Grassroots Public Advocacy For The Protection, Restoration And Conservation Of Great Lakes Beaches And Riparian Property
Rock Revetments Are A Cost Effective Way To Protect Property
Rock revetments offer many advantages to shoreline property owners...
|Cost Effective Compared To Other Structures||Effective At Reducing Wave Erosion|
|Easily Maintained/Quickly Installed||Environmentally Sound "Soft Engineering/Hardening"|
|Retains Bank Soil||Can Be Used With Almost Any Bank/Bluff Profile|
|Can Be "landscaped" for aesthetics||Greatly Reduces Wave Run-Up|
|Flexible, settling/ minor damage allows structure to function||Material Is Readily Available In Most Areas|
|Proper Design & Slope Can Restore Beach||Rip-Rap often provides habitat|
|Can Often Be Vegetated To Cover Material||Can Be Designed To Reduce Flanking|
Causes Of Revetment Failure
There are several causes of revetment failure
|Failure To Use Filter Cloth Or Improper Filter Cloth||Improper Sizing Of Rip-Rap, Armour and/or Splash Stone|
|Incorrectly Calculated Design Height||Design Slope Is Too Steep|
|Not Enough Care In Placing Materials||Not Enough Attention To Toe Protection "Key Stones"|
Filter Cloth (Geo-textiles cloths) are a crucial component of any revetment. Filter cloth separates the rip-rap and amour stone from the bank, bluff and/or beach. It provides a stable base that reduces settling. It also helps capture the fine material (sand and gravel) so that often the revetment partially covers itself and the eroded beach returns to it's previous state. Without proper filter cloth, even very large armor stone can settle or move greatly reducing the effectiveness of the revetment. Filter cloth also helps prevent storm driven waves from washing away the base material supporting the revetment.
Improper sizing of rip-rap and armor stone is another cause of revetment failure. It is easy to calculate the size of the armor stone and rip-rap. The U.S. Army Corps. of Engineers has published a simple way to determine stone sizes used in revetments based on the depth of water 50ft off shore. Since wave energy, reaching the beach, is greater based on the depth of water, the deeper the water, the larger the stone must be. If the water depth 50ft off shore is 3ft-4ft, the size would be 200-500 lbs. and about 1.9 tons per lineal ft would be needed. If the depth is between 5ft-6ft, stone sizes would be 750-2000 lbs. with 4.95 tons per lineal ft. Water depths of 7ft-8ft. require stone that is 2000 to 5000 lbs. and about 7.35 tons per lineal ft. In most cases about 2000 to 6000 lb stone of mixed sizes and a 3 ton per lineal ft. rate is adequate for Lake Ontario private proper.
Keep in mind that the larger the stone and the more tons per lineal foot needed, the larger surface area the revetment will cover. This becomes a regulatory issue as, in most cases, there is only so much fill that a property owner will be permitted to put into the lake. When you add sound engineering practices that require a stable slope, the larger the revetment is the harder it is to get permitted. Also notice in picture that individual rocks have been hand-marked with paint to insure that proper armor and key-stone is selected
Lets Explain Slope And Why It Is So Important
Slope is ratio between the vertical rise (height) and the horizontal run (length). I the greater the run (length) relative to the rise (height) the more stable the revetment will be. The average design height for most locations on Lake Ontario is about 7 ft. If you think of the revetment as a right angle triangle (as in the illustration below) and the RISE is 7 ft. one can easily calculate the distance lake ward at various RUNS. (7ft rise, 1:2 slope=14ft run)
Keep in mind that while greater run will be more stable (gravity is working to pull the material down rather that helping it fall forward) longer runs also mean more materials will be needed and thus higher expense.
Slope is another place where most riparians will run into permit problems. The NY DEC 505 Regulations REQUIRE that a revetment lasts at least 30 years. Given that all other design criteria are met, the greater the run, the greater the probability that the revetment will last. However, the greater the run, the further the revetment will extend lake ward.
In the past the DEC has permitted a 1:3 slope which produces a stable revetment. However you may have to argue as the Fish and Wildlife division almost always want 1:2 which will mean the likelihood of failure and high maintenance. If you have a hard time with this point, tell the DEC you want their engineers involved, they will have to agree that a more gradual slope is a better design. The U.S. Army Corps "Help Yourself" manual clearly states "1:2 or flatter" slope..
Determining Proper Design Height
Design height is another crucial factor that can mean the success of failure of your revetment, If design height is not high enough, the revetment could be over topped and washed away. If the design height is too high you could be wasting money. Keep in mind that design height+slope=total size. Permits become more difficult to obtain with larger revetments. However, the longer the run, the more wave dissipation the revetment provides.
To calculate the proper design height you need to consider things like storm set up values, water level depths 50ft off shore and expected rise in seasonal lake levels.
Most of the storm set-up values for the South shore of Lake Ontario are about .75 ft. Loosely explained, this the addition depth of water on the shore due to effects of wind piling up water against the shore. It's like tipping a bowl, water on one size become more shallow while the other side gets deeper.
If the depth of water 50ft off shore is 4 feet, you would add 4+.75=4.75. Next take the expected increase in seasonal levels and add it to the 4.75 to get OPTIMUM design height. The lake is supposed to be regulated within a 4 foot range. Add the expected increase 3 and the 4ft to arrive at 7 feet.
Next calculate the RUN at 1:3 slope 7X3=21 of run. The proper revetment size is 7ft high X 21ft long @ 1:3 slope.
While you don't want your revetment to be over-topped by high wave action, you also want to reduce the possibility that the revetment will be under-mined at the toe. An undermined revetment will allow the carefully placed material to slip forward and "down" your design slope. This is another reason why you want the flattest slope you can possibly get permitted.
The "Key Stone" (an interlocked row of the largest stone) is the first row of stone and it is placed in a trench excavated so that the top of the key stone is about equal to the lowest expected water level. This reduces the possibility that the key, which holds the revetment from moving lake ward can be undermined by wave action. You want the waves to always break onto the revetment ABOVE the key stone.
Sometimes the proper placement of the key stone will mean that they are placed lake ward and below the high water mark currently defined as 247.3ft. The U.S. Army Corps Of Engineers has jurisdiction in this area. Both the DEC and the Corps will want you to move your revetment back behind the high water mark.
Under current regulations, a MINOR project is one that places no more than 100 cubic yards of material lake ward of 247.3ft. If you consider that 100 cubic yards is an amount equal to 8-10 ten wheel dump trucks, by law, you are allowed to place your revetment lake ward and below high water.
Proper Material Placement.....
Proper placement of material is crucial to the success of your revetment. Rocks cannot be dumped, they must be carefully placed using the proper equipment. An excavator that utilizes an hydraulic "thumb" is the best machine to achieve proper placement. Behind the key-stone row, larger rocks are placed in an inter-locking pattern closest to the lake. These are the armor stones that will dissipate the highest wave energy.
All rocks should be supported on 3 sides by adjoining material. A mix of stone sizes are used to fill and voids or gaps between larger rocks. Material can be of smaller size towards the top and back of the revetment. Careful attention to maintaining the design height and slope is necessary to provide the proper dissipations of run-up.
Placing the stones "one-by-one" prevents tears and damage to the underlying filter cloth. Careful inspection and even hand placement of smaller stones to fill voids will probably be necessary to finish the structure.
A properly designed and built revetment is far more than a "pile of rocks." It is an interlocked structure designed to maximize mass, gravity and slope to intercept and dissipate wave energy.
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