Siting a Dam using GIS tools

In this exercise, you will use the GIS to examine potential dam sites. You will determine the area to volume relationship for one of these. You will also examine the potential impact of building a dam at the proposed sites by determining the area of different land covers and land users that will be flooded by the proposed dam.

Introduction

You will start by copying topographic data from the S: drive to C:\usertemp using these instructions. Once you have these copied, open Arcmap GIS by clicking on the damsite.mxd file in your c:\usertemp folder. Once this opens, immediately save this file, under the same name, to your H: drive. This will save the dam sites for the next lab, which is the week following Spring Break. Once you have this open, you should see a map that looks like this:

The three dam sites are shown. You need to decide which are the best dam sites based on the area/volume relationship for a given site and the amount of useful land flooded by the reservoir. The layers that are shown in the map are below:

The potential_dams layer is a point data set which identifies the potential dam sites you will examine. The dams layer is a line data set which is used by the program. You will will not use this directly, nor see the lines. You will draw lines as graphics, which are stored during processing in the dams layer. The dem90 layer is the data that you will use to build contours to see the topography of the dam site and the program will use to determine the area and volume of the dam, given various dam heights. The dem90 layer has a cells that are 90 by 90 meters. The Hillshade of dem90 layer is the hillshading produced form the dem90 layer. This helps you see the topography. The dem layer is the layer that the program uses to calculate the watershed, based on dams that you draw. The dem layer has cells that are 1000 by 1000 meters. The Hillshade of dem is the hillshading layer produced from the dem layer. As with the 90 meter hillshading, it helps you visualize the topography. Induslandcov is not list above but it is included in your map. It is the same land cover dataset that you used last week.

To see Google Maps representation of this area, try this link.

Flow Direction and Watershed Modeling

Flow direction is the major building block in finding watersheds in a GIS. In this GIS, flow direction is assumed to be in one direction only. For example, in each cell of 90 meters by 90 meters or 1 kilometer by 1 kilometer, all of the runoff from that cell is assumed to flow into one, and only one, cell. The water from that cell flows into only one other cell. It might look like this:

In the upper left cell, with an elevation of 340, water will flow in the cell to the lower right, with an elevation of 332, which represents the steepest slope from the cell to all other cells. There are eight choices:

and the water must flow into one of the cells.

Watershed modeling works with flow direction grids, which are derived from the digital elevation model (DEM). The watershed model assumes a source grid. Your source grid is derived from the line that you will draw that represents the dam. Every cell that flows into the dam is evaluated. Each of the cells that flow into those cells are recorded. This process is repeated to the edge of the watershed.

Using the new tools

Add the new tool by clicking on the Tools menu then clicking on customize. Select both the Spatial Analyst and MIT Engineering for Sustain ability toolbars. The looks like this:

You will use the first three tools in this toolbar during this lab and the fourth in the next lab. It is disabled for today.

The first tool is a simple freehand drawing tool. Once you click on it, you drag the cursor over the map and draw a dam by clicking on the left cursor button, not releasing the cursor, then you drag it across the map, then releasing the button on the cursor. This is a graphic image which is used by the Find Watershed button.

The Find Watershed button will find the extent of the watershed above the dam that you draw. It requires that the dams layer is in the map table of contents and that you have a selected line graphic on the map. Graphics are automatically selected once they are made. If you click on the map using the pointer tool you may have unselected it. Just use the pointer tool to reselect it. When selected, the graphic will be surrounded by teal boxes. The Find Watershed tool will report the area of the watershed in square kilometers before drawing the watershed.

The Area and Volume of Dam button will calculate the volume and area of the dam you selected, based on a dam height that you enter. It depends on the raster layer that was created using the Find Watershed button. It reports the area and volume of the dam in millions of cubic meters.

Please check that the Spatial Analyst extension is enabled. Click on the Tools menu and then extensions. You should see this form:

If Spatial Analyst is not checked, you need to check the box next to it and then close the form.

Setting your map up

You may find it easier to find the best location for a dam when you see the contour lines. To make contour lines, you will first set the options for how big an area you want the lines drawn for. You should first zoom into the area of the dam. To get to the options, click on Spatial Analyst on the Spatial Analyst toolbar:

You should see this form:

As in the image, pick Same as Display as the Analysis extent then click OK. Do this after you have zoomed in to one of the dam sites. Now make the contour lines. To do so, click on Spatial Analyst in the toolbar, the click on Surface Analysis, the on Contour. You should see this image:

Your input surface should be dem90. The contour interval should be 20 (the dem90 elevations are in meters). Once you have these, zoom in even closer the the dam.

Using the tools

<drawing the dam from Hanan>

Once you have drawn the dam, click the Find Watershed button. This may take a several seconds. There are two results. The first is a message box that informs you of the low and high elevations encountered along the dam you have drawn. The low point is accurate however the high point is likely to be on only one side of the dam. You should check the heights using the Identify tool (looking at dem90 or the contour lines) to find the highest point on either side. You will use this to see how high a dam can be physically built.

The second result is a new raster layer that depicts the watershed that results from the dam you have drawn. It is called raster#, where # is an integer. Right click on the name of the layer and click on zoom to layer to see how large this is. It should be the entire Indus watershed above this point.

Once you have the watershed, you need to determine the volume to area relationship for the dam. To do this means that you need to run the Area and Volume of Dam button several times for different heights of the dam. You should start out with 20 (meters) and increase by 5 or 10 meters to 80 meters. When you click the button, you will see this form:

Enter the dam height and then click on Find Area and Volume of Reservoir button. This produces three results. The first is a message box with the area and volume of the reservoir in cubic kilometers. The next result is a grid with the elevations above and below the dam elevation. For programming purposes, the positive numbers show the water depth and the negative numbers imply land that is above the water level. The last result is a grid that shows where the reservoir is (elevations are positive and the value is 1) and where it isn't (elevations are negative and the value is 0).

Once you have determined the optimal dam height you need to find out what you would flood if you were to build this dam. Rather than sorting through all of the grids you created to find the grid that represents the reservoir at the optimal dam height, just rerun the program to create new grids. You will use the reservoir grid in this next section.

To see the reservoir on the landscape, make all of the other raster layers that you created invisible (clicking the checkbox next to each of them). Next, right click on the reservoir grid and then click Properties. Click on the Display tab and then change the transparency from 0 to 50%. This should allow you to see the underlying topography as well as the reservoir.

Finding the area of cultivated land flooded by your reservoir

The last raster image created, which has only 0 and 1 in the legend, shows the area that is flooded by the reservoir. You will use the Raster Calculator (part of the Spatial Analyst toolbar) to do this. Open the toolbar by clicking on Spatial Analyst on the toolbar then clicking on Raster Calculator:

To find the land uses and land covers that are in the reservoir, you need to multiply the induslandcov grid by the reservoir grid (raster20 in this example). This will yield a new grid, called calculation, which shows you all of the land uses that are covered by the reservoir:

What this tells you is that land uses/covers 90, 93, 98, 102, and 159 (values) had areas of 31, 497, 119, 314, 107 (counts) in the area covered by the example reservoir. The areas are in cells. The cells in this grid are 90 by 90 meters, or 8100 square meters. So the area of land cover 159 is 866700 square meters or 0.8667 square kilometers. To translate land covers from the codes to descriptions, use the USGS page you saw last week. Looking at this page, 159 is Crop Land (Winter Wheat). You will need to find the area of all of the land covers that are included in the area of the reservoir and find the text descriptions for each of the land covers.

You should have a two maps, one with the watershed and another with the reservoir.