Evaluation of Rivertools GIS Software for Stream-Profile Analysis of the Irish Hills, San Luis Obispo County, California
May 8th, 2013, 2:00pm
The Diablo Canyon Nuclear Power Plant is located on the coast below the Irish Hills southwest of San Luis Obispo, CA. Built in 1973, the plant is operated by Pacific Gas and Electric (PG&E). In recent years, PG&E has been conducting a large-scale study of tectonic risks to the plant as part of their long term seismic program (LTSP). As part of that program, airborne LiDAR data were collected as a base for topographic evaluation of the landscape, and those data have been released to SDSU for this project.
The Irish Hills are bounded by northwest-trending strike-slip and oblique to reverse faults, which curve westward both to the north and south of the hills. Movement of the Pacific plate toward the northwest is impeded by this bend, so the plate buckles. Fault slip in such a restraining bend generally leads to uplift of the compressed land surface. Evidence from coral dating suggests that marine terraces in the area have been uplifted at an average rate of about 0.2 mm per year for at least the past 120 ka.
Another way of assessing vertical displacement is by analysis of drainage systems. Specifically, the drainage density within the Irish Hills is expected to be different to that in nearby less-uplifted regions. There should also be a regional change in the Hack index, also known as the stream-length gradient or SL index. If lithology, precipitation and other key factors are the same, we should expect to see greater drainage density and SL index values in a recently uplifted region than in a nearby region where uplift is less. If we do in fact see this, we may conclude that recent uplift is a reasonable factor to explain the differences in drainage parameters.
For this project, a one-meter resolution LiDAR elevation model provided by PG&E was analyzed with RiverTools, a specialized geographic information systems (GIS) product designed to extract stream networks and related statistics from digital elevation models (DEMs). The extracted networks were exported to a vector format (ESRI shapefile) and spatially correlated with a one-kilometer square grid. The grid cells were coded to represent drainage density (total length of streams per square kilometer), SL index (values averaged over each square), and Horton-Strahler order, and overlaid on a regional map to display correlations between these values and regions of different uplift rate.
Preliminary results show a definite signal along the crest of the Irish Hills with respect to drainage density. Results for SL index and HS order are less definitive.