Pond Boss Forums GENERAL QUESTIONS Muddy water Rule of thumb for an Emergency Spillway?

As I mentioned in an earlier post, we (like a lot of others) received a TON of rain in the last 48 hours or so.

My big pond's culvert (7+ acre BOW, drawing from several hundred acres for runoff) has in the past appeared to me to be undersized, although it has what NRCS said it was supposed to (18" culvert). I guess some of this could be due to the 3 '100 year' rain events we've had in the last 5 years or so. I've seen the 18" culvert a full 12-18" under water all 3 of those times.

This last time (yesterday morning) the water was within 8-10 inches of spilling over the top of the levee. Now luckily that was at peak and it's down 2+ feed now. The top of the culvert is exposed 3-4 inches as of when I checked it this morning.

Here's my question though. With water within a foot of running over the top of the levee, shouldn't my emergency spillway have come into play? I'm going to get the transit out tomorrow morning and shoot some elevations, but it appears to me that the water will be almost breaching the levee before it's running over the emergency spillway.

My thought is that shortly after the culvert is completely submerged (or maybe EXACTLY then) the water should be running over the emergency spillway. But that's purely my gut feeling - mostly driven from my immense fear of a breach.

Is there an actual rule and/or policy to follow as to the height of the emergency spillway vs levee height?

What I'm planning to do as of right now, and barring any further information gleaned, is when it dries out I'm going to lower the emergency spillway and line it with large riprap. It's probably a 20 yard 'flat' lentgh before it starts wrapping around the back of the levee. I want it to essentially be 15-20 feet wide so that when it does get used there's a good volume of water with not much flow rate...to cut down on any erosion issues as much as possible.

I have no rule of thumb but your plan sounds valid to me. For comparison, our NRCS designed pond is 3 acres, has 8" overflow pipe, watershed no more than 20 acres, emergency overflow one foot above full pool, three feet freeboard above full pool. So our emergency overflow starts flowing after the overflow pipe is covered by four inches and still have two feet of freeboard on the dam at that point.

I would think with your watershed you would surely want no less protection than I have and maybe even more.

Freeboard between the overflow pipe and emergency spillway elevations is very important (in my mind). A lot of different opinions and ways to design these things, but in my case I have 2' of freeboard after my pipe is submerged before the spillway will flow. And several times now, we've had a big enough storm event that the 12" siphon was under water, but the storage capacity between it and the spillway was enough that the spillway never got used. Time will tell, but hopefully the spillway will rarely be necessary and I won't have to worry much about any consequences, such as erosion.

I probably have 3.5 feet of freeboard after the culvert is submerged completely before the levee would have water over the top. And 3.5 feet vertically would be a LOT of water when it spreads out over the topography....

But I'd rather have the spillway in play with say 1.5-2 feet left. Just for the added peace of mind. I think so anyway.

Unfortunately, the rule of thumb is "it depends".

If it was my pond, I'd like to see more freeboard between the emergency spillway and the top of the dam too.

It's a lot of reading and calculating, but start on page 13 and go thru page 50 or so....

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_030362.pdf

Or if you have the calculations that the NRCS did for the dam when it was designed, go back to that and measure. Make sure the dam didn't settle more than was engineered and that the freeboard numbers are what were spec'd out.

It's probably easier to drop the emergency spillway than to add material to the top of the dam, but make sure you make the spillway wide enough to keep the velocity of the water with design specs.

Esshup, I've got the original builder coming back out next week to figure out the issue. There's 2 questions he's looking at - did we get more compaction on top of the levee than expected, and is the emergency spillway up to snuff. Hopefully I don't have to add a bunch of dirt to the spillway...