Using high-frequency monitoring data to quantify city-wide suspended-sediment load and evaluate TMDL goals

Abstract

Excess sediment is a common reason water bodies in the USA become listed as impaired resulting in total maximum daily loads (TMDL) that require municipalities to invest millions of dollars annually on management practices aimed at reducing suspended-sediment loads (SSLs), yet monitoring data are rarely used to quantify SSLs and track TMDL progress. A monitoring network was created to quantify the SSL from the City of Roanoke, Virginia, USA (CoR), to the Roanoke River and Tinker Creek and help guide TMDL assessment and implementation. Suspended-sediment concentrations were estimated between 2020 and 2022 from high-frequency turbidity data using surrogate linear-regression models. Sixty-one percent of the total three-year SSL resulted from five large storm events. The average suspended-sediment yield from the CoR (58.1 metric tons/km2/year) was similar to other urban watersheds in the Eastern United States; however, the yield was nearly five times larger than the TMDL allocation (12.2 metric tons/km2/year). The TMDL allocated load was modeled based on a predominantly forested reference watershed and may not be a practical target for highly impervious watersheds within the CoR. The TMDL model used daily input data which likely does not capture the full range of SSLs during storm events, particularly from flashy urban streams. The average SSL following the five large storm events doubled that of the CoR’s annual allocated load from the TMDL. The results of this study highlight the importance of using high-frequency monitoring data to accurately estimate SSLs and evaluate TMDLs in urban areas.

Urbanization and Suspended Sediment Transport Dynamics: A Comparative Study of Watersheds with Varying Degree of Urbanization Using Concentration-Discharge Hysteresis

“…Excessive flux of suspended sediments has deleterious effects on river ecosystems, including degradation of stream habitat. , In contrast, a lack of suspended sediment flux results in “hungry water”, which can lead to bank erosion, channel widening and incision, and modification of the riverine habitat. − Although alteration in sediment regime and geomorphology are among the primary symptoms of urban stream syndrome and have been documented in earlier studies, , research on sediment transport in urban watersheds remains limited. A significant portion of the limited studies of sediment in urban watersheds − has investigated the issue of increased suspended sediment yield, first documented by Wolman . The understanding of the sources, timing, mechanism, and pathways of fine sediment delivery in built-out urban watersheds remains underexplored.…”