Wheaton Branch Stormwater Management Pond
The below picture is an aerial photo of the Wheaton Branch stormwater management facility. This project significantly improved stormwater control to over 800 acres of developed drainage in the headwaters of Sligo Creek. It controls runoff from a large shopping mall and commercial complex in the Wheaton Central Business district, and other commercial and residential development between Wheaton and Dennis Avenue along the intensively developed Georgia Avenue corridor. The three-celled pond design was used to protect the existing sanitary sewer lines (shown in red) that traversed the original flood control facility site. The pond improvements control runoff from small, frequently occurring storms and larger storms that occur every 1-2 years. Runoff from these storms is detained for a period of 24-40 hours and released slowly, to minimize erosion damages downstream in the Wheaton Branch tributary of Sligo Creek. This project contributes substantially to the success of stream restoration measures installed immediately downstream on the Wheaton Branch of Sligo Creek and on the Sligo Creek main channel. [from DEP's 2003 report on Montgomery County's Commitment to Anacostia Watershed Restoration]
See http://www.fosc.org/WheatonPonds.htm for report on 2004 Dredging Operation.
The Wheaton Branch facility was once a dry detention facility that was retrofitted to provide water quality and channel protection controls. The facility, constructed in 1990, drains an 800-acre watershed that is over 50% impervious. A unique design feature was the three-cell wet pond (constructed around an existing sanitary sewer trunk main) to provide water quality controls. Extended detention controls for the 1.5 inch rainfall event were incorporated for channel protection. The three-cell pond has a complex flow path for both base flows and small stormflows to facilitate maximum settling of solids. Controls for larger storms (i.e. two - 100 year rainfall events) were balanced against upstream backflow constraints and dm safety considerations. Figure 1 illustrates the key operational and design elements of the project.
The first cell of the facility, or forebay provided almost a tenth of an inch of storage per impervious acre within the watershed (this is too small for most retrofits). A 25-foot wide access ramp with a level 30' by 30' pad was provided for future dredging of the forebay would be necessary every five years or so. The first cleanout of the forebay occurred in July 1997, a little over seven years after completion of the project. The second cleanout occurred in 2005.
The Wheaton Branch retrofit facility was part of the larger Sligo Creek watershed restoration project. Downstream habitat improvement and native fish restocking projects accompanied the retrofit and have proved very successful over their seven-year initial period.
The above description is based on a case study by Richard A. Claytor Jr. P.E. An Eight -Step Approach to Implementing Stormwater Retrofitting