Applicability of proposed non-bankfull SQT methodology to headwater streams in the Nashville Basin of Tennessee
Anthony O. Brais, PE
Stantec
Nashville, TN
The Tennessee Stream Quantification Tool (TN-SQT) provides a quantitative framework for stream assessment and design to measure functional loss or lift for restoration projects. Headwater systems (small drainage area, steep slope, non-alluvial valleys, etc.) are difficult to characterize under current TN-SQT methodologies which are based on bankfull processes. As regulatory agencies propose SQT revisions to include non-bankfull methodologies, it is critical to understand how these metrics shift functional scores.
This presentation will explore a study that compares floodplain connectivity metrics in preservation headwater systems on a stream mitigation site in southern Nashville Basin of middle Tennessee under current bankfull and proposed non-bankfull SQT approaches. Bankfull entrenchment and bank height ratios by reach were collected in 2023-2024 under rapid assessment TN-SQT methodology. Collected field data was used to derive non-bankfull floodplain inundation and permissible shear stress metrics. All reaches scored identical or higher with the non-bankfull floodplain inundation metric. The preservation reaches at the study sites were all “A” and “B” stream types. These systems need limited floodprone width to have a fully functioning entrenchment ratio under current bankfull SQT metrics. Regional regression equations were used to develop design storm discharges for the non-bankfull floodplain inundation metric. This approach is not site-specific and may over or underestimate discharge derived from field collected cross-sections. Incorporating a reach-level approach to stream discharge estimates likely would allow a more accurate comparison of bankfull and non-bankfull approaches. For bank height ratio and channel incision metrics, results varied. Reaches with bedrock scored higher under the non-bankfull channel incision approach. Gravel & cobble bed reaches scored poorly on the non-bankfull channel incision metric despite all having a full functioning bank height ratio. If a reach has downcut to bedrock, the non-bankfull approach may underestimate impairment and additional channel degradation may occur. These findings suggest that further refinement of field bed material composition data in non-bedrock reaches likely would allow a more accurate comparison of bankfull and non-bankfull approaches.
About Anthony O. Brais, PE
Anthony is an environmental/water resources engineer with 12 years’ experience in nature-based solutions, including stream and wetland restoration design, stormwater management design, and erosion and sediment control design. He has worked extensively in stream and wetland assessment, design, permitting, and construction for compensatory mitigation in the mid-South having served as the engineer of record for 20 stream and wetland restoration projects in Indiana, Georgia, Kentucky, and Tennessee. Anthony has assessed and designed more than 25 miles of stream channel predominately working under stream quantification tool (SQT) methodologies and utilizing process-based natural channel/floodplain design. He is also a licensed FAA Part 107 Remote Pilot conducting UAV (drone) surveys for construction oversight and as-built. Anthony also leads surveys for state threatened and endangered amphibians and reptiles.