Pond Boss Forums POND CONSTRUCTION Building a dam Spillway Question

I am trying to determine how big my primary spillway needs to
be. I want to use a verticle steel pipe attached to a smaller diameter horizontal steel pipe running through the dam.

I have the estimated runoff for a 100 year flood calculated.
Is there a formula to use that will tell you the size pipe to use?

Hey pops...
Publication 590 should have the tables, charts, and calc's to answer your spillway question.

Maybe its in there, but I could'nt find it. I saw a lot on earthen spillways, which I plan to use as an emergency spillway.

Are you talking about the charts page 38?

pops...
I have the hard copy at home; easier to peruse than the on-line version. If somebody else hasn't already kicked it back, I will review and respond this evening from home.
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btw; 4 posts for pops....a slightly belated welcome from Bski!
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vote, please

Oooo boy; it's been almost 2 years since I went thru alot of this calculation stuff. I don't really miss it.
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I just did a cursory buzz thru Pub 590 and it appears that the steps are:

• establish an estimated "peak discharge rate", assigning a value (page 22 of the book Estimating Peak Discharge rates)
  
• review the introductory on "Pipes through the dam" on Pg 36; select the type of pipe spillway you plan to use
  
• Use tables 14 & 15 on pages 41 & 42, along with the accompanying text
  

Sorry, pops, but it's gonna be alot of focused head-scratching and calculations. I remember it took me a few hours just to analyze and calculate my run-off characteristics. There are alot of variables that need to be known. Knowing the amount of acreage is the easy part. You need the soil types and their hydrology, the anticipated 10 year and 100 year levels of precip, types of vegetation and how many ac of each, the list goes on.
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Is there any chance of getting your county NRCS office to assist?

Hi pops,

In my limited understanding, the so called "principle spillway" pipe is for low flow releases for the primary purpose of maintaining a relatively constant water level in the pond.

The "emergency spillway", usually an earthen structure, is what should be designed to safely carry the peak flow from a specific storm event. It's purpose is to allow safe passage of the flow produced by such a storm event without overtopping of the dam and without erosional damage in the spillway itself. NRCS normally uses a 25-year 24-hour storm event for emergency spillway design on farm ponds.

The volume of flow through a pipe is directly proportional to the head pressure, expressed in feet of head, and is determined using the height of the water above the pipe.

In general, it is not practical to use a pipe to carry design storm overflows unless the dam is being used for a flood control purpose, in which case there would be provision for excessive dry storage between the top of the pipe and the earthen spillway to achieve the required head pressure to make the pipe flow. This is what you see in Corps of Engineer dams, or in the smaller Soil Conservation Service dams, where the concrete inlet for the pipe (principle spillway) is much lower in elevation than the emergency spillway. Of course, the top of the dam still has to be several feet above the emergency spillway to prevent overtopping in case the pipe fails to perform for any unforeseeable reason.

So, are you sure you want to use a pipe to carry the flow produced by the 100-year flood? Or, is the pipe simply going to carry what comes into the pond under low flow conditions, perhaps to prevent trickle flow through the emergency spillway?

Here is what I eneded up doing. I have a 72" vertical pipe about 28 feet to the waterline and attached to it I have a 36" horizontal pipe going through the dam. Based on my calculations if the vertical pipe is full I will be moving +/- 2000gal/sec. I also have an emergency spillway 2 feet below the top of the dam 80 feet wide. I know the primary spillway is probably over designed but I got a good deal on the pipe

Pops what region of texas are you in and how big is the water shed?

East TX around Athens and the watershed is 450 ac

BOOOOIIIINNNNG!!!
Jeesh...I gotta ask. How big is this pond?

about 18 acres

Pops I have a pond and dam similar to yours. Last year we had a 50 year flood that came over both of my emergency spillways 100 feet on both sides of the dam. The water was over two feet deep and had enough force to wash away a gravel road below the dam. The twenty inch steel pipe that goes through the dam helped with the water but not enough. Next week I am going to widen the emergency spillway another fifty feet on the side away from the gravel road because rebuilding the road was not cheap. The reason I am telling you all of this is so you will not make your emergency spillway too small. Here in Texas we have droughts and then we have floods and you have to be ready for both.