2-10-00, Environmental Factors in the Coastal Region, Waves
2-17-00, Breakwaters and Rubble Mound Structure Design
Ref:Shore Protection Manual, USACE, 1984Basic Coastal
Engineering, R.M. Sorensen, 1997Coastal Engineering Handbook, J.B.
Herbich, 1991EM 1110-2-2904, Design of Breakwaters and Jetties,
USACE, 1986Breakwaters, Jetties, Bulkheads and Seawalls, Pile Buck,
1992Coastal, Estuarial and Harbour Engineers' Reference Book, M.B.
Abbot and W.A. Price, 1994, (Chapter 29)
TopicsDefinitions/ Descriptions of Various Coastal
StructuresTypes of BreakwatersRubble Mound Breakwater DesignLayout
Options for Rubble Mound Breakwaters and Jetties General
DescriptionDesign WaveWater Levels and DatumsDesign
ParametersDesign Concept/ ProcedureStructure Elevation, Run-up and
OvertoppingCrest/Crown WidthArmor Unit Size and StabilityUnderlayer
DesignBedding and Filter DesignToe StructuresLow Crested
BreakwatersSlope and Foundation
stability---------------------------------------------------------------------------------------------------------------------
Definitions/ Descriptions of Various Coastal Structures
Breakwater - a structure that protects the area in its lee from
wave attack. Breakwaters can be connected to the shoreline
(attached breakwater) or completely isolated from the shore
(detached breakwater). (rubble mound structure or
composite)Bulkhead, seawall, revetment - structures built to
separate the land from the water to prevent erosion and other
damage primarily due to wave action. Bulkheads are typically
smaller structures designed to retain shore material under less
severe wave conditions than seawalls. Revetments are designed to
protect shorelines and waterways from erosion by currents and small
waves. (generally a rubble mound structure built on sloping bank)
Seawalls are typically large and designed to withstand the full
force of storm waves.Groin - shore perpendicular structure,
installed singly or as a field of groins, designed to trap sand
from the littoral drift system or to hold sand in place. (rubble
mound structure)Jetty - a shore perpendicular structure located
near an inlet or harbor entrance to reduce in-filling of the inlet
or channel, protect the entrance and provide vessel sheltering from
waves. (rubble mound structure)Dolphin - a marine structure
(usually a cluster of piles) for mooring vessels; (1) a mooring
dolphin is designed only as a mooring structure and cannot support
an impact force, (2) a breasting dolphin is designed to support the
impact of a ship when mooringWharf or Quay - a dock consisting of a
reinforced shore or riverbank where ships are loaded or unloaded.
Generally, vessels may only moor on one side of a wharf, but on
either side of a quay.
Various Rubble Mound Structures - breakwaters, jetties and
groinsRelative sizes: breakwater > jetty > groin
Types of BreakwatersRubble 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. 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
Composite or 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.
If forces permit and the foundation is suitable, steel-sheet pile
structures may be used in depths up to about 40 feet. 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.
Floating Breakwaters - potential application for boat basin
protection, boat ramp protection, and shoreline erosion
control.
AdvantagesDisadvantages
Sloped Rubble Mound1. Suitable for irregular bottom2. Suitable
for weak soil (disbursed load)3. Progressive damage4. Low toe
scour5. Simpler construction6. Simpler maintenance1. Required
material increases rapidly with increased water depth2. High
maintenance cost3. Large base cuts into basin size
CompositeVertical1. Material savings (stone required)2. Easy to
maintain (day-to-day)3. Control water depth clearly defined1.
Requires firm soil2. High construction requirements3. Repair
difficult
Low mound1. Suitable for deeper water with less firm soil2. More
economic/ flexible design1. Complicated construction2. More
difficult repair
High mound1. Suitable for deeper water with less firm soil1.
More complicated construction2. More susceptible to breaking
waves
Rubble Mound Breakwater Design Layout Options for Rubble Mound
Breakwaters and Jetties1. Attached or Detached. a. Jetties usually
attached to stabilize an inlet or eliminate channel shoaling. b.
Breakwaters attached or detached. i. If the harbor 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. 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.2.
Overtopped or Non-overtopped. a. Overtopped:crown elevation allows
larger waves to wash across the crest wave heights on the protected
side are larger than for a non-overtopped structure.b.
