-HMS Storm Reproduction (Calibration) PHASE HMS Verification Phase (Unregulated Conditions) Prepared HEC-5 Reservoir Operation Model for Regulated Basin Conditions Final RAS Hydraulic Model(s) for Main Stem Final HMS/ HEC-5/ RAS Model(s) for Main Stem Convert Flood Profiles to Floodplain Inundation Layers for GIS Mapping F. Coordination Efforts During Study 1. Technical Meetings Numerous technical meetings were held during the conduct of the study, including two large meetings. See Volume II, Chapter 7, CD-9 for agenda and minutes of Technical Meetings. On January 17, 2001, a formal Technical Meeting was held at the LCRA. Attendees included staff from the LCRA, Corps of Engineers, FEMA, and the Halff Study Team. This stated objective of that meeting was to discuss the technical issues related to hydrologic and hydraulic modeling of the Colorado River basin and to come to a decision or firm direction on each issue. Issues included discharge-frequency analysis, distribution of rainfall, routing, overall technical approach, and reservoir operation issues. On November 6, 2001, another formal Technical Meeting was held at the Fort Worth office of Halff Associates. Representatives of the LCRA, Corps of Engineers, FEMA s technical consultant (PBS&J), and the Halff Study Team were present. This meeting was primarily held to discuss the Espey Consultants Draft Flood Frequency Analysis Report, the Ron Hula Period-of-Record (SUPER) analysis, and the rainfall generator being developed by Halff Associates for the study. 2. Other Coordination See Volume II, Chapter 7, CD 9 for additional coordination between Halff Associates, Inc. and others. This CD contains Technical Memorandums, transmittal letters, status reports and other documents generated for this study. Halff Associates, Inc. 12 July 2002
G. Previous Studies See the References and Previous Studies on Pages 23-28 of this volume. H. Limitations of Data and Models Used in Study 1. Mapping Data a. Digital contour mapping to national map accuracy standards, 4-foot interpolated to 2-foot in most areas, some urban areas 2-foot interpolated to 1-foot. b. Upstream limits of detailed mapping is the Burnet/Lampasas county line. c. Hydrology (watershed divides) based on USGS 30 meter resolution terrain data, plus or minus 10-foot vertical accuracy. d. Field surveyed cross sections average 2 cross section per mile, channel interpolation based on these surveys. 2. GIS Data a. Individual arcs limited to 500 vertices, effects stationing line. b. Amount of terrain data required the use of multiple TINs to define the main stem. 3. HEC-5 Model a. HEC-5 is a Corps of Engineers program developed primarily for Corps flood control reservoir systems. b. HEC-5, Version 8.0 has certain limitations in modeling a system such as the Highland Lakes along the Colorado River. i. The current HEC-5 model cannot forecast to interim pool elevations and increase releases based on those forecasts. HEC-5 can only increase releases when the top of flood pool is forecast to be exceeded. ii. HEC-5 uses the same forecast time for inflows into Lake Travis and for looking downstream at control points. These limitations could not be addressed in the current HEC-5 model. c. Two other limitations of Version 8.0 were overcome by changing parameters iteratively within the model. i. Currently, the pre-release option must be used universally for all reservoirs. In order to keep inflow = outflow up to the maximum outlet capacity at pass through reservoirs, the model was forced to release inflow (by adding a QA card) for the few time periods that the pre-release (outflow > inflow) occurred. This had no effect on the final pool elevation. ii. Another problem was that the releases from Lake Travis never allowed the river at the Austin gauge to reach its full channel capacity. In order to overcome this limitation, the channel capacity in HEC-5 at Austin was altered until the maximum allowable flow was achieved by the combination of the Halff Associates, Inc. 13 July 2002
Travis releases and Austin local flows. This resulted in the proper peak flow rate at Austin and Travis outflows for various frequency storm events. d. Lake Buchanan Operations The Lake Buchanan Operation, in accordance with the 1990 FEMA/LCRA agreement, could not be input directly into HEC-5. A spreadsheet was used to compute a Buchanan outflow hydrograph based on Buchanan inflows and San Saba gauge flows (from HMS) in accordance with the 1990 FEMA/LCRA agreement. The computed outflow hydrograph was input directly into HEC-5 as Buchanan releases (by adding QA cards). 4. RESPROB Program RESPROB is a basic (single) reservoir operation program. It can only look at one downstream control point with one maximum channel capacity (Columbus in this case), and no routing of flow to the downstream control point is considered. This is another reason that RESPROB and the joint probability analysis should only be used to extrapolate the SUPER period-of-record frequency curve to larger storms. 5. Limitations of Lake Travis Joint Probability Analysis: The Total Probability Theorem was applied to extend the computed period-of-record frequency pool elevation curve for larger events at Lake Travis. The Total Probability Theorem requires independence between the events. In the case of the Lake Travis analysis, the two events are starting pool elevation and inflow hydrograph (storm frequency). Since the starting pool elevation probabilities were determined from the period-of-record analysis, the effects of large storm events influenced these starting pool elevations. This is a major reason why the period-of-record (SUPER) results should be used except for the large (less frequent) events. The joint probability analysis is only used to extrapolate the SUPER period-of-record frequency curve to these larger storms. 6. SUPER Model i. The SUPER period-of-record analysis uses a daily time step which will not capture instantaneous peaks. a. Instantaneous peaks were calculated based on the slope of a line between the daily peak flow and the instantaneous peak flows observed at the gauging stations. These instantaneous peak flows were used in HEC-FFA to calculate unregulated and regulated frequencies. b. Hourly inflow hydrograph ordinates for Lake Travis were estimated based on preserving total daily volume and the general shape of the hydrographs. This hydrograph was used to produce the peak stage for Lake Travis in the Joint Probability procedure. ii. The SUPER simulation assumes that the Reservoir Operation Plan is not deviated from during the period of simulation for all reservoirs. Halff Associates, Inc. 14 July 2002
