2-17-00, Breakwaters and Rubble Mound Structure Design

Ref:Shore Protection Manual, USACE, 1984

Basic Coastal Engineering, R.M. Sorensen, 1997

Coastal Engineering Handbook, J.B. Herbich, 1991

EM 1110-2-2904, Design of Breakwaters and Jetties, USACE, 1986

Breakwaters, Jetties, Bulkheads and Seawalls, Pile Buck, 1992

Coastal, Estuarial and Harbour Engineers' Reference Book, M.B. Abbot and W.A. Price, 1994, (Chapter 29)

Topics

Definitions/ Descriptions of Various Coastal Structures

Types of Breakwaters

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

Slope and Foundation stability

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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 mooring

Wharf 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 groins

Relative sizes: breakwater > jetty > groin

Types of Breakwaters

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.
  • 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.

Advantages / Disadvantages
Sloped Rubble Mound /
  1. Suitable for irregular bottom
  2. Suitable for weak soil (disbursed load)
  3. Progressive damage
  4. Low toe scour
  5. Simpler construction
  6. Simpler maintenance
/
  1. Required material increases rapidly with increased water depth
  2. High maintenance cost
  3. Large base cuts into basin size

Composite / Vertical /
  1. Material savings (stone required)
  2. Easy to maintain (day-to-day)
  3. Control water depth clearly defined
/
  1. Requires firm soil
  2. High construction requirements
  3. Repair difficult

Low mound /
  1. Suitable for deeper water with less firm soil
  2. More economic/ flexible design
/
  1. Complicated construction
  2. More difficult repair

High mound /
  1. Suitable for deeper water with less firm soil
/
  1. More complicated construction
  2. More susceptible to breaking waves

Rubble Mound Breakwater Design

Layout Options for Rubble Mound Breakwaters and Jetties

  1. Attached or Detached.
  2. Jetties usually attached to stabilize an inlet or eliminate channel shoaling.
  3. Breakwaters attached or detached.
  4. 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.
  5. An attached breakwater extended from a natural headland could be used to protect a harbor located in a cove.
  6. A system of attached and detached breakwaters may be used.
  7. 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.
  8. Overtopped or Non-overtopped.
  9. 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.
  10. Non-overtopped: elevation precludes any significant amount of wave energy from coming across the crest.
  11. Non-overtopped breakwaters or jetties
  12. Greater degree of wave protection
  13. More costly to build because of the increased volume of materials required.
  14. Crest elevation determines the amount of wave overtopping expected
  15. Hydraulic model investigation to find the magnitude of transmitted wave heights
  16. Optimum crest elevation  minimum height that provides the needed protection.
  17. Overtopped breakwater
  18. 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).
  19. Overtopped structures are more difficult to design because their stability response is strongly affected by small changes in the still water level.
  20. Submerged Breakwater
  21. Example: A detached breakwater constructed parallel to the coastline and designed to dissipate sufficient wave energy to eliminate or reduce shoreline erosion.
  22. Advantages:
  23. Less expensive to build.
  24. May be aesthetically more pleasing (do not encroach on any scenic view)
  25. Disadvantages:
  26. Significantly less wave protection is provided
  27. Monitoring the structure's condition is more difficult.
  28. Navigation hazards may be created.
  29. Single or Double.
  30. 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.
  31. 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.
  32. 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.
  33. 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.
  34. 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 Description

Multi-layer design. Typical design has at least three major layers:

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

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/hb ~ 0.6 to 1.1
  4. 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/Hb

m = nearshore slope

p = 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 Parameters

hwater depth of structure relative to design high water (DHW)

hcbreakwater crest relative to DHW

Rfreeboard, peak crown elevation above DHW

htdepth of structure toe relative to still water level (SWL)

Bcrest width

Bttoe apron width

front slope (seaside)

bback slope (lee)

tthickness of layers

Warmor 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/ 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, , b
  3. Determine armor unit size/ type and underlayer requirements
  4. Develop toe structure and filter or bedding layer
  5. Analyze foundation settlement, bearing capacity and stability
  6. Adjust parameters and repeat as necessary

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 (%) / a / b / c / d
0.1 / 1.12 / 1.34 / 0.55 / 2.58
2 / 0.96 / 1.17 / 0.46 / 1.97
5 / 0.86 / 1.05 / 0.44 / 1.68
10 / 0.77 / 0.94 / 0.42 / 1.45
50 / 0.47 / 0.60 / 0.34 / 0.82

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 base / 0.8
Gravel / 0.7
Rock rip-rap with thickness > 2D50 / 0.5-0.6

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, , above

for straight smooth slopes (no berms), non-depth limited waves

Slope / 1:1 / 1:1.5 / 1:2 / 1:3 / 1:4
a / 0.008 / 0.010 / 0.013 / 0.016 / 0.019
b / 20 / 20 / 22 / 32 / 47

determine R based on acceptable for the design

Harbor protection

Vehicles on b.w.

Pedestrians

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 (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 Stability

Considerations:

  • Slope: flatter slope  smaller armor unit weight but more material req'd

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)
  • 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.

Considerations

  1. Availability of casting forms
  2. Concrete quality
  3. Use of reinforcing (req'd if > 10-20 t)
  4. Placement
  5. Construction equipment availability

When 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 unit

D = diameter of armor unit

a = 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