How To Study Floodway

Transcription

1 FLOOD INSURANCE STUDY ARLINGTON COUNTY, VIRGINIA COMMUNITY NAME COMMUNITY NUMBER ARLINGTON COUNTY (UNINCORPORATED AREAS) Arlington County Effective: August 19, 2013 Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER 51013CV000A

2 NOTICE TO FLOOD INSURANCE STUDY USERS Communities participating in the National Flood Insurance Program have established repositories of flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study (FIS) may not contain all data available within the repository. It is advisable to contact the community repository for any additional data. Part or all of this FIS may be revised and republished at any time. In addition, part of this FIS may be revised by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS. It is, therefore, the responsibility of the user to consult with community officials and to check the community repository to obtain the most current FIS components. Selected Flood Insurance Rate Map Panels for this community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map panels (e.g. floodways, cross-sections). In addition, former flood hazard zones designations have been changed as follows: Old Zones A1 through A30 B C New Zones AE X X Initial Countywide FIS Effective Date: August 19, 2013

3 TABLE OF CONTENTS Page 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgments Coordination AREA STUDIED Scope of Study Community Description Principal Flood Problems Flood Protection Measures ENGINEERING METHODS Hydrologic Analyses Hydraulic Analyses Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES 27 i

4 TABLE OF CONTENTS continued Page FIGURES Figure 1 Floodway Schematic 18 TABLES Table 1 Flooding Sources Studied by Detailed Methods 3 Table 2 Stream Name Changes 4 Table 3 Letters of Map Revision 4-5 Table 4 Summary of Discharges Table 5 Hydraulic Capacity Analysis 13 Table 6 Manning's "n" Values 14 Table 7 Floodway Data Table 8 Community Map History 28 EXHIBITS Exhibit 1 Flood Profiles Doctors Branch Donaldson Run Four Mile Run Gulf Branch Little Pimmit Run Little Pimmit Run Tributary Lower Long Branch Lubber Run North Branch Donaldson Run Potomac River Spout Run Upper Long Branch Windy Run Panels 01P 02P Panels 03P 04P Panels 05P 12P Panels 13P 14P Panels 15P 16P Panels 17P 18P Panels 19P 20P Panels 21P 22P Panel 23P Panels 24P 27P Panel 28P Panel 29P Panel 30P Exhibit 2 Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY ARLINGTON COUNTY, VIRGINIA 1.0 INTRODUCTION 1.1 Purpose of Study This countywide Flood Insurance Study (FIS) investigates the existence and severity of flood hazards in, or revises and updates previous FIS / Flood Insurance Rate Map (FIRM) in the geographic area of Arlington County, Virginia, and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This FIS has developed flood-risk data for various areas of the community that will be used to establish actuarial flood insurance rates. This information will also be used by Arlington County to update existing floodplain regulations as part of the Regular Phase of the National Flood Insurance Program (NFIP), and will also be used by local and regional planners to further promote sound land use and floodplain development. Minimum floodplain management requirements for participation in the NFIP are set forth in the Code of Federal Regulations at 44 CFR, In some states or communities, floodplain management criteria or regulations may exist that are more restrictive or comprehensive than the minimum Federal requirements. In such cases, the more restrictive criteria take precedence, and the State (or other jurisdictional agency) shall be able to explain them. 1.2 Authority and Acknowledgments The sources of authority for this FIS are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This FIS was prepared to include the unincorporated areas of Arlington County in a countywide format. Information on the authority and acknowledgments for each jurisdiction included in this countywide FIS, as compiled from their previously printed FIS reports, is shown below. Arlington County: Unincorporated Areas The hydrologic and hydraulic analyses from the FIS report dated November 3, 1981, were performed by the U. S. Army Corps of Engineers (USACE) for the Federal Emergency Management Agency (FEMA), under Inter- Agency Agreement No. IAA-H-7-66, Project Order No. 29. This work was completed in April For this FIS revision, the hydrologic and hydraulic analysis for the Potomac River was taken from the FIS for the District of Columbia, Washington D.C, effective September 27, Previously, the flooding effects from the Potomac River were not included in the Arlington County FIS and FIRM. A revised analysis for the Potomac River 1% annual chance tidal elevation was performed for FEMA by USACE under Contract No. HSFE03-04-X This work was completed in 1

