Rubblemound Structures

Breakwater Design

Haryo Dwito Armono

Rubble Mound Structures

Humboldt, California

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Concrete Armour Unit

quadripod stones Core-loc®

dolos Accropod ® Tetrapod

Tetralpod
tribar Concrete block

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Type of Breakwaters

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Types of Breakwaters

• Rubble Mound Breakwater

• Composite Breakwater
• Floating Breakwater
• Submerged Breakwater

Rubble Mound Breakwater (Structure)

– Consist of interior graded layers of stone and an
outer armor layer. Armor layer may be of stone or
specially shaped concrete units.

– Adaptable to a wide range of water depths, suitable

on nearly all foundations

– Layering provides better economy (large stones are

more expensive) and the structure does not
typically fail catastrophically (i.e. protection
continues to be provided after damage and repairs
may be made after the storm passes).

– Readily repaired.
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Rubble Mound Breakwater (Structure)
– Armor units are large enough to resist wave attack,
but allow high wave energy transmission (hence
the layering to reduce transmission). Graded layers
below the armor layer absorb wave energy and
prevent the finer soil in the foundation from being
undermined.

– Sloped structure produces less reflected wave

action than the wall type.

– Require larger amounts of material than most other

types
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Composite / Wall-Type Breakwaters

– Typically consist of cassions (a concrete or steel shell filled with
sand or gravel) sitting on a gravel base (also known as vertical
wall breakwater). Exposed faces are vertical or slightly inclined
(wall-type)

– Sheet-pile walls and sheet-pile cells of various shapes are in

common use.

– Reflection of energy and scour at the toe of the structure are

important considerations for all vertical structures.

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Composite / Wall-Type Breakwaters
– If forces permit and the foundation is suitable, steel-sheet pile
structures may be used in depths up to about 15m.

– When foundation conditions are suitable, steel sheet piles may

be used to form a cellular, gravity-type structure without
penetration of the piles into the bottom material.

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Floating Breakwater

• Potential application for boat basin protection,

boat ramp protection, and shoreline erosion
control.

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 Comparison

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Types of Breakwaters
• Attached or detached ?
• Overtop or Nonovertop?
• Submerged or emerged ?
• Floating or not?
• Single or Double?
• Weir section or not?
• Deflector Vane
• Arrow head
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 Attached vs Detached
i. If the harbour is on the open coastline, predominant wave crests
approach parallel to the coastline, a detached offshore breakwater
might be the best option.

ii. An attached breakwater extended from a natural headland could be

used to protect a harbor located in a cove (bay/inlet).

iii. A system of attached and detached breakwaters may be used.

iv. An advantage of attached breakwaters is ease of access for

construction, operation, and maintenance; however, one
disadvantage may be a negative impact on water quality due to
effects on natural circulation

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Overtop structures
• crest elevation allows larger waves to wash across the crest Æ wave heights
on the protected side are larger than for a non-overtopped structure.

• crest elevation determines the amount of wave overtopping expected

i. Hydraulic model investigation to find the magnitude of transmitted wave
heights

ii. Optimum crest elevation Æ minimum height that provides the needed
protection.

• crest elevation may be set by the design wave height that can be expected
during the period the harbor will be used (especially true in colder climates).

• more difficult to design because their stability response is strongly affected by

small changes in the still water level.

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 Non overtop structures
• elevation precludes any significant amount of wave energy from
coming across the crest.

• Non-overtopped breakwaters or jetties

i. Greater degree of wave protection
ii. More costly to build because of the increased volume of materials
required.

