The recent retreat of the shoreline near Inverloch invites a closer look at the Bruun Rule – whether it is applicable in Gippsland, and if so what it means in practice. Wikipedia states that “The Bruun Rule is a formula for estimating the magnitude of the retreat of the shoreline of a sandy shore in response to changes in sea level. Originally published in 1962 by Per Bruun, the Bruun Rule was the first to give a relationship between sea level rise and shoreline recession.” The ‘rule’ is a simple two dimensional model highly controversial, little known, and as far as I am aware has not been considered in Gippsland.
The rule is often illustrated in a simple diagram (see above). These diagrams seldom accurately depict the actuality of the rule where the retreat is usually illustrated as one or two times the sea level rise rather than the 50 times that rule maintains. Understandably it is difficult to illustrate these dimensions in diagrams of this sort.
The standard approach of measuring sea level rise is static – that is it factors in a vertical rise in the levels but does not take storm surge or coastal erosion into account. This is often applied to images of Lakes Entrance where flood levels (or the sea level rise equivalent) only are taken into account and not the retreat or even disappearance of the coastal dune barrier.
The fact that sand dunes are readily erodible is obvious but the sand when removed has to be deposited elsewhere, either in deeper water or somewhere along the coast. If the latter occurs then there will be a build up of beaches in other parts of the coast. This seems quite likely to occur whilst the sea level rises are small in the short to medium term. On the other hand large rises over a longer period of time are almost certain to be catastrophic.
The measurements taken by citizen scientist Aileen Vening at Inverloch indicates the coastal retreat, at least in this location and at this time, is far greater than the Bruun Rule suggests and that it is at best a very rough guide only. To reiterate the retreat of the coast is a result of a combination of several climate change factors. As well as sea level rise there is the number and intensity of storms all of which are predicted to get worse. It follows that replenishing the sand on the beach as is currently proposed is a waste of time, energy and money and that planning for a long term retreat from the coast is the only viable option. What is happening now at Inverloch will soon be occurring, if it is not already, along most of Gippsland’s coast.