6 August For all other detailed studies in Arlington County, AMEC Earth & Environmental, Inc. used the existing hydraulic analyses for Arlington County to redelineate floodplains based on more detailed and up-to-date topographic information. Culvert capacities were also evaluated in select areas to determine whether floodplain reduction or removal was justified where previous open channels had been piped. This work was done under Contract No. FS EMP-2001-RP-2411, and was completed in April The extents of these analyses are listed in Section 2.0 of this report. Planimetric base map information for all FIRM panels was provided in digital format by the Arlington County s GIS Mapping Center. This information was photogrammetrically compiled at a scale of 1 =50 from aerial photography dated September Digital orthophotographs, published in 2003, were also provided by the Arlington County s GIS Mapping Center. Users of this FIRM should be aware that minor adjustments may have been made to specific base map features to align them to 1":120 digital orthophotographs. The coordinate system used for the production of this FIRM is Universal Transverse Mercator (UTM), Zone 18 North, North American Datum of 1983 (NAD 83), GRS 80 spheroid. Corner coordinates shown on the FIRM are in latitude and longitude referenced to the UTM projection, NAD 83. Differences in the datum and spheroid used in the production of FIRMs for adjacent counties may result in slight positional differences in map features at the county boundaries. These differences do not affect the accuracy of information shown on the FIRM. 1.3 Coordination Consultation and Coordination Officer s (CCO) meetings may be held for each jurisdiction in this countywide FIS. An initial CCO meeting is held typically with representatives of FEMA, the community, and the study contractor to explain the nature and purpose of a FIS, and to identify the streams to be studied by detailed methods. A final CCO meeting is held typically with representatives of FEMA, the community, and the study contractor to review the results of the study. Two initial CCO meetings were held in July and December These meetings were attended by representatives of FEMA, USACE (the study contractor), and county officials. The study was also coordinated with the Arlington County Department of Transportation, the Northern Virginia Planning District Commission, and the U.S. Geological Survey (USGS) to gather background information and obtain other data. On June 30, 1981, the results of the study were reviewed at the final CCO meeting attended by representatives of the study contractor, FEMA, and county officials. For this countywide FIS, an initial CCO meeting was held May 26, This meeting was attended by representatives of FEMA, the study contractors, and Arlington County. A final CCO meeting was held on September 28, 2009 to discuss the results of this study. This meeting was attended by representatives from FEMA, the study contractors, and Arlington County. 2

7 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Arlington County, Virginia. All or portions of the flooding sources listed in Table 1, "Flooding Sources Studied by Detailed Methods," were studied by detailed methods. Limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). TABLE 1 - FLOODING SOURCES STUDIED BY DETAILED METHODS Stream Doctors Branch Donaldson Run Four Mile Run Gulf Branch Little Pimmitt Run Little Pimmitt Run Tributary Lower Long Branch Lubber Run North Branch Donaldson Run Potomac River Spout Run Upper Long Branch Windy Run Length of Study From its confluence with Four Mile Run to approximately 1950 feet upstream From its confluence with the Potomac River to approximately 8100 feet upstream From its confluence with the Potomac River to approximately 44,600 feet upstream From its confluence with the Potomac River to approximately 6850 feet upstream From the Arlington County boundary to approximately 5150 feet upstream From its confluence with Little Pimmitt Run to approximately 1350 feet upstream From its confluence with Four Mile Run to approximately 10,000 feet upstream From its confluence with Four Mile Run to approximately 8550 feet upstream From approximately 5000 feet above the confluence of Donaldson Run with the Potomac River to approximately 2550 feet upstream The entire length along the Arlington County boundary From its confluence with the Potomac River to approximately 5800 feet upstream From its confluence with Four Mile Run to approximately 3850 feet upstream From its confluence with the Potomac River to approximately 3300 feet upstream 3

8 As noted in Section 1.2 of this FIS, hydrologic and hydraulic analysis for the Potomac River was taken from the FIS for the District of Columbia, Washington D.C., effective September 27, Previously, the flooding effects from the Potomac River were not included in the Arlington County FIS and FIRM. A revised analysis for the Potomac River 1% annual chance tidal elevation was performed for FEMA by USACE under Contract No. HSFE03-04-X This work was completed in August Table 2, "Stream Name Changes," lists streams that have names in this countywide FIS other than those used in the previously printed FISs for the communities in which they are located. TABLE 2 - STREAM NAME CHANGES Community Old Name New Name Arlington County Long Branch* Lower Long Branch Arlington County Long Branch* Upper Long Branch *In the previous effective study, dated November 3, 1981, there were two studied streams with the name Long Branch. The name change to Lower Long Branch refers to the stream in the eastern section of the county in the vicinity of Interstate 395. It was shown on FIRM Panels B and B, and is now shown on FIRM Panel 51013C0077C. The name change to Upper Long Branch refers to the stream in the southwestern section of the county in the vicinity of Arlington Boulevard (US Route 50). It was shown on FIRM Panel B, and is now shown on FIRM Panels 51013C0057C and 51013C0076C. This FIS also incorporates the determinations of letters issued by FEMA resulting in map changes (Letter of Map Revision [LOMR]), as shown in Table 3, "Letters of Map Revision." TABLE 3 - LETTERS OF MAP REVISION Case Number Flooding Source(s)/Project Identifier Effective Date FIA Lower Long Branch - November 12, 1986 Between Columbia Pike and 500 feet downstream P Lubber Run - May 15, 1997 Box culvert under Washington Boulevard and Detention Basin P Lubber Run - February 5, 1998 Between Wilson Boulevard and Washington Boulevard P Lubber Run - April 28, 1999 Between Carlin Springs Road and Wilson Boulevard 4

