requirement here (also a very arid environment) is to retain the entire 100-year, 2-hour storm. this is regulated by local government ordinance. you should size your basin according to local requirements

Pond sizing will depend on your design objectives and the applicable stormwater regulations. Why are you building the pond? Where will it be located?

Peter Smart
HydroCAD Software
www.hydrocad.net

It is a 10 acres area located near a harbor for container storage (it is off site) and as per the local authorities, no permit to drain to the local storm water drainage.
The owner is to find a solution for local stormwater management - and of course, for the owner, he is seeking the cheapiest accepted solution.
I found that a detention pond is the cheapiest solution.
Any comments ?

evaporation may not be the recommended or best disposal method. its too slow and may create a hazard with stagnant water and mosquitoes. you should consider a dry well, infiltration system or a bleed off line to drain the basin within 36 hours after the storm

At a minimum, you probably want to retain the 100-year 24-hour event. But regardless of your design event, it will be exceeded at some point, and there will be a discharge which has to go somewhere, so you still need a provision to handle overflow.

Peter Smart
HydroCAD Software
www.hydrocad.net

Yes Psmart - your point is absolutely correct, at some point there will be an overflow - I am studying with the owner to have an overflow tank that will drain to the local sewage line at a controlled rate - that is acceptable by the governing authority.
I needed to confirm what is the runoff to account for - some documents on the net specify 100 yr 2hrs storm - some 100 yr 24 hrs storm, and some (especially in parking lots)it is a 2 yrs 24 hrs storm

In my semi-arid region (Central California), some agencies require designing detention ponds for the 100-yr/10-d event. In the locations I have dealt with on the floor of the San Joaquin Valley, the 100-yr/10-d event ranges from about 4.5" to 7.0" inches of precipitation depth. Other agencies around here have lesser requirements, with the minimum I have seen being a 100-yr/2-d event (about 2.7" to 5.0").

I will echo the above recommendations: design to the standards of the local agency with jurisdiction and provide a good means to deal with overflow.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

Dear fel3 - unfortunately there is no a local guidance concerning this point and also there is shortage in the avialable data - so I have to make a carefull decision.
The site is in Saudi Arabia - North West coast and there is no mention to this point in the local standards at all. it is very hot in summer (around 45 C) and about 20-25 in winter aired environment - but when it rains it gets really bad.
The IDF curves sound unreasonable if I am going to calculate the runoff using the modified rational method - so I have to double check with reasonable requirements if I am going to use those IDF curves
Thats why I am bothering all my colleagues

• https://files.engineering.com/getfile.aspx?folder=4c2e2d51-1924-4de7-a036-6

can you use RCJ standards? its on the east coast but they may have something relevant

RD-Design Engineer...

IDF curves are normally used for estimating peak flow for short duration event and Depth-Duration-Frequency (DDF) tables are normally used for pond design, which must handle longer duration events. I have used the Modified Rational Method and IDF curves for pond design where there was no DDF data available, but that was a long time ago. Now it's easier, at least in California, to get good IDF and DDF data for the locations I deal with.

If you can find local DDF tables, that would be better than using the IDF curves. Your IDF curves end at 6 hrs and you need a longer duration for pond design. I don't like to extrapolate and using data for one to six hours and extrapolating out to two to ten days is not a good idea.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

Dear fel3 - in California or any other loaction in the states you do not have any problem to find the IDF or the DDF - you can visit the NOAA Precipetation Frequency Data Server and you'll find all the data you need to size and design the pond - and a visit to the local county you'll find all the requirements for the detention pond - unfortunately, this is not the case here

and this IDF curve is all what is puplished in the loacl standards - yes thats all

does the met office have something?

Dear cvg - the met office have a one number for avaerage precipitation per month - I checked it and it was not very useful
- and by the way: what do you mean by RCJ standards, I did not get what it refers to

RCJ Royal Commission for Jubail and Yanbu. i believe they have new engineering standards that may include retention basins design

attached is the old standards, they may have an updated one. they were working on it several years ago

• https://files.engineering.com/getfile.aspx?folder=46e586e4-26a0-4caf-8f17-0

Thank you cvg very much.
In conclusion - I can see most of the design standards are specifying a 100 yrs storm - for at least 24 hours.
I tend to use this value - and it seams very rational.

Best Regards

Sorry a little late to the discussion from what I gathered the job is in Saudi arabia and there is no clear local guidance on the design. It is upto the owner to decide which guidance to follow and it is your job to make recommendations. You may consider finding US or any other jurisdictions guidance which has similar precipitation total, flash flood frequency, and landscapes. My experience is in Ontario Canada which has a good design procedure but it is not the same climactic conditions as saudi arabia.

Edit link to Ontario guidance
https://www.ontario.ca/document/stormwater-managem...

Thank you GeoEnvGuy late is better than never
I found a refernce in the manuals of highways design (drainage part) from the ministry of transport