Required Detention Pond Volume? 2

[SoonerCFM]

Can anyone shed some light on the subject of detention pond volume necessary to reduce flow rates to pre-project levels? Of the many methods for determining the preliminary estimate of detention pond volume necessary to begin the pond design,I am most familiar with the SCS TR-55 Method which provides the estimated required detention volume based on curve numbers, rainfall, basin area, and initial abstraction. Is it possible to reduce post-project flows to existing condition levels without providing the estimated required detention pond volume? I have seen SCS (NRCS) Method HEC-1 runs for projects showing the peak flows reduced, while providing considerably less than the recommended SCS pond volume. Any suggestions or comments would be appreciated.

 

[RWF7437]

Sorry,
As far as I can tell, there is no consensus on this. Every regulatory agency, from cities to counties to state highway departments, adopts "standards" which are arbitrary and sometimes uneasonable. They don't agree with each other and often provide little or no flood protection benefit.

If any one out there knows of a universally accepted method, I'd love to hear it.

 

[gbam]

If you are lokking for an initial size of the basin then here is the proceedure I use
1. graph the inflow hydrograph
2. draw a line form begining of hydrograph to the desired outflow from basin.
3. measure or calculate the area between the hydrograph and line that was drawn. This is the volume of H2O that must be stored.
4. increase the volume by 10-20%

This will give a good estimate for the required storage volume. If you have any questions you can contact me at;
gbambaue@hdrinc.com

 

[RWF7437]

Interesting ! But what inflow hydrograph ??? The 100 year - 24 hour storm ? The 50-year 1 hour storm ? The 500 year - 7 day storm ? All of these ?

 

[Bayou]

Another method is to just subtract the existing condition hydrograph from the proposed hydrograph. The volume summation for this difference lends to a good approximate of required detention. I usually add 10%.
The frequency of the event will be dictated by the local government regulations. however it is usually a 100-year, 24-hour storm.

 

[RWF7437]

"The frequency of the event will be dictated by the local government regulations. however it is usually a 100-year, 24-hour storm."

I think you're possibly right Bayou when you make this statement BUT, I'm trying to suggest that some regulatory agencies simply arbitraily "dictate" a design storm without regard to other factors. For example, in our part of the world, the Pacific Northwest, the 24 hour storm almost never causes flooding. Usually it is a 7 or 8 day prolonged rainfall event which causes trouble. In your part of the world, the situation may be quite different.

I'm being argumentative on purpose. I don't hear engineers and others discussing these ideas but I think they need to be discussed vigorusly in this and other forums. We can all do the mechanics of hydrology and hydraulics but serious thought about the size, type, duration and intensity of storm events which should be used for design to lead to ECONOMICAL designs is seldom mentioned.

 

[Bayou]

The design of systems has been limited due to economic reasons. The Corps use to design channels to the Probable Maximum Percentage (PMP) event which in some parts of the US equated to a 500-year, 24hour event.

The standard of 100-year, 24 hour event lends itself from FEMA regulations. Since the FIRMS show this flood plain, most regulatory agencies adhere to the standard.

However, many design criterion are set for different time intervals and frequencies. Many storm sewers design criteria require 1 hour event analysis. Reservoir and dam analysis may require multiple storm events or time analysis of multiple months or even a year (eg. HEC-6 modeling).

Most events are not the hypothetical 24-hour events. Some places can severe flooding with just a one hour event, for example Las Vegas. Other parts of the country require several hours of rainfall for flooding to occur, eg. the southern regions.

Flood occurrence is depended upon the terrain and specifics of the region as well as the frequency. That is why the local engineers within that region must cooperate with the regulatory agencies to establish appropriate criteria and ensure that the criteria is applicable to their region.

 

[PECPESC]

Try WINhydro from the McTrans Center at the University of Fla. It compute storage volumes, basin staging etc.

 

[RWF7437]

Soonercfm,

You may also like SMADA available from the University of Central Florida.

http://engr.ucf.edu

(I think ).

It will allow you to input actual storm hyetographs as well as the "standard" SCS storms.