7. HEC-HMS The HEC-HMS model was developed for the entire Colorado River basin below Lake O.H. Ivie. This basin-wide model should not be applied to smaller tributaries in the basin. These small basins would need to have their own hydrologic model assembled. The same principles could be used for the smaller tributary watersheds but the sub-basins would need to be more defined to produce an acceptable model to predict peak flows in the watershed. 8. HEC-RAS a. No Peak Discharges - Unsteady HEC-RAS models were used primarily to compute the maximum water surface profiles for the 474 studied river miles. Inherently, the unsteady HEC-RAS model generates stage and flow hydrographs at any cross section in the model. We do not recommend taking a peak flow value from these flow hydrographs to be used for any purpose other than this study. Note, for the sections that will be converted to steady HEC-RAS, peak flows will be adjusted to generate the maximum water surface profile developed using the unsteady HEC-RAS models. b. As part of the calibration to frequency events, flow hydrographs at gauged points were compared to those resulting from the hydrology and reservoir operation study components. Routing of the flood frequency hydrographs using the unsteady HEC-RAS models prepared for this study reproduce the HEC-HMS and HEC-5 results at comparison points with less than a 10% difference. Hydrographs computed by the 3 components of the study: HEC-HMS Hydrology, HEC-5 Reservoir Operations, and HEC-RAS flood profiles, for the different points of interest are not expected to be identical. For this study, the peak flows computed with HEC-RAS were generally lower than those computed with HEC-5 or HEC-HMS. However, the stage hydrograph used as a downstream boundary for each RAS model is the maximum stage generated by either HEC-5 at the dams, or by HMS peaks at the various gauges downstream of Longhorn Dam. c. Energy Approach - In order to create stable, robust, unsteady models for the various reaches, most bridges were modeled using the energy approach method for high and low flows. Although, the momentum, Yarnell, or weir equations may yield slightly higher results, the differences were considered negligible, typically ranging below 0.2, although a few values reached 0.5. Tables developed for each bridge comparing results from all modeling approach methods are included in the discussions for each HEC-RAS model reach. d. Relation to Topographic Mapping - Computed water surface elevations are only as accurate as the topography used. See Volume II- Chapter 3- Mapping and Geographic Information System for a detailed description of sources and accuracy of the terrain data used for this study. e. Roughness Coefficients Manning s n values were calibrated using only one historical flood event, either June 1997 or October 1998. Because of changing conditions in the river, n values need to be reviewed when using the models. Halff Associates, Inc. 15 July 2002
II. Engineering Analyses Methodology Colorado River Flood Damage Evaluation Project Phase I A. General Overview Of Technical Approach (Hydrologic And Hydraulic Analyses) See Volume II, Technical Support Data for a detailed description of each major component of the study. These chapters will include documentation of data gathering, model development, software applications, calibration, and findings. The following is a brief description of the technical component, which will be covered, in the referenced chapter. 1. Period-of-Record Flow Analysis (Chapter 2) The Study Team prepared a historical period-of-record analysis for development of unregulated flows on the Colorado River basin, using the best available gauge data and the Corps Southwestern Division Modeling System for the Simulation of the Regulation of a Multi-Purpose Reservoir System (SUPER) program. This analysis provided a full 70-year period-of-record, unregulated and regulated set of historical basin flows (peaks) at all gauges. This analysis also provided a regulated set of historical volume based flows for Lake Travis. The data was in a daily time step format. Another Corps program, RESPROB, was also used as a tool to extend the period-of-record study for less frequent events at Lake Travis. A by-product of this analysis was a historical discharge-frequency analysis for both unregulated and regulated basin conditions, to be used for calibration and for comparison to other historical frequency analyses. Mr. Ron Hula, retired Corps of Engineers hydrologist and contractor to the Fort Worth District Corps of Engineers, prepared this process, which included extensive statistical analysis and joint probability development. Extensive documentation of this analysis is contained in Chapter 2, and a summary of results is contained in the Summary of Findings. 2. Historical Frequency Analysis at Each Gauge (Chapter 1) Utilizing the WRC Bulletin 17B guidelines, other possible criteria, and the HEC-FFA Flood Flow Frequency Analysis software, the Study Team computed frequency versus peak flows for unregulated conditions at 16 gauges on the Colorado River and tributaries. These results were used in conjunction with the period-of-record analysis results and allowed the Study Team to develop an unregulated, data set to use for hydrologic model calibration (HMS). This step provided the basis for calibration of the hydrology models. A summary of results is contained in the Summary of Findings. 3. Initial/Preliminary HEC-RAS Hydraulic Model (Chapter 6) This preliminary model was developed for hydrologic routing purposes, using GISgenerated TIN s (LCRA detailed mapping topo DEM s) and Geo-RAS. The model limits were from the San Saba, Texas gauge on the Colorado River to the mouth at Matagorda Bay. A summary of results is contained in the Summary of Findings. 4. Initial/Preliminary UNREGULATED Basin-Wide HMS Model (Chapter 4) The steps in this process included: a. Finalized sub-basins for the entire study area, starting with UT-CRWR base. Halff Associates, Inc. 16 July 2002
b. Reproduce the UT-CRWR basin delineations using HEC-GeoHMS c. Utilized existing Sub-basin parameter utility program, develop sub-basin parameters: Snyder s unit hydrograph parameters, revised loss rates, urbanization, etc. d. Tributary Routing Used GIS-generated Modified puls routing tables for all routing reaches outside of the main stem Colorado River. Used USGS DEM s and Geo-RAS. e. Main Stem Routing - Used GIS-generated (LCRA detailed mapping topo DEM s) and Geo-RAS, prepared the preliminary RAS (steady) model of river, develop Modified puls routing tables (from Lake Buchanan to Matagorda Bay). From upper Buchanan to Ivie Reservoir, used USGS DEM. f. Routing through the original river portion of the now existing reservoirs (Highland Lakes) was based on using old topographic maps, sediment ranges, or other cross section sources, as available 5. Rainfall Information for HMS Model The steps in this process included: a. Prepared the Hypothetical Storm Rainfall b. Used 96-hour storm duration and a centered temporal storm. c. Rainfall data was obtained from TP-40, TP-49, and Hydro-35. For SPF, used Corps Engineer Bulletin 52-8 (SPF), EM 1110-2-1411. d. STORM CENTERS FOR SPECIAL POINTS OF INTEREST (POI S) Six Special POI s, were at Colorado River near San Saba Gauge, Llano River at Llano Gauge, Buchanan Dam, Mansfield Dam (Travis), Bastrop gauge, and Wharton gauge. e. Contouring for Centering of critical rainfall storms for the Special POI s i. Prepared setup grid pattern and input files for automated, multiple stormcentering HMS runs. ii. One search grid (sub-basin centroids) iii. Shape one elliptical shape iv. Orientation Use one only, the preferred orientation (HMR-51 & 52) f. Post-Contouring Phase - Select final storm centers for each frequency 6. HMS Storm Reproduction (Calibration) PHASE Input was actual NEXRAD rainfall and stream gauge data from floods that occurred in the 1990 s and 2000. a. Used NEXRAD historical rainfall data (for 3 storms June 1997; October 1998; and November 2000). b. Generated flood hydrographs, from the recorded storm rainfall and compared to the historical gauge records for those storms. Halff Associates, Inc. 17 July 2002
