A Regulatory Application of LiDAR Data

Capture South Florida Water Management District (SFWMD) has an agreement with the U.S. Department of Agriculture and the Natural Resources Conservation Service (NRCS) to provide engineering/hydrologic modeling technical assistance to NRCS in the delivery of technical services related to fish and wildlife conservation throughout the state of Florida on non-Federal private and Tribal lands, and to deliver conservation programs that are a part of the Food, Conservation, and Energy Act of 2008, also referred to as the 2008 Farm Bill.

In order to evaluate the water management systems for off-site potential impacts a computer model is generally developed for the existing condition and the peak stages and flows generated are compared to the proposed system to determine impacts.

One of the most data intensive steps for model development is the creation of the topographic information necessary to construct the stage/storage relationship.  The use of digital elevation models (DEM) derived from LiDAR data allows for quick and accurate estimates of basin storage especially in remote areas where conventional topographic surveys are unavailable or cost prohibitive.  The use of these LiDAR derived DEMs allows District staff to accurately delineate and evaluate watersheds because of the large extent of the area covered.  In the past, many watershed hydrology and hydraulics assessments have not been possible because of a lack of topographic data. The following discussion presents a typical project in support of the 2008 Farm Bill.

The project involved the production of volume calculations (based on one foot intervals) for each of the 47 basins identified within a large project encompassing approximately 24,500 acres of land. The steps taken to accomplish this were as follows: Create a file directory for each basin, within each directory, create a file geodatabase to store: the basin boundary, DEMs, contours, TINs, and text files.

The basin boundary becomes the mask for clipping the DEM. blog1.jpg

The clipped LiDAR DEM.

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Using ESRI’s 3D Analyst Extension a contour feature class is created from the clipped DEM.

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A TIN is created using the contour layer as the input feature class. blog4

In some instances the TIN will extend beyond the boundary of the basin so it needed to be edited to remove the excess area. When clipping for this purpose, the surface feature type must be set to Hard Clip. This ensures that the basin boundary defines the clipped extent of the TIN.

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At this point the 3D Analyst tool Surface Volume is needed to obtain the calculated volumes.  Unfortunately, the tool creates a separate results text file for each elevation analyzed and thus can result in a large number of files. To fix this problem the standard ESRI Volume Calculation script was modified to produce one file with the calculated volumes for all elevations analyzed. The output is a comma delimited text file listing the outputs of the surface volume function for multiple depths. You can view the modified script on GitHub. blog6

Here is the output for 13 elevations.blog7 The last step was to QAQC the results by comparing the basin acreage with the Area_2D of the maximum elevation.  In this case, the basin acreage is 464.36 acres and the volume calculated acreage is 20,227,347 / 43,560 = 464.355. With this basin complete its time to start another one…

Linda McCafferty, Geographer 3, Regulation GIS, SFWMD
Shakir Ahmed, Geographer 3, Regulation GIS, SFWMD
Juan Tobar, Supervisor – Geographers, Regulation GIS, SFWMD

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