Depending on the drainage authority or authorities you are operating under, you may be required to conduct a Critical Storm Duration analysis. This analysis is designed to determine the design storm that produces the highest peak flow rate and/or volume, either from a runoff hydrograph or through detention pond routing. It involves running your hydrology model using design storms of varying durations, such as 1, 2, 4, 8, and 24 hours, across the 2, 10, and 100-year return periods.
Navigating through five design storms, each at unique frequencies, can be quite time consuming. Thankfully Hydrology Studio is equipped to easily handle these tasks. The software is inherently capable of computing multiple frequencies simultaneously. Users simply need to select a new Design Storm from an easy-to-use list box and re-compute the same model for that specific duration. This process is straightforward and will allow you to obtain results in minutes rather than hours.
In essence, you’ll create duplicate models for each storm duration. (But see “For Complex Models” at the end of this article.) Compute the first model with one duration, then proceed to the next with a different duration, and continue this process until all durations have been used. Ultimately, we will have several computed models, each corresponding to its designated duration, which we can then analyze to identify the design storm that produces the desired result.
Do not attempt to do this using the Rational method. Its duration is connected to the Time of Concentration (Tc). By definition, the Tc IS the critical duration for Rational method hydrographs.
An Example
Lets work through an example from which you can follow and adapt to any other similar analysis. We will be using the FLDOT design storms below but you can use any other storms such as the IDF-Based Synthetic which offers 1, 2, 3, 6 and 12 hour durations.
In this example, we are required to find the storm duration from the 1, 2, 4, 8 and 24 hour duration storms prescribed by the Florida DOT, which maximizes the peak outflow from a post-development detention pond design.
Create the Basin Model
Below is a basin model of an inflow hydrograph a pond route. It was replicated four times to represent a total of five durations. There is no data attached to the hydrographs yet. You can build the model by clicking the [NRCS] button five times. Then go to each runoff hydrograph and add a Route icon. This gives us a total of five complete models as shown below.

Now follow the steps below.
1. Select the 1st Design Storm
In this case we will choose the Florida DOT – 1 hr design storm from the Rainfall tab.

Check Your Rainfall Data
In case you haven’t added the precipitation amounts to the FLDOT Storms, you should do that now. (If you’re using the IDF-Based Synthetic storms, check your IDF Curves.)
Click on the [Rainfall] button to open the Precipitation Manager. Expand the FDOT grid by clicking on the little arrow. Then add the rainfall amounts for each design storm as shown below.

2. Compute the 1st Runoff Hydrograph
Next we will compute the first NRCS runoff hydrograph. Select Hydrograph #1. Then enter the Name, Area, CN & Tc as shown below, and then Compute.

3. Perform the 1st Pond Routing
Select the Route icon associated with this 1st runoff hydrograph (Hyd. No. 6). This example is going to skip the pond design details for simplicity, so you can stay focused on the Critical Storm Duration procedure. We will assume the pond has been designed and is ready for a final routing.

Enter a Name. Choose your pond and Compute.
4. Repeat Steps 1, 2 & 3 For the Other Durations
We have now finished calculating the model for the 1-hour duration. We will repeat steps 1, 2 & 3, for the remaining durations. That is:
1. Select the next design storm, for example “Florida FDOT 2-Hr”.
2. Select the next runoff Hydrograph. Replicate the data from the previous NRCS Hydrograph but use a different Name, for example “FDOT 2-Hr”. Then Compute.
3. Perform a new pond route. Use the same pond. No need to create duplicate ponds.
Repeat Steps 1, 2 & 3 until you’ve computed all five models.
5. Identify the Critical Storm Duration
All models are now computed. Lets take a look at the charts and reports.


Click the Reports button on the Home tab. Select Return Period Recap and Hydrograph Summary.

Parsing through the charts and numerical reports reveals the 8-hour duration is the storm that produces the maximum peak outflow across all frequencies. Note that the 24-hour duration produced the smallest peak Q on both runoff and pond routings.
Summary
Now you know how to build a model to perform a Critical Storm Duration analysis.
We basically make copies of models for every storm duration. Start by calculating the first model for one duration, then move on to the next one with a different duration, and keep going like that. In the end, you’ll have a bunch of models, each linked to its specific duration, which you can easily identify on the Charts and Reports, the critical storm.
Before moving onward there are some important things for you to note.
1. DO NOT USE THE BATCH RUN FEATURE! Remember that these hydrographs were each computed using the selected design storm and its corresponding precipitation. You subsequently chose a different design storm for each of the other models. Invoking the Batch Run will recompute ALL of the model’s hydrographs using the currently selected design storm, essentially overwriting those hydrographs that were computed using previously designated storm durations. If you inadvertently use Batch Run, you can easily re-set the design storm and recompute each individual runoff hydrograph and its routing, i.e., repeat steps 1, 2 & 3.
2. The individual printed Hydrograph Reports will show the currently selected design storm duration which may be different from the one used at that time. For example, the report for the 1-hour may indicate the 24-hour duration since that was the last design storm used.

To avoid this, you might want to print the Hydrograph Reports for the Runoff Hydrographs as you go along so that these values coincide. Pond Route and other types do not include the Storm Duration.
3. Again, the Rational method does not apply.
For Complex Models
The example above is a simple two-hydrograph model. If you have a complex model using many hydrographs, it is recommended that you do not replicate each one but rather use a single model instead. Compute the single model using one storm duration; print the associated reports. Then select the next design storm duration; recompute the model; and print reports. Repeat for each duration. Note that in this case you can use the Batch Run feature. It will save you a lot of time.