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Submerged Structures
a. Example: A detached breakwater constructed parallel to the
coastline and designed to dissipate sufficient wave energy to
eliminate or reduce shoreline erosion.
b. Advantages:
i. Less expensive to build.
ii. May be aesthetically more pleasing (do not encroach on
any scenic view)
c. Disadvantages:
i. Significantly less wave protection is provided
ii. Monitoring the structure's condition is more difficult.
iii. Navigation hazards may be created.
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 Single or Double?
• Jetties:
• Double parallel jetties will normally be required to direct
tidal currents to keep the channel scoured to a suitable
depth.
• However, there may be instances where coastline
geometry is such that a single updrift jetty will provide a
significant amount of stabilization.
• One disadvantage of single jetties is the tendency of
the channel to migrate toward the structure.
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Single or Double?
• Breakwaters:
• Choice of single or double breakwaters will depend on
such factors as coastline geometry and predominant
wave direction.
• Typically, a harbor positioned in a cove will be
protected by double breakwaters extended seaward
and arced toward each other with a navigation opening
between the breakwater heads.
• For a harbor constructed on the open coastline a single
offshore breakwater with appropriate navigation
openings might be the more advantageous.
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 Weir Section or not?
• Some jetties are constructed with low
shoreward ends that act as weirs.
• Water and sediment can be transported over
this portion of the structure for part or all of a
normal tidal cycle.
• The weir section, generally less than 200m
long, acts as a breakwater and provides a
semi-protected area for dredging of the
deposition basin when it has filled.
• The basin is dredged to store some
estimated quantity of sand moving into the
basin during a given time period. A hydraulic
dredge working in the semi-protected waters
can bypass sand to the downdrift beach. 21

Deflector Vanes
• In many instances where jetties are used to help maintain a
navigation channel,
– currents will tend to propagate along the ocean-side of the jetty and
– deposit their sediment load in the mouth of the channel.

• Deflector vanes can be incorporated into the jetty design to aid in

turning the currents and thus help to keep the sediments away
from the mouth of the channel.
• Position, length, and orientation of the vanes can be optimized in
a model investigation.

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 Arrowhead Breakwaters
• When a breakwater is constructed parallel to the coastline navigation
conditions at the navigation opening may be enhanced by the addition of
arrowhead breakwaters. Prototype experience with such structures
however has shown them to be of questionable benefit in some cases.

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General Design Description

• Multi-layer design. Typical design has at least three major
layers:
1. Outer layer called the armor layer (largest units, stone or specially
shaped concrete armor units)
2. One or more stone underlayers
3. Core or base layer of quarry-run stone, sand, or slag (bedding or
filter layer below)
• Designed for non-breaking or breaking waves, depending on the
positioning of the breakwater and severity of anticipated wave
action during life.
• Armor layer may need to be specially shaped concrete armor
units in order to provide economic construction of a stable
breakwater.
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Cross section Breakwater

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 Concrete Armour Unit

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Design Wave
1. Usually H1/3, but may be H1/10 to reduce repair costs
(Pacific NW) (USACE recommends H1/10)
2. The depth limited breaking wave should be calculated
and compared with the unbroken storm wave height,
and the lesser of the two chosen as the design wave.
(Breaking occurs in water in front of structure)
3. Use Hb/db ~ 0.6 to 1.1
4. For variable water depth, design in segments

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 Breaking Wave
• The design breaker height (Hb) depends on
– the depth of water some distance seaward from the structure toe
where the wave first begins to break.
– This depth varies with tidal stage.

• Therefore, the design breaker height depends on

– the critical design depth at the structure toe (ds),

– the slope on which the structure is built (m),

– incident wave steepness (Hi, T)

– the distance traveled by the wave

during breaking (xP).

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Breaking Wave
• Assume that the design wave plunges on the structure

db/Hb

• If the maximum design depth at the structure toe and the

incident wave period are known, the design breaker height
can be determined from the chart given in Figure 7-4 of the
SPM, 1984.

• Calculate ds/(gT2), locate the nearshore slope and

determine Hb/ds
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Waterlevel and Datum

• Both maximum and minimum water levels are needed for the
designing of breakwaters and jetties.

• Water levels can be affected by storm surges, seiches, river

discharges, natural lake fluctuations, reservoir storage limits, and
ocean tides.

– High-water levels are used to estimate maximum depth-limited

breaking wave heights and to determine crown elevations.

– Low-water levels are generally needed for toe design.

• Structural features should be referred to appropriate low-water

datum planes.
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 Design Parameters
– h water depth of structure relative to design high
water (DHW)
– hc breakwater crest relative to DHW
– R freeboard, peak crown elevation above DHW
– ht depth of structure toe relative to still water level
(SWL)
– B crest width
– Bt toe apron width
α front slope (seaside)
αb back slope (lee)
– t thickness of layers
– W armor unit weight

• DHW varies may be MHHW, storm surge, etc.

