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Sensitivity to Sedimentation
Step Wise Guide
· Which channels in my watershed are most susceptible to sedimentation and associated overbank flooding?
Some channels are more susceptible to sedimentation and flooding than others. Here are the steps you can use in NetMap to identify those locations.
Step 1:Go to NetMap Erosion Module >Generic Erosion Potential. If the data have not been loaded, do so. You are interested in the channel representation of GEP, either at the stream segment scale or the aggregated (accumulated) values. In the drop down list in the tool select "generic erosion potential - segment scale" or "generic erosion potential - summed downstream". Alternatively you can used the "delivered" values, which are more focused on those areas that can deliver sediment (and organic material) directly to stream channels. Display the maps. You can use this to visually identify area of channels that are predicted to be in locations that have a high erosion and thus sediment supply potential. You can also convert GEP into sediment yield values (tons/km2/yr). Go to NetMap Erosion Module > Sediment Yield >GEP to Sediment Yield (this is optional).
Step 2: Go To NetMap Erosion Module > Sediment Yield >Channel Sedimentation Potential. This tool is used to calculate sedimentation potential by comparing the predicted potential for sediment delivery (Step 1) with the ability of streams to transport sediment, using an index of stream power. Display the results.
Predicted sedimentation potential may indicate areas where overbank flooding is more likely, but these areas will likely be in locations of wide floodplains or unconstrained channels.
Step 3: Go To NetMap Fluvial Morphology Module >Floodplain Mapping. Run this tool, using an elevation above channel based on number of bankfull depths. NetMap comes loaded with a floodplain width calculated at 2x bankfull depth. Display the map. If you want to use the default 2x value, simply Go To the Basic Tool Module (add-in) and using the "Maps", select Fluvial Morphology as the "category" and then choose the "floodplain width" attribute.
Step 4: Go To NetMap Basic Tools > Risk Management >Stream Segment Overlap. Here, from the drop down lists, you search for spatial overlaps or intersections between some measure (percentile of the full distribution) of sedimentation potential (for example, the top 10%) and some measure of floodplain width (say top 10%). Make sure you select a map display attribute from the uppermost drop down list (can either be one of your primary attributes, sedimentation potential or floodplain width, or something else like fish habitat or stream gradient, etc.). Run the tool and search for overlaps. Depending on what you selected, you may find none. If so, keep on increasing the percentile magnitudes (i.e., top 20%, top 10%), of you can focus more narrowly, such as the top 1% of sedimentation potential and the top 5% of floodplain width.
This allows you to focus in on those areas in the watershed that are predicted to have the greatest intrinsic potential for sedimentation and flooding. Of course, use of aerial photography, field observations and measurements and local history of flooding are also critically important.