# Management of Small Docks and Piers

# Environmental Impacts and Issues

# 

# Forward---

This and the other related sections of the following workbook---along
with the complementary PowerPoint presentations---were funded through
the National Oceanic and Atmospheric Administration's (NOAA) National
Centers for Coastal Ocean Science (NCCOS) and the Office of Ocean and
Coastal Resource Management (OCRM). Steve Bliven of Bliven & Sternack
prepared the text and slides under contract to those offices. All those
concerned would like to thank the reviewers who offered valuable
comments on drafts of this document.

# Introduction---

Private docks have been implicated in a number of environmental impacts.
Some of these have received scientific investigation and some remain
anecdotal. In an attempt to clarify what is known about environmental
impacts from docks, NOAA's National Centers for Coastal Ocean Science
convened a workshop in January of 2003. This session brought together
scientists and a group of coastal managers from across the country to
discuss the environmental impacts of small docks. The results of that
workshop may be found in Kelty and Bliven (2003).

The workshop identified four general impacts:

-   Impacts to vegetation

-   Impacts from contaminants related to docks

-   Impacts from associated boating use

-   Impacts to sediments and substrate

Impacts to wildlife habitat were discussed as part of the topics above,
as this is a significant cross-cutting issue.

# Impacts to Vegetation from Small Docks---

Marsh plants and submerged aquatic vegetation (SAV) are ecologically
critical as a source of food and nursery habitat for fish, shellfish,
amphibians, reptiles, birds and mammals that live in coastal waters or
the adjacent marsh and uplands (Weigert *et al.,* 1981; Teal, 1962,
1969; Weinstein, 1996). Vegetated areas also stabilize shoreline and
bottom sediments against erosion (Kearney *et al.,* 1983; Teal, 1986)
and take up contaminants and excess nutrients from the water and
sediments (Vernberg, 1996).

Impacts to plant health and productivity from docks generally occur in
one of the following ways:

-   Short-term construction impacts,

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-   Chronic impacts from shading,

-   Chronic impacts from storage of floats and boats and associated foot
    traffic, and

-   Impacts from boat use (covered later in this module as a separate
    topic).

## *Short-Term Construction Impacts---*

Activities during construction can destroy plants either above the tide
line (*e.g.,* marsh grasses such as *Spartina* or *Distichlis*) or below
(*e.g.*, sea grasses such as *Zostera* or *Halodule*) by pulling them
from the substrate or destroying their root systems. The peat beds
underlying salt marshes can be compacted through the improper use of
heavy equipment leading to ponding of saline waters and a resultant loss
or changing of marsh vegetation. Although these impacts are seemingly
evident, limited research appears to have been done on the long-term
effect of these activities.

Setting pilings in sea grass beds may have immediate impacts as well as
causing long-term changes. The act of installing pilings destroys any
vegetation within the footprint of the pilings. The use of "jetting"
with high-pressure hoses typically disturbs adjacent vegetation and
sediments---depopulating grasses existing there prior to construction.
Once areas are depopulated, the presence of pilings may lessen chances
for regrowth. Beal, Schmit, and Williams (1999) suggest that changes in
seagrass communities in the vicinity of pilings may be caused by the
modification of currents, sediment deposition, attraction of
bioturbators, and leaching from chemically treated wood.

Shafer and Robinson (2001) tracked the regrowth of the seagrass
*Halodule wrightii* beneath docks in St. Andrew Bay, FL. They noted bare
areas from 35--78 inches in diameter around pilings, "even though the
age of these docks varied widely" suggesting that regrowth is affected
by the presence of pilings. Use of high pressure jetting produced a six-
to seven-foot diameter hole around each piling, which was then
backfilled to hold the piling in place. The resultant "halo" might
remain for 10 years without seagrass regrowth. The authors found that
where piles were installed using low-pressure jetting techniques there
was, "little or no sand deposition around the pilings and the remaining
seagrasses around the pilings looked healthy and had good growth around
the piling."

Dock construction also impacts marsh grass. Sanger and Holland (2002)
noted a barren path through the marsh along each side of a newly
constructed dock in North Carolina where vegetation had been almost
totally destroyed. However, construction did not appreciably alter the
original marsh elevations. Resurveying the site 15 months later, the
researchers found that *Spartina. alterniflora* had recolonized the area
and substantial recovery had occurred.

