Problem

Why is StormCAD over-designing the conduits during a design run on a system with very shallow slopes? (during constraint based design)

Background

First, it is important to note that with constraint based design in the StormCAD solver, there is a list of priorities that is used during the design run. If you have a case where a smaller conduit size would be better suited but a higher design priority would be violated, the smaller conduit will not be used. For information on the design priorities, refer to the StormCAD Help document "Design Priorities" or this link.

Very shallow slopes in some models can cause issues in the design run. Some may push the limits of a constraint based design solution toward achieving a truly optimal solution. This can happen if the flow travel time is large, with a significant reduction in flow due to attenuation (larger system travel time = smaller intensity). The flow that you see reported in a conduit normally accounts for this attenuation already. In other words, it reports the flow at the downstream end of the conduit. However during the constraint based design calculations, pipes are sized based on the flow at the upstream end of the conduit, before such attenuation is accounted for. Normally this assumption does not cause any problems since the attenuation effect is usually small. However for a shallow-sloped pipe with a significant travel time, that reduction due to attenuation could potentially cause a discrepancy.

For example, suppose you have a long, shallow-sloped conduit whose flow is 10 cfs on the upstream side and 8 cfs on the downstream side. Assuming that the capacity of pipe size A is 11 cfs and smaller pipe size B is 9 cfs, the constraint based design would pick pipe size A since it looks at the flow on the upstream side (10 cfs) and picks a conduit whose capacity is greater (11 cfs). However when running an analysis calculation on the model (or when looking at the Analysis results that are generated after computing the design run), you will see 8 cfs reported in the pipe. This occurs because of the attenuated flow on the downstream side due to high travel time. Given that results, a modeler may wonder why the design run did not select the smaller pipe size whose capacity (9 cfs) was larger than 8 cfs observed in the pipe.

One might argue that using the upstream flow results in a more conservative design, but this can sometimes cause confusion. As a results, starting with version 08.11.03.77 of the StormCAD solver, an enhancement is included that improves on how the design computation works with these shallow slope scenarios by taking into account travel time. Basically it allows you to tell the program to base the pipe sizes on the flow at the downstream end after any attenuation from travel time has occurred. Below are the steps to implement this enhancement.

Solution

Option 1

With StormCAD V8i SELECTseries 4 or later, all that is needed is a simple change to the calculation options. Open the calculation option properties and find the attribute "Include Conduit Flow Travel Time in Design." Set this to True to account for the travel time in the pipes for the design run. The same workflow is used if you are using CivilStorm or SewerGEMS with the StormCAD solver.

Option 2

With StormCAD V8i SELECTseries 3, you will need to use the following steps to account for the travel time:

1) Browse to C:\Program Files (x86)\Bentley\StormCAD8, locate the xml file called IncludeConduitFlowTravelTimeInDesignOptions.xml

2) Place this file in the Local User Setting Directory (C:\Users\FirstName.LastName\AppData\Local\Bentley\StormCAD\8\) or Common Application Settings Directory (C:\ProgramData\Bentley\StormCAD\8\).   Note: As the GFV Rational solver is available in both SewerGEMS and CivilStorm, if you are using the GFV Rational solver in one of these, replace "StormCAD" with "CivilStorm" or "SewerGEMS".

The program will first check the Local User Setting Directory. If the xml file does not exist in Local User Setting Directory, the same file in Common Application Settings Directory will be used. If the file does not exist in either directories, conduit travel time will not be included in the StormCAD design calculations.

The following is the format of the xml file (as seen when opened in a text editor):

<?xml version="1.0"?>
<StorageFile xmlns="http://www.haestad.com">
  <CalculationOption IncludeConduitFlowTravelTimeInDesign = "true"/>
</StorageFile>

Only if the attribute IncludeConduitFlowTravelTimeInDesign is set to “true” does the program include conduit flow travel time in the GVF rational design calculations. If attribute IncludeConduitFlowTravelTimeInDesign is set to other values, conduit flow travel time will not be considered (which is the default/legacy behavior).there is a new file found at C:\Program Files (x86)\Bentley\StormCAD8\ called "Design enhancement instructions.txt". This file will provide instructions on how to use the updated XML file that will utilize the enhanced design algorithm, and conduit travel time will be included in the design process.

These details are also found in the file "Design enhancement instructions.txt" found at C:\Program Files (x86)\Bentley\StormCAD8.

See Also

What is the order of the design priorities used in StormCAD (GVF-Rational solver) and SewerCAD (GVF-Convex solver)?