Infill Development Impact on the Capacity of Regional Drainage Networks

The scope of the dissertation is to identify the hydraulic capacity of the stormwater drainage pipes and if they can accommodate ongoing infill development in two sample catchments in the Bass Coast local government area of Victoria. The research project identifies existing inadequacies of stormwater assets and conversely the areas of the drainage network with additional hydraulic capacity. A hydraulic model has been created in PCSWMM software to examine the capacity of Bass Coast Shire Council’s drainage pipes. One model for each study area has been created based on the existing level of development. A second model has then been created assuming the catchment areas are infill developed to their maximum potential. The increase in impervious surfaces results in a higher volume of runoff, increased flow rates and thus greater pressure on the existing drainage network. This study can assist Council engineers to make more informed decisions around drainage requirements for future infill developments in regional towns. The project has classified individual pipes into a class of drainage network capacity for two sample study areas in Cowes and Inverloch. The resultant capacity classes for the Cowes study area are: • Critical = 26 % • Near capacity = 30 % • Sufficient capacity = 44 % The capacity classes for the Inverloch study area are: • Critical = 7 % • Near capacity = 16 % • Sufficient capacity = 77 % The ‘critical’ class is the pipes that are already surcharging under the existing level of development; the ‘near capacity’ class are the pipes that will surcharge if the catchment is developed to the full potential. These results show that while the Inverloch study area is reasonably well equipped for future infill development, the Cowes study area requires onsite detention or pipe upsizing to accommodate the growth. Despite the results of under-capacity pipes, most of the surcharging is found to be contained within the stormwater pits and thus flooding is minimal. All infill development in Bass Coast is currently required to manage the increase in stormwater runoff via onsite detention systems. This study has found that a ‘one size fits all’ approach to stormwater management is overly conservative for infill development and some areas may not require flow restriction. A risk assessment should be undertaken however before Council allows infill development to proceed without either onsite detention or upsizing of the pipes in the existing drainage network