Coastal Processes

From 2019 to 2021, MWPA engaged a Royal Haskoning (RHDHV) to undertake a study to understand better the coastal processes affecting the beaches to the north and south of the Port of Geraldton and at the Geraldton and proposed Oakajee Port sites.

2021 Mid West Ports Coastal Processes Study - Stage One

2021 Mid West Ports Coastal Processes Study - Stage Two & Three

Mid West Ports has identified and is aware of typos in the report as per the Errata Register.

Frequently Asked Questions

Click on the question for an expanded answer.

1.

In short coastal processes are “any action of natural forces on the coastal environment. ”

These processes occur everywhere along the coastline, moving sand laterally along the coast as well as on- and off-shore, and they vary in intensity and relative importance. Coastal processes also influence the type and distribution of benthic communities and habitats.

The coastal environment is naturally dynamic, and its morphology at any point in time is determined by the interaction between its structures (e.g. rocky cliffs or unconsolidated sand), and the intensity and degree of exposure to key geophysical processes (e.g. wind strength, wave height, currents speed and direction).  These processes occur everywhere along the coastline, moving sand laterally along the coast (longshore sediment transport or littoral transport) as well as on- and off-shore (cross-shore sediment transport), and they vary in intensity and relative importance.

Figure 1-1: Components of a sediment budget on a sandy coast. 

The processes can be cyclic and occur on a daily or seasonal basis, or episodic (e.g. storm events), and these processes change the shape of the seabed and beach profiles at varying rates and degrees. The physical action of water and wind can alter beach profiles over short timescales, while cliffs and rocky platforms will erode and change over longer timescales. Coastal processes also influence the type and distribution of benthic communities and habitats in sub-tidal and inter-tidal zones. For example, seagrass meadows thrive in bays and lagoons that are sheltered from wave energy by offshore reefs or islands. The types and distribution of benthic communities can influence coastal morphology by moderating the effects of coastal processes; for example, mangroves can protect coastlines from storm damage (in Geraldton, seaweed wrack along the coast can provide some protection).

The placement of hard structures on the coast or offshore can alter the effect of coastal processes and hence impact the coastline and nearshore zone, and the environmental values they support. (EPA, 2016).

2.

On average, sediment transport in the Geraldton region moves from the south to the north, driven by the prevailing southerly winds, swell from the SW and the currents they induce, generally building up sand on south-facing shorelines, i.e. northern Mahomets beach.

The Geraldton coastline is unique compared to other continental coastlines due to a relatively shallow nearshore reef system that extends from south of Greenough River to north of Oakajee and reaches out some 5km from the coast in the embayment’s north (Champion Bay) and south of Point Moore.

The nearshore reef systems appear to play a key role in the evolution of the sandy beaches along the Geraldton coastline. These bathymetric features, clearly visible in aerial imagery, dissipate wave energy and promote the growth of tombolo features on both large (e.g. Point Moore) and small (e.g. around Oakajee) scales. In addition, they set up complex nearshore currents and circulation patterns which dictate local sediment transport pathways. (RHDHV, 2023a; section 7.1.1). They also provide relatively protected areas for seagrass meadows which are a source of sediment from biogenic processes.

On average sediment transport in the Geraldton region moves from the south to the north, driven by the prevailing southerly winds, swell from the SW and the currents they induce. There are seasonal reversals of sediment moving to the south driven by episodic events such as strong NW winds associated with the passage of fronts and storms in winter. However, the predominant movement is from south to north.

In detail from Section 5.5 of the Stage 1 Report (RHDHV,2023a): Along most of the Western Australian coast, including the study area, the combination of swell waves and strong sea-breeze wind fields drive the prevailing northward longshore sediment transport. Significant year-round swell waves coming from the south–southwest are attenuated and refracted by coastal limestone ridges, which are common in the study area at ~4km from the shore. Close-to-shore wind waves generated by the strong prevailing south–south-westerly winds are superimposed on the swell. The net longshore sediment transport direction is determined by the relative energies of swell and wind-sea waves and the angle between the local shoreline and dominant incident waves.

Due to refraction, the incident wave direction is not from the south (for example, where waves refract and diffract around shallow fringing reefs). However, there is an overall south-to-north transport pattern.

Three fundamental mechanisms can transport sand towards, away from or along the beach;

  • Longshore sediment transport (Littoral transport);
  • Cross-shore sediment transport; and
  • Wind-blown losses.

The Geraldton coastline is characterised by longshore sediment transport, with a net northwards transport of sand. The wind-blown mechanism can move sand from the beach to nearby dunes, forming a natural buffer to accommodate erosion during severe storms.