## *Chronic Impacts from Shading---*

![](media/image1.pct){width="4.875in" height="3.009027777777778in"}Both
marsh grasses and sea grasses have adapted to living in extended periods
of sunlight. Therefore, shading can have significant impacts on the
health and productivity of these plants. Figure 1 illustrates some of
the potential impacts from dock shading. Two thresholds are shown; the
minimum amount of light necessary for the plant to reach full growth,
and the minimum amount of light for the plant to sustain itself. Below
the first threshold, plants are stunted in growth or grow tall and
spindly in an attempt to reach sunlight. Below the second threshold,
plants die.

**Figure 1.** Light Profile under a dock (From Nightengale and Simenstad
2001. Ron Thom, pers. comm.). The graph illustrates how the shading of
an east-west oriented dock affects light levels and vegetation growth
below a dock in Washington state. The two horizontal lines indicate the
light levels necessary for full, unaffected growth of plants (Minimum
light needed for full growth) and the amount of light for plants to live
(Minimum light for plant maintenance). Note that much of the area
beneath the illustrated dock is below the threshold for plant life. Note
also that the shading is somewhat offset to the north due to the
declination of the sun.

[Amount of Light Required by Vegetation---]{.underline}

Susceptibility to shading varies by species, even those occupying
similar habitats. For example, in marsh grasses, densities of *Spartina
patens* (Salt Meadow Grass or Salt Hay Grass) and *Distichlis spicata*
(Spike Grass or Salt Grass) are more sensitive than *Spartina
alterniflora* (Smooth Cordgrass or Saltwater Coordgrass) (Kearney *et
al.* 1983).

Light levels necessary for plants to sustain themselves generally range
between 12--25 percent of ambient light, depending on the species.
Specifically for seagrasses, a National Marine Fisheries Service (NMFS)
Technical Memorandum reports that "the light requirements of temperate
and tropical seagrasses are very similar" requiring "at least 15 to 25%
of the incident light just for maintenance" (Kenworthy and Haunert,
1991). Light requirements are presumably higher to allow full growth but
there has been limited research that provides specific percentages.

NMFS' summary report is supported by studies on individual seagrass
species. For example, Shaefer and Robinson (2001) report that light
levels of 13--14 percent of mean daily surface irradiance (SI) are
necessary for survival of the seagrass *Halodule wrightii*. Shaefer
(1999) found that seagrass densities were 40--47 percent less in areas
shaded at levels of 16--19 percent SI. Burdick and Short (1999) observed
similar trends in the eelgrass, *Zostera marina,* which required light
levels of at least 15 percent of surface irradiance for survival and
approximately 50--60 percent for healthy beds.

## [Direct Impacts of Shading---]{.underline}

As we have already demonstrated, marsh and submerged aquatic vegetation
need adequate light levels to survive and flourish. Therefore shading
from docks can have significant implications for this vegetation. Recent
studies have shown that shoot density, biomass, and overall plant growth
may be reduced by dock shading (Sanger and Holland, 2002; McGuire, 1990;
Burdick and Short, 1999). In some instances, researchers found an
increase in the height of marsh grasses found under docks, possibly due
to etiolation. (Etiolation is a condition in which plants growing in
reduced light levels elongate much more rapidly than normal as a means
of reaching light. It is characterized by long weak stems and small
leaves.)

![](media/image1.pct){width="4.570138888888889in" height="3.1125in"}

**Figure 2.** Eelgrass density was significantly lower under and near
docks than at sites removed from the dock by various distances (Burdick
and Short, 1999).

Impacts due to shading also appear in fresh waters. Garrison *et al.*
(2005) investigated shading impacts on submerged vegetation and
relationships to fish and invertebrate habitats. They found significant
shading under docks "with a corresponding reduction in aquatic plant
abundance" in Wisconsin lakes. By placing unbaited minnow traps below
piers and within nearby, control vegetated zones, the researchers also
found decreased numbers of macroinvertebrates and vertebrates as well as
changes in species composition.

Wilson (2002), working in fresh-tidal waters in Connecticut, found that
not only did shading have impacts on existing submerged aquatic
vegetation but it also adversely affected its ability to regenerate
after being mechanically disturbed. (Wilson manually clipped the
submerged vegetation in his research but suggests that similar effects
would be found from disturbance by boat propellers.)

