How urban stormwater regimes drive geomorphic degradation of receiving streams

For streams draining urban catchments, sediment transport capacity is the key driver of physical impacts including bed sediment removal and channel incision. The main unanswered question is the relative role of flow alteration compared to sediment supply in influencing sediment transport capacity. With this objective, we computed sand and gravel bed sediment transport capacity using the Wilcock and Kenworthy two-fraction bedload transport relation for nine streams in catchments covering a gradient of urbanisation. Computations were done for typical natural bed surface material, based on conditions in the least urban study streams. We compared transport capacity distributions and cumulative transport capacity over one-year between the streams. Transport capacity was up to three orders of magnitude higher in urban streams than in forested-catchment streams. This was driven overwhelmingly by the urbanisation-induced alterations to the flow regime, with only minor feedback from channel form changes. Transport capacity was two to three orders of magnitude greater than measured bedload transport in all but the least urban streams. This excess bedload transport capacity mobilises and removes bed sediment, produces channel incision and enlargement and reduces channel complexity. Rebalancing transport capacity with sediment supply therefore requires significant flow mitigation towards pre-urban conditions. Other responses, which may theoretically help to regain this balance - channel widening, grade control, increasing roughness, sediment augmentation - are either inappropriate or only feasible following flow mitigation measures.