CE707 Groyne Design

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 11thJan 2016

GROYNES

• A divergent nodal region in longshore transport

 the central area of a crenulated pocket beach,  in the border region of a diffraction shadow zone of a harbor breakwater or jetty,

 the curvature of the coast changes greatly.

• no source of sand, such as

 On the down-drift side of a large harbor breakwater or jetty.

Divergent nodal regions with groin fields

Source: 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

•

Intruding sand is to be managed, such as,

at the updrift side of an inlet entrance, harbor entrance, or

navigation channel

for stabilizing or anchoring the beach

for stockpiling material for bypassing across the inlet

•

sand movement alongshore is to be controlled or

gated,

to prevent undue loss of beach fill, while providing

material to downdrift beaches

T-Head Groins near South Lake Worth Inlet, Ocean Ridge, FL

(http://www.asbpa.org/publications/white_papers/ReintroducingStructuresforErosionControlFINAL.pdf)

COASTAL FEATURES AND PROCESSES

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

LENGTH OF GROYNE

LENGTH OF GROYNE

• Appropriate choice of shapes, dimensions and location of groynes is crucial 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.

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 shoreline

response model GENESIS to simulate the action of single

and 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 the

net 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.

DESIGN OF GROYNE SYSTEMS(Cont.)

For functional groyne design

1. Bypassing should be represented such that the shoreline

response to a groyne, including evolution of the shoreline

in time and its equilibrium plan form, depend on groyne

length (depth at tip of groyne), with an increase in length

increasing the impact of the structure on the shoreline.

2. Different groyne permeabilities should produce different

equilibrium plan forms, with increasing permeability

decreasing the impact of the structure on the shoreline.

3. A permeability of 100% should result in longshore sand

transport and shoreline evolution identical to that with no

groyne 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 the

groyne 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)

Design of Groyne System

In the shoreline response Model GENESIS,

•

The fraction of sand that passes a groyne (F) by being

transported over and through it (Hanson & Kraus 1989), is

given 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

* = F . Q

(2)

where Q

is the potential rate at the groyne

Design of Groyne System(Cont.)

•

For a 100% permeability, i.e. by limi ng P→1, the

calculation should give the same result as for “ no

groyne present” Eq. 2 is required

Bypassing factor, B= 1- D

/D

(3)

where D

= depth at the groyne at a particular time step,

D

is the depth of active longshore sand transport

•

D

= y

,

where y is distance offshore

•

D

= 1.6 H

, where H

=significant breaking wave height

(Hanson & Kraus 1989).

•

Eq.3 suggest that the parameter D

/H

, characterize

the groyne bypassing, where H

is the deep water wave

height.

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 a

plane-sloping profile and two similar distribution on an

equilibrium profile. In the test, median grain size 0.25

mm, was used to determine the equilibrium profile

shape, the groyne was 100 m long on an initially

straight 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 calculated

positions of the shoreline directly updrift of groyne

divided by the groyne length are plotted in Fig 1.

Influence of Gross Longshore

transport for Single Groyne

•

Shoreline change in the vicinity of disturbances that

alter transport alongshore is controlled by the gross

transport rate as well as the net (Bodge 1992).

•

Single groyne was placed on the beach with initially

straight shoreline with the deep wave height as 1m

and period as 8 sec, and wave direction of 10 deg.

•

the net to gross transport ratio were changed from

Q

/Q

= 1, 0.5, 0.33, and 0.25 . The ratio Q

/Q

= 0.5,

with Q

= 300,000 cu m is the design condition for

Westhampton.

•

The length of the groyne Y

was also varied in relation

to the width of the surfzone (to the breakpoint) Y

on

the initially straight beach Y

/Y

= 0.5, 1, and 2.

Single groyne(Cont.)

Shoreline change calculated on the updrift side of the groynefor YG/YB =1.

Shoreline change with Qn/Qg= 0.5

for the three dimensionless groynelengths YG/YB= 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.

