VOLUME 1 OF 4 PRINCE WILLIAM COUNTY, VIRGINIA AND INCORPORATED AREAS

COMMUNITY COMMUNITY NAME NUMBER DUMFRIES, TOWN OF 510120 HAYMARKET, TOWN OF 510121 MANASSAS, CITY OF 510122 MANASSAS PARK, CITY OF 510123 OCCOQUAN, TOWN OF 510124 PRINCE WILLIAM COUNTY, 510119 UNINCORPORATED AREAS QUANTICO, TOWN OF 510232

September 30, 2020 REVISED: TBD FLOOD INSURANCE STUDY NUMBER 51153CV001B Version Number 2.6.4.6 TABLE OF CONTENTS

Volume 1 Page SECTION 1.0 – INTRODUCTION 1 1.1 The National Flood Insurance Program 1 1.2 Purpose of this Flood Insurance Study Report 2 1.3 Jurisdictions Included in the Flood Insurance Study Project 2 1.4 Considerations for using this Flood Insurance Study Report 4 SECTION 2.0 – FLOODPLAIN MANAGEMENT APPLICATIONS 16 2.1 Floodplain Boundaries 16 2.2 Floodways 31 2.3 Base Flood Elevations 32 2.4 Non-Encroachment Zones 33 2.5 Coastal Flood Hazard Areas 33 2.5.1 Water Elevations and the Effects of Waves 33 2.5.2 Floodplain Boundaries and BFEs for Coastal Areas 35 2.5.3 Coastal High Hazard Areas 35 2.5.4 Limit of Moderate Wave Action 37 SECTION 3.0 – INSURANCE APPLICATIONS 37 3.1 National Flood Insurance Program Insurance Zones 37 SECTION 4.0 – AREA STUDIED38 4.1 Basin Description 38 4.2 Principal Flood Problems 38 4.3 Non-Levee Flood Protection Measures 40 4.4 Levees 41 SECTION 5.0 – ENGINEERING METHODS 41 5.1 Hydrologic Analyses 42 5.2 Hydraulic Analyses 55 5.3 Coastal Analyses 71 5.3.1 Total Stillwater Elevations 72 5.3.2 Waves 74 5.3.3 Coastal Erosion 75 5.3.4 Wave Hazard Analyses 75 5.4 Alluvial Fan Analyses 79 SECTION 6.0 – MAPPING METHODS 79 6.1 Vertical and Horizontal Control 79 6.2 Base Map 80 6.3 Floodplain and Floodway Delineation 81

i Volume 2

6.4 Coastal Flood Hazard Mapping 140 6.5 FIRM Revisions 142 6.5.1 Letters of Map Amendment 142 6.5.2 Letters of Map Revision Based on Fill 143 6.5.3 Letters of Map Revision 143 6.5.4 Physical Map Revisions 145 6.5.5 Contracted Restudies 146 6.5.6 Community Map History 146 SECTION 7.0 – CONTRACTED STUDIES AND COMMUNITY COORDINATION 147 7.1 Contracted Studies 147 7.2 Community Meetings 154 SECTION 8.0 – ADDITIONAL INFORMATION 160 SECTION 9.0 – BIBLIOGRAPHY AND REFERENCES 161

Volume 1

Figures Page

Figure 1: FIRM Index 6 Figure 2: FIRM Notes to Users 9 Figure 3: Map Legend for FIRM 12 Figure 4: Floodway Schematic 32 Figure 5: Wave Runup Transect Schematic 34 Figure 6: Coastal Transect Schematic 36 Figure 7: Frequency Discharge-Drainage Area Curves 53 Figure 8: 1% Annual Chance Total Stillwater Elevations for Coastal Areas 73 Figure 9: Transect Location Map 78

Tables Page

Table 1: Listing of NFIP Jurisdictions 2 Table 2: Flooding Sources Included in this FIS Report 17 Table 3: Flood Zone Designations by Community 38 Table 4: Basin Characteristics 38 Table 5: Principal Flood Problems 39 Table 6: Historic Flooding Elevations 39 Table 7: Non-Levee Flood Protection Measures 40 Table 8: Levees 41 Table 9: Summary of Discharges 43 Table 10: Summary of Non-Coastal Stillwater Elevations 54 Table 11: Stream Gage Information used to Determine Discharges 55 Table 12: Summary of Hydrologic and Hydraulic Analyses 56

ii Volume 1 (continued)

Table 13: Roughness Coefficients 70 Table 14: Summary of Coastal Analyses 72 Table 15: Tide Gage Analysis Specifics 74 Table 16: Coastal Transect Parameters 76 Table 17: Summary of Alluvial Fan Analyses 79 Table 18: Results of Alluvial Fan Analyses 79 Table 19: Countywide Vertical Datum Conversion 80 Table 20: Stream-Based Vertical Datum Conversion 80 Table 21: Base Map Sources 81 Table 22: Summary of Topographic Elevation Data used in Mapping 82 Table 23: Floodway Data 83

Volume 2

Tables Page

Table 23: Floodway Data (continued) 101 Table 24: Flood Hazard and Non-Encroachment Data for Selected Streams 140 Table 25: Summary of Coastal Transect Mapping Considerations 141 Table 26: Incorporated Letters of Map Change 143 Table 27: Community Map History 147 Table 28: Summary of Contracted Studies Included in this FIS Report 148 Table 29: Community Meetings 155 Table 30: Map Repositories 160 Table 31: Additional Information 161 Table 32: Bibliography and References 162 Exhibits

Flood Profiles Panel Broad Run Panels 001P-007P Broad Run Side Channel Panel 008P Bull Run Panels 009P-021P Bull Run Tributary A Panels 022P-029P Volume 3 Exhibits Flood Profiles Panel Bull Run Tributary B Panels 030P-033P Bull Run Tributary C Panels 034P-036P Bull Run Tributary D Panels 037P-038P Cannon Branch Panel 039P Catharpin Creek Panels 040P-042P Cedar Run Panel 043P Chestnut Lick Tributary No. 1 Panel 044P

iii Volume 3 (continued)

Flood Profiles Panel Chopawamsic Creek Panels 045P-046P Cockrell Branch Panel 047P Cow Branch Panels 048P-509P Dawkins Branch Panels 051P-053P Farm Creek Panels 054P-055P Flat Branch Panels 056P-059P Flat Branch Tributary A Panels 060P-061P Flat Branch Tributary B Panels 062P-067P Flat Branch Tributary C Panels 068P-071P Holkums Branch Panel 072P-073P Hooes Run Panels 074P-076P Kettle Run Panels 077P-079P Little Bull Run Panels 080P-084P Little Creek Panels 085P-094P Marumsco Creek Panels 095P-097P Marumsco Creek Tributary A Panels 098P-099P Marumsco Creek Tributary B Panel 100P Neabsco Creek Panels 101P-106P Neabsco Creek Tributary A Panels 107P-112P North Fork Broad Run Panels 113P-119P North Fork Broad Run Bypass Spillway Panel 120P Occoquan River Panels 121P-132P

Volume 4 Exhibits

Flood Profiles Panel Powells Creek Panels 133P-138P Purcell Branch Panels 139P-140P Quantico Creek Panels 141P-143P Rocky Branch Panels 144P-147P Russia Branch Panels 148P-149P Tributary No. 1 to Cannon Branch Panel 150P Tributary No. 1 to Flat Branch Tributary Panels 151P-152P B Tributary No. 1 to Flat Branch Tributary Panels 153P-155P C Tributary No. 1 to Marumsco Creek Panels 156P-157P Tributary B Tributary No. 1 to Quantico Creek Panels 158P-159P

iv Volume 4 (continued)

Flood Profiles Panel Tributary No. 2 to Russian Branch Panels 160P-161P Tributary No. 3 to Flat Branch Tributary Panels 162P-163P A Tributary No. 3 to Flat Branch Tributary Panels 164P-165P B Unnamed Tributary to Chestnut Lick Panel 166P Tributary No. 1 Unnamed Tributary to North Fork Broad Panel 167P Run Winters Branch Panels 168P-170P

Published Separately

Flood Insurance Rate Map (FIRM)

v FLOOD INSURANCE STUDY REPORT PRINCE WILLIAM COUNTY, VIRGINIA

SECTION 1.0 – INTRODUCTION

1.1 The National Flood Insurance Program

The National Flood Insurance Program (NFIP) is a voluntary Federal program that enables property owners in participating communities to purchase insurance protection against losses from flooding. This insurance is designed to provide an alternative to disaster assistance to meet the escalating costs of repairing damage to buildings and their contents caused by floods.

For decades, the national response to flood disasters was generally limited to constructing flood-control works such as dams, levees, sea-walls, and the like, and providing disaster relief to flood victims. This approach did not reduce losses nor did it discourage unwise development. In some instances, it may have actually encouraged additional development. To compound the problem, the public generally could not buy flood coverage from insurance companies, and building techniques to reduce flood damage were often overlooked.

In the face of mounting flood losses and escalating costs of disaster relief to the general taxpayers, the U.S. Congress created the NFIP. The intent was to reduce future flood damage through community floodplain management ordinances, and provide protection for property owners against potential losses through an insurance mechanism that requires a premium to be paid for the protection.

The U.S. Congress established the NFIP on August 1, 1968, with the passage of the National Flood Insurance Act of 1968. The NFIP was broadened and modified with the passage of the Flood Disaster Protection Act of 1973 and other legislative measures. It was further modified by the National Flood Insurance Reform Act of 1994 and the Flood Insurance Reform Act of 2004. The NFIP is administered by the Federal Emergency Management Agency (FEMA), which is a component of the Department of Homeland Security (DHS).

Participation in the NFIP is based on an agreement between local communities and the Federal Government. If a community adopts and enforces floodplain management regulations to reduce future flood risks to new construction and substantially improved structures in Special Flood Hazard Areas (SFHAs), the Federal Government will make flood insurance available within the community as a financial protection against flood losses. The community’s floodplain management regulations must meet or exceed criteria established in accordance with Title 44 Code of Federal Regulations (CFR) Part 60, Criteria for Land Management and Use.

SFHAs are delineated on the community’s Flood Insurance Rate Maps (FIRMs). Under the NFIP, buildings that were built before the flood hazard was identified on the community’s FIRMs are generally referred to as “Pre-FIRM” buildings. When the NFIP was created, the U.S. Congress recognized that insurance for Pre-FIRM buildings would be prohibitively expensive if the premiums were not subsidized by the Federal Government. Congress also recognized that most of these floodprone buildings were built

1 by individuals who did not have sufficient knowledge of the flood hazard to make informed decisions. The NFIP requires that full actuarial rates reflecting the complete flood risk be charged on all buildings constructed or substantially improved on or after the effective date of the initial FIRM for the community or after December 31, 1974, whichever is later. These buildings are generally referred to as “Post-FIRM” buildings.

1.2 Purpose of this Flood Insurance Study Report

This Flood Insurance Study (FIS) Report revises and updates information on the existence and severity of flood hazards for the study area. The studies described in this report developed flood hazard data that will be used to establish actuarial flood insurance rates and to assist communities in efforts to implement sound floodplain management.

In some states or communities, floodplain management criteria or regulations may exist that are more restrictive than the minimum Federal requirements. Contact your State NFIP Coordinator to ensure that any higher State standards are included in the community’s regulations.

1.3 Jurisdictions Included in the Flood Insurance Study Project

This FIS Report covers the entire geographic area of Prince William County, Virginia.

The jurisdictions that are included in this project area, along with the Community Identification Number (CID) for each community and the United States Geological Survey (USGS) 8-digit Hydrologic Unit Code (HUC-8) sub-basins affecting each, are shown in Table 1. The FIRM panel numbers that affect each community are listed. If the flood hazard data for the community is not included in this FIS Report, the location of that data is identified.

Table 1: Listing of NFIP Jurisdictions If Not Included, HUC-8 Location Community CID Sub- Located on FIRM Panel(s) of Flood Basin(s) Hazard Data Dumfries, 51153C0304F, 51153C0308F, 51153C0312F, 510120 2070011 Town of 51153C0316F Haymarket, 510121 2070010 51153C0059E, 51153C0067E Town of 51153C0094E, 51153C0113E, 51153C0114E, Manassas, 510122 2070010 51153C0157E, 51153C0159E, 51153C0176E, City of 51153C0177E

Manassas 51153C0111E, 51153C0112E, 51153C0113E, 510123 2070010 Park, City of 51153C0114E, 51153C0118E

Occoquan, 510124 2070010 51153C0217E Town of

2 Table 1: Listing of NFIP Jurisdictions (continued)

If Not Included, HUC-8 Location Community CID Sub- Located on FIRM Panel(s) of Flood Basin(s) Hazard Data

51153C0009E, 51153C0017E, 51153C0025E, 51153C0036E, 51153C0038E, 51153C0039E, 51153C0043E, 51153C0055E, 51153C0058E, 51153C0059E, 51153C0060E, 51153C0065E, 51153C0066E, 51153C0067E, 51153C0068E, 51153C0069E, 51153C0078E, 51153C0079E, 51153C0080E, 51153C0081E, 51153C0083E, 51153C0084E, 51153C0086E, 51153C0087E, 51153C0088E, 51153C0089E, 51153C0091E, 51153C0092E, 51153C0093E, 51153C0094E, 51153C0103E, 51153C0111E, 51153C0112E, 51153C0113E, 51153C0114E, 51153C0116E, 51153C0118E, 51153C0119E, 51153C0150E, 51153C0152E, 51153C0154E, 51153C0155E, 51153C0156E, 51153C0157E, 51153C0158E, Prince William 02070008, 51153C0159E, 51153C0165E, 51153C0170E, County, 510119 02070010, 51153C0176E, 51153C0177E, 51153C0178E, Unincorporated 02070011 51153C0179E, 51153C0181E, 51153C0182E, Areas 51153C0183E, 51153C0184E, 51153C0186E, 51153C0187E, 51153C0190E, 51153C0191E, 51153C0192E, 51153C0193E, 51153C0194E, 51153C0201E, 51153C0202E, 51153C0203E, 51153C0204E, 51153C0208E, 51153C0209E, 51153C0211E, 51153C0212E, 51153C0213E, 51153C0214E, 51153C0216E, 51153C0217E, 51153C0218E, 51153C0219F, 51153C0236F, 51153C0238F, 51153C0275E, 51153C0282E, 51153C0300E, 51153C0301E, 51153C0302E, 51153C0303E, 51153C0304F, 51153C0306F, 51153C0307F, 51153C0308F, 51153C0309F, 51153C0311E, 51153C0312F, 51153C0313F, 51153C0314F, 51153C0316F, 51153C0317F, 51153C0318F, 51153C0326F, 51153C0328F

Quantico, 510232 2070011 51153C0318F Town of

3 1.4 Considerations for using this Flood Insurance Study Report The NFIP encourages State and local governments to implement sound floodplain management programs. To assist in this endeavor, each FIS Report provides floodplain data, which may include a combination of the following: 10-, 4-, 2-, 1-, and 0.2-percent annual chance flood elevations (the 1-percent-annual-chance flood elevation is also referred to as the Base Flood Elevation (BFE)); delineations of the 1-percent-annual-chance and 0.2-percent-annual-chance floodplains; and 1-percent-annual-chance floodway. This information is presented on the FIRM and/or in many components of the FIS Report, including Flood Profiles, Floodway Data tables, Summary of Non-Coastal Stillwater Elevations tables, and Coastal Transect Parameters tables (not all components may be provided for a specific FIS). This section presents important considerations for using the information contained in this FIS Report and the FIRM, including changes in format and content. Figures 1, 2, and 3 present information that applies to using the FIRM with the FIS Report.  Part or all of this FIS Report may be revised and republished at any time. In addition, part of this FIS Report may be revised by a Letter of Map Revision (LOMR), which does not involve republication or redistribution of the FIS Report. Refer to Section 6.5 of this FIS Report for information about the process to revise the FIS Report and/or FIRM. It is, therefore, the responsibility of the user to consult with community officials by contacting the community repository to obtain the most current FIS Report components. Communities participating in the NFIP have established repositories of flood hazard data for floodplain management and flood insurance purposes. Community map repository addresses are provided in Table 30, “Map Repositories,” within this FIS Report.  New FIS Reports are frequently developed for multiple communities, such as entire counties. A countywide FIS Report incorporates previous FIS Reports for individual communities and the unincorporated area of the county (if not jurisdictional) into a single document and supersedes those documents for the purposes of the NFIP. The initial Countywide FIS Report for Prince William County became effective on January 5, 1995. Refer to Table 27 for information about subsequent revisions to the FIRMs.

 FEMA does not impose floodplain management requirements or special insurance ratings based on Limit of Moderate Wave Action (LiMWA) delineations at this time. The LiMWA represents the approximate landward limit of the 1.5-foot breaking wave. If the LiMWA is shown on the FIRM, it is being provided by FEMA as information only. For communities that do adopt Zone VE building standards in the area defined by the LiMWA, additional Community Rating System (CRS) credits are available. Refer to Section 2.5.4 for additional information about the LiMWA.

 FEMA has developed a Guide to Flood Maps (FEMA 258) and online tutorials to assist users in accessing the information contained on the FIRM. These include how to read panels and step-by-step instructions to obtain specific information. To obtain this guide and other assistance in using the FIRM, visit the FEMA Web site

4 at www.fema.gov/online-tutorials.

The FIRM Index in Figure 1 shows the overall FIRM panel layout within Prince William County, and also displays the panel number for each FIRM panel in the county. Other information shown on the FIRM Index includes community boundaries, flooding sources, watershed boundaries, and USGS HUC-8 codes.

5 Figure 1: FIRM Index

0025E

LOUDOUN COUNTY

0009E

0017E 0036E

«¬615

Black Branch

0038E 0039E 0043E «¬600 15 ¤£ FAIRFAX COUNTY

Chestnut Lick

0055E 0060E 705 0081E 0080E «¬ Bull Run HUC8 02070010 «¬601 Middle Potomac- ¤£15 Anacostia-Occoquan Watershed

0058E 0059E Silver Lake 0078E 0079E Catharpin 0083E 0084E North Fork Little Bull Run Broad Run Creek «¬234

TOWN OF PRINCE WILLIAM 55 «¬ COUNTY 66 HAYMARKET ¥ 510119 29 510121 ¤£ Holkums Branch 0065E 0066E 0067E 0086E 0087E Tributary To 0091E 0092E North Fork Broad Run

FAUQUIER COUNTY 66 RAILROAD ¥ ¤£29 ¤£15 Bull Run Tributary D 0068E 0069E 0088E Rocky Branch 0089E 0093E 0094E

Lake Manassas Flat Branch Buckland Broad Run «¬619 Lake Dawkins Branch

Broad Run

CITY OF MANASSAS 510122

1 inch = 7,500 feet 1:90,000 feet COUNTY LOCATOR INDEX LOCATOR NATIONAL FLOOD INSURANCE PROGRAM 0 3,750 7,500 15,000 FLOOD INSURANCE RATE MAP INDEX (Sheet 1 of 3) PRINCE WILLIAM COUNTY, VIRGINIA and Incorporated Areas Map Projection: NAD 1983 UTM Zone 18N Sheet PANELS PRINTED: North American Datum of 1983 1 OF 3 North American Vertical Datum 1988 0009, 0017, 0025, 0036, 0038, 0039, 0043, 0055, 0058, 0059, 0060, 0065, 0066, 0067, 0068, 0069, 0078, 0079, 0080, 0081, 0083, 0084, 0086, 0087, 0088, 0089, THE INFORMATION DEPICTED ON THIS MAP AND SUPPORTING 0091, 0092, 0093, 0094 This Area Shown DOCUMENTATION ARE ALSO AVAILABLE IN DIGITAL FORMAT AT on Index Sheet MAP NUMBER 3 OF 3 This Area Shown on 51153CIND1B HTTPS://MSC.FEMA.GOV Index Sheet SEE FLOOD INSURANCE STUDY FOR ADDITIONAL INFORMATION 2 OF 3 MAP REVISED PRELIMINARY 9/30/2020 6 Figure 1: FIRM Index (continued)

0103E

0111E 0112E 0116E

Bull Run CITY OF MANASSAS PARK Flat Branch 510123

0113E 0114E 0118E 0119E

CITY OF 234 HUC8 02070010 MANASSAS «¬ Middle 510122 Potomac-Anacostia-Occoquan RAILROAD Watershed 0176E 0177E 0181E 0182E 0201E 0202E

«¬663 FAIRFAX COUNTY

0178E 0179E 0183E 0184E 0203E 0204E 0208E 0209E

Broad Run Occoquan River TOWN OF 294 Occoquan 663 Lake Jackson «¬ «¬ Reservoir OCCOQUAN 0186E 0187E 0191E 0192E 510124 0211E 0212E 0216E «¬234 0217E 0236F

PRINCE WILLIAM Hooes Run COUNTY «¬619 510119 Lake Omiscal

0190E 0193E 0194E 0213E 0214E

0218E 0219F 0238F RAILROAD

¤£1 Neabsco Cow Branch Creek Occoquan River Farm Creek 0282E 0301E 0302E 0300E 0306F 0307F 0326F