• SWL may be MSL, MLLW, etc.
• Wave setup is generally neglected in determining DHW
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Cross section picture

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 Procedure

1. Specify Design Condition Æ design wave (H1/3, Hmax,

To, Lo, depth, water elevation, overtopping, breaking,
purpose of structure, etc.)
2. Set breakwater dimensions Æ h, hc, R, ht, B, α, αb
3. Determine armor unit size/ type and underlayer
requirements
4. Develop toe structure and filter or bedding layer
5. Analyze foundation settlement, bearing capacity and
stability
6. Adjust parameters and repeat as necessary
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Elevation, Run up and Overtopping

• Wave breaking on a slope causes up-rush and down-rush.

– The maximum and minimum vertical elevation of the water surface from SWL is called
run-up (Ru) and run-down (Rd).
– Non-dimensionalize with respect to wave height ÆRu/H and Rd/H.

• Overtopping occurs if the freeboard (R) is less than the set-up + Ru.

• Generally neglect wave setup for sloped structures

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 Elevation, Run up and Run down
• Freeboard may be zero if overtopping is allowed. Freeboard may also be set to
achieve a given allowed overtopping.

• Run-up and run-down are functions of surf similarity (ξ), permeability, porosity
and surface roughness of the slope.

• Effects of Permeability - Flow fields induced in permeable structures by wave

action result in reduced run-up and run-down, but increased destabilizing forces
(see diagram).

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 Elevation, Run up and Run down
• Reduction factors are applied to the Run-up formula to account for
roughness, oblique waters and overtopping : RuR/Hs = (Ru/Hs) * γI

• Run-down is typically 1/3 to 1/2 of the run-up, may be used to determine

– the minimum downward extension of the main armor and
– a possible upper level for introducing a berm with reduced armor size.

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Designing to an Allowable Overtopping

• Overtopping depends on
– relative freeboard, R/Hs,
– wave period, wave steepness,
– permeability,
– porosity, and
– surface roughness.

• Usually overtopping of a rubble structure such as a breakwater or jetty

can be tolerated only if it does not cause damaging waves behind the
structure.

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Crest
Concrete Caps
Considered for strengthening the crest, increasing crest height,
providing access along crest for construction or maintenance.
Evaluate by calculating cost of cap vs. cost of increasing
breakwater dimensions to increase overtopping stability

Crest/ Crown Width

Depends on degree of allowed overtopping. Not critical if no
overtopping is allowed. Minimum of 3 armor units or 3 meters for
low degree of overtopping.

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Wave Transmission
• Wave transmission behind rubble mound breakwaters is caused by wave
regeneration due to overtopping and wave penetration through voids in
the breakwater.
• Affected by:
– Crest elevation
– Crest width seaside and lee-side face slopes
– Rubble size
– Breakwater porosity
– Wave height,
– wave length and
– water depth

KT = HT / Hi
– HT = transmitted wave height
– Hi = incident wave height 44
 Armour Unit Size

• Considerations:
– Slope: flatter slope Æ smaller armor unit weight but more
material required
– Seaside Armor Slope - 1:1.15 to 1:2
– Harbor-side (leeside) Slope
– Minor overtopping/ moderate wave action - 1:1.25 to 1:1.5
– Moderate overtopping/ large waves - 1:1.33 to 1:1.5
• harbor-side slopes are steeper, subject to landslide type failure

• Trunk vs. head (end of breakwater)

– head is exposed to more concentrated wave attack Æ want
flatter slopes at head (or larger armor units)
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• Overtopping Æ less return flow/ action on seaward side but more on

leeward
• Layer dimensions Æ thicker layers give more reserve stability if
damaged
• Special placement Æ reduces size required, generally limited to
concrete armor units
• Concrete armor units (may be required for more extreme wave
conditions)
– Advantage
• increase stability, allow steeper slopes (less material
required), lighter weight.
– Disadvantage
• breakage results in lost stability and more rapid deterioration.
Hydraulic studies have indicated that up to 15 percent random
breakage of doles armor units may be experienced before
stability is threatened, and up to five broken units in a cluster
can be tolerated.

