Goal: This project aims to collate information and evidence describing dynamics of the Lower Swan River. This has been developed to support decision-making by regulatory agencies, potentially improving management and engineering design within the estuary.
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Lower Swan River Morphology: quasi-equilibria, human influences, relicts from the distant past and future pressures
Management decisions for the lower Swan River have often been based on the mistaken assumption that microtidal conditions and limited flooding result in stable morphology. This perception has been exacerbated by the presence of ancient features and the persistence of historic works. However, like all estuaries, the lower Swan River morphology is dynamic, responding to changing conditions and human activities. Understanding the nature and pathway of these responses is crucial to our future decision-making, particularly in response to predicted tidal and sea level changes.
We have identified that active estuarine structure, although presently changing slowly, includes morphologic balances which will respond dynamically if disturbed, due to installation of structures or through climate change. Features of interest include areas of sediment transport divergence due to varying wind directions and fetch length; separation of nearshore and offshore dynamics along terraced estuarine beaches; and balancing of tidal flow and cross-sectional area upstream of the Causeway.
Historic examples within the Swan River illustrate the need to consider dynamic behaviour and connectivity scales when undertaking works. Knowledge of potential sensitivity provides a basis for improved site selection and locally-relevant design principles.
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Documents & Presentations:
The aim of the project was to describe the geomorphology and shore stabilisation structures of the South Perth Western Foreshore, between the Cloisters Overpass and the northern limit of Milyu Nature Reserve, in the estuary of the Swan River. The description was intended as a basis for preparation of foreshore management plans and engineering works.
The City of Fremantle identified an issue of intermittent inundation on the area of reserve parkland located immediately to the east of Stirling Traffic Bridge. The park forms a section of the popular walk trail between the Northbank Development and Pier 21 Resort. Repeated inundation during winter storms killed many well established trees, and greatly reduced the amenity of the park due to poor drainage. It was proposed the park be modified through excavation and replanting, to form a tidal wetland, to alleviate the effect of flooding and provide a site of high public amenity.
The morphology of Como Beach in the Swan River Estuary, Western Australia, is described. Como Beach is in a microtidal estuarine environment in which modal wave conditions are extremely low and nontidal fluctuations in water level are principally determined by storm surges and low-frequency changes in ocean water levels. Detailed descriptions of sandy beaches in very sheltered locations, such as Como, are uncommon in the literature, although these beaches are a common feature of coastal environments. In contrast to beaches in wave-dominated environments, those in very sheltered, low-energy locations may support subtidal terraces and beach profiles that differ in form and scale from the bars and intertidal flats in wave- and tide-dominated environments. At Como, beach profiles are superimposed on a subtidal terrace rising steeply from waters several metres deep to 21.5 m (Australian Height Datum; AHD), then with a low gradient to approximately 20.3 m (AHD) at the shore. The profiles range from planar forms on which very small waves (Hb less than 0.1 m) are dissipated to curvilinear forms that reflect higher waves (Hb . 0.2 m) from the beachface. A transitional form with a segmented profile comprising a steep beachface and flat inshore occurs, particularly where littoral drift is apparent.
2012: Sea level variability influencing coastal flooding in the Swan River region, Western Australia
Coastal flooding refers to the incidence of high water levels produced by water level fluctuations of marine origin, rather than riverine floods. An understanding of the amplitude and frequency of high water level events is essential to foreshore management and the design of many coastal and estuarine facilities. Coastal flooding events generally determine public perception of sea level phenomena, as they are commonly associated with erosion events.
This investigation has explored the nature of coastal flooding events affecting the Swan River Region, Western Australia, considering water level records at four sites in the estuary and lower river, extending from the mouth of the Swan River to 40km upstream. The analysis examined the significance of tides, storms and mean sea level fluctuations over both seasonal and inter-annual time scales. The relative timing of these processes is significant for the enhanced or reduced frequency of coastal flooding. These variations overlie net sea level rise previously reported from the coastal Fremantle record, which is further supported by changes to the distribution of high water level events at an estuarine tidal station.