Powells Creek

Lake Montclair South Fork Quantico Creek 0303E 0304F 0308F 0309F 0328F Happyland Camp TOWN OF Five Lake DUMFRIES «¬234 510120 HUC8 02070011 619 Lower Potomac «¬ Watershed 0311E 0312F 0316F 0317F

Quantico Creek

Breckenridge Reservoir Little Creek

CHARLES COUNTY STAFFORD 0313F 0314F COUNTY 1 0318F ¤£ TOWN OF QUANTICO 510232

RAILROAD

1 inch = 10,000 feet 1:120,000 feet COUNTY LOCATOR INDEX LOCATOR NATIONAL FLOOD INSURANCE PROGRAM 0 5,000 10,000 20,000 FLOOD INSURANCE RATE MAP INDEX (Sheet 2 of 3) PRINCE WILLIAM COUNTY, VIRGINIA and Incorporated Areas Map Projection: NAD 1983 UTM Zone 18N This Area PANELS PRINTED: North American Datum of 1983 Shown on 0103, 0111, 0112, 0113, 0114, 0116, 0118, 0119, 0176, 0177, 0178, 0179, 0181, 0182, Index Sheet North American Vertical Datum 1988 1 OF 3 0183, 0184, 0186, 0187, 0190, 0191, 0192, 0193, 0194, 0201, 0202, 0203, 0204, 0208, 0209, 0211, 0212, 0213, 0214, 0216, 0217, 0218, 0219, 0236, 0238, 0282, 0300, 0301, THE INFORMATION DEPICTED ON THIS MAP AND SUPPORTING 0302, 0303, 0304, 0306, 0307, 0308, 0309, 0311, 0312, 0313, 0314, 0316, 0317, 0318, DOCUMENTATION ARE ALSO AVAILABLE IN DIGITAL FORMAT AT This Area Shown on Index Sheet 0326, 0328 MAP NUMBER 3 OF 3 51153CIND2B HTTPS://MSC.FEMA.GOV Sheet 2 OF 3 SEE FLOOD INSURANCE STUDY FOR ADDITIONAL INFORMATION MAP REVISED PRELIMINARY 9/30/2020 7 Figure 1: FIRM Index (continued)

CITY OF MANASSAS 510122 0150E 0155E 0152E Broad Run 0156E 0157E

«¬619 Broad Run «¬28

«¬652 0154E 0158E 0159E

Kettle Run

RAILROAD

0165E 0170E PRINCE WILLIAM COUNTY 510119

Slate Run «¬646

Bagley Pond

Cedar Run FAUQUIER COUNTY

0275E Cedar Run

HUC 02070010 Middle Potomac- Anacostia-Occoquan Watershed

HUC 02070011 Lower Potomac Watershed

STAFFORD COUNTY

1 inch = 7,500 feet 1:90,000 feet COUNTY LOCATOR INDEX LOCATOR NATIONAL FLOOD INSURANCE PROGRAM 0 3,750 7,500 15,000 FLOOD INSURANCE RATE MAP INDEX (Sheet 3 of 3) PRINCE WILLIAM COUNTY, VIRGINIA and Incorporated Areas Map Projection: NAD 1983 UTM Zone 18N This Area PANELS PRINTED: North American Datum of 1983 Shown on Index Sheet 0150, 152, 0154, 0155, 0156, 0157, 0158, 0159, 0165, 0170, 0275 North American Vertical Datum 1988 1 OF 3 THE INFORMATION DEPICTED ON THIS MAP AND SUPPORTING DOCUMENTATION ARE ALSO AVAILABLE IN DIGITAL FORMAT AT Sheet MAP NUMBER 3 OF 3 This Area Shown 51153CIND3B HTTPS://MSC.FEMA.GOV on Index Sheet SEE FLOOD INSURANCE STUDY FOR ADDITIONAL INFORMATION 2 OF 3 MAP REVISED PRELIMINARY 9/30/2020 8 Each FIRM panel may contain specific notes to the user that provide additional information regarding the flood hazard data shown on that map. However, the FIRM panel does not contain enough space to show all the notes that may be relevant in helping to better understand the information on the panel. Figure 2 contains the full list of these notes.

Figure 2: FIRM Notes to Users NOTES TO USERS For information and questions about this map, available products associated with this FIRM including historic versions of this FIRM, how to order products, or the National Flood Insurance Program in general, please call the FEMA Map Information eXchange at 1-877-FEMA-MAP (1- 877-336-2627) or visit the FEMA Flood Map Service Center website at https://msc.fema.gov. Available products may include previously issued Letters of Map Change, a Flood Insurance Study Report, and/or digital versions of this map. Many of these products can be ordered or obtained directly from the website. Users may determine the current map date for each FIRM panel by visiting the FEMA Flood Map Service Center website or by calling the FEMA Map Information eXchange.

Communities annexing land on adjacent FIRM panels must obtain a current copy of the adjacent panel as well as the current FIRM Index. These may be ordered directly from the Flood Map Service Center at the number listed above.

For community and countywide map dates, refer to Table 27 in this FIS Report.

To determine if flood insurance is available in the community, contact your insurance agent or call the National Flood Insurance Program at 1-800-638-6620.

PRELIMINARY FIS REPORT: FEMA maintains information about map features, such as street locations and names, in or near designated flood hazard areas. Requests to revise information in or near designated flood hazard areas may be provided to FEMA during the community review period, at the final Consultation Coordination Officer's meeting, or during the statutory 90-day appeal period. Approved requests for changes will be shown on the final printed FIRM.

The map is for use in administering the NFIP. It may not identify all areas subject to flooding, particularly from local drainage sources of small size. Consult the community map repository to find updated or additional flood hazard information.

BASE FLOOD ELEVATIONS: For more detailed information in areas where Base Flood Elevations (BFEs) and/or floodways have been determined, consult the Flood Profiles and Floodway Data and/or Summary of Non-Coastal Stillwater Elevations tables within this FIS Report. Use the flood elevation data within the FIS Report in conjunction with the FIRM for construction and/or floodplain management.

Coastal Base Flood Elevations shown on the map apply only landward of 0.0' North American Vertical Datum of 1988 (NAVD88). Coastal flood elevations are also provided in the Coastal Transect Parameters table in the FIS Report for this jurisdiction. Elevations shown in the Coastal Transect Parameters table should be used for construction and/or floodplain management purposes when they are higher than the elevations shown on the FIRM.

9 Figure 2. FIRM Notes to Users (continued)

FLOODWAY INFORMATION: Boundaries of the floodways were computed at cross sections and interpolated between cross sections. The floodways were based on hydraulic considerations with regard to requirements of the National Flood Insurance Program. Floodway widths and other pertinent floodway data are provided in the FIS Report for this jurisdiction.

FLOOD CONTROL STRUCTURE INFORMATION: Certain areas not in Special Flood Hazard Areas may be protected by flood control structures. Refer to Section 4.3 "Non-Levee Flood Protection Measures" of this FIS Report for information on flood control structures for this jurisdiction.

PROJECTION INFORMATION: The projection used in the preparation of the map was UNIVERSAL TRANSVERSE MERCATOR Zone 18. The horizontal datum was the North American Datum 1983, GRS 1980 spheroid. Differences in datum, spheroid, projection or State Plane zones used in the production of FIRMs for adjacent jurisdictions may result in slight positional differences in map features across jurisdiction boundaries. These differences do not affect the accuracy of the FIRM.

ELEVATION DATUM: Flood elevations on the FIRM are referenced to the North American Vertical Datum of 1988. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. For information regarding conversion between the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988, visit the National Geodetic Survey website at www.ngs.noaa.gov.

Local vertical monuments may have been used to create the map. To obtain current monument information, please contact the appropriate local community listed in Table 30 of this FIS Report.

BASE MAP INFORMATION: Base map information shown on this FIRM was provided in digital format by United States Department of Agriculture (USDA) Farm Service Agency (FSA) Aerial Photography Field Office, National Agriculture Imagery Program (NAIP). This information was derived from digital orthophotography at a 2-foot resolution from photography dated 2019. Vector information shown on this FIRM was provided in digital format by the Federal Emergency Management Agency, dated 2016, United States Geological Survey, dated 2014, United States Census Bureau, dated 2018, and Virginia GIS Clearinghouse, dated 2016. For information about base maps, refer to Section 6.2 “Base Map” in this FIS Report.

Corporate limits shown on the map are based on the best data available at the time of publication. Because changes due to annexations or de-annexations may have occurred after the map was published, map users should contact appropriate community officials to verify current corporate limit locations.

NOTES FOR FIRM INDEX

REVISIONS TO INDEX: As new studies are performed and FIRM panels are updated within Prince William County, Virginia, corresponding revisions to the FIRM Index will be incorporated within the FIS Report to reflect the effective dates of those panels. Please refer to Table 27 of this FIS Report to determine the most recent FIRM revision date for each community. The most recent FIRM panel effective date will correspond to the most recent index date.

10 Figure 2. FIRM Notes to Users (continued)

SPECIAL NOTES FOR SPECIFIC FIRM PANELS

This Notes to Users section was created specifically for Prince William County, Virginia, effective TBD.

LIMIT OF MODERATE WAVE ACTION: Zone AE has been divided by a Limit of Moderate Wave Action (LiMWA). The LiMWA represents the approximate landward limit of the 1.5-foot breaking wave. The effects of wave hazards between Zone VE and the LiMWA (or between the shoreline and the LiMWA for areas where Zone VE is not identified) will be similar to, but less severe than, those in Zone VE.

FLOOD RISK REPORT: A Flood Risk Report (FRR) may be available for many of the flooding sources and communities referenced in this FIS Report. The FRR is provided to increase public awareness of flood risk by helping communities identify the areas within their jurisdictions that have the greatest risks. Although non-regulatory, the information provided within the FRR can assist communities in assessing and evaluating mitigation opportunities to reduce these risks. It can also be used by communities developing or updating flood risk mitigation plans. These plans allow communities to identify and evaluate opportunities to reduce potential loss of life and property. However, the FRR is not intended to be the final authoritative source of all flood risk data for a project area; rather, it should be used with other data sources to paint a comprehensive picture of flood risk.

11 Each FIRM panel contains an abbreviated legend for the features shown on the maps. However, the FIRM panel does not contain enough space to show the legend for all map features. Figure 3 shows the full legend of all map features. Note that not all of these features may appear on the FIRM panels in Prince William County.

Figure 3: Map Legend for FIRM SPECIAL FLOOD HAZARD AREAS: The 1% annual chance flood, also known as the base flood or 100-year flood, has a 1% chance of happening or being exceeded each year. Special Flood Hazard Areas are subject to flooding by the 1% annual chance flood. The Base Flood Elevation is the water surface elevation of the 1% annual chance flood. The floodway is the channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that the 1% annual chance flood can be carried without substantial increases in flood heights. See note for specific types. If the floodway is too narrow to be shown, a note is shown. Special Flood Hazard Areas subject to inundation by the 1% annual chance flood (Zones A, AE, AH, AO, AR, A99, V and VE) Zone A The flood insurance rate zone that corresponds to the 1% annual chance floodplains. No base (1% annual chance) flood elevations (BFEs) or depths are shown within this zone. Zone AE The flood insurance rate zone that corresponds to the 1% annual chance floodplains. Base flood elevations derived from the hydraulic analyses are shown within this zone. Zone AH The flood insurance rate zone that corresponds to the areas of 1% annual chance shallow flooding (usually areas of ponding) where average depths are between 1 and 3 feet. Whole-foot BFEs derived from the hydraulic analyses are shown at selected intervals within this zone. Zone AO The flood insurance rate zone that corresponds to the areas of 1% annual chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between 1 and 3 feet. Average whole-foot depths derived from the hydraulic analyses are shown within this zone. Zone AR The flood insurance rate zone that corresponds to areas that were formerly protected from the 1% annual chance flood by a flood control system that was subsequently decertified. Zone AR indicates that the former flood control system is being restored to provide protection from the 1% annual chance or greater flood. Zone A99 The flood insurance rate zone that corresponds to areas of the 1% annual chance floodplain that will be protected by a Federal flood protection system where construction has reached specified statutory milestones. No base flood elevations or flood depths are shown within this zone. Zone V The flood insurance rate zone that corresponds to the 1% annual chance coastal floodplains that have additional hazards associated with storm waves. Base flood elevations are not shown within this zone. Zone VE Zone VE is the flood insurance rate zone that corresponds to the 1% annual chance coastal floodplains that have additional hazards associated with storm waves. Base flood elevations derived from the coastal analyses are shown within this zone as static whole-foot elevations that apply throughout the zone.

Regulatory Floodway determined in Zone AE.

12 Figure 3: Map Legend for FIRM (continued)

OTHER AREAS OF FLOOD HAZARD Shaded Zone X: Areas of 0.2% annual chance flood hazards and areas of 1% annual chance flood hazards with average depths of less than 1 foot or with drainage areas less than 1 square mile. Future Conditions 1% Annual Chance Flood Hazard – Zone X: The flood insurance rate zone that corresponds to the 1% annual chance floodplains that are determined based on future-conditions hydrology. No base flood elevations or flood depths are shown within this zone. Area with Reduced Flood Risk due to Levee: Areas where an accredited levee, dike, or other flood control structure has reduced the flood risk from the 1% annual chance flood. Area with Flood Risk due to Levee: Areas where a non-accredited levee, dike, or other flood control structure is shown as providing protection to less than the 1% annual chance flood. OTHER AREAS Zone D (Areas of Undetermined Flood Hazard): The flood insurance rate zone that corresponds to unstudied areas where flood hazards are undetermined, but possible.

NO SCREEN Unshaded Zone X: Areas of minimal flood hazard.

FLOOD HAZARD AND OTHER BOUNDARY LINES

Flood Zone Boundary (white line on ortho-photography-based mapping; gray line on vector-based mapping) (ortho) (vector)

Limit of Study

Jurisdiction Boundary

Limit of Moderate Wave Action (LiMWA): Indicates the inland limit of the area affected by waves greater than 1.5 feet GENERAL STRUCTURES

Aqueduct Channel Channel, Culvert, Aqueduct, or Storm Sewer Culvert Storm Sewer

______Dam Jetty Dam, Jetty, Weir Weir

Levee, Dike, or Floodwall

Bridge Bridge

13 Figure 3: Map Legend for FIRM (continued)

REFERENCE MARKERS

River mile Markers

CROSS SECTION & TRANSECT INFORMATION

Lettered Cross Section with Regulatory Water Surface Elevation (BFE)

Numbered Cross Section with Regulatory Water Surface Elevation (BFE)

Unlettered Cross Section with Regulatory Water Surface Elevation (BFE)

Coastal Transect

Profile Baseline: Indicates the modeled flow path of a stream and is shown on FIRM panels for all valid studies with profiles or otherwise established base flood elevation. Coastal Transect Baseline: Used in the coastal flood hazard model to represent the 0.0-foot elevation contour and the starting point for the transect and the measuring point for the coastal mapping.

Base Flood Elevation Line

ZONE AE Static Base Flood Elevation value (shown under zone label) (EL 16) ZONE AO Zone designation with Depth (DEPTH 2) ZONE AO (DEPTH 2) Zone designation with Depth and Velocity (VEL 15 FPS) BASE MAP FEATURES River, Stream or Other Hydrographic Feature Missouri Creek

Interstate Highway

U.S. Highway

State Highway

County Highway

MAPLE LANE Street, Road, Avenue Name, or Private Drive if shown on Flood Profile

Railroad RAILROAD

14 Figure 3: Map Legend for FIRM (continued)

Horizontal Reference Grid Line

Horizontal Reference Grid Ticks

Secondary Grid Crosshairs

Land Grant Name of Land Grant

7 Section Number

R. 43 W. T. 22 N. Range, Township Number

4276000mE Horizontal Reference Grid Coordinates (UTM)

365000 FT Horizontal Reference Grid Coordinates (State Plane)

80 16’ 52.5” Corner Coordinates (Latitude, Longitude)

15 SECTION 2.0 – FLOODPLAIN MANAGEMENT APPLICATIONS

2.1 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 hazard in the community.

Each flooding source included in the project scope has been studied and mapped using professional engineering and mapping methodologies that were agreed upon by FEMA and Prince William County as appropriate to the risk level. Flood risk is evaluated based on factors such as known flood hazards and projected impact on the built environment. Engineering analyses were performed for each studied flooding source to calculate its 1- percent-annual-chance flood elevations; elevations corresponding to other floods (e.g. 10-, 4-, 2-, 0.2-percent annual chance, etc.) may have also been computed for certain flooding sources. Engineering models and methods are described in detail in Section 5.0 of this FIS Report. The modeled elevations at cross sections were used to delineate the floodplain boundaries on the FIRM; between cross sections, the boundaries were interpolated using elevation data from various sources. More information on specific mapping methods is provided in Section 6.0 of this FIS Report.

Depending on the accuracy of available topographic data (Table 22), study methodologies employed (Section 5.0), and flood risk, certain flooding sources may be mapped to show both the 1-percent and 0.2-percent-annual-chance floodplain boundaries, regulatory water surface elevations (BFEs), and/or a regulatory floodway. Similarly, other flooding sources may be mapped to show only the 1-percent-annual-chance floodplain boundary on the FIRM, without published water surface elevations. 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 is shown on the FIRM. Figure 3, “Map Legend for FIRM”, describes the flood zones that are used on the FIRMs to account for the varying levels of flood risk that exist along flooding sources within the project area. Table 2 and Table 3 indicate the flood zone designations for each flooding source and each community within Prince William County, respectively.

Table 2, “Flooding Sources Included in this FIS Report,” lists each flooding source, including its study limits, affected communities, mapped zone on the FIRM, and the completion date of its engineering analysis from which the flood elevations on the FIRM and in the FIS Report were derived. Descriptions and dates for the latest hydrologic and hydraulic analyses of the flooding sources are shown in Table 12. Floodplain boundaries for these flooding sources are shown on the FIRM (published separately) using the symbology described in Figure 3. On the map, the 1-percent-annual-chance floodplain corresponds to the SFHAs. The 0.2-percent-annual-chance floodplain shows areas that, although out of the regulatory floodplain, are still subject to flood hazards.

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. The procedures to remove these areas from the SFHA are described in Section 6.5 of this FIS Report.

