What are coastal sediment processes?
The term “coastal sediment processes” refers to the forces that erode, transport and deposit sediment along shorelines. The coastal environment consists of constantly changing conditions, caused by the forces of wind, waves, currents and tides. Beaches are composed of sediment of various sizes, from large boulders to fine sand or mud. Erosion occurs when sediment is removed from a particular location (e.g. by wind, rain or waves); deposition occurs where sediment is added to a location.
Where does sediment come from?
Sediment can be derived from a number of sources:
Example of coastal bluffs (Dallas Rd., Victoria) composed of glacial till
- Many sources are deposits of glacial till (mixtures of clay, sand, gravel, and cobbles left behind by glaciers). Seaside cliffs or bluffs composed of glacial till are eroded by the combination of waves and high tides. Inland glacial deposits can be eroded by water and deposited in the ocean by rivers and streams.
- When waves converge on the headlands of rocky shores by the process of refraction (see waves section), they erode the rock and sediment from these areas and deposit it in pocket beaches.
- In tropical areas, deposits of the shells and skeletons of marine organisms contribute to the formation of white sand beaches.
- Deposits of volcanic ash and lava create black sand beaches in some areas.
- In areas near inland deserts, offshore winds can transport sand into the ocean, where it is later washed up on the shore.
- Most of the sediment in the Victoria area comes from glacial till.
How do coastal sediment processes occur?
As described in the waves section, refraction causes waves to bend to meet the shoreline. Yet, when waves initially contact the shoreline on angle, their energy creates a current that travels parallel to the shore, between the breaking waves and the shoreline. This is called a long shore current, as shown below. As these waves break and recede along a shoreline, they erode and deposit sediment in a zigzag pattern called long shore transport.
Image: Coastal sedimentation process
Long shore transport can create landforms such as various types of beaches, bars, spits and barrier islands. A good local example is the spit commonly referred to as Coburg Peninsula, which encloses Esquimalt Lagoon; this was formed by long shore transport with sediment from a gravel pit to the southwest. Farther west, a sand spit that encloses an estuary at Witty’s Lagoon, was formed in a similar way from a different sediment source.
The form shaped by sediment deposition and erosion in turn affects the energy of waves on the shoreline. For example, gently sloping sand beaches dissipate wave energy and help to protect the land behind them from damage by storms; steep beaches reflect wave energy. Cliffs that are undercut from waves can eventually slump and release more sediment.
How do coastal sediment processes affect marine life?
Sand and gravel shorelines provide habitat for a variety of marine life. For example:
- Many types of shorebirds feed on shellfish that burrow in sand, mud and gravel
- Among sand dunes and driftwood, delicate plants grow and help to stabilize the shifting sand
- Sand and gravel bars provide resting and birthing areas for harbour seals and stopover areas for migratory birds
- Eelgrass grows in soft sediments, at and below the low tide line, and supports many types of fish and invertebrate species
Sand and gravel beaches require a constant supply of sediment in order to maintain equilibrium between erosion and deposition. When the sediment supply is cut off due to the construction of seawalls, groynes (a ridged structure that interrupts water flow and limits the movement of sediment), or other structures, these beaches and habitat are often lost.
Spits that form near river mouths can create estuaries such as Esquimalt Lagoon and Witty’s Lagoon, which are highly productive ecosystems characterized by a mix of salt and freshwater influences. Pocket beaches are formed from sediment deposition in small coves and provide diverse habitat among rocky shorelines.
On rocky shorelines, constant erosion keeps sediment from accumulating and creates a different kind of ecosystem characterized by attached species such as kelp, mussels and barnacles.
How do coastal sediment processes affect people?
Throughout history, people have lived on shorelines and beaches, where they benefited from access to rich hunting and fishing grounds and trade routes. Shorelines are still one of the most valued places to live, and people spend significant time and money travelling there for recreation. People also use beaches for subsistence activities such as shellfish and seaweed harvesting. Some shoreline ecosystems such as estuaries and eelgrass beds provide habitat for commercially important species including salmon and herring.