9 TABLE 3 - LETTERS OF MAP REVISION - continued Case Number Flooding Source(s)/Project Identifier Date Issued P Four Mile Run - February 18, feet downstream of Interstate 66 to approximately 650 feet upstream of Fairfax Drive P Little Pimmit Run - April 9, 2010 From Arlington County boundary to approximately 1,000 feet upstream of Rock Spring Road Little Pimmit Run Tributary - From confluence with Little Pimmit Run to approximately 120 feet upstream of confluence P Little Pimmit Run - December 17, 2012 From approximately 170 feet downstream of Williamsburg Boulevard to approximately 200 feet upstream of Little Falls Road The areas studied by detailed methods were selected with priority given to all known flood hazard areas and areas of projected development and proposed construction. All or portions of numerous flooding sources in the county were studied by approximate methods. Approximate analyses were used to study those areas having a low development potential or minimal flood hazards. The scope and methods of study were proposed to, and agreed upon by FEMA and Arlington County. 2.2 Community Description Arlington County is located southwest of the District of Columbia just across the Potomac River, and has developed as an extension of the District. Arlington County is bordered on the northwest by Fairfax County and on the southwest by the independent cities of Falls Church and Alexandria. More than 75 percent of the county is developed, and relatively little space is available for additional capacity. Over 30 percent of the developed land is used for institutional, commercial and other non-residential uses such as Arlington National Cemetery, Washington National Airport and the Pentagon complex. A shift toward multi-family dwellings and the influence of institutional and commercial pressures is expected in the future. Recently constructed high-rise developments are evidence of the county space limitations and indicate an increase in high density development. According to U. S. Census Bureau figures, the population has increased from 174,000 to 207,628 between 1970 and (Census, 1967 and 2011) 5

10 Four Mile Run originates near Brilyn Park in Fairfax County and flows southeast for a distance of 9.3 miles to the Potomac River. It has a total drainage area of 19.5 square miles, 13.7 of which are in Arlington County. The principal tributaries of Four Mile Run are Lower and Upper Long Branch, Doctors Branch and Lubber Run. Doctors Branch flows south from its origin near South George Mason Drive to its confluence with Four Mile Run. Little Pimmit Run emerges from underground conduits just north of Yorktown Boulevard and flows north into Fairfax County to the Potomac River. About 1.55 square miles of the Little Pimmit Run basin are located in Arlington County. Little Pimmit Run Tributary flows north from its origin near Williamsburg Boulevard to its confluence with Little Pimmit Run. Lower Long Branch has a drainage area of 2.7 square miles and is segmented into sewered and open channel portions. It flows south from its origin near the Navy Annex to its confluence with Four Mile Run. Donaldson Run flows northeast from its origin near Lee Heights to its confluence with the Potomac River. North Branch Donaldson Run flows east from its origin near the Washington Country Club to its confluence with Donaldson Run. Gulf Branch originates near Country Club Hills and flows northeast to the Potomac River. Upper Long Branch rises near Lee Boulevard Heights and flows northeast to Four Mile Run. Lubber Run begins near 13th Street and flows south to Four Mile Run. Spout Run emerges from the ground near North Stafford Street and flows northeast to the Potomac River. Windy Run rises near Lorcum Lane and flows northeast to the Potomac River. The climate of this area is temperate and characteristic of the coastal plain, with fairly high relative humidity and moderate rainfall. The summers are rather warm while winters are generally moderate. The Chesapeake Bay has some tempering effect on the extremes of summer heat and winter cold. The average annual temperature is 57 degrees Fahrenheit ( F), with extremes varying from 107 F to 26 F below zero. Average annual precipitation is 42 inches and average annual snowfall is approximately 18 inches. Precipitation is fairly evenly distributed throughout the year. (Commerce, 1975) 2.3 Principal Flood Problems Past history of flooding indicates that floods may occur during any time of the year. Due to the rapid runoff associated with urbanization, the majority of major floods have occurred during intense thunderstorms. Four Mile Run has, in the past, been the principal source of flooding problems in Arlington County. The portion of Four Mile Run from Walter Reed Drive to the confluence with the Potomac River has been out of its banks regularly, inundating this area. The inadequate railroad culverts under the Potomac Yards were the primary cause of this flooding, although the poor hydraulic condition of the channel in the Mount Vernon Avenue vicinity would have caused some flooding even if the culverts were not an obstruction. The Four Mile Run floodplain from U. S. Route 1 to the confluence of Lower Long Branch is subject to flooding from Four Mile Run, and from backwater in the Four Mile Run estuary from fluvial and tidal flows in the Potomac River. The August 1933 hurricane tide, the largest tide of record in the Washington, D. C. 6

