The preferred method of controlling shoreline erosion is through the use of native pants and slope grading. If these methods are not effective against significant erosion, due to wave action or swift currents, it may be necessary to install some other means of shoreline protection. Riprap uses rock, coarse stone or boulders on the edge of a bank or water shore to provide shoreline erosion control. Re-grading of the bank may still be necessary. Riprap can be an integral part of a shoreline landscape and has several advantages over other materials such as concrete, wooden seawall or steel sheet piling. It can be colonized by some insect and native plant species. It can blend into the shoreline and be partially hidden by reestablished vegetation. It does not act as a barrier for organisms that move in and out of the water. If riprap is installed according to Minnesota Department of Natural Resources guidelines, you do not need a permit (be sure to check with your state's Department of Natural Resources). A further benefit of riprap is to prevent the burrowing of animals, such as muskrat, which can promote to bank instability. It is necessary to install a filter layer under the riprap to relieve hydrostatic pressures inside the embankment to help distribute the weight of the riprap, to prevent settling, and to prevent fine materials in the embankment from being pulled through the riprap by water action.
Amounts, Specifications & Suppliers:
The amount and size of rock needed will depend on each site's exposure to erosion. The Minnesota Department of Natural Resources recommends 12-inch diameter, or larger, natural rock; this will assure that the riprap will not be disrupted by waves, ice or currents.
The filter material under the rock can be fabric, gravel, crushed stone or small rock. The size of the filter material will also depend on the site. It should be larger in diameter than the bank or shore material under it and smaller than the riprap. If gravel or other material isn't available, a filter cloth can be used. Many types of cloth are available from Monsanto, DuPont, Celanese and other companies. Check the manufacture's specifications to choose a cloth that is designed for your application.
Both rock and filter material should be available from local landscape firms.
Tools and Equipment:
- A truck large enough to haul the amount of rock needed, unless the rock is to be delivered.
- A spade and rake to regrade the slope. If more extensive grading needs to be done it may be necessary to hire this portion done with mechanized equipment.
- A wheelbarrow, skid loader, or other means of hauling rock and filter material to the shoreline.
The most important part of installing riprap is to properly analyze the degree of erosion, the probability of future erosion and the physical characteristics of the site. This needs to be done before planning an erosion control method. The Minnesota Department of Natural Resources has clear permit requirements and construction guidelines. The amount of water action, both wave and current, need to be known. The slope needs to be calculated and the soil on the slope analyzed. Riprap cannot be installed without a permit on Lake Superior, on a designated trout stream, or in a posted fish spawning area. Other sites can be installed without a permit if Department of Natural Resources (DNR) guidelines are strictly followed.
- Inform yourself of the laws, guidelines and rationale behind using riprap for erosion control. In Minnesota, contact the DNR (1-800-766-6000) to obtain their bulletin "Riprap Shore and Streambank Protection" and to determine if a permit is required. It is illegal to install riprap without a permit unless DNR guidelines are strictly adhered to. Obtain a permit, if needed, from your regional DNR office or the Division of Waters in St. Paul. Outside Minnesota, contact your state DNR headquarters for that state's guidelines and needs for permits. Check with your local government to determine if a local permit is needed. Finally, contact your local Conservation Officer or Regional Hydrologist for guidance. The above agencies should be able to help determine if a homeowner can install riprap according to DNR guidelines by themselves, or if a professional consultant should be used. If a consultant is hired, be sure they specialize in bioengineering and aquascaping.
- Acceptable Installation Method Without a Minnesota DNR Permit: Determine the ordinary high water mark, the average water level, and the lowest water level. The ordinary high water mark on lakes is usually where the natural vegetation changes from predominately aquatic to predominantly terrestrial. For moving water, such as a stream, the ordinary high water mark is defined as the top of the bank or channel.
- Determine the natural alignment of the shore or streambank. Riprap should follow the natural alignment and not attempt to alter the watercourse.
- Determine the slope of the shore or streambank. The slope, prior to adding riprap, cannot be steeper than 3 feet horizontal to 1 foot vertical. Determine the amount of linear feet that requires riprap installation -- this cannot extend beyond the area with erosion problems.
- Measure horizontally from the ordinary high water mark 5 feet towards the water and place marking stakes. No material can be placed into the water beyond this point. Added materials cannot interfere with the water flow. Riprap does need to go below the flow line or lowest water level to prevent undercutting by water action.
- Have the soil tested for stability. Combination of sand, silt and clay are acceptable; peat and muck are not.
- Grade the shore (do not exceed a 3:1 slope) with shovel and rake. Provide for a depressed top "end" area and a flattened "toe" area to hold the riprap in place. The more gentle the slope, the more stable it will be. If mechanized excavation is needed a DNR permit is required.
- Cover the area from toe to end with a filter blanket, crushed stone, or well-graded gravel. Thickness of stone or gravel will depend on existing erosion and type of underlying soil.
- Cover filter material with natural rock 12 inches in diameter or larger. The depth of rock will depend on the degree of erosion beneath it. The more erosion that has occurred, the deeper the rock needs to be. The amount of wave action or current the area will be exposed to will also influence how much rock is needed. A bioengineer can help determine the need thickness. The largest and thickest riprap should be placed over the toe and end to anchor the installation. Dumped rock will best adjust itself to an uneven area. Hand placed rock is easily disrupted by settling.
- Replant the area above the riprap as soon a possible. This will prevent erosion due to terrestrial runoff and will allow the riprap to blend into the shoreline.
- Plan for regular maintenance including watching for erosion behind the riprap. Properly installed riprap will need a minimum of maintenance but some displacement can be expected.
Cost will vary depending on the site. Availability of materials, the need for a professional consultant and the degree of erosion to be controlled will affect cost.
Aquascaping: A Guide to Shoreline Landscaping. Hennepin Conservation District. Minnetonka, MN.
Life on the Edge: Owning Waterfront Property. UWEX-Lakes Partnership, College of Natural Resources, University of Wisconsin.
Riprap Shore and Streambank Protection Information Sheet. State of Minnesota, Department of Natural Resources.