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Western Australian Sea Level Variability

Goal: This research examines the relative contributions of tides, atmospheric surges, long period waves, seasonal, decadal and inter-annual variability to coastal flooding along the Western Australian coast.


Abstract:

This research examines the relative contributions of tides, atmospheric surges, long period waves, seasonal, decadal and inter-annual variability to coastal flooding along the Western Australian coast. The nature of these sources of sea level variability was explored regionally using a state-wide network of 26 coastal tide gauges. Assessment at a regional scale showed the coherence or divergence of sea level processes around the State’s coastline. Anomalous surge events in the southwest, unrelated to local meteorological forcing, were identified as continental shelf waves generated by tropical cyclones on the Northwest Shelf and propagating thousands of kilometres.


Spatial variation in the phase and amplitude of water level processes was observed over both seasonal and inter-annual time scales. These differences affect the nature and likelihood of flood events. Over decadal time scales, the relative influence of storminess, climate-related mean sea level variation and tidal modulations was examined. A relationship to tidal form was established, with the 18.6 year lunar nodal cycle affecting flood recurrence in diurnal conditions, and the 4.4 year sub-harmonic of lunar perigee influencing flooding in semi-diurnal conditions.


Results of the research have confirmed the importance of multiple processes to influence coastal flooding occurrence. Within the Swan River Region, flooding increased substantially between 1990 and 2006 due to mean sea level variability, tidal modulation and the seasonal timing of severe storms. However, these processes were not directly coincident and there is potential for secular change if they peaked simultaneously, although this has not occurred over the historic period.


PhD Thesis:

Chapter 1: Introduction

Chapter 2: Literature Review

Chapter 3: Review of Western Australian Sea Level Data

Chapter 4: Remote Forcing of Water Levels by Tropical Cyclones in Southwest Australia

Chapter 5: Influence of Inter-Annual Tidal Modulation on Coastal Flooding Along the Western Australian Coast

Chapter 6: Sea Level Variability Influencing Coastal Flooding in the Swan River Region, Western Australia

Chapter 7: Discussion & Conclusions

Chapter 8: References

Appendix A: Water Level Cascades

Appendix B: Ancillary Publications


Other Documents & Presentations:



This presentation summarises observations of the 2004 Boxing Day Tsunami along the Western Australian coast, including preliminary interpretation of tide gauge records.



Operations at BP Kwinana refinery since the 1950s have caused hydrocarbons to accumulate within the soil profile and the water table of the superficial aquifer. Passive oil recovery systems (trenches and selective oil skimmers in recovery bores) are used to manage and mitigate this contamination; however, groundwater levels influence the volume of oil recovered. Land use above the aquifer, groundwater abstraction, recharge through rainfall, and sea level variability affect the groundwater levels in a coastal aquifer. Walker (1994) monitored the groundwater levels at Kwinana and concluded that sea level signals propagated through the aquifer to influence the groundwater levels. A non-linear response exists, however, with greater damping for higher frequency ocean signals, such as daily tides. Further, the local hydrogeology is complex, potentially producing a relationship between ocean levels and hydrocarbon movement. Although this relationship is obscure, the history of sea level variability can be compared with extraction management to identify conditions that have adversely affected the extractive process. The volume of extracted hydrocarbons depends on the groundwater level, with lower groundwater levels extracting more efficiently. (A 1-cm decrease in groundwater levels is equivalent to a 4-cm increase in the oil thickness above the groundwater level.) Thus changes of the order of centimetres influence the level of oil recovered from the aquifer. Tide gauge observations from Fremantle over the last 25 years were examined to identify the variability of ocean water levels that may affect groundwater movements at BP Kwinana. Attention was focused on the past three years. Dominant fluctuations, in order of scale, included:

  • a daily tidal range, which was subjected to biannual and interannual (18.6-year) cycles

  • a mean seasonal sea level cycle, which had a low level of interannual variation over several years

  • interannual variations in storminess, which were high in the late 1990s and early 2000s

  • interannual cycles of mean sea level, typically over five to eight years, correlated with El Niño – La Niña fluctuations

  • a gradual long-term increase in mean sea level.

Of these sources of variability, the interannual cycle was enhanced over 2004 to 2006. However, it was notable that the active recovery trenches were installed when mean sea levels, storminess, and tidal potential were low. A high tidal potential, combined with elevated mean sea levels due to La Niña conditions, is expected for the next two to three years. Hence the low efficiency of the extractive process is likely to continue because of the higher water levels. Over the longer-term, the effects of climate change-induced mean sea level change may further impact on the effectiveness of the existing mitigation techniques. Future management of the extraction management system should make allowances for the interannual variability of tides, surges, and mean sea level, as well as the long-term sea level rise associated with climate change.




