LakeOntarioRiparianAlliance

Grassroots Public Advocacy For The Protection, Restoration And Conservation Of Great Lakes Beaches And Riparian Property


Stone Revetments...Frequently Asked Questions

  • Why Choose A Stone Revetment?
  • Will A Stone Revetment Destroy My Beach?
  • Why Do Stone Revetments Fail?
  • What Is Slope And Why Is It Important?
  • What Size & How Much Stone Do I Need?
  • Do I Need To Use Filter Cloth?
  • How High Should The Revetment Be Built?
  • Are There Disadvantages To Building A Stone Revetment?
  • How Much Does A Revetment Cost?
     
    Why Choose A Stone (Rubble Mound) Revetment?
    Stone revetments have many advantages over other methods of erosion protection. In fact, the Corps Of Engineers says, in their Help Yourself guide that, stone revetments are the most effective structure for absorbing wave energy. Stone revetments do this by dissipating wave energy over their many irregular surfaces.

    Stone revetments also reduce wave scour and rebound which are the major causes of erosion control structure failure by allowing waves to "run-up" part of the structure and deplete their energy. The more solid the structure, such as in bulkheads, seas walls or even gabions, the more energy is created as waves crash into the solid structure.

    As waves hit solid structures their energy is deflected rather than absorbed. This means some of the wave energy is pushed down and scours the material from in front of the structure. Rebound, then moves the scoured material away from the toe of the structure causing the lakeward tilt so common in solid structures. Eventually, the structure falls forward or moves into a configuration which no longer protects.

    Stone revetments are also the most economical and quickest to install of all of the methods of erosion control. They are easier to maintain and can settle with little or no affect on their protection value.

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    Won't A Stone Revetment Cause Me To Lose My Beach?

    Only if it is designed or installed improperly. First we need to understand that a row or two of large 5-8 ton stones, placed in a line is not a revetment. A pile of stone dropped over the bank from the back of a dump truck is not a revetment. A bulldozed pile of beach stone is not a revetment. A half dozen loads of field stone from a farmers hedge row is not a revetment.

    A stone revetment is an organized placement of properly sized stone, on the proper slope, placed on a prepared grade, with adequate toe protection, a trenched "key" that is at or below the low water line, is built to a calculated design height and all sits on the proper type of filter cloth.

    A common belief, reinforced by the many, many failed attempts at erosion control is that stone doesn't work and that their is never a beach in front of stone erosion control structures. All one has to do is walk the shoreline to see stone structures that have fallen into the lake, removed the protective beach and no longer functions as any meaningful protection.

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    Generally, stone structures fail for three reasons....
    1) The stone is placed on too steep of a slope or no slope at all, as in the case of "rows" of stone. This type of placement reduces or eliminates the energy absorbing run-up and increases scour and rebound. Too severe a slope also causes gravity to work in two directions, vertically and forward. A gentler slope means the more of the weight of the stone is being forced down onto the stone below it.

    2) Improper or no filter cloth. Not using a filter cloth, which was very common until recently, means the beach material that the stone sits on is more prone to being dislodged or washed away. When this foundation material is lost, the stone moves too far out of the design configuration to provide maximum protection. Improper filter cloth can also not let water through it, fail to trap sediment or decompose. Filter cloth also helps distribute the entire revetment weight more evenly.

    3) Improper stone and/or stone size is another common reason for failure. Not only do you need carefully calculated stone size, as determined by weight, you need a certain volume. In revetments with a steep slope and minimal lakeward intrusion, which is encouraged by State and Federal regulations, it is often impossible to get the proper tons per ft. volume needed to provide a lasting structure.

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    What Do You Mean By Slope?


    Slope is the vertical "rise" (height) as compared to the horizontal "run" (distance). A 1 on 2 slope rises 1 ft. for every 2 ft in length and is steeper than a 1 on 3 slope which rises 1 ft. for every 3 ft of distance.

    Draw a triangle, to scale, that is 6 ft. high and 12 ft long on the bottom. Now draw the same 6 ft. high only make the bottom 18 ft long. You'll notice that the slope and the volume of the triangle changes greatly. Since the design height, (the 6 ft ) is the same in both, the longer the lakeward intrusion (the 12 ft or 18 ft) bottom is the more volume there is for the stone and the gentler the slope is for stability and wave dissipation.

