How urban stormwater regimes drive geomorphic degradation of receiving streams

Abstract

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.

Catchment‐scale urbanization diminishes effects of habitat complexity on instream macroinvertebrate assemblages

“…Urban stormwater runoff increases the frequency and magnitude of in-stream disturbance, both hydraulic disturbance arising from larger, more frequent high-flow events, and water quality disturbance associated with polluted and thermally altered runoff (Walsh et al 2005, Somers et al 2013. It also reduces in-stream habitat complexity, through increased capacity of streams to transport sediments (Russell et al 2020), causing simplification of in-stream habitat, such as the loss of bars, benches, and woody debris (Vietz et al 2014).…”

Real Time Control of Rainwater Harvesting Systems: The Benefits of Increasing Rainfall Forecast Window

“…Therefore, releasing the predicted overflow over a longer period at a lower rate, such as occurs in S3 Longest Discharge and S4 Streamflow Preservation (Figure 4), arguably better imitates the reference flow regime. Doing so also has the benefit of minimizing the hydraulic disturbance and subsequent geomorphic degradation of the channels of receiving streams (Russell et al, 2020). In coupling with real time flow monitoring (Kawanisi et al, 2018), RTC offers the potential to adapt the controlled release to real time flow conditions, thus mimicking the natural streamflow and delivering the flow regime determined appropriate for the ecological objectives of the receiving water.…”

“…Accordingly, the conventional hydraulic efficient drainage network, which directly connects the impervious runoff to receiving water, increases the frequency, magnitude, and volume of storm flow (Leopold, 1968) and reduces storm recession time (Burns et al, 2005). Such a change drives channel erosion (Hammer, 1972; Russell et al, 2020) and ecological degradation in urban streams and leads to a subsequent loss of ecosystem services (Bunn & Arthington, 2002; King et al, 2005; Walsh et al, 2012). Similarly, loss of baseflow results in loss of dry weather wetted habitat, thus further reducing biodiversity (Poff et al, 1997).…”

Real-Time Control of Rainwater Harvesting Systems: The Benefits of Increasing Rainfall Forecast Window

“…Therefore, releasing the predicted overflow over a longer period at a lower rate, such as occurs in S3 Longest Discharge and S4 Streamflow Preservation (Figure 4), arguably better imitates the reference flow regime. Doing so also has the benefit of minimising the hydraulic disturbance and subsequent geomorphic degradation of the channels of receiving streams (Russell et al, 2020). In coupling with real-time flow monitoring (Kawanisi et al, 2018), RTC offers the potential to adapt the controlled release to real-time flow conditions, thus mimicking the natural streamflow, and delivering the flow regime determined appropriate for the ecological objectives of the receiving water.…”

“…Accordingly, the conventional hydraulic efficient drainage network, which directly connects the impervious runoff to receiving water, increases the frequency, magnitude and volume of storm flow (Leopold, 1968) and reduces storm recession time (Burns et al, 2005). Such a change drives channel erosion (Hammer, 1972;Russell et al, 2020) and ecological degradation in urban streams and leads to a subsequent loss of ecosystem services (Bunn & Arthington, 2002;King et al, 2005;Walsh et al, 2012). Similarly, loss of baseflow, results in loss of dry weather wetted habitat, thus further reducing biodiversity (Poff et al, 1997).…”