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 11th Jan 2016
GROYNES • Groynes are a possible component of shore-protection, beachsaving, and sand-management alternatives • 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.
Divergent nodal regions with groin fields Source: http://www.coastalreview.org/2013/08/bald-heads-battle-with-the-sea/ Photo: Olsen Associates Inc.
• no source of sand, such as On the down-drift side of a large harbor breakwater or jetty.
• 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)
WORKING PRINCIPLE OF GROYNE SYSTEMS
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
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 • 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.
The most popular shapes and types of groynes
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 throughpassing) • 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, QG* = F . QG (2) where QG 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- DG /DLT (3) where DG= depth at the groyne at a particular time step, DLT is the depth of active longshore sand transport • DG= y2/3 ,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 , characterize the groyne bypassing, where H0 is the deep water wave height.
Single groyne • Shoreline change prediction at single groyne compared for 4 transport distribution: rectangular on a planesloping 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 deepwater 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.
Single groyne (Cont.)
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 Qn/Q g= 1, 0.5, 0.33, and 0.25 . The ratio Qn/Q g = 0.5, with Qn= 300,000 cu m is the design condition for Westhampton. • The length of the groyne YG was also varied in relation to the width of the surfzone (to the breakpoint) YB on the initially straight beach YG/YB = 0.5, 1, and 2.
Single groyne(Cont.) Shoreline change calculated on the updrift side of the groynefor YG/YB =1.
Over the 5-year calculation interval, the shoreline approaches the tip of the groyne only if the gross and net rates are equal.
Shoreline change with Qn/Qg = 0.5 for the three dimensionless groynelengths YG/YB = 0.5, 1, and 2.
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 H0 = 1 m, period 8 sec, and deepwater 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. • Figure 5 tracks shoreline position over time
• 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 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)
GROYNE DESIGN (Cont.)
Comparison of the Cross-Shore Distribution of Longshore Transport from Two Studies
Functional groyneDesign – Plan View Source: Coghlan et al. 2013
GROYNE DESIGN (Cont.)
Functional Groyne Design – Side View
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
1 in 100 Year ARI(Average Recurrance Interval) Concept Design Conditions
GROYNE DESIGN (Cont.) • 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 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).
long term groyne field- Layout 1
Minimum Groyne Section
Typical Groyne Section