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

Over the last 200 years the condition of Australia’s streams has changed dramatically. The removal of massive volumes of woody debris and the impoverishment of native riparian vegetation has resulted in channels where flow is minimally obstructed. Such hydraulically efficient channels are able to carry larger discharges before flooding commences. For the last two decades the major stream rehabilitation activity in Australia has been to revegetate the riparian zone and to reinstate large woody debris (LWD). However, to date little has been done to understand ramifications of riparian revegetation on flood behaviour. This paper describes a modelling study that seeks to quantify the impact of riparian vegetation on both the shape of a flood hydrograph and the speed at which it propagates down a river reach. A one-dimensional flow-routing model (FLDWAV) is used to solve the fully dynamic formulation of the Saint-Venant equations. The hydraulic properties of riparian vegetation are computed using a simple model of vegetation resistance, where Manning’s ‘n’ is a function of flow depth and 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), showing that smaller floods are more sensitive to vegetation condition than larger floods.