Latest mapping (April 2012) shows sand volumes in Coolangatta Bay are reducing

Introduction

If there were no entrance training walls at the Tweed River entrance and no pumping or dredging was undertaken there, an average quantity of about 500,000 m³ of sand would be pushed northwards past the entrance each year by waves and currents coming mainly from the southeast. The sand would move around Point Danger, through Coolangatta Bay and continue to flow northward along the Gold Coast, almost like a river of sand.

The actual amount of sand pushed northward each year would range from about 250,000 m³ to over 1,000,000 m³, depending on waves and currents which are always changing in direction and intensity, particularly between the seasons. This is why the beaches from Froggies Beach to Kirra Point were known to change from narrow to wide or vice-versa over a matter of months prior to the construction of the Tweed training walls in the 1960s.

 
Coolangatta early 1958 (Photograph by Ray Sharpe, sourced from the Gold Coast City Council Local Studies Library)

Coolangatta late 1958 (Photograph by Arthur Leebold, sourced from Gold Coast City Council Local Studies Library)

After the training walls were built, a lot of the sand that would have been naturally pushed to the north became trapped behind them. Sand began to build up at the Tweed River entrance and the natural sand supply to Coolangatta Bay dwindled. The southern Gold Coast beaches became severely eroded at times and this is part of the reason that the Tweed River Entrance Sand Bypassing Project (TRESBP) was put in place.

The Sand Bypass Project

By 2000, the sand bar at the Tweed entrance (the Tweed Bar) had built up to the point where it was a navigational hazard and the Southern Gold Coast beaches were suffering ongoing depletion of sand.

During the early years of TRESBP pumping and dredging, more than the average natural amount of 500,000 m³ of sand was bypassed around the training walls each year. This was required to clear the severe sand congestion at the Tweed River entrance and to make up for the long-term depletion of sand that the southern Gold Coast beaches were suffering. TRESBP bypasses sand around the training walls by pumping and dredging. The pumping system has been operating since 2001.

With this extra sand being delivered, the beaches from Froggies Beach to Kirra Beach grew in width and the nearshore area (the area from the surfing zone out to a water depth of about 10 m) became shallower.

Reducing Sand Volumes in Coolangatta Bay

Since 2006, TRESBP has been aiming to bypass the same amount of sand that nature would have pushed past the entrance if the training walls did not exist. The yearly sand volumes still vary greatly because of ever-changing wave and current conditions.

With less sand coming in to Coolangatta Bay, the build up of sand that had previously been sitting there has been pushed naturally and more rapidly to the north. As a result, in mid 2009, sand volumes in the Rainbow Bay area returned to conditions similar to those observed in 2000, prior to sand pumping. Over the next year and a half, further reduction in sand volumes occurred and during the summer of 2011 the beach was even narrower than conditions observed prior to sand pumping. Some of these Rainbow Bay sand losses were temporarily deposited on Coolangatta Beach, but by mid 2011all of this sand had migrated north of Kirra Beach. 

Seasonal sand movements were detected in the November 2011 coastal survey which showed a slight increase in the sand volumes in Coolangatta Bay. This was due to the migration of sand into the bay that was scoured from Point Danger during persistent, higher than average south-easterly wave conditions during mid to late 2011. This pulse of sand has now moved through Coolangatta Bay and is most likely responsible for the current build-up of sand at Kirra. The pulse of sand will continue to migrate northwards over time from Kirra towards Bilinga.

Overall sand volumes in the Coolangatta/Greenmount area have also continued to reduce, and approached 2000-like conditions in March 2011. This occurred through an ongoing loss of about 70,000 m³ per year in the Greenmount/Coolangatta area over the last four years, with a similar ongoing loss of about 70,000 m³ per year in the Snapper Rocks East to Rainbow Bay area.

 
Download graph (PDF 122kb)

 

 
Coolangatta 2000 (Photograph © NSW LPMA)

 
Coolangatta 2003 (Photograph © NSW LPMA)

More than 1,000,000 m³ of excess sand has been lost from the bay over the last five years (Jul 2007 - April 2012) and hydrographic survey (seabed mapping) results show that this trend is continuing. About 25% (one quarter) of the sand came from the Snapper Rocks/Point Danger area and about 75% (three quarters) came from the Rainbow Bay/Coolangatta area. A further 285,000 m³ has been lost from the Kirra area over the last five years. More information is provided on the Beach changes along Kirra page.

 
Download the isopach map (JPG 1.43mb)

 

View graph of changes in beach and nearshore sand volumes compared to February 2000 conditions (PDF 122kb).

The sand reduction within Coolangatta Bay has been very difficult to notice as the losses have mainly come from the nearshore zone, in water depths of 2-10 m.

The sand loss is however, beginning to become more evident in the inshore (wading) zone and upper-beach (tidal and dry beach) zone, and beach users may have noticed the difference in beach widths or swimming conditions at Rainbow Bay and Greenmount Beach close to the headland over the last few years.

The beaches from Rainbow Bay to Coolangatta Beach have begun to retreat, but are expected to retreat further over the long term, to match the deeper nearshore zone over time as they work towards their natural state under a complete sand supply, which has not been seen since the 1960's. However, it should be noted that natural variations in waves and currents will still cause short-term changes in beach size and shape. As a result, beaches will continue to show short-term cycles of growth and retreat over the year in response to seasonal changes in wave action and sand drift conditions, while undergoing longer term overall reduction in sand volumes.

Predicted changes to beach conditions under a restored sand supply system, extract from the project's Environmental Impact Study 1997 (PDF 31kB).

It is difficult to say exactly when the beaches will reach their typical states under the restored coastal sand supply, because beach conditions are very dependent on seasonal weather, particularly the occurrence and duration of significant storms, but a major part of the change has already occurred over the last four years. 

 
Coolangatta July 2007 (Photograph © NSW LPMA)

 
Coolangatta March 2011 (Photograph © NSW LPMA)

Now that sand is being pushed out of Coolangatta Bay, the beaches to the north which have been low on sand for many years, will gain sand to protect them against storm erosion. Most of the sand from Coolangatta Bay has already moved past Kirra Beach and Coolangatta Creek and the majority of it has now reached the area to the north of the North Kirra Surf Life Saving Club.

Sand volumes in the Snapper Rocks and Rainbow Bay area are now close to or less than what they were before pumping began. The rate at which sand moves out of the Kirra area has increased as the sand continues to be naturally pushed further north to the North Kirra / Tugun area.

There are no obstructive features such as headlands or groynes along the North Kirra to Tugun stretch of coast so it is not expected that the sand will travel as a large mass of the kind that filled in the beaches from Rainbow Bay to Kirra in the early years of pumping.

As a result of the spreading out of the sand, the beaches to the north of Kirra shouldn't become as wide as the beaches from Rainbow Bay to Kirra have been in recent years, but they will receive some much-needed nourishing sand.