3.

Usually, beaches build up (accrete) during the summer months and lose sand (erode) in the winter months.

During storms, (which occur more frequently in winter) the associated storm surge raises water levels along the coastline this allows more wave energy over the fringing reefs and allows the waves to move further up the beach scouring away at the dunes, the sand is taken off the beach and foredunes then pulled back into the nearshore surf zone building a nearshore sandbar (cross-shore transport) where it is can be quickly moved along the coast by longshore transport. In calmer weather waves move the sand from nearshore sandbars back up onto the beach, which can then be blown further up the beach by onshore winds and re-establish the beach dune system.

Figure 1-2: Beach erosion/accretion cycle - no permanent sand loss or shoreline retreat 

If there is an imbalance in the sediment budget, i.e. more sand moves out of an area than into it, then there is no supply to build the beach back up after an erosional event. If this continues to occur over longer periods, then you will see a long-term beach recession.

Figure 1-3: Long-term beach recession - profile displaced landward due to permanent sand loss 

4.

Climate drivers, such as the Southern Oscillation Index (SOI) can have significant impacts on the Geraldton coastline. During La-Nina years, WA experiences higher than average water levels, which can lead to significant erosion events.

Section 4 of the Stage 1 report (RHDHV, 2023a), focuses on the relationship of metocean parameters with climate indices, such as the Southern Oscillation Index (SOI – an indicator of El Nino / La Nina events), the Southern Annular Mode (SAM) and the Indian Ocean Dipole (IOD).

The correlation between the following metocean conditions was found:

  • Deepwater Waves: - the climate indices with the most correlation was the SAM, followed by a weaker correlation with the IOD; in both cases there was a negative correlation. During times of positive SAM or IOD, wave heights, periods and energy tend to be lower, wave direction is more from the south, and storms are less frequent, so during these times, you would expect less erosion. When the SAM or IOD values are negative; wave, heights, periods and energy are higher, and wave direction is more from the west with a higher frequency of storms, we would expect more erosion during these times but the effects are moderated due to the presence of the Abrolhos and the fringing shallow reef edge in the vicinity of Geraldton. It was found the overall net effect on longshore transport at Geraldton (if any) is difficult to predict, more investigation would need to be undertaken.
  • Water Levels: - it was found there is a strong positive correlation between water levels and the SOI. Water levels could be up to +0.2m higher during years of strongly positive SOI (La-Nina years), (Land Insights, 2022). This has been found to be the case along most of the Western Australian coastline up to approximately Port Hedland. When the mean water level is higher, it allows additional wave penetration past the shallow reefs fringing Geraldton, and waves to reach further up the beach, this allows more wave energy to mobilise and transport beach and subaerial sediments (erosion), potentially increasing net sediment transport rates;
  • Wind: no meaningful correlation was found over the period spanned by the available data; and
  • Nearshore Waves: no meaningful correlation was found over the period spanned by the available data.

5.

The main sources of the sediment that moves into the Geraldton sediment cells include:

  • Long-shore drift of sand coming into the local sediment cells from areas further south;
  • Southgates Dunes;
  • Seagrass-derived sediment (Biogenic);
  • and Chapman River.
Section 5.3 of the Stage 1 (RHDHV, 2023a), report discusses the sediment composition around Geraldton, from which information on where the sediment has come from can be attained. Section 5.5.2 focuses on the sediment sources. Section 5.6 provides more information on how the sand moves through the Geraldton sediment cells, and Section 7 provides an overview of all the sand movement through a conceptual model, which is updated in the Stage 2/3 Report – section 4.7 (RHDHV, 2023b).

In summary, most of the sand in the southern embayment (from Southgates around Point Moore to the Geraldton Port navigation channel) consists of fine modern skeletal (carbonate) sand. ~60% of this sand is derived from modern bioclasts living in association with seagrass meadows, with the remainder coming from longshore transport from further south and the north-western edge of Southgates dunes. Most of this fine skeletal sand is moved via longshore transport to the north and exists mainly within the nearshore reefs ~2km from the shoreline.

The seafloor offshore of the fringing reefs in deeper water is composed primarily of medium to coarse sand that is largely relict and does not interact with the sediment transport mechanisms which occur in the shallower areas inside the fringing reefs <10m deep. This relict sand is mainly derived from the erosion of submersed limestone structures.

In the northern embayment of Champion Bay (from the Ports navigation channel to Drummonds Point), there is a mixture of fine-medium quartzose and modern skeletal sand. It is mostly found off the Chapman River mouth with limited sediment redistribution to the north. Offshore sediment transport occurs in this area and allows the deposition of quartzose sand from the Chapman River mouth up to ~2km offshore at around 10m water depth (near the middle of Champion Bay).

