Altered Shorelines
Example of a typical altered shoreline composed of a seawall and
angular rock (diagram courtesy of Archipelago Marine Research Ltd.)
What are altered shorelines?
Altered shorelines are those shorelines that have been modified by people from their original form, with structures such as seawalls, docks and wharves. In some cases, the alteration has been on a very large scale. For example, many coves and pocket beaches in Victoria Harbour and Esquimalt Harbour were filled in with rock and soil, to create more land on which to build roads, buildings and industrial facilities. Materials used for shoreline alteration may include cement, angular rock (“rip-rap”), metal, wood or soil.
Although altered shorelines do support some communities of marine life, the diversity and quality of available habitats is usually much lower than in the unaltered condition. In many cases where human structures are well-established, restoration of the shoreline to its previous condition is not feasible. However, the habitat values of some altered shorelines can be improved, and environmental design techniques can be used in areas slated for development to reduce impact.
Photo (left): Seawall in Victoria Harbour Photo (right): Rip-rap shoreline
Where are altered shorelines found around Victoria?
Altered shorelines are found in all of the main urban harbours of Victoria (Esquimalt Lagoon, Esquimalt Harbour, Victoria Harbour, Portage Inlet and Gorge Waterway). They form 37% of the total shoreline of these areas. Esquimalt Lagoon has the least amount of altered shorelines.
What lives on altered shorelines?
Lichens, barnacles, periwinkles, mussels, anemones, sea stars and rockweeds can establish on rock and on some structures. Fish sometimes shelter under docks and wharves. Large angular rock (“rip-rap”) can provide slightly more habitat than smooth walls due to the increased surface area and hiding spaces provided by the rocks.
Why are altered shorelines usually less valuable as wildlife habitat?
- Hard structures that interrupt natural sediment movement can cause the loss of sand and gravel beaches and their associated habitat (see also the sand/gravel shorelines and coastal sediment processes sections).
- Altered shorelines are usually much more straight, and materials are relatively smooth, compared to natural shorelines. This reduces the available surface area for attached species.
- Seawalls made of cement or rock are usually much steeper than the natural shorelines they replace. While gently sloping beaches and shorelines dissipate wave energy, seawalls receive the full force of each wave. Few organisms can withstand these extreme conditions.
- Structures like docks and wharves can “shade out” vegetation such as eelgrass and kelp.
- Development can create a pathway for pollutants, from storm drains, runoff from yards, roads and parking lots, and construction materials such as pesticide-treated wood.
- Native shoreline vegetation fulfills several very important functions:
- it helps to hold soil in place with the roots;
- it provides habitat for wildlife such as birds and mammals;
- it helps to filter pollution that is washed off the land, before it enters the ocean;
- salt marsh vegetation in particular can help to absorb the destructive
energy of storm waves.
When natural vegetation is removed, for example in order to expand a lawn right up to the shoreline, these functions are lost or reduced. The results can include increased erosion and property damage, a degraded shoreline ecosystem and a greater vulnerability to invasive plant species.
- Despite these negative aspects of altered shorelines, in certain cases human-made structures can actually increase subtidal and intertidal habitat. If the natural sea bottom is relatively smooth, a structure such as an artificial reef increases the surface area available as hiding places for marine organisms. A local example is the Ogden Point breakwater in Victoria, which is now a world-renowned diving site. The fact that this breakwater is composed mostly of angular rock and cement blocks makes it more valuable as habitat than a smooth-sided breakwater.
How do altered shorelines affect sediment transportation?
Shorelines are most often altered in order to prevent erosion. However, an adage to keep in mind is: erosion is a process, not a problem. This is true because all shorelines naturally erode to some extent. The action of water, wind and waves breaks down rock and removes sediment such as clay, sand and gravel. This material is transported to other areas and deposited, only to be eroded again later. These processes build our treasured sand and gravel beaches. When they are interrupted, problems can arise:
- Seawalls and dikes prevent the natural landward/seaward migration of sediment, and change the energy of waves. This can create zones of increased erosion and scouring, alongside and at the toe of the structure.
- Gradually sloping beaches, dunes and vegetation can all provide natural protection from storm waves. When the shoreline is hardened, for example by building a seawall and a road, the gravel or sand beach is often scoured away and the shore becomes more vulnerable to damage.
- Breakwaters and wharves can prevent longshore currents from replenishing downdrift beaches with sediment (see coastal sediment processes).
- Structures such as houses and roads built on bluffs can cause compaction of the slope, possibly leading to slope failure.
- Removal of natural vegetation and over-watering can saturate soils and cause erosion, slumping and slope failure.
- Dikes and berms in estuaries prevent tidal flow and convert the areas into less productive dry land.
What can be done to improve altered shorelines?
Rocky shorelines are the least erodible and are the most stable for building on. Bluffs composed of clay, sand and gravel are the most erodible, and buildings should be set well back from the edge of these shorelines. By working within the limitations of the landscape, many erosion “problems” can be avoided. A good rule of thumb to preventing erosion and increasing marine habitat is to keep the shoreline as natural as possible. Some tips and resources for naturalizing altered shorelines include the following.
- A buffer of vegetation along the shoreline can be maintained or planted. This protects against erosion, provides wildlife habitat and helps to filter pollutants.
- Vegetation can be planted along the bottom and sides of seawalls, to soften edges and reduce scouring by waves.
- Seawalls can be enhanced with rock and salt marsh benches, to create a more resilient structure that provides greater intertidal habitat; see the diagrams below.
- Seawalls can sometimes be removed and replaced with sloping land and natural vegetation.
- Bioengineering techniques can be used that involve building retaining structures made of natural materials such as bundles of dormant vegetation (e.g. willow stakes). The plants subsequently root and “take over” the role of erosion control from the structure.
- Structures (buildings, roads, paths) on top of erosion-prone bluffs can be set back further from the edge of steep slopes, to reduce compaction and erosion.
- The footprint of shoreline structures such as wharves can sometimes be reduced by replacing supporting fill or concrete slabs with pilings. This creates some habitat under the structure.
- Decking and wharves can incorporate perforations and grating to allow some light to pass through.
- High density foam can be used in place of styrofoam, in dock floats. Styrofoam tends to break apart and can be ingested by animals, particularly birds.
- The roughness of structures can be increased, for example by using concrete or rock in place of steel. More roughness provides greater surface area for the attachment of microorganisms and in turn larger attached species like mussels, barnacles and algae. Structures can also be designed to incorporate habitat, such as crevices and recesses for fish and invertebrates, and ledges for marine mammals to rest upon.
The following diagrams show how intertidal habitat is minimal on a shoreline
with a vertical wall, but can be increased by making a more gentle slope;
a salt marsh bench can also be constructed, to provide habitat and food
sources for invertebrates, fish and birds.
How can I help restore altered shorelines?
- Learn about natural shoreline development techniques that can prevent erosion of your shoreline property and increase wildlife habitat
- Help to educate others about the value of natural shorelines, and encourage development that protects habitat and water quality.
- Retain and replant native shoreline vegetation.
- Leave driftwood and logs on the beach; they help to prevent erosion.
- Learn how to reduce impervious surfaces near shorelines, to increase infiltration of water, and reduce erosion and chemical pollution.
