Physical Model Studies on Breakwaters with Geotextile Sand Container Armour Units
Date
2023
Authors
Elias, Tom
Journal Title
Journal ISSN
Volume Title
Publisher
National Institute Of Technology Karnataka Surathkal
Abstract
Breakwaters are essential constructions providing tranquillity to ports and harbour
structures when natural protection is lacking. Traditionally, these massive structures are
constructed using natural rocks weighing tonnes. In the present scenario, obtaining huge
natural rocks is cumbersome and non-eco-friendly. Harnessing the advantages of geotextile
sand containers (GSCs), numerous submerged breakwaters and shoreline protection
structures have been constructed worldwide. But an emerged breakwater structure with
geotextile armour units, capable of replacing the conventional structures, is rarely
discussed. A 1:30 scaled, monochromatic wave flume physical experimentation is carried
out as a preliminary investigation to test the feasibility of using GSCs as breakwater armour
units. The structural design of the GSC breakwater evolved based on a comprehensive
literature survey. Single-layer GSC breakwater structures armoured with sand-filled units
differing in size and fill percentage (named Bag 1, Bag 2, Bag 3 and Bag 4 ) are investigated
in the initial stage. Studies on hydraulic performance (wave runup, rundown and reflection)
and stability of GSC breakwater are carried out to analyse the efficiency of the structure
against the wave conditions of the Mangaluru coast. The study revealed that the reflection
coefficient (Kr) for GSC structures could range from 0.26 to 0.69. Additionally, reducing
GSC fill percentage from 100 to 80 is found to be more effective (up to 64%) in reducing
reflection, runup and rundown rates than altering GSC size. As far as stability is concerned,
the best-performing single-layer configuration comprising Bag 3 could withstand wave
heights up to 2.7 m in the prototype. The effect of armour unit size and sand fill ratio on
the stability of the structure is analysed, and it is concluded that changing the sand fill ratio
from 80% to 100% shot up the structural stability to a maximum of 14%. Increasing bag
size also resulted in increased stability by up to 8%. Stability curves for all tested
configurations are projected as the significant research outcome and can serve as a practical
guideline for coastal engineers in designing GSC breakwaters.
It's efficacy to be used as the armour units of breakwaters motivated to advance further in
GSC research by adding a second GSC layer to the breakwater model. These double-
layered breakwater models with different placement modes are tested for its hydraulic
iiiperformance and stability. Double layer placement showed 45 to 52% less Kr than single-
layer placement and 38 to 42% lesser than the conventional breakwater, owing to its higher
porosity. It is observed that the stability of the structure increased by up to 17% when
supplemented with double layers. Structure tends to be stable with increasing armour unit
size and fill percentage. Larger bags stacked in double layers is found to be the most stable
configuration. 80% filled, slope parallel placement exhibited the least stability. Double
layer placement with Bag 3 is found to be stable up to a wave height of 0.132 m on the
model scale, which is 3.96 m in the prototype. Stability nomograms for all the tested
double-layer GSC breakwater cases are obtained from physical experimentation.
In the last stage, a pilot study is conducted by filling cement and sand to GSC units of the
best-performing models from the above experiments. When GSC breakwaters are filled
with sand and cement, up to 43% increased stability is observed with a considerable
decrease in wave runup, rundown and reflection. As a result, cement-sand-filled GSC units
can be suggested as a possible alternative to sand-alone-filled units where vandalism has
to be countered.
Description
Keywords
Geotextiles, Coastal Protection, Breakwaters, Wave Flume