VOLUME I HYDROLOGY HYDRAULICS APPENDIX TO CORPS INFORMATION PAPER ON COLORADO RIVER FLOOD DAMAGE EVALUATION PROJECT PHASE I.

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1 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

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3 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 Purpose Phase I - Identification of Existing Conditions Phase II - Detailed Evaluation Of Alternatives (Future Phase) B. Study Area Description Basin Description Climatology...5 C. Historical Flood Data USGS Stream Gauges LCRA Stream Gauges Flood History...9 D. FEMA Flood Insurance Study Discharges...11 E. Study Tasks Overview...11 F. Coordination Efforts During Study Technical Meetings 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) Period-of-Record Flow Analysis (Chapter 2) Historical Frequency Analysis at Each Gauge (Chapter 1) Initial/Preliminary HEC-RAS Hydraulic Model (Chapter 6) Initial/Preliminary UNREGULATED Basin-Wide HMS Model (Chapter 4) Rainfall Information for HMS Model HMS Storm Reproduction (Calibration) PHASE HMS Verification Phase (Unregulated Conditions)...18 Halff Associates, Inc. i July 2002

4 8. HEC-5 Reservoir Operation Model for Regulated Basin Conditions (Chapter 5) Final RAS Hydraulic Model(s) for Main Stem 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 Terrain Data Field Survey Data B. Hydrology Study Applications Pre-Pro (UT-CRWR) HEC-GeoHMS...21 C. Hydraulic Study Applications TIN Development HEC-GeoRAS River Channel Issues River Centerline Issues 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

5 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 Report PDF s Halff Associates, Inc. iii July 2002

6 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

7 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

8 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

9 FIGURE ES-1 Halff Associates, Inc. ES-3 July 2002

10 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 ( ); 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

11 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, ,000 (4) 142, , , , , ,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

12 100-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 (NGVD mean sea level) to the current datum (NAVD ) 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) Inks Lake (1) Lake LBJ (1) Lake Marble Falls (1) Lake Travis (1) Lake Austin (1) Town Lake (1) Austin Gauge Upstream of U.S Bastrop Gauge at Loop Columbus Gauge at U.S Wharton Gauge at U.S. 59 (Business) 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

13 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 ( ) 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

14 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

15 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

16 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

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