Pond Boss Forums INTRO Questions & Observations Is It Safe For Me to Raise my Water Level 1"?

You likely will not even reduce the flow where your pipe looks to be almost horizontal. A horizontal pipe unless it has head pressure from being completely covered a ways by water will not flow full. There will always be an air gap at the top at the outlet. So cutting down the inlet at the bottom edge by a small amount likely would not even affect flow. If the pipe were slanted down and expected to go into siphon mode at some point, then the restriction at the bottom of the inlet might come into play and restrict flow a small amount. But as long as there is an air gap at the top of the pipe at the outlet, I can't see where 1" at the bottom at the inlet would ever make a difference absent significant head pressure.

I raised my pond level above design by 3".

The important factors are:

that the dam is structurally sound enough to hold the extra pressure, which is should be or you are in trouble should the water ever get to emergency overflow height.

That there is sufficient height of the dam that there is no possibility that water will overflow the top of the dam (which has to do with the size of the watershed in relation to the surface area of BOW or in other words the surge capacity of the pond)

That the emergency overflow can handle any excess water flow that the overflow pipe can not (related to watershed size and width of emergency overflow).

As long as you have plenty of capacity to get rid of excess water and have a rather small watershed in relation to pond size and do not have huge rain events I would not worry about raising the pond an inch or two or even a little more.

My emergency overflow was designed to be a foot above full pool with an 8" overflow pipe. In other words when the water got somewhat above 8" full pool the overflow pipe would enter siphon mode and increase its flow rate (it is slanted downward so can get significant increase flow from siphon - horizontal pipes don't have gravity to put them into siphon). Then if the overflow pipe could not handle the flow when water got 1' above full pool or in other words 3" above my pipe the emergency overflow would come into play. Now the way it stands, once I'm above the 8" overflow the emergency spillway will come into play so likely my overflow pipe will never again enter siphon mode. I've had one rain event that brought water level just right at the edge of the emergency spillway. That was 12" of rain in less than 24 hours. My pond also has a very small watershed in relation to the size of the pond. Which is a good thing in normal rainfall years and not so good when droughts come along.

One way I have thought of raising a pond level and making it variable (but don't know how well this would actually work because have never tried it) is to put a 90 degree elbow on the overflow pipe so it could be rotated to the desired water level inlet height. This might change the water flow characteristics of the overflow pipe and give a real boni-fied engineer a heart attack though so I'm not recommending it. Would also probably need to be a smooth PCV type pipe. Not sure if you can get elbows for corrugated pipe.

Right now my old pond has a rip-rap earthen spillway. If I get energetic this summer and build another pond right next to it, I plan on taking some of the extra clay excavation and after cleaning out the rip-rap and getting a solid base plugging this spillway up and putting in a pipe overflow with an emergency spillway further down at the end of the dam. If I do this, am going to raise the water level another foot. I have plenty of dam height for the watershed this pond encompasses.