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.
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.
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?