16

Table 2: Flooding Sources Included in this FIS Report Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Dumfries, Town of; Manassas, City of; Manassas Park, City All flooding sources of; Occoquan, Town mapped as Zone A on Various Various 02070010 796.6 N A 2017 of; Prince William the FIRM County, Unincorporated Areas Prince William Approximately 4,610 County, Confluence with feet upstream of the 02070010 3.1 Y AE 1992 Broad Run Unincorporated Occoquan River confluence with Kettle Areas Run Manassas, City of; Approximately 4,610 Approximately 660 feet Prince William feet upstream of the downstream of the County, 02070010 3.4 Y AE 2018 Broad Run confluence with Kettle southern railway bridge Unincorporated Run on Milford Road Areas Approximately 3,875 Prince William Approximately 660 feet feet upstream of the County, downstream of the Broad Run Nokesville Road bridge 02070010 1.7 Y AE 2018 Unincorporated southern railway bridge located 500 feet west Areas on Milford Road of Piper Lane Approximately 3,875 Prince William Approximately 4,300 feet upstream of the County, feet downstream of the Broad Run Nokesville Road bridge 02070010 0.5 Y AE 1992 Unincorporated confluence with located 500 feet west of Areas Dawkins Branch Piper Lane

17 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Prince William Approximately 4,300 Approximately 480 feet County, feet downstream of the upstream of the Broad Run 02070010 0.9 Y AE 2010 Unincorporated confluence with confluence with Areas Dawkins Branch Dawkins Branch Prince William Approximately 480 feet Approximately 2,350 County, upstream of the feet upstream from the Broad Run 02070010 3.3 Y AE 2020 Unincorporated confluence with Linton Hall Road Areas Dawkins Branch Bridge Prince William Approximately 2,350 County, T. Nelson Elliott Dam at Broad Run feet upstream from the 02070010 2.8 Y AE 1992 Unincorporated Lake Manassas Linton Hall Road Bridge Areas Broad Run Side Convergence with Divergence with Broad Manassas, City of 02070010 0.6 Y AE 2018 Channel Broad Run Run Prince William Approximately 905 feet County, Confluence with northeast of where Bull Run 02070010 9.4 Y AE 1992 Unincorporated Occoquan River Lake Drive meets Pine Areas Street Approximately 2,115 Prince William Approximately 905 feet feet upstream of the County, northeast of where Bull Run State Route 28 02070010 1.3 Y AE 2017 Unincorporated Lake Drive meets Pine (Centreville Road) Areas Street Bridge Approximately 2,115 Prince William feet upstream of the Approximately 100 feet County, 1992 Bull Run State Route 28 upstream of Loudoun 02070010 21 Y AE Unincorporated (Centreville Road) Road Areas Bridge

18 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Approximately 850 Prince William southeast of the County, Confluence with Bull Bull Run Tributary A intersection of Leland 02070010 0.3 Y AE Unincorporated Run 1977 Road and Maplewood Areas Drive Manassas Park, City Approximately 850 Appoximately 395 feet of; Prince William southeast of the southeast of the Bull Run Tributary A County, intersection of Leland intersection of 02070010 0.4 Y AE 2018 Unincorporated Road and Maplewood Maplewood Drive and Areas Drive Spruce Street Manassas Park, City Appoximately 395 feet of; Prince William southeast of the Approximately 605 feet Bull Run Tributary A County, intersection of upstream of the 02070010 1.0 Y AE 1977 Unincorporated Maplewood Drive and Manassas Drive Bridge Areas Spruce Street Prince William Approximately 215 feet County, Confluence with Bull north of the Leland Bull Run Tributary B 02070010 0.9 Y AE 1977 Unincorporated Run Road and Centreville Areas Road intersection Prince William Approximately 215 feet Approximately 437 feet County, north of the Leland east of the Rugby Road Bull Run Tributary B 02070010 0.3 Y AE 1998 Unincorporated Road and Centreville and Leland Road Areas Road intersection intersection Prince William Approximately 437 feet County, east of the Rugby Road Peakwood Court Bull Run Tributary B 02070010 0.7 Y AE 1977 Unincorporated and Leland Road Bridge Areas intersection

19 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis

Approximately 515 feet Prince William Confluence with Bull northeast of the Bull Run Tributary C 02070010 0.6 Y AE 1977 County, Run intersection of Bull Run Unincorporated Road and Rugby Road Areas Prince William Approximately 1,343 County, Confluence with Bull Bull Run Tributary D feet upstream of 02070010 1.4 Y AE Unincorporated Run 1992 Williamson Boulevard Areas Prince William Approximately 680 feet Approximately 1,343 County, east of the intersection Bull Run Tributary D feet upstream of 02070010 0.01 Y AE Unincorporated of State Route 234 and Williamson Boulevard 2008 Areas Davidson Place Manassas, City of; Prince William Confluence with Broad Cannon Branch County, Railroad 02070010 1.3 Y AE 1977 Run Unincorporated Areas Manassas, City of; Prince William Cannon Branch County, Railroad Godwin Drive 02070010 0.7 Y AE 2016 Unincorporated Areas Manassas, City of; Approximately 1,910 Prince William feet east of the Cannon Branch County, Godwin Drive University Boulevard 02070010 0.7 Y AE 1977 Unincorporated and Freedom Center Areas Boulevard intersection

20 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Approximately 1,560 Prince William feet east of the County, Confluence with Little Catharpin Creek intersection of Fishers 02070010 3 Y AE 1992 Unincorporated Bull Run Hill Way and Yellow Areas Tavern Court Approximately 1,670 Prince William feet northeast of the County, Confluence with Cedar Run intersection of Old 02070010 2.4 Y AE 1977 Unincorporated Occoquan River Church Road and Slate Areas Run Lane Prince William Approximately 215 feet Approximately 345 feet Chestnut Lick County, downstream of Logmill upstream of Logmill 02070010 0.1 N AE 2017 Tributary No. 1 Unincorporated Road Road Areas

Prince William Prince William and Approximately 700 feet Chopawamsic Creek County, Stafford County upstream of U.S. Route 02070011 2.1 Y AE 2018 Unincorporated boundary 95 South Areas

Manassas, City of; Prince William Approximately 540 feet Cockrell Branch County, upstream of Hidden State Route 234 02070010 0.8 Y AE 2015 Unincorporated Spring Drive Areas

Prince William Confluence with South side of Rippon Cow Branch County, 02070010 3.5 Y AE 2018 Neabsco Creek Boulevard culvert Unincorporated Areas

21 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Prince William Approximately 22 feet County, Confluence with Broad Dawkins Branch upstream of University 02070010 1.1 Y AE 2010 Unincorporated Run Boulevard Areas

Prince William Approximately 22 feet Approximately 1,180 Dawkins Branch County, upstream of University feet upstream of Prince 02070010 1.6 Y AE 1992 Unincorporated Boulevard William Parkway Areas

Prince William Approximately 590 feet Farm Creek County, Railroad upstream of Farm 02070010 0.4 Y AE 1992 Unincorporated Creek Drive Areas

Prince William Approximately 420 feet Confluence with Bull Flat Branch County, downstream of 02070010 1.9 Y AE 1992 Run Unincorporated Fairmont Avenue Areas

Prince William Approximately 420 feet Approximately 215 feet Flat Branch County, downstream of upstream of State 02070010 0.5 Y AE 2016 Unincorporated Fairmont Avenue Route 234 Areas

Prince William Approximately 215 feet Approximately 65 feet Flat Branch County, upstream of State upstream of Ashton 02070010 0.6 Y AE 1992 Unincorporated Route 234 Avenue Areas

Manassas, City of; Approximately 1,065 Flat Branch Tributary Prince William Confluence with Flat feet upstream of the 02070010 1.4 Y AE 1977 A County, Branch Unincorporated Stonewall Road Bridge Areas

22 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis

Manassas, City of; Manassas Park, City Approximately 565 feet Flat Branch Tributary Confluence with Flat of; Prince William upstream of Portner 02070010 2.4 Y AE 1977 B Branch County, Avenue Unincorporated Areas Manassas, City of; Prince William Flat Branch Tributary Confluence with Flat Approximately 500 feet County, 02070010 1.8 Y AE 1977 C Branch west of Stonewall Road Unincorporated Areas Prince William Approximately 295 feet County, Confluence with Bull Holkums Branch upstream of 02070010 1.6 Y AE 1992 Unincorporated Run Automotive Drive Areas Prince William Approximately 435 feet County, Approximately 295 feet southeast of the Holkums Branch Unincorporated upstream of Automotive intersection of Sudley 02070010 0.01 Y AE 2012 Areas Drive Road and Automotive Drive

Prince William Approximately 1,621 Confluence with Hooes Run County, feet upstream of Lake 02070010 2.9 Y AE 1977 Occoquan River Unincorporated Omisol Dam Areas

Approximately 0.6 mile Prince William Confluence with Broad Kettle Run upstream of Kettle Run 02070010 6.1 Y AE 1992 County, Run Unincorporated Road Areas

23 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Approximately 420 feet Prince William southwest of the Confluence with Bull Little Bull Run County, intersection between 02070010 9.4 Y AE 1992 Run Unincorporated Twin Branch Court and Areas Stepping Stone Drive Approximately 420 feet Prince William southwest of the Approximately 515 feet County, Little Bull Run intersection between upstream of Dominion 02070010 1.8 Y AE 2017 Unincorporated Twin Branch Court and Valley Drive Areas Stepping Stone Drive Prince William Approximately 410 feet County, Confluence with downstream of Little Creek 02070011 3.4 Y AE 1992 Unincorporated Quantico Creek Jefferson Davis Areas Highway Prince William Approximately 410 feet Approximately 540 feet County, downstream of Little Creek upstream of Jefferson 02070011 0.2 Y AE 2017 Unincorporated Jefferson Davis Davis Highway Areas Highway Prince William Approximately 540 feet County, Approximately 40 feet Little Creek upstream of Jefferson 02070011 0.4 Y AE 1992 Unincorporated upstream of Inn Road Davis Highway Areas

Prince William Approximately 330 feet Confluence with Marumsco Creek County, downstream of 02070010 2.9 Y AE 1992 Potomac River Unincorporated Interstate Route 95 Areas Prince William Approximately 150 feet Marumsco Creek County, Confluence with upstream of Prince 02070010 0.7 Y AE 1977 Tributary A Unincorporated Marumsco Creek William Parkway Areas

24 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Prince William Approximately 150 feet Approximately 1,205 Marumsco Creek County, upstream of Prince feet upstream of 02070010 0.3 Y AE 2018 Tributary A Unincorporated William Parkway Church Hill Drive Areas Prince William Approximately 500 feet Marumsco Creek County, Confluence with upstream of Colchester 02070010 1.0 Y AE 1992 Tributary B Unincorporated Marumsco Creek Road Areas Prince William Approximately 1,800 Approximately 1,290 County, feet upstream of the feet downstream of Neabsco Creek 02070010 0.8 Y AE 2018 Unincorporated confluence with Cow Jefferson Davis Areas Branch Highway Prince William Approximately 1,290 Approximately 25 feet County, feet downstream of Neabsco Creek upstream of Jefferson 02070010 0.2 Y AE 1992 Unincorporated Jefferson Davis Davis Highway Areas Highway Prince William Approximately 25 feet Approximately 2,275 County, Neabsco Creek upstream of Jefferson feet downstream of 02070010 0.1 Y AE 2015 Unincorporated Davis Highway Interstate Route 95 Areas

Prince William Approximately 2,275 Approximately 970 feet Neabsco Creek County, feet downstream of upstream of Princedale 02070010 10.2 Y AE 1992 Unincorporated Interstate Route 95 Drive Areas

Prince William Approximately 275 feet Neabsco Creek Confluence with north of the intersection County, 02070010 1 Y AE 1992 Tributary A Unincorporated Neabsco Creek of Kerrydale Road and Areas Kerrydale Place

25 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Approximately 645 feet Approximately 275 feet south of the Neabsco Creek Prince William north of the intersection intersection of 02070010 0.8 Y AE 2002 Tributary A County, of Kerrydale Road and Kerrydale Road and Unincorporated Kerrydale Place Areas Kingswell Drive Prince William Approximately 645 feet Approximately 1,290 Neabsco Creek County, south of the intersection feet downstream of 02070010 1.2 Y AE 1992 Tributary A Unincorporated of Kerrydale Road and State Route 642 Areas Kingswell Drive Haymarket, Town of; Prince William Approximately 1,370 Confluence with Lake North Fork Broad Run County, feet upstream of 02070010 2.3 Y AE 1992 Manassas Unincorporated Jefferson Street Areas Haymarket, Town of; Prince William Approximately 1,370 Approximately 300 feet North Fork Broad Run County, feet upstream of downstream of James 02070010 0.3 Y AE 2007 Unincorporated Jefferson Street Madison Highway Areas Haymarket, Town of; Prince William Approximately 300 feet Approximately 155 feet North Fork Broad Run County, downstream of James downstream of State 02070010 1.5 Y AE 2013 Unincorporated Madison Highway Route 55 Areas

Prince William Approximately 155 feet Approximately 25 feet North Fork Broad Run County, downstream of State downstream of 02070010 0.1 Y AE 1992 Unincorporated Route 55 Interstate Route 66 Areas

26 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis

Prince William Approximately 25 feet Approximately 16 feet North Fork Broad Run County, downstream of downstream of 02070010 0.7 Y AE 2002 Unincorporated Interstate Route 66 Thoroughfare Road Areas

Prince William Approximately 16 feet Approximately 35 feet North Fork Broad Run County, downstream of upstream of Gaines 02070010 0.8 Y AE 1992 Unincorporated Thoroughfare Road Road Areas

Prince William North Fork Broad Run Convergence with Divergence from North County, 02070010 0.3 Y AE 2002 Bypass Spillway Unincorporated North Fork Broad Run Fork Broad River Areas Prince William Approximately 85 feet County, Confluence with Occoquan River upstream of Jefferson 02070010 5.0 Y AE/VE 1992 Unincorporated Potomac River Davis Highway Areas Prince William Approximately 85 feet County, Northernmost Lane of Occoquan River upstream of Jefferson 02070010 0.4 Y AE 2018 Unincorporated Interstate Route 95 Davis Highway Areas Occoquan, Town of; Prince William Northernmost Lane of Confluence with Broad Occoquan River County, 02070010 23.9 Y AE 1992 Interstate Route 95 Run and Cedar Run Unincorporated Areas Prince William County, Confluence with Powells Creek U.S. Highway 1 02070011 2.3 Y AE/VE 1992 Unincorporated Potomac River Areas

27 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Prince William Approximately 1,520 County, Powells Creek U.S. Highway 1 feet downstream of 02070011 0.9 Y AE 2020 Unincorporated Interstate 95 Areas Prince William Approximately 1,520 Approximately 880 feet County, Powells Creek feet downstream of downstream of Spriggs 02070011 6.3 Y AE 1992 Unincorporated Interstate 95 Road Areas Prince William Approximately 880 feet Approximately 5,128 County, Powells Creek downstream of Spriggs feet downstream of 02070011 1.6 Y AE 2018 Unincorporated Road Minnieville Road Areas Prince William Approximately 5,128 Approximately 1,130 County, Powells Creek feet downstream of feet upstream of 02070011 1.2 Y AE 1992 Unincorporated Minnieville Road Minnieville Road Areas Prince William Approximately 1,255 County, Confluence with Purcell Branch feet upstream of Sly 02070010 2.1 Y AE 1992 Unincorporated Occoquan River Fox Lane Areas Dumfries, Town of; Prince William Approximately 4,460 Confluence with Quantico Creek County, feet upstream of Van 02070011 5.8 Y AE 1992 Potomac River Unincorporated Buren Road Areas Prince William County, Confluence with Broad Rocky Branch Worthington Drive 02070010 0.9 Y AE 2020 Unincorporated Run Areas

28 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Prince William Approximately 100 feet County, Rocky Branch Worthington Drive upstream of State 02070010 2.5 Y AE 1992 Unincorporated Route 619 Areas Approximately 130 feet Approximately 4,120 Russia Branch Manassas, City of downstream of Liberia feet upstream of Liberia 02070010 0.8 Y AE 1992 Avenue Avenue Prince William Approximately 2,100 Tributary No. 1 to County, Confluence with feet upstream of 02070010 0.6 Y AE 1992 Cannon Branch Unincorporated Cannon Branch University Boulevard Areas Approximately 1,140 Tributary No. 1 to Flat Confluence with Flat feet upstream of the Manassas, City of 02070010 0.2 Y AE 1992 Branch Tributary B Branch Tributary B confluence with Flat Branch Tributary B Approximately 523 feet Tributary No. 1 to Flat Confluence with Flat Manassas, City of upstream of Grant 02070010 1.6 Y AE Branch Tributary C Branch Tributary C 2005 Avenue Prince William Tributary No. 1 to Confluence with Approximately 445 feet County, Marumsco Creek Marumsco Creek downstream of 02070010 0.3 Y AE 1992 Unincorporated Tributary B Tributary B Longview Drive Areas Approximately 105 feet Prince William Tributary No. 1 To Approximately 445 feet east of the intersection County, Marumsco Creek downstream of of Silverwood Lane and 02070010 0.3 Y AE 2018 Unincorporated Tributary B Longview Drive Jefferson Davis Areas Highway

29 Table 2: Flooding Sources Included in this FIS Report (continued) Length (mi) HUC-8 Sub- (streams or Floodway Zone shown Date of Flooding Source Community Downstream Limit Upstream Limit Basin(s) coastlines) (Y/N) on FIRM Analysis Dumfries, Town of; Prince William Tributary No. 1 to Confluence with Approximately 830 feet County, 02070011 0.4 Y AE 1977 Quantico Creek Quantico Creek upstream of Eby Street Unincorporated Areas Tributary No. 2 to Confluence with Russia Approximately 530 feet Manassas, City of 02070010 0.2 Y AE 1992 Russia Branch Branch upstream of railroad Approximately 560 feet Tributary No. 3 to Flat Confluence with Flat Manassas, City of upstream of Traveller 02070010 0.6 Y AE 1992 Branch Tributary A Branch Tributary A Street Manassas Park, City Approximately 725 of; Prince William feet northwest of the Tributary No. 3 to Flat Confluence with Flat County, 02070010 0.9 Y AE 1992 Branch Tributary B Branch Tributary B intersection of Old Unincorporated Centreville Road and Areas Mclean Way Prince William Approximately 550 feet Unnamed Tributary To Confluence with County, upstream of confluence Chestnut Lick Chestnut Lick Tributary 02070010 0.1 N AE 2017 Unincorporated with Chestnut Lick Tributary No. 1 No. 1 Areas Tributary No. 1 Prince William Unnamed Tributary To County, Confluence with North John Marshall Highway 02070010 1.1 Y AE 2013 North Fork Broad Run Unincorporated Fork Broad Run Areas Approximately 430 feet Approximately 1,500 southwest of the signal Winters Branch Manassas, City of feet upstream of 02070010 1.1 Y AE 1992 Hill Road and Fairview Wellington Road Avenue intersection

30 2.2 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 balancing floodplain development against increasing flood hazard. With this approach, the area of the 1-percent-annual-chance floodplain on a river is divided into a floodway and a floodway fringe based on hydraulic modeling. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment in order to carry the 1-percent-annual-chance flood. The floodway fringe is the area between the floodway and the 1-percent-annual-chance floodplain boundaries where encroachment is permitted. The floodway must be wide enough so that the floodway fringe could be completely obstructed without increasing the water surface elevation 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 4.

To participate in the NFIP, Federal regulations require communities to limit increases caused by encroachment to 1.0 foot, provided that hazardous velocities are not produced. Regulations in Prince William County require communities to limit increases caused by encroachment to 1.0 foot. The floodways in this project are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway projects.

31 Figure 4: Floodway Schematic

Floodway widths presented in this FIS Report and on the FIRM were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. For certain stream segments, floodways were adjusted so that the amount of floodwaters conveyed on each side of the floodplain would be reduced equally. The results of the floodway computations have been tabulated for selected cross sections and are shown in Table 23, “Floodway Data.”

All floodways that were developed for this Flood Risk Project are shown on the FIRM using the symbology described in Figure 3. In cases where the floodway and 1-percent-annual- chance floodplain boundaries are either close together or collinear, only the floodway boundary has been shown on the FIRM. For information about the delineation of floodways on the FIRM, refer to Section 6.3.

2.3 Base Flood Elevations

The hydraulic characteristics of flooding sources were analyzed to provide estimates of the elevations of floods of the selected recurrence intervals. The BFE is the elevation of the 1-percent-annual-chance flood. These BFEs are most commonly rounded to the whole foot, as shown on the FIRM, but in certain circumstances or locations they may be rounded to 0.1 foot. Cross section lines shown on the FIRM may also be labeled with the BFE rounded to 0.1 foot. Whole-foot BFEs derived from engineering analyses that apply to coastal areas, areas of ponding, or other static areas with little elevation change may also be shown at selected intervals on the FIRM.

BFEs are primarily intended for flood insurance rating purposes. Cross sections with BFEs

32 shown on the FIRM correspond to the cross sections shown in the Floodway Data table and Flood Profiles in this FIS Report. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this FIS Report in conjunction with the data shown on the FIRM. For example, the user may use the FIRM to determine the stream station of a location of interest and then use the profile to determine the 1-percent annual chance elevation at that location. Because only selected cross sections may be shown on the FIRM for riverine areas, the profile should be used to obtain the flood elevation between mapped cross sections. Additionally, for riverine areas, whole-foot elevations shown on the FIRM may not exactly reflect the elevations derived from the hydraulic analyses; therefore, elevations obtained from the profile may more accurately reflect the results of the hydraulic analysis.

2.4 Non-Encroachment Zones

This section is not applicable to the Flood Risk Project.

2.5 Coastal Flood Hazard Areas

For most areas along rivers, streams, and small lakes, BFEs and floodplain boundaries are based on the amount of water expected to enter the area during a 1-percent-annual- chance flood and the geometry of the floodplain. Floods in these areas are typically caused by storm events. However, for areas on or near ocean coasts, large rivers, or large bodies of water, BFE and floodplain boundaries may need to be based on additional components, including storm surges and waves.

Coastal flooding sources that are included in this Flood Risk Project are shown in Table 2.

2.5.1 Water Elevations and the Effects of Waves

Specific terminology is used in coastal analyses to indicate which components have been included in evaluating flood hazards.

The stillwater elevation (SWEL or still water level) is the surface of the water resulting from astronomical tides, storm surge, and freshwater inputs, but excluding wave setup contribution or the effects of waves.

 Astronomical tides are periodic rises and falls in large bodies of water caused by the rotation of the earth and by the gravitational forces exerted by the earth, moon and sun.  Storm surge is the additional water depth that occurs during large storm events. These events can bring air pressure changes and strong winds that force water up against the shore.  Freshwater inputs include rainfall that falls directly on the body of water, runoff from surfaces and overland flow, and inputs from rivers.

The 1-percent-annual-chance stillwater elevation is the stillwater elevation that has been calculated for a storm surge from a 1-percent-annual-chance storm. The 1-percent- annual-chance storm surge can be determined from analyses of tidal gage records, statistical study of regional historical storms, or other modeling approaches. Stillwater

33 elevations for storms of other frequencies can be developed using similar approaches.

The total stillwater elevation (also referred to as the mean water level) is the stillwater elevation plus wave setup contribution but excluding the effects of waves.

 Wave setup is the increase in stillwater elevation at the shoreline caused by the reduction of waves in shallow water. It occurs as breaking wave momentum is transferred to the water column.

Like the stillwater elevation, the total stillwater elevation is based on a storm of a particular frequency, such as the 1-percent-annual-chance storm. Wave setup is typically estimated using standard engineering practices or calculated using models, since tidal gages are often sited in areas sheltered from wave action and do not capture this information.

