FLOOD INSURANCE STUDY

Transcription

1 FLOOD INSURANCE STUDY STAFFORD COUNTY, VIRGINIA (ALL JURISDICTIONS) COMMUNITY NAME COMMUNITY NUMBER Stafford County STAFFORD COUNTY (ALL JURISDICTIONS) Please note: this Preliminary FIS Report only includes revised coastal data and riverine updates. The unrevised components will appear in the final FIS report. PRELIMINARY: August 16, 2013 Revised: Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER V000B

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 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 Flood Insurance Study may be revised and republished at any time. In addition, part of this Flood Insurance Study may be revised by the Letter of Map Revision process, which does not involve republication or redistribution of the Flood Insurance Study. It is, therefore, the responsibility of the user to consult with community officials and to check the community repository to obtain the most current Flood Insurance Study components. Initial Countywide FIS Effective Date: February 4, 2005 Revised countywide FIS Date: Please note: this Preliminary FIS Report only includes revised coastal data and riverine updates. The unrevised components will appear in the final FIS report.

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 Coastal Analyses Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES 52 i

4 TABLE OF CONTENTS - continued Page FIGURES Figure 1 Transect Location Map 23 Figure 2 Typical Transect Schematic 26 Figure 3 Floodway Schematic 48 TABLES Table 1 - Summary of Discharges Table 2 Summary of Stillwater Elevations 20 Table 3 Transect Data Table 4 Floodway Data Table 5 - Community Map History 50 EXHIBITS Exhibit 1 - Flood Profiles Accokeek Creek Panels 01P-08P Aquia Creek Panels 09P-17P Austin Run Panels 18P-22P Claiborne Run Panels 23P-27P England Run Panels 28P-36P Falls Run Panels 37P-42P Little Falls Run Panels 43P-46P Potomac Creek Panels 47P-48P Rappahannock River Panels 49P-51P Rappahannock River - Left Channel Panel 52P Rocky Run Panels 53P-54P Tributary 3 to Austin Run Panel 55P Tributary 1 to Chopawamsic Creek Panels 56P-58P Tributary 1 to Rappahannock River Panels 59P-60P Whitsons Run Panels 61P-64P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 1 FLOOD INSURANCE STUDY STAFFORD COUNTY, VIRGINIA (ALL JURISDICTIONS) 1.0 INTRODUCTION 1.1 Purpose of Study This countywide Flood Insurance Study (FIS) revises and updates previous FIS s / Flood Insurance Rate Maps (FIRMs) in the geographic area of Stafford 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 Stafford 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 Stafford County in a countywide format FIS. Information on the authority and acknowledgments included in this countywide FIS, as compiled from previously printed FIS reports, is shown below. For the May 1980 FIS report and November 19, 1980, FIRM, the hydrologic and hydraulic analyses were prepared by Harris-Toups Associates for the Federal Emergency Management Agency (FEMA), under Contract No. H The work for the original study was completed in February The hydrologic and hydraulic analyses for the Rappahannock R i v e r in the original studies were performed previously by the Norfolk District of the U.S. Army Corps of Engineers (USACE). For the June 18, 1990, FIS, updated topographic and hydraulic data were prepared by Bengtson, DeBell, Elkin & Titus, Ltd.; and Dewberry & Davis, LLC, respectively, under agreement with FEMA. The work for that revised study was completed in April

6 In the March 3, 1992, FIS, updated topographic, hydrologic, and hydraulic data were prepared for Austin Run and Tributary 3 to Austin Run by Springfield Engineering Corporation. That work was completed in October The hydrologic and hydraulic analyses for the February 4, 2005, revision were prepared by the USACE, Norfolk District, for FEMA, under Inter- Agency Agreement No. EMW-96-IA-0294, Project Order No. 1. That work was completed in December The base map information shown on the February 4, 2005, FIRM (Exhibit 2) was derived from planimetric digital base map files provided by the Stafford County Geographical Information System Office, 1300 Courthouse Road, P.O. Box 339, Stafford, Virginia The base map information was photogrammetrically compiled at a scale of 1:2,400 from aerial photography dated Within Marine Corps Base Quantico, major roads and drainage was derived from U.S. Geological Survey (USGS) Digitial Orthophoto Quadrangles produced at a scale of 1:12,000 from photography dated 1994 or later. The coordinate system used for the production of the February 4, 2005 digital FIRM is Universal Transverse Mercator (UTM), Zone 18, referenced to the North American Datum of 1983 (NAD 83)/ High Accuracy Reference Network (HARN), Geodetic Reference System 1980 (GRS 80) spheroid. For this new countywide revision, the coastal analysis and mapping for Stafford County were conducted for FEMA by the USACE and its project partners under Project Nos. HSFE03-06-X-0023 and HSFE03-09-X The coastal analysis involved transect layout, field reconnaissance, erosion analysis, and overland wave modeling including wave setup, wave height analysis and wave runup. The base map information shown on this new countywide revision FIRM was provided by the Commonwealth of Virginia through the Virginia Base Mapping Program (VBMP). The orthophotos were flown in 2009 at scales of 1:100 and 1:200. The projection for this new countywide revision is Virginia State Plane South zone. The horizontal datum is the NAD 83/HARN, GRS 80 spheroid. Differences in datum, spheroid, projection, or UTM zones used in the production of the FIRMs for adjacent jurisdictions may result in slight positional differences in map features across jurisdictional boundaries. These differences do not affect the accuracy of this FIRM. 1.3 Coordination An initial Consultation and Coordination Officer's (CCO) meeting is held typically with representatives of FEMA, the community, and the study 2