2009: Trends of Coastal Flooding in the Swan River Estuary (Greenhouse 2009 Presentation)



Tropical cyclones (termed hurricanes and typhoons in other regions), are extreme events associated with strong winds, torrential rain and storm surges (in coastal areas) and cause extensive damage as a result of strong winds and flooding (caused by either heavy rainfall or ocean storm surges) in the immediate area of impact. The eastern Indian Ocean, particularly in the northwest region of Australia, is impacted by up to 10 tropical cyclones during the cyclone season, although direct impact of cyclones along the west and southwest coastlines is rare. However, the sub-tidal frequency component of sea level records along the west and south coasts of Western Australia indicates lagged correspondence with the occurrence of tropical cyclones. It is demonstrated that the tropical cyclones generate a continental shelf wave which travels along the west and south coasts of Australia up to 3500 km with speeds of 450-500 km day(-1) (5.2-5.8 ms(-1)) with maximum trough to crest wave height of 0.63 m, comparable with the mean daily tidal range in the region. The shelf wave is identified in the coastal sea level records, initially as a decrease in water level, 1-2 days after the passage of the cyclone and has a period of influence up to 10 days. Amplitude of the shelf wave was strongly affected by the path of the tropical cyclone, with cyclones travelling parallel to the west coast typically producing the most significant signal due to resonance and superposition with local forcing. Analysis of water levels from Port Hedland, Geraldton, Fremantle and Albany together with cyclone paths over a ten year period (1988-1998) indicated that the tropical cyclones paths may be classified into 6 different types based on the amplitude of the wave.


Diurnal and semidiurnal tides are modulated over a range of time scales, including systematic annual and interannual variations. Although identified for other parts of the world, the effects of interannual tidal modulations have had limited attention on the Western Australian coast. Research described here identified that tidal modulations are a significant and regular factor in the frequency with which high water level thresholds are exceeded. Hence, tidal modulations provide a predictable contribution to the coastal management effort required on a year-to-year basis and allow prediction of periods where there is enhanced risk of flooding to coastal infrastructure. As has been demonstrated elsewhere, these cycles are obscured within conventional harmonic and extreme analysis, and their identification requires dedicated techniques. In this study, annual standard deviations and exceedance frequency have been used to examine both hourly and high-pass-filtered water levels to establish the influence of tidal modulations. The relative contribution of the two principal cycles and their subharmonics varies along the Western Australian coast from north to south and hence is strongly linked to the tidal form. High-tide levels for Western Australian locations with diurnal tidal dominance are dominated by the lunar nodal cycle, with a clear 18.6 year signal in the Fremantle-Bunbury region. The cycle most recently peaked in 2007 with declining tidal peaks expected until 2017. High-tide levels for locations with semidiurnal tidal dominance are mainly affected by the lunar perigean subharmonic, causing a 4.4 year cycle along the Northwest Shelf. The last peak occurred in 2006 with the next peak due in 2011.


Periods of high astronomically generated tides contribute to the occurrence of extreme sea levels. Over interannual time scales, two precessions associated with the orbit of the Moon cause systematic variation of high tides. A global assessment of when these tidal modulations occur allows for the prediction of periods when the enhanced risk of coastal flooding is likely in different parts of the world. This paper uses modeled tides to assess the influence of the 18.61 year lunar nodal cycle and the 8.85 year cycle of lunar perigee (which affects high tidal levels as a quasi 4.4 year cycle) on high tidal levels on a global scale. Tidal constituents from the TPXO7.2 global tidal model are used, with satellite modulation corrections based on equilibrium tide expectations, to predict multidecadal hourly time series of tides on a one-quarter degree global grid. These time series are used to determine the amplitude and phase of tidal modulations using harmonic analysis fitted to 18.61, 9.305, 8.85, and 4.425 year sinusoidal signals. The spatial variations in the range and phase of the tidal modulations are related to the global distribution of the main tidal constituents and tidal characteristics (diurnal or semidiurnal and tidal range). Results indicate that the 18.61 year nodal cycle has the greatest influence in diurnal regions with tidal ranges of >4 m and that the 4.4 year cycle is largest in semidiurnal regions where the tidal range is >6 m. The phase of the interannual tidal modulations is shown to relate to the form of the tide.



