Watershed Management Program

In September 1974, acting on behalf of the four watershed localities, the Northern Virginia Planning District Commission (now NVRC) began a study to develop a basin-wide land management program that would satisfy the Congressional requirement. NVRC developed the Four Mile Run Computer Model, a computer-based planning tool which NVRC staff uses to analyze the potential flooding impacts of land development plans (i.e. stormwater management); staff utilizes the computer model to recommend control measures to mitigate those land development impacts. 

Four Mile Run Watershed Management Program
The Four Mile Run Management Program operates under a Memorandum of Agreement, signed in March 1977 by representatives of the four watershed localities. A Runoff Management Board, consisting of the chief administrative officers of the four jurisdictions, oversees the program's operation and resolves any policy issues or questions that result from the Technical Review Committee's deliberations.

The Technical Review Committee, comprised of engineers and planners from the public works and environmental services departments of local governments, is responsible for the technical investigations required to determine the flooding/SWM impacts of future land use changes and drainage modifications. Technical and administrative support for the program is provided by NVRC staff.

Four Mile Run Computer Model

The Watershed Management Program relies upon the Four Mile Run Computer Model to simulate the watershed's complex hydrology and to forecast interjurisdictional impacts of stormwater runoff from local land use changes and drainage modifications. Cumulative impacts are reviewed on a quarterly basis, and corrective measures are recommended if the model projects flooding problems.

NVRC staff uses the watershed model to develop projections of peak stream flows and water surface (flood) elevations at critical control points in the watershed and recommends ways in which adverse impacts can be eliminated or mitigated. The potential for a proposed land use change or drainage modification to cause flooding in the Arlandria area can be analyzed simply by the following methodology: 1) Revise the computer model's land use database or channel configuration to represent the proposed project and 2) examine the model's output. The computer model also is used to evaluate the effectiveness of various proposed stormwater detention facilities for which downstream effects would not otherwise be known. Finally, the model has been employed to help analyze a number of  "what if" scenarios related to stream flow and flooding at various user-specified locations throughout the watershed.  

To date, the computer-based watershed model has analyzed over 650 local land use changes affecting stream flow, which cumulatively represent the addition of over 200 acres of impervious ground cover.  Reflected with these land use changes in the model are 110 runoff control structures that collectively hold a total of over 32 acre-feet of detention storage. Additionally, the construction of Route I-66 through the watershed resulted in the conversion of 100 acres from open space to impervious cover as well as the addition of 47 acre-feet of detention storage to the watershed, both of which are represented by the Four Mile Run computer model.

Future of the Four Mile Run Watershed Management Program
The Four Mile Run Watershed Management Program assures that benefits achieved by the $63 million regional flood control project are not offset by increased stream flows from future land development and redevelopment projects in the basin. Comparisons with peak flows projected for watershed conditions at the inception of the program indicate net reductions in projected 100-year stream flows of up to 1673 cubic feet per second, with none of the projected peak flows in excess of the design capacity along any portion of the flood control channel. The program demonstrates that regional planning can effectively encourage urban development patterns that minimize the total costs of environmental controls.

Congress and the USACE view the Four Mile Run Watershed Management Program as an important prototype for future watershed management elsewhere in the United States. NVRC views the program as an excellent example of how jurisdictions can collaborate to develop regional solutions to water resources problems which spill over jurisdictional boundaries. In short, the watershed management program constitutes a regional planning effort that continues to function effectively, and one in which all of its participants can take great pride.