####### Geographic Area Affected by Shading---

It is clear that shading from docks can have an adverse impact on the
underlying vegetation but one of the questions coastal managers grapple
with most is how much total marsh grass or seagrass with be affected by
docks, and will the addition of more docks have a significant impact on
the total amount of vegetation present. Sanger and Holland (2002)
compared the area of marsh affected by docks to the total area within
specific creek systems and also estimated the amount of marsh affected
by docks through out coastal South Carolina. Based on the number of
docks present in 1999, they estimated that docks reduced *S.
alterniflora* densities by 0.03--0.72 percent within their study sites.
When they projected to a total possible build-out of similarly sized
docks in the creek systems, Sanger and Holland calculated that docks
would cause a 0.18--5.45 percent decrease in marsh grass. Finally,
Sanger and Holland applied this approach to the projected build-out of
docks for all eight coastal counties by the year 2010. They estimated
that a reduction in marsh density of between 0.03--1.98 percent could be
attributed to dock impacts.

Smith and Mezich (1999, in Shafer 2001) note that up to 50 acres of
seagrass beds had been negatively impacted in the early 1990s by
single-family docks in the Palm Beach County, Florida. Studies in
Georgia (Alexander and Robinson, 2004) suggested a maximum estimate of
4--6 percent of the marsh around Wilmington Island could be shaded if
full build-out should occur under the provisions of current state laws.

It should be noted that the significance of these shading impacts to the
coastal ecosystem as a whole varies by region and the amount of salt
marsh or seagrasses present. In areas where coastal vegetation is
already severely reduced or otherwise impacted (*e.g.,* New England and
Florida) affected areas as a percentage of the entire marsh system could
be higher than those reported for South Carolina and Georgia.

## [Indirect Impacts of Shading---]{.underline}

Changes in vegetation density and hardiness may lead to increasing
sediment erosion and resuspension, and increased undercutting of the
marsh shoreline near the dock because robust healthy marsh and seagrass
vegetation is no longer present to hold the sediments in place (Burdick
and Short 1999).

Burdick and Short (1999) also found that shading of eelgrass beds can
lead to fragmentation, thereby disrupting wildlife habitat. The work of
Garrison *et al.* (2005) mentioned previously showed that the loss of
vegetation due to shading by docks reduced fish and large invertebrate
abundance and altered species composition in Wisconsin lakes.

A subsequent effect of the shading by docks in Connecticut was reported
to be accelerated soil erosion beneath structures passing over *S.
alterniflora* at the edge of the marsh (Kearny *et al.* 1983).

## [Factors that Affect Shading---]{.underline}

Kearney *et al*. (1983) studied impacts to marsh grasses from
walkways/docks from "all the structures" within Connecticut's major salt
marsh regions, collecting data on vegetation density and height beneath
and adjacent to the structures, and the physical dimensions of the docks
(width, height, plank width, and spacing between planking---they did not
include orientation). They found that dock height was the only
statistically significant variable; docks less than 12--16 inches above
the marsh shaded out all vegetation in every study site. Any impacts
from dock width, plank width, and plank spacing were not statistically
significant. However, it should be noted that a National Marine
Fisheries Service study that assessed dock impacts on marsh grass
vegetation in Connecticut, Rhode Island, and Massachusetts cast some
doubt on the methodology and statistical analyses of Kearney *et al*
(1983) (Colligan and Collins, 1995).

In a field study conducted in Waquoit Bay, Falmouth/Mashpee and
Nantucket Harbor (all in Massachusetts), Burdick and Short (1999) found
that dock height was not the only factor influencing shading impacts on
eelgrass beds but that the orientation of the dock and dock width also
played significant roles. According to their data, North-South
orientated docks had less impact than East-West orientated ones.

While orientation may be an important factor in northern latitudes, it
may not play a significant role in lower latitudes. Sanger and Holland
(2002) assessed impacts on *S. alterniflora* from 32 docks in the
Charleston, SC area*.* The structures represented a range of lengths,
orientations, and ages. They found no significant difference in
reduction in *S. alterniflora* density due to shading between
North-South oriented docks and those with an East-West orientation.

The National Marine Fisheries Service suggests that spacing between
decking planks on the order of an inch or two has little effect on
shading impacts, particularly in northern latitudes (Michael Ludwig,
NMFS, Personal Communication, 2003). There appears, however, to have
been little systematic research on this topic.

In summary, it appears that dock height and width are significant
factors that contribute to shading impacts on vegetation. However, but
it is still unclear whether orientation and spacing between decking
boards have measurable impacts. There appear to be differences in these
impacts between northern and southern latitudes.