 

[cvg]

the Corps designs dams for the Standard Project Flood (SPf) and part of the definition is as follows "...the standard project flood is intended to represent a flood that would be exceeded in magnitude only on rare occasions, and thus to constitute a standard for design of structures that would provide a high degree of flood protection as determined by flood potentialities of the drainage system involved, without regard to localized economic or other practical limitations of individual projects ...In most cases, the SPF should equal or approximate the flood that would result under existing or specified conditions of basin development, if the most critical storm of record in the region should occur over the drainage area involved when hydrologic conditions were reasonably favorable for flood runoff" This is not the PMF, and may be only a fraction of it. For example - on the current project I am involved with, the PMF is over 3.5 times the magnitude of the SPF and the peak runoff from the SPF is only marginally greater than the existing 100 year runoff. Also, for this particular project, the multiple day storm event is much less than the PMF (which actually is a 72 hour storm). For other projects in this area, there is some concern that the multiple storm event (probably a series of 2 or more 50 year events) may have a recurrence interval of approximately 100 years and could be more significant than the 100 year - 24 hour storm because of the greater volume of runoff.

 

[gbam]

Assuming that the initial hydrologic study has been completed which ever method is applicable then the inflow hydrograph would be known. So either of the hydrograph methods stated above will give an initial basin size to start the modeling. once the storage routing has been completed, the basin size my need to be tweaked up or down depending on the freeboard requirements

 

[dicksewerrat]

You are missing one part of the equation when you talk about the 24 hr storm. That is the frequency of the event. A one year frequency is nothing, its the 100 year or in some cases the 500 year event that the agency is concerned about. And the intensity of that event causes the flooding, not the rainfall over a whole 24 hrs. In Minnesota the 100 year event is on the magnitude of 2.5 inches per hour. The pipes are not sized for that event, and you get flooding.

 

[RWF7437]

The discussion goes on doesn't it Soonercfm ?

As you can see there are several approaches suggested by responders depending on where they live, their experience, and local regulations. But no one method exists.

Your approach, using TR-55, is as good a place to start as any. But it is suggested you not stop there assuming there is but one answer. If this pond is large and important; say a regional pond, it would be a good idea to route as many storms, hypothetical and actual, as you can through it. Software is available to do this fairly easily so it is not that much more work.

You could for example route storms with durations of 1 hour, 6 hours, 12 hours, 24 hours, 48 hours and 96 hours and frequencies from say, 25 years through and including 500 years. Throw in a couple of actual storms for your area if you have the data available. This will give you a good idea of how your pond will perform under a wide range of conditions. What would be the consequences of failure of your pond? How would you define failure? Merely overflowing might not be disastrous depending upon what is downstream. Doing all this will give you a "feel" for your design and possibly some confidence that it is reasonable.

Just some food for thought.

 

[psmart]

There are a number of techniqies for estimating the required storage, including the TR-55 method and the graphical techniquue already described. But these are all estimates, based on assumptions about the shape of the inflow and outflow hydrographs. To use these results for an actual design requires the addition of a safety margin, as previously suggested, and is still just an approximation.

The best approach is to use one of the estimating techniques to do a trail pond design, and then do an actual hydrograph routing through the pond. This eliminates any assumptions about the outflow hydrograph, since the outflow is being calculated based on the actual inflow, pond storage, and outlet characteristics. The routing process will also determine the peak elevation and storage attained during the storm, from which you can fine-tune the pond design to meet your exact design goals.

Modeling systems like HydroCAD offer both calculations: An initial size estimate, and an interactive hydrograph routing process that makes it easy to fine-tune the design. For details see

http://www.hydrocad.net

Of course, there is still the the bigger issue of pond design standards, but that's another question.

 

[kknowle2]

The pond should hold at a minimum the difference between the pre- and post- discharge volumes. Usually you have to provide more because you also must include a treatment volume in addition to the mitigation volume.

http://www.kesco.sphosting.com

 

[RWF7437]

As you can tell from the 14 replies to your original post, there remains no consensus. To quote myself:

"Sorry,
As far as I can tell, there is no consensus on this. Every regulatory agency, from cities to counties to state highway departments, adopts "standards" which are arbitrary and sometimes unreasonable. They don't agree with each other and often provide little or no flood protection benefit.

If any one out there knows of a universally accepted method, I'd love to hear it."

Still all is not hopeless. My suggestion would be to use all of the approaches suggested and the test each against you experience and judgement. You will, of course, have to satisfy some regulator but ultimately you will need to convince yourself and your client that what you have done is reasonable and effective.

Good luck out there,

Russ