Non-overtopped: elevation precludes any significant amount of wave
energy from coming across the crest.c. Non-overtopped breakwaters
or jetties i. Greater degree of wave protection ii. More costly to
build because of the increased volume of materials required. d.
Crest elevation determines the amount of wave overtopping
expectedi. Hydraulic model investigation to find the magnitude of
transmitted wave heightsii. Optimum crest elevation minimum height
that provides the needed protection.e. Overtopped breakwateri.
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). ii. Overtopped structures are more difficult
to design because their stability response is strongly affected by
small changes in the still water level.3. Submerged Breakwater 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 providedii. Monitoring the structure's condition
is more difficult.iii. Navigation hazards may be created.4. Single
or Double. a. 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. b. 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.5. Weir Section. 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 500 feet
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.6.
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.7. 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.
Jetties with Weir section and Deflector Vanes
Arrowhead Breakwaters
General DescriptionMulti-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 underlayers3. 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.
Design Wave1. 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/hb
~ 0.6 to 1.14. For variable water depth, design in segments
Breaking Wave Considerations (SPM, Chapter 7)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, the
slope on which the structure is built, incident wave steepness, and
the distance traveled by the wave during breaking.Assume that the
design wave plunges on the structure
ds = depth at structure toe = hb/Hbm = nearshore slopep =
dimensionless plunge distance, = breaker travel distance (xp) /
breaker height (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 below (Figure 7-4 of the SPM,
1984). Calculate ds/(gT2), locate the nearshore slope and determine
Hb/ds.
Water Levels and Datums. 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.
a. Tide Predictions, The National Ocean Service (NOS) publishes
tide height predictions and tide ranges. Figure 2-l shows spring
tide ranges for the continental United States. Published tide
predictions are sufficient for most project designs; however,
prototype observations may be required in some instances.
b. Datum Planes. Structural features should be referred to
appropriate low-water datum planes. The relationship of low-water
datum to the National Geodetic Vertical Datum (NGVD) will be needed
for vertical control of construction. The low-water datum for the
Atlantic and Gulf Coasts is being converted to mean lower low water
(MLLW). Until the conversion is complete, the use of mean low water
(MLW) for the Atlantic and Gulf Coast low water datum (GCLWD) is
acceptable. Other low-water datums are as follows: Pacific Coast:
Mean lower low water (MLLW) Great Lakes: International Great Lakes
Datum (IGLD) Rivers: River, low-water datum planes (local)
Reservoirs: Recreation pool levels
Design Parametershwater depth of structure relative to design
high water (DHW)hcbreakwater crest relative to DHWRfreeboard, peak
crown elevation above DHWhtdepth of structure toe relative to still
water level (SWL)Bcrest widthBttoe apron widthfront slope
(seaside)bback slope (lee)tthickness of layersWarmor unit
weight
DHW varies may be MHHW, storm surge, etc. SWL may be MSL, MLLW,
etc. Wave setup is generally neglected in determining DHW
Design Concept/ Procedure1. 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, , b3. Determine armor unit size/ type and underlayer
requirements4. Develop toe structure and filter or bedding layer5.
Analyze foundation settlement, bearing capacity and stability6.
Adjust parameters and repeat as necessary
Structure Elevation, Run-up and Overtopping
Design elevation (peak crown elevation) = DHW + set-up + run-up
+ freeboard
If overtopping is allowed, freeboard is equal to zero and
allowed overtopping is subtracted from design elevation. Generally
neglect wave setup for sloped structures
Run-up determined by surf similarity parameter (m) and core
permeability
, where Lm is the wave length for the modal period, Tm (deep
water assumed) van der Meer (1988)
for m < 1.5
for m > 1.5
for permeable structures (P > 0.4) run-up is limited to
Ru exceedence probability (%)abcd
0.11.121.340.552.58
20.961.170.461.97
50.861.050.441.68
100.770.940.421.45
500.470.600.340.82
Reduction factors are applied to the Run-up formula to account
for roughness, oblique waters and overtopping
Roughness Reduction Factors are:Reduction factor ()
Smooth impermeable (including smooth concrete and
asphalt)1.0
1 layer of stone rubble on impermeable base0.8
Gravel0.7
Rock rip-rap with thickness > 2D500.5-0.6
Overtopping occurs if water level exceeds the freeboard (R),
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.Owen (1980, 1982)
, where
mean overtopping discharge (in m3/s/m or ft3/s/ft):
use run-up reduction factors, , abovefor straight smooth slopes
(no berms), non-depth limited wavesSlope1:11:1.51:21:31:4
a0.0080.0100.0130.0160.019
b2020223247
determine R based on acceptable for the design
Harbor protection
Vehicles on b.w.