11 vicinity prior to the 21 st century, produced an elevation of approximately 9.3 feet at the mouth of Four Mile Run. The March 1936 flood, the largest recorded fluvial flood on the Potomac River, produced an elevation of approximately 8.6 feet at the mouth of Four Mile Run. The October 1942 flood was the second largest recorded fluvial flood on the Potomac River and produced an elevation of 9.5 feet at the mouth of Four Mile Run. While the fluvial discharge was less than the 1936 flood, stages in the Washington, D. C. vicinity were higher as a result of an abnormally high tide which occurred coincident with the peak flow. The 1933 Chesapeake Potomac hurricane, which caused $27.2 million dollars (1933 USD; $457 million (2009 USD)) in damage, was the worst storm to strike Virginia until Hurricane Isabel of 2003, which caused $3.6 billion dollars (2003 USD; $4.28 billion (2009 USD)) in damage (Blake et al, 2007). Tropical storm Agnes produced a then record flood stage at the Mount Vernon Avenue bridge across Four Mile Run on June 22, 1972, and resulted from an estimated peak discharge of 10,000 cubic feet per second (cfs) (Interior, 1975). The maximum stage at the bridge was approximately 19.5 feet, 3.5 feet higher then the previous record of July 22, The 1972 flood stage at Mount Vernon Avenue was not caused by a single record peak discharge, but rather by several smaller peaks occurring at such short intervals that the impounded floodwaters upstream of the inadequate railroad culverts under the Potomac Yards did not fully recede between peaks of inflow. In addition, the discharge through the culverts was reduced due to debris caught at the culvert entrances. Although the flash flood of July 22, 1969 destroyed the USGS gaging station upstream of East Glebe Road, the discharge was estimated by the USGS to be a record high of 14,000 cfs (Interior, 1970). Peak inflow to the pool above the sixbridge constriction near the mouth of Four Mile Run was estimated at 17,000 cfs, with peak outflow of only 5,500 cfs. The result was a nearly flat pool extending more than 4,000 feet upstream. The maximum stage at the Mount Vernon Avenue Bridge was estimated to be 16 feet. The inadequate railroad culverts under the Potomac Yards were the primary cause of the extensive backwater flood in the Arlington-Alexandria area. On September 23, 2003, the USGS gaging station ( ) at Shirlington Road recorded a peak streamflow of 6,040 cfs with a gage height of feet as a result of Hurricane Isabel (USGS Peak Streamflow). The hurricane s eye tracked well west of the Chesapeake Bay, but the storm's 40 to 60 mph sustained winds pushed a bulge of water northward up the bay and its tributaries producing a record storm surge. The Virginia western shore counties of the Chesapeake Bay and the tidal tributaries of the Potomac, Rappahannock and other smaller rivers, experienced a storm surge which reached 5 to 9 feet above normal tides. In many locations, Isabel's surge was higher than the previous record storm known as the Chesapeake-Potomac Hurricane of Impact on the commonwealth of Virginia as a whole is staggering with $1.6 billion in damages with over 1,186 homes and 77 businesses destroyed, 9,110 homes and 333 businesses with major damage and 107,908 homes and over 1,000 businesses affected or impacted with minor damage. An estimated 660,000 dump trucks of debris was generated. At least 10 people were directly killed by the storm with hundreds injured. Almost 2 million electrical customers found themselves without power. Crop losses were calculated to be $59.3 million with another $57.6 million in damages to fences, farm buildings 7

