Typically, reefs initiate around dormant volcanic cones (Pacific Islands). As the apparent sea level rises, the coral grows to the light, forming a ring with gaps for tidal flows in and out of the lagoon. Darwin noted the various stages of these islands on his Beagle journey across the Pacific. The collapse of the lava vented volcanic cone leads to an apparent “rise” in sea level. The lagoon behind the reef slowly fills with material (Coral debris and sands) from the erosion of the reef. These sediments compact and sink as the volcanic cone collapses.
The beach being slowly rebuilt as the sand laden waves break on to the banks of seaweed. The continual reworking of the material allows sand bars to form with a back lagoon and channels, raising the beach profile.
Sand lobes progress across the sea weed / grass banks.
Waves pushing water over the sand banks into the lagoon.
Formation of sand banks with channels
Channel forming with water rushing back to the sea creating a small rip.
Beaches from Grange jetty to the Torrens outlet were heavily scoured by the storm. The predominant winds were from the NW and the waves heavily damaged the new and older dunes alike. Debris was washed south along the coast, opposite to the normal wave transport to the north. Dune wind walls and posts were ripped away and sheared at the beach level. Sand was washed away from between the protective rock walls, as well as the beach itself. Beach level was reduce by between 30 & 50 cm around the Henley Square and SLSC ramp area. Older rock walls, black wooden fences and concrete walls were exposed to the south of Joe’s café where over half the width of the dune was destroyed.
The worst storm for damage in the past 4 years, although Henley has seen many major storms in the past century. This storm event coincided with an additional 1m tide height.
Dunes to Grange destroyed
Same place – dunes wiped out – North to Grange
Old dunes cut back exposing the root system – looking north to the Grange jetty.
Dune root system exposed and sand washed away
Looking south to Henley – new dunes stripped and washed away
Henley Square outlet showing discoloured (rust) stain from pumped water.
The old paint shows the beach erosion and the sun cover disappeared!
Rocks washed clean up against Joe’s Shack.
Over 50cm of sand lost from the beach next the SLSC at Henley
Dunes cut in half exposing the protective rocks, old wooden fence posts (black) and concrete wall.
Sands washed away from the steps and access to the beach was impossible in some locations.
New Dunes washed away exposing protective rocks. Looking south to the Torrens outlet.
Massive dune destruction – posts were at the edge of the dune line.
Trucking in sand to repair this beach will take months.
A better solution is required to knock out the destructive power of the waves before they hit the beach. On offshore barrier – interlinking, concrete tetrahedrons. This is a standard technology used to encircle artificial islands. The water passes through the offshore (300m) barriers but take out the energy of the waves. An offshore barrier would require entry points for boats and markers.
A small price for coastal homeowners to pay for a visual break in the seascape to protect the public beaches and eventually their private homes and business premises from washing away!
A low pressure system in the Great Bight brings high winds in excess of 80km/hr from the NW along with high seas and higher than normal tide. One way to clear out the seaweed or at least moving to the southern beaches of Glenelg, Brighton and Seacliff.
Rising tide and stormHigh tide in 3 hrsLooking to Henley jettyWaves breaking over Henley jetty
After the NW winds the other day in the Gulf and the prevailing SW winds in the southern ocean, the sea conspired to dump inordinate amounts of chopped and ripped up seaweed and sea grasses on to the beaches of SA.
The new Torrens outlet / overflow!
