Hydrological and hydraulic modelling of the Nyl River floodplain Part 2: Modelling hydraulic behaviour

  • AL Birkhead
  • CS James
  • MT Kleynhans
Keywords: Nyl River floodplain, Nylsvlei, Nylsvley Nature Reserve, modelling wetland hydraulics, wetland evapotranspiration, floodplain water balance, LIDAR.

Abstract

Catchment land-use and water resource developments may threaten the ecological integrity of the Nyl River floodplain, a world-renowned conservation area. The effect of developments on the water supply regime to the floodplain can be predicted by hydrological modelling, but assessing their ecological consequences requires transformation of discharge into ecologically-relevant hydraulic characteristics. Studies of Wild Rice (Oryza longistaminata) have indicated that the important variables are the extent, depth, duration and timing of flood inundations. One-dimensional hydraulic modelling has been successfully applied to predict these flood characteristics, using a suite of models comprising Quicksurf, RiverCAD, HEC-RAS and HEC-DSSVue. Separate but linked models were developed for three contiguous portions of the large floodplain. The topography was surveyed by airborne laser mapping, and extensive discharge, water level, rainfall, infiltration and evapotranspiration data were collected for model calibration and verification. The models were successfully calibrated through adjustment of Manning’s n for the main channel and floodplain in each portion, and are able to predict flooding characteristics at a resolution appropriate for ecological interpretation
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eISSN: 0378-4738