Hello, I've been viewing this forum for awhile because of interest in starting our pond project, but this is my first post.
I have a question about sizing a siphon system with an earthen emergency spillway. I think my total freeboard will be 2.5'. I'm thinking I need a 12" pvc for the siphon system, but i'm having difficulty where to set the emergency spillway in relation to the siphon drain. If my total freeboard is 2.5', should I set the emergency spillway at 1.5' above the pond level? That would give me 1.5' of storage until the emergency spillway kicks in.
More specs:
65 acre drainage area, wooded.
3.5 acre surface area for the pond.
1 yr storm runoff is 2.9 cfs
10yr is 37.5 cfs
25yr is 55.7 cfs
100yr is 77 cfs
Comments or suggestions are appreciated.
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Bronco,
I'm not sure about siphon systems, as we have either used straight pipes with hooded inlets, or drop structures going into straight pipes for all our dams. However our pipe inlets are obviously set right at the water level. Then the emergency spillway is 1-2 ft above the top of the pipe inlet (most of our pipes are 18+ inches in diameter). Then we have 2.5-3 ft of elevation between the emergency spillway to the top of the dam. Thus the top of the dam is always at least 5 ft above the normal water level.
These are designed to handle at least 25 year storms completely through the principal spillway (the trickle tubes). In our area that's a 3-4 inch rain in a 24 h period. Last year we had 2 events of 7+ inches overnight, and the water came over the emergency spillway about 1 ft deep, but was not close to over-topping the dam.
I'd always recommend over-building a dam in this way. It is a few more bucks, but well worth the peace of mind.
In your part of the country with more significant rainfall (including hurricane storms) I would think you would want much more than 2.5 ft of freeboard.
Bill
Your local NRCS office should be able to help you with your concerns.
I have a 12" siphon pipe on a pond maybe a little larger than yours with a 150 acre watershed, and have had no problems. My ES area is 1.5 ft above the the pipe on one dam end and 1.0 ft higher on the other end. The freeboard is about 4 ft, however, I close the pipe at the end of spring and allow the elevation to rise about a foot higher for the summer drought months.
Thanks for all of the good advice. Sounds like 4' above the permanent pool is a standard for a pond this size.
I will post some some of the overall design as i get further along.
Siphon pipe may be a bit different but a primary spillway pipe with a canopy inlet needs to be set a minimum of 1 1/2 times the dia of the pipe below the emergency spillway.
The reason for this is it takes that much water over the end of the pipe for the pipe to start sucking without drawing in air. If there is some good fall on the pipe, as there is in most ponds, the pipe will move much more water at this point.
Personally I would go 2' from pipe to spillway. If your around the pond to fix any low spots in the dam should they develop 2' freeboard should be ok. If it won't be practical to ever do maintenance than I would recommend 3' freeboard.