7 2.0 AREA STUDIED 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. For the March 3, 1992, FIS, an initial CCO meeting was held in February 1976, with representatives from FEMA, the county, the Virginia State Water Control Board, and the study contractor to select the county base map and to identify local flooding problems. Regional drainage area-peak discharge relationships used for the detailed and approximate study methods were coordinated with those used elsewhere in the area by the USGS and the USACE. Peak discharges, flood elevations, flood boundaries, and floodway delineations were reviewed by county officials and officials of the State Water Control Board. On January 9, 1980, a final CCO meeting was held with representatives from FEMA, the State Water Control Board, the county, and the study contractor to review the results of the original study. For the February 4, revision, an initial CCO meeting was held on November 21, 1996, and was attended by representatives of FEMA, Stafford County, and the USACE (the study contractor) to explain the nature and purpose of the study, the scope and limits of the work, and to obtain flood information currently available concerning the county. Contacts with various local, state, and Federal agencies were made during the study in order to minimize possible hydrologic and hydraulic conflicts. A search for basic data was made at all levels of Government. On March 6, 2001, the results of this study were reviewed at a final CCO meeting attended by representatives of FEMA, Stafford County, and the USACE. For this new countywide revision, the FEMA Region III office initiated a coastal storm surge study in 2008 for the Atlantic Ocean, the Chesapeake Bay and its tributaries, and the Delaware Bay. Therefore, no initial CCO meeting for the coastal storm surge study was held. For this new countywide revision, a final CCO meeting was held on, with representatives from FEMA, the study contractor, and Stafford County. 2.1 Scope of Study This FIS covers the geographic area of Stafford County, Virginia. In the original study, the following streams were studied by detailed methods: Accokeek Creek, Aquia Creek, Tributary 1 to Chopawamsic 3

8 Creek, Claiborne Run, Falls Run, Little Falls Run, Potomac Creek, the Rappahannock River, Rappahannock River-Left Channel around Laucks Island, and Tributary 1 to the Rappahannock River. The first revision was performed in order to incorporate updated topographic data and an updated hydraulic analysis for Aquia Creek, from a point approximately 2,025 feet downstream of the U.S. Route 1 bridge to the Beaver Dam. The 1992 FIS was performed in order to incorporate updated topographic data and hydrologic and hydraulic analyses for Austin Run, from a point approximately 620 feet downstream of U.S. Route 1 to a point approximately 100 feet upstream of Interstate 95; and Tributary 3 to Austin Run, from its confluence with Austin Run to a point approximately 5,800 feet upstream. 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. For the February 4, 2005 revision, Tributary 1 to Austin Run and Tributary 2 to Austin Run had name changes, and they are now called Whitsons Run and Rocky Run, respectively. For the February 4, 2005 revision, the following streams were studied by detailed methods and revised from the previous FIS dated March 3, 1992: Stream Study Accokeek Creek Aquia Creek Austin Run Claiborne Run England Run Limits of Revised or New Detailed From approximately 3,470 feet downstream of Raven Road, State Route 609 to approximately 350 feet upstream of Ramoth Church Road, State Route 628. From the confluence with the Potomac River to approximately 925 feet upstream of Tacketts Mill Road, State Route 612. From the confluence with Aquia Creek to approximately 285 feet upstream of Winding Creek Road, State Route 628. From the confluence with the Rappahannock River to approximately 0.56 mile upstream of Jefferson Davis Highway, U.S. Route 1. From the confluence with the Rappahannock River to approximately 1.04 miles upstream of Sanford Drive, State Route

9 Stream Study England Run Falls Run Little Falls Run Rocky Run Tributary 3 to Austin Run Whitsons Run Limits of Revised or New Detailed From the confluence with the Rappahannock River to approximately 1.04 miles upstream of Sanford Drive, State Route 670. From the confluence with the Rappahannock River to approximately 1.06 miles upstream of Cardinal Forest Drive. From the confluence with the Rappahannock River to approximately 0.52 mile upstream of White Oak Road, State Route 218. From the confluence with Tributary 3 to Austin Run to approximately 300 feet upstream of Rockdale Road, State Route 617. From the confluence with Austin Run to approximately 1,510 feet upstream of northbound Interstate 95. From the confluence with Austin Run to approximately 0.65 mile upstream of Eustace Road, State Route 751. The February 4, 2005 revision previously incorporated the determination of two Letter of Map Revisions (LOMRs) issued by FEMA. The FIS incorporated a LOMR for Chopawamsic Creek and Tributary 1 to Chopawamsic Creek, effective September 30, 1992, to reflect updated topographic information, hydraulic and hydrologic analyses. The LOMR for Tributary 5 to Aquia Creek, effective July 24, 2002, incorporated updated topographic information. The February 4, 2005 revision also incorporates the as-built conditions data for two conditional LOMRs without the issuance of as-built LOMRs. These conditional LOMRs are as follows: conditional LOMR for Falls Run, issued June 2, 1997, was incorporated to reflect updated topographic information and conditional LOMR for Rocky Run, issued May 24, 2001, was incorporated to reflect updated hydraulic analyses. For this revision, the determination of one LOMR issued by FEMA was incorporated. This FIS incorporates a LOMR for Tributary 5 to Aquia Creek, effective May 17, 2012, to reflect updated topographic information, and hydraulic and hydrologic analyses. 5

10 Limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). The areas studied by detailed m et h od s were selected with priority given to all known flood hazard areas and areas of projected development and proposed construction. This revision incorporates new detailed coastal flood hazard analyses for the Rappahannock River, Aquia Creek, Austin Run, Potomac Creek, and Quantico Creek. Study efforts were initiated in 2008 and concluded in 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 King George County. 2.2 Community Description Stafford County is located in the northern portion of Virginia. The county is bordered by the unincorporated areas of Prince William County to the north; the unincorporated areas of King George County and the Potomac River to the east; the City of Fredericksburg and the unincorporated areas of Caroline County and Spotsylvania County to the south; and the unincorporated areas of Culpeper and Fauquier Counties to the west. The county is approximately 40 miles south of Washington, D.C., and approximately 55 miles north of the City of Richmond. The county has a total land area of 277 square miles. The Quantico Marine Corps Base, located in the northern part of the county, encompasses 21 percent of Stafford County's land (Internet web site, Stafford County, 2000). Stafford County was formed in 1664 and named for Staffordshire, England. Several historical landmark structures can be found in the county. The boyhood home of George Washington, Ferry Farm, is located on the Rappahannock River in southern Stafford County. Others include the 18th century Chatham manor house, the Belmont home of American impressionist artist Gari Melchers, and Aquia Church, built in In Colonial times, Falmouth, located on the north bank of the Rappahannock River, was a major port for the shipment of tobacco, wheat, com, and cotton. During the Civil War, the Union Army of the Potomac camped within the county and left much of the land and farms in ruins (Internet web site, Stafford County, 2000). The location of the county with respect to the main East Coast transportation corridors (U.S. Route 1 and Interstate 95) has been favorable for growth in the county during the 20th century, and this trend is expected to continue. The population was 24,587 in 1970, 61,236 in 1990, 92,446 in 2000, and 128,961 in 2010 (U.S. Census Bureau, QuickFacts, ). The Aquia Harbour subdivision had a population of 6,400, making it the largest subdivision in the county. Much residential and commercial growth is taking place in the Garrisonville area at the northern end of the county and along U.S. Route 17 in southern Stafford County (Internet web site, Stafford County, 2000). Stafford County is located along the Fall Line, an indefinite physiographic boundary separating the crystalline bedrock complex of the Piedmont Plateau to the western half of the county from the sediments of the Atlantic Coastal Plain to the eastern half. The topography is gently rolling to hilly west of the 6