Zooming in on the area just north of the Batavia Coast Marina (BCM) to Chapman River, the sediment has a relatively low carbonate content. Sediment input into the area is a mixture of artificial and natural sources, with the longshore sediment drift following the south-to-north transport system. The relatively high quartz content found along the northern beaches suggests some redistribution of the Chapman River-derived sand to the south.

Figure 1-4: a) Interpolated % of fine sand in Geraldton. b) Sediment facies (types) in Geraldton (Tecchiato et al., 2016)

6.

The main ‘sinks’ for sand in the Geraldton area are:

  • Longshore Transport;
  • Beaches and Dunes;
  • Port Navigation Channel; and
  • Areas within Champion Bay.
The main sinks in the Geraldton coastal system are outlined in section 5.5.3 of the Stage 1 report (RHDHV, 2023a), they being:
  • The Geraldton Port Navigation Channel: as sediment moves north around Point Moore there is an area of active sediment accumulation immediately offshore Pages Beach, this material (inferred sediment pathway indicated by the red arrow in Fig 5.6), appears to highlight the presence of mobile bedforms migrating offshore from Pages Beach and to the navigation channel where sediment accumulates on its western side. The study suggests the channel acts as an effective barrier to the sediment coming from the accumulation area with only very fine suspended sediment able to cross to the northern and eastern side of the channel.
  • Champion Bay: previous studies have suggested that sediment supplied by Chapman River does not contribute significantly to longshore transport but is more likely to have accumulated in a sink in the middle of Champion Bay.
  • Longshore transport: transport of sand from south to north moves sand into areas of accretion where it can build up on the beaches and dunes until large storm events get it moving again. These areas of accretion are found in areas such as north Tarcoola (Mahomets) Beach, west of Separation Point, Point Moore, Pages Beach, North Glenfield and further north out of the study area along the coast, with some of the material being lost to the nearshore system as it's moved offshore into deeper areas along the way.

7.

The natural longshore sediment transport has been interrupted along the coastline by the construction of structures associated with the Port of Geraldton and the Batavia Coast Marina (BCM). It has been estimated that the coastal developments of the Port and BCM have reduced sand supply to beaches to the north of the BCM by 10,000 to 15,000 m3/yr. (Section 5.5.1, Stage 1 report, (RHDHV, 2023a)).

8.

Due to the interruption of the natural longshore sediment transport pathways by port and city infrastructure, Mid West Ports is committed under the Northern Beaches Stabilisation Programme (NBSP) to annually bypass 12,500m3/yr of sand from Pages Beach, relocating it to the northern beaches.

Due to the interruption of the natural longshore sediment transport pathways by port and city infrastructure, Mid West Ports is committed to mechanically transporting sand that accumulates to the west of the Fishing Boat Harbour (FBH) on Pages Beach to the beaches north of the BCM (northern beaches).

This process of mechanical sand bypassing is a mitigation method that is adopted by many local councils, not just in Australia but around the world, where coastal infrastructure interrupts natural sediment pathways.

To visualise this, think of a conveyor belt that transports sand from Point Moore along the coast to Drummonds Point. The port and city infrastructure cuts this conveyor, so all the sand coming around from Point Moore drops off the conveyor at Pages Beach, with limited supply getting to the north of the BCM. By trucking the sand from Pages to the northern beaches, Mid West Ports artificially links these conveyors so the northern conveyor is fed, and a sand source can continue up the coast, renourishing the beaches.

This sand bypassing is undertaken over 2 to 4 trucking campaigns each year to deliver at least 12,500m3 or sand from Pages Beach to the northern beaches under ministerial agreement 600, and outlined in the Northern Beaches Stabilisation Programme, which was formalised in 2006.

Additionally, the 2021 Maintenance Dredging Campaign relocated ~130,400m3 of clean sand that was dredged from the Port Navigation Channel and spread it across a bare sand patch approximately 1.2km west of Bluff Point, which extends from ~ Morris St in the South to Crowtherton St in the north. Mid West Ports chose sustainable sediment relocation into this dredge material placement area (DMPA), reintroducing dredged material into the sediment transport system (back onto the conveyor) to maintain and supplement sediment supply to sustain the natural processes within the system.

