Boat generated waves and erosion

Impact of wash

Although bank erosion is largely a natural process it can be accelerated and accentuated by various human activities in, and around, the marine environment. The consequences of erosion can lead to important habitats being lost and the silt from the disappearing banks clogging up the waterways. The eroded material is also likely to contain high nutrient levels, which further deteriorates water quality. In fast flowing, open coastal areas, wash from recreational craft is unlikely to have any impact on bank erosion. In sheltered areas, there is more potential for impact, although it depends upon the characteristics of both the boat and also the shoreline itself. For example, the British Water Ski Federation estimates that on intensively used sheltered sites less than 0.5% of all waves which break on the shore are produced by water-ski craft.

Boat Generated Waves and Erosion

As a craft moves through the water, the water surface rises and falls, generating waves. At certain speeds (depending on the nature of the craft) waves from the front of the craft will meet with waves generated at the stern and a large trough can be generated behind the craft. In motorised craft where a propeller is fitted the trough may be accentuated. The magnitude of the resulting waves depends on a number of factors:

• the speed of the craft:
• the size of the craft:
• the craft displacement:

Other factors which may contribute to the magnitude of wash created by boats include:

• distribution of weight within the vessel
• the craft’s volume as distributed along it’s length (the prismatic coefficient)
• the underwater shape of the hull

Impact of Wash

Whilst it is the nature of the craft that influences the size of the waves, it is the nature of the water body and shoreline which are the main determinants of the actual impact of the waves. The following factors all influence the magnitude of impact that wash can have on shoreline erosion:

• type of sediment
• the orientation of the shoreline
• the profile of the shore
• bounce back from estuarine shores and hard flood protection schemes

Shorelines which are soft or easily eroded will suffer more from increased wave action than a rock face or rocky shoreline. This is illustrated by a study conducted by Zabawa and Ostrom (1980) which tested the impact of boat waves at five sites in Chesapeake Bay. It found that at four of the sites there was no increase in shore erosion which could be attributed to boating during the summer. In their analysis, the impact of wave energy is seen to crucially depend on the characteristics of the shore. The research found that a combination of the following shoreside characteristics will make the shoreline most susceptible to erosion:

• an exposed point of land in a cove
• a shoreline of easily eroded material
• a steep near shore gradient
• a location which experiences concentrated boating activity near to the shore
• the level of water on the shore

The distance of the craft from the shore has important implications for the possible impacts of its wash. The further a craft is from shore, the less impact its waves will have, as their energy will have dissipated on reaching the shore. The study suggested that a significant contribution to erosion from boat induced wash is likely only when there is a high frequency of boat passages close to the shore.

This illustrates the need to identify not only the nature of the boat induced waves but also the characteristics of the shoreline. The one site in the study which experienced a change in profile during the boating season, attributable to boat-derived wave energy, had a combination of the above four characteristics. These studies, although relevant, should however be treated with caution as they do not take into consideration differences in boat design, speeds and overlapping wash patterns.

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