11 Fall Line and relatively flat to the east. In general, the county's land surface slopes toward the southeast at about 20 feet per mile. Drainage of runoff is provided by streams which flow into the Potomac River to the east and the Rappahannock River to the south. The county's major soil types are deep, poorly to well-drained clays and loams that are well suited for field crop cultivation. Granite, sand, and gravel are present in certain areas. Crops include hay, barley, corn, and wheat. Common types of natural vegetation include Virginia pine, oak, hickory, and scrub oak (Clements, 1991). Sedimentary formations of the Coastal Plain fall into two general categories. The Potomac Group of the Cretaceous Era is formed by fluvial erosion and deposition. The more recent Tertiary formations result from variations in ancient sea levels. Stream drainage patterns in the Coastal Plain are more advanced than in the more erosion-resistant Piedmont Plateau (Stafford County Planning Commission, 1975). The soils in Stafford County can be grouped into eight different soil associations. On the western side of the county along the Fauquier County boundary, located in the Piedmont uplands, the soils belong to the Nason- Elioak Manor association, which are deep, well-drained silt loams with predominantly clay or silt loam subsoils. Sandy loam soils of the Appling- Cecil-Ashlar association lie east of the Nason-Elioak-Manor association and are located in the Quantico Marine Base, the area around Hartwood, and in the area around Rocky Pen Run and the Rappahannock River. These soils are deep to moderately deep, and well-drained to excessively drained, having dominantly clay or fine sandy loam subsoil. One of the larger soil associations in Stafford County extends from the northern boundary south to State Route 17. Lying just west of the Fall Line, this group is known as the Cullen-Mecklenburg-Orange association. These soils are deep, well-drained to poorly drained loams having dominantly clay subsoils. These soils are also found on Piedmont uplands. The Sassafras-Aura- Caroline soil group occupies the northeastern section of the county, generally from the Fall Line eastward to the Potomac River. Found on Coastal Plain uplands, these soils are deep, well-drained sandy loams having sandy clay loam, heavy clay loam, or clay subsoils. The broad, low-lying areas near the mouths of Aquia Creek and Potomac Creek are situated in the Tetotum-Bladen-Bertie association, which consists of deep, moderately well-drained to poorly drained loams and sandy loams having clayey subsoils. Along the Rappahannock River in the southeastern part of the county are soils in the Wickham-Alta Vista-Dogue association. These are deep, well- drained sandy loams found on stream terraces. A broad belt of soils, extending from Horsepen Run southeast to the county line, lies in the Bourne-Caroline association. These are generally deep, moderately well-drained sandy loams with a clay fragipan. The soils in the area north of White Oak and south of the Potomac River are either sandy loam or loamy sand in the Marr-Westphalia association (U.S. Department of Agriculture, 1974). The climate is typical of the temperate mid-atlantic states. The average annual temperature is 56 degrees Fahrenheit ( F), with monthly averages of 35 F and 77 F in January and July, respectively. The average annual precipitation 7

12 is 41 inches. There is some variation in monthly averages; however, this rainfall is distributed uniformly throughout the year. The annual snowfall averages 17 inches. The average growing season extends from late-april to mid-october, approximately 175 days (Clements, 1991). The economy of the area is highly diversified, including industry, agriculture, construction, education, and government. Major highways such as Interstate 95 and U.S. Route 1, and the commuter rail system to Washington, D.C., have played a major role in the rapid urbanization of the county. With the county's many miles of streams and shoreline, there will be pressure for future development in the floodplains. 2.3 Principal Flood Problems Stafford County is bordered on the east by the Potomac River estuary, which is subject to tidal flooding. The tide of record occurred in 1933, when tidal elevations were 9 feet. The next most severe high tide occurred in 1954, when Hurricane Hazel raised tidal elevations to 6 feet. The normal high tide is 1 foot. The USACE intermediate regional high tide, which has an average recurrence interval of 100 years, was estimated at 9.5 feet for the Potomac River at Stafford County (USACE, 1969). For the streams studied in the February 4, 2005 revision, flooding may be caused by heavy rain occurring any time of the year. Flooding may also occur as a result of intense rainfall produced by local thunderstorms or tropical disturbances such as hurricanes, which move into the area from the Gulf or Atlantic coasts. Flood heights for the streams can rise from normal to extreme flood peaks in a relatively short period of time. The amount and extent of damage caused by fluvial flooding depends upon the size of the area flooded, the height of flooding, the velocity of flow, the rate of rise, and the duration of flooding. The rate of rise and duration of flooding depend largely on the time required for flood waters to concentrate at a particular point, and on the duration and intensity of flood- producing rainfall. Stream velocities during floods depend largely on the size and shape of the cross sections, roughness conditions of the stream which tend to retard the flow, and the bed slope, all of which vary on different streams and at different locations on the same stream. During all major floods, high velocity flood flows and hazardous conditions would exist in the main stream channel. There is one stream gaging station on the Rappahannock River near Fredericksburg (No ). Historic floods between 1937 and 1955, their discharges in cubic feet per second (cfs), and their estimated recurrence interval as recorded by this gage are shown in the following tabulation (U.S. Department of the Interior, 1976; U.S. Department of the Interior, 1968). Discharge Year of Flood ( c f s ) _ Recurrence Interval , years , years , years , years 8