Dredge material positioned in underwater berms aligned with the beach is a technique widely used in the Netherlands, where it is known as “shoreface nourishment”. Establishing a nourishing (sacrificial) berm within the DMPA aims to provide a continuous supply of sand to the active beach as the waves and currents progressively erode it. As they erode, the sediments increase the local supply, enhancing the coastal processes of accretion, thus potentially having two types of benefits—coastal protection and enhancement.

Hydrographic and Environmental monitoring of the DMPA, and its surrounds has been underway since the dredging campaign in 2021, along with the ongoing beach profile monitoring.

Figure 1-5: Footprint of the Nearshore Dredge material Placement Area (DMPA) ~1.2km offshore from Bluff Point.

Mid West Ports is currently developing a Sustainable Sediment Management Plan (SSMP), taking into consideration the results of historical studies such as this coastal processes study, the outcomes of the current Northern Beaches Stabilisation Programme, ongoing monitoring and past and future dredging campaigns to determine and optimise the process of ensuring the natural sediment pathways and volumes of sand feed to the northern beaches (by artificial means) are maintained into the future.

9.

Mid West Ports monitors sediment and coastal hazards around Geraldton through a variety of means such as:

  • 132 Cross-shore beach transects from North Mahomets to Oakajee;
  • Aerial Imagery;
  • Photo Monitoring via NACC App “PhotoMon”;
  • Hydrographic Surveys.

This monitoring provides information on the behaviour of the coastline south and north of the port to help with decision making and management planning. For example, it identifies areas of the coastline that may need to be targeted for beach renourishment or where and when maintenance dredging is required to maintain safe navigation.

Mid West Ports monitors sediment and coastal hazards around the Geraldton coast through a variety of methods such as:

Since 2004, cross-shore beach transects (cross-sections) are undertaken by a local contractor twice per year (post-summer and post-winter), from the northern section of Mahomets Beach all the way to just north of Oakajee River. From the 132 separate transects, Mid West Ports can tell how the behaviour of a beach section changes through seasons and years.

Mid West Ports holds a record of aerial imagery dating back to 1942 of the coastline, ranging from south Greenough up the coast to north of Oakajee. In recent years, aerial imagery companies can provide 2-4 updates through each year of high-quality imagery for our area. Coastline behaviour and even subsea features can be tracked over time through these records.

Bi-annual hydrographic surveys covering the main shipping channels, anchorages, harbour, FBH and surrounds have been conducted by Mid West Ports for decades, this is to ensure that our navigational areas are fit for purpose, and to monitor and track the accumulation of sediments – this informs planning of sediment management methods such as dredging or seabed levelling, and contributes to other environmental studies concerning seagrass coverage and health within Champion Bay.

Photo monitoring via the NACC-designed smartphone application ‘Photomon’. The application is for recording and ‘databasing’ environmental change over time.

Figure 1-6: Point Moore Aerial Imagery: 1942(L) and 2023 (R)

This monitoring provides information on the behaviour of the coastline south and north of the port to help with decision-making and management planning. E.g. areas of the coastline that may need to be targeted for beach renourishment, or where and when maintenance dredging is required to maintain safe navigation.

10.

No, there is no evidence to suggest that the sand mining activities at Southgates significantly affect the volume of sand available to the natural coastal systems or influence accretion/erosion around Geraldton.

Specialist coastal engineers, MRA, completed studies of the Southgate Dune system in 2012 and 2013 as part of the approval process for the (then) proposed development of the area by landholder Bayform Holdings. Details of these studies are provided in the report Southgate Dunes—Sediment Feed Analysis (MRA, 2013).

The results of these studies completed by MRA were generally similar to the previous DPUD (1991) study and concluded the following:

  • The Southgate dune system migrated northwards at a rate of around 10m/yr between 2001 and 2010.
  • The volume of feed from the Western Dune to the longshore transport system is estimated to be in the order of 31,000 to 37,800 m3/yr.
  • The estimated volume of feed from the seaward edge of the Northern Dune to the longshore transport system may be between 3,000 and 5,000 m3/yr (MRA,2022)

Summarised in Figure 1 7 below.

Figure 1-7: Summary of estimated sand feed volumes (MRA, 2022)

Section 6.4.1 of the Stage 1 Coastal Processes Report (RHDHV, 2023a), summarises the volumes from these previous studies and states that these numbers have been estimated based on the observed northern migration rate of the dune with the southerly winds providing a mechanism for beach migration to the north. As shown in Figure 1 8, a comparison of average shoreline positions shows, on average, that profiles adjacent to the Southgate Dune and profiles further north at Tarcoola Beach have been steadily accreting over the past 30+ years.

Figure 1-8: A comparison of average shoreline position time series for profiles adjacent to Southgate Dune and at Tarcoola Beach.

