Shoreline engineering in the form of a seawall is sometimes used to reduce wave erosion at the toe of a bluff. Seawalls are an extreme form of stabilization that should be considered only when all other means of stabilization have been ruled out. Typical seawalls are vertical structures of stone, wood, concrete, steel, or some combination of these materials. When properly constructed, seawalls form a physical barrier to forces of erosion such as tides and storm waves. Seawalls achieve their protection by deflecting the energy of the water away from the area being protected by the seawall. The deflected energy travels horizontally along the seawall until it finds a weak point in the seawall, “scouring” or undermining and weakening the integrity the seawall itself. Deflected energy can also cause “end effect erosion” on adjacent properties that are not also protected by a seawall. Extreme care should be exercised when considering the construction of a seawall in order to determine if other BMPs would be effective as an alternative or in combination to ensure that the seawall addresses the erosion issue without unnecessary consequences for the abutting shoreline. Engineering alone cannot prevent some large landslides. In general, human activities that increase the amount or rate of natural processes may, in various ways, contribute to landslide risk.
Eroding bluffs sometimes can be stabilized solely at the base or along the entire bluff surface using a single technique or combination of tree rafts, wattles, geotextile fabrics, rip-rap, or gabion structures. The costs associated with bluff stabilization can be quite high, depending on the size and project design specifications, and a licensed engineer should be contracted for such a design. Permitting may be required for not only the actual activity, but also for staging or seasonal use of equipment, especially if it occurs from the seaward side of the project and is within the “coastal wetland” or below highest annual tide. Maine Department of Environmental Protection rarely approves seawalls as a general practice because an alternative intervention that is less environmentally damaging is generally available. The guide to Maine Erosion and Sediment Control Best Management Practices, contains stabilization techniques applicable to coastal bluff and landslide sites which should be considered as alternatives to seawalls, including:
- land grading and slope protection
- use of geotextile fabric
- stabilizing slopes with rip-rap
- stabilizing slopes with gabions (rock-filled wire baskets)
- stabilizing streambanks
- Additional resources regarding slope stabilization are provided by the US Army Corps of Engineers Coastal and Hydraulics Laboratory
Follow the steps below to gain the environmental and regulatory information needed for decision making. The steps are listed in general order although some steps may be conducted concurrently.
1) Contact local, state and/or federal officials to obtain regulatory advice. Individuals experienced with coastal regulations may not need to seek regulatory advice in all cases; however, if in doubt seek advice before proceeding with a project.
2) Obtain an assessment from a certified engineer, landscape architect, or other qualified environmental professional. In most cases, local, state, and/or federal regulators can help identify the best professional discipline to assist with a specific project.
3) Owners of coastal property along eroding bluffs or near landslide-prone areas should check their insurance coverage to make sure they have adequate liability coverage related to loss due to landslides or shoreline erosion, as well as flood insurance.
4) If the project involves work within 75 feet of Highest Annual Tide (HAT), develop a site stabilization plan.
5) Be neighborly. If the plan involves work at or near a property boundary, consider sharing the plan with the abutter(s) to make sure they fully understand the work to be performed and the potential impact to drainage on their property. This consultation is a courtesy at this stage, and not a regulatory mandate, however obtaining “buy in” from abutter(s) can potentially avoid neighbor disputes that lead to costly permitting and/or construction delays.
6) Share plans with local code enforcement to determine what, if any, local ordinances apply. Local Shoreland Zoning requirements will determine the acceptable location for structures such as drainage features, catch basins, roads/driveways, paths, etc.
7) If the plan involves alterations within 75 feet of highest annual tide (HAT), within or adjacent to another protected natural resource as defined by the Maine Natural Resources Protection Act (NRPA), or within a development permitted by the Site Location of Development Act (Site Law), a permit will likely be required. The construction of new seawalls does not qualify for a Permit-by-Rule; however, the replacement of existing seawalls can qualify for Permit-by-Rule. An individual permit will be required for any plan that does not qualify for a Permit-by-Rule. This step is critical for all situations that involve bluffs that are exhibiting signs of groundwater discharge through the bluff face. In some cases properly installed seawalls can restrict the natural movement of groundwater which may cause super saturation of the soil and lead to catastrophic slope failure or a landslide.
8) If the plan involves work below the HAT, and/or in a freshwater wetland or habitat for endangered or threatened species, a federal permit under the Federal Clean Water Act and Rivers and Harbors Act may be required. Share the plan with all applicable federal authorities to determine which permits may be necessary.
9) It is good general practice to hire contractors that are experienced with coastal stabilization projects and the implementation of Best Management Practices.