An investigation and quantitative analysis of the impacts on stream flow and fluvial erosion associated with urbanisation of a small catchment

Abstract

[Abstract]: The urbanisation of a catchment can have a significant impact on flow, flood and erosion processes. By modelling these processes in a rural and then a hypothetical urbanised catchment quantitative comparisons can be made on how the urbanisation process has impacted on stream flows, flood behaviour and erosion. Hydraulic and hydrologic modelling are techniques frequently undertaken for a range of purposes including estimation of flood extents, development of flood mitigation plans and in determining the effects of hydraulic structures. Hydraulic modelling typically assume fixed cross-section geometries while in reality the processes of erosion and sediment transport results in changing channel geometries over time. The results indicated that the urbanisation of a catchment has some dramatic effects on stream flow, flood and erosion processes. The RORB hydrologic model showed significant increases in peak flows and run-off volume and a reduction in lag time. The HEC-RAS hydraulic model showed significant increases in water surface elevations, shear stresses and flood extents. The channel enlargement modelling indicated a substantial increase in channel area was required to accommodate the increased urban flows. The project has identified the importance of considering the impacts of development in long-term flood and hydraulic modelling as it cannot be assumed that channel geometries and peak flows are fixed particularly as development has been shown to accelerate fluvial erosion processes. The project has also indentified the need for further research to investigate the complex relationship between urbanization, flood behaviour and channel morphology.