I am in the need of opinions. My excavated ½ acre pond has a 68 acre drainage basin. On average once or twice a year we get either a tremendous thunderstorm during the summer or heavy rain prior to the ground thawing in the spring (happened on Sunday Night). During these rain events the 18” culvert shown can handle the flow albeit a little slow.

After the pond was dug in Fall of 2002 (stocked in spring 2003), we had a large rainfall in Late Summer 2003. I found that quite a few of the fish stocked liked to take a ride downstream. They were mainly the perch and bluegills. Some of them I was able to retrieve but most went onto property where nobody with the exception of god (very disagreeable neighbor, hates everybody) is able to go. After about 8 hours the outlet doesn’t have sustainable flow and the channel dries up to the point it can’t sustain fish. To remedy this situation I made a screen for the culvert which can be easily taken out. The problem is that it is also very hard to keep clean. Early in the spring and in the fall it will frequently plug up with floating debris. I am concerned that this may happen when I am not home and it is not a pretty sight when this culvert is plugged and we have a gully washer. It happened once and the water was 2 feet above the top of the 18” which is basically my emergency spillway elevation. My thought is to remove about 6 or 8 feet of the culvert and place rip rap and cobble as shown on the attached drawings so that I can remove the screen and not worry about it plugging up. I know I may still get the flood of all time in which I may get some escapees but I can’t control everything. Do you guys have any suggestions? Does anybody else have a problem like this that they have corrected? Do you this the base 4” cobble should be placed in a bed of mortar or just placed on top of geotextile?
Outlet Design

fozzy:

I use a plastic grid fence (the stuff Stoney Creek Equipment sells, looks kinda like black snow fnce, often used to make fish cages) set out about 18" into the pond from the inlet to my over flow. I have the fence tied to five steel "T" fence posts, two driven into the bank on either side of the drain, three into the pond bottom in front of it, with insulated copper electric wire remnants. The fence forms a semicircular screen in front of the drain.

Under low and medium overflow conditions, the water runs through the screen (without any larger fish) and into the drain. I have the highest, uphill edges of the fence set so that if the water level should rise that high, water will by-pass the fence on the back side. I do not think the water level has gotten this high except for one time right after installation. (The drain pipe is double-walled plastic, corrugated on the outside and smooth on the inside, with a thin collar on the uphill ends to slide the next piece of pipe into. I originally left the collar on the top piece of pipe, and it collapsed during a heavy flow restricting the drain. I cut the collar off this top piece and it solved the problem.) If the water gets high enough to by-pass the fence, I'm willing to lose some fish in order to help save the dam.

Although the fence is frequently clogged with FA and/or floating weeds at the waterline, the below surface portion usually seems to be clear of obstruction. The portion of the fence that is above the normal waterline is always free of clogs when the water rises and goes through it. I haven't ever cleaned it; as water rises it reaches a clean part to flow through.

I must say your design is a lot more visually appealing. I think you will have to place your cobble in cement to keep it from washing away in a heavy flow.

If you use a grate with only horizontal bars maybe 1 to 1.5" apart, it will not clog up as quickly as a horiz/vertical matrix grate. Smaller fish will still get through, which is the case for either type of grating.

I don't think I would constrict the overflow drain in any way. You might lose some small fish in an overflow situation but not any great amount. However, you just might lose them all in a really serious rise.

FWIW, here's a picture of my drain box and the fish straining fence:

Theo, That is the way I wish I could of done my outlet with a catch basin type outlet. I only have 1 foot of elevation drop across my 80 foot long culvert, so i didn't have enough depth to put a structure in.

In Washington I am required to have a 1/4 inch screen on my outlet to prevent larger bluegill and bass from entering the river system nearby. I've also been concerned with the fact that debris will quickly clog the screen. I've been toying with two ideas to combat this problem.

1. Use a multi screen approach. Like above, but start with a 2 inch mesh wire fence to catch bigger debris and then follow it up with my 1/4 inch mesh. before the water enters an 18 inch culvert.

2. Use a pvc Y to lower the intake a couple feet or so below the bottom level of the culvert. This will prevent most of the clogging due to the fact that the debris is usually near the surface. Then I will build a sort of cage around the intake to maximize surface area for intake if some clogging does occur.

Any comments or suggestions?

Just got another idea. What about using a brushpile as a sort of debris blocker in front of the outflow pipe?