The Valley Bottom Extraction Tool (VBET) is a tool used to identify the valley bottom of a riverscape, and roughly separate it into geomorphic units (channel, topographically low-lying floodplain, and elevated (more distal/inactive) floodplain). The tool takes a DEM and channel area polygon as inputs. Two topographic analyses of the DEM are used as lines of evidence in determining what is valley bottom:

• Slope
• Height Above Nearest Drainage (HAND)

(These inputs can be generated using the TauDEM tool)

In addition to being used in generating the HAND raster, the channel area polygon is rasterized and used as an additional line of evidence.

For interpretation as lines of evidence, raster values in each of the rasters are converted to values between 0 (very low likelihood of being valley bottom) and 1 (very high likelihood of being valley bottom) using calibrated transform functions with parameters that vary based on stream size (as determined by drainage area). The logic for this is that in smaller stream settings, valley bottoms are likely to have higher slopes than in larger river settings. Similarly, higher values of HAND (higher elevations above the drainage network) are less likely to be part of the valley bottom in smaller streams. Finally, these transformed inputs are averaged using different weights, resulting in a raster with values between 0 and 1, essentially representing relative probability of being part of the valley bottom. This raster is one of the tool outputs. Additionally, threshold values can be chosen in order to extract a polygon representing the valley bottom from the raster. Different threshold values can be used to estimate the extent of the different geomorphic units. By default, the tool uses a threshold of 0.65 to extract the valley bottom polygon, and a threshold of 0.85 to extract the low-lying portion of the valley bottom. For a detailed description of the algorithm, see the algorithm page.

In addition to delineating the extent of valley bottom, VBET segments the valley bottom output in to discrete polygons, whose size varies based on the size of the river. These polygons are called discrete geographic objects (DGOs). These DGOs create a sample frame that can be used to calculate metrics related to riverscapes, and VBET calculates a suite of such metrics for each DGO. At the center of each DGO, VBET places a point, referred to as an integrated geographic object (IGO), because it can be used to summarize metrics calculated over a moving window (i.e., a combination of DGO polygons) that the point IGO is at the center of. Like with the DGOs, VBET calculates various moving window metrics and adds these attributes to the IGOs. For a detailed description of the metrics, see the metrics page.

References

• Gilbert J.T., Macfarlane W.W. & Wheaton J.M., 2016. The Valley Bottom Extraction Tool (V-BET): A GIS tool for delineating valley bottoms across entire drainage networks. Computers and Geosciences. DOI: 10.1016/j.cageo.2016.07.014.