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