What are coastal sand dunes?
At the most basic level, dunes are simply piles of sand. Sand dunes are present on shorelines where fine sediment is transported landward by a combination of wind and waves, and stabilized with vegetation such as native dune grass
). Primary dunes (or fore dunes) are situated nearest to the ocean and are affected most significantly by waves and salt spray. Secondary dunes (or rear dunes) are located further inland and are not often directly exposed to marine influences.
Dunes undoubtedly are one of the most well-known features of sandy beaches, yet they are also misunderstood and abused. Dunes can help protect coastal property from the destructive forces of storm surges and tsunamis, but human activities have had severe impacts on coastal sand dunes.
Where are coastal sand dunes found in the harbours around the CRD?
Sand dunes are found at Witty’s Lagoon in Metchosin, Albert Head in Colwood, and Island View Beach in Central Saanich. Other well-known sandy beaches with dunes include Long Beach, on the west coast of Vancouver Island, and Savary Island, on the Sunshine Coast.
The only coastal sand dunes in the main urban harbours of Victoria are found on the sand spit known as Coburg Peninsula in Esquimalt Lagoon. This sand dune system is quite small, less than 2.5 hectares, and has been substantially impacted by human use. However, these dunes do still provide plant and wildlife habitat and are worthy of protecting and restoring.
How do coastal sand dunes form?
Dunes are formed when wind and waves transport sand onto the beach. The ultimate source of the sand is often glacial till that resides offshore, or is eroded from nearby cliffs by waves and transported along the shoreline by longshore currents (see coastal sediment proce
sses). Rivers can also transport sand to the coast from inland deposits. Waves wash this sand up on the beach, and also remove it during storms.
One beach can vary quite substantially with the seasons. During the winter, stronger waves pick up more sand (but leave behind larger sediment), causing the beach to be steeper and coarser. In summer, gentle waves transport sand onshore and the beach takes on a shallower slope with finer sand.
The shape and size of dunes formed depends on the shape of the beach. On a shallow-sloped beach, the energy of waves is dissipated and the suspended grains of sand are deposited, resulting in greater dune potential. Steeper beaches do not dissipate this energy as well, and sand is more frequently washed back out to sea. Consequently, dunes on steeper beaches are smaller.
Once sand has been deposited on a beach, it is transported by the wind. When the prevailing wind direction is onshore, the sand gradually migrates landward from the water’s edge. Shallow slopes of sand form, grains move up the slope and are dropped on the steeper lee (“protected”) side of the pile, where the wind velocity is lower. When plants colonize dunes, their roots and stems help to anchor the shifting sand. The structure of plant stems and leaves create small pockets of protection from the wind, causing more sand to accumulate.
It is important to note that sand dunes undergo a continual cycle of erosion (breaking down) and accretion (building up) with the wind and waves. When structures are built so close to a beach as to prevent this natural fluctuation, overall erosion of the beach and loss of dunes can occur. This leaves a shoreline much more vulnerable to damage from storms.
What lives in coastal sand dunes? Native dune grass (Leymus mollis)
Plants and animals that live among sand dunes must be able to tolerate wind, sand abrasion, sand burial, salt spray, water deprivation and constantly shifting sand with low water-holding capacity and little organic matter. These harsh conditions limit the variety of species found in these environments, but surprisingly complex ecosystems still exist in dunes.
One of the first plants to colonize and stabilize newly formed dunes in this area is native dune grass (see photo). It normally establishes on the seaward side of coastal dunes.
Other plants found among sand dunes include sand dune bluegrass (Poa macrantha
), coastal strawberry (Fragaria chileonsis
), seashore lupine (Lupinus littoralis
), sand verbena (Abronia
spp.), sea rocket (Cakile edentula
), seabeach sandwort (Honkenya pepliodes
), beach morning glory (Convolvus soldanella
), kinnikinnick (Arctostaphylos uva-ursi
) and beach pea (Lathyrus japonica
). Some of these plants have beautiful flowers.
In many places, the imported European beach grass (Ammophila arenaria
) has crowded out native dune grass. European beach grass is more successful at stabilizing dunes, often forming a thick vegetation barrier, starving secondary dunes of sand and providing less habitats for other plant and insect species. For example, pink sand verbena (Ambronia umbelatta
), an endangered species in Canada, is threatened in part by the predominance of European beach grass. Although European beach grass has not yet invaded the areas immediately around Victoria, it is a problem in Long Beach near Tofino and in many areas in Washington and Oregon.
Another invasive species
of concern is Scotch broom (Cytisus scoparius
), a hardy legume that grows in the poorest soils and whose seeds can remain viable for up to 30 years. This plant is a major invader around Victoria.
The animal life of sand dunes is fairly inconspicuous, and includes: amphipods; garter snakes; mammals such as mice and shrews; spiders, and insects such as caterpillars, beetles and flies. Birds such as the endangered Horned Lark (Eremophila alpestris strigata
), Semipalmated Plover (Charadrius semipalmatus
) and Killdeer nest among sand dunes.
Why are coastal sand dunes important?
Coastal sand dunes form a natural barrier against wind and waves, protecting inland areas from damage due to storms. They also provide habitat for plants and animals, including rare and endangered species. The structure of sand dunes depends largely on stabilizing vegetation such as native dune grass. Without it, the sand would simply blow away, as it does in areas that have been degraded by trampling and invasive species. Sand dunes, as a component of sandy beaches, are also popular recreation areas.
What threatens coastal sand dunes?
Coastal sand dune systems are highly vulnerable to disturbance by trampling. Pedestrians and motor vehicles can compact the sand and crush vegetation; for example, native dune grass dies when its roots are crushed. Without the stabilizing vegetation, the sand is blown away and dunes disappear. This can leave the shoreline more prone to damage from storm surges. Invasive species
are a major concern for the ecology of sand dunes. Scotch broom and European beach grass are two common examples. Because they are not native to the region, they often have no natural predators or other controls, and can create dense monocultures that crowd out other species. As they did not evolve along with all the other species in the area, they provide limited habitat values. Some invasive species are so well-established that their eradication is not feasible without substantial cost and effort.
Sand dunes can be destroyed when structures are built too close to the shoreline. As the coastline naturally erodes, these structures become threatened, and people often respond by building “protective” reinforcements such as seawalls. This can further degrade the beach habitat, and even distant seawalls can starve downdrift beaches of sediment (see coastal sediment processes
and altered shorelines
Dune habitats are often affected by alteration of shorelines in other areas. For example, bluffs composed of glacial till provide sediment that is eroded by waves and transported by longshore currents to the sand dune beach, where it is deposited. If the bluffs are armored to prevent erosion, with cement or rock, this sediment supply is cut off and the beach is gradually depleted of sand.
How can I help protect coastal sand dunes?
- When walking on beaches with sand dunes, try to stay on the seaward side of the dunes, where the sand has been compacted by the tides. Stay on marked or established trails or boardwalks, when walking through dune vegetation, and observe signs.
- Keep dogs under control and don’t let them dig in dunes or chase wildlife.
- Plant native vegetation along your shoreline property, to help prevent erosion and increase wildlife habitat. Learn about other options for reducing erosion that use natural shoreline development techniques instead of hard structures.
- Leave driftwood in place, rather than “cleaning up” the beach. Logs help to stabilize sand dunes, and provide hiding places for wildlife.
- Get involved with a local community stewardship group that works to protect and restore sand dunes.
- See other tips to help protect shorelines in general.
Additional Information & Links © Iimages courtesy of C. Glasgow & L. Townsend