The hydrologic and hydraulic study of the behaviour of the Nyl River floodplain

Abstract

The main purpose of this study is to model accurately the hydrologic and hydraulic behaviour of the Nyl River floodplain. It is envisaged that the model will not only provide a means whereby possible impacts on the floodplain system can be predicted, should upstream developments in the catchment be undertaken, but will also complement other investigations that have thus far been undertaken in the biological and ecological fields. Two rainfall-runoff models with different time scales, namely monthly and daily were used for the hydrological analysis. The monthly analysis was carried out using WRSM2000 as a means of comparison and for more broad scale planning, whereas the daily analysis was carried out using DAYFLOW in order to provide the required input to the hydraulic model. Hydrological model calibration was achieved on ten gauged catchments. Further tests on the daily models ability to simulate daily flows indicated the model to be adequate for the study, notwithstanding the shortcomings in the distribution of rain gauges, particularly in the middle reaches of the study area. Hydraulic modelling and four modelling tools are used for developing the hydraulic models, viz Quicksurf, RiverCAD, HEC-RAS and HEC-DSSVue. The topography surveying is carried out using an aerial laser scan, and the data is processed using the Quicksurf and RiverCAD software packages. The hydraulic behaviour of the floodplain is shown to be well replicated by the models, with predicted peak stage levels generally agreeing to within 20 cm of observations.