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Little Seneca Reservoir Project

Little Seneca Reservoir was constructed as an emergency drinking water source for local utilities during drought conditions. Water quality and capacity of the reservoir are threatened by the increase of sediment.

Little Seneca Reservoir Background:

Located in the Black Hill Regional Park, which is operated by the Maryland National Capital Park and Planning Commission (M-NCPPC), the Little Seneca Reservoir was built from 1983-1986. This reservoir is part of the Washington Metropolitan Regional Water Supply, with water supply use and costs shared cooperatively by WSSC Water, Washington Aqueduct and Fairfax Water.

The Little Seneca Reservoir holds four billion gallons of water and serves as an emergency raw-water supply during droughts. The reservoir supplements the Potomac River flow via discharge to Little Seneca Creek.

In addition to its purpose of a supplemental water supply, the reservoir also serves as a public amenity, with thousands of visitors taking advantage of the water and surrounding land for recreational purposes.

Project Overview

After 35 years of trapping and preventing sediment from migrating into deeper sections of the reservoir, it is now time to remove sediment to allow forebays to continue functioning as required. (Forebays are located at the northern entrance of the reservoir and allow sediment to settle prior to traveling further into the body of water.) 

Project Components: 

  • Removal method - Drawdown the reservoir water level 15 feet in order to remove sediment from each forebay in a dry condition.
  • Sediment dewatering -  Stockpile excavated sediment within each respective  forebay and allow it to naturally drain and dry.
  • Construction access to each forebay – Cabin Branch will be accessed by the parking area and open space immediately west of the forebay from Black Hill Drive; Little Seneca Creek will be accessed by utilizing an old construction access road used when the reservoir was built in the 1980s; Ten Mile Creek will be accessed by utilizing Ten Mile Creek Road which was abandoned when the reservoir was built in the 1980s.
  • Sediment Disposition - Utilize the sediment as a soil amendment on nearby park land and farmland.  Other alternatives may consider use on nearby construction projects or for the production and sale of topsoil.

Proposed Little Seneca Water Drawback Map

Proposed Little Seneca Water Drawback Map

Tentative Project Schedule

  • Design and permitting: Fiscal Years 2023 – 2025
  • Construction: Fiscal Years 2026 – 2027
  • Post construction monitoring: Fiscal Year 2028
  • Schedule is subject to change and will be dependent upon availability of funding, permits, weather, and other potential factors

Presentations

November 2, 2021 - Little Seneca Reservoir Forebays Sediment Removal Presentation | Little Seneca Reservoir Forebays Sediment Removal Presentation (Español)

Contact

Thomas.Hilton@wsscwater.com - Division Manager, Planning

Questions/Comments about the Project

Last Modified: November 30, 2021, 4:24 pm EST