Assessing the impact of urbanization on streams using ecohydraulics

Abstract

Urban stormwater runoff is a primary degrader of stream ecosystems. Excess stormwater runoff causes altered flow regimes, reduced in-stream water quality, and modified channel form. Recognition of such impacts has renewed interest in the protection or restoration of the hydrologic cycle in urban stormwater management. One of the greatest unknowns, however, is how the urban stormwater impacts on the flow regime translates to hydraulic conditions in the channel.

This understanding is critical to identifying how the hydraulic environment supports complex and dynamic ecosystem functioning. My thesis investigates how the flow regime and channel form, and their interactions, deliver in-stream hydraulic outcomes for urban streams. I use hydrodynamic modelling (TUFLOW) to predict hydraulic outcomes, with the models developed using hydrographic and topographic data from two streams in the Melbourne region.

I firstly predicted a range of ecologically relevant in-stream hydraulic metrics for a natural reach and compared the metrics to those predicted for a downstream urban reach of the same stream. I found that compared to the natural reach, the urban reach experienced much greater channel bed disturbance (~4 times higher), refuge habitat loss (~2 times smaller) and limited floodplain connectivity. This work thus supports with evidence casual physical mechanisms which likely drive urban stream degradation and habitat quality.

I disentangled the relative role that the flow regime and channel form play in influencing in-stream hydraulic conditions. I did this through modelling different combinations of flow regime and channel form, - e.g. altered flow regime in a natural channel versus natural flow regime in a modified channel. I found that both flow regime and channel form play key roles in setting the hydraulic conditions. The work revealed that the hydraulic regime is sensitive to the channel morphology which controlled key aspects of the hydraulic regime (e.g. magnitude, frequency and duration).

After establishing that both flow regime and channel form play important roles in regulating in-stream hydraulics, I investigated potential management actions which could be applied in both developed- and developing landscapes. I tested the benefit of alternative channel rehabilitation design configurations applied to a (synthetic) degraded urban stream. This work showed that channels designed to increase morphological complexity yield ecohydraulic benefits, but not to levels which approach those predicted under natural flow- and channel conditions. With altered flow regime limiting the effectiveness of such channel designs, I tested if this stressor could be mitigated in developing landscapes using alternative approaches towards urban stormwater management. It demonstrates that widespread application of Stormwater Control Measures (SCMs) could influence the flow regime in ways which translate to ecohydraulic conditions remaining at natural levels. This work suggests that for urbanized catchments, management of in-stream hydraulics requires attention to both the flow regime (using SCMs) and channel form (e.g. through sediment regime management to allow appropriate levels of sediment supply and transport). For developing catchments, SCMs should be implemented as part of the construction phase in order to minimize downstream hydrologic impact and thus geomorphic degradation.

My thesis provides an improved mechanistic understanding of why streams draining urban landscapes are commonly physically and ecologically degraded. Consideration of ecohydraulic indicators in urban stream management could provide targets, and help inform efforts towards stream protection or restoration, particularly when ecological objectives are central.