In October 2020, the Sun East Federal Credit Union reached out to D.L. Howell (DLH), asking for help with a recurring Drainage Issue in the rear of their headquarters building located in Aston Township, Delaware County, PA. During significant storm events, flooding occurs in the area circled in red on the below image. This is problematic because the rear entrance to the building is situated here.
Upon initial inspection, DLH noticed that the grade in this area encourages drainage towards the building entrance instead of away from it. Approximately half of the headquarters building was an addition, so this condition is most likely a result of the addition matching the original building’s finished floor elevation. It was also determined that a stormwater seepage bed sits beneath the paving near this area. Per Sun East, the downspouts connect to the yard drains in the rear of the building and enter directly into the seepage bed. When flooding occurs, the yard drains fill up, and water begins to work its way towards the back entrance. According to the Record Plan, the seepage bed contains a 4-Inch overflow pipe.
Based on the information above, DLH decided to perform a hydraulic analysis. The hydraulic analysis was performed to determine if the existing seepage bed can route its drainage area without overflowing (i.e., water bubbling back up the yard drains). As a result of the hydraulic analysis, it was determined that a 12-Inch overflow pipe would need to be installed to prevent overflow.
The project is currently in the process of being reviewed, but DLH’s Drainage Improvement Plan incorporates installing a new 12-Inch overflow pipe for the seepage bed, re-sizing the conveyance piping that connects the yard drains to the seepage bed, and re-grading the area in the rear of the building to encourage positive drainage away from the entrance. As a part of the review process, DLH was requested to evaluate the current and proposed peak flow rates at Point of Interest #1, which can be seen below. The current peak flow rates were evaluated by modeling the drainage area tributary to the seepage bed without routing it, which is an accurate representation of the site conditions when flooding occurs because stormwater bypasses the seepage bed. The proposed peak flow rates were evaluated by routing the drainage area tributary to the seepage bed with the new 12-Inch overflow pipe. Using this methodology, DLH was able to show that there will be no increase in the flow rates for the 1 through 100-year storms.
DLH believes that this course of action will alleviate the recurring drainage issue and expects to receive approval to move forward with construction during the coming weeks. It has been a pleasure working with the Sun East Federal Credit Union on this project. Also, thanks to Chris Daily, P.E., for his oversight.
