Fort Allen flooding mitigation based on dynamic modeling

The Ft. Allen watershed is comprised of 14 separate drainage areas that total approximately 1630 acres or 2.5 square miles. The development of the watershed was largely completed without the benefit of stormwater management (SWM), including the Ft Allen Elementary school, BP, and the Ft. Allen Volunteer Fire Department. There have also been various parcels developed that required SWM including the JAS center, Word of Life Ministries and a couple of other larger parcels.  The history of development in the watershed is important because even the areas with SWM controls in place are not performing adequately enough to reduce peak flows and prevent flooding, with the flooding on 8/9/2007 is proof.

During that event, approximately 4.05 inches of rain fell on the watershed over a 50 hour period and around 8:30 am on 8/9/2007 1.46 inches fell in an hour. The fire department and numerous homes were flooded as a result of the rainfall on 8/9/2007. The primary damage and flooding that occurred was at the Ft. Allen VFD. A calibrated dynamic model was created to allow the Township to reconstruct the event and other events based on real rainfall and flow data.

The model was completed after a year of monitoring rainfall and stream flow conditions. The results of the model show that a number of very costs effective small projects could eliminate the flooding of the Fire Department and the homes on Navajo without increasing any flooding to the downstream portion of the community. However, the area where the stream enters Sewickley creek is in major need of capacity modifications. The stream has had so much dirt settle that there is less than 2 feet of clearance between the bottom of the stream and the bridge. This causes flooding to the intersection of Fosterville and State Route 119. The estimated project costs include a regional detention pond above SR 136, porous asphalt in the parking lot of the fire department, a larger pipe network in the area of SR 136, improved stream hydraulics on Navajo and Sioux drive, on lot infiltration systems for the schools and stream modifications at Fosterville and SR 119.

This project has since been constructed with the state’s largest application of porous asphalt and has successfully abated flooding for 9 years.