12 and equipment. Cost to Virginia's Dominion Power were $128 million, Red Cross outlays $6 million, military bases $283 million, private property $732 million, National Park Service $123 million, and public property $270 million. Arlington had 2 homes destroyed and 46 with major damage. Another 146 residences had minor damage. Costs of flooding and damage from falling trees were estimated at $2.5 million (NCDC). The flash flood of June 25, 2006 caused the closing of more than ten heavily traveled roads, including Interstate 395, State Route 110 and Washington Boulevard. The George Washington Memorial Parkway South at the Ronald Reagan Washington National Airport exit was closed. A weak cold front settled over the Mid Atlantic between June 23 and June 27. Waves of low pressure rode northeast along the front. Flow in the atmosphere was parallel to the boundary, producing several rounds of persistent showers and thunderstorms. As a result, double digit rainfall totals affected parts of the region through the five day period. Scattered areas of flash flooding began on June 23 and continued into June 24. Then, flooding began to take on an even more serious nature since the ground had become saturated in so many spots. A slow-moving line of thunderstorms fired along a tropical moisture plume and dumped between 4 and 7 inches of rain across Northern Virginia, causing extensive urban flooding on June 25 and June 26. Extensive power outages across the region occurred during this event. Major disruption of transportation was experienced June 26 due to the flooding. Property damage was estimated at $3.0 million (NCDC). Peak discharge at USGS Gage # on Four Mile Run registered at 18,100 cfs, with a gage height of feet, the highest value since records were kept at this gage from 1947, exceeding the value reported in June 1972 that was associated with tropical storm Agnes (USGS Peak Streamflow). 2.4 Flood Protection Measures The USACE constructed a flood protection project on Four Mile Run from Interstate 395 to a point 1050 feet downstream of South Arlington Ridge Road. The project, which impacts both Arlington County and the City of Alexandria, consists of 11,850 feet of improved channel, approximately 4,700 linear feet of floodwalls, bridge improvements, drop structures and other appropriate hydraulic modifications to carry the design flow. Although the project was initially designed to protect against the 1% annual chance flooding event, it currently is not certified to provide this level of protection. This project also included improvements to the downstream end of Lower Long Branch at its confluence with Four Mile Run. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the county, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this FIS. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long term average period between floods 8

13 of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1-percent chance of annual exceedance) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the county at the time of completion of this FIS. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for the flooding sources studied in detail affecting the county. Information on the methods used to determine peak discharge-frequency relationships for the streams studied by detailed methods is shown below. Pre-countywide Analyses The unincorporated areas of Arlington County have a previously printed FIS report. The hydrologic analysis described in that report has been compiled and is summarized below. The hydrologic analysis was a modification of the work presented by the USGS in their Water-Supply Paper, Effects of Urban Development on Floods in Northern Virginia, which relates basin characteristics to stream flow characteristics (Interior, 1970). Frequency curves were developed for gaged streams in and around Arlington County which have basin characteristics similar to Four Mile Run and Little Pimmit Run. This analysis followed the standard log-pearson Type III method as outlined by the Water Resources Council (Water Resources Council, 1976). Using a multiple linear regression, discharges at the selected recurrence intervals were then related to basin characteristics (USACE, 1968). Those regression equations were then used to determine discharges on the ungaged portions of Four Mile Run, Little Pimmit Run, Little Pimmit Run Tributary and Doctors Branch. Discharges for Lower Long Branch, Donaldson Run, North Branch Donaldson Run, Gulf Branch, Upper Long Branch, Lubber Run, Spout Run and Windy Run were computed using the Anderson Method described in Water-Supply Paper 2001-C (Interior, 1970). A summary of the drainage area-peak discharge relationships for the streams studied by detailed methods is shown in Table 4, "Summary of Discharges." 9

14 TABLE 4 SUMMARY OF DISCHARGES FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) PEAK DISCHARGES (cubic feet per second) 2-Percent- 1-Percent- Annual- Annual- Chance Chance 10-Percent- Annual- Chance 0.2-Percent- Annual- Chance DOCTORS BRANCH At confluence with Four Mile Run 1.4 1,250 2,000 2,350 3,400 DONALDSON RUN * * * * * FOUR MILE RUN Downstream of confluence of Lower Long Branch Downstream of confluence of Lucky Run Downstream of confluence of Doctors Branch Downstream of confluence of Right Bank Tributary Downstream of confluence of Upper Long Branch Downstream of confluence of Lubber Run Downstream of confluence of Right Bank Tributary Downstream of confluence of Left Bank Tributary Downstream of North Sycamore Street Downstream of North Van Buren Street Downstream of Lee Highway 17.3 * * 26,000 42, ,800 17,000 22,100 37, ,100 11,800 14,900 42, ,200 9,700 12,100 18, ,400 7,800 9,600 14, ,300 5,700 7,000 10, ,200 3,500 7,000 10, ,500 2,400 2,800 4, ,106 1,855 2,211 3, ,024 1,747 2,049 2, ,467 1,709 2,394 GULF BRANCH * * * * * LITTLE PIMMIT RUN At Arlington County Boundary Upstream of confluence of Left Bank Tributary 1.5 1,500 2,500 3,050 4, ,050 1,200 1,550 * Data not available 10

