Pond bank stabilization

[murdock]

(cross-posted to earthwork and storm forums)

I have two ponds in the center of a residential neighborhood that have slumping banks. The soil has eroded in 12" x 12" chunks in the steeper sections, and a more uniform erosion in the shallower sections. The side slopes of the pond range from 3:1 to 1:1.

A couple options I have been considering include filter fabric/rip rap along the pond perimeter (12" avg. diameter, with stone both above and below normal water surface level), and vegetative stabilization using permanent erosion control blankets and a seed mix that will withstand constant inundation of water.

The vegetative option seems attractive from a cost perspective (no need to get a truck in between buildings for stone install), but I worry that the root structure won't be deep enough and soil will erode underneath root level. Will the stone be sturdy enough, or will freeze-thaw action move the stone into the pond?

Thanks for your input.

 

[Riggly]

Murdock I think your slopes are too steep. ie. the 1:1 slopes. You will need a soil with a minimum angle of friction of 45 deg. for this slope to hold upl. However, we should know a little more about the soil, and also on the construction of the pond. What kind of soils are in the slopes, and was the pond cut, or was the slope of the pond filled in? The slumping on the sleeper slopes appears to be shallow slope failures, simply because the soil is too sleep. Clayey soils might hold up on a 1:1 slope under dry conditions, but when wet, shallow failure is quite possible. Is the pond lined? If line placing rip rap might not be a good idea, unless you are using it in combination with some other methods to contain the water. If possible you can cut back the steep slopes. If the water in the pond will not be permanet, a soil stabilization matting together with vegetation-even shallw root system might hold up pretty good once the root system is established. I have used this on shallow slope failures and they are holding pretty good. Good luck.

 

[murdock]

Riggly,

Thanks for the response. The soils in the area are clay (Vergennes & Covington Clays) with a high water table. The ponds are permanent, excavated ponds (1 acre in surface area, about 9' deep). I have been led to believe from the original designer that bentonite was used as a pond liner.

There may be some opportunity to cut back the banks in the steeper sections, but in others it may not be feasible. I have proposed that the metal stand pipe that serves as storm control be cut down 12-18 inches so that the water level can drop and some work can be done below the water level with less mess.

I can see how the failures are due to wet clayey soils. Here in VT, we have had a very wet couple of years, and it has been in these past 2 years that the erosion has accelerated. Do you have any insight on the shallower slope erosion? Same mechanism as the steeper sections? In these areas, it looks uniform, the old perimeter fence is now 2 feet from the edge of the water.

 

[Riggly]

I am not clear on what you are saying about the fence. Did it move? Or did the water line moved up? Erosion is mainly due to fast flowing surface water- that is different from shear failure, as possibly the case on the steeper slopes. Sheet flow rather than concentrated flow is more likely on the gentler slopes, and this may result in sheet erosion which gives a more uniform erosion. For surface erosion, you need to control/slow-down the run-off on the slopes- proper drainage, vegetation, etc.

 

[murdock]

The fence stayed in place, the normal water surface level moved up. It was originally installed along the water's edge. Now, it is in the water.

The area doesn't seem a likely candidate for erosion: there isn't much overland flow to the pond (only about 100' feet), rooftops are guttered, and there's a good vegetative cover. All of the bare earth and slumping is by the water's edge, not on the pond's side slopes. Do you think there could still be some erosion at the water's edge from overland flow without any other signs uphill?

 

[cvg]

depending on how big and deep the pond is, you could have significant wave action that has caused your problems. otherwise, it would seem that slope failure was the problem with initial slopes at 1:1 that were too steep. Especially if you have periods of rapid drawdown

If you cannot make the slopes flatter, you will need soil reinforcement or some sort of retaining structure.