c. At this point, we had a calibrated HMS runoff model (still unregulated). All final hydrographs were sent to DSS for further verification (HMS) and processing by HEC-5 and RAS (unsteady) 7. HMS Verification Phase (Unregulated Conditions) The purpose of this phase was to test and verify the response of our calibrated HMS model using the historical frequency data derived from the FFA Analysis and the Period-of-record analysis. a. Executed HMS for initial unregulated basin calibration (For 6 POI s for each of 7 frequencies 2-, 5-, 10-, 25-, 50-, 100-, 500-year, and SPF). Total of 48 calibrations/verifications. b. Calibrated the HMS model parameters, primarily loss rates, to match, as closely as possible, the peak discharges (historical frequency) at the 6 points of interest, with checks at other stream gauges in basin (at least 16 total gauges). c. Iterate through b. and c. until calibration is completed. d. Product: A calibrated and verified, unregulated HMS model of basin with peak discharges at all points of interest (including all gauges and the six Special POI s). 8. HEC-5 Reservoir Operation Model for Regulated Basin Conditions (Chapter 5) a. Prepared detailed HEC-5 model including all significant reservoirs (Starts at Buchanan and goes downstream to Matagorda Bay. Columbus is last control point). b. Inflow hydrographs came from HMS (DSS), started at Buchanan and include all local contributing area hydrographs from HMS runs. HEC-5 routed the hydrographs downstream, added in the local hydrographs, and routed through the reservoirs, based on LCRA-provided operation policies/rules. These policies/rules were: For Lake Travis - 1996 USACE document Part 208.19, Title 33 CFR, Standard Operating Procedures Mansfield Dam as of December 1996. For Lake Buchanan - Gate operation was in accordance with 1990 FEMA/LCRA agreement. A copy of the Mansfield Dam Operation Plan and 1990 FEMA/LCRA agreement are in Volume II-B, Chapter 5. Other reservoirs - Operated as inflow equals outflow based on rating curve and storage data provided by LCRA. c. Routing with HEC-5 Used the same Modified Puls routing from the HMS model. In the reservoirs, classic reservoir routing methods were used. d. The output from HEC-5 included computed inflows, releases, and pool elevation hydrographs for each reservoir; and computed flow hydrographs for each control point. Output goes to DSS for use with RAS (unsteady). See Step 9. 9. Final RAS Hydraulic Model(s) for Main Stem Prepared the final HEC-RAS unsteady hydraulic model for final flood profiles/delineation. These models have the Corps field surveyed channel sections and include all bridges. Halff Associates, Inc. 18 July 2002
a. Inflow hydrographs at the upstream end of overall study (San Saba gauge) were from HMS/DSS. Downstream boundary condition (stage hydrograph), at Buchanan was from HEC-5. Lateral inflow hydrographs account for intervening areas, and came from HMS/DSS. b. From Buchanan Dam downstream to Lake LBJ, used HEC-5 outflow hydrographs (releases) as upstream boundary condition and Lake LBJ stage hydrograph as downstream boundary. Lateral inflow hydrographs account for intervening areas, and came from HMS/DSS. c. From Lake LBJ downstream to Lake Marble Falls, used HEC-5 outflow hydrographs (releases) as upstream boundary condition and Lake Marble Falls stage hydrograph as downstream boundary. Lateral inflow hydrographs account for intervening areas, and came from HMS/DSS. d. From Lake Marble Falls downstream to Lake Travis, used HEC-5 outflow hydrographs (releases) as upstream boundary condition and Lake Travis stage hydrograph as downstream boundary. Lateral inflow hydrographs account for intervening areas, and came from HMS/DSS. e. From Travis downstream to Tom Miller Dam (Lake Austin), used HEC-5 outflow hydrographs (releases) as upstream boundary condition and Tom Miller Dam stage hydrograph as downstream boundary. Lateral inflow hydrographs account for intervening areas, and come from HMS/DSS. f. From Tom Miller Dam downstream to Longhorn Dam (Town Lake), used HEC-5 outflow hydrographs (releases) as upstream boundary condition and Longhorn Dam stage hydrograph as downstream boundary. Lateral inflow hydrographs account for intervening areas, and come from HMS/DSS. g. From Longhorn Dam downstream to the Intercoastal Waterway (Matagorda Bay) used HEC-5 outflow hydrographs (releases) as upstream boundary condition and normal depth at the terminal point. Lateral inflow hydrographs account for intervening areas, and came from HMS/DSS. h. Iteration between HEC-5 and RAS (unsteady) was required until final/satisfactory calibration was reached. i. At each stream gauge the RAS-routed peak discharge was compared to historical discharge-frequency curves (From Steps 1 and 2). j. Final flood profiles for each frequency event were computed and stored for future uses. See Volume II-D, Chapter 7, CD s 3-6 for detailed description and copies of profiles. 10. Convert Flood Profiles to Floodplain Inundation Layers for GIS Mapping Once the final RAS (unsteady) models were completed and the final flood profiles computed, a Halff-developed ArcInfo AML was used for floodplain delineation (in lieu of grid-based HEC-Geo-RAS), with the existing GIS-based DTM. A quality control check, by hand, for any abnormalities was made between the hydraulic model output and the floodplain delineation. This automated floodplain mapping layer for the various frequency floods was preserved as separate levels in the GIS, for use in other tasks, such as flood damage assessment, and for non-structural alternative analysis. See Chapters 3 and 7 (CD-7) for detailed description and copies of maps. Halff Associates, Inc. 19 July 2002
III. Mapping and Geographic Information System (GIS) Applications (Chapter 3) A. Data Sources Several different types of mapping data were available for this study area. The primary source of terrain data was developed from traditional aerial mapping procedures and included two foot contours in rural areas, one foot contours in some urban areas, and point elevations derived from USGS DEMs. 1. Terrain Data The Lower Colorado River Authority mapped approximately 451 linear miles along the Colorado River, extending from the Burnet - Lampasas County line down to the Gulf of Mexico at Matagorda Bay. Components of this mapping project included: aerial digital othrophotography, digital contour maps to national map accuracy standards (4-foot contours interpolated to 2-foot contours); in urban areas including Matagorda, Garwood, Columbus, La Grange, and Wharton, digital contour mapping to national map accuracy standards (2-foot contours interpolated to 1-ft); parcel maps and parcel data for river front property; planimetric maps (digital coverage layers ) for all visible structures; and integration of mapping data into ESRI s ArcGIS software. The spatial extent of the aerial mapping was the approximate 500-yr floodplain based on FEMA s Q3 data set. To supplement the aerial mapping, U.S. Geological Survey (USGS) 30m Digital Elevation Model (DEM) data was used outside the spatial extent of the aerial