Hydraflow Express - Hydrology: Required Storage Analysis - Total Precip. Problem

[daniel.terryE27M4]

First post...hopefully my screenshot displays properly.

I'm trying to run a quick prelim storage volume calculation by comparing pre and post development conditions and limiting my post-development flow to my pre-development flow via "Target Q" in the inputs.  I am running an SCS analysis with a synthetic distribution so that (as I understand from reading the user manual) it will use the IDF curves I set up by entering regression equation coefficients published by my Dept. of Transportation.  When I run the analysis, the output tells me the total precip. is 32.16 inches, which isn't right.  I'm only running a 50 year storm because the DOT doesn't have regression coefficients for the 100-yr storm, but the 100-yr/24-hr rainfall depth is only about p = 14".  I've read through the manual and can't figure out where this number is coming from, and the storage volume it's telling me is required is larger than I expected.  

[Edit: I did double check the regression coefficients and compared (spot checked) the resulting IDF curve values to my DOT IDFs and they're spot on.  I also included better screenshots.]

[Hidden_Brain]

can you provide a plot of the idf curves?

[daniel.terryE27M4]

Yes, I've attached a PDF of them.  The DOT did not provide regression equation coefficients for the 1-yr storm so I just mad them all .01 (same goes for the 100-yr storm).  Thanks.

[Hidden_Brain]

32 in is the cumulative rainfall based on your IDF.

[Matt.Anderson]

I'm fairly confident you need Total Depth and a different curve not available in Hydraflow Express to complete the estimated volume.

[daniel.terryE27M4]

I couldn't get any results that passed the smell test using the IDFs I developed by entering local regression coefficients...even though the resulting IDFs looked appropriate when I spot checked them.  Because I needed an answer a couple days ago, I just analyzed using SCS by entering precipitation data and using the applicable SCS distribution.  The resulting "total precip. (in)" for the 100-yr analysis matches the 100-yr rainfall depth that I entered in the precipitation data.  This will have to suffice for the conceptual estimate I need right now, but I'd like to figure out how to use local data.

Maybe I misunderstood the user manual & user interface prompts.  As I understand it, if you choose a "synthetic" distribution, it should pull from the IDF curves.  See attached.  Can anyone confirm/clarify?

[wpagl452]

Hmm yes I verified 32" as well. I think your understanding is correct, it should be using the IDF coefficients.

My question would be why are you generating a 24-hr synthetic design storm when SCS has already generated accepted 24-hr design storms? I have typically seen the synthetic storm generation used when a storm with a shorter duration is desired.

[daniel.terryE27M4]

Sorry for the delay - I've been swamped and out of town recently.

I think I'm misunderstanding something.  A couple people say they confirmed 32" cumulative rainfall but I'm not seeing it.  Looking at the local IDF curve, my 100-yr, 24-hr intensity is about 0.56 in/hr.  If it rains at that rate for 24 hours, that is 13.44 inches of precipitation.  

I want to use DOT data because it's local, the regulatory agencies have approved it, and I've found on previous projects that the SCS and local data don't always agree.  I was trying to use Hydraflow to develop a quick preliminary ballpark estimate of required storage on the front end of a project that we do not have under contract yet.  Based on what I read in the manual it seemed pretty straightforward but there appears to be a disconnect somewhere.  Maybe there's some disagreement somehow between the DOT's IDF curves and the regression equation/coefficients.  I guess I just need to use the SCS data and adjust it as I see necessary for preliminary estimates.  In the end, I will end up using another software to route the applicable storms through a pond. 

[Hidden_Brain]

set up an excel table that incrementally calculates the rainfall depth:

Time (min)          in/hr                             **** Depth

  5                   read from IDF                    (in/hr/60)*5

 10                  read from IDF                     (in/hr/60)*(10-5)

.....                       ........                                ..........

                                               TOTAL **** Depth = should be approx 32 inches based on your IDF

If the 50-yr intensities are used from the IDF, the resultant table will yield a 50-yr 1-hr synthetic rainfall distribution.                                   

[Hidden_Brain]

See below, this is set up based on the 50-yr IDF curve that you provided earlier. The sum of col. C happens to be around 32 inches. I do not know why Hydraflow prints that as total rainfall, perhaps someone from ADesk can explain.

Total rainfall depth in the 1 hr storm duration is about 4.25 inches.