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Evaluating Environmental Impacts of Residential Docks

Docks lining the Mississippi Coastline. (J. Brashier)

Docks lining the Mississippi Coastline. (J. Brashier)

Waterfront property owners are building new docks and refurbishing old ones at an unprecedented rate. With Americans continuing their march to live near the nation’s coasts, the trend toward more residential docks is likely to continue.

Leading from the shore to the water and in varying sizes and heights, residential docks often include lengthy walkways over marshy areas, and also boat lifts and moorings and other recreational accessories. Usually supported by pilings, many have a floating section at the water end.

Many coastal property owners view the access docks provide for boating, fishing, swimming and other forms of recreation as a capstone of their overall experience of living near or on the water. They frequently are impatient with what they see as the hassles of the permitting process. However, these same property owners are often quick to complain when a neighbor’s dock interferes with their shoreline views.

Permitting officials face tough choices

Facing an increasing workload, permitting authorities are eager to streamline the process, while basing their decisions on sound scientific understandings that can withstand scrutiny and potential appeals. Those decisions can be difficult to make, let alone explain to a rejected, and often dejected property owner or to a citizens group.

Docks that improve access to the water for their owners can impede navigation and public access for others. (S. Snow-Cotter)
Docks that improve access to the water for their owners can impede navigation and public access for others. (S. Snow-Cotter)

Docks that improve access to the water for their owners can impede navigation and public access for others. (S. Snow-Cotter)

In making permitting decisions, officials consider the individual and cumulative environmental impacts of a proposed dock in the context of potential benefits to the applicant. Unlike the obvious benefits to the applicant in the form of increased coastal access and increased property value, for instance, the potential environmental impacts are poorly understood and often difficult to document:

  • How, for example, do they decide whether the visual impact of a proposed structure is acceptable from an aesthetic standpoint?
  • Will the shade associated with a proposed dock design cause unacceptable damage to sea grasses or other vegetation? How much seagrass damage is “too much”?
  • How do they take into account the impact on the shoreline and on water quality not just of the structure, but also of associated boat traffic?
  • Should certain pristine or historic areas remain off limits for dock construction to preserve scenic vistas?
  • More generally, are docks the main issue, or are they rather a symptom of added stresses from coastal development?

NOAA’s National Centers for Coastal Ocean Science convened a workshop in January 2003 to discuss scientific data related to siting and construction of recreational docks and piers. Some 30 scientists and permitting officials from across the country shared research findings on impacts of recreational docks on coastal vegetation, contaminants, boating, navigation, aesthetics, and quality of life.

Reducing damages to submerged vegetation

The workshop included several presentations on impacts on submerged aquatic vegetation (SAV) under and adjacent to docks as a result of shading, and on steps property owners can take to minimize these impacts.

SAV and marsh grasses are key to overall health of the marine environment. They provide critical habitat, filter nutrients and sediments, provide nursery habitat for fish and shellfish, stabilize bottom sediments, and form the basis for the marine food web. The width of docks and their height above water or marsh determine the extent of shading impacts. Research indicates that limiting docks to a width of four feet – and building them at least four feet above the water or marsh – helps reduce adverse impacts. In addition, a north-south rather than an east-west orientation can minimize shading impacts in some regions of the Northeast.

Construction-related damages to seagrass beds is evident can be minimized by floating, rather than dragging, construction equipment to the site, and by techniques such as using sharpened piling tips installed with a drop hammer.

Assessing visual impacts

Along with various environmental issues, concerns over aesthetics, a sense of “over-development” and resistance to change also arise in the context of increased construction of docks and piers.

Experts at the NCCOS workshop discussed new scientific methods for assessing aesthetic impacts of docks and explained how managers, using science-based data on visual impacts, can evaluate docks’ aesthetic impacts.

While highly dependent on individual preferences and assessments, docks’ and piers’ impacts can be evaluated objectively and in ways that are both reproducible and predictive in terms of aesthetic values. Computer-generated visual impact assessments (VIAs) illustrate impacts of proposed changes on a visual landscape, and those images can be readily altered to show varying impacts. Shown potential impacts on a shoreline, the great majority of people identify the same image as being aesthetically preferable.

Aesthetic or visual impact assessments are being used in weighing permit applications in some states, including Maine and New York, and Maine’s Natural Resources Protection Act specifically requires an applicant to demonstrate that a proposed activity will not unreasonably interfere with existing scenic and aesthetic uses. Regulations Maine is developing to support this act are expected to include an applicant’s visual assessment checklist.

Leaching of contaminants from pressure treated woods

Leaching of wood preservatives is the most common source of dock-related contaminants.

Wood treated with chromated copper arsenate (CCA) is the most commonly used material for pilings and decking for small docks. (Another toxic chemical, creosote, is used primarily in commercial structures like bridges or piers.)

The scientists discussed one set of studies showing toxins moving from the wood into algae growing on the wood, and then being taken up by fish and invertebrates feeding on that algae. The toxins also can leach from the wood to the water and get absorbed by the sediments, where they are passed along to bottom dwellers.

In some areas, elevated levels of copper, chromium, and arsenic were found in organisms living on or near treated pilings and in fine sediments adjacent to bulkheads built with CCA-treated wood.

But in another study comparing South Carolina creeks with and without docks, researchers reported that they did not find significant differences in the presence of leachates in sediments or in the physiology of oysters they sampled.

The scientists are trying to understand what accounts for the observed differences.

Ongoing Activities Planned

In ongoing and future exchanges, the scientists and managers will discuss differing research results and also seek to develop guidelines for building and permitting of residential docks.

In the meantime, NCCOS scientists are disseminating proceedings from the workshop and sharing insights on guidance and recommendations. Along with the proceedings, they are building a searchable online bibliography useful for individual research. Future workshops will address regulatory and construction approaches to minimize adverse impacts and also provide training opportunities.

These approaches can provide a useful “heads up” on the likelihood of permitting approval for a given dock construction project. With visual impact assessment tools, property owners can visualize what their own completed projects would look like superimposed against existing shoreline views. Other tools would facilitate sea grass-friendly design.

Full proceedings from the workshop are posted at

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