Guidelines & Recommendations for Construction of Piers, Wharves, and Docks

The following is intended to assist permit applicants in planning pier, wharf, or dock projects that alter coastal wetlands. “Coastal wetlands” are defined in 38 MRSA 480-B(2). Alteration includes construction or repair of any permanent structure. DMR is responsible for reviewing proposals to construct or repair structures such as piers and wharves below the Highest Astronomical Tide (HAsT) line. The HAsT line denotes any land subject to tidal action during the highest tide level for the year. DMR seeks to avoid or minimize adverse impacts to coastal wetlands and to water bodies that support diadromous fish.

For the purposes of this guidance document, the terms piers, wharves, and docks include seasonal structures such as ramps and floats.

Construction Standards

Alternatives to pier construction for access should be considered (e.g., shared use of piers, a dinghy haul-out with mooring for a larger boat offshore, or use of public access or marina if within a reasonable distance).            

Pier location should be such that it avoids or minimizes potential adverse impacts to the resource (e.g., by choosing a site that has the narrowest fringing marsh, or least distance to water from the upland, or by minimizing the number of pier supports necessary).

Pier, ramp, and float size should not be larger than what would meet legitimate needs.  This reduces the potential for direct and indirect adverse impacts.

Construction should be done in a manner that results in the least adverse impact to the resource (e.g., use of protective mats if habitat warrants, working off a barge without letting the barge ground out, use of erosion/sedimentation controls where warranted).       

Use of construction material that is untreated with chemical preservatives is preferred.  This is of greatest concern where a large number of piles and/or a large deck area is proposed, where water circulation is low, and/or where a pier is in close proximity to vegetation.

Winter storage of floats and ramps should be out of the resource (upland), or on bare ledge/boulder, never on vegetation or mudflat.

Information DMR needs for a complete review of the project:

  • Project location and surrounding area on a legible map – acceptable base layers include satellite imagery, NOAA nautical chart, or a topographic map. 
  • Plan and elevation (top and side view) scale drawings of the proposed pier that include any proposed ramp(s) and float(s) with the height of the pier decking and depth of water at the float(s) indicated.  Plans should include an arrow indicting north, as well as Mean High Water, Mean Low Water, and HAsT.
  • A description of the intended use of the structure including a description of the boats that might use it. Is the structure intended for commercial or recreational use?
  • A set of high-resolution color photographs of the project area taken at low tide, in absence of ice or snow cover (photo coverage should include both the proposed project area and adjacent areas considered as alternatives, as well as adjacent properties from the shore and the water body the pier would access).  Photographs should be identified and located on an application site plan. 
  • An assessment of the resource in the area of a proposed structure, best done through the growing season (May through October).  By this time of year marine vegetation such as fringing salt marsh and submerged aquatic vegetation (SAV) has had a chance to grow and is more visible.

DMR focuses on five primary areas of concern:

  • Habitat loss and degradation
  • Water quality impacts – including the potential need to reclassify shellfish growing areas to restrict harvesting for public health reasons
  • Marine organism impacts – physical harm, toxicity to, and adverse influences on behavior
  • Impacts to existing uses including fishing activity and navigation
  • Impacts from the use of the proposed project including boat traffic to and from the structure

DMR evaluates each of the above areas of concern.  If the proposed project does not avoid or minimize impacts to the greatest extent possible, DMR will recommend alternatives.  If these recommended alternatives are rejected, DMR will expect a thorough alternatives analysis.  In some cases an alternatives analysis might still be inadequate which could result in DMR recommending a permit denial.  DMR and DEP will communicate closely during this process.

Habitat Loss and Degradation

Habitat types likely associated with piers ranked (highest to lowest) in order of value:

  1. Marine vegetation – including submerged and emergent species (e.g., eelgrass, kelps, salt marsh, rockweed)
  2. Intertidal flats (particularly those with species of commercial value e.g., clams and marine worms)
  3. Coarse substrate (coarse sands to cobble) 
  4. Bare ledge or bare boulder

Piers and floats proposed over vegetation or intertidal mudflats require a thorough justification of why alternatives were rejected and what measures will be taken to minimize impact. 

