Environmental Assessment for Design Alternatives for the Potomac Park Levee System.

National Mall and Memorial Parks » Environmental Assessment for Design Alternatives for the Potomac Park Levee System. » Document List

The National Park Service(NPS) in cooperation with the United States Army Corps of Engineers(USACE) and the National Capital Planning Commission (NCPC) proposes to modify the Potomac Park Levee System.

The purpose of this action is to provide a reliable flood protection to a portion of the monumental core and downtown Washington D.C. in a manner that respects the resources and values of the National Mall. Without reliable flood protection measures, many major facilities in downtown Washington D.C., including portions of the monumental core, portions of Pennsylvania and Constitution Avenues, Federal Triangle, and areas south of the U.S. Capitol to Fort McNair are at risk of flooding from a major (100-year plus) flood event.

The current design and plan for flood response, designed in 1938 is a two phased closure system that includes a permanent earthen barrier that has been conformed to the design of Constitution Gardens and National Park Service (NPS) construction of temporary earthen levee across 17th Street, once notified of an impending flood.

This type of temporary levee system has not been favored by the USACE due to the large scope of this type of emergency closure project, coupled by unknown weather conditions and logistic requirements. Based on new policies since Hurricane Katrina (33 CFR 208.10), this system is unacceptable and no longer meets USACE design standards. In 1936, Congress authorized a higher level of flood protection for a 185-year flood event which is a flow of 700,000 cubic feet per second (cfs).

As a result, FEMA recently proposed changes to the 100-year floodplain map for this area (44 CFR Part 67 - Federal Register Volume 72, Number 186, page 54631-54635), which is defined by a flow of 450,000 cfs. This new mapping puts a portion of downtown Washington D.C. and the monumental core within the 100-year flood insurance rate zone, which would require additional flood insurance and/or costly changes to meet associated building codes for those facilities that now fall within the new 100-year floodplain. FEMA has agreed to delay the implementation of this new mapping, so as to allow the USACE, the District of Columbia and the NPS to design and implement an interim solution that will, at a minimum, stop the 100-year flood at 17th Street south of Constitution Avenue, which will remove these facilities from the newly designated 100-year floodplain. Unless a solution is put in place that meets FEMA accreditation by November 2009, then FEMA will issue the proposed floodplain maps and the affected area will be subject to new constraints and more stringent rules for building.

Project Objectives:

- Protect downtown Washington D.C and the monumental core in the event of a 100-year storm.
- Meet the National Flood Insurance Program requirements as administered by the Federal Emergency Management Agency (FEMA).
- Meet the required FEMA level of protection for the 100 year flood event
- Explore USACE authorized level of protection with some alternatives for a long-term solution
- Minimize adverse impacts to the cultural landscapes and other cultural resources of the monumental core.
- Minimizes adverse impacts to the view sheds of the monumental core and the visual quality of the 17th Street corridor.
- Minimize disruption to visitor use and experience in the monumental core
- Minimize adverse impacts to park management and operations.
- Increase the reliability of the NPS response to a flood event (reduce required time and costs).

March 2021 Update:

The U.S. Army Corps of Engineers (USACE), Baltimore District, in cooperation with National Park Service (NPS), has prepared a supplemental Environmental Assessment (EA) that examines alternatives actions and environmental impacts associated with the proposed Washington, D.C., and Vicinity Flood Risk Management Project Improvements (FRM) at the National Mall and Memorial Parks (NAMA or Park) in Washington, DC. The purpose of the proposed project is to improve reliability of the current levee system under extreme flood events. The project provides FRM for portions of the monumental core,
portions of Pennsylvania and Constitution Avenues, and other public and private facilities located south of the U.S. Capitol to Fort McNair. Only those portions within NAMA are discuss within this document. The project would be constructed by USACE and maintained by the NPS. NAMA's existing FRM features include a levee between the Lincoln Memorial and Washington Monument adjacent to the reflecting pool (Potomac Park Levee); a post and panel closure system at 17th Street; and a temporary closure at 23rd Street (sandbags or inflatable bladders).

Proposed improvement to the FRM project components at the Potomac Park Levee and 23rd Street were evaluated in USACE environmental assessments (EA) prepared in 1992 and 1996. However, the improvements were not constructed due to lack of funding. In 2009, the NPS prepared an EA addressing construction of the 17th Street closure structure, which was constructed in 2011. Due to changes in environmental regulations, and the development of detailed project designs since the publications of the pervious EAs, the USACE has prepared the Washington, D.C., and Vicinity Flood Risk Management Project Improvements supplemental EA. The USACE and the NPS have each prepared Finding of No Significant (FONSI) to support of the supplemental EA.

The EA was prepared in accordance with the National Environmental Policy Act of 1969(NEPA; 42 United States Code [USC] 4321 et seq.), and its implementing regulations of the Council on Environmental Quality (CEQ) for implementing NEPA (40 Code of Federal Regulations [CFR] 1500-1508.9), the Department of the Interior NEPA regulations (43 CFR Part 46); and with NPS Director's Order 12: Conservation Planning, Environmental Impact Analysis, and Decision-making (2011) and its accompanying handbook (2015). The statements and conclusions reached in this finding of no significant impact (FONSI) are based on documentation and analysis provided in the EA and associated decision file. To the extent necessary, relevant sections of the EA are incorporated by reference below.

After consultation with the USACE, review of the EA and other supporting documentation, the NPS, in accordance with 43 CFR 46.320, is adopting this EA and making its decision to allow the USACE to carry out the FRM projects improvement at the Potomac Park Levee and 23rd Street on land administered
by the NPS. The EA fulfills the requirements of NEPA and applicable regulations, and it meets the policies set forth in the DO-12, Conservation Planning, Environmental Impact Analysis and Decision-making, and accompanying Handbook.
The Federal action that that will be undertaken by the NPS is the issuance of a Special Use Permit for the construction of the Potomac Park Levee FRM improvements. The decision considers impacts to the Park resources as expressed in statute, including the Park's enabling legislation, regulation, and policy.

NPS FONSI and USACE EA can be found under the Documents List tab.

Contact Information

Tammy Stidham
Deputy Associate Area Director
Office of Lands, Resources & Planning
National Capital Area