13 From 1951 to 1957, three gaging stations were located in the northern section of the county, two on Chopawamsic Creek and one on Beaverdam Run (U.S. Department of the Interior, 1968). These stations were used to develop flow data for the analysis of hydrologic conditions prior to the construction of Lunga Reservoir. There are no damage figures available for the 1943 flood. However, the Rappahannock River crested 45 feet above normal levels, causing a great deal of damage that included two fatalities and the explosion of five gasoline storage tanks (USACE, September 1970). Damage figures are available for the 1972 storm caused by Tropical Storm Agnes. In the Potomac River basin as a whole, damage totaled $129,128,000, most of which was to residential properties. The subbasin of the Potomac River, downstream of Washington, D.C., which includes the northern section of Stafford, suffered business and agricultural losses (USACE, November 1974). Damage figures were unavailable for the Rappahannock River basin. A discontinued stream gage (No ), maintained by the Virginia Department of Environmental Quality (VDEQ), is located along Aquia Creek near Garrisonville. The gage was discontinued in Records from VDEQ show a maximum discharge of 11,600 cubic feet per second (cfs) for the 1972 storm (U.S. Department of the Interior, 1998). Another stream gage (No ), maintained by the USGS, is located in the Aquia Creek watershed along the Beaverdam Run tributary near Garrisonville (U.S. Department of the Interior, 2000). For the other streams studied in the February 4, 2005 revision, records of stream flows are not available; however, the community has experienced minor flooding where structures have suffered some flood damage. In August 2011, Hurricane Irene hit the eastern coast of the United States and caused substantial damage. In September 2011, President Barack Obama declared a Major Disaster Declaration for numerous counties, including Stafford County, which allowed residents affected by the hurricane to apply for federal aid. This declaration followed the August 2011 Emergency Declaration. In October 2012, Hurricane Sandy made landfall north of the Commonwealth of Virginia, but caused substantial damage in Virginia. President Obama declared an Emergency Declaration for numerous counties, including Stafford County, which allowed assistance for emergency work and the repair or replacement of disaster-damaged facilities. 2.4 Flood Protection Measures There are three reservoirs in Stafford County: the Lunga Reservoir on Beaverdam Run, Smith Lake on Aquia Creek, and Abel Lake on Potomac Creek. The impoundment on Beaverdam Run was constructed in 1957 for recreation and water-supply purposes. It serves the Quantico Marine Corps Base, and normally contains 9,600 acre-feet of water (Patterson, 1976). Smith Lake, constructed in 1968, provides water to areas around Aquia. Rehabilitation of Smith Lake was completed in 1996, increasing the normal 9

14 pool elevation from 70 feet to 90 feet. This impoundment contains approximately 6,445 acre-feet of water (Stafford County comment letter, May 14, 2004). Abel Lake, constructed in 1965, also serves as a water-supply impoundment. However, the principal spillway was designed to release that part of the 100-year storm that cannot be confined in the 5,000 acre-feet of storage provided for flood control. Both Lunga Reservoir and Smith Lake serve minor flood control roles. A non-structural flood protection measure can be found in the floodproofing ordinance of the Virginia Uniform Statewide Building Code (Virginia State Board of Housing, 1975). Zoning and building codes provide a means of nonstructural measures for floodplain management. The "Uniform Statewide Building Code" which went into effect in September 1973 states, "where a structure is located in a 100-year floodplain, the lowest floor or all future construction or substantial improvement to an existing structure..., must be built at or above that level, except for non- residential structures which may be floodproofed to that level" (Commonwealth of Virginia, 1973). These requirements will no doubt be beneficial in reducing future flood damages in the county. 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 study. 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-, 2-, 1-, and 0.2-percent annual chance 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 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 1 percent annual chance flood 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 community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. FEMA adopted recommendations by the National Academy of Sciences (NAS) to include prediction of wave heights in FISs for coastal communities subject to storm surge flooding, and to report the estimated wave crest elevations as the base flood elevations (BFE) on the FIRM (National Academy of Sciences, 1977). Previously, FIRMs for these communities were produced showing only the stillwater storm surge elevations due to the lack of a suitable and generally acceptable methodology for estimating the wave crest elevations associated with storm surges. These stillwater elevations were subsequently stipulated in community flood plain management ordinances as the minimum elevation of the lowest floor, including 10

15 basement, of new construction. Communities and individuals had to consider the additional hazards of velocity waters and wave action on an ad hoc basis. Because there has been a pronounced tendency for buildings to be constructed only to meet minimum standards, without consideration of the additional hazard due to wave height, increasing numbers of people could unknowingly be accepting a high degree of flood-related personal and property risk in coastal areas subject to wave action. Therefore, FEMA has pursued the development of a suitable methodology for estimating the wave crest elevations associated with storm surges. The recent development of such a methodology by the NAS has led to the adoption of wave crest elevations for use as the BFEs in coastal communities. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for each flooding source studied in detail affecting the county. Pre-countywide Analyses In the original study, a log-pearson Type III analysis was carried out using the USGS hydrologic computer program based on Bulletin 17 and the annual peaks of each of the gaging stations shown in the following tabulation (U.S. Department of the Interior, 1976; Water Resources Council, 1976). Gaging Station and Location Difficult Run near Fairfax (No. 6457) Difficult Run near Great Falls (No. 6460) Accotink Creek near Annandale (No. 6540) Cedar Run near Catlett (No. 6560) Broad Run at Buckland (No. 6565) Bull Run near Manassas (No. 6570) Occoquan Creek near Occoquan (No. 6575) Gaging Station and Location South Fork Quantico Creek near Independent Hill (No. 6585) Opequon Creek near Berryville (No. 6150) South Fork Shenandoah River at Front Royal (No. 6310) North Fork Shenandoah River near Strasburg (No. 6340) Cedar Creek near Winchester (No. 6345) Passage Creek near Buckton (No. 6355) Rappahannock River near Warrenton (No. 6620) Rush River at Washington, Virginia (No. 6625) Battle Run near Laurel Mills (No. 6628) Hazel River near Rixeyville (No. 6635) Rappahannock River at Remington (No. 6640) Rappahannock River near Fredericksburg (No. 6680) South Fork Shenandoah River (No. 6285) Period of Record 20 years 41 years 29 years 25 years 25 years 25 years 26 years Period of Record 24 years 32 years 51 years 50 years 39 years 43 years 33 years 22 years 17 years 35 years 33 years 67 years 45 years 11

