New Storm Event Scenarios Wizard

Product(s): SewerGEMS, CivilStorm

CONNECT Edition Update 1 (Build

and later
Area: Layout and Data Input


Starting with Connect Edition update 1, a new storm event scenario wizard has been added to the scenario manger. The storm events scenario wizard assists in streamlining the process of creating scenarios, alternatives, calculation options and storm events for a specific set of return periods and durations.

The set of scenarios, alternatives, calculation options and storm events created are then used with the Critical Storm Analysis tool.

This tool is advantageous for users who would like to analyze many different return events in a single storm water model, so using this tool a separate scenario will be created for each return event. In contrast, with regular storm event creation you need to create a scenario for individual return events manually, which can be cumbersome for a large number of scenarios.

This requirement is seen mostly from the UK and Australia region. So, we have included the “UK Storm Profiles.xml” library as well. 

This article will explain this new storm event scenario wizard and how to use it. 

Storm Event Scenarios Wizard

With the introduction of this new storm event scenarios wizard, streamlining of creating scenarios, alternatives, calculation options and storm events in particular for a specific set of return periods and duration has become easier, as you can create everything for a particular return event and duration in one place. Earlier you will have created scenarios, alternatives, calculation options and storm events separately for this.

A dimensionless curve will be used to create a time-depth or time-intensity storm event. A storm event profile can either be a fixed-time dimensionless curve or a true dimensionless curve. 

To access the new Storm Events Scenarios Wizard, open the Scenario manager, click the new icon, and select "New Storm Event Scenarios…"


Once the storm event scenarios wizard is open, on the first page of the wizard, select the IDF Storm Group to use to convert intensities into depths. An IDF Storm Group is defined as a storm event group that creates IDF curves. If there are no IDF Storm Groups listed, using the […] button to the right of the drop-down to open the storm data dialog. In this dialog you can add new IDF Storm Groups.

The following are the available IDF Storm Group types available in the storm data dialog:

  • User Defined IDF Table
  • Hydro-35
  • IDF Table Equation
  • IDF Curve Equation
  • IDF Polynomial Log Equation
  • Flood Studies Report (United Kingdom)
  • Australia Rainfall and Runoff (2016)
  • Flood Estimation Handbook (United Kingdom)

When an IDF Storm Group is selected, the return periods and duration's are populated in the return period and duration lists. At least one return period and duration selection is required to click Next and move to Step 2. 

The climate change factors are an option that adds a percentage to the base depth of the storm event. This allows you to design your system with climate change in mind. The entered climate change factors must be greater than 0, entered in %of climate change. This enables you to quickly create new storm event scenarios for different climate change factors (which can then be used for example with the Critical Storm Analysis tool) When you apply a climate adjustment, when a new scenario is applied, the Climate Change Adjustment will automatically be included in the Global Storm Event dialog and the Rainfall Runoff Alternative.


In the next step, specify the type of storm event to generate that can be either Time - Intensity or Time- Depth. By default it will populate Time - Intensity type of storm event. If you select Time-Depth, you can then specify whether to generate cumulative or increment depth curves. 

Click on the New button to specify storm event profiles, choose Storm event profile from the library in the first column and then the corresponding storm even profile based on the selected dimensionless curve. 

If there are no storm event profiles available, click the […] button on the right side of either cell in the table. This will open the Engineering Libraries dialog box. From here you have limited editing abilities. You can add an existing engineering library. To create and populate a new library, use the Engineering Libraries command on the ribbon.


In step 3 you review a preview of storm events and scenarios that will be generated once you finish all steps of the wizard. 

Each row in the table represents a single storm event and scenario. The depth is calculated by retrieving the intensity at the selected return period and duration from the selected IDF Storm Group on page 1. The depth is calculated by multiplying the intensity by the selected duration in hours. 

In case of an IDF Storm Group, storm events are generated based on the selected storm event profile – either fixed-time or dimensionless. 

You also have the option to add or delete the rows in the table. Click on New and select the return period, and then storm event profile entry from library and storm event profile and duration of storm after which depth will be calculated. You also can change the units here for depth and duration. 


In this step, you can select the additional alternative types you want to create for a particular storm event scenario, which will be assigned to the scenario automatically then. By default New Rainfall - Runoff alternative will be created. 

If an alternative type is selected, the alternatives are created as such:

  • A new base alternative is created of the alternative type using the label of the first storm event profile in the list.
  • For each subsequent storm event profile, a child alternative is created below the base alternative created in the first step.

By looking at the screenshot below - based on the selected storm event profiles, the name of the new base alternative would be "Distribution C". If there is one more profile, then it will be created as a child of the "Distribution C" alternative. 


Here you need to select your calculation options which will then be assigned to the storm event scenarios that are created. Please note this will not make any changes to the existing scenarios and alternatives. 

First you need to select the active solver. By default the Explicit (SWMM) solver will be populated, but you can change it to the Implicit solver. Also the default Routing Step is 5 seconds along with a default output increment of 3 minutes (0.05 hours.) 

The Simulation Duration Multiplier factor will calculate the duration of the simulation, by multiplying the multiplier with the duration of the storm event to be created. If the duration is 24 hours, then the simulation duration is calculated as 48 hours. This allows for the entire storm to run and to account for all rainfall during that simulation. 

With the Explicit solver, you get the option to select whether to use the Bentley transition equation or the EPA - SWMM equation. 

The minimum time interval determines if a user defined time interval is used when generating storm events using dimensionless time curves. This is especially useful for shorter durations. The default is 1 minute. If the option is off, the calculated increment is used. If the option is on, the entered time interval is used for all generated storm events where the calculated time interval is less than the entered value.

When all this data is entered and you click on Finish, a summary of creation of the scenarios, alternatives, calculation options and storm events is started. A preview of how many scenarios and storm events created is shown then, in which each scenario will represent one storm event. 


See Also

What's new in SewerGEMS, SewerCAD, StormCAD and CivilStorm CONNECT Edition Update 1 (Build

Modeling multiple storm events in a single scenario

Using the Critical Storm Analysis feature