Coastal analyses may examine the effects of overland waves by analyzing storm-induced erosion, overland wave propagation, wave runup, and/or wave overtopping.

 Storm-induced erosion is the modification of existing topography by erosion caused by a specific storm event, as opposed to general erosion that occurs at a more constant rate.

 Overland wave propagation describes the combined effects of variation in ground elevation, vegetation, and physical features on wave characteristics as waves move onshore.

 Wave runup is the uprush of water from wave action on a shore barrier. It is a function of the roughness and geometry of the shoreline at the point where the stillwater elevation intersects the land.

 Wave overtopping refers to wave runup that occurs when waves pass over the crest of a barrier.

Figure 5: Wave Runup Transect Schematic

34 2.5.2 Floodplain Boundaries and BFEs for Coastal Areas

For coastal communities along the Atlantic and Pacific Oceans, the Gulf of Mexico, the Great Lakes, and the Caribbean Sea, flood hazards must take into account how storm surges, waves, and extreme tides interact with factors such as topography and vegetation. Storm surge and waves must also be considered in assessing flood risk for certain communities on rivers or large inland bodies of water.

Beyond areas that are affected by waves and tides, coastal communities can also have riverine floodplains with designated floodways, as described in previous sections.

Floodplain Boundaries

In many coastal areas, storm surge is the principle component of flooding. The extent of the 1-percent-annual-chance floodplain in these areas is derived from the total stillwater elevation (stillwater elevation including storm surge plus wave setup) for the 1-percent- annual-chance storm. The methods that were used for calculation of total stillwater elevations for coastal areas are described in Section 5.3 of this FIS Report. Location of total stillwater elevations for coastal areas are shown in Figure 8, “1% Annual Chance Total Stillwater Levels for Coastal Areas.”

In some areas, the 1-percent-annual-chance floodplain is determined based on the limit of wave runup or wave overtopping for the 1-percent-annual-chance storm surge. The methods that were used for calculation of wave hazards are described in Section 5.3 of this FIS Report.

Table 25 presents the types of coastal analyses that were used in mapping the 1-percent- annual-chance floodplain in coastal areas.

Coastal BFEs

Coastal BFEs are calculated as the total stillwater elevation (stillwater elevation including storm surge plus wave setup) for the 1-percent-annual-chance storm plus the additional flood hazard from overland wave effects (storm-induced erosion, overland wave propagation, wave runup and wave overtopping).

Where they apply, coastal BFEs are calculated along transects extending from offshore to the limit of coastal flooding onshore. Results of these analyses are accurate until local topography, vegetation, or development type and density within the community undergoes major changes.

Parameters that were included in calculating coastal BFEs for each transect included in this FIS Report are presented in Table 16, “Coastal Transect Parameters.” The locations of transects are shown in Figure 9, “Transect Location Map.” More detailed information about the methods used in coastal analyses and the results of intermediate steps in the coastal analyses are presented in Section 5.3 of this FIS Report. Additional information on specific mapping methods is provided in Section 6.4 of this FIS Report.

2.5.3 Coastal High Hazard Areas

Certain areas along the open coast and other areas may have higher risk of experiencing structural damage caused by wave action and/or high-velocity water during the 1-percent-

35 annual-chance flood. These areas will be identified on the FIRM as Coastal High Hazard Areas.

 Coastal High Hazard Area (CHHA) is a SFHA extending from offshore to the inland limit of the primary frontal dune (PFD) or any other area subject to damages caused by wave action and/or high-velocity water during the 1-percent-annual- chance flood.

 Primary Frontal Dune (PFD) is a continuous or nearly continuous mound or ridge of sand with relatively steep slopes immediately landward and adjacent to the beach. The PFD is subject to erosion and overtopping from high tides and waves during major coastal storms.

CHHAs are designated as “V” zones (for “velocity wave zones”) and are subject to more stringent regulatory requirements and a different flood insurance rate structure. The areas of greatest risk are shown as VE on the FIRM. Zone VE is further subdivided into elevation zones and shown with BFEs on the FIRM.

The landward limit of the PFD occurs at a point where there is a distinct change from a relatively steep slope to a relatively mild slope; this point represents the landward extension of Zone VE. Areas of lower risk in the CHHA are designated with Zone V on the FIRM. More detailed information about the identification and designation of Zone VE is presented in Section 6.4 of this FIS Report.

Areas that are not within the CHHA but are SFHAs may still be impacted by coastal flooding and damaging waves; these areas are shown as “A” zones on the FIRM.

Figure 6, “Coastal Transect Schematic,” illustrates the relationship between the base flood elevation, the 1-percent-annual-chance stillwater elevation, and the ground profile as well as the location of the Zone VE and Zone AE areas in an area without a PFD subject to overland wave propagation. This figure also illustrates energy dissipation and regeneration of a wave as it moves inland.

Figure 6: Coastal Transect Schematic

36 Methods used in coastal analyses in this Flood Risk Project are presented in Section 5.3 and mapping methods are provided in Section 6.4 of this FIS Report.

Coastal floodplains are shown on the FIRM using the symbology described in Figure 3, “Map Legend for FIRM.” In many cases, the BFE on the FIRM is higher than the stillwater elevations shown in Table 16 due to the presence of wave effects. The higher elevation should be used for construction and/or floodplain management purposes.

2.5.4 Limit of Moderate Wave Action

Laboratory tests and field investigations have shown that wave heights as little as 1.5 feet can cause damage to and failure of typical Zone AE building construction. Wood-frame, light gage steel, or masonry walls on shallow footings or slabs are subject to damage when exposed to waves less than 3 feet in height. Other flood hazards associated with coastal waves (floating debris, high velocity flow, erosion, and scour) can also damage Zone AE construction.

Therefore, a LiMWA boundary may be shown on the FIRM as an informational layer to assist coastal communities in safe rebuilding practices. The LiMWA represents the approximate landward limit of the 1.5-foot breaking wave. The location of the LiMWA relative to Zone VE and Zone AE is shown in Figure 6.

The effects of wave hazards in Zone AE between Zone VE (or the shoreline where Zone VE is not identified) and the limit of the LiMWA boundary are similar to, but less severe than, those in Zone VE where 3-foot or greater breaking waves are projected to occur during the 1-percent-annual-chance flooding event. Communities are therefore encouraged to adopt and enforce more stringent floodplain management requirements than the minimum NFIP requirements in the LiMWA. The NFIP Community Rating System provides credits for these actions.

SECTION 3.0 – INSURANCE APPLICATIONS

3.1 National Flood Insurance Program Insurance Zones

For flood insurance applications, the FIRM designates flood insurance rate zones as described in Figure 3, “Map Legend for FIRM.” Flood insurance zone designations are assigned to flooding sources based on the results of the hydraulic or coastal analyses. Insurance agents use the zones shown on the FIRM and depths and base flood elevations in this FIS Report in conjunction with information on structures and their contents to assign premium rates for flood insurance policies.

The 1-percent-annual-chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (e.g. Zones A, AE, V, VE, etc.), and the 0.2-percent-annual- chance floodplain boundary corresponds to the boundary of areas of additional flood hazards.

Table 3 lists the flood insurance zones in Prince William County.

37 Table 3: Flood Zone Designations by Community Community Flood Zone(s) Dumfries, Town of A, AE, VE, X Haymarket, Town of AE, X Manassas, City of A, AE, X Manassas Park, City of A, AE, X Occoquan, Town of A, AE, X Prince William County, Unincorporated Areas A, AE, VE, X Quantico, Town of AE, VE, X

SECTION 4.0 – AREA STUDIED

4.1 Basin Description

Table 4 contains a description of the characteristics of the HUC-8 sub-basins within which each community falls. The table includes the main flooding sources within each basin, a brief description of the basin, and its drainage area.

Table 4: Basin Characteristics HUC-8 HUC-8 Primary Sub-Basin Sub-Basin Flooding Drainage Area Name Number Source Description of Affected Area (square miles) Encompasses the southern corner of Prince William Lower Potomac 02070011 County and a majority of the 1,812 Potomac River coastal flooding from the Potomac River

Largest watershed within Middle Prince William County, Potomac- Potomac 02070010 encompassing all but the 1,300 Anacostia- River sourthern most corner of Occoquan county

Middle Watershed extends slightly Potomac Potomac- 02070008 past northern boundary of 1,241 River Catoctin Prince William County

4.2 Principal Flood Problems

Table 5 contains a description of the principal flood problems that have been noted for Prince William County by flooding source.

38 Table 5: Principal Flood Problems Flooding Source Description of Flood Problems

On August 2011, Hurricane Irene hit the eastern coast of the United States and All flooding caused substantial damage to Prince William County. (FIS 2015) sources On October 2012, Hurricane Sandy made landfall north of the Commonwealth of Virginia, but caused substantial damage in Virginia. (FIS 2015)

Large floods occurred in Prince William County in 1937, 1942, and 1972 (Tropical Storm Agnes). Based on flood discharges at Bull Run near the Manassas stream gage, Tropical Storm Agnes was the largest flood experienced and was estimated to be much greater than a 100-year frequency Bull Run flood. On June 23, 1972, “The Journal Messenger”, a local newspaper, had reports of the disaster caused by Tropical Storm Agnes in and around the Manassas area. Flood damage occurred to businesses, homes, boats, and automobiles. Mobile homes were literally washed away. Numerous bridges were overtopped, such as State Route 28 crossing Bull Run. Roads were broken up by currents and the road damage was magnified by blocked culverts and other obstructions in the drainage ways. USGS gage 01657000 located near the City of Manassas historic peaks (cfs) of 76,100, 14,900, 13,800 in 1972, 1980, 1975 with recurrence intervals of 1,000, 12, and 10 years respectively. (FIS 2015)

Flooding from the stream itself and tributaries of the Occoquan River in Prince William County may be caused by heavy rains occurring anytime of the year. Flooding may also occur as a result of intense rainfall produced by local Occoquan thunderstorms or tropical disturbances such as hurricanes which move into the River area from the Gulf or Atlantic Coasts. Flood heights on these 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 depend upon the size of the area flooded, the height of flooding, the velocity of flow, the rate of rise, and the duration of flooding. USGS gage 01657500 located near the City of Manassas historic peaks (cfs) of 75,000, 37,000, 29,200 in 1972, 1943, 1937 with recurrence intervals of 100, 11, and 10 years respectively. (FIS 2015)

Potomac The shoreline areas of Prince William County along the Potomac River and its River and estuaries are vulnerable to tidal flooding from major storms commonly referred its estuaries to as hurricanes and nor’easters. Both storms produce winds which push large volumes of water against the shore. (FIS 2015)

Table 6 contains information about historic flood elevations in the communities within Prince William County.

Table 6: Historic Flooding Elevations [Not Applicable to this Flood Risk Project]

39 4.3 Non-Levee Flood Protection Measures

Table 7 contains information about non-levee flood protection measures within Prince William County such as dams, jetties, and or dikes. Levees are addressed in Section 4.4 of this FIS Report.

Table 7: Non-Levee Flood Protection Measures Flooding Structure Type of Source Name Measure Location Description of Measure There are no existing flood Dam located control structures that would approximately 782 provide protection during T. Nelson feet west of the major floods in the study Broad Run Elliott Dam intersection of area. There are a number of Dam Hamill Run Drive measures that have afforded and Glenkirk Road some protection against flooding.

There are no existing flood Dam located control structures that would approximately 251 provide protection during Lake feet north of the major floods in the study Hooes Run Omisol Dam intersection of area. There are a number of Dam Merganser Lane measures that have afforded and Pintail Road some protection against flooding.

There are no existing flood Dam located control structures that would approximately 433 provide protection during Lake Occoquan feet south of the major floods in the study Jackson Dam River intersection of area. There are a number of Dam Coles Drive and measures that have afforded Mercury Drive some protection against flooding.

There are no existing flood Dam located control structures that would approximately provide protection during Upper 1,947 feet north of Occoquan major floods in the study Occoquan Dam the intersection of River area. There are a number of Dam Cathedral Drive measures that have afforded and Riverboat some protection against Drive flooding.

40 Table 7: Non-Levee Flood Protection Measures (continued) Flooding Structure Type of Source Name Measure Location Description of Measure There are no existing flood Dam located control structures that would approximately 313 provide protection during Montclair feet northeast of major floods in the study Powells Creek Dam Dam the intersection of area. There are a number of Dolphin Drive and measures that have afforded Spillway Lane some protection against flooding. Diversion on Little Small Potomac River Seawall Creek away from Minimal efficacy in periods Seawall the Potomac River of high peak discharges. Town of Quantico Quantico Small Minimal efficacy in periods Seawall into Quantico Creek Seawall of high peak discharges. Creek

4.4 Levees

This section is not applicable to the Flood Risk Project.

Table 8: Levees [Not Applicable to this Flood Risk Project]

SECTION 5.0 – ENGINEERING METHODS

For the flooding sources in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude that are expected to be equaled or exceeded at least once on the average during any 10-, 25-, 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-, 25-, 50-, 100-, and 500-year floods, have a 10-, 4-, 2-, 1-, and 0.2-percent-annual-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 that equals or exceeds the 100-year flood (1-percent chance of annual exceedance) during the term of a 30-year mortgage is approximately 26 percent (about 3 in 10); 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.

In addition to these flood events, the “1-percent-plus”, or “1%+”, annual chance flood elevation has been modeled and included on the flood profile for certain flooding sources in this FIS Report. While not used for regulatory or insurance purposes, this flood event

41 has been calculated to help illustrate the variability range that exists between the regulatory 1-percent-annual-chance flood elevation and a 1-percent-annual-chance elevation that has taken into account an additional amount of uncertainty in the flood discharges (thus, the 1% “plus”). For flooding sources whose discharges were estimated using regression equations, the 1%+ flood elevations are derived by taking the 1-percent- annual-chance flood discharges and increasing the modeled discharges by a percentage equal to the average predictive error for the regression equation. For flooding sources with gage- or rainfall-runoff-based discharge estimates, the upper 84-percent confidence limit of the discharges is used to compute the 1%+ flood elevations.

The engineering analyses described here incorporate the results of previously issued Letters of Map Change (LOMCs) listed in Table 26, “Incorporated Letters of Map Change”, which include Letters of Map Revision (LOMRs). For more information about LOMRs, refer to Section 6.5, “FIRM Revisions.”

5.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish the peak elevation-frequency relationships for floods of the selected recurrence intervals for each flooding source studied. Hydrologic analyses are typically performed at the watershed level. Depending on factors such as watershed size and shape, land use and urbanization, and natural or man-made storage, various models or methodologies may be applied. A summary of the hydrologic methods applied to develop the discharges used in the hydraulic analyses for each stream is provided in Table 12. Greater detail (including assumptions, analysis, and results) is available in the archived project documentation.

A summary of the discharges is provided in Table 9. Discharges for flooding sources designated as Zone A on the FIRM are not shown in Table 9 of this FIS report, however, discharge values are included in the FIRM database in the S_NODES and L_SUMMARY_DISCHARGES feature classes. A summary of stillwater elevations developed for non-coastal flooding sources is provided in Table 10. (Coastal stillwater elevations are discussed in Section 5.3 and shown in Table 16.) Stream gage information is provided in Table 11.

42 Table 9: Summary of Discharges Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance At confluence with Broad Run 138.00 13,300 * 21,200 25,100 32,500 Cedar Run Upstream of 110.00 8,930 * 15,300 18,100 26,100 Broad Run confluence of Kettle Run Downstream of Nelson Broad Run 74.70 8,200 * 13,900 16,400 24,300 Elliot Dam At confluence with Bull Run 198.00 15,000 * 24,500 29,000 41,000 Occoquan River Upstream of Bull Run 41,000 confluence of Cub Run 97.00 9,500 * 24,500 29,000 Upstream of Bull Run confluence of Little Bull 40.20 6,000 * 11,000 13,000 18,000 Run Upstream of Bull Run confluence of Black 13.30 2,840 * 5,040 5,930 8,980 Branch Bull Run At confluence with Bull 0.76 660 * 1,180 1,430 2,000 Tributary A Run Bull Run Approximately 450 feet 0.06 30 * 50 65 90 Tributary A north of Connor Drive Bull Run At Lake Drive 0.85 356 * 549 636 875 Tributary B Bull Run At East Rugby Road 0.68 290 * 447 517 712 Tributary B Bull Run At Centerville Road 0.45 333 * 533 632 720 Tributary B Bull Run At Well Street 0.42 319 * 512 608 694 Tributary B

43 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Bull Run At Leland Road 0.36 288 * 467 555 633 Tributary B Bull Run At Spruce Street 0.10 63 * 97 113 154 Tributary B Bull Run At confluence with Bull 0.15 210 * 350 410 560 Tributary C Run Bull Run At confluence with Bull 1.94 1,570 * 2,600 3,070 4,140 Tributary D Run Bull Run At Williamson 0.87 600 * 1,220 1,400 1,740 Tributary D Boulevard Approximately 0.30 Bull Run mile upstream of 0.44 310 * 700 800 970 Tributary D Williamson Boulevard At confluence with Cannon Branch 6.44 1,235 * 2,245 2,835 4,700 Broad Run At Norfolk Southern Cannon Branch 3.83 2,800 * 4,100 4,700 6,100 Railroad Cannon Branch At Nokesville Road 2.15 1,800 * 2,600 3,000 3,900 Approximately 0.42 Cannon Branch mile west of State 0.78 265 * 520 650 960 Route 661 At confluence with Catharpin Creek 9.52 1,280 * 2,280 2,870 4,270 Little Bull Run Catharpin Creek At U.S. Route 15 4.17 1,010 * 1,730 2,100 2,980 At confluence with Cedar Run 191.80 12,400 * 25,500 33,200 53,500 Broad Run Approximately 380 feet Chestnut Lick downstream of Logmill * Tributary No. 1 0.7 530 * 918 1,103 Road Chestnut Lick Just upstream of * Tributary No. 1 Logmill Road 0.4 383 * 616 739

44 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Downstream of Chopawamsic confluence of Tributary 10,848 Creek 1 to Chopawamsic 27.24 2,575 3,865 5,080 6,224 Creek Chopawamsic At U.S. Route 95 10,620 Creek 22.16 2,317 3,446 4,450 5,608 Approximately 200 feet Cockrell Branch 1,335 east of Lucasville Road 0.85 375 * 735 915 Approximately 400 feet Cockrell Branch 1,020 west of Dumfries Road 0.61 290 * 565 700 Cockrell Branch At State Route 234 0.25 110 * 215 270 390 At confluence with Cow Branch 5,332 Neabsco Creek 3.89 1,739 2,385 2,918 3,511 Cow Branch At Rippon Boulevard 3.46 1,688 2,261 2,730 3,256 4,944 Cow Branch At Blackburn Road 3.38 1,693 2,255 2,717 3,242 4,916 At Jefferson Davis Cow Branch 3.22 1,683 2,230 2,678 3,207 4,852 Highway Downstream of Opitz Cow Branch 2.07 898 1,234 1,603 2,042 3,028 Boulevard Upstream of Opitz Cow Branch 1.99 850 1,161 1,545 1,967 2,918 Boulevard At Montgomery Cow Branch 1.81 824 1,126 1,504 1,904 2,796 Avenue At West Longview Cow Branch 1.49 670 915 1,251 1,578 2,301 Drive Upstream of U.S. Cow Branch 1.26 615 834 1,143 1,432 2,073 Route 95 Cow Branch Downstream of Pond 0.97 456 621 893 1,158 1,803 Upstream of Potomac Cow Branch 0.97 586 737 978 1,247 1,899 Mills Pond

45 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Downstream of Prince Cow Branch 0.52 170 250 424 578 977 William Parkway At confluence with Dawkins Branch 4.70 1,850 * 3,100 3,650 4,660 Broad Run Approximately 1,500 Dawkins Branch feet upstream of 2.53 1,713 * 2,505 2,883 3,259 Sudley Manor Drive At Southern Railway Dawkins Branch 0.91 650 * 1,430 1,650 2,100 Bridge At Richmond, Farm Creek Fredericksburg and 0.77 350 * 780 1,060 2,100 Potomac Railroad Farm Creek At Featherstone Road 0.31 200 * 430 600 1,200 At confluence with Bull Flat Branch 6.94 7,870 * 11,650 13,370 17,460 Run Upstream of Flat Flat Branch 2.95 3,970 * 5,920 6,800 9,040 Branch Tributary A Downstream of Flat Branch 2.95 3,970 * 5,920 6,800 9,040 Tributary C Upstream of Tributary Flat Branch 1.04 1,020 * 1,580 1,840 2,480 C Flat Branch At confluence with Flat 1.13 660 * 1,200 1,500 2,100 Tributary A Branch Approximately 500 feet Flat Branch north of Weems Road 0.08 160 * 240 275 360 Tributary A and Weir Road Approximately 0.24 Flat Branch mile upstream of Flat 1.39 584 * 1,095 1,350 1,850 Tributary B Branch Flat Branch At Liberia Avenue 0.08 45 * 80 100 1,020 Tributary B