16 From the results of the 1980 analysis, a regional relationship correlating the drainage area with the peak flow was developed. This method was also used for the streams studied by approximate methods in the county. For the March 3, 1992, FIS, discharges for Austin Run and Tributary 3 to Austin Run were calculated using the Soil Conservation Service s TR-20 computer model (U.S. Department of Agriculture, 1987). The discharge-frequency relationships for each stream studied in detail in the February 4, 2005, revision were determined using the USACE HEC-1 hydrologic computer program (USACE 1991). Each watershed was divided into subareas and the drainage areas, percent imperviousness, times of concentration, and routing times for each subarea were determined. The percent imperviousness was based on soil types and land uses that existed at the time of the study. Based on the above basin parameters and rainfall data from Technical Paper No. 40 (TP-40) (U.S. Department of Commerce, 1961) and NWS HYDR0-35 ( U.S. Department of Commerce, 1977), flood hydrographs were computed for each subarea, routed downstream and combined with other subareas using the HEC-1 computer program. Discharges were modified due to "reservoir effects" encountered at several high embankment railroad and highway structures which have relatively high fills and small culvert capacities. The HEC-1 computer program was used to route the flood hydrographs through these storage areas, thereby reducing the discharge-frequency relationships downstream of these structures. A summary of the drainage area-peak discharge relationships for the streams studied by detailed methods is shown in Table 1, "Summary of Discharges." TABLE 1 -SUMMARY OF DISCHARGES FLOODING SOURCE DRAINAGE AREA PEAK DISCHARGES (cfs) AND LOCATION (sq. miles) 10-YEAR 50-YEAR 100-YEAR 500-YEAR ACCOKEEK CREEK Approximately 1,200 feet downstream of State Route ,880 7,550 8,490 11,050 Downstream of CSX Railroad ,520 4,870 5,310 6,570 Upstream of CSX Railroad ,810 7,540 8,520 11,180 Downstream of Interstate ,110 4,190 4,550 5,570 Upstream of Interstate ,050 8,060 9,200 12,240 Approximately 8,700 feet upstream of Interstate ,660 5,900 6,740 8,980 Approximately 3,200 feet upstream of State Route ,600 2,580 2,950 3,940 AQUIACREEK At Government Island ,440 15,100 17,000 22,900 Downstream of Smith Lake Dam ,190 14,800 16,700 22,600 Upstream of Smith Lake Dam ,400 16,400 18,400 24,400 At State Route ,580 15,000 17,000 22,500 Approximately 7,200 feet upstream of State Route ,860 9,140 10,400 13,700 At State Route ,720 4,350 4,980 6,630 12

17 TABLE 1 - SUMMARY OF DISCHARGES - continued FLOODING SOURCE DRAINAGE AREA PEAK DISCHARGES (cfs) AND LOCATION (sq. miles) 10-YEAR 50-YEAR 100-YEAR 500-YEAR AQUIACREEK At Government Island ,440 15,100 17,000 22,900 Downstream of Smith Lake Dam ,190 14,800 16,700 22,600 Upstream of Smith Lake Dam ,400 16,400 18,400 24,400 At State Route ,580 15,000 17,000 22,500 Approximately 7,200 feet upstream of State Route ,860 9,140 10,400 13,700 At State Route ,720 4,350 4,980 6,630 AUSTIN RUN At confluence with Aquia Creek ,060 6,720 7,260 8,680 At Interstate ,710 3,330 3,570 4,000 At State Route ,590 2,550 2,920 3,880 At State Route CLAIBORNE RUN At confluence with Rappahannock River ,010 4,930 5,640 7,550 Upstream of tributary located. approximately 300 feet down-. stream of State Route ,450 2,470 2,860 3,900 At U.S. Route ,460 1,680 2,290 ENGLAND RUN At confluence with Rappahannock River ,630 2,660 2,970 4,040 Approximately 3,200 feet downstream of State Route ,120 1,560 Approximately 5,300 feet upstream of State Route FALLS RUN At confluence with Rappahannock River ,030 4,410 4,910 6,010 At Interstate ,770 2,820 3,200 3,640 At State Route ,150 1,310 1,790 Approximately 5,600 feet upstream of Cardinal Drive LITTLE FALLS RUN At confluence with Rappahannock River ,920 2,930 3,150 3,660 At State Route ,810 2,920 3,340 4,450 Approximately 2,750 feet upstream of State Route ,240 1,410 1,830 13

18 TABLE 1 - SUMMARY OF DISCHARGES - continued FLOODING SOURCE DRAINAGE AREA PEAK DISCHARGES (cfs) AND LOCATION (sq. miles) 10-YEAR 50-YEAR 100-YEAR 500-YEAR POTOMAC CREEK Approximately 4,000 feet downstream of U.S. Route ,820 3,900 5,255 10,020 At U.S. Route ,610 3,465 4,675 8,940 At Potomac Creek Reservoir ,130 2,200 RAPPAHANNOCK RIVER Approximately 3,000 feet downstream of confluence of Little Falls Run 1, , , , ,500 At inlet to Laucks Island 1, ,850 54,400 68, ,500 At upstream limit of detailed study 1, , , , ,500 RAPPAHANNOCK RIVER - LEFT CHANNEL At outlet to the Rappahannock River 1, ,350 45,900 58,400 99,000 ROCKY RUN At confluence with Tributary 3 to Austin Run ,400 2,000 2,180 2,950 At State Route ,310 TRIBUTARY 3 TO AUSTIN RUN At confluence with Austin Run ,980 2,560 2,770 3,390 Upstream of confluence with Rocky Run ,300 2,080 2,370 3,140 TRIBUTARY 1 TO CHOPAWAMSIC CREEK At confluence with Chopawamsic Creek ,175 2,290 TRIBUTARY 1 TO THE RAPPAHANNOCK RIVER At confluence with the Rappahannock River ,180 1,600 3,095 At Old Ferry Road WHITSONS RUN At confluence with Austin Run ,300 3,490 3,940 5,100 Approximately 1,800 feet upstream of State Route ,140 1,710 1,930 2,490 Approximately 3,400 feet upstream of State Route

