As the hot summer comes to an end, we’re reminded that August is National Water Quality Month. This month is dedicated to bringing awareness to how essential it is to have fresh, clean water. Having a clean water source is not only important to humans, but also to the wildlife in the ecosystems around us.
At Timmons Group, our Stormwater Management teams design sustainable stream bank repairs that accomplish water quality improvements through the reduction of sediment import. These sites also restore terrestrial habitat and improve aquatic habitat. To celebrate National Water Quality Month, we are taking a look at some projects completed by Timmons Group for the City of Raleigh and how they improve the local community’s surface water conditions.
In 2019, Timmons Group was selected to work with the City of Raleigh for an On-Call Drainage Assistance Program to address infrastructure inadequacies and severe erosion. These projects address property owner stormwater issues located beyond the City’s right-of-ways. Two projects highlighted are located on Gainsboro and Edgedale Drives in Raleigh.
Gainsborough Drive
An unnamed tributary (UT) to Hare Snipe Creek was causing a driveway failure due to an active lateral migration of a meander bend in the channel. Approximately 220 feet of stream bed and bank were identified for repair and improvement for this project. The removal of existing woody vegetation along the stream bank contributed to the destabilization of the channel. Maintained lawns provided for at most a 3 to 4-inch root depth. A portion of the bank had been lined with stone to protect the bank along the downstream portion of the reach.
The bank erosion began at the end of the stone and extended upstream through a bend in the channel. The channel had incised (lowered in elevation) over time. A detrimental effect to stream stability through incision is the containment of larger storm events over time. This causes the stream to experience larger volumes of water, higher velocities and greater erosional forces. The erosion had moved the bank horizontally as well as created an unstable vertical 8-foot-high bank. The existing driveway became more undermined each storm creating a safety concern for the residents.
Timmons Group actively coordinated with the City to prepare the best solution to reshape the western bank and restore the stream’s vertical bed and bank stability. Highlights of our stormwater team’s design plan included reshaping the eroded western bank, adding a bankfull bench on the eastern floodplain to lessen the pressure on the outside meander bend during large storm events, and repairing a short segment of the eastern bank. Vegetation was also reintroduced to both sides of the stream and will allow for long term stability in the bank.
Edgegale Drive
This project was completed to mitigate active bank erosion that was encroaching dangerously close to residential structures. The existing drainage infrastructure in the stream is undersized and degraded, resulting in poor stream flow and erosive conditions. The project goal was to stabilize the stream channel and eliminate active erosion of the stream channel bank. The repaired channel was designed to enhance aquatic passage and habitat through the removal of an existing pipe and embankment in the upper reach.
The stabilization of the banks reduced sediment import into the stream. The repaired channel in the lower reach was raised to provide better floodplain connection and preserve the large trees along the stream corridor. The design was minimized to work within the exiting stream pattern with limited pattern changes and raising the channel for better floodplain connection to reduce the stress on the stream banks during large storm events.
The Edgedale Drive project included degraded upper and lower reaches separated by a stable segment of stream between the two restoration segments. The upper reach included removal of an existing pipe and the stabilization of the western bank adjacent to the structure. The lower reach included the replacement of an existing degraded 36-inch corrugated metal pipe with a larger 48-inch reinforced concrete pipe. The lower reach’s channel elevation was raised approximately 2 feet to provide better connection to the floodplain. New riffles were established along the reach to provide long term stability.
Our natural ecosystems rely on clean water sources. Our stormwater team provides designs for municipalities that improve water quality and are sustainable using natural channel techniques throughout the state.
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