The South Wayne neighborhood, near the Radnor Fire Company (RFC) and Radnor Middle School (RMS) on South Wayne and Runnymede Avenues, has experienced flooding for decades. Acres and acres of impervious surface, much constructed at the turn of the century, as well as the topography of the area, contribute to the flooding. To help provide incremental relief to the area, the Township approved the installation of a South Wayne storm water management system. Despite being small in acreage, the Township owned municipal parking lot on South Wayne Avenue proved to be the best ground in the area for the project as well as the only open space in the otherwise densely populated neighborhood.

Meliora Design, the Township’s stormwater infrastructure design consultant, determined that the 10-year, 1 hour rain event (i.e., 1.9” of rain in a one-hour period) is the most frequent and impactful to the community. The project design utilizes underground stormwater storage beneath the South Wayne Municipal Parking Lot. Based on the elevations of the street and pipes, the maximum amount of storage is constructed underneath the parking lot and, when empty, can hold approximately 253,000 gallons runoff.  

The storm water management project is two pronged including the underground parking lot storage and the use of the underutilized storage of the RMS system. In finishing the RMS project, by connecting the runoff from South Wayne and Lancaster Avenues to the RMS system, another 350,000 gallons of underground storage is now utilized.  

As noted from project onset:  

• The capacity [storage] of these systems is finite. Larger storms will over-run the system and flooding will occur. 

• The system releases runoff slowly, so as not to cause issues downstream. Due to this, sequential storms take up capacity in the system. 

• The project is designed to address the 10-year, 1 hour rainstorm, which is 1.9” of rain in one hour.  

So, what happened during the July 9 storm? Why was there flooding?  In short, the rainstorm on July 9 exceeded the capacity of the underground systems. The Township’s real time weather station at this location confirmed the following: 

• The 0.5” of rainfall received on July 8 used some of the system’s storage capacity.  

• At one point, over 2.5" of rain fell in one hour, 31% over capacity of the system. 

• This is on top of the fact that part of the capacity was already utilized from Saturday’s 0.5" of rain. July 9 total rainfall at South Wayne was 4.7” of rain. 

• During the 24-hour period, South Wayne Avenue received 5.2” of rain, far exceeding the capacity of the system. 

July 9 was no summer thunderstorm; it was a severe weather event. It should be noted, there are areas of South Wayne Avenue that have historically flooded and did not during the July 9 storm due to the newly installed storm water management system. Engineering estimates that approximately 450,000 gallons [since the system contained run off from the July 8 rain] of runoff were contained in the underground systems, as opposed to increased flooding.