19 The effects of tidal flooding in Stafford County were determined by a frequency analysis of the tidal gaging station at Washington, D.C., which was in operation from 1931 to 1977 (Darling, 1977). The results of this analysis were then adjusted for distance above the mouth of the Potomac River. The stillwater elevations have been determined for the 10-, 50-, 100-, and 500- year floods have been determined for the Potomac River and are summarized in Table 2, "Summary of Stillwater Elevations." February 4, 2005 Countywide Analyses No new hydrologic analyses were developed for this FIS. This Countywide Analyses No new hydrologic analyses were developed for this FIS. 3.2 Hydraulic Analyses Hydraulic analyses, considering storm characteristics and the shoreline and bathymetric characteristics of flooding from the sources studied, were carried out to provide estimates of the elevations of floods of the selected recurrence intervals along each of the shorelines. Pre-countywide Analyses For the previous study, cross-section data for the streams studied in detail were obtained from aerial photographs and topographic maps at scales of 1:1,200 and 1:2,400, with a 5-foot contour interval (Toups Corporation of McLean, Virginia, 1977); the below-water cross sections were obtained by field measurements. In order to compute the significant backwater effects of bridges and culverts, cross sections were located upstream and downstream of these structures, which were field surveyed to obtain elevation data and structural geometry. For the February 4, 2005 revision, cross sections for the backwater analyses of the streams were obtained from field surveys and topographic m a p s (Stafford County, 1983 and 1988; Air Survey Corporation, 1983) and located at close intervals to bridges and culverts in order to compute the backwater effects of these structures. Elevation data and structural geometry for bridges, dams, and culverts were obtained from field surveys or available engineering plans. 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). 15

20 For the March 3, 1992, FIS, water-surface elevations of floods of the selected recurrence intervals were computed using the USACE HEC-2 step-backwater computer program (USACE, HEC-2 Water-Surface Profiles. Generalized Computer Program, October 1973; USACE, HEC-2 Water-Surface Profiles, Users Manual, October 1973; USACE, May 1974). For the 1980 FIS, starting water-surface elevations for the streams studied in detail were determined by the slope/area method, except for Aquia Creek and the 100-year flood on the Rappahannock River. For Aquia Creek, starting elevations were taken from the tidal data presented in Table 2 for the Potomac River. The 100-year flood starting elevation for the Rappahannock River was taken from the 1970 USACE study (USACE, September 1970). For the 1992 FIS, the starting water-surface elevations for Austin Run were taken from known elevations and hand calculated to the limit of detailed study. Flood profiles were drawn showing computed water-surface elevations for floods of the selected recurrence intervals. For the February 4, 2005 revision, water-surface elevations of floods of the selected recurrence intervals were computed using the USACE HEC- RAS step-backwater computer program (USACE, 1998 and 1999). Flood profiles were drawn showing computed water-surface elevations for floods of the selected recurrence intervals. Starting water-surface elevations for Tributaries 1 (Whitsons Run), 2 (Rocky Run), and 3 to Austin Run were based on backwater effects from the main stem, due to coincidental peak flooding. For all other streams in this revision, starting water-surface elevations were determined using the slope/area method. The February 4, 2005 revision also incorporates the determination of the LOMR for Tributary 1 to Chopawamsic Creek, effective September 30, The water-surface elevations for this LOMR were computed using USACE HEC-2 step-backwater computer program (USACE, HEC-2 Water-Surface Profiles, Generalized Computer Program, October 1973; USACE, HEC-2 Water-Surface Profiles, Users Manual, October 1973; USACE, May 1974). For the March 3, 1992 study, channel roughness factors (Manning's "n") used in the hydraulic computations for the streams studied by detailed methods, except for the Rappahannock River, were assigned on the basis of field inspections and previously published guidelines (U.S. Department of Agriculture, 1963). Roughness factors for the Rappahannock River were determined from the hydraulic analyses of past floods. This analysis consisted of matching discharges with corresponding historical flood profiles derived from high-water marks. February 4, 2005 Countywide Analyses For the February 4, 2005 revision, channel and overbank roughness factors (Manning's "n") used in the hydraulic computations were based on engineering judgment and field observations of the stream and floodplain areas. The ranges of channel and overbank n 16

21 values for all detailed streams are shown as follows: Stream Channel "n" Overbank "n" Accokeek Creek Aquia Creek Austin Run Claiborne Run England Run Falls Run Little Falls Run Potomac Creek Rappahannock River Rappahannock River- Left Channel Rocky Run Tributary 3 to Austin Run Tributary 1 to Chopawamsic Creek Tributary 1 to the Rappahannock River Whitsons Run For the streams studied by approximate methods, flood elevations were determined from a regional relation defined for estimating the depth of flooding on natural-flow streams in Virginia having a recurrence interval of 100 years (U.S. Department of the Interior, 1977). The drainage area is the only independent variable required, and is related to the depth of flow by the following equation: D = 3.2A 0 2 where D is the flood depth of the 100-year flood, in feet, and A is the drainage area, in square miles. The hydraulic analyses for the February 4, 2005, 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. This Countywide Analyses No new hydraulic analyses were developed for this FIS. Qualifying bench marks (elevation reference 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. 17

