PondPack - Storm Data - Varying Time Step

Hello,

I believe (but am not entirely certain) that the table you shared is showing the total depths (in inches) for a particular return event, where you would select the desired year and duration (for example 100 year, 24 hour) and use the option in PondPack to build a time-depth storm event from a dimensionless curve, enter the respective total depth and appropriate distribution (which I suppose would need to be one of the “Huff” distributions for everything except for 24 hours). See: How can I apply a rainfall depth or intensity to a dimensionless curve to create a storm event?

Since each of these would be a separate return event it would take a while to input the entire table. So, I suspect that you may only need a select few of these for your pond design purposes.

I am checking with some of my colleagues on this and will get back to you.

Correct, the precipitation data provided is showing the total depth for a certain storm event. The AHJ is requesting the following to be analyzed for detention "The storage and release rates from a post-construction land use shall be evaluated for the 2-, 25-, and 100-year storm events for the critical duration of the 3-, 6- and 24-hour durations that results in the maximum or near maximum peak flow." 

I believe I will need to create 9 storm events. Can you please provide more information on how I can input the precipitation data given into the PondPack program to correctly create the events? 

Hi, thanks for confirming. Yes, it sounds like you need to nine storm events to your Storm Event Group. 

For the steps on how to do this, see the link in my previous reply: How can I apply a rainfall depth or intensity to a dimensionless curve to create a storm event?

Also see Quick Start Lesson 1 under Help > Quick Start Lessons.

You would first create a new Time-Depth Storm Event Group on the left side of the Storm Data Window (Components > Storm Data) . Then on the right side, for each of the nine storm events, you would click the new button, choose to add from dimensionless curve, select the dimensionless distribution (you will need to determine what the AHJ requires for this), enter the return event year, total depth and duration, then click OK. This will create the hyetograph for that storm based on the shape of the distribution, total depth and duration. 

You can then store the Storm Event Group in the Engineering Library for use with other models, and can use the Pre/Post-development scenario creation wizard to create all the necessary scenarios for analyzing all of your storm events.

Hi Dynamic Engineering, 

I'm going to try to piece together what the AHJ is looking for here based on the info you've provided and what I've been able to find online. Hopefully this will provide some clarity to help you move forward, but it would probably be a good idea to get confirmation from a reviewer in the jurisdiction before you get too far along.

In the original post, you provided rainfall depth data for a range of storm frequencies and durations. The reference for this is STORM DRAINAGE (nctcog.org), which says the data came from NOAA Atlas 14. 

So now we have the precipitation depth data we need for our 9 storm events, but there's still the question of where we get the data for rainfall distributions that define how the precipitation is distributed over time. Unfortunately, iSWM doesn't seem to provide a lot of help for this--they have examples that use the old SCS 24-hour distributions (like Type II), not the newer ones for  the 3-, 6- and 12-hour durations like what you say you're being asked for in your last response. So we need to go to NOAA Atlas 14 itself to find that.

You can go to PF Map: Contiguous US (noaa.gov) and move the crosshair to a location near your project, which is Texas Region 2 if it's Tarrant County. Then, scroll down the page and click on the Supplementary Information tab. Here, you'll be able to download files with the rainfall distribution data for the different storm durations for this region, and pull up a document with a lot of information about what it all means.

Specific guidance on how to use this information to create your storm events in PondPack is given in (+) How to incorporate NOAA Atlas 14 rainfall distributions? - OpenFlows | Hydraulics and Hydrology Wiki - OpenFlows | Hydraulics and Hydrology - Bentley Communities (caveat: I'd probably use the 50% "percent of occurrence" distributions, not the 90% ones as used in the article's example...more below).

The remaining question is, which of the many available rainfall distributions do you use? There are four groups of distributions based on which storm quartile most of the rain fell in, and these are broken down further into occurrence percentages. It would be great if your jurisdiction could provide the specifics on what they want to see, but if not, here's my best guess/judgement. (I'm basing my rationale on some documentation I found on applying the Huff distributions, which were the midwest-US precursor to the current NOAA Atlas 14 distributions.)

• If you dig into the document referenced on the Supplementary Information tab, on page 269 there is a table that tells the you the percentage of quartile classifications for actual storm events of various durations. For Region 2, for any duration, it looks like the most storms were 1st Quartile, so that sounds like a good choice for your analysis.
• As far as Percent of Occurrence, the data provided ranges from 10 to 90%, with 50% representing the median. I'd probably go with 50%. 

I hope you find this explanation helpful. The newer rainfall distributions require a bit of setup the first time, but ultimately, I do think they result in better-performing detention designs.

Regards,

Kristen Dietrich

Senior Product Manager, OpenFlows

Bentley Systems, Incorporated

Hi Dynamic Engineering, 

I'm going to try to piece together what the AHJ is looking for here based on the info you've provided and what I've been able to find online. Hopefully this will provide some clarity to help you move forward, but it would probably be a good idea to get confirmation from a reviewer in the jurisdiction before you get too far along.

In the original post, you provided rainfall depth data for a range of storm frequencies and durations. The reference for this is STORM DRAINAGE (nctcog.org), which says the data came from NOAA Atlas 14. 

So now we have the precipitation depth data we need for our 9 storm events, but there's still the question of where we get the data for rainfall distributions that define how the precipitation is distributed over time. Unfortunately, iSWM doesn't seem to provide a lot of help for this--they have examples that use the old SCS 24-hour distributions (like Type II), not the newer ones for  the 3-, 6- and 12-hour durations like what you say you're being asked for in your last response. So we need to go to NOAA Atlas 14 itself to find that.

You can go to PF Map: Contiguous US (noaa.gov) and move the crosshair to a location near your project, which is Texas Region 2 if it's Tarrant County. Then, scroll down the page and click on the Supplementary Information tab. Here, you'll be able to download files with the rainfall distribution data for the different storm durations for this region, and pull up a document with a lot of information about what it all means.

Specific guidance on how to use this information to create your storm events in PondPack is given in (+) How to incorporate NOAA Atlas 14 rainfall distributions? - OpenFlows | Hydraulics and Hydrology Wiki - OpenFlows | Hydraulics and Hydrology - Bentley Communities (caveat: I'd probably use the 50% "percent of occurrence" distributions, not the 90% ones as used in the article's example...more below).

The remaining question is, which of the many available rainfall distributions do you use? There are four groups of distributions based on which storm quartile most of the rain fell in, and these are broken down further into occurrence percentages. It would be great if your jurisdiction could provide the specifics on what they want to see, but if not, here's my best guess/judgement. (I'm basing my rationale on some documentation I found on applying the Huff distributions, which were the midwest-US precursor to the current NOAA Atlas 14 distributions.)

• If you dig into the document referenced on the Supplementary Information tab, on page 269 there is a table that tells the you the percentage of quartile classifications for actual storm events of various durations. For Region 2, for any duration, it looks like the most storms were 1st Quartile, so that sounds like a good choice for your analysis.
• As far as Percent of Occurrence, the data provided ranges from 10 to 90%, with 50% representing the median. I'd probably go with 50%. 

I hope you find this explanation helpful. The newer rainfall distributions require a bit of setup the first time, but ultimately, I do think they result in better-performing detention designs.

Regards,

Kristen Dietrich

Senior Product Manager, OpenFlows

Bentley Systems, Incorporated