Using the Critical Storm Analysis feature

Product(s): SewerGEMS, SewerCAD, CivilStorm
Version(s): CONNECT Edition Update 1 and later
Area: Output & Reporting


This TechNote explains how to use the Critical Storm Analysis feature, to supplement the information in the related Help documentation topic.

The Critical Storm Analysis feature is available starting with the CONNECT Edition Update 1 version of the OpenFlows storm and sewer products.


The Critical Storm Analysis feature analyzes the results of the selected scenarios to determine which scenario is the most "critical" for each selected node. This type of analysis is common in some particular regions. The node element represents a manhole or a catch basin. The critical scenario is the one with the highest Hydraulic Grade Line (Maximum) value. If more than one scenario has the same Hydraulic Grade Line (Maximum) value, then the scenario with the largest volume of overflow is the critical one.

Critical Storm Analysis is an efficient tool to determine the most critical storm which is subjected to a storm-sewer system. Whenever a system is designed, it is optimized so that it can successfully convey the runoff from multiple storms occurring for different durations and intensities. The Critical Storm Analysis feature shows the capability of the system to sustain multiple storm events without surcharging, overflowing or flooding.

The Critical Storm Analysis tool allows you to understand the adequacy of the system for different cases represented as scenarios. With this tool the user can find out the flooding / overflowing of manholes for a scenario and any mitigation measures / design changes that need to be undertaken to resolve these issues. Also, the designer can check for the design adequacy based on the allowable limits for which the system can be designed based on the status messages appearing for critical storm analysis.

Before running the critical storm analysis, the user must create and run all the scenarios that will be evaluated in the analysis. The analysis is based on time simulations so that GVF-Rational and GVF-Convex (Steady State) solvers cannot be used. However, the Implicit, Explicit (SWMM), and GVF-Convex (EPS) solvers are valid for this analysis.

The hydraulic grade for the node is analyzed for all selected scenarios in the Critical Storm Analysis feature. The Critical Storm Analysis determines the worst status for each node over the course of all the scenarios. The status for a node can be any one of four (4) values: OK, Surcharged, Overflow Risk, and Overflow depending on the hydraulics of the system. To determine the status of a node, the following conditions are checked for in each selected scenario;

• “OK” = the maximum hydraulic grade is less than the downstream pipe's soffit (inside top of pipe)
• “Surcharged” = the maximum hydraulic grade is greater than the downstream pipe's soffit, but less than the node's rim elevation minus the node's freeboard depth.
• “Overflow Risk” = the maximum hydraulic grade is greater than the node's rim elevation minus the node's freeboard depth, but less than the rim elevation.
• “Overflow” = the maximum hydraulic grade is greater than the rim elevation.

The status index ranges from “OK” to “Overflow”. The severity of the status is as follows,

OK < Surcharged < Overflow Risk < Overflow

The scenario for which a node has the worst status, i.e. most severe is the critical scenario for that node.

Configuring a Critical Storm Analysis

The Critical Storm Analysis feature can be found in the Calculation tab under the Home tab. When the dropdown menu under Compute is accessed, Critical Storm Analysis feature is available.

As an example, we will work with a simple storm-sewer model consisting of a base scenario as “2 YR” storm and two child scenarios as “10 YR” and “25 YR” storm events. When the Critical Storm Analysis feature is selected the “Scenario Chooser” will show up like this;

Not all scenarios may be valid, for reasons including:

• Scenarios with the disabled scenario icon do not have results. These scenarios are grayed out, and cannot be checked.
• Scenarios with a small red “x” in the corner do not use an appropriate solver (they use either GVF-Rational, or GVF-Convex using steady-state analysis) and cannot be checked.

Valid scenarios, therefore, are those which:

• Have results.
• Use the Implicit, Explicit or the GVF-Convex (using EPS) solvers.

There are four buttons in the toolbar at the top of the dialog. These are the “check” and “uncheck” options for choosing the scenarios and the expansion and collapse of the hierarchy trees.

The “Check All Valid Scenarios” option must be selected when the User wishes to compare all scenarios which have the Implicit or Explicit-SWMM engines as their numerical solvers. This feature is useful when you have multiple scenarios using different solvers. As pointed out earlier in this tech-note; the Critical Storm Analysis works on the principle of time simulations and hence scenarios using the GVF-Convex and GVF-Rational engines won’t be considered for analysis.

