Revised Data FOURMILERUN LOCAL FLOOD PROTEGTION CITY OF ALEXANDRIA & ARLINGTON COUNTY, VIRGINIA SUPPLEMENT TO FINAL ENVIRONMENTAL STATEMENT Prepared by U.S. Army Engineer District, Baltimore March 1974 REVISED DATA TO THE SUPPLEMENT TO FINAL ENVIRONMENTAL STATEMENT FOURMILE RUN LOCAL FLOOD PROTECTION CITY OF ALEXANDRIA AND ARLINGTON COUNTY, VIRGINIA Prepared by: US ARMY ENGINEER DISTRICT, BALTIMORE BALTIMORE, MARYLAND March 1974 REVISED DATA TO THE SUPPLEMENT TO FINAL ENVIRONMENTAL STATEMENT FOURMILE RUN LOCAL FLOOD PROTECTION CITY OF ALEXANDRIA AND ARLINGTON COUNTY, VIRGINIA Table of Contents Page INTRODUCTION ~I I. PROJECT DESCRIPTION A. GENERAL 2 1. Project Purpose 2 2. Location 2 3. Benefit-Cost Ratio 2 B. SPECIFIC PROJECT CHANGES 2 1. Flood Channel Capacities 4 2. Levees and Walls 4 3. Bridge Modifications 5 4. Slope Protection 5 5. Disposal of Material 5 6. Recreation 6 7. Construction Sedimentation Control 6 8. Additional Terms of Local Cooperation 6 II. ENVIRONMENTAL SETTING WITHOUT THE PROJECT 7 III. ENVIRONMENTAL IMPACT OF THE PROPOSED ACTION Should The project be implemented 7 A. GENERAL 7 B. IMPACTS OF SPECIFIC MODIFICATIONS 7 1. Mouth of Fourmile Run Upstream to Route 1 7 2. Route 1 Bridge Complex Upstream 8 to Mt. Vernon Avenue 3. Mt. Vernon Avenue Upstream to Long Branch 9 4. Long Branch Upstream to West Glebe Road Bridge 10 5. West Glebe Road Bridge Upstream to Shirley Highway 10 6. Watershed Management for the Basin 11 Page IV. ADVERSE ENVIRONMENTAL EFFECTS WHICH CANNOT BE AVOIDED SHOULD THE PROPOSAL BE IMPLEMENTED 13 A. GENERAL 13 B. AESTHETICS 13 C. SOCIAL 13 D. ECONOMIC 14 E. PHYSICAL 14 F. ECOLOGICAL 14 V. THE RELATIONSHIP BETWEEN LOCAL SHORT-TERM USES OF MAN'S ENVIRONMENT AND THE MAINTEXAnCE AnB ENHANCEMENT OF LONG-TERM PRODUCTIVITY 14 PLATES 1. Location Map 2. Existing Conditions 3. Existing Conditions 4. Project for Which Supplement was Prepared 5. Project for Which Supplement was Prepared 6. Project Proposed Under Present Authorization 7. Project Proposed Under Present Authorization 8. Project Proposed Under Present Authorization 9. Project Proposed Under Present Authorization 10. Proposed Recreation Plan 11. Proposed Recreation Plan 12. Proposed Recreation Plan 13. Proposed Recreation Plan REVISED DATA TO THE SUPPLEMENT TO FINAL ENVIRONMENTAL STATEMENT FOURMILE RUN LOCAL FLOOD PROTECTION CITY OF ALEXANDRIA AND ARLINGTON COUNTY, VIRGINIA INTRODUCTION The final Fourmile Run Environmental Statement, dated 24 September 197C, was filed with the Council on Environmental Quality on 20 October 1970. The final Environmental Statement was prepared for the pre­ authorization study which responded to a resolution by the Committee on Public Works of the House of Representatives on 5 October 1966. The resolution requested a review of the report on the Potomac River and Tributaries and others with a view to determining if improvements in the interests of flood control are advisable at this time, with par­ ticular reference to Holmes Run, Fourmile Run, and other streams that flow through the City of Alexandria, Virginia. Following sub­ mission of the study report, the Fourmile Run project was authorized under Section 201 of the 1965 Flood Control Act (Public Law 89-298) by resolutions of the Committees on Public Works of the United States Senate and House of Representatives dated 25 June and 14 July 1970, respectively. Subsequent to authorization of the project and the accompanying final Environmental Statement, a post-authorization study indicated that design changes were necessary. As a result, a Supplement to the final Environmental Statement was prepared to outline the changes from the authorized plan and describe the resulting environmental impact changes. The Supplement, dated April 1973, was coordinated and then filed with CEQ on 24 April 1973. In addition, response to EPA’s comments of 4 June 1973 on the Supplement were forwarded to CEQ for filing with the Supplement. Subsequent to the formulation of the postauthorization project for which the Supplement was prepared, increased costs for the Federal interests indicated that re-authorization was necessary. As a result of current Congressional re-authorization, design modifications have been made to the local flood protection project to make the present project conform with the conditions of re-authorization. The information provided in this report describes the current project and revises the pertinent in­ formation provided in the Supplement. The modifications required by current authorization have been made to reduce costs and consideration has been given to the positions taken by various agencies which commented on the earlier Statements and Supplement. This report incorporates the response to EPA comments, dated 4 June 1973, which have been previously furnished the Council. This information described all the modifications of the project since the Supplement was filed and the resulting changes in environmental impacts. I. PROJECT DESCRIPTION A. General. 