The emergency overflow on my pond has created a serious erosion problem called a headcut, which is a small creeping waterfall.
If not fixed, this erosion will evenually reach the pond and cause a much bigger problem. USDA is more into putting a larger pipe into the dam, and are not available right now anyway. Has anyone had any experience with this type of erosion ? I have some ideas like just dumping 12 to 15 tons of oversize white rock over the problem area or creating a concrete sturcture to direct the water flow over a spillway..

Never heard of it.

A concrete spillway would fix it. Make a deeper pour, like a footing along the pond side, and the bottom side, to prevent undermining. Big rocks get dirt in between eventually, and weeds or willows grow in, and impossible to mow. Concrete is neater, and may not be any more expensive.

Finny,

Sorry to hear about your nasty erosion problem. Yes the propagation of headcuts is kind of amazing to watch as the damage just keeps migrating.

I assume water is currently running into your pond at a rate greater than your normal pond outlet can discharge the water?

To give a sense of scale, what is the size of your normal pond outlet? What percentage of your current flow is going through your emergency spillway? What is the surface acreage and average depth of your pond? How deep is the fall of the headcut? (The height from the flat spot in the crest of your emergency spillway to the flat area below where the waterfall turns back into just a stream.)

There are lots of possible solutions. Let's get some more info and figure out a good short-term band-aid fix and then a longer term economical solution.

How big is your gully? I think 12-15 tons is ~ one and a half 12 yard dump truck loads.

When I was looking the closest source for oversized rock was in Oklahoma which isn't that far for you.



After much trial and error, this is what I ended up doing. It replaces a 8'-10'-deep gully.

https://i.ibb.co/hKpMXFR/IMG-2346.jpg

Water runs through over size rock. Concrete it.

The gully is about 16' across, but the headcut erosion that i need to fix is only about 5 ft right now., but will increase over time.
The headcut fix must allow the water flow a safe place to go without self destructing. Pasture drain is also a water contributor to the problem. The pond has a 12" siphon pipe , but NRCS office thinks that a larger gravity fall type pipe would be better. But, regardless of the dam pipe, its essential that the erosion problem be addressed now. I've added some pics below of the problem :


headcut_pics by lee harvey, on Flickr

Are those your pics, or stock photos of an eroding headcut?

Is your 12" siphon discharging at full bore? It should be releasing an incredible amount of water right now. If it is not, then you have some clogging in your siphon system. DO NOT GET CLOSE TO THE INLET NOW, THE SUCTION ON A 12" IS STRONGER THAN A HUMAN.

Yes, I agree you need to fix your emergency spillway right now. The force eroding the headcut is due to gravity moving the water. You need to get the water from the high elevation at the crest of your spillway down to the low elevation at the bottom of the headcut.

How much water flow do you estimate is going through the emergency spillway now? If it could pass through a 6" pipe, one option would be to buy some 6" flexible corrugated drain tile pipe and build a sandbag wall for the inlet side on narrowest stretch of flow along your emergency spillway. Pound some rebar into the ground in a v-shape as a debris guard. Run the outlet of the pipe down the headcut into a stilling pool.

If needed for a sufficient rate, you could probably add a second run of 6" pipe through your sandbag wall.

If your flow rate is much higher than that, then you probably need to move up to double-wall plastic culvert. Unsupported ells in culvert are difficult to maintain against high flows. You may have to install your culvert angled down from the top of the headcut down to the bottom. You will have to build a bigger headwall in that configuration (of sandbags or dirt) to direct your waterflows down the culvert.

If your current flows are bigger than that, then dumping a load of rock may be your best short-term fix. However, as Dave points out, that will slow the erosion rate of your headcut, not stop it entirely.

After you get past the emergency fix, you can come back with more pics and dimensions for a cost-effective repair when your ground is dry.

(Even if my suggestions don't work when you survey your specific situation, hopefully they help you create a workable solution.)

Thanks for your input. The pics are actual photos that I took of my existing erosion problem. The drawings are two of several that I made using an old version of Shetchup,
primarily, to get NRCS's opinion of the possible solutions.
The pond is 19 years old , and overflow water always dispersed and spread out over a large brush and high grass area to a drainage ravine that runs to bottom of the property
and into a creek that borders the south boundary of the property.. The current problem showed up a year and half ago. The first pic shows the initial attempt to fix the problem.
Small rain events were never a problem , but 5" over soaked soil was too much.
The current 12" siphon pipe was set 20" too low when the dam was built, and if left on permanently would lower the pond to a level that would leave my floating dock setting in mud.
I try to open the pipe on large rain events , but sometimes it's a hit and miss situation. My current thinking is to install two 36" hdpe culvert pipes , requiring six 20 ft sections..
That would allow ~14 sq ft to void water flow. Incidently, the concrete structure in photo would allow 15 sq ft with a 5 ft spillway and 3' high walls. Regardless, permanent fixes
will probably have to wait for summer when the ground is bone dry.

If you are a little handy with concrete, then an "elevator shaft" drop structure made with cinder blocks is cheaper than long sections of large diameter culvert.

