Coplay explores plan for flooding at 12th, Tapler streets
Borough engineer Daniel Witczak addressed the Tapler Street flooding problems during Coplay council’s regular meeting Dec. 12.
“We evaluated the site and plugged data into our modeling software and got similar results to what residents report,” he said.
Witczak drafted his colleagues at Acela Engineering to do an extensive data collection effort. The detail included size of piping, drainage fields, the number of acres in the drainage field and size of inlets.
“Once we had the data, we looked at the annual rainfall in the area,” he said.
Rainfall calculations range from the expected rainfall on an annual basis to the “100-year rainfall” occurrence.
“A 100-year rainfall means there is a one percent chance that the amount of rain will occur in a given year. It doesn’t mean that after a heavy rainstorm, we won’t see another one for 99 years,” said Witczak.
Based on the documented information, the existing system is expected to flood every year.
“That is pretty much what we see. It means that the systems we have are inadequate for even the expected amount of rainfall,” he said.
The study focused on the area of 12th Street and Keystone Alley and Ruch and Center streets. The engineers came up with a phased solution.
The goal is to come up with a solution that would allow the system to handle a 25-year rainfall event. Handling the 25-year event means that all the rainfall is captured in the system and the water does not come out through the tops of manhole covers.
The first phase of the system replaces the existing 30-inch stormwater line with a 36-inch line. Witczak is recommending a 36-inch line because the Coplay line must tie into the Whitehall line, which is 36 inches.
“We need more capacity than that, but we are limited to what we can connect to in Whitehall,” he said. To give an indication of how bad the existing conditions are, increasing the size of the line from 30-inch to 36-inch increases the capacity by more than 40 percent. The increase will give the system capacity to handle a two-year storm. Instead of flooding every year, the system would flood every other year. Estimated cost is $250,000.
Increasing capacity further will require extending the 36-inch line further north, parallel to Keystone Alley, and adding holding areas along Magna Drive and Coplay Street. The two streets were selected because it was assumed they may have less buried utilities than others. Any expansion would require a more detailed engineering review. The holding areas proposed would catch water during high-water events and gradually release water into the system.
The goal to reach 25-year capacity may not be reached even with these improvements. Adding the holding areas and extending the 36-inch line costs an additional $650,000.
There are other details that could drive the costs higher. All of the problems can be identified in the details once council gives the go-ahead.