Increases in turbidity

Variables in boat-induced turbidity

Sub-aqua induced turbidity

The relative impact of boat-induced turbidity

Water aeration

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

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 induced turbidity

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

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 column.

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 variables

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 turbidity.

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.

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