Modeling runoff dynamics from an urbanizing catchment to support the development of a management plan for a treatment wetland

PCSWMM was used in combination with locally-sourced aerial photography as a first step in assessing a water management plan to protect an internationally important Ramsar site, Connewaree Wetland, near Geelong, Australia. Geelong is about 75km southwest of Melbourne and is rapidly urbanizing as the area has become a commuting community for Melbourne. The local government has implemented an extensive water-sensitive urban design (WSUD) master plan to manage runoff quantity and quality. As part of this master plan, former agricultural land was converted into a storage wetland, known as the Sparrovale Wetland, to protect the Connewaree Ramsar site from water quality impacts and flow fluctuations associated with urban development in South Geelong. The contributing catchment area of about 4000ha currently consists of agriculture and open space (~80%) as well as smaller but rapidly expanding residential, institutional, and commercial land uses (~20%). A total of 567 sub-catchments were established in PCSWMM with a drainage network consisting of underground pipes primarily in the residential areas and open channel conveyance in the agricultural areas that ultimately discharge into the 207ha Sparrovale Wetland. Sparrovale was represented as a storage node in PCSWMM 1D. Model results were compared with monitored water level data in the wetland for the period of June 2021 to December 2022. The total rainfall for this period was 1007mm. Model results were good for the period June 2021 to January 2022, which was slightly wetter than average, with the Nash–Sutcliffe Efficiency (NSE) being 0.812. However, the model underestimated water levels for the extremely wet period July to December 2022. Because of the large open space in the catchment, infiltration loss played a significant role in the sub-catchment water budget. For the Sparrovale Wetland, modeled exfiltration loss was similar to the evaporation loss; however we need to further evaluate the seepage estimates under extreme wet conditions to improve model performance. The aerial photography is being processed to confirm the Sparrovale water surface area and depth as a validation for PCSWMM. Given the importance of the sub-catchment infiltration characteristics and the uncertainty for exfiltration loses in Sparrovale, the next step in this project will be to implement multiple approaches (continuous IoT monitoring and temporally-constrained measurements), aerial photographs (infrequent but available with subscription), satellite imagery (free but need to be processed and takes time) for better model validation. PCSWMM 2D also will be applied to the wetland as a comparison with the 1D model results. The final part of the project is to optimize the water level regulation plan in Sparrovale that will maximize the environmental benefit to the downstream Ramsar site.

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