Though not absolute this indicates that the contribution of sand from Southgates dunes to the longshore transport system has not been significantly affected by the sand mining in the northeast area of the Southgates system.

Section 7.1.4 of the Stage 1 report (RHDHV, 2023a) also states that while Southgates Dunes represents a source of sediment to the South Geraldton coastal cell, the evidence presented in this report suggests that there remains some uncertainty as to the magnitude of Southgates Dunes' contribution to the stability of Geraldton beaches and the sediment transport pathways of this fine sandy material.

It should be understood that one of the main reasons for the sand mining at Southgates is to manage the northward movement of the dune system, which is threatening the Brand Highway and residential properties in the area.

Further information can be found in the Southgate Dunes Management & Decommissioning Plan (2022 to 2027). The document is included in the published Council minutes and is accessible as attachment DS003.

11.

Driving on the beach above the high-tide mark can impact the beach and foredunes leading to erosion, though if managed effectively a compromise between off-road vehicle access and the health of the beach system can be reached.  Refer to the City of Greater Geraldton’s Coast and Waterways webpage on offroad vehicles.

12.

Land reclamation on the northern end of the Fishing Boat Harbour (FBH) has provided more land for the beach at Pages to build up against, resulting in its lengthening and widening.

Land reclamation on the northern side of the FBH has resulted in a significant accumulation of sediment within Pages Beach. The figure below shows that, while Pages Beach has been steadily accreting, the period with the most accretion was between 1942 and 1967 as the beach initially adjusted to the port structures.

The breakwater structure to the east of Pages Beach presents a large barrier to sediment pathways transporting sand to the northeast. Pages Beach has been steadily accreting despite large volumes of sediment being removed for the nourishment of Town Beach, Marina Beach and the northern beaches. With the shoreline now reaching the northern edge of the land reclamation (behind FBH), it appears the shoreline has stabilised since about 2013.

This suggests that the accretion of material at Pages Beach is related to sediment piling up against the groyne forming the western boundary of Pages Beach and then being able to bypass into Pages Beach and up against the Port. (Section 6.4.3, Stage 1 Report (RHDHV, 2023a).

Figure 1-10: Point Moore and Pages Beach coastline over time (L) Current coastline of Pages Beach 2023 (R)

13.

Point Moore has historically been an area of sand accretion, a combination of factors over the last few years, not least, three years of La-Nina, has led to massive erosion of the western shoreline. It is expected the shoreline will recover in the longer term, but more erosion events may occur in the short-term.

  • Historical Trends: Point Moore has historically experienced accretion on its western and northern shores, with evidence showing coastline growth since 1942 (Figure 1 10). Further information on Point Moore Coastal Processes can be found on the CGG Website.
  • Seasonal Changes: The width of the beach at Point Moore typically fluctuates throughout the year, diminishing in winter and expanding in summer. Recent Erosion: Despite its historical accretionary trend, Point Moore has suffered significant erosion over the last three years, a phenomenon not limited to Geraldton but observed at other areas along the West Australian Coast. 
  • Natural Sediment Transport: South of Point Moore, there are no man-made coastal structures that would disrupt natural sediment pathways, indicating that sediment feed to Point Moore remains uninterrupted by human intervention.

Figure 1-11: Point Moore Beach, 23 Apr 2020(L), and 26 Sep 2023 (R)

  • Climate Drivers: Increased erosion over the past three years can be attributed to heightened monthly mean residual water levels caused by the La Niña climate driver. La Niña years in Western Australia typically result in elevated sea levels and more frequent storm events, allowing greater wave energy to reach further up the beach, leading to erosion.

Figure 1-12: Triple-dip La Ninas appear as wide blue dips in Oceanic Nino Index charts, which show the three-month running mean of surface temperature anomalies in the tropical Pacific. (NOAA, 2023)

  • Lunar Nodal Cycle: Additionally, the Earth has been moving towards the peak of the lunar nodal cycle over the last few years. This cycle, which occurs over approximately 18.61 years, affects tidal amplitudes globally and contributes to extreme sea levels, enabling the sea to penetrate further inland along the beach.

Figure 1-13: Predicted nodal modulation at nine selected locations with tides in (a) semidiurnal form and (b) diurnal/mixed and mainly diurnal form. the numbers in the x-axis of the figures are the integer years when the nodal cycle is at peak. (Peng et al, 2019)

  • Point Moore Recovery: Based on its historical behaviour, Point Moore should recover, but it is hard to gauge when. With the possibility of another La-Nina in 2024 and a peak in the lunar nodal cycle in 2025, it may experience more erosion before significant recovery occurs.

