John Mills 24/11/17
Vertical wall versus Stepped wall
A focus on wave energy dissipation and the need to limit sand loss to the beach.
The Paekakariki foreshore has suffered many damaging breaches in recent and living memory especially since the large storms in mid nineteen seventies. Technology borrowed from the Netherlands was chosen as being the most appropriate at the time for the Kapiti Coastal Protection program. It consisted of water jetted vertical timbers tied back at the top to prevent forward rotation activated through vibration and settlement of the supporting backfill all a consequence of wave impact.
Effects of the Vertical Timber Wall:
As I have said previously the vertical wall causes predictably dynamic wave turbulence. This turbulence causes sand scour which leads to random lowering of the sea bed level during a certain chain of events and loss of sand in difficult to predict places. The observer can easily conclude that when violent wave motion is stopped abruptly with a solid vertical barrier the forces are deflected up, and down. Waves are also reflected seaward which sometimes incidentally crashes into and reduces the force of the next incoming wave. The vertical wall is an excellent example of the “unstoppable force charging against the immovable object”. All this action is the cause of vibration, further turbulence, and disturbance creating water borne sand. The sand loss occurs by the downward rush of highly pressured deflected water in the lower part of the wave scours at the toe of the wall and carries sand seaward. This sand loss is also randomly affected by occasionally observable cross currents which result in accumulation of sand in some places and loss in other adjacent places.
See the photographic evidence of Bride Coe’s vertical wall under strong wave attack.
The vertical timber wall is a good model to observe the forces applied by the violence of waves and the kinetic energy of the water. Bolt heads are pulled into the timber and strong stainless steel washers cup as the timbers flex and move. Sand is washed in between the structural timbers while under pressure which forces them apart. The supporting material behind the wall does not resist the force of the wave as would be expected and vibrations have been felt more than 100 meters landward.
The case to consider for Stepped Concrete:
It is entirely probable that the large variance in beach levels experienced in Paekakariki would be moderated by the alternative stepped shape which does not deal with the whole wave at once as at present. Also on the www.seawalls.co.nz homepage site compare the Auzcon stepped wall immediately adjacent to a vertical wall. Both systems are shown dealing with the same wave during storm conditions. At the point where a stepped wall adjoins a vertical wall the wave collides with both wall shapes. At the vertical wall the water is sprayed high in the air while right next to it the same wave is suppressed by the stepped wall with no dramatic consequence.
The stepped wall maximizes energy dissipation. It breaks the kinetic energy of the wave down by whatever number of steps is showing at the wall/water interface. The sand bed disturbance and scouring by deflected water is reduced accordingly.
(That logical sequence of events would not be so measureable with a rock revetment – a very different action entirely randomly unpredictable with the scouring water funneled at pressured force deep between the rocks similar to a fire hose or water blaster).
What can we be sure of:
It is easy to quantify from experience gained over 35 years the negative effects of the Paekakariki vertical wall and the sand loss leading to many wall breaches and high maintenance costs, not to mention the threat to the road as well. The vertical wall causes loss of sand and if it wasn’t for the concrete toe extension shown in the photos the devastation would be even greater.
The effects of scour reduction with the stepped option are difficult to quantify from local experience as we have no examples of stepped walls on the Kapiti Coast. Therefore we are forced to closely study all the sources of information we can get on this issue.
The Foxton Beach Surf Lifesaving car park was threatened by sand erosion which was protected by a stepped concrete wall. With little other information at this point I have observed the steps to be completely buried beneath the naturally restored sand bank.
Photos and information on this wall can be checked on the following links.
I have assembled five studies of erosion control and protection methods. One of them is from the US Army Corps of Engineers which states that stepped walls reduce the amount of wave run-up that can be expected. There is a drawing of an identically shaped stepped concrete option to the Auzcon design on page 8 which I believe might allay concerns engineers have of the construction principles of the system. That document suggests multiple times that toe scour is caused by wave reflection which undercuts foundations, and “the nearer to vertical the face of a seawall is the greater the height of run-up”. Also ‘the steeper the face, the more apt the foundation is to be undercut by wave scour at the toe”. We have experienced this phenomenon in Paekakariki and it has been the bane of our life since the original wall was built.
From my reading of other overseas reports the 3 most important issues to consider for seawalls aside from the obvious erosion which may apply to us in Paekakariki.
• Beach accessibility.
• Public use.
• To take whatever measures are affordable within the budget to limit the known and experienced loss of sand from the beach.
The four other studies also which generally support these statements one of which can be downloaded for $35 Euros. I only read the available summary, but the gist is clear.
Making the right call:
• Although the expense is greater, the longer life expectancy of concrete speaks forcefully of benefit to the generations of Paekakariki residents that follow us.
• Much consideration needs to be given to the negative effects of the known deflective and reflective performance of vertical seawalls.
• The positive effects of the stepped concrete wall are compelling. It would be a travesty not to explore the verifiable certainty of managed, scaled back, and dissipated wave energy of the stepped shape at the wave impact zone.
• The Rock revetment option is so fraught with negative effects it should not be considered whatsoever. These negative effects are listed elsewhere except to say that the only beneficiaries of this depopulation of the Paekakariki foreshore as a public playground will be The Parade property owners, who will be able to enjoy their own seaside view without those pesky families messing up the place.
Whatever option is finally adopted overtopping will be experienced. But if it ever became a problem, if the sea level does rise whether man made or other, a curved reflective wall at the curb side would be a not too expensive solution to the inundation/erosion of the road.
As always it is impossible to completely eliminate risk and unknown effects from anything as dynamic as the sea.