Introduction to coastal structures and design of breakwater

1. CHAPTER 5: INTRODUCTION TO
COASTAL STRUCTURES
DR. MOHSIN SIDDIQUE
ASSISTANT PROFESSOR
1
0401444 - Intro. To Coastal Eng.
University of Sharjah
Dept. of Civil and Env. Engg.

2. INTRODUCTION TO COASTAL STRUCTURES
• Coastal protection, shore protection and sea defense
• Protection of the coast and the shore against the erosive forces of waves,
currents and storm surge can be performed in many ways, and protection
of coast areas against flooding adds ever more types to the protection
defense measures.
• A summary of the various types of protection and management
measures are categories as follows:
• Coastal protection
• Shore protection
• Beach construction
• Management solution
• Sea defense
2

3. INTRODUCTION TO COASTAL STRUCTURES
• Coastal protection, shore protection and sea defense
• Coastal protection
• Sea wall, revetment, emergency protection, bulkhead
• Shore protection
• Regulation of coastal land scape, nourishment, beach de-watering or
beach drain
• Mixed Coastal/Shore protection
• Groynes, detached breakwaters, headlands, perched beach, cove
and artificial pocket beach
• Beach construction
• Artificial beach, beach parks, and beach reclamation
• Management solution
• Land use restrictions
• Sea defense
• Sea dike, artificial dune, Marsh/Magrove platform restoration
3

4. INTRODUCTION TO COASTAL STRUCTURES
Coastal Structures
Sea dike
Sea wall
Revetment
Emergency protection
Bulkheads
Groynes
Jetty
Breakwater
Headland
4
Today’s
content for
discussion

5. INTRODUCTION TO COASTAL STRUCTURES
Sea dike
• Sea dikes are onshore structures with the principal function of protecting
low-lying areas against flooding.
• Sea dikes are usually built as a mound of fine materials like sand and clay
with a gentle seaward slope in order to reduce the wave runup and the
erodible effect of the waves.
• The surface of the dike is armored with grass, asphalt, stones, or concrete
slabs.
5
Sea dike near Westkapelle
(the Netherlands)
Cross-section of a sea dike

6. INTRODUCTION TO COASTAL STRUCTURES
Sea wall
• A seawall is defined as a structure separating land and water.
• It is designed to prevent coastal erosion and other damage due to wave
action and storm surge, such as flooding.
• Sea walls are normally very massive structures because they are
designed to resist the full force of wave and storm surge.
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7. INTRODUCTION TO COASTAL STRUCTURES
Sea wall
Applicability: A sea wall is passive structure, which protects the coast
against erosion and flooding.
Seawalls are often used at location off exposed city fronts, where good
protection was needed and where space was scarce.
7

8. INTRODUCTION TO COASTAL STRUCTURES
Revetments:
• Revetments are onshore structures
with the principal function of
protecting the shoreline from
erosion.
• Revetment structures typically
consist of a cladding of stone,
concrete, or asphalt to armor
sloping natural shoreline profiles.
• In the Corps of Engineers, the
functional distinction is made
between seawalls and revetments
for the purpose of assigning project
benefits; however, in the technical
literature there is often no
distinction between seawalls and
revetments.
8
http://www.pilebuck.com/highways-coastal-environment-second-
edition/chapter-6-coastal-revetments-wave-attack/

9. INTRODUCTION TO COASTAL STRUCTURES
Revetments can be an exposed as well as a buried structure.
9

10. INTRODUCTION TO COASTAL STRUCTURES
Damaged flex slab revetment South of Merang in Terengganu, Malaysia
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11. INTRODUCTION TO COASTAL STRUCTURES
Revetments:
11
Concrete block revetment Damaged Gabion revetment
Revetment constructed using geotextile
and tubes
Revetment constructed using grain bags
and piles

