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County Criteria 100yr_5day storm
3

County Criteria 100yr_5day storm

County Criteria 100yr_5day storm

(OP)
I have been asked to design a detention pond for the 100 year_ 5 day storm event by the local governing body. Typically the 100 year_24 hour storm is used as the design criteria.  I am wondering why this would be requested.  It seems to me that the rainfall intensity would be less and thereby allowing for greater infiltration, a smaller peak runoff and thereby requiring a smaller storage volume. Is there a logical and/or technical reason for this request that I am just missing?

Thanks so much.

RE: County Criteria 100yr_5day storm

Hello Drawleigh,

You are correct that a 5 day storm is likely to produce a lower peak flow.  But, it is also likely to produce a greater runoff volume which may require more storage.  The only way to be sure is to do a preliminary design for both durations; 1 day an 5 day.

You can get precip/duration data from NOAA's Hydrometeorlogical Studies Data center website. Google it.

Good luck

RE: County Criteria 100yr_5day storm

It really depends on your time of concentration whether your peak flow will be higher for the 5-day storm or not. For a construction site, you're usually lucky to get a time of concentration greater than half an hour.  For a stream, it would be longer and for a large river it could be days.

RE: County Criteria 100yr_5day storm

2
This is a case where your project is probably affected by downstream lag effects rather than just upstream lag times.  It sounds like you are located in a coastal area.  I know some communities in California look at a ten day event just to make sure that during the rainy season they can get through a long wet period.  It can take a while for downstream water to move through the delta so you can flush out your city.

If you are modeling anything over 72 hours I would be careful how you use the NWS data.  You probably want to look at not just nesting your storms.  While you would want to handle short 100 yr duration events you most likely don't need to nest them in the models for the long periods also.  For multiple day events it may be useful to use a multiple peaking storm.  Your regulator should have info on this.

RE: County Criteria 100yr_5day storm

(OP)
you would be correct blueoak, i am near the tampabay area.  

i have run the model for both events (100_24 and 100_120) and am oddly needing the same storage space for both events.  the basin is considered closed and grounwater is withing 8 feet. i am supposing these are the cause for the lack of significant change in the storage requirements.

i am more accustomed to working with rural mountainous terrains. the high water table, wetlands, karst topography are new to me.

RE: County Criteria 100yr_5day storm

Are you allowed to discharge during the 5 days?  If so it makes sense that they would be the same.  What are the rainfall depths?  Your infiltration loss may be too high in your model.  After the 1st day of rain I doubt you have much loss in the watershed.  Your total volume should keep increasing.
Typically peak discharges tend to reach an asymptote around 1 to 3 days in areas with large, mesoscale storms.  I am in the desert and we don't see the asymptote if we nest our storms.  What model are you running?

RE: County Criteria 100yr_5day storm

(OP)
No discharge is allowed.  This is a case of "no adverse impact" due to floodplain ordinance. For the 24-hr storm I have flows at the 12th hour at 53.17 cfs and 11.38 inches. For the 5-day the peak is at the 60th hour at 17.56 cfs and 11.38 inches. I am using TR-55 with Type II FL Modified distribution and Interconnected Channel and Pond Routing (ICPR)Version 3.  This a relatively small site with a contributing basin of 9.76 acres.

RE: County Criteria 100yr_5day storm

I was concerned that was the case.  The NRCS type II is a 24-hr hyetograph and TR-55 assumes a 24-hr storm unless tricked up.  If you want to stick with NRCS methods you probably have to run TR-20.  Your precip depths should be different.  Check the NWS data.  I have no clue where Tampa is, but your 4 day is somewhere between 14 and 18 in. on the 1964 maps.

It doesn't seem like you really need to model the hydrograph, just get your precipitation minus estimated loss (maybe about 20% but it is dependent on soils and paving and length of storm) and multiply it by your 9.76 acres for your storage area.  If your values are the same you have the wrong precip numbers.  Ask your regulator, they can give you the total precip. values.

If you want to run the hydrograph you need to use a different hyetograph for the 5 day.  I bet the FL modification is just to better reperesent your storms and not to increase the length of the event.

RE: County Criteria 100yr_5day storm

(OP)
thanks everyone for your input. this has helped a great deal.

RE: County Criteria 100yr_5day storm

(OP)
Mystery solved with there being absolutely no change with the 24 and 5-day event, the program was not reading the hydrology file.  But all of the information you gave me has been helpful, useful and eductaional.

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