A relatively simple tool that delineates valley bottoms based on water-level analysis. This model does not account for floodplain hydraulics.

• Difficulty: 2
• Technical level: 2
• Expense: 1
• Scale: Typically 10 m or 30 m
• Accuracy: 3

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The Valley-Bottom Mapping Tool (VBMT) (Goetz, 2001) is a relatively simple tool that can be used to delineate valley bottoms and focus riparian mapping efforts. This tool is frequently used in conjunction with a vegetation mapping method such as CART analysis. The VBMT utilizes a topographic analysis that is bound by maximum width, height, and slope parameters.

The VBMT delineates valley bottoms around stream networks using a flood-fill algorithm that operates on digital elevation models (DEMs). This analysis models water levels at a given elevation above a stream channel to establish those portions of the landscape that are in the bottom of the valley and are likely to be under the influence of the stream system. Depending on the topography this method can over-estimate the extent of the valley bottom. To compensate for this over-estimation, the model is constrained by three topographic parameters that are defined by the analyst and vary according to the Strahler stream order. These parameters are:

• Buffer distance—places a limit on the maximum width of the valley bottom
• Change in height—places a vertical limit on the height above the stream
• Slope threshold—places an upper limit on the slope a riparian area can have

The buffer distance and change in height parameters operate on the assumption that riparian areas will only occur within a certain distance from the stream itself. Thus, if the topography around the stream results in an extremely wide or deep flood, the tool will exclude areas at an unreasonable distance from the streambed. Likewise, the slope threshold will exclude areas with steep topography that are unlikely to be influenced by the stream. These parameters help link the size of the riparian area to the size of the stream. This is accomplished through a user-prepared table that relates each stream order to the appropriate parameter size.

The VBMT produces a raster dataset depicting the modeled valley bottoms. This dataset should be examined by an Image Interpretation analyst to verify the spatial accuracy of the model.

• RSAC Riparian Mapping Tool
• Land Facet Cooridor Designer
• Buffers

• Digital Elevation Model (DEM)

The VBMT produces a raster dataset depicting the modeled valley bottoms.

Figure1 : Example valley bottom product (from Walterman and others, 2008).

Walterman, M.; Fisk, H.; Lachowski, H.; Maus, P. 2008. Mapping Valley Bottoms for Resource Management. RSAC-0081-TIP1. Salt Lake City, UT: U.S. Department of Agriculture Forest Service, Remote Sensing Applications Center. 4 p.

A riparian mapping effort was conducted in the Chattahoochee-Oconee National Forests. The investigation utilized the VBMT to delineate valley bottoms then validated the mapping products using GPS field surveys and manual GIS topographic analysis.

Goetz, W. 2001. Developing a predictive model for identifying riparian communities at an ecoregion scale in Idaho and Washington. Logan, UT: Utah State University. 65 p. Thesis.

• Predictions are limited by the scale and quality of the DEM.
• The method is dependent on user-defined parameters to place boundaries on the flood-fill analysis. While this buffer varies with Strahler stream order, a buffer is still an inexact tool for modeling the physical environment.
• Uncertain performance in low gradient landscapes.

• ESRI’s ArcGIS
• ERDAS Spatial Modeler
• VBMT AMLs, ERDAS models, and instructions are available here:Chattooga.zip

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