The Wilson Inlet Catchment Committee Inc (WICC)

Wilson Inlet 6



Cover page

Introduction

History

Outside the bar

Inside the bar

Natural variability

Management options

Summary of Findings

Actions

References

 

Managing the bar and the Inlet

The ocean side of the sand bar: Coastal processes

The movement of sand at the mouth of Wilson Inlet is one of the most important processes determining bar opening and closure. The major mechanisms for sand transport include the scour of the bar and delta during the breaching of the bar, and the redistribution of sands in the nearshore zone due to wave action.

Figure 4: This series of photos, shot 1 day, 2 days, 8 days and 10 days after the 1997 bar opening, illustrates the initial turbid outflow from the Inlet as sand is carried from the delta, the strong outflow of darkly tannin stained catchment derived water, the weak outflow of estuarine waters once the Inlet has drained to mean sea level and finally the inflow of marine water on a rising tide. Photo: N. Boughton

Scour during bar opening

It is estimated from surveys that most of the scour of sand from the bar and delta occurs in the first few days after opening. This scoured sand is deposited in and behind the surf zone in Ratcliffe Bay where it is later redistributed back onto the beach by wave action, rebuilding the bar and closing the Inlet off from the ocean once more.

The greater the difference in the water levels between Inlet and ocean at the time of opening, the greater the energy available to scour a channel through the bar and delta.

In 1998 approximately 50,000 m3 of sand was scoured from the bar and delta and deposited in Ratcliffe Bay in the first 5 days after opening; the scour rate was similar in 1997.

The main influences on the volume of scour are the mean sea level at opening, the Inlet level at opening, the sand build up in the delta from previous openings and the river flow following opening.

Figure 5: Initial outflow after bar opening (1997). Photo: N. Boughton

Channel dimensions after opening

Surveys in 1997 and 1998 indicate that that within a week of opening the bar channel during a western opening is 50 m to 75 m wide with a depth of 1.5 m to 3 m below mean sea level. The throat of the channel through the bar continues to slowly deepen for about 4 to 6 weeks subsequent to opening, after which the bar channel begins to fill from the east. There is limited information concerning the size of the bar channel with an eastern opening. A survey following the 1991 eastern opening indicated that this eastern opening created a bar channel that was slightly deeper and wider than the channels in the western openings that have been surveyed. However, the data is not extensive enough to conclude that eastern openings regularly create deeper and wider channels.

Effect of rip cells

Aerial photographs of both western and eastern openings show that offshore currents called rip cells are commonly set up and there can be four to five rip cells between Wilson Head and Dunsky Reef, located about 700 m from the cliff (Figures 7 to 11). From the photographs it is clear that both western and eastern openings can link to rip cells and that the linkage between rip cells and bar openings reinforces both features.

   
Figure 7: Aerial photo of bar in October 1986 showing migration of the bar channel from original opening position of 300 m.
Figure 8: Bar in January 1992 showing multiple rip cells, narrow beach in front of SLSC and migration of the bar channel from opening position of 450 m.

Figure 9: Aerial photo of bar in January 1991 showing migration of the bar channel from original opening position of 450 m.
Figure 10: Bar in November 1996 showing multiple rip cells, narrow beach in front of SLSC and migration of the bar channel from opening position of 100 m.
Figure 11: Aerial photo of bar in December 1996 showing much wider beach berm than in November 1996 and January 1992 as seen in Figures 8 and 10.

Effect of waves on sand movement

Waves in Ratcliffe Bay play an extremely important role in sand transport on Ocean Beach. Waves may be generated by the wind conditions in the Bay or by swells. The waves then set up the longshore, onshore and offshore currents that move sand. In general the smaller summer waves tend to push sand from offshore up onto the beaches while storm waves tend to be erosive dragging sand offshore.

Longshore currents move sand back and forth along the beach. It is estimated that the net longshore drift of sand in Ratcliffe Bay is about 2,000 m3/year in a westerly direction. The gross movement of sand back and forth along the beach is likely to be about 10,000m3/year. These are small rates of longshore drift, for example the net longshore transport at Dawesville is estimated to be about 80,000 m3/year and at Mandurah about 100,000m3/year.

