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.
Thanks so much.





RE: County Criteria 100yr_5day storm
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
http://www.weather.gov/oh/hdsc/
RE: County Criteria 100yr_5day storm
RE: County Criteria 100yr_5day storm
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
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
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
RE: County Criteria 100yr_5day storm
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
RE: County Criteria 100yr_5day storm