46 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Flat Branch At confluence with Flat 1.81 920 * 1,750 2,150 3,200 Tributary C Branch At Stonewall Road approximately 700 feet Flat Branch southeast of 0.12 140 * 250 305 425 Tributary C intersection of Godwin Drive and Sudley Road At confluence with Bull Holkums Branch 1.17 920 * 1,510 1,740 2,300 Run Holkums Branch At Battleview Parkway 0.65 500 * 720 800 930 Holkums Branch At Sudley Road 0.31 220 * 350 400 520 At confluence with Hooes Run 5.43 1,100 * 2,000 2,550 4,200 Occoquan River Hooes Run At Lake Omisol 1.57 480 * 900 1,150 1,950 At confluence with Kettle Run 25.80 5,600 * 9,060 10,760 14,010 Broad Run Approximately 0.24 Kettle Run mile upstream of State 17.10 4,330 * 7,040 8,350 10,940 Route 656 At confluence with Bull Little Bull Run 28.00 2,990 * 5,220 6,440 9,420 Run Downstream of Little Bull Run confluence of 18.70 2,580 * 4,520 5,660 8,360 Catharpin Creek Upstream of Little Bull Run confluence of 9.22 1,410 * 2,510 3,000 4,280 Catharpin Creek Little Bull Run At U.S. Route 15 4.87 940 * 1,740 2,150 3,090 Approximately 2.23 Little Bull Run mile upstream of U.S. 1.88 110 * 180 350 750 Route 15

47 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance At confluence with Little Creek 3.50 1,870 * 3,180 3,800 5,020 Quantico Creek Approximately 350 feet Little Creek downstream of Old 1.05 690 * 1,170 1,400 1,870 Triangle Road Little Creek At Interstate Route 95 0.31 220 * 365 460 615 At confluence with Marumsco Creek 4.29 1,830 * 2,930 3,470 4,570 Potomac River At Richmond, Marumsco Creek Fredericksburg and 2.66 1,350 * 1,900 2,150 2,570 Potomac Railroad Marumsco Creek At U.S. Route 1 2.50 2,270 * 3,420 3,950 5,220 Approximately 0.41 Marumsco Creek mile upstream of 1.06 650 * 1,160 1,400 1,950 Hylton Ave. Marumsco Creek At confluence with 0.66 320 * 705 960 1,900 Tributary A Marumsco Creek Marumsco Creek At confluence with 0.90 390 * 865 1,175 2,300 Tributary B Marumsco Creek Neabsco Creek At U.S. Route 1 16.40 8,270 * 11,400 12,800 15,500 Neabsco Creek At Darbydale Avenue 10.20 5,270 * 6,690 7,320 9,630 Neabsco Creek At Lindendale Road 4.21 2,470 * 3,370 3,720 5,150 Neabsco Creek At Prince Dale Drive 1.25 800 * 1,110 1,250 1,600 185 feet upstream of Neabsco Creek confluence with 1.95 1,057 * 1,506 1,667 2,344 Tributary A Neabsco Creek 3730 feet upstream of Neabsco Creek confluence with 1.8 1,032 * 1,414 1,573 2,072 Tributary A Neabsco Creek

48 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Neabsco Creek Downstream of culvert 1.55 939 * 1,279 1,418 1,861 Tributary A at Dale Boulevard Neabsco Creek Upstream of culvert at 1.55 1,247 * 2,164 2,677 4,029 Tributary A Dale Boulevard Neabsco Creek At Dale Boulevard 1.49 1,080 * 1,380 1,560 2,210 Tributary A Neabsco Creek 3330 feet upstream of 1.3 972 * 1,771 2,159 3,192 Tributary A Dale Boulevard Neabsco Creek 5845 feet upstream of 0.6 129 * 369 514 1,137 Tributary A Dale Boulevard North Fork Broad At U.S. Route 29 7.32 2,070 * 3,670 4,410 5,840 Run North Fork Broad At U.S. Route 15 4.93 2,830 * 4,446 5,234 8,828 Run Approximately 0.33 North Fork Broad mile upstream of State 4.90 1,840 * 3,030 3,600 4,780 Run Route 703 North Fork Broad At Interstate 66 1.33 712 * 1,150 1,366 2,375 Run North Fork Broad At Gaines Road 0.72 490 * 905 1,118 2,037 Run At confluence with Occoquan River 604.00 29,600 * 54,700 65,800 96,500 Potomac River Downstream of Occoquan River 561.00 35,000 * 61,000 74,000 105,000 confluence of Bull Run Upstream of Occoquan River 363.00 22,700 * 40,000 48,700 69,400 confluence of Bull Run Downstream of Occoquan River confluence of Cedar 332.00 24,000 * 42,000 50,000 71,500 Run

49 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance At Richmond, Fredericksburg, and Powells Creek 18.40 2,210 * 3,540 4,230 5,540 Potomac River Railroad Downstream of Lake Powells Creek 10.90 980 * 2,310 2,950 5,070 Montclair Dam Upstream of Lake Powells Creek 10.90 2,660 * 4,440 5,370 7,000 Montclair Dam Downstream of State Powells Creek 7.83 2,753 * 4,445 5,276 7,090 Route 643 Powells Creek At State Route 640 5.67 2,140 * 3,710 4,460 5,920 At confluence with Purcell Branch 3.39 1,520 * 2,620 3,140 4,130 Occoquan River Approximately 0.97 Purcell Branch mile upstream of 2.02 1,210 * 3,710 4,460 5,920 Purcell Road Approximately 0.96 Quantico Creek mile downstream of 30.50 6,110 * 10,600 12,600 17,300 U.S. Route 1 Downstream of Quantico Creek confluence of South 23.80 5,070 * 8,950 10,700 14,600 Fork Quantico Creek Upstream of Quantico Creek confluence of South 6.55 2,000 * 3,450 4,100 5,570 Fork Quantico Creek At confluence with Rocky Branch 6.29 3,030 * 4,780 5,570 7,130 Broad Run Approximately 0.60 Rocky Branch mile upstream of Broad 3.52 1,590 * 2,570 3,040 4,000 Run Rocky Branch At State Route 619 0.78 670 * 1,050 1,230 1,520

50 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Approximately 400 feet Russia Branch 0.74 360 * 705 875 1,280 east of Quarry Road Approximately 0.19 Russia Branch mile west of Liberia 0.40 170 * 330 410 605 Avenue Approximately 300 feet Russia Branch north of State Route 0.02 10 * 15 20 30 689 Tributary No. 1 At confluence with To Cannon 0.68 420 * 810 1,000 1,470 Cannon Branch Branch Tributary No. 1 At confluence with Flat To Flat Branch 0.02 100 * 195 240 370 Branch Tributary B Tributary B Tributary No. 1 To Flat Branch At Centreville Road 0.01 10 * 15 20 30 Tributary B Tributary No. 1 At confluence with Flat To Flat Branch 0.59 509 * 849 1,018 1,430 Branch Tributary C Tributary C Approximately 200 feet Tributary No. 1 west of intersection of To Flat Branch 0.25 282 * 465 548 745 West St. and Portner Tributary C St. Tributary No. 1 At confluence with To Marumsco Marumsco Creek 0.32 200 * 440 600 1,200 Creek Tributary B Tributary B Tributary No. 1 At confluence with To Quantico 0.26 135 * 260 325 475 Quantico Creek Creek

51 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Tributary No. 1 Approximately 600 feet To Quantico 0.10 50 * 100 125 185 upstream of Eby Drive Creek Tributary No. 2 At confluence with To Russia 0.13 55 * 105 135 195 Russia Branch Branch Tributary No. 2 Approximately 350 feet To Russia north of Liberia Avenue 0.06 25 * 50 50 90 Branch and Euclid Street Tributary No. 3 To Flat Branch At confluence with Flat 0.17 190 * 330 395 585 Tributary A Branch Tributary A Tributary No. 3 To Flat Branch At Weir Street and 0.06 110 * 190 225 310 Tributary A Traveller Street Tributary No. 3 At confluence with Flat To Flat Branch 0.38 350 * 630 760 1,080 Branch Tributary B Tributary B Approximately 300 feet Tributary No. 3 northwest of To Flat Branch intersection of 0.11 175 * 300 365 490 Tributary B Manassas and Martin Drives Unnamed At the confluence with Tributary To Chestnut Lick Tributary 0.3 245 * 418 503 855 Chestnut Lick No.1 Tributary No. 1 Unnamed Tributary To At Thoroughfare Road 1.71 1,309 * 2,083 2,462 4,099 North Fork Broad Run

52 Table 9: Summary of Discharges (continued) Peak Discharge (cfs) Drainage Area 0.2% (Square 10% Annual 4% Annual 2% Annual 1% Annual Annual Flooding Source Location Miles) Chance Chance Chance Chance Chance Approximately 0.21 Winters Branch mile south of 0.66 520 * 950 1,160 1,710 Wellington Road At Liberty Street and Winters Branch 0.26 210 * 390 475 700 Grant Avenue At Fairview Avenue Winters Branch 0.02 40 * 70 85 125 and Signal Hill Road

Figure 7: Frequency Discharge-Drainage Area Curves

[Not Applicable to this Flood Risk Project]

53 Table 10: Summary of Non-Coastal Stillwater Elevations

Elevations (feet NAVD88) 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual Flooding Source Location Chance Chance Chance Chance Chance Prince William County, Lake Manassas * * * 291.2 * Unincorporated Areas Prince William County, Lake Omisol * * * 173.2 * Unincorporated Areas *Not calculated for this Flood Risk Project

54 Table 11: Stream Gage Information used to Determine Discharges Agency Drainage Period of Record that Area Gage Maintains (Square Flooding Source Identifier Gage Site Name Miles) From To Aquia Creek Near Aquia Creek 01660400 USGS 35 6/22/1972 2/24/2016 Garrisonville, VA Cedar Run Near Cedar Run Near 01656000 USGS 93 1/1/1951 1/1/2015 Catlett, VA Catlett, VA Rappahannock Rappahannock River Near 01668000 USGS 1,598 1/13/1908 2/17/2016 River Fredericksburg, VA South Fork Quantico Creek South Fork 01658500 USGS Near 7 9/1/1952 2/24/2016 Quantico Creek Independent Hill, VA

5.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. Base flood elevations on the FIRM represent the elevations shown on the Flood Profiles and in the Floodway Data tables in the FIS Report. Rounded whole-foot elevations may be shown on the FIRM in coastal areas, areas of ponding, and other areas with static base flood elevations. These whole-foot elevations may not exactly reflect the elevations derived from the hydraulic analyses. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this FIS Report in conjunction with the data shown on the FIRM. The hydraulic analyses for this 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.

For streams for which hydraulic analyses were based on cross sections, locations of selected cross sections are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 6.3), selected cross sections are also listed in Table 23, “Floodway Data.”

A summary of the methods used in hydraulic analyses performed for this project is provided in Table 12. Roughness coefficients are provided in Table 13. Roughness coefficients are values representing the frictional resistance water experiences when passing overland or through a channel. They are used in the calculations to determine water surface elevations. Greater detail (including assumptions, analysis, and results) is available in the archived project documentation.

55

Table 12: Summary of Hydrologic and Hydraulic Analyses

Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations All flooding Regression Effects of hydraulic structures sources mapped HEC-RAS 5.0 Various Various Equations 1/30/2017 A were not considered in the as Zone A on (USACE 2016) (USGS 2013) model. the FIRM Approximately 4,610 feet Countywide vertical datum Confluence with USACE HEC-1 USACE HEC-2 AE w/ Broad Run upstream of the 8/1/1992 conversion of -0.8 applied to Occoquan River (USACE 1990) (USACE 1982) Floodway confluence with NGVD29 to convert to NAVD88 Kettle Run Approximately Approximately 660 feet 4,610 feet downstream of HEC-RAS AE w/ Revised by LOMR 17-03- Broad Run upstream of the USACE HEC-1 9/13/2018 the southern (USACE n.d.) Floodway 2321P confluence with railway bridge on Kettle Run Milford Road Approximately Approximately 3,875 feet 660 feet upstream of the downstream of AE w/ Revised by LOMR 18-03- Broad Run Nokesville Road USACE HEC-1 HEC-RAS 11/15/2018 the southern Floodway 0611P bridge located railway bridge on 500 feet west of Milford Road Piper Lane Approximately Approximately 3,875 feet 4,300 feet upstream of the Countywide vertical datum downstream of AE w/ Broad Run Nokesville Road USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to the confluence Floodway bridge located NGVD29 to convert to NAVD88 with Dawkins 500 feet west of Branch Piper Lane

56 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Approximately 4,300 feet 480 feet downstream of AE w/ Revised by LOMR 09-03- Broad Run upstream of the USACE HEC-1 HEC-RAS 3/4/2010 the confluence Floodway 1773P confluence with with Dawkins Dawkins Branch Branch Approximately Approximately 480 feet 2,350 feet AE w/ Revised by LOMR 19-03- Broad Run upstream of the upstream from USACE HEC-1 HEC-RAS 6/18/2020 Floodway 0954P confluence with the Linton Hall Dawkins Branch Road Bridge Approximately Just downstream 2,350 feet Countywide vertical datum of T. Nelson AE w/ Broad Run upstream from USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Elliott Dam at Floodway the Linton Hall NGVD29 to convert to NAVD88 Lake Manassas Road Bridge Broad Run Side Convergence Divergence from AE w/ Revised by LOMR 17-03- USACE HEC-1 HEC-RAS 9/13/2018 Channel with Broad Run Broad Run Floodway 2321P Approximately 905 feet Countywide vertical datum Confluence with northeast of AE w/ Bull Run USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Occoquan River where Lake Drive Floodway NGVD29 to convert to NAVD88 meets Pine Street Approximately Approximately 905 feet 2,115 feet northeast of upstream of the AE w/ Revised by LOMR 16-03- Bull Run USACE HEC-1 HEC-RAS 1/2/2017 where Lake State Route 28 Floodway 0196P Drive meets Pine (Centreville Street Road) Bridge

57 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately 2,115 feet Approximately Countywide vertical datum upstream of the 100 feet AE w/ Bull Run USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to State Route 28 upstream of Floodway NGVD29 to convert to NAVD88 (Centreville Loudoun Road Road) Bridge Approximately 850 southeast of Countywide vertical datum Bull Run Confluence with the intersection AE w/ USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary A Bull Run of Leland Road Floodway NGVD29 to convert to NAVD88 and Maplewood Drive Appoximately Approximately 395 feet 850 southeast of southeast of the Bull Run the intersection AE w/ Revised by LOMR 17-03- intersection of USACE HEC-1 HEC-RAS 2/15/2018 Tributary A of Leland Road Floodway 0746P Maplewood Drive and Maplewood and Spruce Drive Street Appoximately 395 feet Approximately southeast of the 605 feet Countywide vertical datum Bull Run AE w/ intersection of upstream of the USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary A Floodway Maplewood Drive Manassas Drive NGVD29 to convert to NAVD88 and Spruce Bridge Street Approximately 215 feet north of Countywide vertical datum Bull Run Confluence with AE w/ the Leland Road USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary B Bull Run Floodway and Centreville NGVD29 to convert to NAVD88 Road intersection

58 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Approximately 215 feet north of 437 feet east of TR-20 Bull Run the Leland Road AE w/ the Rugby Road (USDA-SCS N/A 11/26/1998 Revised by LOMR 98-03-163P Tributary B and Centreville Floodway and Leland Road n.d.) Road intersection intersection Approximately 437 feet east of Countywide vertical datum Bull Run Peakwood Court AE w/ the Rugby Road USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary B Bridge Floodway and Leland Road NGVD29 to convert to NAVD88 intersection Approximately 515 feet Countywide vertical datum Bull Run Confluence with northeast of the AE w/ USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary C Bull Run intersection of Floodway NGVD29 to convert to NAVD88 Bull Run Road and Rugby Road Approximately 1,343 feet Countywide vertical datum Bull Run Confluence with AE w/ upstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Tributary D Bull Run Floodway Williamson NGVD29 to convert to NAVD88 Boulevard Approximately Approximately 680 feet east of 1,343 feet Bull Run the intersection AE w/ Revised by LOMR 08-03- upstream of USACE HEC-1 N/A 2/20/2008 Tributary D of State Route Floodway 0053P Williamson 234 and Boulevard Davidson Place Countywide vertical datum Confluence with AE w/ Cannon Branch Railroad USACE HEC-1 USACE HEC-2 4/1/1977 conversion of -0.8 applied to Broad Run Floodway NGVD29 to convert to NAVD88 HEC-1 4.0.1 AE w/ Revised by LOMR 15-03- Cannon Branch Railroad Godwin Drive HEC-RAS 8/11/2016 (USACE 1998) Floodway 2702P

59 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately 1,910 feet east of the University Countywide vertical datum AE w/ Cannon Branch Godwin Drive Boulevard and USACE HEC-1 USACE HEC-2 4/1/1977 conversion of -0.8 applied to Floodway Freedom Center NGVD29 to convert to NAVD88 Boulevard intersection Approximately 1,560 feet east of Countywide vertical datum Confluence with the intersection AE w/ Catharpin Creek USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Little Bull Run of Fishers Hill Floodway NGVD29 to convert to NAVD88 Way and Yellow Tavern Court Approximately 1,670 feet northeast of the Countywide vertical datum Confluence with AE w/ Cedar Run intersection of USACE HEC-1 USACE HEC-2 4/1/1977 conversion of -0.8 applied to Occoquan River Floodway Old Church Road NGVD29 to convert to NAVD88 and Slate Run Lane Approximately Approximately Chestnut Lick 215 feet 345 feet HEC-HMS 4.1 HEC-RAS 10/19/2017 AE LOMR 17-03-0682P Tributary No. 1 downstream of upstream of (USACE 2015) Logmill Road Logmill Road Approximately Prince William Chopawamsic 700 feet AE w/ and Stafford USACE HEC-1 USACE HEC-2 12/17/2018 Creek upstream of U.S. Floodway County boundary Route 95 South Approximately 540 feet AE w/ Revised by LOMR 15-03- Cockrell Branch upstream of State Route 234 USACE HEC-1 HEC-RAS 11/19/2015 Floodway 1081P Hidden Spring Drive

60 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations South side of Confluence with HEC-HMS 3.0 HEC-RAS 3.1.1 AE w/ Cow Branch Rippon 4/1/2018 Neabsco Creek (USACE 2005) (USACE 2003) Floodway Boulevard culvert Approximately 22 Confluence with feet upstream of AE w/ Revised by LOMR 09-03- Dawkins Branch USACE HEC-1 HEC-RAS 3/4/2010 Broad Run University Floodway 1773P Boulevard Approximately Approximately 22 1,180 feet Countywide vertical datum feet upstream of AE w/ Dawkins Branch upstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to University Floodway Prince William NGVD29 to convert to NAVD88 Boulevard Parkway Approximately 590 feet AE w/ LOMR 16-03-0467P revised Farm Creek Railroad upstream of USACE HEC-1 USACE HEC-2 8/1/1992 Floodway the floodway Farm Creek Drive Approximately Countywide vertical datum Confluence with 420 feet AE w/ Flat Branch USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Bull Run downstream of Floodway NGVD29 to convert to NAVD88 Fairmont Avenue Approximately Approximately 420 feet 215 feet HEC-RAS 4.1.0 AE w/ Revised by LOMR 15-03- Flat Branch USACE HEC-1 8/4/2016 downstream of upstream of (USACE 2010a) Floodway 1042P Fairmont Avenue State Route 234 Approximately Approximately 65 Countywide vertical datum 215 feet AE w/ Flat Branch feet upstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to upstream of Floodway Ashton Avenue NGVD29 to convert to NAVD88 State Route 234 Approximately 1,065 feet Countywide vertical datum Flat Branch Confluence with AE w/ upstream of the USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary A Flat Branch Floodway Stonewall Road NGVD29 to convert to NAVD88 Bridge

61 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Countywide vertical datum Flat Branch Confluence with 565 feet AE w/ USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary B Flat Branch upstream of Floodway NGVD29 to convert to NAVD88 Portner Avenue Approximately Countywide vertical datum Flat Branch Confluence with AE w/ 500 feet west of USACE HEC-1 USACE HEC-2 3/1/1977 conversion of -0.8 applied to Tributary C Flat Branch Floodway Stonewall Road NGVD29 to convert to NAVD88 Approximately Countywide vertical datum Confluence with 295 feet AE w/ Holkums Branch USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Bull Run upstream of Floodway NGVD29 to convert to NAVD88 Automotive Drive Approximately Approximately 435 feet 295 feet southeast of the AE w/ Revised by LOMR 11-03- Holkums Branch USACE HEC-1 HEC-RAS 4.1.0 1/18/2012 upstream of intersection of Floodway 0494P Automotive Drive Sudley Road and Automotive Drive Approximately Countywide vertical datum Confluence with 1,621 feet AE w/ Hooes Run USACE HEC-1 USACE HEC-2 4/1/1977 conversion of -0.8 applied to Occoquan River upstream of Lake Floodway NGVD29 to convert to NAVD88 Omisol Dam Approximately Countywide vertical datum Confluence with 0.6 mile AE w/ Kettle Run USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Broad Run upstream of Floodway NGVD29 to convert to NAVD88 Kettle Run Road Approximately 420 feet southwest of the Countywide vertical datum Confluence with intersection AE w/ Little Bull Run USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Bull Run between Twin Floodway NGVD29 to convert to NAVD88 Branch Court and Stepping Stone Drive

