Sediment Volume
August 2008

I have recently completed a short project for Oregon State University to estimate the volume of sediment contained at the bottoms of several river valleys in western Oregon. The subject areas consisted of about 12 reaches located in the Knowles Creek and Golden Ridge Creek watersheds/stream-networks, which have planar areas of about 58 and 48 square kilometers respectively. I wrote several Python/Wx programs to read and display digital elevation model data acquired from LIDAR (LIght Distance And Range) surveys taken from small aircraft, and to calculate synthetic bedrock bases for the valleys based on the slope and elevation of the valley walls and scattered field measurements of the location of a small number of bedrock points located in stream beds. The valley boundaries were determined semi-automatically with another program which iteratively looked for cross-valley profiles starting from an initial centerline guess entered with the mouse. Plots of down- and cross-valley elevation were created, volumes were calculated from irregular polyhedral prismatic cells, and the resulting sediment thicknesses were saved and displayed as another DEM surface. The results of these analyses will be used to track sediment flow and recycling over geologic timescales.

Here is an elevation plot of one of the study areas (Knowles Creek):

Here is a subset of the valley (upper middle in top plot), which is about 3 km long ('down' is toward the upper left):

Here's the valley in 3d:

Here's the geometry as calculated by one of the programs. The valley walls are in blue, the streambed (thalweg) is in yellow, as are the cross-valley profile lines. The green crosses are bedrock points obtained on-site, which correspond well with the calculated streambed:

Here's the calculated down-valley elevation. The synthetic bedrock base is in orange, and is at or below all known bedrock and calculated streambed elevations:

Here are the cross-valley profiles, with the thalweg in white. Each cross-section consists of the surface elevation (from DEM), the synthetic bedrock base, and the extrapolated slopes of the valley walls:

The sediment volume at any down-valley position is calculated from polyhedral 'cells' bounded by the profile cross-sections, the surface elevation, and the bedrock elevation. The program prints out width, length, thickness, area, and volume for coordinates every 10 m. Here is a composite plot of the estimated sediment thickness at any {x, y} point in the valley:

A similar analysis was performed on the other reaches of both study areas.

Link to research log

©Sky Coyote 2008