Placing Rocks Around Pond Inlet

Discussion in 'Landscape Architecture and Design' started by eskals, Jun 18, 2001.

  1. eskals

    eskals LawnSite Member
    Messages: 210

    I have been doing a bit of work in a subdivision with a retention pond for storm water. The 8-year old engineering drawings call fot the pond to be 4 feet deep in the middle, with a gradual slope to it. The edges are only about 6-12" deep, and the middle looks pretty silted up. It may be 2.5 feet deep only. Anyway, thats the background info. Heres the problem. As you can see in the attached photo, there is an erosion problem around the pond inlet. I think that the water comes out of the inlet and swirls around a bit causing the erosion around the inlet. The idea is to add a bunch of boulders around the inlet to prevent the water from eroding the soil.

    First question: Is this a good idea? Will this help? I sure think it would. I know that Paul has had alot of experience in this type of work. What do you think?

    Second: I think it will take about 5 tons of boulders (1-2'). At first I was just thinking of dumping them close and rolling them into posistion by hand and levers. Now I am wondering if a Mini X with a grapple would be a better idea. How hard are these things to run? Paul, Guido??? Also, would I need the clamp or the grapple attachment?

    Thanks for all the help.


  2. eskals

    eskals LawnSite Member
    Messages: 210

    Another view...

  3. greens1

    greens1 LawnSite Senior Member
    Messages: 352

    This is actually an ideal situation to use the geoweb cellular cofinement system. link at

    The geoweb mats could be placed into the water and brought up arround the discharge flume. The portions in the water could be filled with crushed concrete and then fill the land sections with soil and then sod over them.

    Pauls the expert on this but I would think that with bolders you would need a lot more than 5 tons. You need to stabilize the entire mouth of the outlet! In most situations like this the construction details call for riprap arround the slopes and spillway of the flume.

    One quick word of caution. Whatever you put down make certain that you are not dealing with muck soil, I did a job very much like this one once and I had to remove 75 yards of muck with an excavatior and replace it with crushed concrete to prevent slippage of the geoweb into the pond.

    Good Luck,
    Jim L
  4. paul

    paul Lawnsite Addict
    Messages: 1,625

    This is typical, the contractor installing the pipe backfilled with clay and put 6" of dirt over the clay the grass never had a chance to root thru the clay before the topsoil started to slide down it. There are a number of different ways to stop this. First I need to know if water is in the pond all the time or it just holds water after a rain? Second, is the subdivision looking at improving the whole area? Reason picture two shows wave action has eroded the banks passed the outfall. Is this the only structure?
  5. Scag48

    Scag48 LawnSite Fanatic
    Messages: 6,067

    eskals-I think it would be easier to use a mini excavator to place the rocks with a clamshell grapple or a just a bucket and a hydraulic thumb. The only problem with this method is that you'll either have to track over the lawn alot with the mini x or you'll have to dump the boulders close enough to the bank to just sit there with the mini x and place the rocks where you need them. It will be a bit expensive to rent the excavator, but it might be faster. It would sure save alot of hand labor.
  6. eskals

    eskals LawnSite Member
    Messages: 210

    Paul and Jim:

    The pond is basically a holding pond for the storm water from the streets and such in the subdivision. Rather than dump all the storm water in to the the rouge river all at once, the pond provides a cushion and silt depository. But you guys probably knew all that.

    The pond retains water year round. The water level is fairly stable, even after a bunch of rain. It may rise a few inches, but it isn't rising feet. The pond is in a L-shape, with about 40,000 sq ft as pond. There is another 40,000 sq ft of marsh to the right of the picture titled "inlet3". The marsh area is used as an overflow (although the water never really gets very high).

    There are two structures in the pond itself. One is the inlet, which you have seen. The other is the outlet. I have attached a picture of the outlet.

    Paul, when you say improve the whole area I am not sure exactly what you mean. They want to stop/prevent the erosion problem.

    The pond also has a terrible smell to it. They have been adding copper sulfate to keep the alge and smell down. I know this doesn't seem very realted, but I thought it might be of some interest.


  7. eskals

    eskals LawnSite Member
    Messages: 210

    Another view of the inlet, looking down.


  8. paul

    paul Lawnsite Addict
    Messages: 1,625

    Eric, it looks like you need a mixture of hard armor and soft armor. For soft armor I'd drain the lake back about 2' and install a Turf Renforcement Mat(TRM) and hard armor, boulders around the head walls on the outlets. this combination should elimate the bank erosion that you are getting and stop the pipes from standing out. Boulders use 2'-3' on the bottom and 18" to 24" on the top. This keeps kids from rolling them in to the lake.
  9. eskals

    eskals LawnSite Member
    Messages: 210


    Not to sound like the moron that I am, but you are saying to put boulders behind the "ears" of the inlet? Is this correct?

    Also, two questions on the TRM. Can I install that with water in the pond. I really don't want to get into draining that thing, and I don't think they have the budget for something that. Finally, where do I get the TRM from? I checked online and found a bunch of manufactures. Should I just call them?

    BTW, Any idea on how many ton of boulders i need? I an really bad at estimating boulders.

    Thanks much,

  10. paul

    paul Lawnsite Addict
    Messages: 1,625

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