The selection of suitable gauge sites, river reaches, and event for analysis is presented in this chapter.
As noted by Thukela Basin Consultants (2001), the Department of Water Affairs and Forestry (DWAF) initiated the Vaal Augmentation Planning Study (VAPS) in 1994 to determine alternative development options to meet the increased demand for water in South Africa. DWAF has found that inter-basin transfer schemes, together with other strategic actions, offer a possible and affordable means for augmenting water supplies to the Vaal River system. The Thukela River, along with several other options, was investigated for a further inter-basin transfer scheme.
According to Thukela Basin Consultants (2001) and Encyclopaedia of Nationmaster (2004), the Thukela River has its source in the Drakensberg Mountains near Bergville, where mountain peaks rise to over 3000 m. The river descends rapidly dropping 947 m, down to the Thukela falls, the mean annual rainfall (MAR) exhibits significant variation, ranging from 1500 mm or more in the Drakensberg region to as low as 50 mm in the dry central areas of the catchment.Specifically, in the mountainous Drakensberg area, the MAR exceeds 1500 mm, indicating a high level of precipitation. In contrast, the dry central regions experience considerably lower rainfall, with the mean annual rainfall dropping below 700 mm.
The Thukela catchment was selected to conduct flood routing studies due to its numerous gauging stations, long records of flow, and extensive rivers and tributaries. Additionally, the research would supplement other current research activities in the School of Bioresources Engineering and Environmental Hydrology. This disparity in rainfall distribution underscores the diverse climatic conditions within the Thukela catchment, with implications for water resource management and ecosystem dynamics.
The factors considered when selecting suitable gauging stations for streamflow records include the availability and quality of data, as well as the suitability of the river reach to estimate flood routing parameters. This involves assessing the presence of constrictions, dams, or backwater effects resulting from inundations. Additionally, the appropriateness of the reaches to apply the Muskingum flood routing methods is evaluated. The data required for flood routing analysis typically includes recorded inflow and outflow hydrographs, river slope, channel network, channel roughness, and ice jam conditions (Fread, 1993). Reaches of different lengths were selected to assess the influence of river length on the application of flood routing methods.
River reach length (ΔL) and slope (S) were derived from a 200 m Digital Elevation Model (DEM) provided by the South African Atlas of Agrohydrology and Climatology (Schulze et al., 1997), using the ArcView 3.2a (ESRI, 2000) software package. Base values for Manning's roughness coefficient were estimated from field observations and additional sources. The catchment area and location of gauging weirs were obtained from the Department of Water Affairs and Forestry (DWAF) hydrology database available online.
As illustrated in Figures 3.1 and 3.2, the Thukela catchment spans latitudinally from 27.410 to 29.400 S and longitudinally from 28.960 to 31.440 E, covering a total area of approximately 29,036 km2. The catchment comprises 86 interlinked and cascading Quaternary Catchments, as defined by the Department of Water Affairs and Forestry (DWAF) (Schulze and Taylor, 2002).
Figure 3.1 Thukela location map.
Figure 3.2 Selected gauging stationsstations.
Figures 3.3, 3.4, and 3.5 depict selected sub-catchments, gauging stations, and river networks within the Thukela catchment. Due to the limitations of Muskingum methods outlined in Sections 2.1 and 2.2, as well as the lack of comprehensive data for many gauging weirs in the Thukela catchments, this study focuses on only three of the sub-catchments. The selection of these specific sub-catchments is primarily based on data availability.
A summary of weir site locations and other relevant information for each gauging station are included in Table 3.1.
Table 3.1 Summary of reaches and gauging stations used.