14.

As part of the Northern Beaches Stabilisation Programme (NBSP), the sand placed on Beresford Beach is not meant to stay permanently in the location but to act as a feed to the natural sediment transport system in the area and to be naturally redistributed up the coastline to renourish beaches further north.

As mentioned in Question 8 above “What does the port do to help the sand return to its natural transport system?” Mid West Ports has a commitment under the NBSP to transport a minimum of 12,500m3 of sand each year from Pages Beach to the northern beaches as part of the Northern Beaches Stabilisation Programme. The sand is not meant to stay in the location but acts as a sand source to feed the longshore transport system, which redistributes the sand north along the coast and renourishes the northern beaches. Without providing the sand for beach renourishment, the northern beaches would be subject to long-term erosion and shoreline recession. To maintain this feed to the northern beaches' longshore transport system, sand bypassing needs to continue. In Geraldton, the current methodology is to truck sand, though other means of transporting the sand to the northern beaches (such as a pipeline) are being investigated.

It must be noted that mechanical sand bypassing around coastal infrastructure is not unique to Geraldton but is undertaken at numerous locations in Western Australia for the same reasons, as summarised on the SWASH website which outlines sand bypass and transfer systems around Australia.

Figure 1-14: Summary of Sand Bypass and Transfer Systems in WA

The NBSP document was written in 2006 and states where sand is to be sourced, how it is extracted, volumes required and where it should be placed, providing 4 locations; one of these locations is “the eroding zone between Mitchell & Brown and 170 Chapman Road”

Through monitoring of annual beach transects facilitated by Mid West Ports it has shown that since the completion of the foreshore redevelopment in 2016, placement of the sand opposite Mitchell and Brown has provided stability to the beaches from Beresford to St Georges. (There are seasonal differences, though year to year the beaches appear stable). Further information on the dynamic nature of Beresford foreshore is stated in section 5.6 of the Stage 2/3 Report (RHDHV, 2023b).

Figure 1-15: Beach profile transect N10, (South Beresford Beach) from the October 2023 survey (Quantum Surveys, 2023)

15.

No, there is no clear evidence linking sediment accumulation in the main shipping channel to erosion experienced north of the seawall located at Drummond Cove. Analysis over the last 30 years shows a gradual advance (increase of sediments) in the southern part of the cove’s shoreline, providing evidence that the sediment feed to the area has not been severely affected, meaning other factors may have caused the shoreline recession in the north of the cove.

  • Sediment Trapped in the Channel: There is no clear evidence indicating that sediment that accumulates in the Channel contributed to the erosion experienced in Drummonds Cove, particularly north of the seawall and community centre.

  • Sediment Supply to Drummonds: Glenfield Beach (south of Drummonds) is steadily accumulating sediment. Since the sediment transport system generally moves sand northward, this suggests that the sediment supply closest to Drummonds Cove has not been significantly altered and other factors are at play.
  • Shoreline Changes: Analysis over the last 30 years shows a gradual advance (increase of sediments) in the southern part of the cove’s shoreline. The northern area, directly north of the seawall, remained stable until 2012, when shoreline recession appears to have started in Drummonds Cove (Figure 1-16).
  • Sediment Transport Fluctuations: Historically shoreline trends show Drummond Cove to have short-term fluctuations in beach width, likely caused by discrete large wave events (storms). Multiple wave events have likely prevented sediment buildup against the sea wall, leading to a continued recession since 2012.
  • Need for Management: Interventions will probably be necessary to address erosion in this area to prevent future occurrences. CGG has several proposed coastal adaption projects being investigated for this area.

Figure 1-16: Average shoreline position at the northern (top) and southern (bottom) ends of Drummond Cove (RHDHV, 2020)

16.

Yes, wave energy has been changed, and this contributes to the erosion north of the port. This was anticipated during the planning of the Port Enhancement Project, which resulted in the NBSP and annual sand bypassing campaigns of 12,500m3/yr to replenish the reduced sediment feed to the northern beaches.

The report discusses how the model simulated wave conditions with and without the Port’s access channel in place (Section 3.3.2 Stage 2/3 (RHDHV, 2023b)). The percentage change in wave height resulting from the channel's deepening and widening was predicted using the FUNWAVE model, as shown in Figure 1-17.

The model predicted, during a large swell event from the southwest:

  • There is a 10% reduction in significant wave height in the nearshore area along the Beresford Foreshore, extending north to St Georges Beach.
  • A 10 to 20% increase in wave height to the north and south of the outer section of the channel resulted in nearshore increases adjacent to the BCM.
  • Up to 10% increase in wave height along St Georges Beach and Sunset Beach.