## [Models to Predict Shading---]{.underline}

McGuire (1990) measured the effects of shading by open pile structures
on *S. alterniflora* density in a fringe marsh in the York River Estuary
(VA). She subsequently developed a computer program to calculate the
total number of hours of shading produced by each structure based on
height, width and orientation of the structure and compared the computer
projections with the results of her field studies. The computer program
developed as part of this project appears to hold promise as a
predictive tool. Unfortunately, no electronic copies of the program
remain (the text of the program is available) and it is written in
Pascal. To be effective the program would have to be rewritten in a
contemporary, and more user-friendly, format.

Burdick and Short (1998) modeled the impact of shading on eelgrass. They
presented their results in an informational CD entitled "Dock Design
with the Environment in Mind: Minimizing dock impacts to eelgrass beds."
The CD contains illustrative estimates of impacts to *Zostera* from
docks of specific height, width, and orientation. They did not attempt
to develop a process to assess the impacts from differently sized and
oriented docks but feel that a computer model could be produced to
predict impacts from any combination of design factors (personal
communication, 2005).

## [Cumulative Impacts from Shading---]{.underline}

The issue of cumulative impacts to vegetation from shading or dock
construction has not been heavily researched. Consequently is it not
clear whether such impacts are additive or have some greater effect.

# *Chronic Impacts from Storage of Floats and Boats---*

Floats, boats, or any other solid structure stored, either permanently
or seasonally on the marsh face will significantly shade, and therefore
destroy, any vegetation present.

# *Ramifications of Impacts to Vegetation---*

Shading of vegetation, to the point where its health is impaired, can
have several adverse impacts.

####### 

####### Lessening of input to the Aquatic Food Web---

Marsh and seagrass vegetation and the detritus they produce constitute a
major portion of food available to the base of the aquatic food web. For
example, these habitats are critical to the life cycle of shellfish and
juvenile finfish that inhabit embayments and estuaries. Significant loss
of vegetation may adversely affect populations of these species (Teal,
1986).

# [Modification of topography and lessening productivity of the marsh---]{.underline}

Compaction of marsh peat from construction or continually walking to and
from a dock changes the marsh topography and may lead to long-term
changes in marsh vegetation and drainage (Hruby, 1990). The distribution
and species of marsh vegetation are strongly linked to elevation in
relation to tidal flooding. Ponding of salt water on the marsh face will
eventually lead to changes in vegetation to less productive species
(Lefor, 1992).

####### *Fragmentation of habitat---*

Marshes are important for many species, including fish, birds, mammals
and reptiles. Similarly many aquatic organisms, including game fish,
shellfish, and the food they eat, depend on submerged aquatic
vegetation. Docks, piers and associated walkways to docks fragment these
valuable wetland habitats. The presence of docks or subsequent damage to
the surrounding vegetation can deter wildlife from frequenting the area.
Small docks also fragment eelgrass beds (Burdick and Short,
1999)---primarily through shading of the grasses. There are,
unfortunately, limited research results available to quantify the
impacts due to habitat fragmentation.

There is a body of empirical evidence showing that fragmentation of
habitat causes changes in species diversity and composition (Wilson,
2002). These changes may be in small beds or an entire estuary. Rare or
specialized species tend to be the first to disappear from impacted
environments (Wilson, 2002.

**Impacts from Contaminants Related to Docks---**

The most common contaminant-related concern associated with small docks
is leaching of wood preservatives. Wood continuously exposed to water
can decay rapidly. Pilings are also subject to wood-boring and fouling
organisms that speed their break-down. To protect the wood and ensure
docks will have a reasonable lifespan, the wood is typically treated
with preservative chemicals that, in turn, can leach into surrounding
waters. Historically, the most commonly used materials were oil-based:
creosote or pentachlorophenols. Presently, wood products
pressure-treated with chromated copper arsenate (CCA) are the most
common material used for dock construction.

# *Creosote and Pentachlorophenol---*

Oil based preservatives containing creosote (CRT) or pentachlorophenol
(PCP), applied to the surface of wood materials, leach readily and have
wide-spread environmental and human health impacts Most states have
banned their use for small docks and piers.