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Considerations
1. Availability of casting forms
2. Concrete quality
3. Use of reinforcing bar (required if > 10-20 t)
4. Placement
5. Construction equipment availability
**When using special armor units, under layers are
sized based on stone armor unit weight

Note: See also

• Jeff Melby’s Presentation on Concrete Armour Unit
• Delft Breakwater Course Pictures on Armour Units (PIANC)

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Armour Unit Stability

• Hudson’s Formula is based on a balance of forces to
ensure each armor unit maintains stability under the forces
exerted by a given wave attack.
• W = median weight of armor unit
• D = diameter of armor unit
• γa = unit weight of armor
• H = design wave height (note affect of cubic power on
armor wt.)
• KD = stability coefficient (Table 1 below, from SPM)
• SG = γa/γw = ρa/ρw
(generally. SG = 2.65 for quarry stone, 2.4 for concrete)

• α = slope angle from the horizontal

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Hudson’s Limitations
• Restrictions on Hudson equation:
1. KD not to exceed Table 1 (from SPM) values
2. Crest height prevents minor wave overtopping
3. Uniform armor units Æ 0.75W to 1.25W
4. Uniform slope Æ 1:1.5 to 1:3
5. 1.9 t/m3 ≤ γa ≤ 2.9 t/m3

• Not considered in Hudson equation

– incident wave period
– type of breaking (spilling, plunging, surging)
– allowable damage level (assumes no damage)
– duration of storm (i.e. number of waves)
– structure permeability
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Bottom Elevation of Armour Layer
• Armor units in the cover layer should be extended downslope to
an elevation below minimum still water level equal to 1.5H when
the structure is in a depth greater than 1.5H.
• If the structure is in a depth of less than 1.5H, armor units should
be extended to the bottom.
• Toe conditions at the interface of the breakwater slope and sea
bottom are a critical stability area and should be thoroughly
evaluated in the design.
• The weight of armor units in the secondary cover layer, between
-1.5H and -2H, should be approximately equal to one-half the
weight of armor units in the primary cover layer (W/2).
• Below -2H. the weight requirements can be reduced to
approximately W/l5 .
• When the structure is located in shallow water, where the waves
break, armor units in the primary cover layer should be extended
down the entire slope. 51

• The above-mentioned ratios between the weights of armor

units in the primary and secondary cover layers are applicable
only when stone units are used in the entire cover layer for the
same slope.
• When pre-cast concrete units are used in the primary cover
layer, the weight of stone in the other layers should be based
on the equivalent weight of stone armor

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Armor Layer Thickness

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 Modified Allowable Wave Height
• The concept of designing a rubble-mound
breakwater for zero damage is unrealistic,
because a definite risk always exists for the stability
criteria to be exceeded in the life of the structure.

• Information presented in table 3 may be used to

estimate anticipated annual repair costs, given
appropriate long-term wave statistics for the site.

• If a certain level of damage is acceptable, the design

wave height may be reduced.

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• H/HD=0 is a function of the percent damage, D, for various armor units.

• H is the wave height corresponding to damage D.
• HD=0 is the design wave height corresponding to 0 to 5 percent damage,
generally referred to as the no-damage condition.

Example :
Rough quarry stone breakwater with a design wave height for D = 0% of H = 3 m and
acceptable D = 10-15% Æ H/HD=0 = 1.14
If the 10-15% damage at H = 3 m is acceptable, the design wave height may be reduced to
3m /1.14 = 2.6 m.
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Underlayer Design

