CE707 Groyne Design

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CE 707

Coastal, Port and Harbor Engineering

DESIGN OF GROYNE SYSTEMS

Source: Photograph by Randy Schaetzl, Professor of Geography - Michigan State University(http://geo.msu.edu/extra/geogmich/coastalerosion.html) last accessed 11th Jan 2016

GROYNES

• A divergent nodal regionin longshore transport the central area of a

crenulated pocket beach,

in the border region of adiffraction shadow zoneof a harbor breakwater or jetty,

the curvature of the coastchanges greatly.

• no source of sand, such asOn the down-drift side of

a large harbor breakwateror jetty.

Divergent nodal regions with groin fieldsSource: http://www.coastalreview.org/2013/08/bald-heads-battle-with-the-sea/ Photo:

Olsen Associates Inc.

• Groynes are a possible component of shore-protection, beach-saving, and sand-management alternatives


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• Intruding sand is to be managed, such as,

at the updrift side of an inlet entrance, harbor entrance, ornavigation channel

for stabilizing or anchoring the beach

for stockpiling material for bypassing across the inlet

• sand movement alongshore is to be controlled orgated,

to prevent undue loss of beach fill, while providingmaterial to downdrift beaches

T-Head Groins near South Lake Worth Inlet, Ocean Ridge, FL(http://www.asbpa.org/publications/white_papers/ReintroducingStructuresforErosionControlFINAL.pdf)

WORKING PRIN IPLE OF GROYNE SYSTEMS


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COASTAL FEATURES AND PROCESSES

Typical coastal profile and distribution of the littoral drift along the coastal profile.

LENGTH OF GROYNE

Typical groyne position with respect to coastal processes


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LENGTH OF GROYNE

• Appropriate choice of shapes, dimensions and location of groynes iscrucial for effectiveness of shore protection.

• Groynes length is usually related to mean width of the surf zone and

on the other hand to their longshore spacing.

• An active length of the groyne basically increases together with the

growth of wave-to-shoreline angle.

• They should not trap the whole longshore sediment flux.

• The groynes spread seawards not further than to 40-50% of the storm

surf zone width.

HEIGHT OF GROYNE

• Effectiveness of the groynes depends also on their permeability. The

groynes which are either structurally permeable or submerged

(permanently or during high water levels) allow more sediment to

pass alongshore through them, in comparison to impermeable or high

groynes.

• Pile groynes are usually permeable structures which does not affect

their efficiency.

• The groynes height influences the amount of longshore sediment

transport trapped by the groynes.


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TYPES OF GROYNE

The most popular shapes and types of groynes

• Generally, the groynes are designed to stick out about hs = 0.5-1.0 m

above the beach and the mean sea level (MSL).

• Too high groynes cause wave reflection, resulting in local scours.

• Considering the shape in plan view, the groynes can be straight, bent

or curved, as well as L-shaped, T-shaped or Y-shaped.

FUNCTION DESIGN OF GROYNE SYSTEMS

• Functional design is demonstrated by applying shorelineresponse model GENESIS to simulate the action of singleand multiple groyne

• Functional design of Groyne involves

• bypassing

• permeability,• evolution of the shoreline in the groyne field and groyne

tapering

• Groyne functioning depends on the balance between thenet and the gross longshore transport rate

•Permeable groynes are large rocks, bamboo or timber

•impermeable groynes (solid groynes or rock armour

groynes) are constructed using rock, gravel, gabions.


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DESIGN OF GROYNE SYSTEMS(Cont.)

For functional groyne design

1. Bypassing should be represented such that the shorelineresponse to a groyne, including evolution of the shorelinein time and its equilibrium plan form, depend on groynelength (depth at tip of groyne), with an increase in lengthincreasing the impact of the structure on the shoreline.

2. Different groyne permeabilities should produce differentequilibrium plan forms, with increasing permeabilitydecreasing the impact of the structure on the shoreline.

3. A permeability of 100% should result in longshore sandtransport and shoreline evolution identical to that with nogroyne present.

DESIGN OF GROYNE SYSTEMS(Cont.)