# *Chromated copper arsenate (CCA)---*

CCA-treated wood comes in a variety of "strengths" (the amount of
preservative retained in the wood after treatment. The Southern Pine
Council (2004) makes the following recommendations for specific uses:

+--------------------------------------+-------------------------------+
| # C                                  |                               |
| oncentrations of CCA Wood Treatments |                               |
|                                      |                               |
| # Recommended for Various Uses       |                               |
+--------------------------------------+-------------------------------+
| #                                    | #                             |
|                                      |                               |
| # Retentions                         | # Uses/Exposures              |
|                                      |                               |
| *(lbs./cu.ft.)*                      |                               |
+--------------------------------------+-------------------------------+
| 0.10 -- 0.25                         | Above ground                  |
|                                      |                               |
| 0.21 -- 0.41                         | Soil & Freshwater use         |
|                                      |                               |
| 0.31-- 0.61                          | Permanent Wood Foundation     |
|                                      |                               |
| 2.50                                 | Saltwater use                 |
+--------------------------------------+-------------------------------+

> **Table 1.** Concentrations of CCA wood treatments recommended for
> various uses.

As can be seen, protection of wood products, generally pilings, in
marine waters requires a far heavier treatment than in most other
environments.

Weis *et al.* (1991, 1992), in laboratory studies, found that leaching
occurs in saline waters and that it can have toxic effects. The leaching
rate decreases by about 50% daily once the wood is immersed in seawater.
Approximately 99% of the leaching occurs within the first 90 days in the
marine environment. (Cooper, 1990; Brooks, 1990; in Sanger and Holland,
2002).

The metals that leach from CCA-treated woods (Copper, Chromium, and
Arsenic) adsorb more readily onto fine-grained sediments (silts and
clays) than sand (Luoma and Davis, 1983). Field studies by Weis *et al.*
(1992, 1998) found elevated concentrations of metals in fine sediments
adjacent to (within 1 meter) bulkheads (solid walls of treated lumber,
as opposed to dock pilings) constructed of CCA-treated material. The
distance at which elevated levels could be found varied according to the
sediment types. At most test sites, the impacts were limited to one
meter from the structure. In some other sites where fine-grained
sediments were predominant, the elevated levels could be found out to
approximately 10 meters (Weis *et al.*, 1998).

Elevated concentrations of metals from CCA-treated wood can be found in
organisms living on treated pilings and in the areas near to the pilings
(Wendt *et al.*, 1996; Weis and Weis, 1996). In sediments with higher
contaminant levels, species richness was depressed (Weis and Weis,
1998).

Snails in the laboratory fed marine algae gathered from CCA-treated
pilings became inactive in 3-4 weeks; they initially curled up inside
their shells and then died (Weis *et al.*, 1991). A field study of
oysters living on CCA-treated wooden bulkheads showed that they were
smaller than control populations and had taken up measurable levels of
copper (Weis *et al.* 1993). When copper concentrations are high enough,
the oysters' digestive glands shrink, leading to death. Wendt *et al.,*
(1996), however, evaluated uptake of metals by white shrimp, mud snails
and two species of fish (mummichogs and red drum) and found no increase
in mortality in individuals placed adjacent to 5--12 month old docks for
96 hours. As noted previously, 99% of all leaching occurs within the
first three months; the 5--12-month age of the docks in Wendt *et al.'s*
study is outside the period when significant leaching occurs.

Factors involved in impacts to biota appears to include sediment type
(mentioned above), amount of CCA-treated material (piers vs. bulkheads),
length of time the CCA-treated material has been immersed in marine
waters (more than or less than 3 months), and the flushing rate of the
water body. In an unpublished "gray literature" study prepared for the
New Jersey Department of Environmental Protection, Weis and Weis (1998)
looked at sediments and shellfish in the Navesink-Shrewsbury River
system (NJ) in relation to distance from docks constructed with
CCA-treated materials. They found that concentrations of metals in
sediments adjacent to pilings at their test sites "were generally not
significantly elevated ... it appears that leachates from pilings, in
reasonably flushed areas have negligible ecological effects, while those
from bulkheads, particularly new ones and ones in poorly flushed
regions, have demonstrated, clear-cut, ecological effects." Sanger and
Holland (2002) report that, "it is unlikely that the bioaccumulation of
dock leachates by marine biota is having or is likely to have an impact
on living resources in South Carolina estuaries and tidal creeks."
Reasons given are that approximately 99% of the leaching takes place in
the first three months after installation, that the size of the area
around the dock that might be affected is small, and high rates of tidal
flushing will dilute and flush any accumulations in the water column.

While Weis *et al.* (1991) noted mortality in snails fed algae grown on
CCA-treated wood in laboratory tanks, there have been, thus far, no
reports of the transfer of metals from the CCA treatment up the food
chain to higher predators (P. Weiss in Kelty and Bliven, 2003).

