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Addressing the urban stream disturbance regime

Thresholds for particle entrainment and natural disturbance frequency vary across hydrogeomorphic settings, but urbanization increases the rate and extent of channel erosion and sediment transport in alluvial channels. The urban disturbance regime is a change in the frequency, magnitude, and duration of hydrologically induced disturbance on the stream channel and ecosystem that can lead to geomorphic and ecological degradation. To preserve stream stability and ecological function, stormwater management systems should be optimized to maintain the natural disturbance regime of streambed material within the context of societal and environmental goals. Our proposed framework, based on data from 195 sites across 2 continents, shows that the tools of river mechanics can be used to develop relatively simple, regionally appropriate, streambed-specific guidance for stormwater designers, so that engineers can calibrate stormwater facilities that address the urban stream disturbance regime. In the absence of detailed hydrogeomorphic data, practitioners can use our model to predict an order-of-magnitude approximation of the critical discharge for bed particle entrainment (Qc) based solely on bed material class (e.g., cobble vs sand) and the respective 2-y peak discharge (Q2). The estimate also can guide practitioners toward the types of stormwater management strategies that are likely to be most effective at protecting stream stability in a given setting. For example, duration controls for large events (≥Q2) may be very important for preserving stability in gravel/boulder streams where Qc is expected to be >∼0.1 to 1 × Q2, but could have relatively little effect on the overall stability of sand-dominated systems where Qc is likely to be <0.01 × Q2 and controls of much more frequent events (<<Q2) may have a greater influence.