Changes in sand movement at Snapper Rocks 2009 - 13

Introduction

As with any beach system, the behaviour of a particular surfing break is dependent on the seabed morphology. At Snapper Rocks, the shape of the seabed is constantly changing as sand moves in response to the wave conditions.

When waves arrive from the South East, northerly sediment transport rates are high and sand in deeper water is transported naturally across the Tweed River entrance, while sand in more shallow water is collected by the jetty and pumped to the Snapper Rocks East outlet. During these conditions the sand bank at Snapper Rocks is well maintained.

When waves arrive from the North East, northerly sediment transport is very low and reduced amounts of sand pass naturally over the Tweed River entrance and are available to be pumped by the jetty. During these times, it is not possible for the project to pump large amounts of sand to Snapper Rocks East and the Snapper Rocks surfing bank can somewhat deteriorate.

Storm events are also particularly unfavourable for maintenance of the sand bank as large waves can cause erosion immediately offshore of Snapper Rocks and deposition of sand offshore of Rainbow Bay. The following will give a description of sand changes at snapper Rocks over the last few years.

Conditions

As surfers observed in 2009, the autumn 2009 storms had a major impact on the sand bank at Snapper Rocks.

Seabed mapping by the Tweed River Entrance Sand Bypassing Project showed that the major storms which occurred from February to May 2009 scoured approximately 70,000m3 of sand from the seabed off Snapper Rocks / Little Marley's.  The broader Snapper Rocks area deepened by approximately 3m within 100m of the shore, and a hole up to 8m deep formed in the surfing take-off area at the head of the Snapper Rocks sand bank.

Such an eroded condition had not been seen at Snapper Rocks since the early 1990s, well before the project began.

Beach and offshore sand levels in the vicinity of Snapper Rocks

Condition pre-project
April 1995 
Condition pre-2009 storms
July 2008
Condition post-2009 storms
July/August 2009
Condition five months after 2009 storm
October 2009

Five months after the project's post-storm seabed mapping was completed, and despite consistent pumping of sand to the Snapper Rocks East and West outlets by the project, very little natural recovery was seen at Snapper Rocks. Pumping outlet locations can be viewed on the current activities page.

At this time, about 10,000m3 of sand had migrated back into the Snapper Rocks/Little Marley's area, but sand volumes were still down by 60,000m3 compared to pre-storm conditions.

About 11,000m3 of sand was pumped directly into this area in late July/early August 2009 from the Snapper Rocks West outlet and while improving eroded upper beach conditions, the sand did not significantly improve the eroded sand bank.

In October 2009, the seabed profile remained very flat and the 'Snapper Point Break' that had moved westward towards Rainbow Bay, while straighter than it was in June 2009, did not have a steep enough seaward face for waves to peel nicely over it. The hole at the head of the Snapper Point Break had filled in slightly, but it was still too deep to be reinstated as the surfing take-off zone for the main bank in front of Rainbow Bay.  However, under small to average wave conditions, surfers were enjoying short breaks peeling through to Little Marley's.

Condition eight months after 2009 storm

January 2010

In January 2010 the seabed profile remained very similar to the October 2009 condition as sea conditions during this three month period remained relatively calm with respect to wave height and direction for the transport of sand around Snapper Rocks.

Condition one year after 2009 storm

May 2010

By May 2010, the seabed profile had changed from that observed in January 2010. Although wave conditions had remained generally calm, the hole at the head of the Snapper Point Break had in-filled by approximately 30,000m3 due to the predominantly South East waves. By this time, the hole had been filled by more than half of what was lost during the May 2009 storms. The shallow water bed contours had also been smoothed to promote a higher quality peeling wave, closer to the point. 

Condition two years after 2009 storm

March 2011

When compared to the average regional wave conditions, the remainder of 2010 continued to be a low wave activity, low sediment transport year with slow regeneration of sand volumes at Snapper Rocks and Coolangatta Bay. In addition, a storm with wave heights over 3.5m from the east-northeast in October 2010 caused further erosion of the head of the Snapper Rocks sand bank.

However, the first few months of 2011 saw a seasonal return to predominantly large wave conditions from the South East and associated high volumes of sediment being transported towards the sand pumping jetty at Letitia Spit enabled the project to deliver this sand to the Snapper Rocks East outlet. In response, the March 2011 seabed mapping revealed a fully recovered snapper rocks sand bank, with conditions similar to those seen before the autumn 2009 storms.

Condition three years after 2009 storm

April 2012

Approximately three years after the May 2009 storm, in April 2012 there was minor erosion offshore of Snapper Rocks, with the seabed map looking very similar to that of May 2010. This erosion was most likely the result of ex tropical cyclone Fina which brought large waves, up to 4m in height from the east-northeast, over Christmas and Boxing Day 2011. By April 2012 the hole had somewhat recovered due to persistent South Easterly wave conditions allowing sand to be pumped by the project to the Snapper Rocks East outlet.

The April 2012 seabed map above shows a significant amount of sand in water depths of 4-5m offshore of Duranbah Beach. This sand deposition was most likely due to the natural movement of a large slug of sand across the Tweed River entrance bar which continued to travel northwards along the coastline.