As of 2004, pressure-treated lumber intended for residential and
recreational (including docks in freshwater) is no longer treated with
CCA. (CCA-treated materials are still used in marine waters.)
Alternative treated wood intended for freshwater applications include:
Alkaline Copper Quat (ACQ) and Copper Azole (CA, "Wolmanized"®). These
are not recommended for marine use.

To summarize, issues to consider in management decisions relating to
CCA-treated materials include:

-   the area of exposed surface of CCA-treated materials (bulkheads have
    greater surface exposed to water than dock pilings so have a greater
    potential to leach contaminants into the environment),

-   the age of the materials used (most leaching occurs within the first
    90 days,

-   the types of sediments in the area (fine-grained sediments with high
    organic content take up more contaminants than larger grained
    sediments), and

-   the flow of water through the system.

Despite these research efforts however, a tidal flushing threshold for
contaminant impacts has not been identified, and data do not exist to
evaluate the importance of dilution in high flow areas with different
benthic community composition.

# *Impacts from Flotation Materials---*

Plastic, non-enclosed foam billets are occasionally used as floatation
material for docks. Sometimes referred to by the tradename Styrofoam or
"beadboard." Open-cell foam absorbs water over time and reduces
flotation support. More importantly to environmental effects, it breaks
down easily into small beads that are virtually indestructible. Pieces
of these billets may litter the shoreline or be ingested by wildlife. It
may choke air-breathing species or take up considerable space in the
digestive tract of species that ingest it, lessening their ability to
take up nutrients. Use of this material as dock flotation has been
banned in many jurisdictions (Burns, 1999).

*Impacts from Painting and Seasonal Upkeep---*

Painting, staining, scraping or other seasonal maintenance to docks can
introduce contaminants into the water column. To avoid potential
pollution caused by this type of dock maintenance activity, the Maine
State Planning Office (1997) discourages painting and staining,
suggesting that "all coatings pose a local environmental threat, damage
floatation materials, and have only minimal effect on a structure's
longevity."

*Impacts from Fuel Leakage---*

Fueling that takes place at small docks generally consists of pouring
fuel from a portable tank into an outboard engine's fuel tank---often
with the engine attached to the stern of the boat directly over the
water. This offers the opportunity for spillage or overflows. Poorly
designed or maintained engines also may discharge fuel during operation.
Petroleum products in marine waters can have significant impacts to be
discussed further in the following section on boating impacts.

**Impacts of Small Docks on Sediments and Sedimentation---**

During a permit review or planning exercise, coastal managers sometimes
hear concerns that small, pile supported docks may cause changes to
sediments topography and composition in the vicinity of the structure.
This may be attributed to erosion, increased sedimentation, or
resuspension and movement of specific particulate sizes or types.
Generally, one of the following three mechanisms are suggested:

1.  Changes in water movement due to pilings redirecting water flow or
    speeding movement around the pile resulting in scour,

2.  Disruption of sediments during piling installation,

3.  Suspension of sediments as floats or boats attached to docks touch
    or approach the bottom at low tides and lift sediments as they rise
    with the tide ("pumping").

# *Altering currents---*

Structures placed in moving water have the capability to disrupt the
water's flow. Piles may cause increased flow rates immediately around
their base leading to scour and erosion. They may also lead to a general
slowing of flow over the area of the dock, resulting in settling out of
sediments carried by the current. The resulting changes in sediments
caused by scour or deposition may affect fish shellfish or habitat.

There appears to be very little in the way of research results available
on the impacts on sedimentation from small pile supported structures.
What research has been reported was done in open ocean settings, not in
embayments, and most focused on the morphological changes to adjacent
shorelines and bottom topography---no information was located on the
nature of sediment type change, if any, over time in the vicinity of
pile-supported piers.

What literature was located was done in the 1970s. Noble (1978) assessed
the impacts of 20 piers---all situated within the Southern California
Bight. These piers ranged from 625--2,500 feet in length and 15--300
feet in width---far larger than the small recreational facilities under
consideration here. All of the piers studied had pile spacing greater
that 4 times the diameter of the piles. Noble found that these piers
"had a negligible effect" on sedimentation and erosion of adjacent
shorelines. He notes that his results support prior findings of Johnson
(1973) and Evert and DeWall (1975).

