Duck River and Buchanan Branch Mitigation Banks – Lessons Learned
Jeff Shaver
Civil & Environmental Consultants, Inc.
Franklin, TN
The Duck River Mitigation bank is intended to provide stream and wetland mitigation credits to satisfy compensatory mitigation requirements for adverse impacts to Waters of the United States. The Duck River Mitigation Bank is located in Shelbyville, TN, and consists of the preservation of approximately 15,000 linear feet of unnamed tributaries to Buchanan Branch using natural channel design techniques. A subsequent bank, the Buchanan Branch Mitigation Bank, has been proposed immediately adjacent and to the south of the Duck River Mitigation Bank. This presentation will demonstrate the lessons learned from the Duck River Mitigation Bank which are being implemented in the design of the Buchanan Branch Mitigation Bank.
The Duck River Mitigation Bank includes the design of low-gradient C-type channels as well as smaller and steeper headwater streams that contribute to the baseflow of the larger channels. We found that the low-gradient channels with a riffle-pool bed morphology aligned well within the Tennessee SQT reference parameters, and the SQT scored the geomorphic parameters for these design streams well. However, the steeper headwater channels dominated by cascade or step-pool bed morphology produced lower SQT scores within the geomorphology section, primarily due to the lack of true rifle-pool bed morphology found in the reference reaches for these types of streams. These streams reinforce the notion that the SQT is not a design tool, and appropriate step-pool or cascade morphology is critical to channel stability. It was also found that areas where floodplains converge or groundwater seeps are identified, there is elevated risk of nick points forming in the outer banks of pools and leading to erosion during overbank events, and wood toe was found to be an effective structure to prevent this from occurring. Other lessons learned include the selection of performance standards, best management practices and design considerations for steeper floodplains and in-stream structure selection.
About Jeff Shaver
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