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Pebble/Sand Shoreline

Mortimer Spit - North Side Facing Port Browning, South Pender Island

Port Browning Area, North Pender Island
Pebble Sand Pebble Sand .2NPender Port Browing Area


Description

Pebble/sand shoreline are dynamic sediment beaches, with large supply of smaller sediments. The form of the beach is determined by the surrounding coastal processes, and beaches can be stable, accreting or eroding depending on wave energy and currents.  Finer sediments are usually also present in these beaches, ranging from coarse to fine sands. 

Beaches with larger portions of pebbles are typically more stable than sand beaches due to their larger size but also because the permeability of pebbles absorbs energy in breaking waves and reduces the offshore transport. 

The pebble/ sand shore type is less than 9% of the total shoreline in the Islands Trust area based on the twelve major islands that had shoreline mapping completed.  Denman Island has the most at 18%, while Lasqueti Island has the least at 3%.   17% of South Pender Island's shoreline has been mapped as the pebble/ sand shore type.

Physical Features 

Pebble/sand shorelines are a common sediment shore types in the Gulf Islands and have a significant supply of easily erodible materials that can be transported by wave and current action. 

Pebble/sand shorelines can have large sediment transport rates along the shore. Interruption of the sediments feeding longshore transport can starve downstream beaches, spits and coastal lagoons of material leading to increased erosion. 

In the Gulf Islands most pebble/sand shores are small pocket beaches; however some large beaches such as Tribune Bay on Hornby Island or Walker Hook on Salt Spring Island occur. Many pocket beaches are composed entirely of bleached white shell fragments. 

Pebble/sand shorelines are very dynamic on a human time scale. 

Biological Features

Pebble/sand shore types are usually bare of attached algae and invertebrates in the intertidal zone, however clams, worms and other burrowing species can be common. Mobile invertebrates such as crabs, sea stars and other shellfish are also present.  

In the lower intertidal zone and nearshore of the protected pebble/sand shore type, eelgrass meadows are often established. These habitats are highly productive as well as vulnerable to disturbance, and are important nursery areas for many species of nearshore fish including herring and juvenile rockfish. 

Upper intertidal zones of the pebble/sand shore types are potential spawning habitat for sand lance and surf smelt. These small forage fish are important foundation species in the marine food chain as they are prey for many other larger species including salmon and seabirds. Beach spawning sites are particularly sensitive to disturbances from shoreline changes so potential suitable spawning beaches need to be identified when modifications to pebble/sand beaches are proposed.  

The backshore vegetation and coastal riparian fringing trees and shrubs often overhang the uppermost intertidal zone. Not only does this vegetation help to stabilize the high water line, but it also contributes to shading and nutrients to the upper beach used by beach spawning fish (sand lance and smelt). 

Protected bays with shallow nearshore beaches are important forage areas of diving sea ducks and other waterfowl. 

PebbleSandBCWEB

Page last updated: 01/10/15
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