mapping to fill in a one half mile buffer zone. For hydrology uses, the base data for watershed sub-basin delineation came from available USGS 30-meter DEM data. 2. Field Survey Data The aerial mapping did not provide any elevation data below the water surface so 165 channel cross sections were field surveyed along the river. In addition, data from bathymetry surveys were collected for the lakes within the study area. Using GIS tools, all of these data sets were combined to form a seamless terrain model, in the form of a TIN. B. Hydrology Study Applications 1. Pre-Pro (UT-CRWR) The original GIS based HEC-HMS basin file was developed by the University of Texas, Center for Research in Water Resources (UT-CRWR) using their basin delineation program Pre-Pro in ArcView 3.2. Pre-Pro is actually a set of GIS commands arranged in a step-by-step procedure that produces a basin input file as the final product. The base data for the sub-basin delineation was the 30-meter USGS DEM data. The GIS-generated basin model for the study area was broken into two separate delineation areas due to the size of the basin and software limitations. The break occurred at Mansfield Dam (Lake Travis).The Colorado River basin was originally subdivided into 232 sub-basins, averaging about 79 square miles. This original basin file lacked unit hydrograph parameters and loss rate parameters. Halff Associates, Inc. 20 July 2002
2. HEC-GeoHMS Colorado River Flood Damage Evaluation Project Phase I Based on other parameters, it was decided that there was a need for a greater number of sub-basins. Pre-Pro was not programmed to calculate a few key parameters (basin centroid, basin centroidal length, and longest flow path) needed to develop input parameters using the Corps utility program. The study team also wanted the delineations to be reproducible with the Corps of Engineers program HEC-GeoHMS. The Halff Study Team took some of the original Pre-Pro generated grids and re-delineated the watershed. Several basins were added to the original delineation. The final basin delineation produced 290 sub-basins, averaging 63 square miles. The Corps sub-basin parameter utility program was executed to generate parameters for the HEC-HMS model. Another utility program was used to extract the sub-basin parameters from the text file and insert them into the basin file created using HEC-GeoHMS. The initial hydrologic model parameter estimates were then assembled into the initial HEC-HMS model. C. Hydraulic Study Applications 1. TIN Development TINs for the Colorado River basin were generated for use in HEC-GeoRAS for the purposes of a ground surface elevation model and to extract river cross sections. Due to the amount of topographic data, the detailed study area had to be broken into 20 subsets to stay within the processing limitations. The topographic data layers used for the TINs included (see Volume II-B, Chapter 3 for detailed TINing process): USGS 30m DEMs (Maidment) Spot elevations from aerial survey (LCRA) Derived 2-foot and 1-foot contour lines from aerial survey (LCRA) Spot elevations from lake bathymetry survey (LCRA) Channel field surveys (Corps) 2. HEC-GeoRAS To assist in moving data from the GIS environment to a HEC-RAS hydraulic model geometry file, the Corps of Engineers has developed a software extension for ArcView GIS, developed by ESRI, Inc., called HEC-GeoRAS. This extension is designed to take GIS data representing stream centerlines, cross sections, bank lines, flow paths, land cover, and terrain data in the form of a TIN and process them into a HEC-RAS geometry file. This extension works very well, but some limitations were found with the data and the extension s capabilities when working with a project of this size. 3. River Channel Issues In addition to combining different types of elevation data, a GIS utility was developed to generate interpolated channel geometry between the survey locations. Unlike the cross section interpolater built into HEC-RAS, which can only interpolate in a straight line, the utility that was developed accounted for bends in the river. This interpolated channel geometry was incorporated into the TIN along with all the other terrain data sets. Incorporating the channel geometry into the TIN provided the benefit of being able to take cross sections at any point along the river and not just at survey locations. Halff Associates, Inc. 21 July 2002
4. River Centerline Issues Colorado River Flood Damage Evaluation Project Phase I HEC-GeoRAS was found to have a limitation when processing arcs representing the river centerline. The original line, stored in an ArcView shapefile, was based on the USGS National Hydrography Dataset (NHD). This shapefile was represented by multiple arc segments for the purpose of defining the changes that occur along the river in accordance with the NHD database. Approximately 940 arc segments existed within the 480-mile study reach. When HEC-GeoRAS processes a river centerline a junction is placed at each node in the shapefile (a node is where two or more arc segments are connected). This causes a problem because HEC-RAS only expects to have junctions where a tributary connects to the main stem of the river. HEC-RAS does have the ability to delete junctions, but that would have required deleting 940 junctions by hand. To work around this problem the number of arc segments that represent the river needed to be reduced. The tools to combine arcs already existed within the GIS, but a procedure was needed to reduce the number of vertices that defined the line. To accomplish these tasks a utility was developed that analyzed the angle formed by three vertices, if that angle was within a user specified tolerance for forming a straight line then the middle vertex was deleted. During the processing, this utility also combined arc segments up to the software limit of 500 vertices. This utility was able to reduce the number of arc segments defining the centerline from 940 arcs to 59 arcs. 5. Floodplain Delineation Issues HEC-GeoRAS uses a rasterization procedure used to delineate the floodplain after running HEC-RAS. After running HEC-RAS each cross section in the hydraulic model has a water surface elevation assigned to it for a given flood profile. This elevation data can be brought back into the GIS environment and then used to delineate floodplain polygons. The floodplain is based on the intersection of the ground surface and the water surface. The ground surface is already represented by the terrain TIN. A water surface TIN is generated by TINing the cross sections based on the flood profile elevation assigned to each cross section. When HEC- GeoRAS goes to intersect the ground and water surfaces it first rasterizes the two TINs (rasterization is the process of converting a TIN to grid or DEM). The cells representing the same spatial location are then compared and a new grid is then generated with cells being marked as wet or dry. All cells marked as wet represent the inundated area. The first problem with this procedure is that the resulting inundation area has a blocky appearance. The second problem is that HEC- GeoRAS has a one million cell limitation. For a small area this is not a problem, but when very large areas are being analyzed the cell sizes start to get very large; sometimes cell sizes can reach 1,000 feet or more on a side. When this occurs poor inundation areas are defined. The solution to this problem was to develop a procedure that intersects the ground and water surface TINs without rasterization. Using GIS tools outside of HEC-GeoRAS a utility was developed to intersect the two TINs. The results of using this utility allowed for large areas to be analyzed at one time and smoother, more natural looking, inundation polygons were generated to represent the floodplains. Halff Associates, Inc. 22 July 2002