22 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 (e.g., mounted in bedrock) Stability B: Monuments which generally hold their position/ elevation (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) In addition to NSRS benchmarks, 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 Coastal Analyses Coastal analyses considering storm characteristics and the shoreline and bathymetric characteristics of the flooding sources studied, were carried out to provide estimates of the elevations of floods for the selected recurrence intervals along the shoreline. Users of the FIRM should be aware that coastal flood elevations are provided in Table 2, Summary of Stillwater Elevations, in this report. If the elevation on the FIRM is higher than the elevation shown in this table, a wave height, wave runup, and/or wave setup component likely exists, in which case, the higher elevation should be used for construction and/or floodplain management purposes. Development is moderate along shorefront areas of Stafford County and includes residential development, agricultural areas, small commercial facilities and military facilities. These areas are bisected, in numerous locations, with expansive areas of parklands and undeveloped woodlands. Elevations vary from sea level to approximately 280 feet NAVD 88. Behind the shoreline, development is light. An analysis was performed to establish the frequency peak elevation relationships for coastal flooding in Stafford County. The FEMA Region III office, initiated a study in 2008 to update the coastal storm surge elevations 18

23 within the states of Virginia, Maryland, and Delaware, and the District of Columbia including the Atlantic Ocean, Chesapeake Bay including its tributaries, and the Delaware Bay. The study replaces outdated coastal storm surge stillwater elevations for all FISs in the study area, including Stafford County, and serves as the basis for updated FIRMs. Study efforts were initiated in 2008 and concluded in The storm surge study was conducted for FEMA by the USACE and its project partners under Project HSFE03-06-X-0023, NFIP Coastal Storm Surge Model for Region III and Project HSFE03-09-X-1108, Phase II Coastal Storm Surge Model for FEMA Region III. The work was performed by the Coastal Processes Branch (HF-C) of the Flood and Storm Protection Division (HF), U.S. Army Engineer Research and Development Center Coastal & Hydraulics Laboratory (ERDC-CHL) (USACE, 2012). The end-to-end storm surge modeling system includes the Advanced Circulation Model for Oceanic, Coastal and Estuarine Waters (ADCIRC) for simulation of 2-dimensional hydrodynamics (Luettich and Westerink, 2008). ADCIRC was dynamically coupled to the unstructured numerical wave model Simulating WAves Nearshore (unswan) to calculate the contribution of waves to total storm surge. The resulting model system is typically referred to as SWAN+ADCIRC (Luettich and Westerink, 2008). A seamless modeling grid was developed to support the storm surge modeling efforts. The modeling system validation consisted of a comprehensive tidal calibration followed by a validation using carefully reconstructed wind and pressure fields from three major flood events for the Region III domain: Hurricane Isabel, Hurricane Ernesto, and extratropical storm Ida. Model skill was accessed by quantitative comparison of model output to wind, wave, water level, and high water mark observations. The tidal surge for those areas affected by the Potomac River affect the entire shoreline within Stafford County. The entire Potomac River shoreline coastline, from Potomac Creek to Chopawamsic Creek, is more prone to damaging wave action during high wind events. The storm-surge elevations for the 10-, 2-, 1-, and 0.2-percent annual chance floods were determined for the flooding sources shown in Table 2, Summary of Stillwater Elevations. The analyses reported herein reflect the stillwater elevations due to tidal and wind setup effects. 19

24 TABLE 2 - SUMMARY OF STILLWATER ELEVATIONS ELEVATION (feet NAVD 88*) FLOODING SOURCE AND LOCATION 10-PERCENT 2-PERCENT 1-PERCENT 0.2-PERCENT POTOMAC RIVER At confluence of Potomac Creek At confluence of Aquia Creek At confluence of Chopawamsic Creek *North American Vertical Datum of 1988 The methodology for analyzing the effects of wave heights associated with coastal storm surge flooding is described in a report prepared by the NAS (National Academy of Sciences, 1977). This method is based on three major concepts. First, depth-limited waves in shallow water reach maximum breaking height that is equal to 0.78 times the stillwater depth. The wave crest is 70 percent of the total wave height above the stillwater level. The second major concept is that wave height may be diminished by dissipation of energy due to the presence of obstructions, such as sand dunes, dikes and seawalls, buildings and vegetation. The amount of energy dissipation is a function of the physical characteristics of the obstruction and is determined by procedures prescribed in the NAS report. The third major concept is that wave height can be regenerated in open fetch areas due to the transfer of wind energy to the water. This added energy is related to fetch length and depth. The coastal analysis and mapping for Stafford County was conducted for FEMA by RAMPP under contract No. HSFEHQ-09-D-0369, Task Order HSFE The coastal analysis involved transect layout, field reconnaissance, erosion analysis, and overland wave modeling including wave setup, wave height analysis and wave runup. Wave heights were computed across transects that were located along coastal areas of Stafford County, as illustrated on the FIRM. The transects were located with consideration given to existing transect locations and to the physical and cultural characteristics of the land so that they would closely represent conditions in the locality. Each transect was taken perpendicular to the shoreline and extended inland to a point where coastal flooding ceased. Along each transect, wave heights and elevations were computed considering the combined effects of changes in ground elevation, vegetation, and physical features. The stillwater elevations for a 1% annual chance event were used as the starting elevations 20

