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Rubblemound Structures

Breakwater Design

Haryo Dwito Armono

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Rubble Mound Structures

Humboldt, California

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Concrete Armour Unit

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quadripod

dolos

tribar

stones Core-loc®

Accropod ® Tetrapod

Tetralpod

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

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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 verticalwall 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 crestsapproach parallel to the coastline, a detached offshore breakwater might be the best option.

ii. An attached breakwater extended from a natural headland could beused 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 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 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 Structuresa. 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 200mlong, 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.

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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 structureshowever 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 armorunits 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 waveduring breaking (xP).

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

• 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

db/Hb

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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 design wave (H1/3, Hmax, To, Lo, depth, water elevation, overtopping, breaking, purpose of structure, etc.)

2. Set breakwater dimensions h, hc, R, ht, B, α, αb

3. Determine armor unit size/ type and underlayerrequirements

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 Ru/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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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 forlow degree of overtopping.

Crest

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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

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Armour Unit Size

• Considerations: – Slope: flatter slope 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 want

flatter slopes at head (or larger armor units)

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• Overtopping less return flow/ action on seaward side but more on leeward

• Layer dimensions thicker layers give more reserve stability if damaged

• Special placement 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 0.75W to 1.25W

4. Uniform slope 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 extendeddown the entire slope.

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• 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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• 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.

Modified Allowable Wave Height

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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% 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 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 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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