November 2012

November 2012

The most recent November 2012 survey once again revealed significant erosion of the Snapper Rocks sand bank and shoaling in the middle of Rainbow Bay. This was due to the effects of two large swell events that occurred in close succession in June 2012.

The first was from the East with waves over 2.5m on June 7 and the second was from the North East with waves reaching up to 5.5m, a week later on June 13. The storm moved a significant amount of sand, stored offshore of Duranbah, as well as additional sand travelling northwards along Letitia Spit in deeper water, around Snapper Rocks, and deposited the sand in a large shoal directly offshore from Marley Rocks and Rainbow Bay.

The current sea bed map is very similar to what existed before the project commenced in April 1995, and after the May 2009 storm. This sand configuration is a natural deep hole at the head of the Snapper Rocks sand bank, and sand shoaling in the middle of Rainbow Bay. These conditions are typical of what naturally occurred from time to time in the 1960's as an impact of major ocean storms, before the Tweed River entrance training walls were extended.

Autumn traditionally sees the return to more south easterly wave events which will assist in moving the large quantities of sand that are currently sitting offshore of Duranbah and Frog's beach to provide much needed sand to the Snapper Rocks sand bank.

Sand Pumping

Snapper Rocks East is the primary outlet for the sand bypassing system. In the months that followed the autumn 2009 storms, the project pumped sand primarily to this location to replenish sand that had been eroded from Snapper Rocks. In August 2009, pumping to the secondary Snapper Rocks West outlet was also trialled to see if pumping to this location would assist in this process. A total amount of 91,000m3 of sand was delivered to Snapper Rocks between the July/August and October 2009 seabed mapping.

The project is only able to pump the amount of sand that is delivered to the pumping jetty by natural transport. September to December is typically the calmest time of year for natural sand supply due to persistent North Easterly waves. Following the October 2009 survey, a total of approximately 38,000m3 of sand was pumped to Snapper Rocks East over the three months to the end of January 2010 which is well below the average conditions for this relatively low supply time of year. Under these very low sand delivery conditions, there was little change in the eroded bank at Snapper Rocks, as shown by the October 2009 and January 2010 surveys compared to the storm-eroded conditions surveyed in August 2009.

February to May are typically high natural sand supply months due to the predominantly large wave conditions coming from the South East. Over the four months from February to May 2010 a total of approximately 171,000m3 was pumped to the primary Snapper Rocks East outlet. Again this quantity is well below the average natural supply of approximately 240,000m3 during these typically high supply months. While pumping volumes from February to May 2010 were less than the average natural sand supply conditions, they were sufficient to promote partial recovery of the surfing bank at Snapper Point, but the take-off area at the point was still depleted when compared to the pre-May 2009 storm conditions as surveyed in July 2008.

When compared to the average conditions, the remainder of 2010 from May onwards was a low wave activity and sediment transport year with approximately 196,000m3 being delivered to Snapper Rocks East from June through to December 2010. During the first few months of 2011 there was a return to seasonally high wave conditions from the South East and as a result 110,000m3 was delivered to Sapper Rocks East in just January, February and March 2011 alone. The high delivery volumes were able to assist in fully restoring the head of the Snapper Rocks sand bank, and further grooming of the nearshore contours.

For the remainder of 2011, wave conditions were average, especially throughout the winter months with approximately 350,000m3 being delivered to Snapper Rocks East from April to December. As a result, the Snapper Rocks sand bank was well maintained and surfing conditions at Snapper Rocks were consistently good during this period.

At the end of December over Christmas and Boxing Day 2011, ex-tropical cyclone  Fina saw wave heights over 4m arriving from the East-Northeast which scoured the area immediately offshore of Snapper Rocks. Sand volumes were however, quickly restored with seasonally high wave conditions from the South East allowing just over 120,000m3 of sand to be pumped to the Snapper Rocks East outlet from January to April 2012.

The swell events of June 2012 significantly eroded the Snapper Rocks sand bank and in an effort to restore conditions as quickly as possible the project has been pumping sand as a priority to the Snapper Rocks East outlet (with the exception of the routine Duranbah sand placement). Sand transport conditions have not been highly favourable surfing the second half of 2012 due to calm wave conditions that are generally associated with the spring months. This has meant that the large sand stores that are currently sitting offshore of Duranbah and Frog's beach have not been able to move around the Snapper Rocks headland to nourish the sand bank. In addition, there has been increased erosion in early 2013 due to the effects of Cyclone Oswald on Australia Day and a further East Coast Low in Mid February.

A new pumping strategy will be trialled in 2013 where during a storm event sand will be pumped off the northern Tweed River entrance training wall into the south Duranbah nearshore areas instead of to Snapper Rocks East. It is hoped that this strategy will reduce the amount of sand that is deposited offshore of Rainbow Beach during a severe storm event and assist in more timely recovery of sand stores at both Snapper Rocks and Duranbah.

Monthly pumping records can be viewed on the Sand delivery page.

Page last updated/reviewed: 05 May 2014