Charles City County Flow Concentration Analyses

 This week was spent performing drainage analysis on 9 fields in Charles City County, Va. Using the 10m DEMs on the USGS’s Seamless server I was able to determine flow concentration values along the edge of each field. Charles City County is not one of the 11 counties covered by the newly available LIDAR. However, as mentioned in my previous posts, we are able to use this data with confidence after doing comparison analyses on several fields. The 9 fields will also be used in our test of GIS flow concentration areas with actual field observations. I was able to obtain information on the field ownership via the GIS parcel data on the county’s website and am working on contacting the owners for permission to visit the fields. During the visit my teammates and I will walk the parameter of the field and record the location of channels exiting the field with a GPS unit. Then back at the lab I can put these locations into ArcMap and determine their relation to high accumulation locations. This is a key step in our project. If there is a strong relationship between actual channels forming and high flow accumulation values in GIS then these steps can be used easily to find target points for buffer bypassing on a broad scale, without field visitation being necessary.

After a brainstorming session with my advisor Greg Hancock and Stu Hamilton from the GIS department, we were able to devise a method of quantitatively determine the relationship between our accumulation values from each resolution data sets. We decided to focus on the highest values on each field to eliminate the issue of sample size discrepancies. I will use excel to determine the variance in the accumulation values and in GIS I will find the distances between the high value points of each data set. Hopefully this will give a good estimation of the relationship between the two resolutions.

Click on Image for larger view

Included with this post is an image of a field in Charles City County. Using symbology tools in ArcMap I displayed the flow accumulation values as graduated circles. This allows one to easily see the points of the field where large amounts of flow are entering or exiting the field. The values of these points allow me to determine area of field drained, as each point in the total value belongs to one pixel with a known area. The majority of the fields analyzed thus far have shown large percentages of the field draining by a few points but hopefully in a few weeks we’ll have more data and a quantitative determination of the extent of the problem.