The channel's influence reduces around Sunset Beach, and the wave climate normalises to conditions without the channel near the beaches north of Sunset Beach.

Figure 1-17: % Change in Hs due to the creation of the Port access channel. The plot shows "access channel" minus "without access channel", so a positive change represents an increase in wave height due to the channel.

Section 4.6 of the Stage 2/3 report (RHDHV, 2023b), discusses the effects of the changed wave conditions and the trapping of sediments in the channel stating: ‘Overall, the results from the potential sediment transport modelling suggest that the capital dredging of the Port access channel in 2003 created a barrier to sediment transport, effectively blocking the natural transport pathway across where the channel was dredged. Although suspended sediment may bypass the channel to some extent, evaluation of samples from northern beaches suggests that the amount (of suspended sediments bypassing the channel) is insignificant and the material is not compatible with the beach sediments to the north of the port and thus, will not be able to prevent shoreline erosion’.

The model also indicates that changes to the channel altered wave patterns, causing less wave energy in its lee. This has led to decreased sediment movement towards the shore, and along with a change in wave direction, it has possibly reduced the net northerly longshore transport.

Since the 1930s, sediment supply to the area had been progressively reduced due to coastal construction activities, such as the first Port breakwater. Erosion observed in 2003, when the port access channel was widened and deepened, has contributed to this reduction. The erosion on the northern beaches is primarily caused by the differences in sediment transport rates along the foreshore, rather than the absolute amounts of sediment.

If sediment transport is interrupted in one area, then there is no sediment being supplied to the longshore transport system, creating a deficit that can lead to erosion in other areas. This erosion is worsened if there’s also a reduction in sediment supply from offshore sources.

Detailed modelling at Beresford (RHDHV, 2015) indicated that during high energy events, sediment is removed from the beaches to reef areas due to high water levels and wave-driven currents. However, there’s no ongoing natural mechanism to replenish sediment after a storm event, either from the south or offshore.

The potential impact on beaches to the north of the Batavia Coast Marina (BCM) was recognised when the Port Enhancement Project (PEP) was proposed. As an impact mitigation, it was recommended that sand bypassing be undertaken on an annual basis. Since 2003, an average of 12,500 m3/yr has been placed on beaches at Beresford and North. This sand supply is greater than any loss of sand supply from offshore sources which have been conservatively estimated by others to be in the region of 1,200 m3/yr for the region between the marina and the Chapman River.

According to the Shire of Northampton’s 2017 Coastal Management Strategy (CMS), increased sedimentation at the jetty occurred from 2001 to 2012, with refuelling at the jetty no longer possible from 2012. Sediment is supplied from the south, from cliff erosion and intermittent supply from the Hutt River, and river mouth. Due to the role of reefs, this coast is also highly sensitive to variations in water level with higher mean sea levels occurring in the period of 2008 to 2013 associated with a strong La Nina. This period of higher water levels may have mobilised sediment over the reef. It is anticipated that sedimentation will continue but at a slower rate.

As above, the CMS outlines sources of sediment to Port Gregory, though there is no link to the sand coming from Drummonds. Community members had questioned if the dredging from Geraldton Harbour contributed to the sedimentation at Port Gregory, and it was explained that the dredging had no impact.

Figure 1-18: Variability in beach position and beach width at Port Gregory Jetty 2009-2015 (Source: Landgate)

18.

Coastal development, including dredging within ports, can have both direct and indirect impacts on seagrass ecosystems. It has been observed that port dredging can have both positive and negative impacts on seagrasses. Monitoring during more recent maintenance dredging programs has demonstrated that these short-term, well-planned activities have had no impact on seagrasses. 

Direct impacts: can result from the physical removal of seagrasses during dredging operations or the smothering of seagrass during the relocation and placement of dredged sediments on the seafloor. Mid West Ports completes detailed seabed mapping exercises to identify marine habitats and ecosystems prior to dredging. All attempts are made to avoid disturbing seagrasses by restricting maintenance dredging activities to previously disturbed areas and optimising capital project designs to minimise seagrass removal during construction activities.

Indirect impacts: on seagrasses can arise from changes to water quality, natural hydrology, or coastal processes or through the creation of turbidity, which reduces sunlight penetration to the seabed, impeding seagrasses’ ability to photosynthesis and interfering with their growth and reproduction. MWPA plans dredging activities for Autumn and Winter months when seagrasses are dormant to minimise the impacts of turbidity. The dredging methodology chosen and the management of water from dredge overflow and sediment dewatering activities can significantly improve water quality during these operations. Mid West Ports completes detailed impact assessments that inform environmental approvals and dredge management plans.

