Environmental Assessment for Design Alternatives for the Potomac Park Levee System.
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.
- 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).
National Capital Region
Office of Lands, Resources & Planning