IV. Summary Of Findings A. General This hydrologic and hydraulic analysis of the Colorado River basin includes 482 river miles of the Colorado River, covers 18,000 square miles of watershed, includes seventy years of historical flood data, and delineates floodplains for eight different flood events (2-year to 500-year floods and the SPF). The Technical Support Data in Volume II contain detailed descriptions of the results or findings of each major task of this study. Following is a brief summary of pertinent study results from this support data: B. Flood Peak Discharges A summary of 100-year frequency flood peak discharges at selected locations is shown in Table IV-1. In general, the peak discharges computed for this study were slightly lower than the published FEMA flood insurance study values. In some cases, lower peak discharges do not always produce lower flood elevations, due to updated modeling data and techniques. Earlier studies utilized steady-state hydraulic models while this study uses unsteady modeling along the Colorado River. TABLE IV-1 Summary And Comparison Of 100-Year Flood Peak Discharges Colorado River At Selected Locations (cfs) Location On the Colorado River Red Bluff Gauge Near San Saba Tom Miller Dam (Lake Austin) Austin Gauge Upstream of U.S. 183 Below Mouth of Onion Creek Bastrop Gauge at Loop 150 Columbus Gauge at U.S. 90 Wharton Gauge at U.S. 59 (Business) Current Study Computed 100-year Discharge (1) FEMA 100-year Discharge 237,100 N/A 90,100(2) 170,000 (3) 90,300(2) 170,000 (3) 138,300 210,000 (4) 142,000 149,300 135,200 136,000 114,100 139,500 (1) Computed values used to determine flood elevations. (2) Releases from Mansfield Dam, 90,000 cfs. (3) Value in Published Flood Insurance Study is 170,000 cfs. Values in the effective FEMA models range from 90,000 to 100,000 cfs. (4) Value from Travis County FIS at Travis-Bastrop County Line. C. Flood Elevations A summary of the peak flood elevations on the Highland Lakes and Town Lake is shown in Table IV-2. Halff Associates, Inc. 23 July 2002
TABLE IV-2 Colorado River Reservoir Summary (All elevations are computed at upstream face of the dam. Flood elevations on each lake will rise along the river upstream from the dams. See flood profiles in Volume II, Chapter 6) Lake Buchanan (Historic High = 1021.6, NAVD88) Frequency Current Study Computed Elevation (Feet NAVD88) FEMA Elevation (Feet NAVD88) Difference Current FEMA (Feet) 2-Year 1020.0 N/A N/A 5-Year 1020.0 N/A N/A 10-Year 1020.5 1020.7-0.2 25-Year 1020.5 N/A N/A 50-Year 1020.5 1020.7-0.2 100-Year 1021.0 1021.2-0.2 500-Year 1022.7 1022.1 +0.6 SPF 1025.9 N/A N/A Inks Lake (Historic High = 903.0, NAVD88) Frequency Current Study Computed Elevation (Feet NAVD88) FEMA Elevation (Feet NAVD88) Difference Current FEMA (Feet) 2-Year 890.0 N/A N/A 5-Year 892.9 N/A N/A 10-Year 895.0 895.2-0.2 25-Year 896.9 N/A N/A 50-Year 900.0 900.2-0.2 100-Year 901.7 901.9-0.2 500-Year 909.0 908.7 +0.3 SPF 912.4 N/A N/A Lake LBJ (Historic High = 836.4, NAVD88) Frequency Current Study Computed Elevation (Feet NAVD88) FEMA Elevation (Feet NAVD88) Difference Current FEMA (Feet) 2-Year 825.0 N/A N/A 5-Year 825.0 N/A N/A 10-Year 825.0 825.2-0.2 25-Year 825.0 N/A N/A 50-Year 825.4 826.2-0.8 100-Year 828.1 828.1 0.0 500-Year 839.4 841.2-1.8 SPF 843.3 N/A N/A Halff Associates, Inc. 24 July 2002
Lake Marble Falls (Historic High = 756.6, NAVD88) Frequency Current Study Computed Elevation (Feet NAVD88) FEMA Elevation (Feet NAVD88) Difference Current FEMA (Feet) 2-Year 738.0 N/A N/A 5-Year 738.0 N/A N/A 10-Year 742.8 739.2 +3.6 25-Year 748.0 N/A N/A 50-Year 752.2 749.7 +2.5 100-Year 754.3 753.2 +1.1 500-Year 762.1 762.7-0.6 SPF 771.4 N/A N/A Elevation based on old rating, LCRA to provide new rating. Lake Travis (Historic High = 710.2, NAVD88) Frequency Current Study Computed Elevation (Feet NAVD88) FEMA Elevation (Feet NAVD88) Difference Current FEMA (Feet) 2-Year 685.2 N/A N/A 5-Year 691.1 N/A N/A 10-Year 697.0 691.2 +5.8 25-Year 713.7 N/A N/A 50-Year 716.7 710.2 +6.5 100-Year 722.0 716.2 +5.8 500-Year 732.6 728.7 +3.9 SPF 734.4 N/A N/A Lake Austin (Historic High = 495.5, NAVD88) Frequency Current Study Computed Elevation (Feet NAVD88) FEMA Elevation (Feet NAVD88) Difference Current FEMA (Feet) 2-Year 492.8 N/A N/A 5-Year 492.8 N/A N/A 10-Year 492.8 493.1-0.3 25-Year 492.8 N/A N/A 50-Year 492.8 493.3-0.5 100-Year 492.8 493.3-0.5 500-Year 506.3 503.5 +2.8 SPF 507.7 N/A N/A Halff Associates, Inc. 25 July 2002
Town Lake Current Study Computed Elevation (Feet NAVD88) Colorado River Flood Damage Evaluation Project Phase I Difference Current FEMA (Feet) FEMA Elevation Frequency (Feet NAVD88) 2-Year 428.3 N/A N/A 5-Year 428.3 N/A N/A 10-Year 428.3 429.1-0.8 25-Year 430.3 N/A N/A 50-Year 438.6 436.5 +2.1 100-Year 438.6 439.8-1.2 500-Year 458.2 450.3 +7.9 SPF 459.6 N/A N/A 100-year Flood Elevations - A summary of 100-year frequency peak flood elevations at selected locations is shown in Table ES-2. Note that the peak flood elevations computed for this study differ from earlier FEMA flood insurance study values. For the computed pool elevations at the upstream face of the dams, this study has equal or lower flood elevations at the upstream face of five dams (Buchanan, LBJ, Inks, Austin, and Town Lake); and higher elevations on two dams (Marble Falls and Travis). In the Austin area the current study elevations are slightly higher. At Bastrop, the estimated flood elevation is lower and at Wharton the estimated flood level is below the earlier studies. Some minor differences in the vertical elevation datum from the previous studies (NGVD29-1929 mean sea level) to the current datum (NAVD88-1988) does occur as noted in Table IV 4 of this Volume and in Volume II B, Chapter 3. TABLE IV-3 Summary And Comparison Of 100-Year Peak Flood Elevations Colorado River At Selected Locations Location on the Colorado River Current Study Computed 100-year Elevation (Feet NAVD88) FEMA 100-year Elevation (Feet NAVD88) (3) Difference Current FEMA (Feet) (2) Lake Buchanan (1) 1021.0 1021.2-0.2 Inks Lake (1) 901.7 901.9-0.2 Lake LBJ (1) 828.1 828.1 0.0 Lake Marble Falls (1) 754.3 753.2 +1.1 Lake Travis (1) 722.0 716.2 +5.8 Lake Austin (1) 492.8 493.3-0.5 Town Lake (1) 438.6 439.8-1.2 Austin Gauge Upstream of U.S. 183 437.0 435.3 +1.7 Bastrop Gauge at Loop 150 352.2 353.9-1.7 Columbus Gauge at U.S. 90 190.2 194.1-1.9 Wharton Gauge at U.S. 59 (Business) 102.4 103.3-0.9 (1) Flood Elevation computed at upstream face of the dam. Flood elevations on each lake will rise along the river, upstream of the dam. See flood profiles in Section IV and in Volume II, Chapter 6. (2) See Table IV 4 for explanation of vertical datum differences. (3) Current effective FEMA 100-year elevations adjusted to NAVD88. Halff Associates, Inc. 26 July 2002
