.Brandan Scully 11/3/09 MEA 592 ? GIS Analysis & Modeling Assignment 9 Dr. Mitasova Task The assigned task was to derive flow accumulations and watersheds in GRASS and ArcGIS. Approach: Figure 1 shows watersheds and streams derived from 30m NED using r.watershed and r.streams command in GRASS. Figure 2 shows depressions in the 30m NED raster that were filled. Figure 3 shows contributing watershed of a defined outfall, computed using r.water.outlet. Figure 4 shows flow lines and topo lines for comparison. Figure 5 shows derived streams carved into the elevation raster. Figures 6 and 7 show lake area at elevation 113.7 and 115, respectively. Discussion Figure 1 shows watershed delineation based on flow accumulation and stream development. This is exceptionally useful. When we perform this operation in consulting, we typically to this by hand, which can take hours for complicated topography. Two issues that I can see immediately are that this may perform poorly in very flat areas at high resolution. This would be typical for commercial site development such as can be found in Johnston County, NC, where we routinely work. The other issue, is this likely won?t handle storm sewer networks very neatly, which is an application where I would appreciate having a tool like this. One option would be to carve in the network, but doing so will distort surface topography in a detrimental way. This would be very useful on a watershed planning scale, and likely lends itself nicely to surface weighting methods, such as NRCS Curve Number analysis. Figure 1 Figure 4 Figure 2 Figure 2 Figure 3 Figure 4 Figure 5 Area for the watershed corresponding to Figure 3 was reported using the r.report command. The output areas were described as ?0? and ?1?. I assume that the value ?1? corresponds to the area within the watershed, making the watershed 58.9 acres. Figure 4 has flow lines that appear to converge in valleys (downslope). Uplsope flowlines accumulate on ridges. Figure 5 shows stream carving, which I alluded to already. This looks useful for major streams and rivers, and can be useful for bypassing structures like bridges. It works in that instance because a bridge is much smaller in scale than the entire watershed, and is unlikely to significantly affect runoff flow regime. This is not the case for storm sewer networks, where surface flow may be designed to carry runoff to a collector that runs below and in the opposite or oblique direction to the surface. Figure 6 and 7 are useful for inundation modeling, or computing a bank fill volume of for beach nourishment. It?s likely more accurate and less time consuming to use this method than the average end area method. I was unable to get the sequence of commands at the end of the ArcGIS to run. I received the error ?Output raster: Streams_der30's workspace is an invalid output workspace.? And ?Invalid parameters. Failed to execute (Con).? I tried to use the con command manually, but without success. Conclusion: The hydrology tools explored here seem like quick and powerful alternatives to common traditional tools. Results are highly dependent on data quality and choice of tool for the application. Combining these methods with some of the statistical methods we have explored earlier can extract very useful data, such as the average number of people at risk of sea level rise, or the socioeconomic status of populations that experience flooding during heavy rains in urban areas. ScullyBM Microsoft Word - bmscully.assign9.docx
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