Drainage modeling case study

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Drainage modeling case study

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In the following discussion we use the Pentland Hills NT04 test DEM to illustrate the procedures involved. If a relatively flat and/or poorly bounded region is selected, with integer height values, the output from hydrological modeling may be ambiguous, so here we have selected a more mountainous test dataset.

Flow accumulation

Figure 6‑28A shows the Pentland Hills source DEM as a hillshaded relief map. This version of the relief has been pre-processed to remove small pits in the surface, as described in Section 6.4.1, Drainage modeling. Using this amended grid file as input, a local drainage direction (ldd) grid has been generated (Figure 6‑28B), which is similar to, but not precisely the same as, the aspect map in this case. This is because it is based on the simple D8 algorithm rather than more complex computations used in aspect determination. With the ldd map as input, flow accumulation modeling can proceed. Figure 6‑28C shows a close-up of a part of the NT04 grid after the accumulation process has been run. Lighter pixels are those with higher accumulation values, and the pixelated nature of the accumulations and the boundary between accumulation zones can be clearly seen.

Stream network construction

The flow accumulation grid provides the basis for stream network construction, typically marking all accumulation values greater than some pre-defined figure (e.g. 10) as being streams or potential streams. This process is illustrated in Figure 6‑29A. Note that an assignment rule (e.g. proximity) is required for flat regions. Figure 6‑29B shows those regions of the source DEM (prior to pit-filling) that may warrant closer attention. This may be because their values indicate that they are either flat (shown here in yellow) or local extrema (maxima in red, minima in blue, saddles in green). Each stream is identified by coloring the appropriate pixels; in this case without making allowance for accumulation value in the number of pixels colored (i.e. unlike Figure 6‑28C).

Figure 6‑28 Flow direction and accumulation

A. Source DEM after pit removal

B. ldd grid — D8 algorithm

C. Flow accumulation — selected area

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Figure 6‑29 Stream identification

A. Streams

B. Flat areas and extrema

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Stream basin construction

Surrounding collections of streams are the upstream limits of flow, which identify the watersheds, as shown in Figure 6‑29A and without the stream overlay in Figure 6‑30A. Figure 6‑30B illustrates the construction of stream basins. These are shown here as yellow/white polygons, and are subdivisions of the main watershed polygons. The stream branching patterns shown and the identification of stream basins are both determined by flow threshold parameters, e.g. by setting the parameter lower a larger number of smaller stream basins will be identified, whilst increasing this parameter will reduce the number of such basins within each watershed.

Figure 6‑30 Watersheds and basins

A. Watersheds

B. Stream basins

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