Design of Breakwater

Question 1

types

Answer 1

rubble mound
-natural/local material
-smaller construction equipment
-natural reef and less environmental impact
-a lot of material and continually maintained

composite/vertical
-minimal material so its good for deep water
-all energy is reflected with no transmission
-hard and expensive to repair
-needs a level seabed so susceptible to erosion and tilting
-needs specific towing equipment

Question 2

some rubble mound properties

Answer 2

R = elevation above DHW
B = crest width
h = DHW
ht = from toe to SWS

Question 3

rubble mound design procedure

Answer 3

1. determine the design wave, check for breaking before the mound
2. set dimensions
   *structure elevation
   *wave transmission
   *armour layer
   *underlayer
   *core
   *toe
   *bedding/filtering layer
3. check bearing capacity, settlement and stability and material availability
4. iterate if required

Question 4

structure elevation

Answer 4

-no overtopping so R>=Ru
-permeability reduces Ru but increases the destabilising forces
-Rd is 1/2 to 1/3 of Ru

Question 5

wave transmission

Answer 5

caused by penetration or overtopping

reduced by
-face slopes
-porosity of BW
-rubble size
-B,R
-wave height, wavelength and water depth

Question 6

armour layer

Answer 6

-provides stability against waves by gravity, interlocking and surface friction
-made of stones or conc. blocks
-slopes
*1:1.15 to 1:1.2 on the seaside
*1.25 to 1.25 on the leeside
*start with 1:2
*flatter slopes decrease the armour weight but increase the material

Hudson formula
-semi-empirical and based on the balance of forces on each armour unit
*balance of weight and wave lift/drag
-for t: n=No. layers
-for B: n=No. stones (3)
-Assumes
*uniform armour units with 0.75W to 1.25W
*uniform slope of 1:1.5 to 1:3
-doesnt account for permeability and type of breakers

Question 7

underlayer

Answer 7

-prevents core and base material from escaping
-sufficient porosity to avoid excess PP inside BW
-should be at least 2 stones thick

for sorted material
-calm: D15/d85 < 5, D for top layer and d for bottom layer
-dynamic loads: D50/d50 < 2.5 to 3, leads to W/w_base < 15 to 25 with W directly proportional to D^3
*for 1 underlayer: W/10

Question 8

core

Answer 8

-prevent wave/sediment transmission
-increases the structures volume
-made from quarry run
*too coarse increases transmission
*too fine increases washout so more filtering in needed and there is more vulnerability to erosion

Question 9

toe

Answer 9

-prevents sliding of the armour layer
-protects against scouring
*leads to return flow and vortex creation
-prevents leaching out of underlying material
-provides structural stability against circular slip surface
-failure often leads to major structural failure

No rigorous design criteria
-require lab testing
-interaction with structure, soil and hydrodynamics make it complicated
-berm of 3 armour stones
-height of 2 armour stones

Question 10

bedding/filtering layer

Answer 10

-stops the leaching of base material and PP build up with D15/d85 < 4 to 5
* D for filter
* d for base

-stops excessive settlement with D60/d10 < 10 and well sorted material is preferred

-Geotextiles and fabrics can be used
*needs protective layer of crushed rock to prevent puncture
*susceptible to weathering and clogging
*has uniform properties and qualities