Elevation of a 2-yr storm? 3

[kenn100]

Hi guys,

I feel really silly asking this question. But, how do you find what the storm event elevation is?
for example, a 2-yr storm runnoff is 2-inch, peak discharge 2.3 cfs. I have a pond with bottom elevation at 100-ft, surface area 200 sf.
what would be the 2-yr storm elevation in this pond?

 

[kmatte]

Does the pond have and outlet? If there is an outlet, what type is it, ie pipe, weir, oriface, etc. What type of storm event are you working with? Does the pond have a permanent pool volume, or does it drain dry? How big is the drainage area, and are you using a time of concentration? These are some of the things unknown to fully answer your questions.

However, if you want a very simple answer. Neglecting any of the information above. You would simple multiply the peak rate by the duration of the storm. This would give you the total runoff volume. Next you would need to compare the volume of your pond to this.

Lets say you have a squaure pond for simplistics. The bottom surface area is 200 sf. So if the pond is 4 feet deep, you have 800 cf of storage. If your runoff last for 2 hours, 2.3 cfs * 7,200 seconds = 16,560 cf. The pond would need to be bigger.

Without an outlet, ponds need to be very large. If you have and outlet, very simply you could assume a full capacity of the outlet. If you outlet had a full capacity of 1.5 cfs, then you could subtract the volume of outflow from the inflow. This would give you your storage volume, and then your height.

I would like to reiterate that this is very simple, as in addition to the questions I already asked, outlets perform differently as the headwater changes. As the water raises in your pond and the outlet, the capacity of the outlet will change. The change in rate over time for the outlet, runoff entering the pond, possible headwater, etc, make the calculations to get the true elevation complicated.

My suggestion, if you need the true elevation, is to download a free copy of HyrdoCad or other hydrology program. The demo copy will allow you to calculate the pond elevation fairly quickly.

 

[kenn100]

The pond is a 24 hrs detention pond with a Riser structure (8-ft) from pond invert. The riser structure has three stages: a 8"x12" weir opening, a 12" orifice, and the principle spillway (top of riser structure).
The riser structure is connected to a 24" diam culvert outlet.

The drainage area is divided into two: 3.0 acres (CN=98, parking lot), and .5 acrs (CN=61, grass).
The drainage areas are separate, and reaches the pond at two different inlets. So thats confuses me about what the Tc should be. Should I use the least Tc or add both Tc's together?

 

[cvg]

peak rate times storm duration will not give you the total volume, it will give you a much larger number. Download HEC1 (free) and you can analyze your basin with that. For an area this small, I'm not sure I would complicate things by using two separate subareas.

 

[francesca]

WinTR-55 is also free and much easier to use than HEC-1. It will generate the two hydrographs you need and combine them. It can also do detention volume calculations.

http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/Tools_Models/WinTR55.html

 

[elev8848]

ken100:
No question is silly.

Using your data, the max. theoretically possible pond elevation is 227ft - that is if there were no outlets and if all of the 3.5 acre surface was totally impervious. Here is why.

Total Vol. from rain possible in 24 hrs = (2/12 ft*3.5 acres*43560) = 25410 cu. ft

Depth required = 25410 cu. ft/200 sq. ft=127 ft

Elevation of water surface =100+127 ft = 227 ft!!!

Obviously, this is just to understand the calculation.

Because you have 3 outlets in the pond, all the water does not need to be stored. Whatever run-off keeps on entering the pond will have option to either stay or flow out. The "flowing out" is governed by how the elevation of the rectangular weir opening and that of the orifice is set up.

Now, these things can be modelled in SWMM5 if you must. However, a 2 inch in 24 hours is a relatively small storm. The peak intensity of such a storm will be 1.2 inch per hour (if you were in SCS Curve III Zone). You said the outlet riser is connected to a 24" culvert. If the 24" pipe can discharge without much downstream constraint (tailwater), then I can safely assume that the 24" pipe can carry any peak flow generated by this storm.

So I would conclude that water level in this pond will not rise much above the riser.

If your question is-where exactly will the maximum water level be? Then, you can us a SWMM5 model. The modelling for your scenario is easy if one is familiar with SWMM5. (If not, can you mention the height of the rectangular weir and the orifice here? And also the water level at the end of the culvert?)

If I misunderstood you, please correct me.

 

[kenn100]

I was able to do some calculation following what I read from the discussion with some actual numbers. It really helped to get some input from other people because I am not really confident in what I am doing.
The real problem is this:
I have a parking lot that is been redeveloped with additional impervious area.
The runnoff goes to a nearby wet-detention pond. I want to evaluate the impact to the pond due to the additional impervious area. Can the existing detention pond handle new drainage area?
I don't know how to approach this.
Should I run a TR-55/ TR-20? when I run TR-20, what values should I be looking for. I know the post-construction peak discharge can not be greater than the pre- constr. peak discharge. I don't know how to route the hydrograph thru the pond to keep it below pre-construction discharge.
Most books I read end up confusing me and don't show how they got their tables and values.

 

[RWF7437]

If you MUST do this because of some local detention requirement, you would do well to get some software designed to do these calculations for you. HydroCad, Hydraflow Hydrographs, and SMADA will all do these things. Even the free program SWMM can do this. To understand what you are doing read the hydrology and hydraulic documentation that comes with these programs and with TR-55. Mainly they will tell you what the limitations of these uncalibrated models are and what inherent errors may be expected. Read other stuff too !

For your tiny site, these errors are likely to be plus or minus 100%, in my experience.

good luck

 

[kenn100]

Thanks guys,
I was able to read-up on some of the documentation and decided to use Hydrocad to model the system and everything worked great.
The output from Hydrocad is alot easier to understand and makes sense.
However, I need to reduce the post construction peak discharge for the 10-year storm (38.84 cfs) to pre-construction discharge of 4.45 cfs.
How much acre-feet or sq. feet to I need to manage outfall?

Bottom of pond = 376.00 ft,
available storage = 3.44 acre-feet.

 

[psmart]

If you open the HydroCAD pond sizing report, it will give you an estimate of the storage required to meet your target discharge. For details see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

sounds like you need to retain almost the entire storm. elev8848 gave the formula for this previously -

Total Vol. from rain possible in 24 hrs = (2/12 ft*3.5 acres*43560) = 25410 cu. ft

 

[kenn100]

The hydrocad sizing report was very helpful.
Just to make sure that I understand the sizing table output.
Should I use same table to estimate storage required for all the storm events.
Here are the limiting outfall discharges:
Q-2yr storm = 3.2 cfs
Q-10yr storm =4.45 cfs
Q-100yr storm =15.04 cfs.

I have attached the hydrocad sizing table.

 

[psmart]

The overall pond size is generally based on the largest event. Then you will need to design outlet controls that achieve the desired discharge for each event.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kenn100]

The existing pond has been sized for the largest storm event. However, I am trying to find out how to control the desired discharges for each event.
As mentioned above, the limiting discharges are the desired discharges for each event.
what I am trying to do is find out what the storage volume needs to be for each desired discharges.
Can I use the pond sizing table for this?

 

[psmart]

Yes, you could use the sizing report to estimate the storage for each event, but what you really need to do is design the outlet controls for each event. Remember that the sizing report is only intened to provide an initial estimate. Once you setup a trial pond your design will be based on the actual hydrograph routing.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kenn100]

I am going to put in a request for my company to buy Hydrocad.
I was wondering if you have any suggestions.

tx.

 

[psmart]

What kind of "suggestions" are you looking for?


Peter Smart
HydroCAD Software

http://www.hydrocad.net