River and estuary foreshores are dynamic places. In the Swan Canning Riverpark, foreshores respond to seasonal variations in water level, storm surges, wave action, sediment flux, tidal currents and river flow.
Landward movement of foreshores (erosion) can effect human amenity value and threaten valuable infrastructure. Offshore movement of foreshores (accretion) may affect navigation, smother benthic habitats and decrease channel carrying capacity.
Problems with foreshore movement generally occur when instability threatens social, environmental or economic values. Much of Perth is built on reclaimed foreshores and there is a history of locating important infrastructure close to the river without adequate setback. In order to protect that infrastructure from erosion, river walls and revetments have been built along shorelines. These are often inadequate due to their age, insufficient maintenance or inappropriate design for the conditions.
The presence of trees and sedges helps to naturally stabilise foreshores by reducing bank sediment mobility and buffering erosion from wave action and river flow. Widespread clearing for urban and agricultural activity means foreshore vegetation often exists as only a single line of mature trees bordering the river. This provides no room for vegetation retreat and makes the vegetation itself vulnerable to erosion.
Waves derived from boat wakes that exceed the intensity and duration of natural waves also undermine fringing vegetation. Structures on foreshores such as drains can disturb natural sediment transport patterns along shorelines. Discharge from drains on to or above riverbanks can also undermine bank stability.
Through the Riverbank Program, Parks and Wildlife is working in partnership with 21 local and six State Government agencies to address localised areas of erosion.
The cumulative increase in vessel wake can damage riverbanks and infrastructure and adversely affect other aquatic users. With an increase in boat ownership in Perth and greater proportions of large recreational vessels, the effects of boat wash on shorelines stability is becoming increasingly topical. However, it should be noted that in some areas of the river, affects from boat wash may have less influence on shoreline erosion than tidal movements, wind waves and drainage.
In 2009, research and trials were undertaken to investigate boat speeds and wash in the Riverpark.
Based on these results a speed reduction from 8 knots to 5 knots was introduced to the upper Swan River upstream of Belmont Ski area in November 2011. The speed reduction is expected to reduce shoreline erosion and damage to wildlife habitat. It also reduces conflict between boat users and other users of the river.
The results have also been used to help inform a major review of aquatic usage on the Swan and Canning rivers in the draft Aquatic Use Review and Management Framework.