Comparison of Surface Area to Volume of Lake Buchanan

Emily Wieweck GEO 327g Fall 2011

1. 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

ArcCatalog

I. 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/bucha

nan/, -> 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 is

disconnected 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 snapping

toolbars -> 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 field

with 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 -> TIN Management -> Create

TIN -> Save output as tin_bath -> Import Spatial reference from any of

the lake shapefiles -> From the drop down menu under input feature

class choose the river -> click ok and the TIN is added to the table of

contents.

Figure 9- Creating at TIN from ArcToolbox.

g. Next a DEM is created from the TIN we just made. Open ArcToolbox

-> 3D Analyst Tools -> Conversion -> From TIN -> TIN to Raster ->

From drop down menu under input TIN choose tin_bath -> Save

output raster as bath_DEM -> set output data type to integer ->set

method to linear -> set sampling distance to cell size -> set z factor to

1.

Figure 10- Converting the TIN to a raster to calculate volume.

h. Now we will calculate the volume of the lake under each of the

contour lines. Open ArcToolbox -> 3D Analyst Tools -> Functional

Surface -> Surface Volume -> From the drop down menu under input

surface choose bath_DEM -> save the output surface as 920_vol in the

project folder -> For reference plane choose below -> for plane

height type in 920 -> leave the Z factor as 1. When the new attribute

table has loaded open it and copy and paste the volume into the excel

sheet. Do this for all of the contour line intervals typing in the

corresponding plane height for each interval.

Figure 11- Calculating surface volume of lake under each contour line.

i. Add a column in the excel table labeled Volume in acre-feet and

convert the volume in cubic feet to volume in acre feet using 1 acre-

foot = 43,560 cubic feet.

j. Open ArcToolbox -> Spatial Analyst Tools -> Surface -> Hillshade -.

Choose bath_DEM as input raster -> save output raster as hs_DEM ->

keep all other values the same -> click ok. When the hillshade is

generated open its properties -> Change stretch type to Standard

Deviations -> change n to 4-> change color ramp to blue bright and

check the invert box -> click ok.

Figure 12- Hillshade raster for DEM.

k. Open ArcToolbox -> Spatial Analyst Tools -> Surface -> Slope -> In

drop down menu under input raster choose bath_DEM -> save output

raster as slope_map -> choose degree for output measurement and

keep Z factor at 1 -> click ok. Move the slope_map above the hill shade

in the table of contents. Open the properties of the slope_map ->

symbology -> click on label -> format labels -. Click on number of

decimal place and change it to 1 -> click ok.

Figure 13- Slope map of Lake Buchanan.

l. Maps can now be made to see what the underwater slope looks like in

todays drafts compared with when it is full. Open the properties of

the slope map and change the color gradient to slope -> insert the

1010_poly shapefile from ArcCatalog and symbolize it with a 1 point

black line -> Do this same thing with the 980_poly in a new map.

These will both show the slope under the water dependent upon

where the lake level is.

4. Conclusion

a. By plotting the surface areas and volumes of Lake Buchanan at each

10 ft interval of lake level drop a correlation can be seen. Initially it

was anticipated that although surface area and volume would have a

linear relationship that at a certain lake level the surface would level

off as the volume decreased because of a steeper channel at the

middle of the river. However this wasn’t quite the case. There is a

small change in the slope of the line where surface area begins to

decrease less with less volume but it is not a noticeable as thought

previously (Figure 14).

b. Individual maps were also created showing the slope profiles of the

river bottom with the 1010 ft contour on one map representing the

lake being full and one map with the 980 ft contour representing what

the slope under Lake Buchanan looks like currently. These maps

show a comparison between what the lake bottom looks like at

maximum capacity and what it looks like during a bad drought. They

show a gradual steepness toward the center of the main channel. As

expected the farther away from the main channel of the river the

shallower the slope becomes.

Table 1- Surface area and volume changes with lake level drops.

Figure 14- Plotted volumes and surface areas at 10 ft lake level drops of Lake Buchanan.

lake level drop (ft)

Area (sq. miles)

Volume (acre-feet)

0 30.353751 657122.3898

10 24.868444 483038.2537

20 19.949113 343506.9197

30 15.890157 229777.8619

40 11.763802 143185.1365

50 8.080659 80993.90935

60 5.063369 40413.49905

70 2.850487 16321.37809

80 1.358369 4152.828845

90 0.444021 0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

657122 483038 343507 229778 143185 80994 40413 16321 4153 0

Su

rfa

ce A

rea

(sq

. mil

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Volume (acre-feet)

Volume vs. Surface Area of Lake Buchanan