Increases in turbidity
Variables in boat-induced turbidity
Sub-aqua induced turbidity
The relative impact of boat-induced
The interdependence of the variables
In addition to the potential contribution boats
make to bank erosion, the movement of boats through water can
also cause disturbance to the bed of the water body, either
through direct contact or through the effect of turbulence created
by the vessel's passage. This disturbance is most evident in
the form of the stirring up of fine sediments from the bottom
of the water body which decreases water clarity in the water
column. Such turbidity can cause potential problems
for both aquatic flora, which depend on light transmission through
the water column for growth, and fauna which feed on the submerged
vegetation. It can also smother and suffocate benthic communities.
Variables in Boat Induced
Turbidity can be a result of a number of factors,
some natural and some human induced. The extent of boat/propeller
induced turbidity appears to be influenced by a number of variables
relating both to the boat and engine and to the characteristics
of the water body. These variables include the depth of the
water, the speed and power characteristics of the craft, the
characteristics of the propeller/water jet, the distance the
boat travels from the shore, the duration and extent of boating
activity and the characteristics of the water body sediments.
The depth of the water - boats have a greater
opportunity to create turbidity when travelling in shallower
water as the downward pressure of water created by the craft
reaches the sediment with greater energy.
The effect of engine power - a more powerful
engine has the capacity to create more turbidity than a smaller
one. It is thought, however, that a planing boat may create
less turbidity as the craft is effectively lifted out of the
water and, therefore, causes less underwater disturbance.
Anchoring - the potential effects of permanent
anchoring provisions such as swing moorings and pontoons are
discussed in greater detail in chapter 5. However temporary
anchoring while a craft is in use can have consequences for
seabed vegetation and communities. Anchor drag caused by inappropriate
anchoring can disturb the upper layers of the seabed sediment
and caused localised particle suspension. Such drag can also
cause damage to seabed habitats and species by cutting or breaking
plants as it drags over the surface.
Propeller and water jet action - the speed,
size and direction of a propeller will all influence its possible
impact on the water body bed. In general, a horizontally angled
propeller will have a lesser effect on the bottom sediments
as its energy will be more dissipated when reaching the water
body bed than that of a propeller which is more vertical in
orientation. Water jets, as used in personal watercraft, are
likely to cause similar disturbance to propellers.
Duration and extent of boating activity - the
overall sediment disturbance may be related to the time that
it has to settle between re-suspension.
Sediment characteristics - lighter sediments,
such as silt, will be more easily suspended than larger particles
such as gravel.
Sub-aqua induced turbidity
Sub-aqua activities can cause localised turbidity
through the effects of finning, the kicking action
of the feet. This can cause sediments to rise into the water
column and, in popular diving areas, act to reduce the amount
of sunlight filtering though the water column. This effect is
likely to be only temporary in nature and mainly related to
novice divers - experienced divers usually fin in a manner which
does not cause disturbance.
The relative impact of boat
Turbidity decreases the amount of light in
the water column so affecting aquatic flora and fauna. However
the relative contribution of boat-induced turbidity is still
relatively unknown, particularly in coastal locations.
Although boat-induced turbidity may be relatively
slight, the sensitivity of submerged aquatic vegetation to light
changes makes it vulnerable to small changes in light penetration.
Boat induced turbidity is often found to be the greatest during
the most sensitive growth phases of submerged vegetation. Low
densities of boat traffic are reported to be beneficial to vegetation
conservation in some low flow areas because they arrest the
process of natural succession and maintain a diversity of species.
It seems likely, however, that the increase
in suspended sediments caused directly by boat activity may
be small compared with the suspended sediment load present from
other sources such as sediment increases during high runoff
periods and changes due to tidal scour etc. Many site specific
variables will play an important role in determining the overall
contribution of boating to turbidity, and in particular the
sensitivity of the features to increased turbidity.
Water aeration is often cited as being a beneficial
impact of boating. The presupposition is that the turbulence
caused by the action of the propeller causes an increase in
the dissolved oxygen content with resulting benefits for vegetation
and wildlife. However, there is little research available which
either confirms or contradicts this theory.
This issue has been addressed scientifically
by Yousef (1974), who found that under certain conditions increases
in the total dissolved oxygen content were noted following boating
activity, although he noted that the oxygen transfer to the
water body depends primarily on the oxygen deficit in the water
However, in comparison to natural wave generation,
it is unlikely that motorised craft can contribute significantly
to aeration in water bodies. Even if an individual boat does
aerate the water this is likely to be more than offset by the
turbidity created by the same craft and oxygenation is likely
to be localised and temporary.
The Interdependence of the
It should be noted that many of these effects
are interdependent and cannot be separated. For example, the
energy dispersed at the shore from boat-induced waves may not
only have an effect on shoreline profiles, but may also cause
an increase in sediment with a consequent intensification of
In assessing the impact of boating on the physical
characteristics of the water and the surrounding areas, it is
essential that as many variables as possible are included in
the analysis. Boating activities can affect the physical properties
of the water but it is important that the impact is seen in
the context of other variables with a similar impact. For example,
the impact of boating on water clarity may, if observed in isolation,
be large, but in comparison to the effect of a rainstorm, with
consequent run-off and increase in suspended sediment, such
impact may be negligible.
Non powered craft such as sailboards and dinghies
tend to have less overall impact on marine features than powered
craft. As they are not mechanically powered, they are not associated
with issues such as engine emissions and noise, and oil and
fuel spillage. In addition, they do not discharge sewage, although
participants may be responsible for littering as with large
craft. These craft can, however, have localised impacts on features
through physical disturbance particularly at launch sites and
in shallow areas.