Distributing a lump sum demand or load over a large area

Product(s): WaterGEMS, WaterCAD, SewerGEMS, SewerCAD
Version(s): 08.11.XX.XX, 10.XX.XX.XX
Area: Layout and Data Input


How can you distribute a lump sum demand over a large area? For example you have a single polygon shapefile with a total demand/load or total population, or you just have the number without a file, and need to distribute it to model nodes.

Problem # 54617


The solution to this depends on the type of data that you have.

LoadBuilder - Polygon Shapefile

To distribute this demand or load over the model, one approach would be to use the Loadbuilder tool. Loadbuilder has several different methods of distributing demand information over the nodes in your model, based on spatial information. For example, you could use the Proportional Distribution by area method. With this method, you could first generate a Thiessen polygon (using the Thiessen Polygon tool) to create polygon areas representing each junction in the model (service areas). Loadbuilder would then overlay the lump sum demand polygon shapefile that you have with the service areas and distribute the demand to each node based on the overlapping areas. You can read more about Loadbuilder and Thiessen polygon in the help documentation.

If you have the total population in the shapefile, you could use the Load Estimation By Population method, entering the appropriate load density (flow per capita).

Demand Control Center / Sanitary Load Control Center - no Polygon Shapefile

Alternatively, if you don't have spatial information (no polygon shapefile) and just know the total demand/load amount, you could divide it by the number of nodes in the model to find the demand per node, then use the "Initialize for all elements" option in the Demand Control Center (WaterCAD/WaterGEMS) or Sanitary Load Control Center (SewerGEMS/SewerCAD). This would create a single demand/load entry for every element, with the same demand amount. This would be the least accurate way to distribute the demand.

LoadBuilder - no Polygon Shapefile

If you don't have spatial information, you could create a single polygon shapefile and distribute the demand/load using LoadBuilder per the "Loadbuilder" section further above. To do this, draw a catchment or LID element (which are both polygons) around the entire model, then create a new flextable under the "tables-hydraulic model" section of the Flextable manager, filtered as needed to only show that one catchment, and only containing the field that has the lump sum load/demand or population. Then, use LoadBuilder per the above to distribute (you may need to create a Thiessen Polygon as well).

Units Loads and Extreme flow Factors

Note that if you need to use Extreme Flow Factors (peaking factors) with the GVF-Convex (SewerCAD) solver after import, then the loads will need to be unit loads. LoadBuilder will always import loads and pattern-based loads. See: Using Unit Loads and Extreme Flow Factors after a LoadBuilder run

See Also

How do each of the Loadbuilder methods work? 

Troubleshooting and Understanding LoadBuilder