Miller *et al.,* (1983), researching the impacts of an 1,840-foot long,
20-foot wide pier near Duck, NC on the Atlantic coast found that the
pier produced a permanent trough under the pier reaching a maximum depth
of 9.9 feet. Scour around individual pilings was noted to be on the
order of 3.3 feet in depth. The pilings in this case are 30 and 36
inches in diameter spaced 15 feet on center across the pier and 40 feet
on center along its length.

In an engineering study related to Lagoon Pond on Martha's Vineyard, MA,
Poole (1987) suggests that, "At a wind angle of 90º to a 50-foot pier
with 5 pilings on each side \[diameter of pilings not noted--ed.\] can
\[sic\] produce eddy currents and flow friction 2 times the diameter of
the pilings---minimally. This means...a 30 percent reduction in flow.
The area or parallel shoreline affected by the flow reduction would be a
factor of 2 to 3 times the pier length. Properties within 100 feet to
150 feet of a 50--foot pier could be subjected to wrack algae
accumulation, sand deposition and shellfish population changes." This
evaluation cites no research results and appears to be based on
predictive engineering calculations.

# *Disruption during pile installation---*

Anecdotal evidence suggests that the method of piling installation can
produce changes in sediment type and bottom morphology in the vicinity
of a dock (Ziencina, 2002 pers. com.) Jetting of pilings (jetting uses a
high pressure water pump to blow a deep hole in the bottom. The piling
is set into the hole and sand packs back around the piling) tends to
cause greater disruption than driving piles with a drop hammer. Jetting
suspends sediments and can disrupt adjacent vegetation resulting in bare
areas around pilings that are subject to scour. Shaefer (2001) found
bare areas with a diameter of 35--78 inches around pilings in St. Andrew
Bay, FL. Using a low pressure pump to produce a starter hole and
subsequent insertion of a sharpened pile with a drop hammer in a sandy
area "reduces the physical removal and disturbance" of seagrasses in the
area of the piling and results in little to no sand deposition around
the pilings (Shaefer, 2001)

# *Pumping of sediments from floats or docks resting on or near the bottom---*

Observational evidence indicates that changes in sediments occur when
floats or boats are allowed to settle on the bottom at low tide
(Ziencina, 2002, pers. com.). As the floats rise they create a suction
that resuspends sediments---the sediment is "pumped" into resuspension.
Additionally wave refraction in a downward direction may also resuspend
some sediments (Ludwig, 2003, pers. com.).

**Impacts from Boating Uses Associated with Small Docks---**

######## 

Most small docks are associated with boat traffic. Being situated at the
interface between land and water, at least a portion of each dock is in
the intertidal zone and extends into or through shallow areas. In many
cases this can lead to environmental impacts. Because docks are in the
shallowest areas of an embayment and are the location where refueling
may take place and engines are started and stopped, impacts are apt to
be particularly significant. Propeller scarring of vegetation and "prop
dredging" of sediments are perhaps the most visible impacts in the
shallow waters adjacent to docks.

In 1994, a workshop on the impacts of boating was held at the Woods Hole
Oceanographic Institution (Crawford *et al*., 1998). A number of
potential boating-related impacts were discussed although no
differentiation was made between general boating activities and those
taking place in the vicinity of docks. While noting that there were
adverse impacts, the presentations revealed that there were limited
quantitative data available that could be used as the basis for
management decisions---although it was agreed that sufficient data exist
to "substantiate the inference that recreational ... motor boat traffic
is far from a benign influence on aquatic and marine environments." A
second symposium on the topic, "Impacts of Small Motorized Watercraft on
Shallow Aquatic Systems" was held in 2000 at Rutgers University. The
results of this symposium were published in Kennish (2002).

Both workshops identified several issues of concern regarding boating
activity including:

-   Impacts to submerged aquatic vegetation,

-   Contamination from fuel discharges,

-   Erosion on shorelines, and

-   Resuspension of bottom sediments and turbidity.

