Using Unit Loads and Extreme Flow Factors after a LoadBuilder run

  Applies To 
  Product(s): SewerCAD, SewerGEMS, WaterCAD, WaterGEMS
  Version(s): 08.11.XX.XX through 10.01.XX.XX
  Area:  Modeling
  Original Author: Jesse Dringoli, Bentley Technical Support Group


Is it possible to import unit loads with LoadBuilder (instead of pattern loads), for example to apply extreme flow factors in SewerCAD or SewerGEMS?

Problem ID# 57927

Background on unit loads

Unit loads (SewerCAD/SewerGEMS) or unit demands (WaterCAD/WaterGEMS) are a type of load that is specified by way of the count of a number of something. In the node properties (or control center), the unit count is entered along with the corresponding unit load type. For example a unit load type might be "apartment" for example. Within the unit load type, you specify the population unit corresponding to the unit count, along with the total flow (demand / load) represented by one of those units. For discharge-based unit loads, you would also enter the equivalent population for each unit.

As an example, you may have a unit load type of "Apartment" with population unit of "resident" and unit load of 70 Gal/day, which means that the number you enter for the unit load count in a node (with this unit load type selected) represents the number of apartment residents represented by that node. So, if you have 50 residents, you would enter 50 for the unit load count and "apartment" as the unit load type. The total load/demand for that node would then be 50 residents times 70 Gal/day = 3500 Gal/day.

Unit loads are required if you need to analyze extreme flow factors in a steady state simulation with the GVF-Convex (SewerCAD) solver (available in SewerCAD and SewerGEMS). The reason is because the extreme flow method adjusts a peaking factor based on the total contributing population at each point in the network. Unit loads are required in order to determine the population.


LoadBuilder currently imports loads/demands as pattern loads/demands, which is a base flow plus pattern.

However, for SewerCAD (GVF-Convex solver), extreme flow factors can only be applied to unit loads. So, if you need to use LoadBuilder to import loading information from a shapefile and then use extreme flow factors, you will need to adjust the loading so that the loads are unit loads instead instead of pattern loads. To do that, you can try the following options.

Option 1

For a case where the value being imported is a count of population, you can try the following after importing through loadbuilder. Open the Sanitary Load Control Center, initialize unit loads for all elements using the first icon, then copy the base flow value into the Loading Unit Count field, and populate the Unit Sanitary load (the type). The pattern load should then be deleted from the Sanitary Load Control Center if it is not to be included in the calculation. The screenshot below is what the Sanitary Load Control Center may look like after updating the model with LoadBuilder and copying the data in the Load Unit Count field, but before deleting the pattern loads.

Option 2

If the value imported by LoadBuilder is a total flow, you will need to multiply the base flow by a certain factor to achieve the desired loading count (you will need to know how many units/population is represented). Then you can enter the correct corresponding unit load. 

Option 3

If you have shapefiles with land use type and population, you may consider a different approach, using the "Load Estimation by Population" Loadbuilder method to distribute the loads based on population density, load density and service area. This requires that the land use polygons overlap the areas occupied by your manholes, since the loads are distributed based on manholes that fall within the polygons. This also means that you will still end up with Pattern based loads (not unit loads, required for Extreme Flow Factors). So, this method will only work for you if you are able to use Pattern loads. For details, see the below article and also Option 4 for "peaking" pattern based loads:

Import loads from shapefile containing polygons with land use type and total population

Option 4

Consider using the Pattern Based loads imported by Loadbuilder, applying "peaking" to the base load. For example, after importing pattern loads with Loadbuilder, copy/paste the information from the Sanitary Load Control Center into an external application such as Excel, calculate the peak flows there, then paste back into the sanitary load control center (or re-import using Modelbuilder)

Option 5

For sewer modeling applications - if you do not need to run a steady state with the GVF-Convex (SewerCAD) solver (required for extreme flow factors) but rather an extended period simulation (EPS) or a dynamic simulation (Implicit or Explicit solver), but want to look at worst-case or peak conditions, the extreme flow methods may not be appropriate. Instead, you may consider creating a new sanitary loading alternative in a new scenario, and global edit the base flow for your loading and apply a multiplication factor there.