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area and volume of flood water is a complex one determined by details of topography. Of necessity, the model simplifies this complexity. At many nodes, the Pardeshipura model assumed SPIDA's simple 'double cone' storage volume, in which the user specifies the surface area of flooding at each of two flood depths. Between the ground level and the lower depth, the storage volume is represented by an inverted cone with its apex at the node centre; the base of this inverted cone has an area equal to the surface area of flooding at the lower flood depth. Above this lower flood depth, the incremental volume is defined by the frustum of a second concentric inverted cone, the angle of which is defined by the slope between the two user-specified depths and areas. At these 'cone' nodes, all flood water is stored, and then returned to the channel at the same node as the water level recedes. Field observation confirmed that flood waters did recede back into the channel in many areas. |
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In other parts of the catchment, however, most of the water flooding out of the drain was seen to flow across the ground and return to the drain further downstream. For these cases, a hypothetical 'overflow weir' was assumed between the two nodes at the upstream and downstream ends of the overland flow. Finally, at a few downstream nodes in the catchment (M100, M120 and S90), any water leaving the channel effectively leaves the system, as the channel here is the boundary of the catchment. This effect was included in our model. |
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Solids levels in modelled drains were checked twice weekly, with periodic spot checks to find out if rapid variations took place during the week. These measurements were part of a programme to record solids buildup since a cleaning in September 1993. Solids depths measured in the open drainage channels in the Pardeshipura catchment were high. On average, approximately 30 per cent of the channel depth was filled with solids. However, in some locations, especially towards the upstream end, solids depths were as high as 85 per cent. |
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Towards the end of the study period, samples were taken from five evenly spaced stations. At each station, a 1 m length was cleaned out, and the contents analysed for their size distribution. |
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