• Armor Layer provides structural stability against external forces

(waves) Underlayers prevent core or base material from
escaping.
• Requirements:
1. Prevent fine material from leaching out.
2. Allow for sufficient porosity to avoid excessive pore pressure build-
up inside the breakwater that could lead to instability or liquefaction
in extreme cases
Note: requirements are in conflict, Engineers must provide an optimum
solution
• Armor layer units are large Æ satisfy (2) above readily •
• Based on spherical shape geometry , core material cannot
escape the cover layer if the diameter ratio of the cover material
(D) to the core material (d) is less than six. (i.e. D/d < 6)
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• First Underlayer (directly under the armor units)
minimum two stone thick (n = 2)
1. under layer unit weight = W/10
• if cover layer and first underlayer are both stone
• if the first underlayer is stone and the cover layer is
concrete armor units with KD ≤ 10
2. under layer unit weight = W/15 when the cover layer is of
armor units with KD > 10
• Second Underlayer - n = 2 thick, W/200
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Bedding / Filter Layer Design

• Layer between structure and foundation or between

cover layer and bank material for revetments.
• Purpose is to prevent base material from leaching out,
prevent pore pressure build-up in base material and
protect from excessive settlement.
• Should be used except when:
1. Depths > 3Hmax, or
2. Anticipated currents are weak (i.e. cannot move
average foundation material), or
3. Hard, durable foundation material (i.e. bedrock)

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• Cohesive Material:
May not need filter layer if foundation is cohesive material.
A layer of quarry stone may be placed as a bedding layer
or apron to reduce settlement or scour.
• Coarse Gravel:
Foundations of coarse gravel may not require a filter
blanket.
• Sand:
a filter blanket should be provided to prevent waves and
currents from removing sand through the voids of the
rubble and thus causing settlement.
• When large quarry-stone are placed directly on a sand
foundation at depths where waves and currents act on the
bottom (as in the surf zone), the rubble will settle into the
sand until it reaches the depth below which the sand will
not be disturbed by the currents.
Large amounts of rubble may be required to allow for the
loss of rubble because of settlement. This, in turn, can
provide a stable foundation.

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 • General guidelines for stability against wave attack.
– Bedding Layer thickness should be:
• 2-3 times the diameter for large stone
• 10 cm for coarse sand
• 20 cm for gravel
• For foundation stability Bedding Layer thickness
should be at least 2 feet
• Bedding Layer should extend 5 feet horizontally
beyond the toe cover stone.

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• Geotextile filter fabric may be used as a substitute for a bedding

layer or filter blanket, especially for bank protection structures.
• When a fabric is used, a protective layer of spalls or crushed
rock (7-inch maximum to 4-inch minimum size) having a
recommended minimum thickness of 2 feet should be placed
between the fabric and adjacent stone to prevent puncture of
the fabric.
• Filter criteria should be met between the protective layer of
spalls and adjacent stone.
– Advantages: uniform properties and quality.
– Disadvantage: susceptible to weathering, tearing, clogging and
flopping.

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Toe Structure
• No rigorous criteria.
• Design is complicated by interactions between main
structure, hydrodynamic forces and foundation soil.
• Design is often ad hoc or based on laboratory testing.
• Toe failure often leads to major structural failure.
• Functions of toe structure:
1. support the armor layer and prevent it from sliding (armor
layer is subject to waves and will tend to assume the
equilibrium beach profile shape)
2. protect against scouring at the toe of the structure
3. prevent underlying material from leaching out
4. provide structural stability against circular or slip failure
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 Toe Structure Stability
• For larger ht Æ smaller stone sizes are required (wave action is
reduced as depth increases). From experiments (CIAD report, 1985):

• Above equations are guidelines.

• CEM/SPM recommends berm width at toe be at least 3 armor stones and the
height at least 2. Actual width and height should be checked by circular
stability analysis.
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Scour Considerations
• If no Toe Structure is used, armor layer should extend below maximum
scouring depth and the breakwater slope may require adjustment to reduce
scour.

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• For Vertical Breakwater

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Low Crested Breakwater

• Highest part of breakwater is at or below MSL
1. Stabilize beach/ retain sand after nourishment
2. Protect larger structures
3. Cause large storm waves to break and dissipate energy
before reaching the beach

• Traditional high-crested breakwaters with a multi-layered

cross section may not be appropriate for a structure used
to protect a beach or shoreline.
• Adequate wave protection may be more economically
provided by a low-crested or submerged structure
composed of a homogeneous pile of stone.

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