Hydrological assessment to predict velocity-flow classes in the lower Thukela River

Abstract

This study investigates the use of a two-dimensional hydrodynamic model (River2D) to determine environmental flow requirements in the lower Thukela River, KwaZulu-Natal. A digital elevation model (DEM) was developed by combining bathymetric data from field surveys with topographic data in ArcGIS. HECGeo-RAS was used to delineate cross-sections, flow boundaries, river banks and flood plains from the DEM. Data were imported to HEC-RAS were a series of flows were simulated to generate a stage-discharge curve. The predicted stage generated by HEC-RAS was used to set the downstream boundary conditions in River2D. The 2-dimensional modelling techniques used in this study make use of a combination of three different programs namely: BED, MESH and River2D to create a river bed profile that can be used for complex calculations. To determine the habitat requirements and preferences, 19 freshwater and estuarine fish species relevant to the lower Thukela River were used in the analyses. Multivariate statistical analysis showed that some species community structures changed significantly with a change in substrate and velocity. Labeobarbus natalensis and Eleotris fusca were the identified indicator species for this study. Preference files were generated for each species as well as habitat suitability according to field data. To determine the environmental flow requirements (EFR) of the lower Thukela River, historic and habitat methods were used and compared. River2D make use of the PHABSIM concept to calculate weighted usable area (WUA) (m2/m) by combining habitat suitability with velocity and depth preferences. The EFR suggested by the historic methods for the lower Thukela River is too low and does not consider the anthropogenic changes upstream of the study site and therefore the habitat method in the form of WUA was recommended.