12. INTRODUCTION TO COASTAL STRUCTURES
Revetments:
Applicability: A revetment is a passive structure, which protects
against erosion caused by wave action, storm surge and currents.
• The main difference in the function of a sea wall and a revetment
is that a seawall protects against erosion and flooding, whereas a
revetment only protects against erosion.
• A revetment is thus a passive coastal protection measure and is
used in locations exposed to erosion or as a supplement to
seawalls or dikes at location exposed to both erosion and flooding.
• Revetments are used on all types of coasts
12

13. INTRODUCTION TO COASTAL STRUCTURES
Emergency protection:
Emergency protection is a quick
installation of a temporary
revetment type structure made by
available material as “response” to
unexpected coastal erosion.
It is normally applied for securing
building or infrastructure against
unexpected erosion.
There are by nature quickly build
and not well designed.
Rock dumping
Sand bagging
Dumping of any other easily
available material
13
Emergency revetment constructed by
rubble concrete

14. INTRODUCTION TO COASTAL STRUCTURES
Emergency protection:
Applicability: Private and public land owners are sometime forced
to “construct” emergency protection at locations where “unexpected”
erosion occurs.
• The emergency protection is installed in order to prevent further
damage to coastal installation.
• “Unexpected” can have different causes as discussed below:
• It can be in forms of a rare extreme events
• It can be development of ongoing chronic erosion at locations
where it has not been possible to provide fund for a proper and
timely protection.
14

15. INTRODUCTION TO COASTAL STRUCTURES
Bulkheads:
• A bulkhead is a structure or
partition used to retain or prevent
sliding of the land.
• A secondary purpose is to protect
the coast against damage from
wave action.
• Bulkhead area normally smaller
than seawalls, as their primary
function is to retain fill at locations
with only limited wave action and
not to resist coastal erosion.
• A bulkhead cannot really be
characterized as a coastal
protection structure; it is rather a
structure that is used to retain fills
along the water perimeter of
reclaimed areas and in port basins.
15
Bulkhead structure constructed by
Gabion mesh boxes

16. INTRODUCTION TO COASTAL STRUCTURES
Bulkhead:
Applicability: These function well as a separation between land and
sea in marine basins and along protected shorelines.
These are used along natural shorelines and along filled area, where a
well defined separation between land and sea is required.
These can be used against sea level rise if adjusted in height and if the
area is still protected against wave despite the sea level rise.
These are not used to protect against erosion.
16

17. INTRODUCTION TO COASTAL STRUCTURES
Groynes:
• These are normally straight structures perpendicular (could be slightly
oblique) to the shoreline. They work by blocking the littoral drift (sediment
transport), whereby they trap/maintain sand on their upstream side.
17

18. INTRODUCTION TO COASTAL STRUCTURES
Groynes:
18
Groyne field at south coast of Beira,
Mozambique
An old groyne field at Danish north sea
coast

19. INTRODUCTION TO COASTAL STRUCTURES
Groynes:
• They can have special shapes and they can be emerged, sloping
or submerged, they can be constructed as single structure or as a
groyne field.
• They are build a rubble mound structures but they can be
constructed in other materials such as concrete, timber, geo-tube
etc.
Applicability: These are generally applicable against chronic
erosion as groynes are active when there is a net longshore
transport. Groynes are not applicable against acute erosion.
• Groins retain their protective capability on the upstream coast
during sea level rise provided they are high enough and that they
are not backcut but the leeside erosion will increase.
19

20. INTRODUCTION TO COASTAL STRUCTURES
Jetty or Jetties:
• They are shore-normal stone structures commonly used for
training navigation channels and stabilizing inlets.
• They prevent intrusion of long shore sediment transport
• They cause higher flow velocity that scour the channel to a depth
required for safe navigation
20The panru jetty, Estonia

21. INTRODUCTION TO COASTAL STRUCTURES
Detached breakwaters-Emerged breakwaters;
It is structure parallel or close to parallel, to the coast, build inside or
outside the surf-zone.
Detached breakers are mainly with two purposes, either to protect a
ship wharf from wave action or as a coast/shore protection measure.
21
Breakwater
coastalresearch.sakura.ne.jp
Segmented detached breakwater scheme

22. INTRODUCTION TO COASTAL STRUCTURES
Detached breakwaters-Emerged breakwaters;
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23. INTRODUCTION TO COASTAL STRUCTURES
Detached breakwaters-Emerged breakwaters:
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A detached breakwater provides
shelter from waves, whereby the
littoral transport behind the
breakwater is decreased and the
transport pattern adjacent to the
breakwater is modified.