• Shoreline Response= f [groyne(s);beach; waves, wind,&tide]

• Spacing of Groyne on sandy beach =2 to 4 times thegroyne length (SPM suggests a spacing of 2 to 3)

• Optimal spacing and groyne functioning depends on• Groyne length (depth at the groyne tip, which controls the

sand bypassing)• Groyne permeability or porosity (control sand through-

passing)• Groyne elevation and tidal range (control sand overpassing)• Predominant wave direction and height• Net and gross longshore transport• Sediment grain size ( transported as suspended load or bed

load)


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Design of Groyne System

In the shoreline response Model GENESIS,

• The fraction of sand that passes a groyne (F) by beingtransported over and through it (Hanson & Kraus 1989), isgiven by

F = P(1 - B) + B (1)

where 0≤ P≤ 1 and 0 ≤ B ≤ 1 and

P = Permeability Factor

B = bypassing factor(amount passing around the seaward end)

• Actual transport rate at the groin, Q G* = F . Q G (2)

where Q G is the potential rate at the groyne

Design of Groyne System(Cont.)

• For a 100% permeability, i.e. by liming P→1, thecalculation should give the same result as for “ nogroyne present” Eq. 2 is required

Bypassing factor, B= 1- DG /DLT (3)

where DG= depth at the groyne at a particular time step,DLT is the depth of active longshore sand transport

• DG= y 2/3

,where y is distance offshore

• DLT = 1.6 Hs, where Hs =significant breaking wave height(Hanson & Kraus 1989).

• Eq.3 suggest that the parameter DG /H0 , characterizethe groyne bypassing, where H0 is the deep water waveheight.


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Single groyne• Shoreline change prediction at single groyne compared

for 4 transport distribution: rectangular on a plane-sloping profile, triangular with peak at the shore on aplane-sloping profile and two similar distribution on anequilibrium profile. In the test, median grain size 0.25mm, was used to determine the equilibrium profileshape, the groyne was 100 m long on an initiallystraight shoreline, and waves were constant with deep-water height of 1 m, period of 8 sec, and angle of 20

deg. The model was run for 15 years and calculatedpositions of the shoreline directly updrift of groynedivided by the groyne length are plotted in Fig 1.

Single groyne (Cont.)


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Influence of Gross Longshore

transport for Single Groyne• Shoreline change in the vicinity of disturbances that

alter transport alongshore is controlled by the grosstransport rate as well as the net (Bodge 1992).

• Single groyne was placed on the beach with initiallystraight shoreline with the deep wave height as 1mand period as 8 sec, and wave direction of 10 deg.

• the net to gross transport ratio were changed fromQ n/Q g= 1, 0.5, 0.33, and 0.25 . The ratio Q n/Q g = 0.5,with Q n= 300,000 cu m is the design condition forWesthampton.

• The length of the groyne YG was also varied in relationto the width of the surfzone (to the breakpoint) YB onthe initially straight beach YG/YB = 0.5, 1, and 2.

Single groyne(Cont.)

Shoreline change calculatedon the updrift side of thegroynefor YG/YB =1.

Shoreline change with Qn /Qg = 0.5for the three dimensionlessgroynelengths Y G /Y B = 0.5, 1, and 2.

Over the 5-year calculation interval, the

shoreline approaches the tip of the

groyne only if the gross and net rates are

equal.

The updrift shoreline moves seaward

more rapidly as the relative groyne

length increases.


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Mutiple GroyneTests

• The shoreline changes were calculate for a field of 7groins with P=10% placed on an straight beach.The groins were 100 m long with a spacing of 400m. Waves were Raleigh distributed in heightwith significant H0 = 1 m, period 8 sec, and deep-water direction 10 deg. Grid spacing was 50 m andtime step was 6 hr. Fig. 4 shows calculatedshoreline change after 5 and 10 years.

• Figure 5 tracks shoreline position over time


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• Westhampton Beach is composed of fine to mediumsands, and the net transport rate has been estimated tobe on the order of 300,000 cu m/year to the west(Panuzio 1968). Fig. 7 is an oblique aerial view of the

Westhampton groin field, looking east, with Groin 15 inthe foreground. Over the years, the groin field has verysuccessfully performed its intended local function ofreinforcing the historically weak section of barrierbeach by building a wide beach at the groin field and tothe east (updrift) (Nersesian et al. 1992). However, thebeach immediately to the west has eroded significantlyand was breached on December 18,1992, during astrong subtropical storm.