D. Reason for Changes in Flood Elevations Colorado River Flood Damage Evaluation Project Phase I There are several reasons that the 100-year flood elevations have changed along the Colorado River and especially on the Highland Lakes: 1. This is the first detailed, comprehensive, basin-wide approach for modeling, simulating, and computing frequency-based rainfall, runoff, reservoir elevations, and flood elevations along the entire river corridor. 2. There is an additional 25 years of historical flood and rainfall records that have been collected since the previous flood studies of the mid to late 1970 s. This provides a more comprehensive statistical database for developing flood frequency estimates. 3. The calibration and verification of the flood models used in the study has been enhanced significantly by the additional historical rainfall and flood data and the computational power of large capacity computers. The use of NEXRAD radar and GIS tools in the collection of data, development of computer models, and display of results has provided a greater degree of accuracy in the floodplain delineation and overall flood analysis process. 4. A more realistic assumption of the long-range river flood forecasting abilities of reservoir operators has had an effect on predicted 100-year pool levels. For example, in earlier flood studies to determine FEMA pool elevations on Lake Travis, an unrealistic assumption of a reliable 36-hour forecast time was used. Even with advanced NEXRAD radar and additional rainfall and stream gauges, a 12-hour flood forecast is considered by the LCRA and the Corps as the maximum time that can be safely used in dam gate operations. 5. Within the historical period of record (1930-1999) used in this study, the 1938 flood would have caused Lake Travis to reach approximately the projected 100-year flood pool (722) if the lakes had been in place. This 1938 flood, which was a high volume event, is statistically considered to be approximately the 100-year flood. In addition, the 1936, high volume flood, would have reached an estimated 719 elevation on Lake Travis. 6. There are some minor vertical elevation datum differences throughout the study area as shown on Table IV 4, and in Volume II B (Chapter 3). The changes in datum from the previous studies to this study vary from near zero in the lower basin to a maximum of 0.3 feet in the Highland Lakes area. Halff Associates, Inc. 27 July 2002
TABLE IV-4 Vertical Datum Comparison (Ngvd29 Vs. Navd88) Elevation feet Datum Shift (ft) Gauge/Dam Name NGVD29 NAVD88 NAVD88 - NGVD29 Colorado River at Winchell 1335.77 1336.15 0.38 Colorado River near San Saba 1137.00 1137.20 0.20 Llano river at Llano 982.58 982.72 0.14 Pedernales River near Johnson City 1134.67 1134.92 0.25 Lake Austin at Davenport Ranch 483.48 483.72 0.24 Town Lake near Longhorn Dam 439.31 439.58 0.26 Colorado River at Austin 407.50 407.77 0.27 Onion Creek at Buda 681.81 682.13 0.32 Onion Creek at Hwy 183, Austin 471.85 472.12 0.27 Colorado River at Bastrop 332.10 332.30 0.20 Colorado River above La Grange 256.10 256.25 0.15 Colorado River at Columbus 195.43 195.49 0.06 Colorado River at Wharton 100.31 100.30-0.01 Colorado River at Bay City 46.80 46.71-0.09 Pedernales River near Fredericksburg 1597.14 1597.33 0.19 Llano River near Junction 1665.51 1665.61 0.10 Pecan Bayou near Mullin 1239.68 1239.96 0.28 San Saba River near Brady 1549.68 1549.88 0.20 Tom Miller Dam 479.29 479.55 0.26 Mansfield Dam 510.54 510.74 0.20 Starcke Dam 703.85 704.03 0.18 Wirtz Dam 762.42 762.60 0.18 Inks Dam 852.90 853.08 0.18 Buchanan Dam 1019.92 1020.10 0.18 E. Floodplains Based on the computed flood elevations from this study, the total 100-year floodplain for the Colorado River, from the mouth to the Red Bluff gauge, is about 449 square miles or 287,000 acres. Since this is the first time much of the river has been studied in detail, there are no comparisons from previous studies. Volume II-D, Chapter 7, CD-7 contains a complete set of 100- and 500-year floodplains. Volume II-D, Chapter 7, CD-9 Profiles includes computed flood profiles of the 2-, 5-, 10-, 25-, 50-, 100-, 500-year frequency floods and the Standard Project Flood (a very large Corps defined flood). F. Flood Profiles Plates of flood profiles along the Colorado River are shown on the following figures. Halff Associates, Inc. 28 July 2002
1250 1200 1150 From Hwy 190 to Buchanan Dam Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 1100 Elevation (ft) 1050 1000 950 900 Buchanan Dam Fall Creek Cherokee Creek FM 580 US Hwy 190 2200000 2250000 2300000 2350000 2400000 2450000 2500000 2550000 Main Channel Distance (ft) FIGURE IV-1 Profiles for the Buchanan HEC-RAS reach. Halff Associates, Inc. 29 July 2002
From Buchanan Dam to Inks Dam Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR-U 7) P9-500YR-U 8) P10-SPF 940 920 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR-U WS Max WS - P8-100YR-U WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 900 Elevation (ft) 880 860 840 Inks Dam 820 2185000 2190000 2195000 2200000 2205000 2210000 2215000 Main Channel Distance (ft) SH 29 - Existing FIGURE IV-2 Profiles for the Inks HEC-RAS reach. Buchanan Dam Halff Associates, Inc. 30 July 2002
880 860 840 From Inks Dam to Wirtz Dam Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 820 Elevation (ft) 800 780 760 Wirtz Dam Sandy Creek 740 2060000 2080000 2100000 2120000 2140000 2160000 2180000 2200000 Main Channel Distance (ft) LLano River SP RR-Kingsland FIGURE IV-3 Profiles for the LBJ HEC-RAS reach. Inks Dam Halff Associates, Inc. 31 July 2002
800 780 From Wirtz Dam to Starcke Dam Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 760 Elevation (ft) 740 720 700 680 Starcke Dam US Hwy 281 Cold Spring Cr Wirtz Dam 2040000 2045000 2050000 2055000 2060000 2065000 2070000 2075000 2080000 Main Channel Distance (ft) FIGURE IV-4 Profiles for the Marble Falls HEC-RAS reach. Halff Associates, Inc. 32 July 2002
From Starcke Dam to Mansfield Dam Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 750 700 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 650 Elevation (ft) 600 550 500 Mansfield Dam Big Sandy Creek Cow Creek Pedernales River Hamilton Creek Starcke Dam 1700000 1750000 1800000 1850000 1900000 1950000 2000000 2050000 Station (ft) FIGURE IV-5 Profiles for the Travis HEC-RAS reach. Halff Associates, Inc. 33 July 2002
From Mansfield Dam to Tom Miller Dam Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 1 Legend WS Max WS - P10-SPF 550 WS Max WS - P9-500YR WS Max WS - P5-10YR WS Max WS - P6-25YR WS Max WS - P7-50YR WS Max WS - P8-100YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 500 Elevation (ft) 450 Tom Miller Dam Bull Creek SH 360 Mansfield LWC FM 620 1580000 1600000 1620000 1640000 1660000 1680000 1700000 Station (ft) FIGURE IV-6 Profiles for the Lake Austin HEC-RAS reach. Halff Associates, Inc. 34 July 2002
From Tom Miller Dam to Longhorn Dam Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 500 480 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 460 Elevation (ft) 440 420 400 Longhorn Dam IH-35 East Front Congress Ave. First St. Union Pacific RR Lamar Ped./Bike 380 1555000 1560000 1565000 1570000 1575000 1580000 1585000 1590000 1595000 Barton Creek Main Channel Distance (ft) FIGURE IV-7 Profiles for the Town Lake HEC-RAS reach. N/B Mopac Redbud Trail Tom Miller Dam Halff Associates, Inc. 35 July 2002