Seasonally, coastal flooding events observed in the Swan River region are largely restricted to the period from May to July due to the relative phases of the annual mean sea fluctuation and biannual tidal cycle. Although significant storm surge events occur outside this period, their impact is normally reduced, as they are superimposed on lower tidal and mean sea level conditions.
Over inter-annual time scales tide, storminess and mean sea level produce cycles of enhanced and depressed frequency of coastal flooding. For the Swan River region, the inter-annual tidal variation is regular, dominated by the 18.6 year lunar nodal cycle. Storminess and mean sea level variations are independent and irregular, with cycles from 3 to 10 year duration. Since 1960, these fluctuations have not occurred in phase, suggesting that recent historic records may not provide a real indication of inundation risk, exclusive of factors linked to climate change.
The burst-like nature of coastal flooding incidents, with respect to frequency, has implications for both public perception and coastal management effort. The result, when combined with sea level rise, produces step-like change, with short periods of frequent coastal flooding, followed by extended, slowly varying quiescent periods. This presents challenges for coastal managers to incorporate variability into projections of future management needs, and to ensure that public and political recognition of coastal flooding hazard is not downplayed during quiet periods.
River engineering works along the lower Swan and Canning Rivers conducted from the 1830s until 1980s continue to influence the foreshore dynamics to the present day. This brief presentation summarises some of the major actions and drivers.
This WESROC foreshore management plan has been prepared to assist local governments protect and enhance riverbanks. It is intended to assist planning for foreshore structure maintenance, renewal and capital works over a range of time-scales and to help forecast funding needed to undertake the works. The plan attempts to reduce the reliance upon reactive management. Proposed present and future management required consideration of existing management and adaptation pathways, with an aim to develop and improve foreshore resilience. Management recommendations are required at an asset level to facilitate successful delivery of on-ground projects.
This presentation summarises the February 2017 runoff flooding event, in context of previous evaluations of river flooding mechanics for the Swan River. Information is also presented regarding foreshore assessment tools developed as part of Foreshore Erosion Management Best Management Practices project.
This summary document was developed on behalf of the Department for Biodiversity, Conservation and Attractions Rivers and Estuaries Division. It outlines factors that should typically be considered when interpreting submergence curves in the Swan River Region, including those associated with tide gauges from Fremantle, Barrack Street (Perth) and Meadow Street (Guildford). Submergence Curves are an illustrative diagram used by the Department of Transport (DOT) to summarise substantial information for each tide gauge station. Curves from Fremantle and Perth are available from DOT. Submergence curve diagrams are widely used for preliminary evaluation of site constraints, such as indicating the percentage of time a navigation channel may be accessible for a vessel of a certain draught or the frequency riparian vegetation may be flooded. Although highly valuable for broad use and planning for inundation hazard, submergence curves provide a potentially biased representation of emergence or submergence for practical applications.
This presentation outlines principles and practices for undertaking small-scale engineering works to enhance riparian vegetation stability, building natural resilience of foreshores. Foreshore restoration through planting of native vegetation is a common target on degraded foreshores, as riparian vegetation can significantly improve foreshore stability and provides valuable habitat. However, without additional support or management of pressures, a replanted foreshore is likely to return to a degraded state. Small-scale engineering works may be appropriate to enhance vegetation stability and build natural resilience. To identify suitable actions, understanding the site and attributes of riparian vegetation is crucial when selecting a bioengineering approach. It is important to recognize that riparian vegetation is not a seawall, with different species tolerances to hydroperiod, hydrodynamic stress, trampling, pollution, and undercutting. Different forms of intervention may be appropriate during an establishment phase, and damage during extreme events may be offset by natural recovery. Techniques to enhance foreshore restoration can include regrading, nodal hard-points, toe revetments, matting, large woody debris, brushwalls and brush mattresses.
Foreshore assessment and implementation of nature-based approaches has been undertaken in the Swan River for over 20 years, supported by the Best Management Practice guidelines for foreshore stabilisation.
2024: Swan-Canning Foreshore Risk Mapping
The approach being undertaken considers that processes contributing to dynamics of the river and estuarine system change across different domains. Interpretation of change is complicated by extensive modification since European settlement, and climate change, with a significant decline in rainfall and river flow.
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