Physical Model Study of Living Breakwaters

Physical Model Study of Living Breakwaters: Stability and Ecological Analysis of Green-Grey Hybrid Structure Concept for Climate Change Adaptation

Raza, Nauman (TU Delft Civil Engineering and Geosciences)

Lubbad, Raed (mentor)
Hofland, B. (graduation committee)
A. Arntsen, Øivind (graduation committee)
Sasikumar, Athul (graduation committee)

Delft University of Technology
Norwegian University of Science and Technology (NTNU)

Coastal and Marine Engineering and Management (CoMEM)

2019-08-05

A vast majority (84%) of all countries in the world have coastlines and 80-100% of their population resides within 100 km of the shoreline. Studies show a major growth in population in low-elevation coastal zones and a scenario of rising sea level may force millions of people to relocate. To deal with the increased frequency of extreme events and sea level rise, coastal vegetation (mangroves, salt marches and coral reefs) has been observed to act as an effective natural barrier. Coral reefs are believed to reduce upto 90% of wave energy but increasingly warming oceans and acidification are destroying this barrier by coral bleaching. Apart from a social, ecological and environmental damage, this will also result in an increase in environmental loading on coastal structures. This study focuses on the development of a climate change adaption measure for existing structures on the principles of Sustainability. In order to do so, a representative existing breakwater at Kiberg Norway is chosen. A brief ecology study of the area is conducted and based on economic value and vulnerability, Red King Crabs and Capelin are chosen as target species. A green-grey hybrid structure consisting of an existing breakwater with additional 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 them to enhance the stability of existing breakwater may create tension between hydrodynamic and ecological performance. In order to investigate the hydraulic behaviour of hybrid structure, physical model study is conducted. A traditional method of using transmission coefficient to quantify energy dissipation over submerged/non-submerged AR breakwater is not suitable for this hybrid structure. 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 experimental setup are finalized with four types of AR units (AR1, AR2, AR3 and AR4) and in total 175 tests are carried out. Behaviour of breaking and non-breaking waves is observed to be different especially over config-3 and config-4. Landward vortex and breaker tongue are not fully developed in config-3 due to depth limited scenario. Additional non-linearities in the 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 for plunging and surging waves at lower wave heights. However, higher waves exhibit greater damage reduction and formulas show larger deviations. Results indicate that one row of AR placed as toe, does not reduce much damage (10%). A comparison of all the configurations indicate that config-3 and config-4 show an average damage reduction of 38% and 51% 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 15m reef length a damage reduction upto 45% is observed. Reef porosity is observed to have dependency on placement location and reef length. Ecological performance is predicted to increase by 25% in 10 years of construction. However, differently chosen indicator species might have shown better results. It is concluded from the study that green-grey hybrid structures can be a suitable short-term climate change adaption measure.

CoMEM
Living Breakwaters
Climate Change Adaption
Artificial Reefs

http://resolver.tudelft.nl/uuid:39d037d6-10a0-4031-ba65-09bda58cb617

2020-08-01

Student theses

master thesis

© 2019 Nauman Raza