Sediment processes therefore have many important social implications. Natural erosion and deposition are necessary to maintain the desired values of shorelines; human development can drastically alter these processes.
How do people affect coastal sediment processes?
The usual strategy to prevent or reduce erosion is to “armour” shorelines with rock, seawalls, bulkheads and other hard structures.
Seawalls or bulkheads are built along shorelines and are used to reduce erosion caused by waves. As discussed in the waves section, they can be effective in this task in the short term, but they substantially alter sediment processes.
Over the long term, seawalls may actually increase erosion due to increased scouring at the base. In a long seawall, this can result in a total loss of beach sediment. The fine sediment is removed first, and gradually, a beach that was predominantly sand changes to gravel, cobble and finally bedrock or clay. When the beach in front of a seawall is lost, the productive intertidal habitat is also destroyed, except for a very limited zone along the wall where the tide rises and falls.
Small seawalls in front of individual lots can also cause erosion, particularly at the sides, where wave energy is concentrated. This can lead to a “chain reaction” of seawall installation as adjacent property owners feel compelled to protect their own shorelines from erosion caused by neighbouring seawalls. This process has been at work in the Gorge Waterway and in Victoria Harbour.
Image: Human impacts on coastal sedimentation processes
Bulkhead (seawall) causing a loss of beach sediment
Armouring shorelines with rock, cement or other hard surfaces can prevent them from supplying sediment to other locations. For example, coastal planners in California are grappling with the prospect of losing many of their fine sand beaches.
Groynes are walls that jut out perpendicular to the beach. As shown below, groynes cause sediment to be deposited on one side and eroded on the other. While this strategy protects the property on the left, the down-drift beach on the right becomes eroded. As with seawalls, one groyne can cause a “chain reaction” and result in numerous others erected in a short time by neighbouring property owners.
Alteration of sediment transport by a groyne
How can coastal erosion be managed or avoided?
Wisdom is growing about how to best enjoy coastal living without damaging fragile ecosystems. The processes of erosion and deposition are beginning be viewed as part of the natural dynamics of shorelines, rather than as problems that can be “engineered” away. Damage to property and habitat can often be avoided by learning what types of processes to expect in a particular shoreline, and using natural techniques to increase its ability to withstand disturbances.
Options to decrease shoreline erosion include the following:
- Community planning can prevent risk of erosion damage in the first place, for example by requiring developments to be set back from the shoreline a minimum distance.
- Natural landscaping techniques, such as reducing water use, managing drainage, replacing drift logs and planting native vegetation along shorelines can help to prevent erosion. (See the shoreline development techniques section.)
- Paving on the top of slopes and bluffs can be reduced or limited, to prevent excessive runoff of rainwater, which can increase erosion.
- Living fences can be created with vegetation such as willows, that keep soil from eroding on slopes, first with the structure of the fence and then with plant roots.
- In some cases obsolete dams can be removed, which can help to replenish coastal areas with sediment.
- Government regulation and policies can be enacted to support “managed retreat.” In some areas the cost of trying to stabilize a volatile shoreline may exceed the benefits, and retreat is the best option.
- “Beach nourishment” involves replenishing the local sediment supply with foreign sources. This technique has been successful in some cases, however sediment must be carefully chosen to be of the same size and shape as the native sediment; otherwise, it can be washed away.
- Offshore structures such as breakwaters can be constructed parallel to the shore, in sections, so that long shore transport is not completely blocked. This can help to prevent beach erosion. However, sediment transport can still be significantly affected, and these structures are expensive to build and maintain.
- In some established industrialized areas, standard shoreline armouring (with rock, seawalls, etc.) may be the only option to maintain current land use and prevent erosion.
A full cost-benefit analysis, that takes into account the ecological and recreational values of an area, can help to weigh the options.