24. INTRODUCTION TO COASTAL STRUCTURES
Detached breakwaters-offshore breakwaters:
• These are located relatively far outside the surf zone x*>3. The purpose
of an offshore breakwater is normally to protect an offshore ship wharf
against wave action, which means that offshore breakwater is a special
type of port.
24
Sand accumulation forming a salient in
shore line behind offshore breakwater

25. INTRODUCTION TO COASTAL STRUCTURES
Detached breakwaters-coastal breakwaters:
• These are located within a distance from the shoreline of half the width of
surf-zone, up to twice the width of the surf-zone, 2>x*>0.5.
• Such breakwaters trap sand within the part of the littoral zone they cover,
thus ensuring that part of the coastal profile against erosion.
25
Coastal breakwater, which has form (an unstable)
tombolo, (Sri lanka)

26. INTRODUCTION TO COASTAL STRUCTURES
Detached breakwaters-beach breakwaters:
• These are located within less than half the width of the surf-zone from the
shoreline, x*<0.5.
• Beach breakwater traps sand on the foreshore without interfering
significantly with the overall transport pattern.
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27. INTRODUCTION TO COASTAL STRUCTURES
• A segment breakwater
scheme provides many
possibilities ranging from
total coastal protection to
mid shore protection
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28. INTRODUCTION TO COASTAL STRUCTURES
Applicability:
Breakwater are able to protect sections of shoreline in a more
diversified and less harmful way than groynes
A breakwater can, for example, trap sand on a coast line with a
perpendicular wave approach, which is hardly the case for a groyne.
The applicability of breakwater to different types of needs to be studies
before opting a particular design.
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29. INTRODUCTION TO COASTAL STRUCTURES
Other type of breakwater:
Submerged breakwater
Floating breakwater
Special type breakwater
29

30. INTRODUCTION TO COASTAL STRUCTURES
Other type of breakwater:
Submerged breakwater: They function by provoking wave-breaking
and by allowing some wave transmission so that a milder wave
climate is obtained in lee of the submerged structure, although it is
not as mild as if the structure was emerged.
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31. INTRODUCTION TO COASTAL STRUCTURES
Other type of breakwater:
Floating breakwater: They work by dissipating and reflecting part of
wave energy. No surplus water is brought into the sheltered area in
this situation.
31
Example of floating breakwater
(Fezzano,SP-Italy)
http://www.coastalwiki.org/wiki/Floating_bre
akwaters
Floating breakwater and wave attenuator
http://www.wavebrake.com/

32. INTRODUCTION TO COASTAL STRUCTURES
Other type of breakwater:
Special type breakwater: Various specially designed concrete
element type submerged and permeable breakwaters and flexible
tube breakwaters have been introduced onto the market.
32
Geo-textile Tubes for Breakwaters and Beach
Restoration in Mexico [https://www.geosynthetica.net/]

33. INTRODUCTION TO COASTAL STRUCTURES
Headlands or modified
breakwater:
A series of breakwater and
groynes constructed in an
“attached” fashion.
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34. INTRODUCTION TO COASTAL STRUCTURES
Headlands or modified breakwater:
34
http://www.vims.edu/

