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Coastal Processes

The Physical Shoreline is a Result of Three Main Natural Processes

Waves:  waves are the main energy source in the coastal zone and create most of the erosion, sediment transport and deposition shaping beaches, sand spits and other features.  The force of waves is a result of the wind velocity, the straight distance (or fetch) that winds travel over water, and the length of time a wind blows.  Generally the longer the fetch, the larger the waves.  Most shorelines in the Gulf Islands are classed as medium exposure where wave fetches are limited to 10-50 km.

   Wave Fetch for South Pender Island

Example of Wave Fetch

Sediment Movement:  sediments along a shoreline are constantly moved by waves and currents along, onto and away from the shore. Longshore drift is the movement of sediments parallel to the shoreline where a sediment source, transport zone and deposition (or accretion) zone creates what is called a drift cell.  There can be many drift cells along a shoreline and they are mainly determined by the direction of the predominant currents and waves.  Sediment on the shore comes from eroding shorelines, freshwater streams, or drainage outfalls.  Headlands and pocket beaches affect the longshore current strenght and direction.  The perpendicular movement of sediments onto and away from the shore is referred to as the onshore/offshore transport and is typically a seasonal occurence where winter storms moving sediments away from the shore and calmer summer waves move sediment back onto the beach. 

             Longshore Drift Cell

Example of a Longshore Drift Cell

Water Levels: tides and storm surges can have immediate impacts on the shoreline, while gradual increases due to climate change may have longer term impacts.  The highest tides occur in December and when combined with winter storms they can have dramatic results moving large amounts of sediment,  heavy logs and debris.

Site Conditions

The energy system of a particular site will be determined by the wave and wind exposure, the predominant direction of storms, and whether the shoreline is eroding, stable or depositional.  For example, on South Pender Island the majority of strong winds and storms come from the southeast resulting in the southeastern shoreline having a medium exposure rating while the rest of the island has a low exposure rating.   Although some winter storms come from the northeast, South Pender Island is protected from winds from that direction by North Pender Island. 

Energy Exposure Categories

High energy exposure sites are exposed to the full force of the predominant wind with a considerable fetch. There are limited high energy locations in the Gulf Islands with most occurring on the east facing coasts of Hornby, Galiano, Mayne, and Saturna where maximum wave fetches are greater than 50 km. Large waves generated during south easterly storms are capable of moving cobbles and small boulders.       

Medium energy exposure sites are shores parallel to or at an angle to the predominant winds. Sites may be exposed to summer winds. This is the most common exposure in the Gulf Islands where wave fetches are limited to 10-50 km. Waves generated by storms may move cobble, but generally cobble is sufficiently stable that attached algae and fauna are common.            

Low energy exposure sites are protected from the predominant waves.  They have maximum wave fetches of 1-10 km. They are typically stable enough that shore pebbles can support attached algae and fauna.

Very low energy exposure sites are highly protected from wave exposure with a fetch less than 1 km. They may have poor water circulation which increases vulnerability to pollutants.  These sites are often favoured by boaters for anchorage and marina locations.

Types of Beaches in a Longshore Drift Cell

Eroding shores are typically associated with higher energy environments such as headlands or points and sites with exposed sediments as well as in high-current tidal channels. Eroding shorelines feed the longshore system and preventing erosion can severely impact downstream beaches.

Transport shores are located between erosion and depositional areas. They are relatively stable but blocking any sediment movement along these shores can have significant downshore impacts.

Depositional shores are typically associated with lower energy environments. They include shoreline features such as sandy beaches, pocket beaches, mudflats, and estuaries. Because sediments are accreting, there is a potential to accumulate contaminants.

Coastal Process Summary

How is the type of beach influenced by the energy exposure?

 

Page last updated: 01/10/15
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