A vast majority (84%) of all countries in the world have coastlines and 80-100% of theirpopulation resides within 100 km of the shoreline. Studies show a major growth in populationin low-elevation coastal zones and a scenario of rising sea level may force millionsof people to relocate. To deal with the increased frequency of extreme events and sea levelrise, coastal vegetation (mangroves, salt marches and coral reefs) has been observed to actas an effective natural barrier. Coral reefs are believed to reduce upto 90% of wave energybut increasingly warming oceans and acidification are destroying this barrier by coralbleaching. Apart from a social, ecological and environmental damage, this will also resultin an increase in environmental loading on coastal structures.
This study focuses on the development of a climate change adoption measure for existingstructures on the principles of Sustainability. In order to do so, a representative existingbreakwater at Kiberg Norway is chosen. A brief ecology study of the area is conductedand based on economic value and vulnerability, Red King Crabs and Capelin are chosenas target species. A green-grey hybrid structure consisting of an existing breakwater withadditional Artificial Reefs (AR) as toe elements is hypothesized to be the suitable solution.However, hydraulic performance of AR is still not understood properly and to utilize themto enhance the stability of existing breakwater may create tension between hydrodynamicand ecological performance.
In order to investigate the hydraulic behaviour of hybrid structure, physical model studyis conducted. A traditional method of using transmission coefficient to quantify energydissipation over submerged/non-submerged AR breakwater is not suitable for this hybridstructure. Therefore, stability of existing breakwater is measured in terms of damage level(Ahrens and Cox, 1990) and indirectly by turbulent kinetic energy (Mukaro and Govender,2013) for 9 plunging and 6 surging wave conditions. Four configurations of experimentalsetup are finalized with four types of AR units (AR1, AR2, AR3 and AR4) and in total175 tests are carried out. Behaviour of breaking and non-breaking waves is observed to bedifferent especially over config-3 and config-4. Landward vortex and breaker tongue arenot fully developed in config-3 due to depth limited scenario. Additional non-linearities inthe flow, due to interaction of incoming and secondary waves, are observed for config-4,which resulted into higher reflection coefficient than other configurations.
Behaviour of a hybrid structure can be predicted by Van der Meer stability formulas forplunging and surging waves at lower wave heights. However, higher waves exhibit greaterdamage reduction and formulas show larger deviations. Results indicate that one row ofAR placed as toe, does not reduce much damage (10%). A comparison of all the configurationsindicate that config-3 and config-4 show an average damage reduction of 38% and51% respectively. Critical stability number of config-4 (i.e. 1:45) is lower than of config-1(i.e. 1:7), indicating that disturbing forces are becoming weaker due to the presence of AR. Residence time of wave on reef is believed to be of much importance and with a 15mreef length a damage reduction upto 45% is observed. Reef porosity is observed to havedependency on placement location and reef length. Ecological performance is predicted toincrease by 25% in 10 years of construction. However, differently chosen indicator speciesmight have shown better results.
It is concluded from the study that green-grey hybrid structures can be a suitable short-termclimate change adoption measure.