20-2 Suspended Sediment Transport Regimes in Urban Streams are Governed by Natural and Human-Modified Sediment Supply

Abstract:

Urbanization reshapes stream channels and sediment supply, and this transformation often degrades ecosystems and threatens infrastructure. To reduce these environmental and economic costs, understanding suspended sediment supply and transport is essential. We combined geospatial analysis of 117 US urban watersheds with a combined 50,000 storm events for which turbidity and discharge data were available. We found that the longitudinal distribution of geomorphometry, soil texture, and riparian impervious cover, which we called the erosion and sediment supply potential (ESSP) index, is associated with four distinct suspended sediment transport regimes, characterized by their hysteresis behavior. The ESSP index was validated with field mapping in three Ohio urban streams, where bedrock outcrops and hardened reaches modulated sediment dynamics. The national analysis also revealed that suspended sediment response (turbidity-discharge slope) is suppressed in watersheds with >40% imperviousness, likely due to extensive channel hardening. This result was validated in a Vermont urban watershed, where stream burial reduced suspended sediment loads in a >40% impervious tributary, relative to the mainstem where mostly natural banks were present.  Stormwater management also modulated suspended sediment response across a range of storm sizes. Combined, our results show that infrastructure- and geologically-mediated bank sediment supply governs suspended sediment transport regimes in urban streams, pointing the way to strategies for proactive management that protects communities and the environment. 

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