NetMap's Technical Help Guide

Mapping Wide Valley Floors

Delineating floodplains and other valley floor landforms in NetMap can be done for all channels in a watershed at once. However, be aware that during delineation, an analyst must decide whether to evaluate (rank) by area, by elevation (tool interface here) or by bankfull depths. (more detail here).
However, in valley floors that are very wide (multi-kilometers) and that contains multiple channels, including distributaries or confluencing and parallel running tributaries, floodplain delineation of all channels at once can produce errors. This is because the model has difficulty determining which floodplain is associated with which channel.
Hence, when delineating floodplains in wide valley floors, it is recommended to use only a single channel (G in Figure 1).
Figure 1. When mapping floodplains along wide valley floors, it is recommended that you use the single channel only option in NetMap's tool interface (G).
Note that the number of bankfull depths (elevation) chosen controls the lateral extent of the delineated floodplain (C, and see Figures 2 and 3 below).
Figure 2. Specifying the number of bankfull depths (elevations) (C in Figure 3) controls the lateral extent of the floodplain that is delineated. Note that interventing high points, if laterally extensive, will limit identification of floodplains as shown in the figure. Hence, a larger number of bankfull depths will capture a larger extent of the floodplain. See Figure 5.
Figure 3. Use of an increasing number of bankfull depths leads to an increase mapped extent of the floodplain (specifying C in Figure 3). Example is from the Matanuska-Susitina watershed in Alaska.

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