35. INTRODUCTION TO COASTAL STRUCTURES
Headlands or modified breakwater:
Applicability:
• The curved breakwater, the breakwater connected to the shore, and
headland are useful substitute for traditional groyne and breakwaters
on coasts.
• The curved breakwater can be used for all types of coasts, while
choice of traditional breakwater depends on coast type.
• Neither the breakwater connected to the shore nor the headland can
be used as replacement for traditional segmented breakwater with
small gaps and pocket beaches.
• The impact of sea water level on curve breakwater is same as for the
traditional breakwater
• Headland are generally applicable against chronic erosion as
headlands are active when there is a net longshore transport.
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36. SUMMARY
36

37. REFERENCE
Shore protection guideline, DHI
Coastal Engineering manual
37

38. THANK YOU
38

39. DESIGN OF COASTAL STRUCTURES
Detached breakwaters-Emerged Breakwaters;
It is structure parallel or close to parallel, to the coast, build inside or
outside the surf-zone.
Detached breakers are mainly with two purposes, either to protect a
ship wharf from wave action or as a coast/shore protection measure.
39
Breakwater
coastalresearch.sakura.ne.jp
Segmented detached breakwater scheme

40. DESIGN OF COASTAL STRUCTRUES
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Types of Breakwaters
Floating Breakwater

41. DESIGN OF COASTAL STRUCTURES
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Types of Breakwaters:
• Rubble Mound Breakwater - consists of interior graded layers of stone
and an outer armor layer. Armor layer may be of stone or specially
shaped concrete units.
www.fao.org

42. DESIGN OF COASTAL STRUCTURES
42
Types of Breakwaters:
• Rubble Mound Breakwater
• 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

43. DESIGN OF COASTAL STRUCTURES
43
• 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)
http://constructionmanuals.tpub.com/

44. DESIGN OF COASTAL STRUCTURES
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• Composite or Wall-Type Breakwaters
• 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.

45. DESIGN OF COASTAL STRUCTURES
The main design concern for
composite breakwaters is
stability of the vertical section.

46. DESIGN OF COASTAL STRUCTURES
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Advantages and disadvantages of breakwaters

47. DESIGN OF COASTAL STRUCTURES
47
Floating Breakwaters: potential application for boat basin protection, boat
ramp protection, and shoreline erosion control.
Example of floating breakwater (Fezzano,SP-Italy)
http://www.coastalwiki.org/wiki/Floating_breakwaters

48. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Layout Options for Rubble Mound Breakwaters and Jetties
• General Description
• Design Wave
• Water Levels and Datums
• Design Parameters
• Design Concept/ Procedure
• Structure Elevation, Run-up and Overtopping
• Crest/Crown Width
• Armor Unit Size and Stability
• Underlayer Design
• Bedding and Filter Design
• Toe Structures
• Low Crested Breakwaters

49. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Layout Options for Rubble Mound Breakwaters and Jetties
Attached or Detached.
a. Jetties: usually attached to stabilize an inlet or eliminate channel shoaling.
b. Breakwaters: attached or detached.
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 prevents any significant amount of wave
energy from coming across the crest.
Emerged or submerged.

50. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• General description

51. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• General 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

52. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Design Wave
1. Usually H1/3, but may be H1/10 to reduce repair costs (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 1.1
4. For variable water depth, design in segments

53. RUBBLE MOUND STRUCTURE DESIGN
53
• Rubble Mound Breakwater Design
• Design 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, 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 >>
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.
h=d

54. RUBBLE MOUND STRUCTURE DESIGN
Figure 7-4 of the SPM, 1984

55. RUBBLE MOUND STRUCTURE DESIGN
55
• Rubble Mound Breakwater Design
• Water level 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.
• Tide prediction
• Datum Plane

56. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Design Parameters

57. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• 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

58. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Design Concept/ 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 underlayer requirements
4. Develop toe structure and filter or bedding layer
5. Analyze foundation settlement, bearing capacity and stability (not
discussed here)
6. Adjust parameters and repeat as necessary

59. SUMMARY
59

60. SUMMARY
60

61. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Structural 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.

62. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Structural elevation, run-up and overtopping
• Overtopping occurs if the freeboard (R) is less than the [set-up + Ru]
• Generally neglect wave setup for sloped structures
• 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 parameter, 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).

