HECHMS HMR 52 Storm

[u/Throwaway20230510]

Can someone ELI5 if I need to not use the standard HMR52 storm if I am interested in durations less than 72 hrs hours? I don't think I fully understand how other durations are nested within. Thanks

[u/NewToTheGPU]

Hey! Assuming you are not in a state with a PMP study available, I would refer you to HMR-52 Section 2.3. I did this for a very small site (<10 sq miles), so I didn't have to worry about storm position.

My interpretation of the steps layed out in Section 2.3 are below:

1. Get HMR-51 Duration-Depth data for all durations up to the chosen storm duration

For example: If I want to create a 24-hr PMP, I would get the 6, 12, and 24 hour PMP depths from HMR-51.

1. You need to create 6-hr incremental depths. This provides your hyetograph shape and if done correctly, result in the correct total volume.

For my example: I need 4, 6-hr incremental depths for a 24-hr duration storm. Let's say the HMR-51 values for the 6, 12, and 24 hr are 30, 35, and 40 inches, respectively for my location.

The incremental depths for a 24-hour PMP can then be defined as follows: a.) 6-hr PMP Depth b.) 12-hr minus 6-hr PMP Depth c.) 24-hr minus 12-hr PMP Depth Divided by 2 d.) 24-hr minus 12-hr PMP Depth Divided by 2 (again)

This makes it so the applicable PMP intensities are experienced during the duration of the 24-hr PMP event.

1. Arrange the incremental depths to form hyetograph

This has some rules as well as subjectivity.

The rule is: The two least intense 6-hour intervals must be placed at the beginning and end.

The subjectivity is: The most intense 6-hour interval can be placed at any interval that is not the beginning or end, so in the case of the 24-hr PMP, we can either place it from hours 6-12 or 12-18.

So two valid organizations of the incremental depths set in Step 2 are possible for a 24-hour PMP:

c,a,b,d c,b,a,d

Note: c=d

You can then choose organization based off sensitivity testing. Let's say in my example, c, b, a, d was the order that caused the highest flow at my watershed outlet.

So in the end, my incremental 24-hr PMP hyetograph would be:

0-6hr: 2.5 inch 6-12hr: 5 inch 12-18hr: 30 inch 18-24hr: 2.5 inch

This nesring accomplishes our main goals with the design storm based on our depth-duration information:

Experience 30 inches in 6-hours Experience 35 inches in 12-hours Experience 40 inches in 24-hours

You could likely do a similar methodology for the 48-hour. For 6-hour or lower durations, I think you are at the mercy of your state funding a PMP study.

Hope this helps!

[u/Throwaway20230510 → u/NewToTheGPU]

Thank you, so does the HMR52 storm in HECHMS actually automate this process to determine the most critical depth and duration, or is it just computing what you described for a 72hr duration?

[u/NewToTheGPU → u/Throwaway20230510]

From my understanding, it is more or less doing what I described. However, in HMS it has a bunch of parameters for positioning (X, Y, Orientation, etc.). The subjectivity piece of my explanation is handled in the "Peak Intensity" drop down. The 1 to 6 hour ratio shapes that really intense 6-hour intensity. For my project, I did not consider the 1 to 6 hour ratio and assumed the peak intensity 6-hour period was uniformly applied.

From my understanding, HMS is not finding critical depths. It is only alternating the depths from the HMR-51 depth duration curves to come up with a 72-hour. hyetograph.