Impact Monitoring: critical dredge management tools are the environmental monitoring programs conducted pre-, during and post-dredge operations to assess changes in seagrass health and distribution to help identify impacts. The seagrasses of Champion Bay have proved to be extremely resilient in recovering strongly after the Port Enhancement Project and capital dredging in 2001/2, as documented by BMT’s 2021 seagrass health survey covering the seagrass communities of Champion Bay and surrounding areas. Monitoring during more recent maintenance dredging programs has demonstrated that these short-term, well-planned activities have had no impact on seagrasses. Mid West Ports publishes monitoring reports on its Geraldton Port Maintenance Dredging webpage.

19.

Stemming from lessons learned from the last main harbour and channel dredging campaign in 2021, Mid West Ports is developing a Long-Term Dredge Management Plan and a Sustainable Sediment Management Plan. Similar plans are found at other ports around Australia.

Delivery of the two documents will provide guidance for future dredging campaigns and the sustainable management of sediments in and around the Geraldton Port, aligning with the current Northern Beaches Stabilisation Programme and Mid West Ports’ Asset Management Plan.

The main objective of Sustainable Sediment Management is to minimise interruption to the natural sediment pathways and processes around the Geraldton Port and mitigate any effects from Mid West Ports operations. This is to ensure our northern beaches have a steady supply of sand and to keep our navigable channels and harbours safe and fit for purpose.

Bathymetric features - Relates to the depths and shapes of underwater terrain, e.g., sand ripples, sand-waves, reef.

BCH - Benthic Communities and Habitat: Benthic communities are biological communities that inhabit the seabed and are important for primary and secondary production. Benthic habitats are areas of the seabed that do or ‘can’ support these communities.

BCM - Batavia Coast Marina

Bioclast - A fragment of a shell or fossil forming part of a sedimentary rock.

Biogenic - Produced or bought about by living organisms, i.e., biogenic sediments.

CSM - Coastal Management Strategy

Cross-Shore Sediment Transport - Cross-Shore Sediment Transport is the transportation of sediment in an onshore / offshore direction. It can be both destructive, resulting in erosion of the shoreline and deposition of material into the nearshore area, or constructive, resulting in deposition of sediment onto the beach.

EPA - Environmental Protection Agency

FBH - Fishing Boat Harbour

IOD - Indian Ocean Dipole is defined as the difference in sea surface temperature between the Arabian Sea (western Indian Ocean) and the eastern Indian Ocean south of Indonesia. The IOD affects the climate in Australia and other countries that surround the Indian Ocean. When IOD and ENSO are in phase the impacts of El Nino and La Nina events are often most extreme over Australia, while when they are out of phase the impacts can be diminished.

Longshore Sediment Transport - Longshore Sediment Transport (aka littoral transport) is the transportation of sediment along the shoreline. This form of transport occurs when waves approach the beach at an angle. This results in wave breaking forces acting along the direction of the beach and driving sediment transport in this direction.

LTDMP - Long Term Dredge Management Plan – MWPA document currently in development.

MRA - M P Rogers and Associates

MWPA - Mid West Ports Authority

SAM - Southern Annular Mode, also known as the Antarctic Oscillation, describes the north-south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the southern hemisphere. The changing position of the westerly wind belt influences the strength and position of cold fronts and middle-latitude storm systems.

Sediment Cells - Sediment cells are spatially discrete areas of the coast within which marine and terrestrial landforms are likely to be connected through processes of sediment exchange, often described using sediment budgets. They include areas of sediment supply (sources), sediment loss (sinks), and the sediment transport processes linking them (pathways). Sediment transport pathways include both alongshore and cross-shore processes, and therefore cells are best represented in two dimensions.

Skeletal sands - Skeletal sands are made of the remain of animals and plants and generally have not travelled very far from their source.

SOI / ENSO - Southern Oscillation Index / El Nino Southern Oscillation, measures the difference in surface air pressure between Tahiti and Darwin. It is one of the key atmospheric indices for gauging the strength of El Nino and La Nina events and their potential impacts on the Australian region. Sustained positive SOI values above +7 indicate a La Nina event, while sustained negative values below about -7 indicate an El Nino. Values between +7 and -7 typically indicate neutral conditions.

SSMP - Sustainable Sediment Management Plan – MWPA document currently in development.

Tombolo - A tombolo is a sandbar or spit that connects an island or nearshore reef to the mainland.