In this example, the upper side of the dam will be at an altitude of 3360 m; the freeboard of the dam is 20 m; the actual water level will be at 3340 m altitude.
The Create Raster Map dialog box appears:
- Fill in the new map name (e.g. DAM);
- Choose the Georeference which is used in the DEM (accept the default);
- Select the system Value domain; then click OK.
The Pixel editor is opened. Zoom in on the area where you want to build the dam. Then select pixels which will form the dam in map DAM. The pixels indicating the dam should be properly 4-connected (see figures below). When pixels are only 8-connected, pixels at the opposite side of the dam are also used in the calculation later on.
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Use the left mouse button to select the pixels while you hold down the Ctrl key of the keyboard. When the correct pixels are selected, give the dam the value of the altitude of the top of the dam (in this example 3360).
DEMNEW = IFUNDEF(DAM,DEM, DAM)
This means that if a pixel in map DAM is undefined, the value of the DEM is assigned, else the value given in map DAM. Thus the altitude of the dam (in this example 3360m) is included in the Digital Elevation Model.
Display the output map DEMNEW to force calculation.
First, display map DEMNEW and choose File, Create, Create Raster Map from the menu in the map window.
Then fill out the Create Raster Map dialog box:
- In this example, type START for name of the new raster map;
- Select the same georeference as was used in DEMNEW;
- Select the system Bool domain.
Select a pixel in the area which will be flooded, double-click this pixel and mark it 'True'.
Leave the pixel editor: the map START has only one defined pixel, the rest is undefined.
Display the map START to force calculation.
Calculating the flooded area
To start the calculation, double-click the Iteration item in the Operation-list.
The Iteration dialog box appears:
- For Start Map, select START.
- Type for the Expression:
IFF (DEMNEW > 3340, START, NBMAX(START#))
This means: if the altitude in the new Digital Elevation Model is higher than 3340 m (the desired water level), then return the pixel values of raster map START (which are undefined). Else, assign the maximum value of the neighbouring pixels found in raster map START.
In the first iteration there is only one starting pixel which is 'True'. Then in every next iteration, the neighbours of the True pixel(s) will be assigned True as will, as long as these pixels have an altitude less than 3340 m.
- Accept the default Until No changes as the Stop criterium;
- Keep the option Propagation selected because then the new pixel value is immediately used in the calculation of the next line;
- Type FLOODED as the name of output raster map;
- Select the Show check box;
- Select the system Bool domain;
- Then click OK.
Calculating the water volume
Therefore, you can Cross the newly calculated map FLOODED with map DEMNEW. The crossing of two value maps may be a relatively time consuming process.
To reduce calculation time and the size of the output cross table, you can create another DEM which only shows the original height values of the area that will be flooded, while all other pixels have the undefined value. To do so, type the following expression on the command line of the Main window:
DEMSMALL = IFF(FLOODED, DEMNEW, ?)
This means that for the flooded area the values of the DEM will be used, while all other pixels are assigned the undefined value; these undefined values are not taken into account in the cross operation.
The Cross dialog box appears.
- Select for the first map FLOODED;
- Select for the second map DEMSMALL;
- Type FLOODDAM as the name for the output cross table;
- Select the Show check box.
- Do not select the Output Map check box.
- Click OK.
The cross table is calculated and is displayed on the screen. It shows the combinations of input values of the map FLOODED and the map DEMSMALL, the number of pixels that occur for each combination and the area for each combination.
Type the command line of the table window:
DEPTH = 3340 - DEMSMALL
Then calculate the volumes of the depths as follows:
VOL = DEPTH * Area
Finally, sum all volumes of all different water depths together. This can be done using the aggregation function AGGSUM.
SUM = AGGSUM (VOL)
Column SUM shows the total volume of water in cubic meters for the entire area which will be flooded after creating the dam.