63. RUBBLE MOUND STRUCTURE DESIGN
63
• Rubble Mound Breakwater Design
• Structural elevation, run-up and overtopping
Run-up may be determined by surf similarity parameter (ξm) and core
permeability (Abbot and Price, 1994)

64. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Structural elevation, run-up and overtopping
Run-down is typically 1/3 to ½ of the run-up and 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.

65. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Structural elevation, run-up and overtopping
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.

66. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Structural elevation, run-up and overtopping
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

67. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• 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.

68. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Wave Transmission
Wave transmission behind rubble mound breakwaters is caused by wave
regeneration due to overtopping and wave penetration through voids in the
breakwater.
It is affected by: Crest elevation; Crest width; seaside and lee-side face
slopes; Rubble size; Breakwater porosity; Wave height, and wave length
and water depth
Given an acceptable lee-side wave height, the crest elevation (hc) and width
(B) can be determined by using the diagram below (note: the diagram is
based on experiments by N. Tanaka, 1976, on a symmetric breakwater with
1:2 seaside and lee-side slopes.)

69. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability

70. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
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

71. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
Considerations:
• 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 requirements, generally limited to
concrete armor units
• Concrete armor units (may be required for more extreme wave
conditions)

72. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
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.
Considerations
• 1. Availability of casting forms
• 2. Concrete quality
• 3. Use of reinforcing (required if > 10-20t)
• 4. Placement
• 5. Construction equipment availability
**When using special armor units, underlayers are sized based on stone armor unit weight

73. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
Hudson's Formula for Determining Armor Unit Weight:
Ref. Hudson, R. Y. (1959)
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
γ
a = 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 (gen. SG = 2.65 for quarry stone, 2.4 for concrete)
α = slope angle from the horizontal

74. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
Hudson's Formula for Determining Armor Unit Weight:
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. 120 pcf ≤ γa ≤ 180 pcf (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

75. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
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.

76. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
The previously-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.
From Table
γstone=165lb/ft3
SGstone=2.34

77. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability

78. RUBBLE MOUND STRUCTURE DESIGN
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79. RUBBLE MOUND STRUCTURE DESIGN
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80. RUBBLE MOUND STRUCTURE DESIGN

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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
Modified Allowable Wave Height Based on Damage
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.
Table 3 shows results of damage tests where 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.
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.

82. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Armor Unit Size and Stability
3

83. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Under layer design
Underlayers 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, Engineer must provide an optimum
solution

84. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Under layer design

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• Rubble Mound Breakwater Design
• Under layer design
(Guidance from Shore Protection Manual)
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
• Minimum two stone thick, (n = 2)
• Under layer unit weight= W/200

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• Rubble Mound Breakwater Design
• Bedding of 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)

87. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Bedding of Filter Layer Design
• 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.

88. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Bedding of Filter Layer Design

89. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Bedding of Filter Layer Design
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.

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• Rubble Mound Breakwater Design
• Toe Structures
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

91. RUBBLE MOUND STRUCTURE DESIGN
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• Rubble Mound Breakwater Design
• Toe Structures

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• Rubble Mound Breakwater Design
• Toe Structure

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• Rubble Mound Breakwater Design
• Toe Structure

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• Rubble Mound Breakwater Design
• Toe Structure

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• Rubble Mound Breakwater Design
• Low Crested Breakwater (from Sorensen)
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.
** Failure occurs by loss of stones from the crest.

