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A GIS - Remote Sensing compatible rainfall-surface runoff model for regional level planning
The TIN map is overlaid on the land-use and soil maps, if possible again, through GIS software. Even if done manually, each triangular surface (also called a facet) of the TIN is associated with a corresponding land-use type for which a correlation table would assign an "equivalent overland flow roughness coefficient" required for surface runoff simulation. Similarly, each triangle is associated with a particular soil type which is correlated to a corresponding infiltration rate which, when subtracted from the design rainfall, produces the effective rainfall over the triangular surface.
The "equivalent overland flow roughness coefficient" can be expressed in various forms like the Darcy-Weisbach's, Chezy's, or Manning's. In the present model, the Manning's coefficient is used. Numerical values of infiltration rates for specific soil types can be had from agricultural handbooks. However, in the absence of field data, an assumed value may be adopted for the entire watershed, which is then later optimized.
The channels require slope, roughness and cross-sectional data, the first of which is obtained from analyzing the TIN model. Roughness data for the channel bed is based on the well documented values as given by any standard literature of channel hydraulics. The cross-sectional areas of the channels, at least for the main streams, have to be obtained from field observations. In the absence of such data, some guidance may be taken from literature on channel morphology.
For a given TIN, the drainage network is identified and numbered by the algorithms used in this model. Typically, the network composes of channel reaches emanating from ridges, which combine at confluences from where further reaches originate in a recursive pattern. The network of reaches at the outlet of catchment is assigned the first confluence number. This is because sometimes two reaches may meet at the outlet point of the catchment. According to the definition adopted for the model, a reach comprises of a number of channel segments and span between a ridge and a confluence or between two confluences.
The effective rainfall over the triangular facets may be assumed to move as an equivalent sheet flow down to the valley below. In the algorithms it is recognized that though the flow is two-dimensional globally, it may be considered one-dimensional if the local axes of each triangle are directed along the triangle's steepest slope. Accordingly, the triangles are divided into overland flow planes that cascade down from the higher to the lower facet till it meets a channel segment. The shape of the cascade plane is calculated and stored in relation to the connected channel reach/segment identification.
The flow on the overland cascade planes as well as that in the channel reaches is assumed to be modelled by the kinematic wave assumption for simplicity. The flow of each channel segment is treated as inflow to the segment connected just downstream and the outlet flow of a reach is input as the inflow to the confluence connected to its lower end. The equations are solved by an explicit finite difference scheme.
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