Any engineers in da house?
I have a friend who wants to build a pond on his property and needs some design help.
He will be constructing an eathern dam and would like to build a box culvert riser and drain to handle the average flow as well as the flow up to 100 cubic feet per second. Anything more (like a 25 year rain) would be handled by a concrete spillway on the top of the dam.
He is proposing a horizontal concrete box culvert that would be 4’x4′ and would run at the pitch of the original drainage (approx. 10% slope).
The question: Would this culvert handle 100 CFS, and what size of a vertical riser would it take to load the horizontal portion? Would a 4’x8′ vertical be enough?
Where would one go to research such a project? This one has me going, trying to figure out fluid dynamics, and I dropped out of engineering school.
How large a culvert does it take to handle 100 cfs of water? Does the velocity of the flow change the answer?
Any dam builders out there??
Replies
I don't have the slightest idea....but I know the crew at my local ASCS office US dept. of AG/Department of Conservation can do it for me.
Should have an office in your county! Look under United States Government in the phone white pages.
.....................Iron Helix
My friend already went to his local county for info. A kid that kinda knew how to run the computer program gave him some estimates for flow rates that seem ludicrous, like a 1000 cfs 25 year runoff.
That amount of runoff (draining approx 640 surface acres) would wash out every bridge and culvert along this watercourse.
Observing the high water mark we estimate, at most, 100 cfs.
Thanks for the suggestion.
What the HECK was I thinking?
1 inch/hour rain on 640 acres is 641 cubic feet per second. 1"on 640 acres is 2.3M cubic inches
Your 4'x4' culvert is 16 square feet in section. If the flow thru it is 6 linear feet per second, that will be 96 cubic feet per second flow.
If you got water to the top of that 4' high opening, I garundamtee you will be flowing a heck of a lot more than 100 cubic feet per second.
If you want to know everything about water measurement
http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/index.htm
If you just want math
http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/chap07_13.html
One more link
http://www.usbr.gov/pmts/hydraulics_lab/pubs/
Google for weir gate flow rates
I'm used to seeing water flows in the 10acre feet range. This would be in a concrete ditch 1' wide on the bottom, 4' wide at the top and 3' deep. 2-3%slope
SamT
Thank you,. SamT !You have given me a lot to look at. I appreciate the help.I will learn a lot about water flow (hydraulics) and I will probably never need it again, but it keeps the ol' synapses snappin'........
Thanks again,
heckWhat the HECK was I thinking?
Iron Helix is right about the ASCS office being a good source of info.
I gotta tell ya, if it's draining 640 acres, 100CFS is extraordinarilly low. The 1,000 CFS number sounds more reasonable to me. I can't tell you how many times I've seen culverts and dams wash out because the water flow was underestimated.
Has your friend checked into possible government programs? If you can write a grant request that shows this will reduce erosion, or something like that, he might get some financial help on it. The ASCS office would be a good place to ask about it.
There was a thread about a fire pond a while back that had some good info in it:
http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=39532.1
We had gay burglars the other night. They broke in and rearranged the furniture. [Robin Williams]
Boss, my buddy didn't leave the paperwork with me, but the 1000cfs flow was calculated for a 25 year rain of 7.25" in 24 hours, and took into account the slope of the terrain and the porousity of the soil, the vegetation, etc.
The flow rate from this amount was described to be 3.3" in 24 hrs. That's less than .15 inches per hour.
I just can't see how there can be 1000 cfs produced using those figures, but then I don't understand fluid mechanics...that's why I'm askin'.Thanks for the input, the idea about grant assistance is good, also thanks for the link.
heckWhat the HECK was I thinking?
I was re-reading my first post and I don't think the slope I gave is correct...I stated 10% and I think it is less slope than that.. I will try to get a more accurate figure.What the HECK was I thinking?
The HECK of it is, is that it is not slope you need to think about, but rather, HEAD.
The head of water is measured by the difference in elevation of LEVEL water (10 feet or so before the overflow) and the elevation of the overflow (dam). As water flows over the dam, there will be a slope in the water from the dam back into the pond.
That slope is what gives you HEAD.
HEAD is what energizes water to move.
Remember those irrigation ditches I was talking about? In some so-called Super Flat fields, the irrigation system was so level it followed the curvature of the earth. Zero, zip, nada slope. HEAD made everything work jest find.
SamT
You lost me there - You said "the 1000cfs flow was calculated for a 25 year rain of 7.25" in 24 hours" - Then you said "The flow rate from this amount was described to be 3.3" in 24 hrs."
.
I'm not trying to be critical of your reasoning. But what I said about seeing a lot of things get washed out was true. Even one of my Dad's erosion control dams that was engineered by the ASCS isn't high enough.
So I tend to be a little over-cautious about stuff like this. Especially as big as this pond must be. (You didn't mention the approximate size of the pond, unless I missed it)
Remember that water is relentless - It can do amazing things given enough time.I've never seen a monument erected to a pessimist [Paul Harvey]
Boss, the flow rate is the amount of run-off created by the 7.25" and accounting for absorption ,slope, etc.These are the figures generated not by the engineer, but by the office clerk punching numbers into a program. My friend will be trying to speak directly to the engineer to clarify the details.
The size of the pond will be about 2.5 surface acres (if memory serves), I forget how many acre feet of storage.
I agree with you that water can do some horrible things, so we would want to be cautious in our planning.
He was also incorporating a concrete spillway to handle what was beyond the culvert capacity.
Obsevation of the high water mark would suggest a 30' width and a 3' height, so 90 cubic feet....but at what flow rate? This answer would give us the CFS for the high water mark, would it not?What the HECK was I thinking?
O.K. - You didn't mention the 30' spillway before. (Unless I missed it) That would make all the difference in the world.
What's the purpose of the pond? Recreation, flood control, irrigation?
If it's for recreation, it might be a good idea to consider some sediment control basins in the main runoff channels coming into the pond. That would help keep the water clearer, free of chemicals, and reduce sedimentation. Depends on the soil type(s) and land useage above the pond.
The ASCS would also have some info on these. Around here' they'll pay up to 60% of the cost of building them.Sometimes you've got to specifically go out of your way to get into trouble. It's called fun. [Robin Williams]
The pond will be primarily for recreation.Good input, Ron, thanks...I gotta go, but later I would like to explore ideas on the standpipe/culvert design.
What the HECK was I thinking?
Locally the "pond" builders avoid the vertical/horizontal box culvert configuration.
The wisdom that is alluded to is that vertical shafted overflows produce turbulent flows that are not efficient at allowing for large volumes to flow smoothly to an exit.
The norm now is a slanted pipe/culvert at the slope of the backside of the dam that will allow for "laminar flow" of excess water.
That's what I am told.......seems reasonable....I can't rerfute the logic with the knowledge I possess at the moment.
Check it out.......local ASCS for me. If your local has a bunch of neophytes...look in the next county for some "old-timers".
..................Iron Helix