Mutiple GroyneTests

•

The shoreline changes were calculate for a field of 7

groins with P=10% placed on an straight beach.

The groins were 100 m long with a spacing of

400m. Waves were Raleigh distributed in height

with significant H

= 1 m, period 8 sec, and

deep-water direction 10 deg. Grid spacing was 50 m and

time step was 6 hr. Fig. 4 shows calculated

shoreline change after 5 and 10 years.

•

Westhampton Beach is composed of fine to medium

sands, and the net transport rate has been estimated to

be 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 in

the foreground. Over the years, the groin field has very

successfully performed its intended local function of

reinforcing the historically weak section of barrier

beach by building a wide beach at the groin field and to

the east (updrift) (Nersesian et al. 1992). However, the

beach immediately to the west has eroded significantly

and was breached on December 18,1992, during a

strong subtropical storm.

Design Problem

Design a groyne structure for Kingscliff Beach, NSW

The net annual longshore sand transport at the southern end

of Kingscliff Beach (Sutherland Point) is 518,000 m

_/year

northward

The cross-shore distribution of littoral drift transport at

Kingscliff Beach was approximated from two other studies in

the region (Figure shown)

Comparison of the Cross-Shore Distribution of Longshore Transport from Two Studies

GROYNE DESIGN

Functional groyneDesign – Plan View Source: Coghlan et al. 2013

GROYNE DESIGN (Cont.)

Development of Groyne Field Concept

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 depth limited wave conditions. The 1 in 100 year ARI event was selected 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 structures from wave impacts.

• IMPERMEABLE type groins were selected for concept groyne design

GROYNE DESIGN (Cont.)

•

GROYNE Length

• Beach stabilization using groins is generally feasible in areas characterized by a dominant direction of littoral drift transport

• The net annual longshore sand transport at the southern end of Kingscliff Beach (Sutherland Point) is 518,000 m3_/year

northward

• The cross-shore distribution of littoral drift transport at Kingscliff Beach was approximated from two other studies in the region (Figure shown)

• Based on these studies it was assumed that the groynes would extend seaward to the -3 m AHD(Australian Height Datum) contour for concept design of the long term groyne field

Summary of Design conditions adopted for the

groyne field concept design

• Groyne Spacing

• Groynes on sandy beaches perform best if their spacing is two to four times the groyne length(Kraus et al.,1994, also suggested by CEM (USACE, 2006))

• 2 to 3 times groyne length(based on SPM 1984)

• Spacing is dependent on the trade-off between total groyne length and nourishment volume, as shown in Figure

Effect of Groyne Spacing on Nourishment Volume

GROYNE DESIGN (Cont.)

GROYNE DESIGN (Cont.)

• Groyne Orientation

• the SPM (shoreline protection manual) (1984) recommendation of orientation 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 which will minimize the amount of construction materials used, control sand movement over the top of the groins and accommodate land-based construction equipment that might operate directly on the structures.

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

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

• The resulting crest level would vary from 2.7 m AHD at the landward end to 3.2 m AHD 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 access during construction.

GROYNE DESIGN (Cont.)

•

Design Scour Level

At each groyne head, scour depth was determined based

on the following

• Historical measurements of beach profile movement on natural beaches;

• Historical measurements of scour at the head of an existing groyne; and

• Erosion modelling

A design scour level of -5 m AHD was adopted on the

basis that the typical bed elevation at the head of each

groyne would be -3 m AHD (allowance for 2 m scour

depth)

GROYNE DESIGN (Cont.)

•

Groyne Field Layouts

•

Groyne locations were determined through

consideration of the location of existing structures

•

Groyne Construction Materials

Four different construction materials were assessed for

suitability for the long term groyne field, as follows:

• Rock (greywacke or basalt); • Sand-filled geotextile containers; • Piles (timber or concrete); and • Concrete (Hanbars).

Groyne Design(Cont.)

long term groyne field- Layout 1