Tanyard Branch Stormwater Management Alternatives Analysis
The Town of Chapel Hill contracted with NCSU to evaluate stormwater management alternatives for the downtown area draining to Tanyard Branch. This study will assist the Town in determining how best to manage stormwater in this area and how to approach any future stream restoration projects that may occur there.
What are the problems?
Tanyard Branch is being severely eroded by large volumes of stormwater surging through the outfall (two 48” pipes) into the stream. The majority of downtown Chapel Hill drains to this one outfall, and as a result of the drainage area's very high amount of impervious surface, considerable runoff with great erosive energy is generated from even small rains.
An impaired segment of stream below Caldwell Street was identified in the Bolin Creek Watershed Geomorphic Assessment as a potential site for a stream restoration. However, there is the potential to “blow out” or destroy a newly restored stream if stormwater flow above the restoration site is not controlled.
Best management practices (BMPs) such as ponds, wetlands, and rain gardens are often installed to slow down stormwater before it reaches creeks with its destructive energy. Unfortunately, in this highly urbanized area there is little space to install BMPs where runoff can be intercepted before it reaches the stream. One alternative is to install a wet pond below the stormwater outfall. But because Tanyard Branch is a piped perennial stream, EPA requires an “alternatives analysis” to compare best options for installing stormwater management “inline” with the stream.
What are the possible solutions?
NCSU's study evaluated different combinations of stormwater management practices including an undersized wet pond (not large enough to handle flows regularly handled by modern practices, but as large as can be installed in current available area), a full-sized wet pond (which would require land acquisition and sanitary sewer relocation), residential stormwater management practices, UNC stormwater practices, and downtown stormwater practices. The study compared costs of BMP installation, land acquisition, maintenance, sanitary sewer relocation, and stream restoration. It examined exports of nitrogen and phosphorus under the different scenarios. Most importantly, a model was developed to predict what flow levels and what durations of these flows are erosive to the channel below the proposed BMPs, and from there suggested which alternatives would be most protective of the downstream channel if they were installed.
What is the status of the project?
The stormwater management alternatives analysis is complete. The most cost-effective stormwater management scenarios are in the $400,000 to $600,000 range, not including stream restoration downstream of the installation.
What are the next steps?
A decision must be made on how and whether to proceed with any stormwater management and/or stream restoration in the watershed to reduce the severe erosion and scouring. One suggestion was for this work to be part of a larger proposal for the creation of a downtown park with a pond, but would depend on private property owner interest and cooperation.
Though repairing the stream to reduce sedimentation is a primary goal, reducing pollution through dumping, illicit discharges, and sanitary sewer-storm drain cross-connections is also important to improve the overall water quality and biotic integrity of Tanyard Branch. The Town’s Stormwater Management Division conducts training of restaurant staff and guidance to businesses in the downtown area to reduce direct inputs to the storm drain system.
What monitoring or data collection have we done?
Some flow and channel morphology monitoring was conducted as part of the alternatives analysis. Stream macroinvertebrate monitoring is being conducted on Tanyard Branch, one of several sites around Town. No further monitoring is planned until projects in the watershed are proposed.
What reports or other resources are available?
• Thesis excerpt, Tanyard Branch alternatives analysis only
• Thesis excerpt, Tanyard Branch appendix
• Full Master’s thesis which includes the Tanyard Branch alternatives analysis: Tillinghast, Erica Desmond. 2011. Thesis: Stormwater Control Measure Discharge Standards to Improve Stream Geomorphic Stability. NC State University.
• Morgan-Little Creek Study