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• Rubble Mound Breakwater Design
• Low Crested Breakwater (from Sorensen)

98. DESIGN EXAMPLE
Rubble Mound Breakwater Design

99. RUBBLE MOUND STRUCTURE DESIGN
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• Design Example

100. RUBBLE MOUND STRUCTURE DESIGN
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0
• Solution:

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Specify Design Condition
Still water level (SWL) = 5.5 m,
Design high water (DHW) = 1.7 m
Water depth h (or d) = 5.5 + 1.7 = 7.2m
Assume listed conditions are at structure toe.
Hs = H1/3 = 2 m
T = 8 sec
Lo = 100 m
Using dispersion relation, calculate L at h =7.2m [take d=h]
Lm=62m

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2
Specify Design Condition
Calculate depth limited breaking wave height at structure toe, compare with
unbroken wave height and use less of the two for design
Hb/hb ~ 0.78** [simplified breaking criteria]
at DHW: Hb = 0.78×7.2 = 5.6 m [hb=db]
at SWL: Hb = 0.78×5.5 = 4.3 m
**Alternative breaking methods may be applied.
Both waves heights are greater than Hs which means waves are not
breaking and design H=Hs=2m

103. RUBBLE MOUND STRUCTURE DESIGN
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3
Set BW Dimensions (controlled by height & slope):
Set-up: waves are not breaking per the previous calculation therefore no set-
up
NOTE: there will be a set-down, but this will be neglected and considered an
added factor of safety unless required to reduce the structure size
Wave set up,
Overtopping Discharge (CEM VI-5, pp. 19-33)
0=η

104. RUBBLE MOUND STRUCTURE DESIGN
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Overtopping Discharge (CEM VI-5, pp. 19-33)
From Table VI-5-8 ( given above):
slope 1:2 >> a = 0.013, b = 22
rock riprap > 2D50 thick >> γr ~ 0.55

105. RUBBLE MOUND STRUCTURE DESIGN
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Solving we get;
Qq =
Note

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Run up:

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Run up:

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Run up:

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BW Dimension Summary:
• Assumed
• structure is symmetric, α = αb
• no set-down
• no crown, hc = R
• total settlement = 0.1 m (adjust later)

110. RUBBLE MOUND STRUCTURE DESIGN
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Armor Unit Design:
Assume Armor unit is rough quarry stone, 2 layers, no breaking >> Table
VI-5-22 applies
non-breaking waves, 0-5% damage, random placement: KD = 4

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Armor Unit Design:
sg = γa/γw = (2.5 t/m3)/(1 t/m3) = 2.5

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Armor Thickness:

113. RUBBLE MOUND STRUCTURE DESIGN
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Armor Thickness:

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Under Layer Design:
The goal to reduce the size of the stone to at point where W/wcore ≤ 15-25,
where W is the stone in the layer covering the core.
Roughly, this gives a size of ~W/4000 for the core >> ½ lb stones, with 2 inch
diameter. If some other size is readily available, that might be the goal. Must
check to ensure the W/wcore ≤ 15-25 is met once the core over-layer is
known.

115. RUBBLE MOUND STRUCTURE DESIGN
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Under Layer Design:

116. RUBBLE MOUND STRUCTURE DESIGN
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First Under layer:
~0.7m

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First Under Layer:

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Second Under-Layer:

119. RUBBLE MOUND STRUCTURE DESIGN
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Second Under-Layer :

120. RUBBLE MOUND STRUCTURE DESIGN
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Core:
• Dynamic load requirement based on layer above:
• W/wcore ≤ 15 to 25
• W = 4.5 kg
• wcore ≥ 4.5/25 – 4.5/15 = 0.18 – 0.3 kg
• Based on Armor Layer Thickness
• W4000 = 0.75 t/4000 = 0.00019 t × 1000 = 0.2 kg
• next larger available size is 0.23 kg
W4000 =W/4000
W=

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Core design:

122. RUBBLE MOUND STRUCTURE DESIGN
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Toe Design:

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Toe Design:
See next slide
Toe height, D = 0.76~0.8m

124. RUBBLE MOUND STRUCTURE DESIGN

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Toe Design: (Tanimoto, K., Yagyu, T., and Goda, Y., 1982)

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Toe Design:

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Filter Bed Design:

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Structure Summary:

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Settlement & Bearing Capacity:
Similar to foundation eng…

130. THANK YOU