Many rubble mound coastal breakwater structures have primary armour rock and/or Concrete Armour Units CAUs that are undersized, and require regular (and costly) repairs after storm events. This ongoing challenge will be further exacerbated by sea level rise. Rock or CAU protected structures cannot be simply retrofitted with an additional layer of larger armour because different sized primary armour (rock nor CAU) units may not be available or do not necessarily interlock well.
Breakwater design equations predict that the stability of breakwater armour is proportional to the cube of the design wave height and inversely proportional to the submerged density of the armour material. Therefore modest increases in wave heights (due to changes in wave climate and sea level rise with climate change) and material density will lead to either large losses or gains in stability respectively.
A series physical modelling in wave flumes over many years at Water Research Laboratory have been undertaken to provide guidance on Upgrading of Seawalls with Climate Change –
Placing larger rock or CAUs over existing rock or CAUs armour can be undertaken but stability is dependent upon achieving sufficiently high placement densities during construction – model testing is essential.
The density of conventional concrete can be increased by replacing gravel aggregate (SG = 2.6) with steel furnace slag (SG = 3.0), but the concrete product is prone to cracking because of chemical reactions between Portland cement and the steel furnace slag. Recent advances in concrete technology have enabled the development of new products where Portland cement can be entirely replaced with geopolymer cement. Geopolymer cement does not react with steel furnace slag, making it suitable for use in high-density concrete.
High density concrete Hanbar CAUs have been found useful in model testing for upgrading existing CAU Hanbar structures, because they provide additional stability while retaining the same dimensions (ensuring good interlocking with existing armour units). This important result provides a potential upgrade pathway for all concrete armour unit structures in response to sea level rise. Prototype installation of 20t CAU Hanbars in a trial repair to a major coastal port in NSW Australia is planned
The paper will summarise the extensive physical modelling undertaken over many years at Water Research Laboratory , synthesizes it with international literature and provides guidance for Upgrading of Seawalls with Climate Change, including financial optimisation for both new and upgraded options.