Dig out your headcut erosion and pour a foundation below grade. Then build a cinder block shaft to above grade of the top surface of your emergency spillway. The shaft outlet flow could either be two large culverts that you cement in, or just staggered gaps in your cinder block wall on the lowest 2-3 levels to let water pass.

T-bar's ramp outlet is much more attractive, but does require a lot more work with concrete forms and more cost in concrete.

Your oversize rock fix would also be fairly cheap if you are big/young enough to man-handle the oversize rock. I think that would stop your headcut erosion, IF you put very heavy geofabric under your stones, build some wings to the structure, and hand place the large stones for a stable base and hand fill in with smaller stones to lock everything in place. (Like the people that build stone fences that last for hundreds of years.)

Good luck making it through to the "dry" times!

I have a similar project to do for a customer, its about a 4 acre pond, that has no overflow pipe, they have been letting it overflow off to the side of the dam, goes across to the next valley, not around behind the dam, according to the size of trees on this dam the pond has to be thirty plus yrs old.
Its still about 40 feet from the actual high water mark of the pond then is dropping straight down in a 12 ft deep by probably 15 wide gully, obviously undercutting back toward the pond a few ft every yr, I plan on regrading the gully into a slope down to the lower elevation then lining that slope with a riprap channel, using a fabric underlayment under the riprap, to let the overflow water down to the lower elevation without erosion, just about exactly like Tbar's picture with concrete but an a much bigger scale and using riprap. just waiting till it is dry enough to get trucks back semi close to it to deliver the rock, about a 1/4 of a mile thru a brushy field.
Am not as big a fan of the concrete version of that because I have seen, and had to fix way too many that didnt work, unless it is done perfectly, like Tbar's appears to be, you can get a small pencil thin stream to trickle under it and that pencil thin stream will undercut the whole slab in very little time, and concrete will do what it does and bridge that undercut so that you dont even know its doing it until a lot of damage has been done, rip rap will keep settling to the deepest hole and will not allow under-cutting. its definitely not as aesthetically pleasing as concrete but this is in the back of his hunting farm so he doesnt mow or brush hog anything anyways. Good Luck, just my 2 cents

Years ago I read an article about how slow water had to flow over a grassy area to prevent erosion and cutting away of the soil. I cannot remember where I saw it, and I don't have the time now to do an extensive search for that information. That information IS out there, somewhere. I dont' know if it is in here or not.
https://damsafety.org/dam-owners/open-channel-spillways

i.e. if a 3' wide 6" deep flow of water was eroding the emergency spillway, making it 18' wide and only 1" deep would slow down the speed of the water flow and stop the erosion.

In any case, I would put the thickest geotextile fabric down on the ground before you put any rocks down, and make sure that the geotextile fabric is under the outflow of any drain pipe or culvert so the water won't go under it.

Off the top of my head I don't know what a 12" siphon pipe will equate to flow wise to an open discharge culvert. I know that a siphon will flow more water than a culvert.

Could you put a gate valve in the 12" siphon pipe to allow you to open/close it when required? Just throwing ideas out there. Let me get in touch with Rex and see if he can weigh in on this. He's done a lot of work with siphon systems, more than I have.

Thanks for all the input - I'm open to anything that will work. I'd like to see a pic of the cinder block drop stucture. I'm aware of water
running under a concrete slab - I've experienced , it first hand, the water ran under the slab and than washed it down into the large gulley. But, the spillway slab will have an 8" x 4" footing on the flow entry back side. Thie recessed footing will be 11" below grade .
I'm limited in the size and amount of rock that is available. Most of the rock that I get comes from Sawyer, Ok, and I'm not certain that they have anything larger than the oversized limestone rocks. Also, only a Bobtail truck(12 yds) could access the erosion area.

I've included a couple of pics from 18 months ago of the original fix to the first headcut erosion problem - worked fine for nearly 18 months before washing out. The embankment had an 18" angled culvert pipe, that I told the contractor was not near large enough,
but what do I know ( 4 of those pipes wouldn't have been enough). Incidently, the concrete structure shown earlier, is equivalent to 8.6 18" culvert pipes

Oldfix by lee harvey, on Flickr

You might consider talking to an engineer and do some research as to just how many acres you are draining and what most of the cover and grades are on that acreage, that will tell you a lot on the proper sizing of the overflow, pipe or spillway.
Also a concrete or similar drop inlet box with the drain pipe coming out of the bottom of it will drain much faster then just a standard pipe sticking into the pond.

NRCS engineer is supposedly coming out to assess the situation. But, that was 3 weeks ago , so I can only guess if he will ever show up. However, my immediate concern is to fix the headcut erosion, otherwtse there won't be a pond . The dam pipe fix would most likely will need to wait until dry weather comes back, maybe even summer.

With them, the squeaky wheel gets the grease. Be nice, but call them back and try to set up an appointment date and time. If you don't hear back from them that day, call again tomorrow. You are paying for their services through your tax dollars, so you might as well get what you are paying for.

Agree about tax dollars but I wouldn’t remind them of that. The ones I’ve met have been pretty good guys.