62 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately 420 feet Approximately southwest of the 515 feet intersection AE w/ Revised by LOMR 16-03- Little Bull Run upstream of USACE HEC-1 HEC-RAS 6/15/2017 between Twin Floodway 1829P Dominion Valley Branch Court Drive and Stepping Stone Drive Approximately 410 feet Countywide vertical datum Confluence with AE w/ Little Creek downstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Quantico Creek Floodway Jefferson Davis NGVD29 to convert to NAVD88 Highway Approximately Approximately 410 feet 540 feet AE w/ Revised by LOMR 16-03- Little Creek downstream of upstream of USACE HEC-1 HEC-RAS 8/24/2017 Floodway 1619P Jefferson Davis Jefferson Davis Highway Highway Approximately 540 feet Approximately 40 Countywide vertical datum AE w/ Little Creek upstream of feet upstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Floodway Jefferson Davis Inn Road NGVD29 to convert to NAVD88 Highway Approximately 330 feet Countywide vertical datum Marumsco Confluence with AE w/ downstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Creek Potomac River Floodway Interstate Route NGVD29 to convert to NAVD88 95 Approximately Marumsco 150 feet Countywide vertical datum Confluence with AE w/ Creek Tributary upstream of USACE HEC-1 USACE HEC-2 4/1/1977 conversion of -0.8 applied to Marumsco Creek Floodway A Prince William NGVD29 to convert to NAVD88 Parkway

63 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Approximately Marumsco 150 feet 1,205 feet AE w/ Revised by LOMC 17-03- Creek Tributary upstream of USACE HEC-1 HEC-RAS 6/28/2018 upstream of Floodway 1826P A Prince William Church Hill Drive Parkway Approximately Marumsco Countywide vertical datum Confluence with 500 feet AE w/ Creek Tributary USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Marumsco Creek upstream of Floodway B NGVD29 to convert to NAVD88 Colchester Road Approximately Approximately 1,800 feet 1,290 feet HEC-RAS 5.0.1 AE w/ Revised by LOMR 17-03- Neabsco Creek upstream of the downstream of USACE HEC-1 4/26/2018 (USACE 2016) Floodway 1502P confluence with Jefferson Davis Cow Branch Highway Approximately Approximately 25 1,290 feet Countywide vertical datum feet upstream of AE w/ Neabsco Creek downstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Jefferson Davis Floodway Jefferson Davis NGVD29 to convert to NAVD88 Highway Highway Approximately Approximately 25 2,275 feet feet upstream of HEC-2 4.6.2 AE w/ Revised by LOMR 15-03- Neabsco Creek downstream of USACE HEC-1 8/4/2015 Jefferson Davis (USACE 1991) Floodway 2125P, 14-03-0598P Interstate Route Highway 95 Approximately Approximately 2,275 feet Countywide vertical datum 970 feet AE w/ Neabsco Creek downstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to upstream of Floodway Interstate Route NGVD29 to convert to NAVD88 Princedale Drive 95 Approximately 275 feet north of Countywide vertical datum Neabsco Creek Confluence with the intersection AE w/ USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Tributary A Neabsco Creek of Kerrydale Floodway NGVD29 to convert to NAVD88 Road and Kerrydale Place

64 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Approximately 275 feet north of 645 feet south of Neabsco Creek the intersection the intersection AE w/ USACE HEC-1 USACE HEC-2 5/16/2002 Revised by LOMR 01-03-123P Tributary A of Kerrydale of Kerrydale Floodway Road and Road and Kerrydale Place Kingswell Drive Approximately 645 feet south of Approximately Countywide vertical datum Neabsco Creek the intersection 1,290 feet AE w/ USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Tributary A of Kerrydale downstream of Floodway NGVD29 to convert to NAVD88 Road and State Route 642 Kingswell Drive Approximately Countywide vertical datum North Fork Confluence with 1,370 feet AE w/ USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Broad Run Lake Manassas upstream of Floodway NGVD29 to convert to NAVD88 Jefferson Street Approximately Approximately 300 feet North Fork 1,370 feet AE w/ Revised by LOMR 05-03- downstream of USACE HEC-1 USACE HEC-2 1/4/2007 Broad Run upstream of Floodway A398P James Madison Jefferson Street Highway Approximately Approximately 300 feet North Fork 155 feet PondPack V8i AE w/ Revised by LOMR 12-03- downstream of HEC-RAS 3/18/2013 Broad Run downstream of (Bentley 2012) Floodway 0457P James Madison State Route 55 Highway Approximately Approximately 25 Countywide vertical datum North Fork 155 feet feet downstream AE w/ USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Broad Run downstream of of Interstate Floodway NGVD29 to convert to NAVD88 State Route 55 Route 66 Approximately 25 Approximately 16 North Fork feet downstream feet downstream AE w/ USACE HEC-1 USACE HEC-2 4/3/2002 Revised by LOMR 01-03-077P Broad Run of Interstate of Thoroughfare Floodway Route 66 Road

65 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately 16 Approximately 35 Countywide vertical datum North Fork feet downstream AE w/ feet upstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Broad Run of Thoroughfare Floodway Gaines Road NGVD29 to convert to NAVD88 Road North Fork Convergence with Divergence from AE w/ Broad Run North Fork Broad North Fork Broad USACE HEC-1 USACE HEC-2 4/3/2002 LOMR 01-03-077P Floodway Bypass Spillway Run River Approximately 85 Countywide vertical datum Confluence with feet upstream of AE w/ Occoquan River USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Potomac River Jefferson Davis Floodway NGVD29 to convert to NAVD88 Highway Approximately 85 Northernmost feet upstream of AE w/ Revised by LOMR 18-03- Occoquan River Lane of Interstate USACE HEC-1 HEC-RAS 10/12/2018 Jefferson Davis Floodway 0171P Route 95 Highway Northernmost Confluence with Countywide vertical datum Lane of AE w/ Occoquan River Broad Run and USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Interstate Route Floodway Cedar Run NGVD29 to convert to NAVD88 95 Countywide vertical datum Confluence with AE w/ Powells Creek U.S. Highway 1 USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Potomac River Floodway NGVD29 to convert to NAVD88 Approximately 1,520 feet HEC-RAS 5.0.6 AE w/ Revised by LOMR 19-03- Powells Creek U.S. Highway 1 USACE HEC-1 1/16/2020 downstream of (USACE 2018) Floodway 0792P Interstate 95 Approximately Approximately Countywide vertical datum 1,520 feet 880 feet AE w/ Powells Creek USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to downstream of downstream of Floodway NGVD29 to convert to NAVD88 Interstate 95 Spriggs Road Approximately Approximately HEC-HMS 3.5 880 feet 5,128 feet AE w/ Revised by LOMR 17-03- Powells Creek (USACE HEC-RAS 7/12/2018 downstream of downstream of Floodway 1866P 2010b) Spriggs Road Minnieville Road

66 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Approximately Countywide vertical datum 5,128 Feet 1,130 Feet AE w/ Powells Creek USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to downstream of upstream of Floodway NGVD29 to convert to NAVD88 Minnieville Road Minnieville Road Approximately Countywide vertical datum Confluence with 1,255 feet AE w/ Purcell Branch USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Occoquan River upstream of Sly Floodway NGVD29 to convert to NAVD88 Fox Lane Approximately Countywide vertical datum Confluence with 4,460 feet AE w/ Quantico Creek USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Potomac River upstream of Van Floodway NGVD29 to convert to NAVD88 Buren Road Confluence with Worthington AE w/ Backwater revised by LOMR 19- Rocky Branch USACE HEC-1 USACE HEC-2 6/18/2020 Broad Run Drive Floodway 03-0954P Approximately Countywide vertical datum Worthington 100 feet AE w/ Rocky Branch USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Drive upstream of Floodway NGVD29 to convert to NAVD88 State Route 619 Approximately Approximately Countywide vertical datum 130 feet 4,120 feet AE w/ Russia Branch USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to downstream of upstream of Floodway NGVD29 to convert to NAVD88 Liberia Avenue Liberia Avenue Approximately Tributary No. 1 2,100 feet Countywide vertical datum Confluence with AE w/ to Cannon upstream of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Cannon Branch Floodway Branch University NGVD29 to convert to NAVD88 Boulevard Approximately 1,140 feet Tributary No. 1 Confluence with Countywide vertical datum upstream of the AE w/ to Flat Branch Flat Branch USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to confluence with Floodway Tributary B Tributary B NGVD29 to convert to NAVD88 Flat Branch Tributary B

67 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Tributary No. 1 Confluence with 523 feet Anderson AE w/ to Flat Branch Flat Branch HEC-RAS 1/19/2005 LOMR 04-03-111P upstream of Method Floodway Tributary C Tributary C Grant Avenue Tributary No. 1 Approximately Confluence with Countywide vertical datum to Marumsco 445 feet AE w/ Marumsco Creek USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Creek Tributary downstream of Floodway Tributary B NGVD29 to convert to NAVD88 B Longview Drive Approximately 105 feet east of Tributary No. 1 Approximately the intersection To Marumsco 445 feet AE w/ Revised by LOMR 17-03- of Silverwood USACE HEC-1 HEC-RAS 4.1.0 7/12/2018 Creek Tributary downstream of Floodway 1825P Lane and B Longview Drive Jefferson Davis Highway Approximately Tributary No. 1 Countywide vertical datum Confluence with 830 feet AE w/ to Quantico USACE HEC-1 USACE HEC-2 4/1/1977 conversion of -0.8 applied to Quantico Creek upstream of Eby Floodway Creek NGVD29 to convert to NAVD88 Street Approximately Tributary No. 2 Countywide vertical datum Confluence with 530 feet AE w/ to Russia USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Russia Branch upstream of Floodway Branch NGVD29 to convert to NAVD88 railroad Approximately Tributary No. 3 Confluence with Countywide vertical datum 560 feet AE w/ to Flat Branch Flat Branch USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to upstream of Floodway Tributary A Tributary A NGVD29 to convert to NAVD88 Traveller Street Approximately 725 feet Tributary No. 3 Confluence with northwest of the Countywide vertical datum AE w/ to Flat Branch Flat Branch intersection of USACE HEC-1 USACE HEC-2 8/1/1992 conversion of -0.8 applied to Floodway Tributary B Tributary B Old Centreville NGVD29 to convert to NAVD88 Road and Mclean Way

68 Table 12: Summary of Hydrologic and Hydraulic Analyses (continued) Hydrologic Hydraulic Date Study Limits Study Limits Model or Method Model or Method Analyses Flood Zone Flooding Source Downstream Limit Upstream Limit Used Used Completed on FIRM Special Considerations Approximately Unnamed 550 feet Confluence with Tributary To upstream of Chestnut Lick HEC-HMS 4.1 HEC-RAS 10/19/2017 AE LOMR 17-03-0682P Chestnut Lick confluence with Tributary No. 1 Tributary No. 1 Chestnut Lick Tributary No. 1 Unnamed Confluence with Tributary To John Marshall AE w/ North Fork Broad PondPack V8i HEC-RAS 3/18/2013 LOMR 12-03-0457P North Fork Highway Floodway Run Broad Run Approximately 430 feet Approximately Countywide vertical datum southwest of the 1,500 feet AE w/ conversion of -0.8 applied to Winters Branch signal Hill Road USACE HEC-1 USACE HEC-2 8/1/1992 upstream of Floodway NGVD29 to convert to NAVD88 And Fairview Wellington Road Avenue intersection

69 Table 13: Roughness Coefficients Flooding Source Channel “n” Overbank “n” All Streams Studied By Approximate Methods - Middle 0.045 0.045 – 0.120 Potomac-Anacostia-Occoquan All Streams Studied By Approximate Methods - Lower 0.045 0.045 – 0.120 Potomac All Streams Studied By Approximate Methods - 0.045 – 0.055 0.045 – 0.120 Rapidan-Upper Rappahannock Watershed Broad Run 0.045 0.100 Bull Run 0.040 – 0.065 0.080 – 0.100 Bull Run Tributary A 0.020 – 0.050 0.050 – 0.080 Bull Run Tributary B 0.017 – 0.100 0.050 – 0.100 Bull Run Tributary C 0.020 – 0.055 0.060 – 0.080 Bull Run Tributary D 0.045 0.080 Cannon Branch 0.020 – 0.040 0.060 – 0.080 Catharpin Creek 0.045 0.100 Cedar Run 0.040 0.080 Chopawamsic Creek 0.045 – 0.050 0.013 – 0.120 Cockrell Branch 0.020 – 0.045 0.060 Cow Branch 0.041 – 0.060 0.060 – 0.120 Dawkins Branch 0.045 0.100 Farm Creek 0.025 – 0.030 0.050 Flat Branch 0.040 – 0.050 0.020 – 0.100 Flat Branch Tributary A 0.025 – 0.050 0.060 – 0.090 Flat Branch Tributary B 0.020 – 0.050 0.030 – 0.050 Flat Branch Tributary C 0.020 – 0.050 0.030 – 0.060 Holkums Branch 0.035 – 0.045 0.020 – 0.100 Hooes Run 0.020 – 0.040 0.080 Kettle Run 0.045 0.080 Little Bull Run 0.050 0.100 Little Creek 0.015 – 0.060 0.030 – 0.130 Marumsco Creek 0.045 0.100 Marumsco Creek Tributary A 0.030 – 0.055 0.100 Marumsco Creek Tributary B 0.025 – 0.050 0.090 Neabsco Creek 0.035 – 0.050 0.075 – 0.100 Neabsco Creek Tributary A 0.050 0.080 North Fork Broad Run 0.045 – 0.065 0.100 – 0.135 North Fork Broad Run Bypass n/a n/a Spillway Occoquan River 0.025 – 0.045 0.012 – 0.100 Potomac River 0.030 0.060

70 Table 13: Roughness Coefficients (continued) Flooding Source Channel “n” Overbank “n” Powells Creek 0.050 – 0.060 0.100 Purcell Branch 0.050 0.100 Quantico Creek 0.020 – 0.060 0.030 – 0.100 Rocky Branch 0.045 0.100 Russia Branch 0.020-0.050 0.050-0.080 South Run 0.020-0.035 0.050-0.080 Tributary No. 1 to Cannon 0.020 – 0.050 0.040 – 0.050 Branch Tributary No. 1 to Flat Branch 0.020 0.040 Tributary B Tributary No. 1 to Flat Branch 0.020 – 0.035 0.030 – 0.100 Tributary C Tributary No. 1 to Marumsco Creek Tributary B 0.030 – 0.050 0.100 Tributary No. 1 to Quantico 0.020 – 0.030 0.030 – 0.060 Creek Tributary No. 2 to Russia Branch 0.030 – 0.070 0.080 – 0.110 Tributary No. 3 to Flat Branch 0.020 – 0.060 0.070 – 0.110 Tributary A Tributary No. 3 to Flat Branch 0.040 – 0.050 0.020 – 0.030 Tributary C Winters Branch 0.040 – 0.050 0.070

5.3 Coastal Analyses

For the areas of Prince William County that are impacted by coastal flooding processes, coastal flood hazard analyses were performed to provide estimates of coastal BFEs. Coastal BFEs reflect the increase in water levels during a flood event due to extreme tides and storm surge as well as overland wave effects.

The following subsections provide summaries of how each coastal process was considered for this FIS Report. Greater detail (including assumptions, analysis, and results) is available in the archived project documentation. Table 14 summarizes the methods and/or models used for the coastal analyses. Refer to Section 2.5.1 for descriptions of the terms used in this section.

71 Table 14: Summary of Coastal Analyses Model or Date Analysis Flooding Study Limits Study Limits Hazard Method was Source From To Evaluated Used Completed Fairfax Stafford Potomac County/ County/ River/ Prince Prince Storm Surge ADCIRC 05/13/2011 Occoquan William William River County County Boundary Boundary Fairfax Stafford Potomac County/ County/ River/ Prince Prince Waves SWAN 05/13/2011 Occoquan William William River County County Boundary Boundary Fairfax Stafford Potomac County/ County/ River/ Prince Prince Statistical JPM/EST 05/01/2010 Occoquan William William Analyses River County County Boundary Boundary Fairfax Stafford Potomac County/ County/ Overland River/ Prince Prince Wave WHAFIS 08/03/2015 Occoquan William William Propagation River County County Boundary Boundary Fairfax Stafford Potomac County/ County/ River/ Prince Prince SPM/TAW/ Wave Runup 08/03/2015 Occoquan William William CSHORE River County County Boundary Boundary

5.3.1 Total Stillwater Elevations The total stillwater elevations (stillwater including storm surge plus wave setup) for the 1- percent-annual-chance flood were determined for areas subject to coastal flooding. The models and methods that were used to determine storm surge and wave setup are listed in Table 14. The stillwater elevation that was used for each transect in coastal analyses is shown in Table 16, “Coastal Transect Parameters.” Figure 8 shows an example of the total stillwater elevations for the 1-percent-annual-chance flood that was determined for this coastal analysis.

72 Figure 8: 1% Annual Chance Total Stillwater Elevations for Coastal Areas

73 Astronomical Tide

Astronomical tidal statistics were generated directly from local tidal constituents by sampling the predicted tide at random times throughout the tidal epoch.

Storm Surge Statistics

Storm surge is modeled based on characteristics of actual storms responsible for significant coastal flooding. The characteristics of these storms are typically determined by statistical study of the regional historical record of storms or by statistical study of tidal gages.

When historic records are used to calculate storm surge, characteristics such as the strength, size, track, etc., of storms are identified by site. Storm data was used in conjunction with numerical hydrodynamic models to determine the corresponding storm surge levels. An extreme value analysis was performed on the storm surge modeling results to determine a stillwater elevation for the 1-percent-annual-chance event.

Tidal gages can be used instead of historic records of storms when the available tidal gage record for the area represents both the astronomical tide component and the storm surge component. Table 15 provides the gage name, managing agency, gage type, gage identifier, start date, end date, and statistical methodology applied to each gage used to determine the stillwater elevations.

Table 15: Tide Gage Analysis Specifics

Managing Agency of Tide Gage Statistical Gage Name Record Gage Type Start Date End Date Methodology 8638863 NOAA Tide 01/26/1975 08/31/2008 EST 8638610 NOAA Tide 07/01/1927 08/31/2008 EST 8575512 NOAA Tide 08/01/1928 08/31/2008 EST 8574680 NOAA Tide 07/01/1902 08/31/2008 EST 8557380 NOAA Tide 01/01/1957 08/31/2008 EST 8536110 NOAA Tide 11/01/1965 08/31/2008 EST 8594900 NOAA Tide 04/01/1965 08/31/2008 EST

Wave Setup Analysis

Wave setup was computed during the storm surge modeling through the methods and models listed in Table 14 and included in the frequency analysis for the determination of the total stillwater elevations. The oscillating component of wave setup, dynamic wave setup, was calculated for areas subject to wave runup hazards.

5.3.2 Waves

The methodology for analyzing the effects of wave heights associated with coastal storm surge flooding is described in the National Academy of Sciences (NAS) report (NAS).

74 This method is based on three major concepts. First, depth-limited waves in shallow water reach a maximum breaking height that is equal to 0.78 times the stillwater depth, and the wave crest is 70 percent of the total wave height above the stillwater level. The second major concept is that the wave height may be diminished by the dissipation of energy due to the presence of obstructions such as sand dunes, dikes, seawalls, buildings, and vegetation. The amount of energy dissipation is a function of the physical characteristics of the obstruction and is determined by procedures described in reference (NAS). 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.

5.3.3 Coastal Erosion

This section is not applicable to this Flood Risk Project.

5.3.4 Wave Hazard Analyses

Overland wave hazards were evaluated to determine the combined effects of ground elevation, vegetation, and physical features on overland wave propagation and wave runup. These analyses were performed at representative transects along all shorelines for which waves were expected to be present during the floods of the selected recurrence intervals. The results of these analyses were used to determine elevations for the 1- percent-annual-chance flood.

Transect locations were chosen with consideration given to the physical land characteristics as well as development type and density so that they would closely represent conditions in their locality. Additional consideration was given to changes in the total stillwater elevation. Transects were spaced close together in areas of complex topography and dense development or where total stillwater elevations varied. In areas having more uniform characteristics, transects were spaced at larger intervals. Transects shown in Figure 9, “Transect Location Map,” are also depicted on the FIRM. Table 16 provides the location, stillwater elevations, and starting wave conditions for each transect evaluated for overland wave hazards. In this table, “starting” indicates the parameter value at the beginning of the transect.

Wave Height Analysis

Wave height analyses were performed to determine wave heights and corresponding wave crest elevations for the areas inundated by coastal flooding and subject to overland wave propagation hazards. Refer to Figure 6 for a schematic of a coastal transect evaluated for overland wave propagation hazards.

