Intertidal erosion

Re-suspension of sediments

Turbulence and aeration

The movement of ships through water may potentially affect the features of a marine SAC under certain circumstances, both through the generation of waves and propeller-induced turbidity in the water column. Ships generate waves, which get bigger and more energetic the faster the ship goes relative to its length. The magnitude of the waves generated by a vessel are related to the following variables:

• the speed of the vessel (as the speed of the vessel increases, the waves generally increase in size),
• the size and displacement of the vessel, and
• the distance between the vessel and the marine feature of interest (clearly the wave energy at the foreshore and hence the potential for erosion will be dependent upon the distance from the source of the wash, the form of the seabed and any other obstacles).

The energy in the waves is a function of speed and displacement. Therefore, the generation of ships’ wash will be highly specific to the type and design of vessel, and it cannot be assumed that the larger or faster a vessel the greater the wash generated as this is not always the case (i ). The wave energy generated by moving vessels should be considered in relation to the background wave climate in an area.

Examples of how vessel type can affect wash include the following considerations:

• Small fast power cruisers proceeding just ‘off the plane’ will make more wash than if they were in a fully planing mode at maximum speeds.
• High speed hulls, such as planing hulls or narrow low-wash catamaran hulls may produce little wash.
• Hovercraft make a depression in the water under the cushion which can have poor wash characteristics.

To date little research has been undertaken to investigate the potential impacts of vessel movements and ships’ wash on marine habitats, although this matter has recently received increasing attention. Review of the limited information available at present has identified the following more common effects:

• intertidal erosion of estuaries which may have a minor or adverse impact,
• resuspension of sediments which may have either adverse or beneficial effects, and
• aeration of the water column which would be considered beneficial.

A connection between ships wash and potential impacts on the erosion of intertidal flats and saltmarsh is difficult to establish because of the natural variability of the marine system, although the wash itself may be obvious as it breaks on the intertidal. Other potential causes of erosion include reduction in sediment supply and natural storm events. However, there are cases in UK estuaries where ships’ wash is considered to exacerbate rates of erosion of intertidal and shallow subtidal habitats. For example, there are concerns about the high speed ferry service operating between Harwich and the Hook of Holland which is known to create wave problems along the shores and shallow sandbanks of the Stour/Orwell Estuary. The ferry operator is reported to have taken appropriate action to address the ships’ wash problem that only occurs at critical depths and speeds (HR Wallingford & Posford Duvivier Environment 1998).

The distance of the designated marine feature to be protected from the main navigation channel is an important consideration in assessing the potential for erosion of a given shoreline, as wash energy dissipates relatively quickly. A boat that produces large waves some distance from the shoreline may have less impact then a vessel producing small waves closer to the shoreline (Zabawa & Ostrom 1980). This effect will rely to some extent on other variables such as the form of the seabed coming into play. The depth of water over which the vessel moves affects the size and energy of the vessel-induced waves. Within deep water the waves produced will be relatively smaller than in shallower waters where wave heights increase.

Rate of shoreline erosion is also critically dependent upon the composition of the shoreline. A shoreline that is soft and easily eroded will suffer more from increased wave action than a rocky shore. In coastal SACs the existence of intertidal foreshore will protect a feature behind it, such as Atlantic salt meadows or dune systems, by dissipating wave energy. Research has also shown that the following characteristics increase the susceptibility of a shoreline to erosion (Zabawa & Ostrom 1980):

• an exposed point of land in a narrow river,
• a steep near shore gradient,
• water level in proximity to vulnerable areas of the shoreline, and
• high levels of boating activity concentrated near to the shore.

The re-suspension of sediments from the bottom and margins of navigation channels as a result of vessel movements may present an issue in certain estuary, shallow inlets and bays, intertidal flat and subtidal sandbank habitats. Suspended sediment decreases the amount of light that penetrates the water column and therefore has an impact on plants and algae. This reduction in plant productivity has knock on effects to the rest of the ecosystem. The re-suspension of sediments may cause disturbance to sensitive marine animals, particularly due to a smothering effect as the sediments settle. Depending on the quality of the sediments, organic matter, nutrients, and contaminants may be re-released affecting water quality, by the removal of oxygen for example, with possible detrimental effects on marine animals and plants in the area. The potential impacts of sediment re-suspension are discussed in more detail in the dredging section (Section 5). In comparison with natural events, such as storms, which often cause large amounts of sediments to be lifted into the water column over large areas, shipping and boating activity represents only a minor source of localised re-suspended sediments.

Boat and propeller induced turbidity appears to be influenced by a number of variables including depth of water, levels of activity and sediment characteristics (case study).

In areas with high levels of suspended sediments, such as the Severn Estuary, the resuspension of sediments from vessel movements is likely to have little or no additional environmental effects on the benthic communities living in these turbid environments. Similarly, where the temporary resuspension of sediments occurs on a regular basis within the proximity of the navigation channel it is unlikely to cause any observable effects on the communities present which will be adapted to living with the disturbance in these locations. However, the impact of re-suspending sediments on communities in areas with low suspended sediment levels is potentially higher.

The potential for problems exist if ships’ movements result in erosion at the margins of the channel, and depending on the depth and characteristics of the sediments, this can cause temporary resuspension of sediment, which may be transported away from the site of erosion. The amount of sediment mobilised will depend on the speed, size and position of the vessel causing the ship wash in relation to the erosion site. Generally, resuspended material is likely to be deposited on to the channel bed rather than back onto the mudflats. A special case where sediment resuspension may occur, is when a vessel passes through a narrow channel, occupying a large proportion of its cross sectional area. Where this occurs the waves generated and the proximity of the hull to the bottom, ‘under-keel clearance’, can result in greater mobilisation of sediment from the bottom and margins of navigation channels.

Turbulence caused by the action of the propeller results in aeration of the water column. The increase in the dissolved oxygen content of the water column would be beneficial to the surrounding flora and fauna. Unfortunately, there has been little research into this area to establish a link (UK CEED 1993).

Concerns have recently been expressed over the potential impacts of large waterjet propelled vessels on marine life, in particular that plankton and marine micro-organisms would be destroyed by the rapid pressure changes as water passes through water jet propulsion units. However, concerns have been defused by a recent study into the effect of fast ferry operations carried out by the Danish Environmental Studies Institute which found that turbulence caused by waterjet propulsion units of fast ferries do not constitute a threat to marine micro-organisms (Hynds 1997). Even assuming the worse case that all micro-organisms that pass through the waterjets would die, the mortality rate would still only be a very small proportion of the total numbers of plankton in the vessel’s track and considering the short life span of a generation of plankton (generally less than 14 days), fast ferry movements are unlikely to have any noticeable effects. Further information

Next section