# 

# *Impacts on submerged vegetation---*

Boat propellers can directly damage submerged aquatic vegetation in
shallow waters (Phillips, 1960; Thayer *et al.,* 1975; Zieman, 1976;
Eleuteruis, 1987; Kruer, 1998; Burdick and Short, 1999); impacts that
may take years to heal. *Thallasia sp.,* for example, can take four to
six years to recolonize a prop scar (Kruer, 1998). Damage to the plants
and their rhizome system

![](media/image2.jpeg){width="2.9298611111111112in" height="1.93125in"}

![](media/image3.jpeg){width="2.9319444444444445in"
height="1.9083333333333334in"}

**Figure 3.** Prop scarring in Waquoit Bay Massachusetts. From Crawford
(2002)

######## 

######## often leads to both reduced wildlife habitat and destabilized sediments. Zieman (1976) reported that most propeller scarring takes place in water less that 1 meter (3.3 feet) in depth. Research in and around Corpus Christi Bay found that 39 percent of the seagrass meadows were either moderately (5--20 percent) or heavily (\<20 percent) scarred based on the percentage of the area of the beds compared with the area of the propeller scars (Dunton and Schonberg, 2002).

######## 

######## 

######## *Contamination from fuel discharges---*

Outboard motors have long been associated with polluting of waterways.
Milliken and Lee (1990) provide a good summary of the early literature.
Two-cycle engines release up to 20 percent unburned fuel along with
exhaust gases (Moore, 1998). Moore (1998) compared the polycyclic
aromatic hydrocarbon (PAH), a carcinogenic organic molecule found in
petroleum products, output from a two-cycle outboard engine with that
from a four-cycle engine. The tests were run in tanks containing fresh
water. The two-cycle motor discharged five times as much PAH as the
four-cycle engine based on levels in the tanks. Most of this difference
was due to a reduction in discharge of 2- and 3-ring compounds in the
four-cycle. However, he found little difference between the levels of
discharge of 4- and 5-ring compounds---those generally related to
chronic toxicity. Albers (2002) notes that PAH concentrations in the
water column are "usually several orders of magnitude below levels that
are acutely toxic," but those in sediments may be much higher.

Even when PAHs are found in coastal waters it is difficult to relate
them directly to small dock use. Sanger and Holland (2002) looked at PAH
levels in tidal creeks in South Carolina but were not able to
distinguish PAHs from dock-related activities from other anthropogenic
sources. Additionally, it is difficult to differentiate between general
recreational boat use and that associated with small docks (Sanger, in
Kelty and Bliven, 2003)

######## *Shoreline erosion---*

Boat wakes, which lap at the shoreline, can contribute to increased
shore erosion (Zabawa *et al.* 1980; Camfield *et al.* 1980; Hagerty *et
al.*, 1981). Most of these relate to boats moving at or near maximum
speed through waterways. If boats are moving at a speed slow enough to
avoid leaving a wake, there will not be shoreline erosion. There was
little found in the literature that pertained specifically to boats
maneuvering near docks or landing areas

######## *Resuspension of bottom sediments and turbidity---*

Running a motorized boat through shallow waters produces two distinct
types of wake (Crawford, 1998):

-   The primary wake (or bow wake) that is related to water displacement
    by the boat that moves out to the side and can cause bank erosion,
    and

-   The secondary wake (or prop wash) related to engine and propeller
    effects that moves behind the boat and down and causes sediment
    resuspension and damage to submerged aquatic vegetation.

The secondary wake does not fan out as does the surface wake and
consequently has localized impacts. Hartge (1998) compared prop-driven
boats with those that were water-jet propelled and noted no major
differences between the amount of resuspension of sediments; he did note
that slow-moving, heavy laden boats caused more turbidity than lighter,
faster moving boats. Modern planning hulls (hulls designed to climb
towards the surface of the water as power is applied, thus reducing the
amount of wetted hull surface and reducing the friction or drag) also
have a far lesser impact on bottom sediments (Crawford, 1998; Hartge,
1998). Secondary wake impacts are difficult to quantify accurately
because they vary widely from boat to boat and based on environmental
conditions. Propeller thrust characteristics are highly variable
depending on:

-   Propeller size,

-   Thrust angle,

-   Clearance over bottom,

-   Engine power,

-   Hull shape,

-   Operating conditions (*e.g.,* speed, state of the tide, weather,
    number of passengers, and

-   Operator choices. (Crawford in Kelty and Bliven, 2003).

Despite the ongoing research described above, there has been limited
progress in finding quantifiable, predictable impacts from boating uses.
This led Crawford (in Kelty and Bliven, 2003) to offer the following
conclusions.

-   Using sediment resuspension to assess impacts is not recommended
    because of the wide range of factors involved.

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-   Small-scale measurements of wave impacts are too variable; the
    broader the scale the better.

-   It is difficult to ascribe generic impacts to an activity like
    boating that has such a wide range of variables.

-   More research is needed---however the research is expensive and very
    time consuming.

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