Aston Township was hit by two back to back severe storms during the week of August 3, 2020. The storm which occurred on Tuesday, August 4th resulted from the remnants of Tropical Storm Isais and was widespread over the entire county. The storm on Friday, August 7th was more localized, with Aston and other western Delco communities hit the hardest. The storm on Tuesday dropped 4.8 inches of rain over a 6-hour period which would be between a 50 and 100-year storm event. The Friday storm resulted in 3 inches of rain over a 1-hour period which would be classified as a 100 and 200-year storm event. This was based on the reading at the USGS Chadds Ford River Station on the Brandywine so the actual rainfall in Aston may have been higher. The fact that storms of this magnitude both occurred a few days apart is unprecedented.
When we design inlets and storm sewers, they are designed for 10-year frequency storms, meaning that statistically storms of that magnitude should occur every ten years. This does not mean that you cannot have 2 in a month but over a long period of time, they occur every ten years. Retention basins are designed for 100-year storms as they are generally at the terminus of the storm sewer system.
Putting this in perspective, Friday’s storm was on par with Hurricane Sandy in 2012 given the large amount of rain in the short period of time. Storm sewers within communities like Aston, are designed to handle most storms that we are expected to receive in any given year. However, it is not economically feasible to design them for storms of the magnitude that we saw in August. The fact that these large storms were “back to back” added to the problem and then created other issues. The ground was already saturated from all the rains received this summer, even prior to the first storm on the Tuesday. Retention basins that are meant to hold storm water and release it slowly for several days after a normal storm, did not have sufficient time to completely discharge between Tuesday and Friday. This is what happened here.
The two storms also played havoc with West Branch Chester Creek 1,000’ in from Birney Highway. Here significant tree trunks, branches and other debris build up at the rock dam that created the water fall, caused the stream level to rise and eventually breaching the dam to the south side, severely eroding the creek bank and creating a new channel.
The Township is very much aware of the concerns of our residents. We are already taking steps in the short run and are exploring remedial options for the long run investigating to improve the stormwater systems to handle future events to the best extent possible. They include but are not limited to the following current and future actions:
- The ongoing review, cleaning and repair of defective stormwater inlets and storm drains.
- Cleaning stormwater retention ponds and drainage lines to improve percolating into the ground and faster discharge.
- Explore the feasibility of adding one or more additional retention ponds upstream from Newsom’s Pond (Weir & Blackthorne) to reduce the water volume on Newsom’s stormwater system.
- Explore the feasibility of adding one or more rain garden drainage systems in targeted areas to reduce the amount of stormwater overflow on properties and township streets.
- Explore the feasibility of changing the alignment and enlarging the drainage line under the Weir and Blackthorn intersection through to the creek adjacent to the Weir Park Baseball complex to better handle the flow of stormwater from the Newsom’s Pond stormwater system.
- Evaluate the PADEP requirement to remove the rock dam that created the West Branch Chester waterfalls and submit options to the PADEP requirement calling for removal of the dam. Additionally, oversee restoration of the eroded south bank of the creek, and stabilization of the south bank against future storm erosion.
The Township will continue to keep all our residents up to date on our stormwater remediation efforts and on the progress of the improvements. Information will be posted on the Township website at www.astontownship.net, the Township Facebook page, Aston Township and at the monthly Commissioners Legislative Meeting.