Four Northern Virginia jurisdictions have agreed to cooperate in a program designed to relieve flooding in the Four Mile Run watershed area.

Fairfax County, Falls Church, Alexandria and Arlington County signed the cooperative agreement early this month. The agreement puts into operation a complex computer model able to determine how development in each of the jurisdictions will increase water runoff into Four Mile Run.

Commercial and residential areas between I-95 and the Potomac River, particularly the Arlandria district of Arlington County, have suffered more than $40 million in property damages in the past 12 years from Four Mile Run flooding, according the the Northern Virginia Planning District Commission. The Commission initiated the stormwater monitoring program on the request of the four jurisdictions, which helped fund a $190,000 study during the past two years to develop the mathmatical model.

Much of the flooding, say commission planners, results from increased watershed area of Four Mile Run. As development increases, so does stormwater runoff flowing at faster velocities into Four Mile Run than the channel can accomodate.

The computer model, which simulates present physical conditions in the four jurisdictions, will be used to show how any changes in those conditions would affect the Four Mile Run area. For example, date concerning the development of a new shopping center would be plugged into the model to show how runoff might increase by adding more cement and structures to a lesser developed area.

While one shopping center in one jurisdiction might have no significant effect on water flowing into Four Mile Run, the combined development in all of the jurisdictions could worsen the flood-prone nature of the channel, said Austan Librach, director of regional resources at the Northern Virginia Planning District Commission.

Under the agreement, a board made up of representatives from each of cooperating jurisdictions will review a quarterly report prepared by the planning commission staff to determine how information gained from using the computer model is being put to use.

"By signing this agreement, all of te jurisdictions are committing themselves to minimizing the extent of stormwater runoff originating in their jurisdictions," Librach said. "How they accomplish this is up to them. It may be by preventative measures, such as stricter controls for building on areas that might generate a lot of runoff, or corrective measures, like building water impoundments at locations already experiencing heavy runoff.

Operating the mathmatical model will cost about $50,000 a year, funded by each of the four jurisdictions. The cost includes using field technicians to gauge amounts of stormwater drainage, updating and upgrading the model as physical changes occur in the watershed, running the computer and adapting the model for use in each of the jurisdictions.

The agreement is to last for the 100 year economic lifetime fo the Army Corps of Engineers' flood prevention project underway to widen the mouth into the estuary of the Potomac River. The effort to control stormwater runoff fulfills a requirement by Congress that the four Northern Virginia jurisdictions develop long-range land management programs to preserve the Corps flood prevention project.

"We believe this is the first time Congress has required such guarantees to prevent the obsolecence of a federal project," Librach said.

He added that additional concentrated development in Northern Virginia could cause substantially more stormwater runoff and undermine the corps' project without runoff controls.

"All of the jurisdictions have been working on a local basis to minimize runoff," Librach said. "But this is the first time they will try to prevent it as a regional body, realizing what one jurisdiction does effects them all."