15 TABLE 4 SUMMARY OF DISCHARGES - continued PEAK DISCHARGES (cubic feet per second) DRAINAGE 10-Percent- 2-Percent- 1-Percent- 0.2-Percent- FLOODING SOURCE AREA Annual- Annual- Annual- Annual- AND LOCATION (sq. miles) Chance Chance Chance Chance LITTLE PIMMIT RUN TRIBUTARY 1 At confluence with Little Pimmit Run /1, /1, /1, /2,550 LOWER LONG BRANCH At confluence with Four Mile Run 2.7 * * 4,550 7,200 NORTH BRANCH DONALDSON RUN * * * * * POTOMAC RIVER * * * * * SPOUT RUN * * * * * UPPER LONG BRANCH * * * * * WINDY RUN * * * * * * Data not available ¹ Discharge in open channel/total discharge including that portion in storm sewers This Countywide Analyses No new hydrologic analyses were performed for this FIS. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the source studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS report. For construction and/or floodplain management purposes, users are encouraged to use the flood elevation data presented in this FIS in conjunction with the data shown on the FIRM. Cross sections were determined from topographic maps and field surveys. All bridges, dams, and culverts were field surveyed to obtain elevation data and structural geometry. All topographic mapping used to determine cross sections is referenced in Section

16 Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the FIRM (Exhibit 2). The hydraulic analyses for this countwide FIS were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. Pre-countywide Analyses For the November 3, 1981, FIS, cross sections for the backwater analyses of Four Mile Run, Little Pimmit Run and Little Pimmit Run Tributary were obtained from field surveys. Cross sections for the hydraulic analyses of Donaldson Run, North Branch Donaldson Run, Gulf Branch, Long Branch, Lubber Run, Spout Run and Windy Run were also obtained from field surveys. All bridges, dams and culverts were field surveyed to obtain elevation data and structural geometry. Bridge symbols for Interstate Route 66 and North Roosevelt Avenue were not shown on the flood profiles (Exhibit 1) because they are high level bridges and do not affect flood flows. Numerous bicycle path bridges along Four Mile Run were not shown on the Flood Boundary and Floodway Map (Exhibit 3), Flood Insurance Rate Map (published separately) and the Flood Profiles (Exhibit 1).- this will need to be revised. These bridges are located beneath the 10-year flood level and would be washed out when flooding occurs. Cross sections for the analysis of Doctors Branch and Long Branch were developed from the Arlington County base maps (Arlington, 1974). Water-surface profiles of floods of the selected recurrence intervals on Four Mile Run, Doctors Branch, Little Pimmit Run and Little Pimmit Run Tributary were computed through use of the COE HEC-2 step-backwater computer program (USACE, October 1973). Water-surface profiles for floods of the selected recurrence intervals on Long Branch were computed on the basis of hand calculations done by the Baltimore District of the COE. Starting water-surface elevations for Four Mile Run were a continuation of the flood flow lines used in the hydraulic design of the COE's Four Mile Run Local Flood Protection Project (USACE, April 1973). Starting-water surface elevations for Doctors Branch were obtained from the Four Mile Run profiles. Starting water-surface elevations for Little Pimmit Run Tributary were obtained from the Little Pimmit Run profiles. Starting water-surface elevations for Long Branch were determined from a rating curve developed for the Drop Structure on Long Branch. This Countywide Analyses No new hydraulic analyses were performed as part of this FIS revision. Revised information for the Potomac River was taken from the Flood Insurance Study for Washington D.C. where the USACE Engineer Research and Development Center (ERDC) Coastal and Hydraulics Laboratory (CHL) performed a cursory-level frequency-of-occurrence analysis of the storm surge for the tidally influenced reach of the Potomac River. This reach extends from the Potomac s confluence with the Chesapeake Bay to Washington D.C. The 1% annual chance storm surge elevation along the lower Potomac was estimated using the Advanced CIRCulation (ADCIRC) numerical model. In order to estimate the 1% annual chance storm surge elevation, an event emulating Hurricane Isabel was dynamically simulated. This 12