The “Check Children” option selects only the child scenarios of your base scenario. If you have more than one base scenario, then the child scenarios of the highlighted base scenario will be selected. For example, if the highlighted base scenario is “2 YR” then the child scenarios for that particular base scenario will be chosen i.e. “10 YR” and “25 YR”.

The “Check Hierarchy” option selects the currently highlighted scenario and its entire hierarchy. So, as shown in the case above, all the scenarios including base would be selected for comparison.

The “Uncheck” option similarly has sub-options such as “Uncheck All Valid Scenarios”, “Uncheck Children” and “Uncheck Hierarchy” which perform the opposite action to the checking sub-options. So, the user can quickly uncheck the scenario which should not be considered for a Critical Storm Analysis.

Performing a Critical Storm Analysis

Once the scenarios required for analysis are selected, the Critical Storm Analysis for them is performed and the results are displayed in a tabular format. For our example, we have considered the “2 YR”, “10 YR” and “25 YR” scenarios for analysis.

If a particular node is selected in the drawing pane prior to analysis, the critical storm analysis would be performed for that node only. If nothing is selected, the nodes in the entire model are considered.

The results would appear in a tabular format like this;

The above table shows that out of the three scenarios selected for analysis, the scenario “25 YR” is the critical scenario for all the nodes in the system. This is a simplified example though, since the 25-year event likely has more runoff than the 2 and 10. In other situations, you may have a range of durations, return frequencies and possibly other runoff parameters.

MH-1 is showing an Overflow Status i.e. the hydraulic grade at this node for the 25-year storm event is above the rim elevation. The manhole can be termed as “Overflowing."

MH-2 is showing an “Overflow Risk” Status, i.e. the hydraulic grade at this node for the 25-year storm event is below the rim elevation, but is greater than the rim elevation minus the freeboard depth. The manhole can be termed as “Overflow Risk.”

MH-3 is showing a “Surcharged” Status i.e. the hydraulic grade at this node for the 25-year storm event is below the rim elevation, but is greater than the downstream pipe’s soffit. The manhole can be termed as “Surcharged.”

In the model one can check which of the nodes are surcharged or overflowing by selecting the properties of “Is Surcharged?” or “Is Overflowing?” for annotation under Element Symbology. This article explains the difference between these two properties in detail.

Starting the the CONNECT Edition Update 2 release of the storm-sewer products, a new “Freeboard (Required)” field is included to allow you to help determine the lower limit of the overflow risk status when you are running a Critical Storm Analysis. The default value for this is listed as 1.0, but the user can input another value. The Critical Storm Analysis will determine overflow risk.

Note: The “Freeboard (Required)” field in only used for Critical Storm Analysis, and not used during the hydraulic analysis.

In the left-most column (labeled with an asterisk) the color associated with the “Status” is displayed. Here are the colors and the status associated with them;

The user can change the colors as required. The option to change the color coding system is available in the Critical Storm Status Options under the Option button available in the Critical Storm Summary Results.

Note: For the rest of the options available in the Critical Storm Summary Results and details of the result columns please refer the Help document for details (Calculating your Model > Critical Storm Analysis).

The user can change the color associated with a status by simply clicking on the ellipsis button next to the color in the Critical Storm Status Options dialog box. Different options are available for choosing the desired color.

Starting with the CONNECT Edition Update 2 release, you can now sort the rows of elements in the Critical Storm Analysis tool dendritically. This way selected rows are arranged not only by their upstream to downstream connectivity but also grouped by branch.  Links of a dendritic stormwater network can be well-labeled with the Branch Labeling tool, resulting in a clear designation of links organized into tributary branches.  After this sort, the first rows appearing will be the elements within the upstream most branch, and appearing in upstream to downstream order within that branch.

This will allow you to better see the impact of the critical storm analysis as you move downstream through your system.


When trying to compute one or more scenarios using this tool, the program crashes with a "stopped working" error

This is a known issue (Defect #1062070) that has been resolved with a patch for version Either contract Technical Support for the latest cumulative patch set, or upgrade to a newer version when available (which will include the fix).

See Also

Difference between a "surcharged" and an "overflowing" node