Valley Gradient
Valley gradient describes an increase or decrease in the elevation of the valley bottom over a longitudinal distance.
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Why is this metric important?
- Is a predictor of channel morphology
Metric info:
Machine Code: VALGRAD
Analysis Window
| Small | Medium | Large |
|---|---|---|
| 100 m | 250 m | 1 km |
Lines of Evidence
| LOE | Measurement |
|---|---|
| DEM | elevation start and end points in meters for each valley bottom segment length |
| Valley bottom segment | length in meters of each valley bottom segment |
Calculation
(top of segment elevation - bottom of segment elevation) / (valley bottom centerline length)
Process
In small riverscapes, we look upstream and downstream 50 meters, to build a 100 meter analysis window. At the top and bottom of this window, we buffer the end point by 25 meters to find the lowest value cell in our input DEM. This ensures that we are actually looking at the channel elevation, instead of a potential floodplain or terrace elevation, which could happen if the stream network and DEM don't align perfectly. We then measure the length of the stream network within the analysis window. From there it is a simple rise over run calculation:
(top of segment elevation - bottom of segment elevation) / (valley bottom centerline length)
The process is the same for medium and large riverscapes, except that the analysis windows are 250 meters and 1,000 meters, respectively, and the elevation search buffers are 50 meters and 100 meters, respectively.