17 model was calibrated to replicate the 1% annual chance storm surge elevation at the Washington, DC National Ocean Service gauge no (Haines Point). For all other detailed studies in Arlington County, the existing hydraulic analyses were used to redelineate floodplains based on more detailed and up-to-date topographic information. In select areas where effective flood hazards were not able to be effectively redelineated due to culvert and storm sewer implementation, hydraulic capacities were evaluated. If warranted, floodplains were either reduced or removed in some of these areas. A summary of the areas evaluated for storm sewer or culvert capacity is shown in Table 5, "Hydraulic Capacity Analysis." TABLE 5 HYDRAULIC CAPACITY ANALYSIS Stream Name Donaldson Run Spout Run Lower Long Branch Location From a point downstream of 31st Street North to a point upstream of Military Road From a point just downstream of Interstate 66 to a point just upstream of Kirkwood Road and from a point just downstream of Kirkwood Road to North Taylor Street From a point downstream of 28th Street to a point just upstream of Interstate 395 Donaldson Run and the lower reach of Spout Run (from Interstate 66 to Kirkwood Road) were evaluated using the CulvertMaster hydraulic analysis tool. Culvert dimensions were obtained from available as-built plans and county planimetric files and were field verified. The results of these analyses determined that the Donaldson Run system contained the 1% annual chance discharge while the lower reach of Spout Run was not capable of containing the 1% annual chance flood. As a result, the floodplain for the aforementioned reach for Donaldson Run was removed. For the lower reach of Spout Run, the effective floodplain was maintained. The upper reach of Spout Run (Kirkwood Road to North Taylor Street) is a complex storm sewer system that replaced an area of previous open channel flow. Upon reviewing this area and the capacity of the storm sewer system, FEMA and Arlington County Officials concurred that this floodplain could be removed from Kirkwood Road to North Taylor Street. The Northern Virginia Regional Commission (NVRC) provided verification that the Lower Long Branch storm sewer system was capable of containing the 1% annual chance flood discharge. The NVRC verified this conclusion by providing the results of a Storm Water Management Model (SWMM) analysis of this system. As a result, the effective floodplain for Lower Long Branch from 28 th Street to Interstate 395 was removed. Please note that the linear distance between cross sections on the profile do not match the linear distance on the FIRM. This discrepancy is due to changes made to 13

18 streamline orientations that have resulted from using more detailed topographic mapping provided by Arlington County. Flood profiles were drawn showing computed water-surface elevations for floods of the selected recurrence intervals. Roughness factors (Manning's "n") used in the hydraulic computations were chosen by engineering judgment and were based on field observations of the streams and floodplain areas. Roughness factors for all streams studied by detailed methods are shown in Table 6, "Manning's "n" Values." TABLE 6 - MANNING'S "n" VALUES Stream Channel "n" Overbank "n" Doctors Branch Donaldson Run * * Four Mile Run Gulf Branch * * Little Pimmit Run Little Pimmit Run Tributary Lower Long Branch Lubber Run * * North Branch Donaldson Run * * Potomac River * * Spout Run * * Upper Long Branch Windy Run * * * Data not available All qualifying bench marks within a given jurisdiction that are cataloged by the National Geodetic Survey (NGS) and entered into the National Spatial Reference System (NSRS) as First or Second Order Vertical and have a vertical stability classification of A, B, or C are shown and labeled on the FIRM with their 6- character NSRS Permanent Identifier. Bench marks cataloged by the NGS and entered into the NSRS vary widely in vertical stability classification. NSRS vertical stability classifications are as follows: Stability A: Monuments of the most reliable nature, expected to hold position/elevation well (e.g., mounted in bedrock) Stability B: Monuments which generally hold their position/elevation well (e.g., concrete bridge abutment) Stability C: Monuments which may be affected by surface ground movements (e.g., concrete monument below frost line) Stability D: Mark of questionable or unknown vertical stability (e.g., concrete monument above frost line, or steel witness post) 14

19 In addition to NSRS bench marks, the FIRM may also show vertical control monuments established by a local jurisdiction; these monuments will be shown on the FIRM with the appropriate designations. Local monuments will only be placed on the FIRM if the community has requested that they be included, and if the monuments meet the aforementioned NSRS inclusion criteria. To obtain current elevation, description, and/or location information for bench marks shown on the FIRM for this jurisdiction, please contact the Information Services Branch of the NGS at (301) , or visit their Web site at It is important to note that temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with this FIS and FIRM. Interested individuals may contact FEMA to access this data. 3.3 Vertical Datum All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum used for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD 29). With the completion of the North American Vertical Datum of 1988 (NAVD 88), many FIS reports and FIRMs are now prepared using NAVD 88 as the referenced vertical datum. All flood elevations shown in this FIS report and on the FIRM are now referenced to NAVD 88. In order to perform this conversion, effective NGVD 29 elevation values were adjusted downward by 0.80 foot. Structure and ground elevations in the community must, therefore, be referenced to NAVD 88. It is important to note that adjacent communities may be referenced to NGVD 29. This may result in differences in base flood elevations across the corporate limits between the communities. NGVD = NAVD88 The BFEs shown on the FIRM represent whole-foot rounded values. For example, a BFE of will appear as 102 on the FIRM and will appear as 103. Therefore, users that wish to convert the elevations in this FIS to NGVD 29 should apply the conversion factor to elevations shown on the Flood Profiles and supporting data tables in this FIS report, which are shown at a minimum to the nearest 0.1 foot. For more information on NAVD 88, see Converting the National Flood Insurance Program to the North American Vertical Datum of 1988, FEMA Publication FIA- 20/June 1992, or contact the National Geodetic Survey at the following address: 15