From Longhorn Dam to Bastrop Gauge Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 480 460 440 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 420 400 Elevation (ft) 380 360 340 320 300 Loop 150 - Old Big Sandy Creek Wilbarger Creek FM 969 Dry Creek Gilleland Creek Onion Creek FM 973 Walnut Creek Gravel Pit Rd. US 183 - N/B Longhorn Dam 1250000 1300000 1350000 1400000 1450000 1500000 1550000 1600000 Station (ft) FIGURE IV-8 Profiles for the Bastrop HEC-RAS reach. Halff Associates, Inc. 36 July 2002
From Longhorn Dam to Bastrop Gauge Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 480 460 440 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 420 400 Elevation (ft) 380 360 340 320 300 Loop 150 - Old Big Sandy Creek Wilbarger Creek FM 969 Dry Creek Gilleland Creek Onion Creek FM 973 Walnut Creek Gravel Pit Rd. US 183 - N/B Longhorn Dam 1250000 1300000 1350000 1400000 1450000 1500000 1550000 1600000 Station (ft) FIGURE IV-9 Profiles for the La Grange HEC-RAS reach. Halff Associates, Inc. 37 July 2002
280 260 240 From La Grange Gauge to Columbus Gauge Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 220 Elevation (ft) 200 180 160 140 Gauge @ Columbus Cummins Creek Bus71-Columbus D/SHwy71Columbus 120 700000 750000 800000 850000 900000 950000 Main Channel Distance (ft) FIGURE IV-10 Profiles for the Columbus HEC-RAS reach. Hwy 77-LaGrange Buckners Creek MK&T RR-Fayette Gauge @ La Grang Halff Associates, Inc. 38 July 2002
From Columbus Gauge to Garwood Gauge Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 220 200 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 180 Elevation (ft) 160 140 120 Gauge @ Garwood Skull Creek US Hwy 90A - Alt 100 560000 580000 600000 620000 640000 660000 680000 700000 720000 Main Channel Distance (ft) FIGURE IV-11 Profiles for the Garwood HEC-RAS reach. IH-10 E/B Gauge @ Columbus Halff Associates, Inc. 39 July 2002
From Garwood Gauge to Wharton Gauge Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 160 140 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 120 Elevation (ft) 100 80 60 Wharton Gauge US Hwy 59 N/B FM 960 40 300000 350000 400000 450000 500000 550000 600000 Main Channel Distance (ft) FIGURE IV-12 Profiles for the Wharton HEC-RAS reach. FM 950 - Garwood Garwood Gauge Halff Associates, Inc. 40 July 2002
From Wharton Gage to Bay City Gage Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 120 100 80 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 60 Elevation (ft) 40 20 0 Gauge at Bay Cit D/S SH 35-20 150000 200000 250000 300000 350000 Main Channel Distance (ft) FIGURE IV-13 Profiles for the Bay City HEC-RAS reach. Halff Associates, Inc. 41 July 2002
60 40 From Bay City Gage to last cross section Plan: 1) P3-2YR 2) P4-5YR 3) P5-10YR 4) P6-25YR 5) P7-50YR 6) P8-100YR 7) P9-500YR 8) P10-SPF 1 Legend WS Max WS - P10-SPF WS Max WS - P9-500YR WS Max WS - P8-100YR WS Max WS - P7-50YR WS Max WS - P6-25YR WS Max WS - P5-10YR WS Max WS - P4-5YR WS Max WS - P3-2YR Ground 20 Elevation (ft) 0-20 Intercoastal FM 521-40 20000 40000 60000 80000 100000 120000 140000 160000 180000 Main Channel Distance (ft) FIGURE IV-14 Profiles for the Matagorda HEC-RAS reach. MP RR- Matagorda Bay City Halff Associates, Inc. 42 July 2002
V. References and Previous Studies References Federal Emergency Management Agency. 1991. Flood Insurance Study for Llano County, Travis County and Unincorporated Areas Volumes 1-6, and Burnet County, Texas. Lower Colorado River Authority. 1997. Flooding on the Colorado River, An Overview of Flood Management and Critique of the June 1997 Flood. Lower Colorado River Authority. 1992. Flooding on the Colorado River, The Story of the Winter Floods As Told by the Texas Media 1991-1992, Volumes I and II. Lower Colorado River Authority. June 1993. Water Management Plan for the Lower Colorado River Basin. Lower Colorado River Authority. June 1995. Rainfall Survey Report, Sandy Creek Watershed, Blanco, Gillespie, and Llano Counties. Lower Colorado River Authority. Undated. Colorado River Flood Warning Guide. Lower Colorado River Authority. August 1997. High Water, A Guide to the Colorado River/Highland Lakes Floodplains from Lampasas to Bastrop Counties. Lower Colorado River Authority. December 1997. Llano/Colorado River Storm Report, June 1997 Flood Event. LTM Engineering, Inc. February 1998. Gauge Placement Plan for the Flood Warning Preliminary Engineering Investigation, Phoenix, Arizona. Maidment, David R. PhD, and Ferdinand Hellweger, HEC-PrePro: A GIS Preprocessor for Lumped Parameter Hydrologic Modeling Programs, CRWR Online Report 97-8, 1997. National Oceanic and Atmospheric Administration (NOAA). December 1991, May 1995, June 1997. Hourly Precipitation Data, Texas, Asheville, North Carolina. Perry, Greg R. and Shafer, Kevin L., Frequency-Related Temporally and Spatially Varied Rainfall, Journal of Hydraulic Engineering, Vol. 116, No. 10, October 1990, pp. 1215-1231. Placer County, California. 1990. Placer County Flood Control and Water Conservation District Stormwater Management Manual. Ravi S. Devulapalli and Juan B. Valdes, Volume-Duration-Frequencies for Ungaged Catchments in Texas, Volume I. Calculation of Regional Regression Equations, Texas Water Resources Institute, Texas A&M University, March, 1996. Halff Associates, Inc. 43 July 2002
References (Continued) Ravi S. Devulapalli and Juan B. Valdes, Volume-Duration-Frequencies for Ungaged Catchments in Texas, Volume II. Computations of Flood Volumes of Varying Durations and Frequencies for Catchments with Areas Greater Than 300 Square Miles, Texas Water Resources Institute, Texas A&M University, March, 1996. Texas Water Development Board. February 1971. Dams and Reservoirs in Texas, Report 126, Part III. Austin, Texas. U.S. Army Corps of Engineers, Hydrologic Engineering Center. 1992. DSSMATH Utility Program for Mathematical Manipulation of HECDSS Data, User Manual, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. August 1997. UNET One- Dimensional Unsteady Flow Through a Full Network of Open Channels, User's Manual, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. 1990. HEC-1 Flood Hydrograph Package, User's Manual, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. October, 2001. HEC-HMS Hydrologic Modeling System, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. January, 2001. HEC-RAS River Analysis System, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. 1995. HECDSS User's Guide and Utility Program Manuals, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. 1990. PREAD Functions, Macros, Menus and Screens, User Information, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. 1991. Real-Time Water Control, Volumes I and II of Training Course Notes, Davis, California. U.S. Army Corps of Engineers, Hydrologic Engineering Center. 1989. Water Control Software, Forecast and Operations, Davis, California. U.S. Department of Agriculture Soil Conservation Service. June 1974. Soil Survey of Travis County, Texas. U.S. Department of Agriculture Soil Conservation Service. January 1982. Soil Survey of San Saba County, Texas. U.S. Department of Agriculture Soil Conservation Service. October 1992. Soil Survey of Menard County, Texas Halff Associates, Inc. 44 July 2002
