Waves are disturbances that cause energy to be transported through a medium (e.g. air or water), with very little transport of the medium itself. In other words, although a wave can appear as a moving “wall” of water, the water molecules themselves are not being transported. Waves are defined with respect to their height (measured from trough to crest), length (measured from crest to crest), the frequency (how many crests pass a point in a given amount of time) and the wave period (the time it takes two successive crests to pass a given point).
How are waves formed?
Most waves are formed by the wind (the exception is tsunamis, which are caused by earthquakes). Downdrafts of wind depress the ocean surface momentarily. A ridge is formed along with the depression, and, like a ripple in a pond, the wave travels away from its point of origin. As waves join together, they grow larger. The size of waves is controlled by three factors:
- the strength of the wind
- the duration of the wind
- the distance of open water the wave travels (the fetch)
What causes waves to break?
Waves in deep water have a symmetrical “sine-wave” shape, where the crest and trough are smooth curves of equal size and shape. When a wave approaches the shore, the trough becomes depressed by the sea bottom, and the wave grows taller. At the same time, friction with the bottom causes the lower part of the wave to slow down while the top continues to move at the same speed. Eventually, the wave topples over or “breaks.”
How a wave breaks is determined by the shape of the shoreline. Shallow, sloping beaches cause waves to “spill” or gently break down the face. Slightly steeper beaches cause “plunging” breakers with a curling break or tunnel; these are the classic surfing waves. Beaches that are steeper still can create “collapsing” waves that break by falling all at once along the crest. “Surges” are very powerful waves that form on steep beaches during storms. They don’t actually break, rather they heave or roll up the beach with tremendous force that can cause damage to property and shorelines.
Since the harbours in the CRD are relatively protected from the open ocean, the fetch is limited and large waves do not usually affect the shorelines. An exception is along Coburg Peninsula in Esquimalt Lagoon, where winter storms with southeasterly winds sometimes cause large waves and occasionally storm surges. Other areas along the Victoria waterfront are more exposed to large breaking waves.
How do waves affect shorelines?
In calm conditions, waves generally “build” beaches that have mobile sediment (sand, gravel and cobble) by transporting sediment onshore. During storms, strong currents under the breakers (“undertows”) draw water and sediment back from the beach; this causes beach erosion. Therefore, waves can substantially change the shape of beaches according to the seasons. In the Victoria area, this change is apparent when beaches that are sandy and have shallow slopes during the summer shift to having more pebbles and steeper slopes in the winter. Coburg Peninsula in Esquimalt Lagoon and beaches along the Victoria waterfront undergo these changes.
Extreme storms can have greater effects, for example by completely destroying sand bars or other sediment deposits. Generally, the harbours of the CRD are protected from major storm waves. However, sometimes the sand spit known as Coburg Peninsula in Esquimalt Lagoon is exposed to southeasterly winds during the winter that can cause storm surges. Historically, these waves probably washed over the spit.
On a longer time scale, waves also erode bedrock and sandstone cliffs, and create forms such as sea caves and sea stacks; these can be seen in some locations on the west coast of Vancouver Island. They also erode more easily erodible bluffs and transport the sediment offshore or along the shore. This process builds sand and gravel beaches nearby. Unstable bluffs make poor building sites because of their vulnerability to erosion and slope failure. In order to avoid property damage, developers are well advised to preserve as much natural vegetation as possible, and to set structures well back from the shore. Examples of erodible bluffs in the Victoria area include Royal Bay, in Metchosin, and the waterfront along Dallas Road between Clover Point and Finlayson Point.
Refraction of waves is an important shoreline process. This occurs when a wave encounters the shore on an angle. The part of the wave that first touches the bottom slows down, whereas the rest of the wave in deep water travels at its original speed. This causes the wave to bend to meet the shoreline. Refraction causes waves to converge at convex points such as headlands, where the increased energy causes erosion. Conversely, divergence and deposition of sediment occur at concave areas such as coves. This process is responsible for the formation of pocket beaches, which can be seen in many areas around Victoria.
Image: Wave refraction along a straight shore
How are waves patterns altered by development?
In an effort to protect the shoreline from erosion caused by waves, people often construct groynes, bulkheads, seawalls and breakwaters (for example, the Ogden Point in Victoria).
These structures are sometimes necessary, for example in highly industrial areas, and they can be successful in reducing damage by waves. However, they significantly alter natural wave patterns and sediment transportation. For example, armouring soft sediment shorelines with rock and cement prevents them from acting as sediment sources to replenish nearby beaches.
Coastal Sediment Processes
Sometimes overall erosion increases when seawalls or bulkheads are installed, because in place of a gently sloping beach that dissipates energy, waves encounter vertical walls. These reflect waves back offshore, resulting in larger waves with more energy, and increase scouring at the base of the wall. A local example of this effect is at Ross Bay in Victoria, where the seawall has become undercut. Particularly along shorelines that have not been altered on a large scale, there are alternatives to seawalls and bulkheads that can be more effective at preventing erosion. (See Related Information, below.)
How do waves affect marine life?
Waves, in combination with currents and tides, help to supply nutrients to marine plants and animals that live along the shorelines. Waves throw water up the shore, allowing intertidal plants and animals to survive in niches that would otherwise be dry. In areas with larger waves, the intertidal zone extends farther up the shore.
In controlling the transportation of sediment, waves affect the type of habitat that develops along a particular shoreline. For example, sand and gravel shorelines occur because waves and currents have eroded sediment from one source and deposited it another. The types of plants and animals that live on these shorelines differ substantially from those on rocky shorelines.
When beach slopes are transformed by waves, this also changes the area of the beach that is exposed at low tide, and determines the available habitat for intertidal species. For example, steep beaches have relatively limited intertidal habitat, compared to mud flats.
Waves are also responsible for natural cycles of death and rebirth in intertidal and subtidal areas. For example, storms and large waves can tear up kelp beds, and deposit the plant material on beaches where it nourishes shoreline plants and animals. Strong waves can also tear attached species such as mussels from the rocks, allowing other individuals or species to colonize that space.
In the harbours around Victoria, the areas most exposed to waves are located at the entrances to Esquimalt Harbour and Victoria Harbour, along the Victoria waterfront, and on shorelines west of Sooke. Except for sheltered coves in these areas, the shorelines are generally rocky, and dominated by animals and algae well adapted to the pounding forces of waves.