Wave heights and wave crest elevations were modeled using the methods and models listed in Table 14, “Summary of Coastal Analyses”.

Wave Runup Analysis

Wave runup analyses were performed to determine the height and extent of runup beyond the limit of stillwater inundation for the 1-percent-annual-chance flood. Wave runup elevations were modeled using the methods and models listed in Table 14.

75 Table 16: Coastal Transect Parameters

Starting Stillwater Elevations (ft NAVD88) Starting Wave Conditions for the Range of Stillwater Elevations 1% Annual Chance (ft NAVD88) Significant Peak Wave Flood Coastal Wave Height Period 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual Source Transect Hs (ft) Tp (sec) Chance Chance Chance Chance Chance Occoquan 4.9 * 5.9 6.3 9.1 1 1.7 2.7 River 4.9-4.9 * 5.8-5.9 6.3-6.3 9.1-9.2

Potomac 4.9 * 5.9 6.3 9.0 2 1.6 2.7 River 4.9-4.9 * 5.8-5.9 6.2-6.3 9.0 -9.1

Potomac 4.8 * 5.8 6.2 8.8 3 2.2 2.9 River 4.8-4.8 * 5.8-5.8 6.2-6.2 8.8-8.9

Potomac 4.8 * 5.8 6.2 8.8 4 2.8 3.3 River 4.8-4.8 * 5.8-5.8 6.2-6.2 8.8-8.8

Potomac 4.8 * 5.9 6.2 8.9 5 3.1 3.4 River 4.8-4.9 * 5.8-5.9 6.2-6.2 8.9-8.9

Potomac 4.8 * 5.8 6.2 8.8 6 3.1 3.4 River 4.8-4.8 * 5.8-5.8 6.2-6.2 8.8-8.8

Potomac 4.8 * 5.8 6.2 8.7 7 3.2 3.5 River 4.8-4.8 * 5.8-5.8 6.2-6.2 8.7-8.7

Potomac 4.8 * 5.8 6.2 8.6 8 3.4 3.7 River 4.8-4.8 * 5.8-5.8 6.2-6.2 8.5-8.6

Potomac 4.8 * 5.8 6.2 8.4 9 3.3 3.7 River 4.8-4.8 * 5.8-5.8 6.1-6.3 8.4-8.7

Potomac 4.7 * 5.8 6.2 8.4 10 3.4 3.7 River 4.7-4.8 * 5.8-5.8 6.2-6.2 8.4-8.4

76 Table 16: Coastal Transect Parameters (continued)

Starting Stillwater Elevations (ft NAVD88) Starting Wave Conditions for the Range of Stillwater Elevations 1% Annual Chance (ft NAVD88) Significant Peak Wave Flood Coastal Wave Height Period 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual Source Transect Hs (ft) Tp (sec) Chance Chance Chance Chance Chance Potomac 4.7 * 5.8 6.1 8.3 11 3.2 3.4 River 4.7-4.7 * 5.8-5.8 6.1-6.1 8.3-8.3

Potomac 4.7 * 5.8 6.1 8.3 12 3.1 3.4 River 4.7-4.8 * 5.8-5.9 6.1-6.2 8.3-8.5

Potomac 4.7 * 5.8 6.1 8.1 13 3.0 3.3 River 4.7-4.7 * 5.8-5.8 6.1-6.1 8.1-8.1

Potomac 4.7 * 5.8 6.1 8.0 14 3.0 3.5 River 4.6-4.7 * 5.8-5.8 6.1-6.1 8.0-8.0

Potomac 4.6 * 5.8 6.1 7.9 15 3.1 3.8 River 4.6-4.7 * 5.8-5.8 6.1-6.2 7.9-8.7

Potomac 4.6 * 5.8 6.1 7.9 16 3.3 4.0 River 4.6-4.6 * 5.7-5.8 6.1-6.1 7.9-7.9

Potomac 4.6 * 5.8 6.1 7.8 17 3.4 4.1 River 4.6-4.6 * 5.8-5.8 6.1-6.1 7.8-7.9 *Not calculated for this Flood Risk Project

77 Figure 9: Transect Location Map

Occoquan River Unnamed Tributary Lake Omiscal No. 2 to Lake Omisol Unnamed Tributary FAIRFAX COUNTY Unnamed to Occoquan River Tributary No. 4 Neabsco Creek Tributary A Marumsco Creek to Neabsco Creek Hooes Run (1 Marumsco Creek Unnamed Tributary A 2 Tributary No. 3 ( to Neabsco Creek (3 Marumsco Creek (5 Tributary B 4 PRINCE WILLIAM ( Neabsco Creek COUNTY 510119 Marumsco Acres Lake 6 Cow Branch ( FAIRFAX COUNTY Powells Creek Farm Creek (7 (9

8 Lake ( Montclair Neabsco Creek Quantico Creek (12 10 Potomac River Powells Creek ( 11 Quantico Creek ( Tributary 2

Unnamed Tributary to Quantico Creek (15

PRINCE WILLIAM South Fork 13 TOWN OF COUNTY ( Quantico Creek ( DUMFRIES 510119 14 510120

Unnamed Tributary No. 1 to Quantico Creek Quantico Creek

Chopawamsic Creek Tributary 6

Little Creek

Chopawamsic Creek Tributary 5

TOWN OF CHARLES COUNTY Chopawamsic Creek QUANTICO MARYLAND Tributary 3 510232 16 (17 ( Chopawamsic Creek

STAFFORD COUNTY

1 inch = 5,000 feet COUNTY LOCATOR NATIONAL FLOOD INSURANCE PROGRAM 0 4,000 8,000 Transect Locator Map Feet PANELS WITH TRANSECTS: Map Projection: 0219, 0236, 0238, 0304, 0306, 0307, 0308, 0309, NAD 1983 UTM Zone 18N 0312, 0316, 0317, 0318, 0326

78 5.4 Alluvial Fan Analyses

This section is not applicable to the Flood Risk Project.

Table 17: Summary of Alluvial Fan Analyses [Not Applicable to this Flood Risk Project]

Table 18: Results of Alluvial Fan Analyses [Not Applicable to this Flood Risk Project]

SECTION 6.0 – MAPPING METHODS

6.1 Vertical and Horizontal Control

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 (NGVD29). With the completion of the North American Vertical Datum of 1988 (NAVD88), many FIS Reports and FIRMs are now prepared using NAVD88 as the referenced vertical datum.

Flood elevations shown in this FIS Report and on the FIRMs are referenced to NAVD88. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. For information regarding conversion between NGVD29 and NAVD88 or other datum conversion, visit the National Geodetic Survey website at www.ngs.noaa.gov.

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 archived project documentation associated with the FIS Report and the FIRMs for this community. Interested individuals may contact FEMA to access these data.

To obtain current elevation, description, and/or location information for benchmarks in the area, please visit the NGS website at www.ngs.noaa.gov.

The datum conversion locations and values that were calculated for Prince William County are provided in Table 19.

79 Table 19: Countywide Vertical Datum Conversion

Conversion from Quadrangle NGVD29 to Quadrangle Name Corner Latitude Longitude NAVD88 (feet) Catlett NE 38.750 -77.625 -0.792 Gainesville NE 38.875 -77.500 -0.745 Independent Hill NE 38.750 -77.375 -0.817 Joplin NE 38.625 -77.375 -0.798 Midland NE 38.625 -77.625 -0.812 Nokesville NE 38.750 -77.500 -0.818 Quantico NE 38.625 -77.250 -0.837 Somerville NE 38.625 -77.500 -0.800 Stafford NE 38.500 -77.375 -0.856 Thoroughfare Gap NE 38.875 -77.625 -0.766 Widewater NE 38.500 -77.250 -0.811 Average Conversion from NGVD29 to NAVD88 = -0.805 Feet

Table 20: Stream-Based Vertical Datum Conversion [Not Applicable to this Flood Risk Project]

6.2 Base Map

The FIRMs and FIS Report for this project have been produced in a digital format. The flood hazard information was converted to a Geographic Information System (GIS) format that meets FEMA’s FIRM Database specifications and geographic information standards. This information is provided in a digital format so that it can be incorporated into a local GIS and be accessed more easily by the community. The FIRM Database includes most of the tabular information contained in the FIS Report in such a way that the data can be associated with pertinent spatial features. For example, the information contained in the Floodway Data table and Flood Profiles can be linked to the cross sections that are shown on the FIRMs. Additional information about the FIRM Database and its contents can be found in FEMA’s Guidelines and Standards for Flood Risk Analysis and Mapping, www.fema.gov/media-library/resources-documents/collections/361.

Base map information shown on the FIRM was derived from the sources described in Table 21.

80 Table 21: Base Map Sources

Data Data Type Data Provider Date Data Scale Data Description National U.S. Geological Water area, water lines and Hydrography 2014 1:24,000 Survey attribute information (USGS 2013) Dataset Road center lines, rail center lines Roads and U.S. Census 2018 1:100,000 and attribute information (USCB Railroads Bureau 2018) Virginia Political area and boundaries for Administrative Virginia GIS Prince William County, Manassas 2018 1:6,000 Boundary Dataset Clearinghouse Park, and City of Manassas 2018 (VGIN 2018) U.S. Department of Agriculture, Virginia NAIP Digital Orthoimagery for basemap layer National 2019 1:6,000 Ortho Photo Images (NAIP 2019) Agriculture Imagery Program

6.3 Floodplain and Floodway Delineation

The FIRM shows tints, screens, and symbols to indicate floodplains and floodways as well as the locations of selected cross sections used in the hydraulic analyses and floodway computations. For riverine flooding sources, the mapped floodplain boundaries shown on the FIRM have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated using the topographic elevation data described in Table 22. For each coastal flooding source studied as part of this FIS Report, the mapped floodplain boundaries on the FIRM have been delineated using the flood and wave elevations determined at each transect; between transects, boundaries were delineated using land use and land cover data, the topographic elevation data described in Table 22, and knowledge of coastal flood processes. In ponding areas, flood elevations were determined at each junction of the model; between junctions, boundaries were interpolated using the topographic elevation data described in Table 22. 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. The floodway widths presented in this FIS Report and on the FIRM 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. Table 2 indicates the flooding sources for which floodways have been determined. The results of the floodway computations for those flooding sources have been tabulated for selected cross sections and are shown in Table 23, “Floodway Data.”

Certain flooding sources may have been studied that do not have published BFEs on the FIRMs, or for which there is a need to report the 1-percent-annual-chance flood elevations

81 at selected cross sections because a published Flood Profile does not exist in this FIS Report. These streams may have also been studied using methods to determine non- encroachment zones rather than floodways. For these flooding sources, the 1-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 the topographic elevation data described in Table 22. All topographic data used for modeling or mapping has been converted as necessary to NAVD88. The 1- percent-annual-chance elevations for selected cross sections along these flooding sources, along with their non-encroachment widths, if calculated, are shown in Table 24, “Flood Hazard and Non-Encroachment Data for Selected Streams.”

Table 22: Summary of Topographic Elevation Data used in Mapping

Source for Topographic Elevation Data Vertical Horizontal Community Flooding Source Description Accuracy Accuracy Citation Dumfries, Town of; Haymarket, Town of; All flooding Manassas, City of; Lidar source sources except Manassas Park, 2011 USGS 16.5 cm compiled to for portions of City of; Occoquan, and FEMA CVA at meet 3.28 VA Lidar Cannon Branch, Town of; LiDAR for 95th feet 2012 Trib 2 to Russia North Virginia percentile horizontal Prince William Branch, and RMSE County, Winters Branch Unincorporated Areas; Quantico, Town of Prince William Cannon Branch, 2 feet County, Trib 2 to Russia Topographic Toups 1:100 horizontal Unincorporated Branch, and Maps 1976 RMSE Areas Winters Branch Prince William Cannon Branch, 2 feet County, Trib 2 to Russia Topographic USDI 1:1,200 horizontal Unincorporated Branch, and Maps 1972 RMSE Areas Winters Branch BFEs shown at cross sections on the FIRM represent the 1-percent-annual-chance water surface elevations shown on the Flood Profiles and in the Floodway Data tables in the FIS Report. Rounded whole-foot elevations may be shown on the FIRM in coastal areas, areas of ponding, and other areas with static base flood elevations.

82 TABLE 23: FLOODWAY DATA

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Broad Run A 100 325 6,155 4.1 172.4 168.23 169.2 1.0 B 1,000 425 7,750 3.2 172.4 168.83 169.8 1.0 C 2,000 450 8,325 3.0 172.4 169.33 170.3 1.0 D 3,000 725 13,375 1.9 172.4 169.83 170.7 0.9 E 4,000 1,100 19,815 1.3 172.4 170.03 170.9 0.9 F 5,000 1,100 19,065 1.3 172.4 170.23 171.1 0.9 G 6,000 700 12,020 2.1 172.4 170.33 171.2 0.9 H 7,000 375 6,350 4.0 172.4 170.53 171.3 0.8 I 8,000 500 7,745 3.2 172.4 171.43 172.2 0.8 J 9,000 450 8,565 2.9 172.7 172.7 173.4 0.7 K 10,000 1,000 17,655 1.4 173.2 173.2 173.9 0.7 L 11,000 1,250 21,090 1.2 173.3 173.3 174.0 0.7 M 11,900 1,450 24,195 1.0 173.4 173.4 174.1 0.7 N 13,000 9502 17,175 1.1 173.5 173.5 174.2 0.7 O 14,000 2,3502 37,970 0.5 173.6 173.6 174.3 0.7 P 15,000 2,7502 41,960 0.4 173.6 173.6 174.3 0.7 Q 16,000 2,000 30,000 0.6 173.6 173.6 174.3 0.7 R 17,000 2,300 27,665 0.7 173.7 173.7 174.4 0.7 S 18,000 2,384 30,650 0.6 173.7 173.7 174.5 0.8

1Feet above confluence with the Occoquan River 2Combined Kittle Run/Broad Run floodway 3Elevation computed without consideration of backwater effects from the Occoquan River

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: BROAD RUN

83 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Broad Run (continued) T 19,000 488 6,710 0.1 173.7 173.7 174.5 0.8 U 26,400 583 5,981 3.0 175.1 175.1 175.9 0.8 V 28,055 600 6,098 2.9 177.1 177.1 177.6 0.5 W 31,440 1,385 13,409 1.3 178.6 178.6 179.2 0.6 X 33,160 840 7,211 2.5 179.4 179.4 180.0 0.6 Y 34,496 850 8,531 2.1 180.2 180.2 180.7 0.5 Z 35,000 258 3,137 5.6 180.4 180.4 180.9 0.5 AA 36,087 322 3,849 4.6 182.7 182.7 182.9 0.2 AB 37,588 868 6,254 2.8 183.8 183.8 184.6 0.8 AC 38,736 573 6,234 2.8 185.4 185.4 185.8 0.4 AD 40,313 1,196 13,848 1.2 187.7 187.7 188.2 0.5 AE 41,766 453 3,572 4.8 188.6 188.6 189.0 0.4 AF 43,117 207 1,870 9.2 191.5 191.5 192.0 0.5 AG 44,915 300 3,250 5.3 196.6 196.6 196.8 0.2 AH 45,405 300 2,830 6.1 197.2 197.2 197.4 0.2 AI 45,880 258 2,470 7.0 198.1 198.1 198.5 0.4 AJ 47,075 650 6,241 2.8 201.1 201.1 201.4 0.3 AK 47,810 650 5,356 3.2 201.6 201.6 201.9 0.3 AL 48,780 400 3,481 4.9 202.6 202.6 203.0 0.4 AM 50,655 500 4,914 3.4 204.0 204.0 204.8 0.8 AN 53,948 879 8,398 2.0 207.4 207.4 207.8 0.4 AO 57,183 917 5,651 3.0 208.6 208.6 209.5 0.9 AP 58,018 366 3,310 5.1 209.6 209.6 210.6 1.0

1Feet above confluence with the Occoquan River

TABLE TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA

23 AND INCORPORATED AREAS

FLOODING SOURCE: BROAD RUN

84 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Broad Run (continued) AQ 59,1081 257 2,573 6.5 211.9 211.9 212.6 0.7 AR 60,1441 338 2,929 5.7 214.6 214.6 215.1 0.5 AS 61,2741 330 3,373 5.0 217.5 217.5 217.7 0.2 AT 63,7691 506 4,365 3.8 219.4 219.4 219.8 0.4 AU 64,3731 342 3,455 4.8 220.2 220.2 220.6 0.4 AV 65,7911 500 5,088 3.2 222.4 222.4 222.7 0.3 AW 66,6631 215 2,117 7.8 223.1 223.1 223.8 0.7 AX 68,4801 301 2,470 6.6 227.9 227.9 228.5 0.6 AY 68,9601 223 1,940 8.5 229.7 229.7 229.9 0.2 AZ 69,6251 266 2,660 6.2 232.8 232.8 232.9 0.1 BA 71,0501 300 3,970 4.1 235.5 235.5 235.5 0.0 BB 71,5251 500 6,645 2.5 236.2 236.2 236.3 0.1 BC 72,4701 500 5,865 2.8 236.7 236.7 236.9 0.2 BD 75,6101 600 5,555 3.0 238.8 238.8 239.4 0.6 BE 78,0601 900 6,970 2.4 240.6 240.6 241.4 0.8 BF 80,4651 400 2,730 6.0 242.8 242.8 243.8 1.0 BG 82,3751 240 2,655 6.2 248.4 248.4 248.8 0.4

Broad Run Side Channel A 1,9202 1,315 9,994 1.7 173.8 173.8 174.5 0.7 B 3,1902 510 4,736 3.7 174.1 174.1 174.8 0.6

1Feet above confluence with the Occoquan River 2Feet above convergence with Broad Run

TABLE TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA 23 AND INCORPORATED AREAS FLOODING SOURCE: BROAD RUN – BROAD RUN SIDE CHANNEL

85 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH2 WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run A 90 549/258 10,070 2.9 135.8 132.83 133.8 1.0 B 1,405 579/305 10,895 2.7 135.8 133.03 134.0 1.0 C 2,480 500/329 9,480 3.1 135.8 133.13 134.0 0.9 D 3,525 352/232 7,445 3.9 135.8 133.23 134.1 0.9 E 4,660 747/314 14,640 2.0 135.8 133.63 134.5 0.9 F 5,740 453/257 8,745 3.3 135.8 133.63 134.5 0.9 G 6,430 960/428 19,045 1.5 135.8 133.83 134.7 0.9 H 7,840 425/215 8,830 3.3 135.8 134.63 135.4 0.8 I 9,015 425/208 8,725 3.3 135.8 134.83 135.6 0.8 J 10,340 425/247 8,830 3.3 135.8 135.03 135.8 0.8 K 11,610 500/159 11,525 2.5 135.8 135.23 135.9 0.7 L 12,970 500/61 8,400 3.5 135.8 135.33 136.0 0.7 M 14,480 400/324 6,915 4.2 135.8 135.63 136.3 0.7 N 16,670 400/346 7,100 4.1 136.7 136.7 137.5 0.8 O 17,640 300/101 5,160 5.6 136.9 136.9 137.7 0.8 P 18,680 500/414 8,110 3.6 137.9 137.9 138.7 0.8 Q 20,445 500/34 8,355 3.5 138.6 138.6 139.4 0.8 R 21,545 500/81 6,655 4.4 139.1 139.1 139.8 0.7 S 22,415 500/157 7,980 3.6 140.0 140.0 140.7 0.7

1Feet above confluence with the Occoquan River 2Width/width within county boundary 3Elevation computed without consideration of backwater effects from Occoquan River

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS FLOODING SOURCE: BULL RUN

86 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH2 WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run (continued) T 23,305 350/249 6,470 4.5 140.4 140.4 141.1 0.7 U 24,535 300/89 5,470 5.3 141.0 141.0 141.7 0.7 V 26,750 250/118 4,915 5.9 142.7 142.7 143.4 0.7 W 28,155 200/61 4,185 6.9 143.8 143.8 144.5 0.7 X 30,195 275/109 5,110 5.7 145.9 145.9 146.4 0.5 Y 31,305 275/189 5,560 5.2 146.9 146.9 147.4 0.5 Z 32,635 225/97 4,340 6.7 147.7 147.7 148.2 0.5 AA 34,025 275/187 5,470 5.3 149.2 149.2 149.6 0.4 AB 35,410 225/81 4,265 6.4 150.2 150.2 150.6 0.4 AC 37,280 198/55 3,625 7.6 151.9 151.9 152.3 0.4 AD 37,745 200/55 3,940 7.0 152.8 152.8 153.3 0.5 AE 39,850 200/99 4,000 6.9 154.8 154.8 155.2 0.4 AF 41,000 200/87 4,075 6.8 156.0 156.0 156.4 0.4 AG 42,050 300/122 6,070 4.5 157.3 157.3 157.7 0.4 AH 44,050 300/195 5,880 4.7 158.4 158.4 158.8 0.4 AI 45,000 425/52 9,250 3.0 159.3 159.3 159.7 0.4 AJ 46,300 600/130 11,880 2.3 159.7 159.7 160.1 0.4 AK 48,525 405/372 8,663 3.2 160.1 160.1 160.6 0.5 AL 50,800 600/516 12,980 2.1 161.2 161.2 161.6 0.4