    In fact, the proper revetment slope encourages rather beach building rather than beach removal by allowing wave to push material from the lake into an then onto the revetment, actually elevating and stabilizing the beach.

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    What Size Do The Rocks Have To Be And How Much Rock Will I Need?
    In order to determine the size and number of tons per ft. that are required, you first need to determine what the water depth is 50 ft. off shore. Deeper water depths mean that more wave energy is able to reach shore. Shallower depths mean that wave energy is dissipated faster as waves are forced to "trip" or break in nearshore shallows.

    The U.S. Army Corps of Engineers has determined, as referenced in their "Help Yourself" discussion of erosion problems on the Great Lakes, that a depth of 2-4 ft., 50 ft. off shore requires app. 1.89 tons per running foot. Water depths of 5-6 ft. require app. 4.95 tons and depths of 7-8 ft. need app. 7.36 tons per foot.

    Stone size needs to vary between app. 200 and 5,000 pounds and a mix of sizes means that their will be more and multiple surfaces to dissipate wave energy. In general we have found that app. 3 tons per lineal ft of stone is adequate for most South shore private property projects. This would mean that an 80ft wide property would need about 240 tons for lake ward face of the revetment. Right and left flanks (areas where revetment ties back to shore) may require a "slight" amount of additional material depending on whether or not you would be tying into adjacent property protection.

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    Do I Need To Use A Filter Cloth?
    YES! Using the proper filter cloth is crucial to achieving a stable long lasting revetment. Not only does the cloth help distribute weight and prevent settling, it also helps retain sediment by allowing water to pass through it while retaining sand, gravel and beach material.

    How High Should I Build The Revetment?
    To calculate the design height of the revetment you must consider "3" important variables. The first number is the depth of water 50 ft. off shore. The second is the expected increase in lake levels. The third is the storm setup value.

    These variables can be determined from reference materials that are available from various sources such as you local Corps Of Engineers Offices, your State Dept of Natural Resource/Conservation or on-line via the internet.

    Basically, it works like this. A dept of 4 ft. 50 ft off shore along with a predicted level fluctuation of +2 ft and a storm setup value of 1 ft would mean an app. design height of 7 ft. This was arrived at by adding all three variable together.  

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    Are There Any Disadvantages To A Stone Revetment?
    None that can't be overcome! Actually, besides the fact that access with heavy equipment is needed and that stone is not always readily available nearby, most objections people have with stone revetments falls into two categories, the way they envision them looking and the way they believe they will limit access to the beach.

    As for looks, properly sloped revetments most often cover themselves, from the beach level, due to wave energy depositing material on them. The top of the revetment can be blended into the bank or bluff and plants may even be able to be established over the top edge of the stone by covering some of the stone with soil. This could mean only a 2 -3 ft section of the stone would be visible

    Access to the beach, something most people with erosion problems already have difficulty with, can be overcome by placement of flat stones, within the revetment, to act as stairs or steps.

    When you consider that properly designed and installed stone revetments are, by far, the most economical and functional form of erosion control, and have almost no disadvantages, that cannot be easily overcome, the stone revetment is a wise choice.

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    How Much Does A Revetment Cost?
    Revetment costs are dependent on several factors. The amount of material needed, the cost of material, the cost of trucking to site, the cost of the proper heavy equipment needed to excavate site and place material, filter cloth and general labor.

    The easiest way to calculate cost is to determine how many tons of material will be needed. In most cases it will be very close to 3 tons per lineal ft. This is the amount of material it "typically" takes to build a revetment using a proper slope and built to proper design height.

    Higher lake level regulation could mean that higher design heights will need to be factored into the design and increase the amount of material needed.

    The typical size of the revetment is a right angle triangle that is 7 ft high at back, 21ft long with a 1:3 slope.

    In "general" you should be able to build a quality structure for between $50 and $75 per ton. If you want to know how much it would be per foot, just multiply the number of feet by the number of tons and then by the per ton cost.

    For example, an 80ft wide property would need 240 tons of rock (80X3=240). At an average of $62.50 per ton, this would mean a turn-key job could be done for about  $15,000.

    Keep in mind that trucking of 2,000 to 10,000 pound material is expensive and quite hard on equipment. If you are located far away for sources of material, costs can increase. Also if access to your shore is limited it could mean that material may have to be "staged" in one place and them moved a second time thus making the project more expensive.

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