1. Project Purpose. The purpose of the proposed project remains unchanged. The flood control works will protect against a fluvial flood of 27, 000 cubic feet per second (c.f. s.) on Fourmile Run having a frequency occurrence of once in 100 years. Refined engineering studies at this stage, however, have resulted in changes in channel capacity at specific areas within the project as will be described in paragraph B-l. The basic configuration of the project remains unchanged, but construction elements of the specific areas have been altered as a result of the changes in channel capacities. 2. Location. The location of the project remains unchanged, extending from the Potomac River upstream along Fourmile Run to 1-95, Shirley Highway. 3. Benefit-Cost Ratio. The project has a benefit-cost ratio of 1.4 to 1.0, based on July 1973 prices and a 100-year project life at 5 5/8 percent interest rate for both Federal and non-Federal investme nt costs. B. Specific Project Changes. Changes to the project from that described in the Supplement to the final Environmental Statement include flood channel capacities; revisions to channels, walls and levee dimensions and configuration; revised interior drainage facilities; revision to spoil disposal plans; bridge modifications; revisions to the recreation plan; and potential restoration of sedimentation from construction. Additions to the project include additional terms of local cooperation. Comparative data for the project described in the supplement and the presently authorized project are presented in Table 1 and in the following paragraphs. Existing conditions are showm on Plates 2 and 3. The project for which the Supplement was prepared is shown on Plates 4 and 5. The project proposed under present authorization is shown on Plates 6 and 7. TABLE 1 COMPARISON OE PLAN FOR WHICH SUPPLEMENT WAS PREPARED AND PROJECT UNDER PRESENT AUTHORIZATION FOURMILE RUN FLOOD CONTROL PROJECT Project (EIS Present Item Supplement) Project Design Discharge, Ft Per Second Fourmile Run 27, 000 27,000 Long Branch 4, 700: 4, 700 Potomac River Design Stage*, msl 11.2 11.2 Length of Improved Channel, Feet Fourmile Run 1 1, 880 11,880 Long Branch 280 400 Length of Levee, Feet Fourmile Run 2, 250 000 Long Branch 100 GOG Flood/Retaining Wall Fourmile* Run 3, 145 3, 833 Long Branch 000 890 Interior Drainage Facilities No. Pumping Station 1 000 Ponding Area, Acre-Feet 000 000 Bridge Modifications No New Highway Bridges 2 2 No. New Railroad Bridges 4 4 No. Modified Existing Highway Bridges 2 1 Drop Structures Fourmile Run 1 000 Long Branch 1 1 -This figure was incorrectly presented in the Supplement to the final Environmental Statement as 4, 100 cfs. 1. Flood Channel Capacities Compared to the previous plan, channel bottom widths have been gen­ erally decreased from station 474+50 (at the downstream end of the pollution control plant) upstream to tne upstream limit of the project at Shirley Highway. The amount of decrease varies. From the pollution control plant upstream to Mt. Vernon Bridge, the proposed channel is reduced 25 feet in width from the earlier plan; from Mt. Vernon Bridge upstream to Long Branch, the proposed channel is reduced 50 feet from the earlier plan; from Long Branch upstream to West Glebe Road, the proposed channel is reduced by 55 feet; from West Glebe Road upstream to Shirley Highway, the amount of decrease varies from 0 feet up to 85 feet. However, in most cases, the reduction in channel capacity represented by reduced channel bottom widths has been com­ pensated for by changes in dimension and configuration of walls and side slopes of the channel as described in rhe following paragraph. The areas where this is not the case include the portion from Shirley Highway downstream to Long Branch, except for the immediate West Glebe Road bridge vicinity, and from Mt. Vernon Avenue Bridge down­ stream to the pollution control plant. It is expected that some increase in overbank flooding will occur at these points during major floods than would have occurred under the original plan. Flows in excess of 16-18, 000 c.f. s. will leave the channel in the vicinity of West Glebe Road, flow down old South Glebe Road (now a residential access road) and return to Fourmile Run through South Glebe Road. Entrance and exit structures will be provided for this flow. In addition to the changes in channel bottom widths, the channel bottom gradient from Shirley Highway downstream to near West Glebe Road bridge has been increased from gradients of 0.005 and 0.00519 to a gradient of 0.00662. This increase in gradient is the result of elimi­ nating a proposed drop structure from the earlier plan. 2. Levees and Walls. a. Levees. As shown on Table 1 and on the plates, the presently authorized plan has deleted 2, 250 linear feet of levee and the associated pumping station located in the vicinity of Mt. Vernon Avenue in Alexandria.