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

Nauman Raza
Master thesis
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no.ntnu:inspera:43478875:46716724.pdf (33.73Mb)
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http://hdl.handle.net/11250/2620466
Utgivelsesdato
2019
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  • Institutt for bygg- og miljøteknikk [2808]
Sammendrag
 
 
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 adoption 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 adoption measure.
 
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