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UT GEO 371C - Comparison of Surface Area to Volume of Lake Buchanan

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Comparison of Surface Area to Volume of Lake Buchanan Emily Wieweck GEO 327g Fall 20111. Introduction a. Problem: This project aims to pose a question that can be answered by using downloaded data that can be processed and analyzed through the ArcGIS software. The question that will be addressed in this report is how the surface area and volume of Lake Buchanan changes as the lake level drops. The lake level drop will be looked at in ten-foot intervals starting at 1010 ft to 920 ft above mean sea level. b. Background: Lake Buchanan is the first in a series of seven dammed lakes in the Texas hill country along the Colorado River. Buchanan Dam was constructed in 1937 and stands at 1025,35 feet above mean sea level. A historic low in the drought of the 1950’s brought the lake level down to 983.7 feet above mean sea level. Currently the lake surface is at 988.72 msl. Although this isn’t the record low for lake Buchanan it is extremely close and represent the serious drought that our state is currently in. 2. Data Collection a. The first step is to create a folder for the project. b. Download bathymetry data from the web. Go to www.capcog.org -> Click on the Information Clearinghouse tab on the left hand side -> Click on Geospatial Data -> Scroll down to the bathymetry and download the shapefile and save it to the project folder c. Open up a blank ArcMap and insert the bathymetry shapefile from ArcCatalogI. Using the select tool select all contours that apply to Lake Buchanan, I used a map from the following website to choose the correct contours www.tpwd.state.tx.us/fishboat/fish/recreational/lakes/buchanan/, -> Export features by right clicking on the layer and choosing export -> data -> make sure selected features is chosen in the drop down box. Figure 1- Selected river data exported into new file.II. Change the symbology of the new lake file: open up the layers properties-> click on the symbology tab-> select categories -> unique values -> in the value field drop down menu select contour -> add all values-> change all lines to be a thickness of 1.5 -> uncheck the all other values box and click ok Figure 2- Symbology of the lake file categorized by 10 ft contour intervals. d. The height of the dam is needs to be known so that the highest surface area and volume of the lake can be calculated. Buchanan Dam is registered at 1025.35 feet above mean sea level. This information can be found at http://www.lcra.org/water/dams/buchanan.html.3. Data Processing a. Since the highest contour downloaded in the bathymetry shapefile is 1010 ft. above msl we will think of this height as the lake being full. Turn on river as a selectable feature -> Select the 1010 ft. contour line from the river shapefile -> Export the selected feature and save shapefile as lake_full in the project folder. Figure 3- Exporting and saving selected contour interval. b. Open a new blank ArcMap and insert lake_full from ArcCatalog -> Open the Editing and Snapping toolbars -> Turn on snapping -> Start Editing -> Select part of the contour line and zoom in to where it isdisconnected to the next part of the contour line -> select edit vertices -> add vertices -> add a vertex to snap the disconnected lines together-> do this to the whole contour line connecting any points that may be disconnected as to form polygons -> save edits. -> Repeat the last two steps for each contour line in the river shapefile saving them all to the project folder. Figure 4- Editing vertices to connect missing segments. c. To make sure that the polygon is completely closed in with no errors a topology needs to be created. I. Create a geodatabase for the project including all contour shapefiles. In ArcCatalog right click on folder -> New -> personal geodatabase -> create new feature dataset by right clicking on geodatabase -> add in all contour shapefiles.II. Right Click on the new feature dataset and choose create new topology -> name it lakes_Topology -> leave the cluster tolerance and click next -> select all feature classes associate with a contour interval e.g. lake_920 -> add the rules of no dangles -> click next and then finish after it reports the summary. Figure 5- Summary of Errors. III. Drag the newly created topology above everything in the table of contents -> turn on the editing, topology, and snappingtoolbars -> start editing -> open the error inspector under the topology toolbar -> search now for all errors -> For a dangle error zoom in closer to the error and see if it needs to be trimmed or extended to be corrected -> These will all need to be extended most likely -> try snapping or extending the line by right clicking on the error and choosing either of these options -> type in a guess e.g. 10 and see if it snaps to the other line -> if it fails try and larger number. d. Each feature (individual contour interval shapefiles) must be converted to polygons to calculate area and volume. Open ArcToolbox -> Data Management Tools -> Features -> Feature to Polygon -> In input features choose lake_full shapefile and in output features save as 1010_ poly to a new folder created in the project folder named polygons -> This must be repeated for all contour shapefiles.Figure 6- Feature to Polygon Tool. e. The next step is to calculate Area for each of the polygons that represent the lake at a certain level. Create an excel table with three columns that represent lake level drop in feet, Area in square miles and volume in acre feet-> in the rows under lake level drop label in 10 feet increments from 0 to 90 which represents that lake level drop below it being what we are considering full. Save it to the project folder. I. Open up 920_poly from the polygons folder -> right click on the 920_poly layer and choose open attribute table -> add fieldwith the name Area -> the type in the drop down menu should be set to double -> click ok. Figure 7- Adding a field to an attribute table. II. Right click on the area column title in the attribute table and choose calculate geometry -> keep everything the same but make sure the property is set to area and the units are set to square miles. Do this for all of the maps in the polygons folder and insert the corresponding areas into the excel table.Figure 8- Calculating the surface area at each lake level drop. f. The next step is to calculate the volumes for each of these polygons. Open ArcToolbox -> 3D Analyst Tools ->


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