Design flood estimation in small catchments using twodimensional hydraulic modelling
Steve Muncaster, Warwick Bishop, Andrew McCowan
Abstract: Design flood estimates are often required for small catchments as part of development planning and/or infrastructure design. Traditionally, the probabilistic Rational Method has been the principal approach to estimate design peak flow with simple hydraulic calculations employed to size culverts and bridge waterways. The application of the Rational Method, while well ingrained in engineering practice, relies on significant simplifications of the catchment runoff process. These simplifications can lead to uncertainty surrounding design flood estimates. More sophisticated analysis is possible through the use of runoff routing models such as RORB, Rafts or Urbs. Such models allow greater detail to be incorporated into the analysis at a subcatchment level, although some simplifications of the runoff process are still necessary.
Recent developments in two dimensional hydraulic models enable the direct application of rainfall excess onto the computational grid. Increasingly high resolution topographic data is becoming available, often associated with proposed development of a particular area. These two developments facilitate the application of two dimensional hydraulic models as a runoff routing model in small catchments. This paper discusses the key aspects of this application including appropriate topographic data sources, computational grid
resolution and effective roughness values. Results from a preliminary application to small rural catchments in the Geelong region are presented and used to illustrate key aspects. In particular, the selection of appropriate hydraulic roughness is critical. Better representation of catchment storage is provided through the use of topographic data in the computational grid. The use of a two-dimensional hydraulic model integrates the hydrologic and hydraulic aspects into a single model. Further investigation is required to assess the role of hydraulic roughness in determining surface runoff rates.
Read the full publication here: Muncaster et al – 2D hydraulic modelling