PedestriansConcrete 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 (note: crown may extent above the breakwater
crest)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.
, where W = median weight of armor unit, a = unit weight of
armor, k = layer thickness coefficient (see Table 2)
Armor Unit Size and StabilityConsiderations: Slope: flatter
slope smaller armor unit weight but more material req'dSeaside
Armor Slope - 1:1.15 to 1:2Harbor-side (leeside) Slope Minor
overtopping/ moderate wave action - 1:1.25 to 1:1.5Moderate
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) 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 req'ts, gen. limited to concrete
armor units Concrete armor units (may be required for more extreme
wave conditions)Advantage - increase stability, allow steeper
slopes (less mat'l req'd), lighter wt.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.Considerations1. Availability of casting forms2. Concrete
quality3. Use of reinforcing (req'd if > 10-20 t)4. Placement5.
Construction equipment availabilityWhen using shaped concrete armor
units, underlayers are sized based on stone armor unit weight
Hudson, R. Y. (1959) Laboratory Investigations of Rubble-Mound
Breakwaters, Proceedings of the American Society of Civil
Engineers, American Society of Civil Engineers, Waterways and
Harbors Division, Vol. 85, NO. WW3, Paper No. 2171.W = median
weight of armor unitD = diameter of armor unita = unit weight of
armor (gen. a = 2.65 for quarry stone, 2.4 for shapes)H = design
wave height (note affect of cubic power on armor wt.)KD = stability
coefficient (Table 1 below, from SPM)SG = a/w = slope angle from
the horizontal
Rough analysis of forces give formula for a "dimensionless wave
height" or stability number
Experiments related the stability number to the face slope and
armor unit shape
Combining give Hudson's equation for required armor unit
weight
Restrictions on Hudson equation:1. KD not to exceed Table 1
(from SPM) values2. Crest height prevents minor wave overtopping3.
Uniform armor units 0.75W to 1.25W4. Uniform slope 1:1.5 to 1:35.
120 pcf a 180 pcf (1.9 t/m3 a 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
Bottom elevation of Armor Layer (How deep should armor
extend?)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.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. For example:tetrapods armor designconditions: 20
foot non-breaking wave attack on a structure trunka = 150 lbf/ft3
for tetrapods SG = 150/64 = 2.34slope = lV:2HKD = 8.0 for tetrapod
armorKD = 4.0 for rough angular stone
for tetrapod:
for stone armor: The secondary cover layer from -1.5H to the
bottom should be as thick as or thicker than the primary cover
layer and sized for W = 21 tons.
Armor layer thickness (t) use to calculate size of layer
, where n = number of layers
number of units per surface area A,
Modified Allowable Wave Height Based on Damage (can be used to
estimate maintenance costs)
H/HD=0, where HD=0 is the design wave height corresponding to
0-5 % damage (no-damage condition)
See Table 3 below for H/HD=0 values
Table 1, Stability Coefficient, KD (breaking occurs before the
wave reaches the structure)Structure TrunkStructure Head
KD(b)KDSlope
Armor unitsn(a)PlacementBreaking WaveNon-breaking waveBreaking
WaveNon-breaking wavecot
Quarry stone
Smooth rounded2Random1.22.41.21.91.5 to 3.0
Smooth rounded>3Random1.63.21.42.3(c)
Rough angular1Random (d)(d)2.9(d)2.3(c)
Rough angular2Random2.04.01.93.21.5
1.62.82.0
1.32.33.0
Rough angular>3Special (e)2.24.52.14.2(c)
Rough angular2Special (e)5.87.05.36.4(c)
Parallelepiped (f)2Random7.0 - 20.08.5 - 24.0----(c)
Tetrapod andQuadripod2Random7.08.05.06.01.5
4.55.52.0
3.54.03.0
Tribar2Random9.010.08.39.01.5
7.88.52.0
6.06.53.0
Dolos2Random15.0 (g)31.0 (g)8.016.02.0 (h)
7.014.03.0
Modified Cube2Random6.57.5--5.0(c)
Hexapod2Random8.09.55.07.0(c)
Toskanes2Random11.022.0----(c)
Tribar1Uniform12.015.07.59.5(c)
Quarrystone (KRR) Graded angular--Random2.22.5------
(a) n is the number of wits comprising the thickness of the
armor layer.(b) Applicable to slopes ranging from 1 on 1.5 to 1 on
5.(c) Until more information is available on the variation of KD
value with slope, the use of KD should be limited to slopes ranging
from 1 on 1.5 to 1 on 3. Some armor units tested on a structure
head indicate a KD slope dependence.(d) The use of a single layer
of quarry stone armor units subject to breaking waves is not
recommended, and only under special conditions for non-breaking
waves. When it is used, the stone should be carefully placed.(e)
Special placement with long axis of stone placed perpendicular to
structure face.(f) Long slab-like stone with the long dimension
about three times its shortest dimension.(g) Refers to no-damage
criteria (~5 percent displacement, rocking, etc.); if no rocking (
10Second Underlayer - n = 2 thick, W/20
Bedding or 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, or2.
