Detention Pond design examples

[ashhafPE]

Does anyone know where I can get examples of detention pond designs? I purchased HydroCad Software but seem to have a problem with the results. I review consultants work (not the detention pond part). I am curious because I do not allow consultants to release more Q than is existing once property is developed. Some of the detention ponds I am seeing do not seem right compared to HydroCad results. Again, the detention pond part is not my concern, however, I feel that as a registered engineer to bring it to the attention of the consultant firm submitting the design or to the owners attention.

 

[bltseattle]

It could be different computational methods. Often conveyance system flows are estimated by the Rational Method and detention pond flows estimated by a hydrograph method. Rational Method yields higher peak runoff rates than hydrograph methods usually, but is not very suited for detention design. To compare "their pond" to "your pond" make sure the hydrologic methods are the same.


You say you are not using the detention part of HydroCAD, but you are comparing pond designs to HydroCAD results... what HydroCAD results are those?

 

[ashhafPE]

I meant I do not use HydroCad at work (not authorized by the office). I use it for my own purpose and compared it to consultants results on my own time. The consultants use the rational method with different time of concentrations. From that they choose the Q (Qprop-Qexist)with the highest peak volume for detention pond design. So basically its not the flow peak they use. It does not make sense? Other problems I am encountering are drainage areas > 200 acres where regression equations are used or TR55 is used. I need sold documentation that can backup what I argue. I do not think some of these consultants are correct with the approach they are using. Detention Ponds are new in our area. Therefore, I would like to learn it the right way and apply it. Thanks for any info I can get.

 

[LHA]

Rational doesn't give a volume at all, only a flow. Modified Rational gives a volume, sort of. There is a lot of difference between Modified Rational and Rational. Make sure you are clear on which you are reviewing.

Detention/Retention volumes must be sized using an inflow rate and routed (again, at a rate, not a volume) through outlet devices, given a storage volume. It is a dynamic system; as such, you can't size a storage volume based on a static peak volume in - volume out equation.

I have used HydroCAD and many competitors for over 5 years, and have never found any errors in HydroCAD's computational methods.

HydroCAD does not allow a modification of the Rational Method which I have ever been able to get to agree with other methods. As a result, I don't use Rational with HydroCAD. If I want to design or review Modified Rational calcs, I use a standard, widely-accepted method to create a Modified Rational hydrograph, and manually input the flows into a HydroCAD link, then route that. It is a pain, but it works.

The consultant is correct, in my opinion, to use TR55 for 200 acre watersheds, and statistical models for very large 'sheds. But, be advised: this is a subject of fierce debate in the H&H world, so if you challenge someone's results based on their method, make sure you have lots of evidence to make your case. The consultant most likely chose the hydrograph generation method for a reason.



Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[ashhafPE]

Steve, thanks for the info. If you have a lot 100'x100' that results in a Q=CIA=12 cfs (Proposed). I am only allowing them to release Q=5 cfs. Therefore, they have to detain 7 cfs. The 5 cfs will be picked up by a grate Inlet and a storm sewer pipe and outfall into either a ditch or tie into an existing system. Now the question is! do you use the 7 cfs to determine the volume for the detention pond? or do you use the modified rational method to determine the volume even if the Q determined from the modified rational is less than 7 cfs.

 

[LHA]

Modified rational method uses the Rational peak, modified rational does not yield a peak.

Using the Modified Rational approach, the Rational peak is ascended to constantly over a multiple of Tc. It lasts at that peak for a duration, then descends constantly to zero, again, over a multiple of Tc. There are many approaches; some hold the peak for the duration of the rainfall, others hold it for a critical intensity duration, others triangulate at the peak. None are incorrect, but any given one must be substantiated.

The rationale for these are all complex, so, again, if you are challenging someone's hydrographs, be prepared. If you challenged mine, I would bring stacks of references to back up my position. I would also insist on seeing yours.

You can't use 7 cfs by itself to analyze volume. You can't detain a peak flow, you can only detain a volume, and the delta volume is determined by the dynamic Qin:Qout, not by Qp alone.

You need the above hydrograph in, the stageutlet ratios (governed by the outlet structure) and the volume's stage:storage values.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[queque]

Please check if I understand this--
Q=CIA = 12 CFS
12= .9* (I) * .23 (ACRES)
I = 55 IS THAT CORRECT?
The intensity = 55 inches per hour appears wrong.

Suggest you use the HydroCad Model to generate 12 cfs.
Then develop a detention basin that stores 7 CFS.

 

[ashhafPE]

queque,
That was not an actual analysis, it was just an example.