Brett Anderson

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

Abstract Over the last 200 years the condition of Australias 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 Mannings ‘ 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.

A review of empirical equations for estimating stream roughness and their application to four streams in Victoria

Abstract Measured Manning’s n values at four sites in Victoria, Australia, the Acheron River at Taggerty, the Merrimans Creek at Stradbroke West, the Mitta Mitta River at Hinnomunjie Bridge and the Tambo River downstream of Ramrod Creek were compared to n values given by empirical equations found in the literature. For the Merrimans Creek, the empirical equations underestimated the measured n values for all discharges. For the Mitta Mitta and Tambo Rivers, equations suggested by Dingman and Sharma1 and Riggs2 provided good estimates of n. Where information on substrate size was used equations suggested by Limerinos3, Froehlich (in Coon4), Bray (1979, in Phillips and Ingersoll5) and Griffiths6 provided reasonable estimates of n for some discharges for the Acheron River and Mitta Mitta River, but these same equations did not perform well for the Tambo River. The procedure suggested by Stewardson7,8 for predicting Manning’s n at a range of discharges, given one measured value of n at a known discharge, was also tested. It was found that this approach, although based on a large international data set7,8 was not useful for these streams. In fact, Manning’s n values showed little variation with discharge.

Flow resistance in four rivers in Victoria, Australia *

Reach-representative estimates of Mannings n are presented for a range of discharges in four rivers in Victoria, Australia: Acheron River at Taggerty, Merrimans Creek at Stradbroke West, Mitta Mitta River at Hinnomunjie Bridge, and Tambo River at Ramrod Creek. These Mannings n values have been determined from discharge and water surface slope measurements at gauging stations on these four rivers. Mannings n was found to remain almost constant over a range of common discharges, and was found to be a better descriptor of flow resistance than Darcy Weisbach f, Chezy C and log-law Zo for these rivers.

Investigating Spatial and Temporal Variability in Runoff and Sediment Generation Using a Physically-based Model, Thales

Abstract Limited within-day rainfall data availability has led to a paradigm of daily hydrological modelling that conflicts with key process timescales, resulting in poor modelling of runoff and particularly erosion. A new hydrological modelling framework that accounts for variation in rainfall, runoff and erosion across the landscape, and within each day using a statistical approach, has been developed. The ultimate aim is to develop a model for estimating runoff and erosion that is consistent with key process time and space scales. Central to this aim is to be able to run the model using daily precipitation data with a distribution function (DF) model to account for sub-daily variations in precipitation and with an algorithm that accounts for routing processes in the catchment. A series of DF models have been developed based on 6-minute interval precipitation data from pluviographs at the point scale. This paper provides insight into the effects of scaling up from hillslopes to whole catchments, with the results to underpin an extension of the existing point-scale models to catchment scale. An analysis framework that allows the various effects to be separated was developed. The Thales physically-based hydrological model was employed with grid-scale (25 m2) runoff and sediment generation processes of varying complexity specified. A series of simulations was performed to investigate both the temporal and spatial effects of rainfall and catchment variability on runoff and erosion generation. The results were analysed and compared across different scales to determine whether the scaling of rainfall in space and time could be linked to the scaling of runoff and erosion. In addition, the results were examined to identify an approach to account for the routing effects that become important in medium-sized catchments.