References (Continued) U.S. Department of Agriculture Soil Conservation Service. August 1979. Soil Survey of Blanco and Burnet Counties, Texas. U.S. Department of Agriculture Soil Conservation Service. May 1975. Soil Survey of Gillespie County, Texas. U.S. Geological Survey, Water Resources Data, Texas, Volume 3, Water Year 1991, 1995, and 1997. U.S. Army Corps of Engineers, Southwestern Division, Southwestern Division Modeling System for the Simulation of the Regulation of a Multi-purpose Reservoir System (SUPER), January 2000. U.S. Army Corps of Engineer, Hydrologic Engineering Center, HEC-GeoHEC-RAS, An Extension for Support of HEC-RAS Using ArcView, User s Manual, Version 3.0, April, 2000. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-5 Simulation of Flood Control and Conservation Systems, Users Manual, Version 8.0, October, 1998. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-FDA, Flood Damage Reduction Analysis, Users Manual, Version 1.0, January, 1998. Previous Studies This is a partial list of studies compiled by Halff Associates, Inc. or the LCRA staff as a reference library. Many additional studies are available and are listed in the LCRA library. 1. The Floods in Central Texas in September 1921, C.E. Ellsworth, USGS. 2. Isohyetal Map Storm of Sept 8-11, 1921 from the Corp of Engineers Report of Survey of the Colorado River and Tributaries. 3. Excessive Rainfalls In Texas: State Reclamation Department Bulletin 25, Lowry, USGS, 1929. 4. Major Texas Floods of 1935, Dalrymple, USGS. 5. Major Texas Floods of 1936, Dalrymple, & Tate, USGS, 1937. 6. Colorado River Project: Texas Flood Control by Marshall Ford Reservoir, Lowry, Reclamation Department, 1937. Halff Associates, Inc. 45 July 2002
Previous Studies (continued) 7. Colorado River Flood-July-August 1938, Report of the State Board of Water Engineers to Senate Investigating Committee of the 45 th Legislature, State Board of Water Engineers, September 19, 1938. 8. Texas Floods of 1938 and 1939, Dalrymple, & Tate, USGS, 1940. 9. La Grange, Texas Area Subject to Flooding, USACE, 1944. Map showing Highwater marks thru La Grange for the 1869,1913,1935,1938 floods in La Grange. 10. Columbus, Texas Area Subject to Flooding, USACE, 1944. Map showing Highwater marks thru Columbus for the 1913,1935,1938 floods in Columbus. 11. Reservoir Operation Mansfield Dam, Flood of June 1935, Army Corps of Engineers, 1944. 12. Hydrographs & Mass Curves, Flood of Sept.-Oct. 1936, Army Corps of Engineers, 1944. 13. Reservoir Operation Mansfield Dam, Flood of Sept-Oct 1936, Army Corps of Engineers, 1944. 14. Reservoir Operation Mansfield Dam, Flood of July & August 1938, Army Corps of Engineers, 1944. 15. Texas Floods of 1940, Breeding, USGS, 1948. 16. Floods of September 1952 in the Colorado and Guadalupe River Basin, Breeding, USGS, 1954. 17. General Storm Report Covering the storms of April May and June 1957 in Texas, OK & AK, USDA, SCS, September 1957. 18. Texas Floods of April, May, June 1957, Yost, Texas Board of Water Engineers, 1958. 19. Rainfall & Floods of April, May, & June 1957 in the South-Central, United States-TP 33, US Weather Bureau, 1958. 20. Memorandum Report- Flood of 1957, US Army Corps of Engineers, 1959. 21. Flood Flows of Texas Rivers, C.E. Ellsworth, USGS, 1957. 22. Special Flood Hazard Information Report: Barton Creek Austin, Texas, 1969, Army Corps of Engineers. 23. Inflow Design Flood Study, Mansfield (Marshall Ford) Dam Colorado River Project, Texas Examination of Existing Structures Program, US Department of Interior Bureau of Reclamation, 1972. 24. Floodplain Information Colorado River and Country Club Creek, Austin, Texas, Army Corps of Engineers, 1975. Halff Associates, Inc. 46 July 2002
Previous Studies (Continued) 25. Floodplain Information Colorado River La Grange, Texas, Corps of Engineers, 1975. 26. Floods in Central Texas August 1978, Schroeder, Massey, USGS, 1979. 27. The Disastrous Texas Flash Floods of August 1-4,1978: A Report to the Administrator, National Weather Service, 1979. 28. Special Flood Hazard Information Report-Pedernales River, Lyndon B. Johnson National Historic Site and State Park, Gillespie County, Texas, Corps of Engineers, December 1979. 29. Special Flood Hazard Information Report, Pedernales River Lyndon B. Johnson National Historic Site and State Park, Gillespie County, Texas. U.S. Army Corps of Engineers, Fort Worth District. December 1979. 30. The Austin, Texas Flood of May 24-25, 1981, Moore, National Academy of Science, 1982. 31. Flood of May 24-25, 1981, in Austin Texas Metropolitan Area, Massey & Reeves, USGS, 1982. 32. Colorado River and Tributaries, Texas, Boggy Creek Austin, Texas Design Memorandum No. 1. U.S. Army Corps of Engineers, Fort Worth District. October 1984. 33. Real-Time Flood Forecasting Model for the Lower Colorado River-Highland Lake System, Unver, UT 1985. 34. The Austin, Texas, Flash Flood: An Examination from Two Perspectives-Forecasting and Research, Maddox & Grice, NOAA, 1986. 35. Floods in Central Texas August 1-4,1978, Schroeder, Massey, USGS, 1987. 36. A Report on June 1987 Storm Event for Lake Travis and Downstream, Unver, LCRA, 1987. 37. Flood Simulation for A Large Reservoir System, Mays 1988, National Water Summary. 38. Reconnaissance Report Central Colorado River Basin, Colorado, Texas, U.S. Army Corps of Engineers, September 1989. 39. Reconnaissance Flood Protection Report, Central Colorado River Watershed, Colorado River Basin, Texas, U.S. Army Corps of Engineers, Fort Worth District. September 1989. 40. Report on Flooding April-May 1990, US Army Corps of Engineers, 1990. 41. Re-Evaluation of the Probable Maximum Floods for LCRA s Highland Lakes Projects, RAC Engineers and Economist, LCRA, 1991 Halff Associates, Inc. 47 July 2002
Previous Studies (Continued) 42. Texas floods, December 1991-January 1992, National Weather Service, 1992. 43. 1991-1992 Flood Event Report, Frithiof, LCRA, 1992. 44. Disastrous Floods on the Trinity, Brazos, Colorado and Guadalupe Rivers in Texas Dec 1991-Jan 1992, National Weather Service, 1993. 45. Floods in Southeast Texas, October 1994, USGS Fact Sheet, 1995. 46. Inks Flood Plain Study, Mays and Carriaga, 1995. 47. Sandy Creek Storm Report May 1995 Flood Event, Work in Progress by LCRA. 48. Floods of December 20-26, 1991 in Central Texas, Heji, Slade, Jennings, USGS, 1996. 49. Llano/Colorado River Storm Report October 1996 Flood Event, LCRA, 1997. 50. Llano/Colorado River Storm Report February-March 1997 Flood Event, LCRA, 1997. 51. Llano/Colorado River Storm Report June 1997 Flood Event, LCRA, 1997. 52. Flooding on the Colorado River: An Overview of Flood Management and Critique of the June 1997 Flood, LCRA, 1997. 53. Flood Operations Procedure Review Project for the Lower Colorado River Authority. Halff Associates, Inc., December 1998. 54. South Texas Floods October 17-22,1998, NOAA, Feb. 1999. 55. Storm Report Lower Colorado River Central & Southeast Texas October 1998 Flood Event, LCRA, 1999. 56. Storm Report Lower Colorado River Oct.-Nov. 2000 Flood Event, LCRA, 2000. 57. Preliminary Report on Floods in Central Texas November 15-16, 2001, USGS, 2001. FEMA Flood Insurance Studies: Bastrop, Blanco, Burnet, Colorado, Llano, Matagorda, San Saba, Travis, Wharton Counties and Cities of Lampasas, Sunrise Beach, Bay City, Palacios, El Campo, and Wharton. Halff Associates, Inc. 48 July 2002