Pond Boss Forums POND CONSTRUCTION Building a dam Trench core / high water table

Gents, I'm having about 30 loads of red clay to be hauled in for a core that will be cut through a 3ft wide/6in deep creek. As my excavator starts cutting the trench, it fills with water as the water table is almost as high as the creek! The operator states that he just starts dumping and compacting the clay immediately, which displaces the water as he goes?

My thoughts are that you need a pretty dry area for your dam AND core, as that's the whole purpose of the core! He's a trusted guy, but I'm wondering if a water pump may be needed to keep water out of his workspace and my $10k worth of dirt?

If you intend to compact the clay as it is installed in lifts about 6” per layer, you will absolutely need to pump out the water. I don’t think you will get good results without compacting and in the presence of that much water.

Agree with liquidsquid, it is not possible to compact soupy clay.

If your creek bottom is sand and silt, then I doubt you can pump it dry very effectively.

Does your creek ever go dry in August, or in a drought year? I hate to say it, but you might do much better on the cost and quality of your dam if you wait for a dry creek.

I believe most dams constructed on running water create a bypass and then build half of the dam. One option would be to take the excavator and cut a trench in the stream bed near one bank to a level below the base of your core trench. You will also have to extend this trench downstream far enough that the spillpoint in the streambed is lower than the base of your core trench. (This will make sense once you start observing the water. Basically, the top of the creek bed 200' downstream will still act as a "dam" and keep your trench full of water until the top of the creek bed is lower than the bottom of your core trench for the entire length of your drainage trench.)

Build one half of your dam on the opposite side of the creek and install your outlet piping system. When that is completed, backfill a section of your drainage trench to protect the uncompleted side of your core trench and build a bypass from the outlet to the drainage trench. Build a berm to divert all of the creek flow through your outlet pipe and then finish the other half of your core trench.

Other considerations:

Have you observed this creek during heavy rain events? Typically a dam in a flowing creek is going to require an exceptional emergency spillway for the dam to survive flooding events. The spillway will have to be large enough so that no floodwaters run over the top of your earthen dam, AND the spillway needs to be constructed to survive very erosive, high velocity water flows.

Also, in many states the requirements to build a dam on a flowing creek are much more restrictive than just gathering surface water coming in from a small watershed. Make sure you are in compliance with all state and federal regulations.

Good luck on your new pond project!

Thanks for the replies! This is what I'm working with- the stream runs year round, and has never been affected by a drought as the stream is 4ft wide, and roughly 6in deep. We have a large earthen spillway that was utilized as the primary pond level, but now we're looking at using a tandem 10in siphon system. FishinRod, I'll continue to read your description, but can you see what I'm dealing with?

The dam will run at a 45 degree from the left bank, and run alongside where the bank on the right is cut at an angle. That creek is roughly 5ft wide for perspective. (That dirt/stick crossing is temporary, and not the dam site.)

I trust my grader, but I get one shot to understand the logistics!

Thanks for the picture. I was envisioning a project pinned between two narrow stream banks. You have plenty of "elbow" room to operate, and that should make things much easier.

It looks like there is a fair bit of clay in your existing soils? (Although the eye is easily deceived.)

What type of sub-soil will exist at the base of the core trench? Has your contractor dug a few deep test trenches to sample the materials? It would literally take him 5 minutes with an excavator of any size.

I would create a sump (deep pool) at the upper end of your designed pond and intercept the running water at that point. Get a trash pump (or two) and a few hundred feet of lay-flat hose, and pump the stream water all of the way around your entire pond area plus the dam and core trench. Wet clay in a construction project always slows things down and increases your costs. This includes not only the core trench and dam, but any work in the pond basin itself.

Another option would be to excavate a trench just one bucket wide that goes around your entire pond area and re-connects to your stream bed well past the dam location. (Your eyeballs on the actual site should be able to determine with your contractor which option is easier.)

The core trench (and any other clay seals) needs to be built in 6" lifts of clay-rich material, wetted to the optimal moisture content, and then compacted (preferably with some type of padfoot roller). You then move on to the next lift and repeat the process

Does your pond bottom have enough clay content to seal after you scarify (till) and re-compact?

How deep will your pond be at full pool? If about 10' deep, then a 6" lift of clay compacted throughout the pond basin, plus another 6" lift spread over the first layer and compacted should be capable of sealing your pond.

All of that wood debris in the picture must NOT be included in any of the clay sealing areas! That wood will decay, and then leave voids in your dirt fills which can later turn into leaks.

I hope that helps a little bit more with your vision of the process.

FishinRod, you've given me some great options on this- yes, the soil is a sandy/clay mix, but I am having 30 truckloads of pure Southeast clay brought in specifically for the core.

My main concern is not so much the stream (great ideas, btw), but when my grader scoops dirt from the bank, the water table seems to be as high as the stream almost. When he digs, it starts to fill with water. So diverting the stream is one task, but mitigating pools of water is the other.

If everywhere you dig, you come up with water-bearing subsoils, then you might need to switch your design (and thinking) to a groundwater pond.

It will be difficult to seal up any sandy horizons in your pond basin with a clay seal if you can never create a dry pond basin.

Can you dig a test trench at a couple of spots inside of the boundaries of your new pond design dimensions? Looking at your mixture of sand and clay, they should probably fill with water rather quickly.

Do you have the means to shoot a level between two water-filled pits? If they have the same level the next morning, then you are probably on a regional aquifer. If that is correct, and you have several sandy horizons, then I doubt you can pump your pond basin dry without serious pump horsepower.

However, that does potentially make your pond project much easier. I expect the overnight water elevation will be significantly lower than the water elevation you designed for after damming the stream? Is it a water level you can accept, based on aesthetics like your wife's view from the deck?

If your subsoils are well-connected to the regional aquifer, now you simply need to excavate a pond basin to create your pond. This will create a LOT of spoils! Do you have a place on the property to spread your spoils? It does not need to be flat over a large area. It is usually easier to build some attractive berms or other landscaping nearer to the pond to limit your costs in just transporting sub-soils.

I don't mean to throw a curveball into your plans. However, your local soil geology may have thrown that curveball, and you will need to build a pond that matches what the ground gives you. I hope my additional information above makes sense. Keep talking to the local contractor such that what he says to you makes sense, since he should have already dealt with similar situations if he has constructed many ponds in the area. Local knowledge is almost always more valuable than "internet" knowledge!

Very good advice and insight from FishinRod. Strongly consider it.

FishinRod, once again- thank you for your knowledge, replies, and time. I can't get this kind of detailed info around here without paying someone, so I made a small donation to the PondBoss site out of respect.

In my experience, you need to mix the new stuff with the old dirt and then compact it. Note that I said my experience. I fixed mine several times before I quit dumping clay and compacting.