By Bernie McComb
THIS is about coastal erosion. It’s to challenge the usual solutions involving “world’s best practice” ideas, from the biggest consultancies, usually involving a huge budget and huge concrete and/or rock walls.
Recently, it’s been reported that $29 million is needed to protect properties at east Cowes from storm surge inundation. It’s said that sea level rise is the cause of the problem, either this or the level of the beach is falling.
THIS is about coastal erosion. It’s to challenge the usual solutions involving “world’s best practice” ideas, from the biggest consultancies, usually involving a huge budget and huge concrete and/or rock walls.
Recently, it’s been reported that $29 million is needed to protect properties at east Cowes from storm surge inundation. It’s said that sea level rise is the cause of the problem, either this or the level of the beach is falling.
More recently, Regional Roads Victoria is asking for $20 million for more sea wall stabilisation to enable road widening at San Remo. One local person, with a 50-plus year memory, advises that the beach west of the bridge was eroded and dumped east of the bridge, past the fishing co-op, where you often see whitecaps over big shallow water where the beach used to be.
Are these related and how can they be explained?
I walk the beach and observe, morning and evening, hail, rain or especially storm. These days the internet makes it easy to investigate all kinds of things. It’s always sensible to analyse the problem before jumping to solutions.
The nature of waves approaching a beach might look like water is moving horizontally but it’s not. A wave is like the single ripple which travels the length of a whip of a whip cracker. Do kids still try to do this with a rope? The point is that the rope rises and falls but the material of the rope never actually travels the length of the rope/whip. It’s exactly the same with a wave.
The slope of a beach depends on the size and roughness/smoothness of sand grains. Grains drop out of suspension in water as the height of the waves is reduced, until the height of the beach is higher than crest of the wave.
Problems arise when the slope runs out such that the wave rises against a vertical wall. The volume of water in the wave, from trough to peak to trough, is then slammed into a skinny upwards vertical waterfall. Whatever goes up must come down. As it falls, the water needs to change direction, from vertical to horizontal. There’s an old saying: “When an irresistible force meets an immovable object, something needs to give.” The closer into the corner at the foot of the wall, the more the water wants to cut the corner. This causes the sand to give. It’s the same kind of suction that makes house roofs fly off in heavy winds, except that water is 1000 times heavier than air, so the suction is extreme. Specialists in the business of marine/civil engineering call this suction “toe scour”.
Toe scour rips sand grains from the foot of a wall, even under footings/foundations, or even behind a wall. Swirling of water keeps grains suspended until the water is calm. In the case of east Cowes, this means sand grains stay suspended while strong tidal currents transport them east. The photo attached looks towards Mussel Rocks, actually showing water flowing east to west, self-repairing the beach in December. Obviously damage is done in winter when the flow is east to west. There’s a ridge between the Bay and this channel. When waves are higher than the ridge, there’s nothing to stop them slamming into the vertical face, sucking sand out from roots of trees, destabilising further. Further west, sand is sucked out from under the yacht club rock wall.
The point of all this is to ask whether Bass Coast Shire Council, or either of our state or federal MPs, is brave enough to find a few bob to chuck at a test before winter. Using either a sand pump or big dozer (rather than the small excavator used previously) for this section, we need to shift the sand between the yacht club and Mussel Rocks so that the natural slope can be filled in to the top of the vertical sand face.
Let’s see if the energy of the waves will be dissipated, without damage, without toe scour, simply against the slope of the beach.
For longer term stabilisation, it will help if geotextile can be spread at the top edge and suitable seedlings planted.
Bernie McComb trained as a mechanical engineer. He walks on the Cowes beaches every day.
Are these related and how can they be explained?
I walk the beach and observe, morning and evening, hail, rain or especially storm. These days the internet makes it easy to investigate all kinds of things. It’s always sensible to analyse the problem before jumping to solutions.
The nature of waves approaching a beach might look like water is moving horizontally but it’s not. A wave is like the single ripple which travels the length of a whip of a whip cracker. Do kids still try to do this with a rope? The point is that the rope rises and falls but the material of the rope never actually travels the length of the rope/whip. It’s exactly the same with a wave.
The slope of a beach depends on the size and roughness/smoothness of sand grains. Grains drop out of suspension in water as the height of the waves is reduced, until the height of the beach is higher than crest of the wave.
Problems arise when the slope runs out such that the wave rises against a vertical wall. The volume of water in the wave, from trough to peak to trough, is then slammed into a skinny upwards vertical waterfall. Whatever goes up must come down. As it falls, the water needs to change direction, from vertical to horizontal. There’s an old saying: “When an irresistible force meets an immovable object, something needs to give.” The closer into the corner at the foot of the wall, the more the water wants to cut the corner. This causes the sand to give. It’s the same kind of suction that makes house roofs fly off in heavy winds, except that water is 1000 times heavier than air, so the suction is extreme. Specialists in the business of marine/civil engineering call this suction “toe scour”.
Toe scour rips sand grains from the foot of a wall, even under footings/foundations, or even behind a wall. Swirling of water keeps grains suspended until the water is calm. In the case of east Cowes, this means sand grains stay suspended while strong tidal currents transport them east. The photo attached looks towards Mussel Rocks, actually showing water flowing east to west, self-repairing the beach in December. Obviously damage is done in winter when the flow is east to west. There’s a ridge between the Bay and this channel. When waves are higher than the ridge, there’s nothing to stop them slamming into the vertical face, sucking sand out from roots of trees, destabilising further. Further west, sand is sucked out from under the yacht club rock wall.
The point of all this is to ask whether Bass Coast Shire Council, or either of our state or federal MPs, is brave enough to find a few bob to chuck at a test before winter. Using either a sand pump or big dozer (rather than the small excavator used previously) for this section, we need to shift the sand between the yacht club and Mussel Rocks so that the natural slope can be filled in to the top of the vertical sand face.
Let’s see if the energy of the waves will be dissipated, without damage, without toe scour, simply against the slope of the beach.
For longer term stabilisation, it will help if geotextile can be spread at the top edge and suitable seedlings planted.
Bernie McComb trained as a mechanical engineer. He walks on the Cowes beaches every day.