Migration of bar channel

Aerial photographs show that both western and eastern openings can migrate in a westward direction; with western openings doing so much faster. There are two processes at work governing this western migration. Firstly the movement of the entrance channel following opening could be caused by stream flow eroding bends; particularly at the back of the bar due to the different angle of the channels through the delta and the bar. As stream flow erodes the outside of the bend the bar will migrate in a south-westerly direction. This process could be particularly strong for western openings, as the channels are more curved than for eastern, causing western openings to realign much faster than eastern through this mechanism.

A second process is the transport of sand along the beach from east to west under the action of southeasterly winds (this is the direction of the net longshore drift described above). This may fill the channel through the bar from the east. As the channel is reduced in cross section by sand from the eastern side, the currents increase and scour the channel on the western side.

Closure of bar channel

Using computer models of sediment transport it was concluded that:
the onshore transport of sand by summer swell is the dominant mechanism in the closure of the entrance.
As the river flow subsides sand is washed into the Inlet from the ocean by waves. From survey work the rate of sand deposition in the bar during the closure of 1998 was estimated to average 300 m3 of sand infill per day over several months.

Importance of the delta

Hydraulic modelling has shown that the shallow area of the delta immediately behind the bar is the major control on water flow between the ocean and the Inlet.
Flow in the channel through the bar itself, for either an east or west opening is controlled by the delta.

The delta channels are dynamic and vary in size depending in a large part upon how much scour has occurred during opening and how much infill has occurred in previous years.

The position and dimensions of the bar channel affect the delta infill, as a wide and deep entrance channel may permit sand to be deposited further into the deltaic area during the closure. Increased infill amounts may take longer to scour out in the next bar opening. Regular changing of the location of the opening could lead to the progressive siltation of channels through the delta and over a number of years reduce the hydraulic capacity of the area.

Repeated openings in the one location could lead to the improvement of the scour channels through the delta and an improvement in the hydraulic capacity of the area. There is some evidence to suggest that this deepening occurred between 1974 and 1980 and between 1994 and 1999.

Width of Ocean Beach

Figure 6: Western opening shortly after breach. The photo shows the location of the surf club building (in left foreground) on the fore-dune vegetation line (1996). Photo: T. Carruthers There have been concerns about the effect of the opening on the width of the beach berm in front of the Denmark Surf Life Saving Club. With the current understanding of the coastal processes at the mouth of the Inlet, it is not possible to accurately distinguish between the relative merits of a western or eastern opening in relation to the width of Ocean Beach. East or west opening aside, the Surf Life Saving Club is situated on the coastal vegetation line and consequently, even with the protection of Wilson Head, is at significant risk from storm erosion.

Most beaches experience significant fluctuations caused by the changing balance of coastal processes and their effects on sand transport. Aerial photographs show that the width of Ocean Beach in front of the Surf Life Saving Club can vary substantially. There is generally a 20 m to 30 m beach berm but, for example, the December 1996 photograph shows a 50 m wide berm (Figure 11).

The major driver of the width of Ocean Beach is believed to be the interannual variation in storm activity.

During stormy years there can be significant loss of the beach berm and the vegetated dunes as sand is eroded and deposited offshore. On other southwest WA beaches recession of the vegetated dunes can easily be 10 m to 20 m in severe storms. On the south coast there appears to be a pattern of a few very stormy winters separated by relatively calm winters.

This interannual variation in storminess means that in stormy years large amounts of sand can be moved offshore in short periods of time with significant loss of the beach berm, while little erosion may occur in calm years.

The amount of sand feed to Ocean Beach from the bar opening is likely to be related to the vigour of the scouring processes during opening.
As noted above, the scouring process is influenced by the Inlet water level at the time of the opening, the size of the flow channels through the delta, as well as the river flow following the opening.

Because there is usually significant wave energy present along the beach, it would be reasonable to expect that the sand deposited in the bay by the opening process could be spread along the beach in the months following the opening. If this is the case, then the position of the opening may not be the dominant influence on the amount of sand feed to Ocean Beach near the Surf Life Saving Club.


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