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kenn100 (Civil/Environme)
1 Aug 08 13:06
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 (Civil/Environme)
1 Aug 08 13:49
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. 字串8
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. 字串4
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 (Civil/Environme)
1 Aug 08 14:50
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 (Civil/Environme)
1 Aug 08 17:01
字串7
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 (Civil/Environme)
1 Aug 08 21:07
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 (Civil/Environme)
2 Aug 08 3:16
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. 字串8
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. 字串2
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.
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