Anticipated currents are weak (i.e. cannot move average foundation
material), or3. Hard, durable foundation material (i.e. bedrock)
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.
Criteria for granular filter design: To prevent material from
leaching out: d85 = dia. exceeded by the coarsest 15% of the base
mat'l D15 = dia. exceeded by the coarsest 85% of the filter
mat'l(important in breakwater design) To prevent pore pressure
build-up: (important for embankment design)
To maintain filter layer internal stability: (i.e. well sorted
material is preferred). Poorly sorted material is not suitable for
filters (internally unstable too much washes out)
Stability against wave attack of the exposed bedding material
has been found to be analogous to the stability of the armor layer
of a rubble mound structure, with the exceptions that the slope of
the seaward face () vanishes from the problem and the local
wavelength (L) is considered. The required median weight (W50) can
be calculated from the following equation:
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.
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.
Toe StructuresNo 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 structure3. prevent underlying
material from leaching out4. provide structural stability against
circular or slip failure
Toe Structure StabilityFor larger ht smaller stone sizes are
required (wave action is reduced as depth increases). From
experiments:
for 50% confidence level
for 90% confidence level
SPM recommends berm width at toe be at least 3 armor stones.
Actual width and height should be checked by circular stability
analysis. (see discussion below on width design for scour
considerations)
Scour ConsiderationIf no Toe Structure is used, armor layer
should extend below maximum scouring depth and the breakwater slope
may require adjustment to reduce scour.
Generally:, with 1.0 at ~ 2.7The following design equations are
based on preventing or minimizing scour in front of vertical
structures
Toe Apron Width (Bt) - width should be the maximum of Bt = 2H or
Bt = 0.4h
Toe Stone Weight
where Ns = stability number is the maximum of
orNs = 1.8where K = a parameter associated with the maximum
horizontal velocity at the edge of the toe apron
Additional Toe Structure Design References:Headquarters,
Department of the Army. (1985) Design of Coastal Revetments,
Seawalls, and Bulkheads, Engineer Manual 1110-2-1614, Washington,
DC, Chapter 2, pp. 15-19.Hudson, R. Y. (1959) Laboratory
Investigations of Rubble-Mound Breakwaters, Proceedings of the
American Society of Civil Engineers, American Society of Civil
Engineers, Waterways and Harbors Division, Vol. 85, NO. WW3, Paper
No. 2171.Shore Protection Manual. (1984) 4th cd., 2 Vols., US Army
Engineer Waterways Experiment Station, Coastal Engineering Research
Center, US Government Printing Office, Washington, DC, Chapter 7,
pp. 242-249.Tanimoto, K., Yagyu, T., and Goda, Y. (1982) Irregular
Wave Tests for Composite Breakwater Foundations, Proceedings of the
18th Coastal Engineering Conference, American Society of Civil
Engineers, Cape Town, Republic of South Africa, Vol. III, pp.
2144-2161.
Low Crested BreakwatersHighest part of breakwater is at or below
MSL1. Stabilize beach/ retain sand after nourishment2. Protect
larger structures3. Cause large storm waves to break and dissipate
energy before reaching the beachTraditional 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.** Failure occurs by loss of stones from the crest.
Use a modified stability number
L is the wave length at the structure depth and is calculated
using peak period (Tp) for random waves.
Damage Level (S) is defined as: , where As = area of damage (see
diagram) and D50 = median stone size of the breakwater
Given S, hc, h determine Ns* from hc = height of the wave crest
above the sea floorh = water depth at the structure