Modeling Dam Construction
Global Mapper’s extensive collection of advanced terrain editing tools is ideally suited for planning engineering or construction projects. Combining the terrain editing tools with the analysis options for watershed modeling and volume calculation, Global Mapper is a perfect tool for the modeling of a dam creation project.
Dams can serve many purposes, but the most common use is for the creation of a reservoir or buildup of water upstream of the manmade structure. Using Global Mapper’s terrain processing tools, the paths of existing streams and rivers can be identified by conducting a Watershed Analysis. By increasing the minimum stream length and minimum drainage area that defines a local catchment in the Watershed Analysis setup, only larger waterways are identified and created as line features in Global Mapper.
Using contour lines generated in Global Mapper, the local terrain is discretely described, and the desired dam height and water level can be identified. Ideally, the dam will span a narrower section of the river at a height that will create a reservoir upstream.
Diverting River Flow
To build a permanent dam structure across an existing river, the water needs to be diverted from the building site. For the construction of a concrete dam, the river diversion is often carried out with a cofferdam and a channel creating an alternate path for the river. These two alterations to the terrain can be modeled and measured using the terrain editing and analysis tools in Global Mapper.
A useful tool for modifying a terrain grid, Global Mapper’s Terrain Painting tool provides operations to lower, raise, and set the terrain height, among many others. Using these operations, an earthen cofferdam and channel can be created in the terrain grid, and using the Watershed Analysis tool, a change in the natural flow of the river can be confirmed.
Starting upstream of the permanent dam site, the Terrain Painting operation Slope terrain along line is used to cut a channel into the terrain, creating a bypass for the river flow. A negative slope value is applied to allow for the flow of water around the intended construction site.
Using a similar method, a cofferdam is built on the terrain grid using the Set Terrain Height operation. This process creates a mound across the river bed to divert the flow of water into the created channel and around the planned construction site.
To confirm that the modeled cofferdam and channel will effectively divert the existing river flow around the permanent dam site, a local watershed analysis can be conducted to create streamlines showing the predicted flow path of water.
Using watershed analysis, the efficacy of the modeled cofferdam and channel is shown, and any needed alterations can be made to the terrain. If needed, the Terrain Painting tool can also be used to revert any changed terrain values allowing the process of simulating the creation of the cofferdam and channel to be reinitiated with different values.
The Terrain Painting tool makes manually altering the terrain easily, but this method does not provide any quantifiable measure of the change applied to the data. To calculate the difference between the painted, or changed terrain and the original surface, a version of the original terrain surface is loaded into the workspace, and a volume calculation between the surfaces is executed.
Using the Measure Volume Between Surfaces tool, the altered surface is subtracted from the original surface to provide both cut and fill volume measurements. These measurements will use the volume units set in the Measure/Units section of Global Mapper’s Configuration options, and this calculation can be executed again if the units used need to be adjusted.
Permanent Dam Modeling
With the implementation of the cofferdam and channel, the river flow will be diverted from the permanent dam construction site. To model the construction of the permanent dam, an area feature representing the top of the dam is created using Global Mapper’s digitizer tools. Ensuring that this feature is of a uniform width, it is derived from a line, using the buffer feature creation tool, which expands the centerline of the dam to the desired width.
After creating an area representing the dam, an elevation attribute can be added to denote the height of the feature. This height will be based on the contour lines that were previously created during the initial planning. The dam polygon, now with an elevation, can be extruded into the terrain to model the dam as a vector feature.
To burn the dam into the terrain, the Create Flattened Plan Site tool is used to apply the elevation of the previously created polygon as a locally modified surface. This tool creates a flattened grid area inside the selected polygon, connecting it to the surrounding terrain at a user-defined slope angle.
With the dam polygon selected, the Create Flattened Plan Site tool uses the elevation of the 3D vector feature to create a gridded dam model with a minimal slope angle so that the dam is slightly wider at the base than at the top. This method of terrain creation also generates a report containing the calculated cut and fill volumes for the dam model.
As shown in the Path Profile view, the gridded dam model is created as a separate terrain layer in Global Mapper. To combine the dam model with the original terrain, the two layers can be displayed as a cohesive model and exported to a single file in any supported elevation format.
Reservoir Volume Calculation
The design of the dam stipulates that the maximum water level of the reservoir will be half a meter below the height of the dam structure. This precise elevation can be used to model the extent of the reservoir and calculate the maximum volume of water it can hold.
Using the contour creation tool, a contour at a single specific elevation is generated, outlining the maximum water level derived from the terrain. Since the dam is now incorporated into the terrain, the upstream side of the dam creates one segment of the reservoir outline, and the rest is determined by the surrounding terrain.
After using Global Mapper’s Digitizer tool to convert the reservoir outline to a polygon, the full extent of the maximum reservoir size can be shown on the map in 2D and 3D. The polygon can also be used to calculate the maximum volume of the reservoir using the Pile Volume Calculation tool.