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Flood routing in ungauged catchments using Muskingum methods

MH Tewolde, JC Smithers

Abstract




River stage or flow rates are required for the design and evaluation of hydraulic structures. Most river reaches are ungauged and a methodology is needed to estimate the rates of flow, at specific locations in streams where no measurements are available.
Flood-routing techniques are utilised to estimate the stages, or rates of flow, in order to predict flood wave propagation along river reaches. Models can be developed for gauged catchments and their parameters related to physical characteristics such as slope, reach width, reach length so that the approach can be applied to ungauged catchments within the same region.
The objective of this study is to assess the Muskingum-Cunge method for flow routing in ungauged river reaches, both with and without lateral inflows. The Muskingum-Cunge method was assessed using catchment-derived parameters for use in ungauged river reaches. Three sub-catchments in the Thukela catchment in KwaZulu-Natal, South Africa were selected for analyses, with river lengths of 4, 21 and 54 km. The slopes of the river reaches and reach lengths were derived from a digital elevation model. Manning\'s roughness coefficients were estimated from field observations. Flow variables such as velocity, hydraulic radius, wetted perimeters and flow depth were determined both from empirical equations and assumed cross-sections
of the reaches. Lateral inflows to long river reaches were estimated from the Saint-Venant equation. The performance of the methods was evaluated by comparing both graphically and statistically the simulated and observed hydrographs. The results obtained show that the computed outflow hydrographs generated using the Muskingum-Cunge method, with variables estimated using both the empirical relationships or assumed cross-sectional shapes, resulted in reasonably accurate computed outflow hydrographs with respect to volume, peak discharge, timing of peak flow and shape of the hydrograph.
From this study, it is concluded that the Muskingum-Cunge method can be applied to route floods in ungauged catchments using derived variables in the Thukela catchment and it is postulated that the method can be used to route floods in other ungauged rivers in South Africa.

Water SA Vol.32 (3) 2006: pp.379-388



http://dx.doi.org/10.4314/wsa.v32i3.5263
AJOL African Journals Online