An analysis of the influence of riparian vegetation on the propagation of flood waves

Over the last 200 years, streams throughout Australia have been channelised to increase their hydraulic conveyance, and so reduce flood peaks and durations. One of the main channelisation activities has been to remove large woody debris (LWD) and bank vegetation. Ironically, the major stream rehabilitation activity in Australia, and in many countries, over the last two decades, has been to revegetate the riparian zone and to reinstate LWD. Yet there has been no consideration of the flood implications of returning much of this in-channel and floodplain roughness. This abstract describes a modelling study that quantifies the effects of catchment-scale riparian revegetation on the shape of a flood hydrograph and the speed at which it propagates down river reaches of varying slope and cross-section. A one-dimensional flowrouting model (FLDWAV) is used to solve the fully dynamic formulation of the Saint-Venant equations. The hydraulic properties of a range of riparian assemblages are computed using a model of vegetative resistance similar to HMODEL2 proposed by Darby (1999). This model computes Manning’s ‘n’ as a function of flow depth based on parameters describing the dimensions and distribution of plants in the riparian zone and on the geometry of the cross-section. This study demonstrates that channel roughness, and hence riparian condition, is a significant determinant of wave celerity, hydrograph dispersion and skewness. The impact of roughness is moderated by the magnitude of the hydrograph (peak discharge), with smaller magnitude floods being more sensitive than larger floods.