Author

C J. Wright

Year

2018

Degree Name

BEnviSci Hons

Department

School of Earth & Environmental Sciences

Advisor(s)

Kerrylee Rogers

Abstract

Urban stormwater presents a significant environmental flow problem. Increased flows generated from impervious urban surfaces, coupled with conventional drainage designs, consistently results in erosion and ecological degradation of receiving waterways. A large portion of the Marshall Mount Creek (MMC) catchment in the Illawarra, New South Wales (NSW), has been allocated for urban development. Unless properly managed, MMC will be subject to the effects of urban stormwater and ecological decline. This is particularly concerning as the catchment drains to Lake Illawarra, which has experienced ongoing water quality issues since European settlement. The stream erosion index (SEI) is a commonly applied target for managing stormwater flows in NSW. The objective for the SEI is based on a generic flow threshold of erosion, which have not been theoretically justified. Mechanisms of erosion are not simply generalised, which called for an investigation into the effectiveness of the target in achieving its desired outcome. A desktop analysis of geomorphology and hydrologic and hydraulic modelling were undertaken to evaluate the effectiveness of the SEI in preventing from excessive erosion and to investigate optimal environmental flow standards for the developments. The system exhibits a disequilibrium in flow and geomorphology and the significance of a given discharge is spatially and temporally variable. This implies that any hydraulic assumption based exclusively on a discharge metric, such as the SEI objective, is questionable. The results of this study indicated that all SEI threshold flows would allow mobilisation of substantial proportions of channel sediments. Thus, not representative of stability and inadequate in protecting the system. Essentially all events sufficient to produce runoff were associated with sediment entrainment, which implies any change in flow has the potential to create excessive erosion. Consequently, management approaches may be better aligned focusing on more frequent events.

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