25 for these computations. Wave heights were calculated to the nearest 0.1 foot, and wave elevations were determined at whole-foot increments along the transects. The location of the 3-foot breaking wave for determining the terminus of the Zone VE (area with velocity wave action) was computed at each transect. A review of the geology and shoreline type in Stafford County was made to determine the applicability of standard erosion methods, and FEMA s standard erosion methodology for coastal areas having primary frontal dunes, referred to as the 540 rule, was used (FEMA, 2007a). This methodology first evaluates the dune s cross-sectional profile to determine whether the dune has a reservoir of material that is greater or less than 540 square feet. If the reservoir is greater than 540 square feet, the retreat erosion method is employed and approximately 540 square feet of the dune is eroded using a standardized eroded profile, as specified in FEMA guidelines. If the reservoir is less than 540 square feet, the remove erosion method is employed where the dune is removed for subsequent analysis, again using a standard eroded profile. The storm surge study provided the return period stillwater elevations required for erosion analyses. Each cross-shore transect was analyzed for erosion, when applicable. Wave height calculations used in this study follow the methodologies described in the FEMA guidance for coastal mapping (FEMA, 2007b). Wave setup results in an increased water level at the shoreline due to the breaking of waves and transfer of momentum to the water column during hurricanes and severe storms. For the Stafford County study, wave setup was determined directly from the coupled wave and storm surge model. The total stillwater elevation (SWEL) with wave setup was then used for simulations of inland wave propagation conducted using FEMA s Wave Height Analysis for Flood Insurance Studies (WHAFIS) model Version 4.0 (FEMA, 2007c). WHAFIS is a one-dimensional model that was applied to each transect in the study area. The model uses the specified SWEL, the computed wave setup, and the starting wave conditions as input. Simulations of wave transformations were then conducted with WHAFIS taking into account the storm-induced erosion and overland features of each transect. Output from the model includes the combined SWEL and wave height along each cross-shore transect allowing for the establishment of BFEs and flood zones from the shoreline to points inland within the study area. Wave runup is defined as the maximum vertical extent of wave uprush on a beach or structure. FEMA s 2007 Guidelines and Specifications require the 2% wave runup level be computed for the coastal feature being evaluated (cliff, coastal bluff, dune, or structure) (FEMA, 2007b). The 2% runup level is the highest 2 percent of wave runup affecting the shoreline during the 1- percent-annual-chance flood event. Each transect defined within the study area was evaluated for the applicability of wave runup, and if necessary, the appropriate runup methodology was selected and applied to each transect. Runup elevations were then compared to WHAFIS results to determine the dominant process affecting BFEs and associated flood hazard levels. 21

26 Computed controlling wave heights at the shoreline range from 2.6 feet to 7.9 feet. The corresponding wave elevation at the shoreline varies from 7.9 feet NAVD 88 to 9.3 feet NAVD 88. Vertical reinforced coastlines serve to reduce wave height z. Between transects, elevations were interpolated using topographic maps, land-use and land cover data, and engineering judgment to determine the aerial extent of flooding. The results of the calculations are accurate until local topography, vegetation, or cultural development within the community experience major changes. Table 3, Transect Data, provides the 10%, 2%, 1% and 0.2% annual chance stillwater elevations and the starting wave conditions for each transect. Figure 1, Transect Location Map, provides an illustration of the transect locations for Stafford County. 22

27 QUANTICO MARINE CORPS BASE Chopawamsic Creek Potomac River STAFFORD COUNTY 16 9 Aquia Creek Potomac Creek Ü 23 FEDERAL EMERGENCY MANAGEMENT AGENCY Miles STAFFORD COUNTY, VA ALL JURISDICTIONS TRANSECT LOCATION MAP FIGURE 1 FIGURE 1 23

28 Table 3 Transect Data Starting Wave Conditions for the 1% Annual Chance Starting Stillwater Elevations (feet NAVD 88) Flood Source Transect Coordinates Significant Wave Height H s (ft) Peak Wave Period T p (sec) 10% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance POTOMAC RIVER 1 N W POTOMAC RIVER 2 N W POTOMAC RIVER 3 N W POTOMAC RIVER 4 N W POTOMAC RIVER 5 N W POTOMAC RIVER 6 N W POTOMAC RIVER 7 N W POTOMAC RIVER 8 N W POTOMAC RIVER 9 N W AQUIA CREEK 10 N W AQUIA CREEK 11 N W AQUIA CREEK 12 N W AQUIA CREEK 13 N W AQUIA CREEK 14 N W AQUIA CREEK 15 N W

29 Table 3 Transect Data - continued Starting Wave Conditions for the 1% Annual Chance Starting Stillwater Elevations (feet NAVD 88) Flood Source Transect Coordinates Significant Wave Height H s (ft) Peak Wave Period T p (sec) 10% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance AQUIA CREEK 16 N W POTOMAC RIVER 17 N W POTOMAC RIVER 18 N W POTOMAC RIVER 19 N W POTOMAC CREEK 20 N W POTOMAC CREEK 21 N W POTOMAC CREEK 22 N W POTOMAC CREEK 23 N W POTOMAC CREEK 24 N W Areas of coastline subject to significant wave attack are referred to as coastal high hazard zones. The USACE has established the 3-foot breaking wave as the criterion for identifying the limit of coastal high hazard zones (USACE, 1975). The 3-foot wave has been determined the minimum size wave capable of causing major damage to conventional wood frame of brick veneer structures. The one exception to the 3-foot wave criteria is where a primary frontal dune exists. The limit the coastal high hazard area then becomes the landward toe of the primary frontal dune or where a 3-foot or greater breaking wave exists, whichever is most landward. The coastal high hazard zone is depicted on the FIRM as Zone VE, where the delineated flood hazard includes wave heights equal to or greater than 3 feet. Zone AE is depicted on the FIRM where the delineated flood hazard includes wave heights less than 3 feet. A depiction of a sample transect which illustrates the relationship between the stillwater elevation, the wave crest elevation, 25

30 and the ground elevation profile, and how the Zones VE and AE are mapped is shown in Figure 2, Typical Transect Schematic. Post-storm field visits and laboratory tests have confirmed that wave heights as small as 1.5 feet can cause significant damage to structures when constructed without consideration to the coastal hazards. Additional flood hazards associated with coastal waves include floating debris, high velocity flow, erosion, and scour which can cause damage to Zone AE-type construction in these coastal areas. To help community officials and property owners recognize this increased potential for damage due to wave action in the AE zone, FEMA issued guidance in December 2008 on identifying and mapping the 1.5-foot wave height line, referred to as the Limit of Moderate Wave Action (LiMWA). While FEMA does not impose floodplain management requirements based on the LiMWA, the LiMWA is provided to help communicate the higher risk that exists in that area. Consequently, it is important to be aware of the area between this inland limit and the Zone VE boundary as it still poses a high risk, though not as high of a risk as Zone VE (see Figure 2). The AE and VE zones were divided into whole-foot elevation zones based on the average wave crest elevation in that zone. Where the map scale did not permit delineating zones at one foot intervals, larger increments were used. In cases where the 1- and 0.2-percent annual chance floodplain boundaries are close together, only the 1-percent annual chance 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. Figure 2 - Typical Transect Schematic 26