1Feet above confluence with the Occoquan River 2Width/width within county boundary

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: BULL RUN

87 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH2 WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run (continued) AM 52,780 998/748 17,833 1.5 161.8 161.8 162.2 0.4 AN 53,225 950/878 13,659 2.0 162.0 162.0 162.4 0.4 AO 55,030 950/235 13,775 2.0 162.3 162.3 162.9 0.6 AP 57,680 1,950/531 33,205 0.8 162.7 162.7 163.3 0.6 AQ 57,950 1,650/804 28,670 1.0 163.0 163.0 163.6 0.6 AR 60,120 1,400/55 19,700 1.4 163.1 163.1 163.7 0.6 AS 64,875 1,800/1,251 21,665 0.9 163.8 163.8 164.4 0.6 AT 68,500 1,700/213 18,625 1.1 164.2 164.2 164.9 0.7 AU 70,415 1,650/207 14,395 1.4 164.6 164.6 165.3 0.7 AV 73,120 850/772 7,935 2.5 165.8 165.8 166.5 0.7 AW 75,035 1,225/1,112 12,050 1.7 166.9 166.9 167.6 0.7 AX 75,710 1,000/902 9,670 2.1 169.7 169.7 170.1 0.4 AY 78,295 550/51 5,875 3.4 171.2 171.2 172.0 0.8 AZ 80,300 600/33 6,415 3.1 173.4 173.4 174.3 0.9 BA 82,115 800/326 9,185 2.1 174.7 174.7 175.6 0.9 BB 83,435 1,150/1,085 12,750 1.5 175.2 175.2 176.2 1.0 BC 83,990 1,475/1,367 17,920 1.1 178.1 178.1 178.7 0.6 BD 86,875 750/34 7,975 2.4 178.6 178.6 179.2 0.6 BE 89,595 850/418 8,955 2.1 180.0 180.0 180.7 0.7

1Feet above confluence with the Occoquan River 2Width/width within county boundary

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: BULL RUN

88 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH2 WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run (continued) BF 93,925 850/75 8,200 2.3 182.5 182.5 183.2 0.7 BG 96,135 600/57 4,565 4.2 184.6 184.6 185.2 0.6 BH 99,405 600/43 5,880 3.2 189.0 189.0 189.7 0.7 BI 100,950 209/10 2,825 6.7 190.7 190.7 191.4 0.7 BJ 101,500 450/35 5,365 3.5 192.4 192.4 192.9 0.5 BK 103,235 350/272 4,065 3.2 193.9 193.9 194.5 0.6 BL 104,275 450/16 4,215 3.1 194.9 194.9 195.5 0.6 BM 105,530 300/51 2,585 5.0 196.5 196.5 197.1 0.6 BN 107,130 300/182 2,720 4.8 200.9 200.9 201.4 0.5 BO 107,450 400/107 4,480 2.9 203.1 203.1 203.4 0.3 BP 110,075 400/333 3,820 3.4 205.5 205.5 206.0 0.5 BQ 111,190 271/33 2,055 6.3 207.2 207.2 208.1 0.9 BR 112,115 300/201 2,715 4.8 211.4 211.4 211.5 0.1 BS 114,965 500/18 4,860 2.7 215.4 215.4 215.6 0.2 BT 117,160 500/409 4,550 2.9 217.1 217.1 217.6 0.5 BU 119,070 325/48 2,585 5.0 219.5 219.5 220.3 0.8 BV 121,005 500/424 5,000 2.6 222.4 222.4 223.3 0.9 BW 123,320 500/185 4,345 3.0 224.5 224.5 225.3 0.8 BX 127,145 600/526 4,815 2.7 229.0 229.0 229.8 0.8

1Feet above confluence with the Occoquan River 2Width/width within county boundary

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: BULL RUN

89 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH2 WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run (continued) BY 130,390 500/296 3,190 4.1 234.9 234.9 235.8 0.9 BZ 132,650 400/46 3,235 4.0 241.9 241.9 242.4 0.5 CA 134,420 500/2 3,295 3.9 246.9 246.9 247.4 0.5 CB 135,570 300/208 3,255 4.0 253.7 253.7 254.0 0.3 CC 137,575 450/38 3,780 3.4 256.8 256.8 257.1 0.3 CD 138,850 338/114 1,700 7.6 260.5 260.5 260.7 0.2 CE 140,320 200/40 1,720 3.4 269.5 269.5 269.6 0.1 CF 141,465 225/72 1,240 4.6 272.0 272.0 272.3 0.3 CG 142,610 248/21 1,405 4.0 276.7 276.7 277.0 0.3 CH 143,600 300/160 1,835 3.0 281.0 281.0 281.8 0.8 CI 144,000 300/260 2,145 2.6 281.9 281.9 282.5 0.6 CJ 145,250 300/8 1,785 3.0 283.8 283.8 284.3 0.5 CK 145,425 300/40 1,925 2.8 284.2 284.2 284.6 0.4 CL 146,900 250/14 1,400 3.7 286.9 286.9 287.9 1.0 CM 148,230 250/23 1,520 3.3 290.9 290.9 291.8 0.9 CN 149,690 250/0 1,400 3.5 295.4 295.4 295.9 0.5 CO 151,385 250/89 1,520 3.1 299.7 299.7 300.3 0.6 CP 152,570 150/31 755 2.9 301.9 301.9 302.8 0.9 CQ 153,790 150/27 510 4.1 306.5 306.5 307.0 0.5

1Feet above confluence with the Occoquan River 2Width/width within county boundary

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: BULL RUN

90 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH2 WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run (continued) CR 155,000 200/103 770 2.6 311.6 311.6 312.3 0.7 CS 155,860 150/0 500 3.8 315.0 315.0 315.7 0.7 CT 156,790 150/106 665 2.7 319.2 319.2 319.7 0.5 CU 157,890 150/24 515 3.2 322.7 322.7 323.5 0.8 CV 158,985 150/3 580 2.7 327.1 327.1 327.5 0.4 CW 160,000 150/52 400 3.6 331.3 331.3 331.9 0.6 CX 160,980 100/13 370 3.6 338.6 338.6 338.7 0.1 CY 161,700 150/82 240 5.3 345.2 345.2 345.2 0.0 CZ 162,275 200/121 465 2.6 351.0 351.0 351.7 0.7 DA 162,685 400/147 1,630 0.7 357.4 357.4 357.4 0.0 DB 163,265 150/60 205 5.4 361.8 361.8 361.8 0.0 DC 163,845 100/16 245 4.3 369.8 369.8 369.8 0.0 DD 164,640 61/6 135 7.0 379.7 379.7 379.8 0.1 DE 165,435 100/20 245 3.6 391.3 391.3 391.3 0.0 DF 165,635 150/48 395 2.2 394.5 394.5 395.2 0.7 DG 166,205 123/7 160 5.0 400.5 400.5 400.5 0.0 DH 166,775 125/42 250 2.9 408.3 408.3 408.6 0.3 DI 166,975 100/27 275 2.6 411.6 411.6 411.7 0.1

1Feet above confluence with the Occoquan River 2Width/width within county boundary

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: BULL RUN

91 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run Tributary A A 1,600 31 142 10.1 161.4 161.4 162.2 0.8 B 2,360 80 698 1.6 180.6 180.6 181.0 0.4 C 3,006 36 125 8.7 180.6 180.6 180.7 0.1 D 3,726 77 214 5.1 188.1 188.1 188.1 0.0 E 4,400 93 341 3.2 195.6 195.6 196.5 0.9 F 5,500 36 126 5.9 202.2 202.2 202.6 0.4 G 6,400 32 93 8.1 216.0 216.0 216.0 0.0 H 6,900 35 134 5.6 222.1 222.1 223.0 0.9 I 7,545 23 177 2.3 247.4 247.4 247.8 0.4 J 7,975 10 28 8.2 252.9 252.9 253.2 0.3 K 8,795 28 45 1.6 275.7 275.7 275.8 0.1

Bull Run Tributary B A 410 248 3,970 0.2 159.9 159.02 160.0 1.0 B 610 160 2,000 0.3 159.9 159.02 160.0 1.0 C 920 177 2,110 0.3 159.9 159.02 160.0 1.0 D 1,630 128 1,020 0.6 159.9 159.12 160.1 1.0 E 2,330 70 320 1.7 159.9 159.62 160.6 1.0 F 2,780 50 170 3.0 161.9 161.9 162.9 1.0 G 2,980 190 910 0.6 165.0 165.0 166.0 1.0 H 3,580 60 160 3.0 168.6 168.6 169.6 1.0 I 4,240 17 72 8.8 175.3 175.3 175.8 0.5

1Feet above confluence with Bull Run 2Elevation computed without consideration of backwater effects from Bull Run

TABLE 9 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS FLOODING SOURCE: BULL RUN TRIBUTARY A – BULL RUN TRIBUTARY B

92 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run Tributary B (continued) J 4,440 26 146 4.3 177.9 177.9 178.3 0.4 K 5,040 53 130 4.7 182.2 182.2 182.5 0.3 L 5,240 34 147 4.2 185.5 185.5 186.0 0.5 M 5,800 15 60 5.1 191.4 191.4 192.4 1.0 N 6,260 18 70 3.2 196.4 196.4 197.4 1.0 O 6,460 97 410 0.5 199.8 199.8 200.8 1.0 P 7,060 11 40 3.1 204.6 204.6 205.6 1.0 Q 7,560 11 30 3.4 213.8 213.8 214.8 1.0 R 7,760 90 590 0.2 219.8 219.8 220.8 1.0 S 8,450 10 30 3.1 230.5 230.5 231.5 1.0 T 9,250 7 20 3.5 247.0 247.0 248.0 1.0

Bull Run Tributary C A 1,110 30 96 3.1 167.5 167.5 167.6 0.1 B 1,260 52 279 1.1 172.1 172.1 172.2 0.1 C 1,600 42 49 6.2 173.5 173.5 173.5 0.0 D 2,000 40 53 5.7 183.5 183.5 183.5 0.0 E 2,400 29 37 5.4 192.0 192.0 192.1 0.1 F 2,570 81 371 0.5 200.4 200.4 200.4 0.0 G 3,030 20 29 6.9 206.8 206.8 206.8 0.0

1Feet above confluence with Bull Run

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS FLOODING SOURCE: BULL RUN TRIBUTARY B – BULL RUN TRIBUTARY C

93 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Bull Run Tributary D A 690 400 1,135 2.7 164.8 158.32 159.3 1.0 B 1,190 200 665 4.6 164.8 160.52 161.3 0.8 C 1,820 100 430 6.8 164.8 164.62 165.1 0.5 D 2,350 100 530 5.5 168.1 168.1 168.6 0.5 E 2,985 100 535 5.5 171.1 171.1 171.9 0.8 F 4,205 75 335 4.2 177.0 177.0 177.7 0.7 G 4,655 75 275 5.1 179.2 179.2 179.6 0.4 H 5,145 60 230 6.0 182.8 182.8 182.8 0.0 I 5,680 60 230 6.1 186.3 186.3 186.6 0.3 J 6,405 206 1,720 0.9 200.6 200.6 200.6 0.0 K 6,980 110 420 1.9 200.6 200.6 200.6 0.0 L 7,150 92 270 3.0 200.8 200.8 200.8 0.0

1Feet above confluence with Bull Run 2Elevation computed without consideration of backwater effects from Bull Run

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: BULL RUN TRIBUTARY D

94 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Cannon Branch A 3,900 256 948 3.0 176.2 176.2 176.6 0.4 B 4,900 170 694 4.1 179.0 179.0 179.4 0.4 C 6,000 376 1,462 1.9 181.3 181.3 181.8 0.5 D 6,900 162 722 3.9 182.6 182.6 183.0 0.4 E 7,625 317 3,216 1.5 191.3 191.3 191.3 0.0 F 8,867 598 8,748 0.5 193.8 193.8 193.8 0.0 G 9,623 312 5,115 0.6 193.8 193.8 193.8 0.0 H 11,387 115 728 2.7 194.0 194.0 194.5 0.5 I 12,250 62 364 4.1 194.6 194.6 195.3 0.7 J 13,780 59 218 6.9 196.8 196.8 197.7 0.9 K 14,300 24 84 7.7 201.2 201.2 202.1 0.9 L 15,000 106 215 3.0 206.9 206.9 207.2 0.3

1 Feet above confluence with Broad Run

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: CANNON BRANCH

95 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Catharpin Creek A 1,0381 150 950 3.0 263.4 260.13 261.1 1.0 B 4,0381 150 760 3.8 268.8 268.8 269.3 0.5 C 6,3381 200 895 3.2 276.0 276.0 276.7 0.7 D 7,2681 100 750 3.8 281.0 281.0 281.3 0.3 E 7,7531 100 595 4.7 281.6 281.6 282.4 0.8 F 11,8031 100 565 4.6 294.8 294.8 295.6 0.8 G 15,7731 100 535 4.5 307.9 307.9 308.2 0.3

Cedar Run A 8502 339 4,510 7.4 172.4 167.34 167.3 0.0 B 1,9002 358 5,789 5.7 172.4 169.34 169.6 0.3 C 2,9002 404 6,232 5.3 172.4 170.54 170.9 0.4 D 3,9002 519 8,083 4.1 172.4 171.64 172.1 0.5 E 4,9002 419 7,363 4.5 172.4 172.24 172.7 0.5 F 5,9002 808 13,294 2.5 172.9 172.9 173.4 0.5 G 6,9002 576 8,460 3.9 173.1 173.1 173.6 0.5 H 7,9002 751 13,462 2.5 173.8 173.8 174.3 0.5 I 8,9002 393 6,934 4.8 173.8 173.8 174.4 0.6 J 10,1002 703 11,961 2.8 174.8 174.8 175.4 0.6 K 11,2002 694 13,163 2.5 175.1 175.1 175.7 0.6 L 11,9002 564 10,323 3.2 175.3 175.3 175.9 0.6 M 12,6002 697 13,003 2.6 175.6 175.6 176.2 0.6

1Feet above confluence with Little Bull Run 2Feet above confluence with Occoquan River 3Elevation computed without consideration of backwater effects from Little Bull Run 4Elevation computed without consideration of backwater effects from Occoquan

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS FLOODING SOURCE: CATHARPIN CREEK – CEDAR RUN

96 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

A 13,633 670/1302 2,611 2.4 7.3 7.3 8.0 0.7 B 14,126 815/532 3,171 2.0 9.0 9.0 9.9 0.9 C 14,615 900/172 3,800 1.6 10.3 10.3 11.2 0.9 D 15,148 1,010/552 4,412 1.4 11.3 11.3 12.2 0.9 E 15,930 1,128/02 4,437 1.4 12.7 12.7 13.5 0.8 F 16,675 1,012/02 3,768 1.7 14.4 14.4 15.2 0.8 G 17,333 1,140/02 4,231 1.3 15.9 15.9 16.7 0.8 H 17,862 1,031/02 3,984 2.3 17.7 17.7 18.4 0.7 I 17,943 1,038/02 4,946 1.1 19.2 19.2 20.1 0.9 J 18,948 800/92 3,308 1.7 20.9 20.9 21.8 0.9 K 19,594 968/132 3,563 1.6 22.3 22.3 23.1 0.8 L 20,032 679/492 2,141 2.6 23.7 23.7 24.2 0.5 M 20,151 676/502 5,075 1.1 28.3 28.3 28.4 0.1 N 20,555 1,126/122 6,373 0.9 28.3 28.3 28.5 0.2 O 21,099 948/272 4,159 1.3 28.6 28.6 28.9 0.3 P 21,589 829/422 3,809 1.5 29.0 29.0 29.5 0.5 Q 22,167 433/292 1,554 3.6 29.8 29.8 30.4 0.6 R 22,634 389/522 1,289 4.4 31.7 31.7 32.2 0.5 S 23,087 99/532 1,031 5.4 36.5 36.5 36.7 0.2 T 23,544 69/292 759 7.4 37.6 37.6 38.1 0.5 U 23,817 78/282 876 6.4 39.4 39.4 39.5 0.1

1Feet above confluence with Potomac River 2Total width/width within county

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS

FLOODING SOURCE: CHOPAWAMSIC CREEK

97 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Chopawamsic Creek (continued)

V 24,211 95/532 1,142 4.9 41.1 41.1 41.2 0.1 W 24,429 229/2072 2,192 2.6 41.6 41.6 41.9 0.3 X 24,815 382/3612 3,423 1.6 41.9 41.9 42.4 0.5

1Feet above confluence with Potomac River 2Total width/width within county

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS FLOODING SOURCE: CHOPAWAMSIC CREEK

98 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC)

Cockrell Branch A 9,6001 116 601 4.7 233.8 233.8 233.8 0.0 B 10,3621 247 2,046 1.1 238.5 238.5 238.5 0.0 C 10,6211 97 414 4.9 238.5 238.5 238.5 0.0 D 10,8981 140 383 5.6 240.1 240.1 240.1 0.0 E 11,6781 52 593 3.7 246.3 246.3 246.3 0.0 F 12,6921 67 290 8.2 249.3 249.3 250.1 0.8 G 13,3531 92 969 1.7 262.1 262.1 262.7 0.6 H 13,7211 101 925 1.0 262.2 262.2 262.7 0.5

Cow Branch A 5262 299 1,385 2.5 6.5 6.5 7.4 0.9 B 1,6082 252 737 4.8 11.0 11.0 11.7 0.7 C 2,1142 231 862 3.8 14.7 14.7 15.7 1.0 D 2,3862 60 391 8.3 15.9 15.9 16.9 1.0 E 3,6762 150 559 5.8 21.6 21.6 22.2 0.6 F 4,0812 171 803 4.0 23.7 23.7 24.6 0.9 G 5,1752 198 923 3.5 29.6 29.6 30.0 0.4 H 5,9242 64 406 7.9 33.7 33.7 34.1 0.4 I 6,1932 63 432 7.4 35.8 35.8 35.8 0.0 J 7,0962 73 302 6.8 40.5 40.5 41.1 0.6 K 7,3862 53 295 6.9 43.2 43.2 43.5 0.3

1Feet above confluence with Broad Run 2Feet above confluence with Neabsco Creek

TABLE 23 TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA AND INCORPORATED AREAS FLOODING SOURCE: COCKRELL BRANCH – COW BRANCH

99 TABLE 23: FLOODWAY DATA (continued)

1% ANNUAL CHANCE FLOOD WATER SURFACE LOCATION FLOODWAY ELEVATION (FEET NAVD88)

SECTION MEAN CROSS WIDTH WITHOUT WITH DISTANCE1 AREA VELOCITY REGULATORY INCREASE SECTION (FEET) FLOODWAY FLOODWAY (SQ. FEET) (FEET/ SEC) Cow Branch (continued) L 7,703 82 342 6.0 47.2 47.2 47.8 0.6 M 7,917 87 279 7.3 48.9 48.9 49.0 0.1 N 8,066 119 411 5.1 51.7 51.7 51.7 0.0 O 8,338 190 570 3.6 54.1 54.1 54.1 0.0 P 8,541 108 445 4.6 56.0 56.0 56.0 0.0 Q 9,133 65 399 4.9 62.1 62.1 62.1 0.0 R 9,359 94 459 4.3 63.0 63.0 63.3 0.3 S 9,498 39 185 10.3 63.0 63.0 63.3 0.3 T 9,711 33 204 9.3 65.5 65.5 65.6 0.1 U 9,905 31 153 12.5 67.1 67.1 67.1 0.0 V 10,316 35 228 8.3 76.9 76.9 76.9 0.0 W 10,890 34 214 8.9 80.9 80.9 81.0 0.1 X 11,262 27 155 12.3 84.1 84.1 84.3 0.2 Y 11,783 33 206 9.3 90.5 90.5 90.7 0.2 Z 12,254 34 168 9.4 95.9 95.9 96.0 0.1 AA 12,613 44 202 7.8 101.2 101.2 101.2 0.0 AB 13,333 158 1,868 0.8 126.4 126.4 127.1 0.7 AC 14,052 111 1,168 1.4 126.4 126.4 127.1 0.7 AD 14,417 100 899 1.8 126.5 126.5 127.2 0.7 AE 15,325 101 946 1.3 137.9 137.9 137.9 0.0 AF 16,009 138 1,311 1.0 144.7 144.7 144.7 0.0 AG 16,251 117 761 1.6 144.7 144.7 144.7 0.0 AH 16,798 94 219 7.4 145.9 145.9 146.0 0.1 AI 17,210 105 202 6.2 152.2 152.2 152.7 0.5 AJ 17,577 84 144 8.7 156.5 156.5 156.8 0.3 1 Feet above confluence with Neabsco Creek

TABLE FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA PRINCE WILLIAM COUNTY, VA

23 AND INCORPORATED AREAS FLOODING SOURCE: COW BRANCH

100