This paper examines the inter-annual and longer-term changes in mean sea level around Western Australia using monthly mean sea level records from 14 tide gauge sites, with the longest record at Fremantle covering from 1897 to 2008. The tide gauge records demonstrate considerable (up to 25 cm) inter-annual fluctuations in mean sea level around the coast, which are of a comparable order of magnitude to projected sea-level rise over the next 50 years. A large part of the variability is coherent across the region and is strongly correlated to the Southern Oscillation Index (a descriptor of the El Niño-Southern Oscillation), although the strength of the correlation has not been constant over time. The extended record at Fremantle indicates a rate of mean sea level rise that is comparable with estimates of global mean change over the 20th Century. However, comparison with other stations around Western Australia indicates regional variation in the rate of rise, with the southern sites showing a rate of mean sea level change less than the global average and the northern sites a rise greater than the global average. This regional pattern is not wholly explained by estimated rates of vertical land movement. Mean sea level rose rapidly in the 1920’s and 1940’s and again in the 1990’s, but was relative stable between about 1950 and 1990. The recent acceleration from 1990 onwards is not unusual compared to that occurring at other times in the 20th century.


The seasonal cycle of sea level (SCSL) is highly variable around the coast of Australia in terms of annual amplitude and timing of the maximum level. We investigated this using monthly mean sea level (MMSL) datasets from satellite altimetry, tide gauges and a barotropic hydrodynamic model for the period 2000 to 2010. The altimeter data and model output were combined to map the distribution of the barotropic and baroclinic MMSL and SCSL around Australia. The estimated baroclinic signal (altimeter minus model) was compared with steric heights derived from temperature and salinity climatology obtained from the World Ocean Atlas (WOA-09).


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.


We used a two-dimensional, barotropic, numerical model to examine the complex behaviour of the tides in Bass Strait. Bass Strait, located in south-east Australia between the Australian continent and Tasmania, has the appropriate dimensions to create a half-wave tidal resonance for the M2 tides. The M2 tidal wave (amplitude < 0.4 m) enters the strait from both strait openings, increasing the M2 tidal amplitude towards the central northern Tasmanian coast to a maximum of ∼1.1 m. The tidal phases and current ellipses in the strait showed the M2 tides resembled a half-wavelength resonance in a curved, open basin. The semidiurnal, S2, and diurnal, K1 and O1, tidal amplitudes were comparatively small (<0.2 m), progressive in nature and mostly constant through the strait. The model simulations revealed that when only open boundary tidal (OBT) forcing was included in the model, the model over predicted the M2 amplitudes by ∼10–15% in the strait's central region; when OBT and direct gravitational tidal (DGT) forcing were included, the model accurately reproduced the observed tidal amplitudes. DGT forcing was used to generate resonantly amplified M2 tides (amplitudes of 0.1–0.3 m) locally, with destructive interference between the OBT-forced and DGT-forced tides occurring in the centre of the strait. The model simulations also revealed that storm systems propagating west to east attenuated the M2 tidal amplitudes in the strait. The greatest decrease in the tidal amplitudes occurred along the central northern Tasmanian coast and was attributed to tide surge interaction and resonance behaviour in the strait.


2016: Design Storms for Western Australian Coastal Planning: Tropical Cyclones

This document summarises the approach of using Design Storms for coastal planning in Western Australia, specifically for the coast potentially affected by tropical cyclones. Definition of design storms aims to support scenario modelling for extreme storm impacts of inundation and erosion, providing a more cost‐effective approach to assessment of development setbacks than more comprehensive synthetic storm database modelling. Design tropical cyclones have been identified for each of the main town sites along the Western Australian coast.


2017: Comparison of Tropical Storm Water Level Modelling using

an Ensemble Approach

This abstract was prepared for the 3rd International Conference on Advances in Extreme Value Analysis and Application to Natural Hazard (EVAN). It is based upon analyses undertaken as part of the technical report "Design Storms for Western Australian Coastal Planning: Tropical Cyclones"... later released in 2018...

Many thanks to Jenny Hornsby for presenting the work on my behalf.


This technical report outlines how the characteristics of sea level variability influence submergence rates for the micro-tidal, mainly diurnal Swan River region. An application-dependent framework to support refined estimates of inundation is presented.


Evaluation of nearshore and estuarine transitions of tide and surge in southwest Australia provides improved understanding to support inundation management. Comparison between sites indicates the combination of processes likely to cause extreme water levels varies geographically, around the southwest, and spatially, through ocean-estuary transitions. Identified behaviour provides an evidence-based context for development of site-relevant tools for inundation assessment and management.

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