Reduce impacts to marine vegetation:

  • If avoiding areas of marine vegetation is not entirely possible, the project should be located in areas that have less vegetation or the project should be designed to “bridge” those areas. 
  • Pier decking should be at least one and a half times as high as it is wide (1.5:1) to reduce shading of salt marsh or eelgrass below.  This is particularly important if the pier has an east – west, or southeast – northwest orientation.
  • If over marine vegetation, greater spacing between decking boards is desirable (¾ inch spacing is typical, use of alternative decking material, such as grating, might also be considered).  This allows increased air circulation and sunlight penetration to vegetation below.
  • If pilings will be installed directly in marine vegetation such as salt marsh, helix anchors are the least disruptive method of anchoring.
  • There should always be a minimum of 4’ of water depth at any tide stage under a float that terminates over SAV.
  • For any float that terminates near SAV, the type of anchoring used is critical. For example, chains to mooring blocks can cause scouring of eelgrass.

Reduce impacts to intertidal flats from floats and associated boats:

  • Float size and number should be the minimum needed.
  • Floats on intertidal flats should be supported off the flat by float skids to reduce compaction and/or hydraulic pumping and loss of fine sediments from the float rising and falling.  Generally, 8 to 12 inches of elevation off the flat is considered adequate.
  • Larger boats should be placed on a mooring rather than left to ground out at the float during low tides. 
  • Boats should approach and depart floats at slow speed during lower tides to minimize direct impacts from propellers and turbidity from propeller wash where there is SAV.

Reduce impacts to habitat from pier supports over bare ledge:

  • Piers and pier supports over bare ledge present fewest concerns.
  • Pier supports in the resource (piles or cribs) should be minimized as much as possible by increasing span length between supports and/or by placing landward supports above HAsT. 
  • Pile supports are preferable to crib supports, particularly where there is soft-bottom habitat. 
  • When pile supports are not feasible, “open” fashion cribs of granite are preferable to solid block pier supports which have no space between blocks.  “Open” cribs provide interstitial space and increased attachment area for marine organisms and generally last longer than timber/stone crib work.
  • Rock filled timber cribs are the least preferable structural support. Rock filled cribs will inevitably fail at some point in the future permanently changing the intertidal substrate beneath to a cobble pile. These structures also provide habitat for the invasive green crab.
  • If for some legitimate reason timber/stone crib work is deemed the best design solution, stone infill should be brought in from off-site and not taken from below the HAsT.     

Build for resiliency

To reduce impacts from storm severity, frequency, and storm surge/sea level rise:

  • Pier structures should be constructed at a height to mitigate overtopping and uplift.
  • Utilize bolting and heavy galvanized hardware at physical connections.
  • Utilize “tie down” practices for dead weight connections.
  • Use heavier decking – 4” dimensional lumber, width appropriate for application, appropriate gaps between deck boards (at least ½”) – to allow for unimpeded water flow both up and down.

Impact on Water Quality

Pier proposals in or adjacent to shellfish harvest areas that appear to present a risk to water quality will be scrutinized in more detail.  Materials used in construction that may result in toxicity is discussed below.  Activities and associated infrastructure such as entertainment decks with hot and cold running water, showers, outdoor kitchens, etc. will have to address incidental wastewater and debris disposal concerns.

Impact on Marine Organisms 

Toxicity concerns arise when antifouling paints and wood preservatives are used in or near the water.  New generation pressure-treated wood has a greater concentration of copper to compensate for decreased amounts of arsenic and pressure-treated wood for marine use has higher level of preservatives than that used for typical upland use.  Only approved preservatives that are labeled for use in water by the U.S. EPA are allowed.  Creosote and tributyl-tin, for example, are prohibited. 

Direct impacts from construction include displacement and interference with behavior such as upstream migration of diadromous fish (those that spend some portion of their life in marine or estuarine waters and some in fresh waters) from noise generated from pile driving.  These potential impacts are evaluated on a case-by-case basis.  In riverine systems the period between March 15 and July 15 is when diadromous fish in Maine can be expected to migrate upstream.  There are also some fall migration considerations.  Smaller projects of limited construction duration, those not in streams, or in streams with no known diadromous fish resource are generally not a significant concern in regards to fish migration.  

Existing uses in the area which may conflict with pier construction:

  • Traditional fishing activity includes clam digging, marine worm digging, mussel dragging, and lobster fishing.  Other fishing activity such as scallop dragging, and trawling for ground fish occur further offshore where proposed piers would unlikely have an impact.
  • Navigational concerns include the potential for interference with access to other piers and wharves in the area, and navigation through channels and passages where the project is proposed.  Protecting navigational access related to commercial fishing is of particular concern to DMR.

Impact of the proposed use of the structure:

The following should be considered:

  • Uses of the proposed pier, ramp(s), and float(s) as to what would be temporarily stored (e.g., motor vehicles, fishing gear) or permanently situated on the pier (e.g., buildings).
  • The potential impact of boat traffic to and from the proposed structures, and the docking of boats.
  • The potential impact from associated moorings.