Pond Boss Forums POND CONSTRUCTION Selecting a site Measuring potential inflow

I'm planning to build a one to three acre pond by damming a small arroyo. The creek bed is dry except after heavy rains. The primary channel is about ten feet wide with cut banks about three feet high. I've seen it run full and assume it occasionally overflows the banks but could not find any evidence of such. I'd estimate the flow speed at about five to seven mph when full. After heavy rains, it runs full for a couple of hours then tappers off to nothing within 24 hours after the rain stops.

I'm not sure how to convert the above estimates into potential water volume going into my planned pond. I'm also not sure how to determine the proper size for spillways. I'd prefer to install a culvert pipe(s) for the primary spillway and add an emergency spillway to overflow around the side of the dam.

Determining the expected inflow will help me decide how large and deep to make the pond. No sense digging a big hole that has no realistic potential of filling with water.

I guess I should also mention that the top of the dam will double as a road bed for crossing the arroyo. Hence the desire to use culvert pipes as the primary spillway.

I plan to seal the pond using locally mined bentonite clay.

Any answers, comments, suggestions, or questions would be appreciated.

I won't pretend to be smart enough to give you an accurate estimate of the volume of water (although a 10' x 3' by 1' wide section is 30 square feet, which is 224.4 gallons of water) But I'll take a stab at it...
With your description 1,000 gallons per second isn't out of the question. An hour long flow at this rate would bring in 3,600,000 gallons of water. That kind of flow coming in would present an incredible amount of energy and would require a dam and overflow engineered to handle that kind of power every time it rains and to hold water after many sequences of being submerged then being dry as the water recedes between rain events. I imagine the build for a dam/road that could handle this is going to feel like overkill. But anything less wouldn't last long under these conditions. Definitely a challenge.

I get 7.1 million gallons/hour at a 6 MPH speed.

Daryl:

If you haven't seen it already, look at both links in this archived thread:

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92487#Post92487

I think in #590 there is a formula to calculate the flow of water over your emergency spillway, so you don't exceed "X" flow speed, which creates washouts. i.e. it will tell you how to figure out your emergency spillway size.

Care to elaborate?

Removed

Darly,
You really need more information and accurate information to make determination on spillways. Based on what you provided I get, but check the math:

@5mph- 219.999 cfs
@7mph- 308.001 cfs

which is a CRAP ton of water. No pipe, unless it is 20' (slightly exaggerated or maybe not) in diameter is going to handle that much water. Other factors that need to be considered, area of watershed, rainfall numbers of 25, 50, and 100 year storms, type of spill way (hooded, drop inlet, siphon, etc.), length of pipe, and free-board between pipe and emergency spillway.

Note: The more freeboard you have, the more temporary water storage capacity you have which in turn lowers your required outlet pipe flow requirement. Many factors to consider.

My advice: I am not an expert but that is a LOT of water, so to prevent future complications I would contact your county extension office or someone who has experience with these calculations and your local conditions.

Thanks for the replies. I'm starting to think my estimations were too high. I wanted go with high estimates to be safe. Maybe I went overboard. Let me back up and clarify a bit.

Channel width: Originally estimated as ten feet. That's the maximum width. The average width is closer to six or seven feet.

Channel depth: Originally estimated as three feet. Again, that's the maximum. Average depth is probably a little under two feet.

Flow speed: Originally estimated as 5-7mph based on walking beside the flow and being outpaced by the water. That was in one of the steeper slope areas with narrower channel. The flow is certainly slower in the wider and deeper sections. Also the terrain is rough, so I was likely over estimating the speed of my walk and thus the flow speed. Perhaps 3mph would be more realistic.

Flow duration: Originally estimated as two hours at full flow then tapering off to nothing within 24 hours. High flow is typically closer to one hour and dropping about 20% an hour. Down to a few inches in width and depth within five hours and nothing within 24 hours.

So, perhaps I should revise the parameters to the following:
Width: 6.5'
Depth: 1.75'
Flow speed: 3mph
Flow duration: 100% 1st hour, 80% 2nd hour, 60% 3rd hour, 40% 4th hour, 20% 5th hour, 5% or less for 15 hours.

I'll try to calculate some estimates based on the above and report back later.

Width of channel 6.5 feet
Depth of channel 1.75 feet
Speed of flow 3 mph
/ 11.375 cubic feet of water per linear foot of flow at 6.5 x 1.75
/ 5280 feet per mile
/ 15,840 linear feet of flow per hour
/ 180,180 cubic feet of inflow per hour

Duration of flow per heavy rain event
1st hour @ 100% = 180,180 cubic feet
2nd hour @ 80% = 144,144 cubic feet
3rd hour @ 60% = 108,108 cubic feet
4th hour @ 40% = 72,072 cubic feet
5th hour @ 20% = 36,036 cubic feet
next 15 hours @ 5% = 135,135 cubic feet

Totals
Total Cubic Feet per heavy rain 675,675 / / Cubic feet per acre foot 43,559.9
Total Acre Feet per heavy rain 15.51 / / Gallons per acre foot 325,851.0
Total Gallons per heavy rain 5,054,404.96 / / Gallons per cubic foot 7.48052
Maximum Flow Rate 1,347,840.09 GPH

I did a tr-55 analysis for my pond. It was a bit tedious, but it gave me some real confidence in the design. I used a free computer program, wintr-55, made by the nrcs to do the nitty gritty calcs. You just have to measure your watershed using county mapping with soil types. Then input some other parameters, and wala, it spits out flows for 10, 25, 100 year storms. You can even input spillway parameters for testing.

It told me that during a 25 year storm my water level would rise 1.5 feet to right at the emergency spillway. For a 100 year storm it would flow through emergency. This was enough info for the county lady who gave me the driveway permit to cross the dam.

There are older nrcs techniques using watershed, soil types, slope, etc, that are simpler than the full tr-55.

Based on what I've seen, you can expect a 25 year storm in the next 3 years, a 100 year storm in 5 to 10 years, and a 500 year storm in 10 to 15 years.