Detention Question 1

[Rookie2]

I am modeling a detention pond. The detention is provided by building a dam on a stream. A large diameter culvert is proposed in the stream bed.

This was permitted by the USACE. Two of the conditions of the permit were:
1. The detention time must be less than 2 hours
2. The pipe must be embedded 1 foot below the natural stream bed.

My questions are-
1. What exactly is the definition of detention time; i.e. how do I calculate it? And can someone tell me a publication that addresses this so that I can provide a reference to the local reviewer?
2. What is the best way to model the embedded culvert? I am thinking I should use an equivalent area pipe and model the invert 1' above the actual design invert. Is this correct?

Also, is there any free detention modeling software that allows you to use a rating curve for the pond tailwater depth?

I hope I was clear with my questions. TIA, any help would be appreciated.

 

[francesca]

I would call the USACE and ask them. They will likely be very, very helpful. You need to know the design storm and what headwater/flow rate conditions they consider to be no longer detaining. In a 24 hour storm, the culvert would have to pass almost all but the peak flow through to have only a 2 hour detention. The traditional "detention basin" modeling event is the 100-year, 24-hour storm.

For free detention modeling software, use WinTR-55. However, I strongly urge you to also model your culvert in HEC-RAS and check the scour. You will likely need some serious scour protection on the downstream side.

 

[psmart]

As francesca says, you definitely need to ask what they mean by detention time. There are a number of ways this can be measured, some of which are discussed at

http://www.hydrocad.net/detentiontime.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

HEC-1 is a good free modeling tool for the detention hydrograph routing and may be preferred by the USACE since this program was developed by them. I'm not sure why you want to change your pipe invert elevation for modeling. During normal river flow, the pipe invert may remain partially buried in sediment. During a flood, higher velocities will probably scour out the sediment and it will run full. If I was to define detention time it would be the time that the water surface in the creek is above the predicted elevation without the culvert in place.

 

[froude]

I have trouble explaining it, but I have always thought detention time is the time required to detain and discharge a flood of a given return event back down to the "normal" operating characteristics of your pond. Sorry if that's too vague.

As far as software, I use PondPack, but that's not free. I think HEC1/HEC-HMS would probably work well and you can use rating curves. EPA-SWMM 5.0 is another good free software. In EPA-SWMM, you can probably model your stream, the dam, outfall, etc.

Hope this helps!

 

[TerryScan]

In addition to their detention time approach, be sure to determine what storm event and flow data (guaged, regression equations, SCS methods) one is to use for this evaluation.

While one can rationalize scenarios other than what is specified regarding embedding the pipe, I would model it as described in the permit. There are several software packages that will model an embedded pipe, including the current version of HY-8:

http://www.fhwa.dot.gov/engineering/hydraulics/software.cfm

As to last question, I am not personnally familliar with HEC-1. The manual does indicate it can account for tailwater in computations, but I don't know how it's tailwater is established.

I work with HydroCAD (affordable) as well as ICPR (not as affordable). Both of these packages can account for tailwater in calculations and can accept user defined rating curves.

 

[cvg]

I would not use HEC1 for developing a rating curve for the culvert. HY8 would be my first choice. Use HEC1 for the routing only.

 

[Rookie2]

Thank you all for your responses. This is very helpful. I guess the USACE would need to answer the question about detention time.

I will give HEC-1 and HEC-HMS a try. I've been using an older version of Hydraflow Hydrographs which only allows a fixed tailwater elevation.

Again thanks for your help