Improving Calibrated SWMM Routing Parameters By Channel Process Disaggregation: Diurnal Flows In The Ash River, South Africa.

William James

ABSTRACT

The Lesotho Highlands Water Scheme comprises several large dams, weirs, intakes, tunnels, outfalls and the Muela hydropower plant, which delivers diurnally fluctuating flows to the Ash/Wilge River system in South Africa. A PCSWMM model of the receiving catchment to the Saulspoort weir near Bethlehem in the Orange Free State was developed by a co-worker Hanlie Pretorius for investigating the use of satellite rainfall data. Diurnal flow variations in the Ash/Wilge rivers, caused by hydroelectric demands, are similar to unidirectional low tidal waves and daily fill and drain flat and wet areas and banks in the channel between the tunnel outlet and the Saulspoort weir, lagged in tune with modified electricity demand in Lesotho. Flow and depth observations are available at both ends of the Ash/Wilge river reach, providing a dataset for investigating the dynamic effect of small-scale and bank storage when routing dynamic waves in steep, rocky river channels. Additionally the local climatic seasonality in rainfall results in prolonged periods of dry weather, which allows study of parameters governing recovery of soil storage, for example. The presentation covers a semi-automatic procedure for developing groundwater-aquifer compartments in PCSWMM, and the systematic improvement of SWMM hydraulic stream routing parameters for these dry season waves over a 2-year continuous period. The emphasis is on calibration and the convergence of Manning’s n and trapezoidal cross-section towards generally accepted values, and reduction of the Integral Square Error when sequentially taking into account the following processes: (a) unmeasured seeps in the headwaters, (b) in-stream flat wet areas and small dams, and (c) active river bank storage.

Evidently the methodology could be extended to an integrated PCSWMM model of the South African transbasin water system (Lesotho, Vaal, Orange, Tugela, Great Fish, Sundays catchments), which can be semi-automatically built to an arbitrary, specified level of complexity, using existing public domain topography (e.g. Google Earth).


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