20 NGS Information Services NOAA, N/NGS12 National Geodetic Survey SSMC-3, # East-West Highway Silver Spring, Maryland (301) To obtain current elevation, description, and/or location information for benchmarks shown on this map, please contact the Information Services Branch of the NGS at (301) , or visit their website at FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1-percent-annual-chance (100-year) flood elevations and delineations of the 1- and 0.2-percent-annual-chance (500-year) floodplain boundaries and 1-percent-annualchance floodway to assist communities in developing floodplain management measures. This information is presented on the FIRM and in many components of the FIS report, including Flood Profiles and Floodway Data Table. Users should reference the data presented in the FIS report as well as additional information that may be available at the local map repository before making flood elevation and/or floodplain boundary determinations. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1-percent annual chance (100-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent annual chance (500-year) flood is employed to indicate additional areas of flood risk in the county. For the streams studied in detail, the 1-percent and 0.2-percent annual chance floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:600 with a contour interval of two feet (Arlington DOT, 1974). For this countywide FIS, floodplain boundaries were redelineated using effective water surface elevations and topographic data at scales of 1:600 with a contour interval of two feet (Arlington, 2003). For the flooding sources studied by approximate methods, the boundaries of the 1-percent annual chance floodplains were delineated using topographic maps provided by Arlington County. The 1-percent and 0.2-percent annual chance floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 1-percent annual chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A and AE), and the 0.2-percent annual chance floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 1-percent and 0.2-percent annual chance floodplain boundaries are close together, only the 1-percent annual chance floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. 16

21 For the streams studied by approximate methods, only the 1-percent annual chance floodplain boundary is shown on the FIRM (Exhibit 2). 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 1-percent annual chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1- percent annual chance flood can be carried without substantial increases in flood heights. Minimum federal standards limit such increases to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this FIS are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this FIS were computed for certain stream segments on the basis of equal conveyance reduction from each side of the floodplain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. The results of the floodway computations are tabulated for selected cross sections (Table 7). The computed floodways are shown on the FIRM (Exhibit 2). In cases where the floodway and 1-percent annual chance floodplain boundaries are either close together or collinear, only the floodway boundary is shown. No floodways were computed for Donaldson Run, Gulf Branch, Lower Long Branch, North Branch Donaldson Run, Potomac River, Spout Run, Upper Long Branch, and Windy Run. Encroachment into areas subject to inundation by floodwaters having hazardous velocities aggravates the risk of flood damage, and heightens potential flood hazards by further increasing velocities. A listing of stream velocities at selected cross sections is provided in Table 7, "Floodway Data." In order to reduce the risk of property damage in areas where the stream velocities are high, the community may wish to restrict development in areas outside the floodway. The area between the floodway and 1-percent-annual-chance floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increasing the Water Surface Elevation (WSEL) of the 1-percent-annual-chance flood more than 1-foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 1, Floodway Schematic. 17

22 Figure 1: FLOODWAY SCHEMATIC 18

23 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) WITHOUT FLOODWAY WITH FLOODWAY INCREASE Doctors Branch A B C D 1, E 1, F 1, G 1, H 1, Feet above confluence with Four Mile Run TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA DOCTORS BRANCH 19

24 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) WITHOUT FLOODWAY WITH FLOODWAY INCREASE Four Mile Run A 10, , B 10, , C 12, , D 13, , E 13, , F 13, , G 14, , H 15, , I 17, , J 18, K 19, L 20, M 21, N 22, , O 23, , P 24, , Q 24, R 24, S 25, Feet above Jefferson Davis Memorial Highway TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA FOUR MILE RUN 20

25 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH2 (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) WITHOUT FLOODWAY WITH FLOODWAY INCREASE Four Mile Run (continued) T 26, U 26, V 26, W 27, X 27, Y 27, , Z 28, , AA 28, AB 28, AC 29, AD 29, AE 30, AF 30, AG 31, AH 31, AI 31, AJ 31, AK 33, Feet above Jefferson Davis Memorial Highway TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA FOUR MILE RUN 21

26 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) WITHOUT FLOODWAY WITH FLOODWAY INCREASE Four Mile Run (continued) AL 33, AM 33, AN 34, AO 35, AP 35, AQ 36, AR 36, AS 36, AT 37, AU 38, AV 39, AW 40, AX 40, AY 41, AZ 43, BA 44, Feet above Jefferson Davis Memorial Highway 2 This width extends beyond county boundary TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA FOUR MILE RUN 22