Pond Boss Forums POND CONSTRUCTION Soil questions How steep can I make a clay bank?

I'm digging a pond in heavy clay soils. The pond was originally designed with a 3:1 slope in many areas, however I'm wondering if I can go steeper? What I'm more specifically trying to find out is if there is a slope where the clay soil will tend to just collapse and slide to the bottom of the pond? I've recently just read that bentonite for instance won't do well on a very steep slope and will slide to the bottom. Sorry I don't have very much more info on the clay that I have other than its mostly black and tan and it holds water very well. Probably the only good thing about my pond location.

The "Why" behind this is for multiple reasons. First is that my contractors screwed up on a few aspects of the pond and now it holds far less volume of water than originally planned. My pond only fills via rain water so I would like to try and rectify that. Second is that I'm looking at my shelfs and other features that I have for my spawning beds and just wondering if these vertical angles will eventually just collapse and spill all my pea gravel or if they will actually hold up for a good amount of time. Over time I would assume that things would degrade, however I'm hoping for the pond to have a good 25-30 year life span before it needs to be drained and given an overhaul.

I know I pop in and out of these forums sporadically. When I'm finished with this pond in hopefully less than 45-60 days I'll post up pics along with all of my costs that went into it.

IMO the steepest slope for good dense dark clay would be maybe 2:1 providing the builder uses a good sheepsfoot roller technique for compaction of the clay for the basin. Steeper than 2:1 makes it very difficult to get the optimum clay compaction which is necessary to get a good clay sealed basin to minimize leakage. Once the walls start sloughing you are loosing the compaction layer and leaks begin to continually increase over time. To be on the safe and conservative side, I would go for 2.5:1 for side walls with the beach spawning area at around a 3:1 slope. IMO you don't need a large spawn area when BG produce around 10000 - 30000 eggs/fry per nest. Good dense shallow habitat determines how fast the fry disappear. Don't let the LMB or predators overpopulate as they do for about every pond owner. Line 20-25% of the bank distance with good types of habitat.

Have contractor for the beach spawn area perimeter push up a dirt curb to outline it to hold the spawning substrate in place. The less shallow water the pond has this minimizes the amount of algae & submerged weed problems that will occur down the road. The more nutrients and organics that get into the pond the more plant growth that will develop as the pond ages. MINIMIZE NUTRIENT INPUTS FOR THE CLEANEST, BEST WATER QUALITY POND. Aerate the pond well for 1 turnover per day and you won't need to rebuild it for 35-40 years. If it were mine I would oversize the aerator and run it less each day.

I tried steeper than 2:1 and it was not a good idea for me but i suppose that depends on the type of clay, if yours is black and tan then I'd worry about the black being organic in nature as that is what it is here but might differ from what you are dealing with. The 2:1 slope even in a low wind area did erode and started to settle to the foot of the slope -making the slope even steeper...also if someone falls in its pretty darn hard to get out - not sure if you have ice there but it can make it dangerous. I found the 3:1 to be the best structurally, and the sunfish spawn on it in areas.

Have fun!

Clay is unlike any of the granular materials that are found in sub-soils. Clays have "plasticity" and can typically be molded into simple shapes.

You can actually cut a vertical wall in moist clay. If the clay has good cohesion, the wall will not slough into the cut until it starts to dehydrate - which causes the clay to shrink and crack.

However, that is not your only consideration. Pond slopes are almost always subject to wave erosion. This force is not very strong in a small pond, but it is relentless. A clay slope that gets slapped with enough tiny waves will eventually start to erode - and the likelihood of erosion increases with the steepness of the slope.

There is a picture in one of the Pond Boss books where they cut some vertical clay walls in the deeper sections of the pond. I thought that might be an interesting option in my clay-rich soils.

I believe I even posed the question on the forum about the long-term sustainability of a vertical clay wall at depth. It got some responses, but no one was definitive either way that they would work OR that they were sure to fail.

In my non-expert opinion, I would probably try to keep the shore slopes at about 3:1. However, if you are excavating in pure clay AND your pond will not be subject to significant water level changes, then you could try some steep cuts at depth.

If you have cobble size (or larger) rocks on your property, you could probably even cut a vertical wall and face it with stone. That might be an excellent fish-concentrating feature that you could create in a few places during the excavation phase.

Since it will not be visible from the land, you could even go ugly and dump concrete construction rubble against the wall. Also, if you are clearing some good wood like hedge trees, you could even crib a steep wall.

Good luck on your pond build!

A customer has clay that he can make pots from. He had the contractor dig the pond deeper and he did it with water in the pond. Vertical walls in the pond down to 20; deep. They lasted for about 2-3 years, then they started to slough off and slide into the pond. His pond now, 5-6 years later went from 28' deep to 15' deep, and his banks caved into the pond a good 5'-8' back from the original pond banks.

My pond called for 3:1 and 4:1 slopes however in the rush to finish we wound up with some underwater ledges that are pure drop offs. Not much I could do, we ended up running out of time with the equipment and the entire goal was to just get dirt out of the pond which was accomplished. Wish I had stumbled upon this thread again sooner because I did build one of my spawning beds pretty close to the drop off edge which means that bed will likely just tumble to the bottom as the edge erodes.

Saj,

Thanks for the follow up post.

The clay banks are already in place, so no use fretting over spilt milk. Perhaps you can report back in a few years and tell us how long they lasted.