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Design Problem

Design a groyne structure for Kingscliff Beach, NSW

The net annual longshore sand transport at the southern endof Kingscliff Beach (Sutherland Point) is 518,000 m3/yearnorthward

The cross-shore distribution of littoral drift transport atKingscliff Beach was approximated from two other studies inthe region (Figure shown)

Comparison of the Cross-Shore Distribution of Longshore

Transport from Two Studies

GROYNE DESIGN (Cont.)


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GROYNE DESIGN

Functional groyneDesign – Plan View

Source: Coghlan et al. 2013


Functional Groyne Design – Side View


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Development of Groyne Field oncept

Designs for Kingscliff Beach

• Planning Horizon• A nominal design life of 50 years was adopted for the long term

groyne field• the maximum significant wave height that can reach the

structure is a function of design water level due to depthlimited wave conditions. The 1 in 100 year ARI event wasselected for both wave conditions (height, period and direction)and water level conditions (tide plus anomaly)

• Groyne Permeability• Based on the fact that there are no long-lasting permeable

groins on marine coastlines in Australia or worldwide and that

there are problems associated with damage to these structuresfrom wave impacts.• IMPERMEABLE type groins were selected for concept groyne

design


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• GROYNE Length• Beach stabilization using groins is generally feasible in areas

characterized by a dominant direction of littoral drifttransport

• The net annual longshore sand transport at the southern endof Kingscliff Beach (Sutherland Point) is 518,000 m3/yearnorthward

• The cross-shore distribution of littoral drift transport atKingscliff Beach was approximated from two other studies inthe region (Figure shown)

• Based on these studies it was assumed that the groyneswould extend seaward to the -3 m AHD(Australian HeightDatum) contour for concept design of the long term groynefield

Summary of Design conditions adopted for thegroyne field concept design

1 in 100 Year ARI(Average Recurrance Interval) Concept Design Conditions


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• Groyne Spacing • Groynes on sandy beaches

perform best if theirspacing is two to fourtimes the groynelength(Kraus et al.,1994,also suggested by CEM(USACE, 2006))

• 2 to 3 times groynelength(based on SPM1984)

• Spacing is dependent onthe trade-off betweentotal groyne length andnourishment volume, asshown in Figure

Effect of Groyne Spacing on Nourishment

Volume



• Groyne Orientation

• the SPM (shoreline protection manual) (1984) recommendation oforientation perpendicular to the coast was adopted for concept design

• Groyne Crest Level and Width

The crest level of each of the proposed groins is influenced by several factors whichwill minimize the amount of construction materials used, control sand movement

over the top of the groins and accommodate land-based construction equipmentthat might operate directly on the structures.

• For practical construction (above high tide level), a crest level of 1 m AHD wasadopted for core material along the full length of each groin.

• Two layers of secondary armor would be placed over this core material and thenfinished with a concrete slab roadway.

• The resulting crest level would vary from 2.7 m AHD at the landward end to 3.2 mAHD at the seaward end of each of the proposed groins.

• A crest width for the core material of 3.0 m was adopted to facilitate accessduring construction.


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• Design Scour Level

At each groyne head, scour depth was determined basedon the following

• Historical measurements of beach profile movement onnatural beaches;

• Historical measurements of scour at the head of an existinggroyne; and

• Erosion modelling

A design scour level of -5 m AHD was adopted on thebasis that the typical bed elevation at the head of eachgroyne would be -3 m AHD (allowance for 2 m scourdepth)


• Groyne Field Layouts• Groyne locations were determined through

consideration of the location of existing structures

• Groyne Construction MaterialsFour different construction materials were assessed forsuitability for the long term groyne field, as follows:

• Rock (greywacke or basalt);

• Sand-filled geotextile containers;

• Piles (timber or concrete); and

• Concrete (Hanbars).


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Groyne Design(Cont.)

long term groyne field- Layout 1

Minimum Groyne Section


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Typical Groyne Section

Documents

CE707 Groyne Design