An active structure that protrudes into the sea from the shore and is often perpendicular to or slightly oblique to the shoreline is called a groyne. Major requirements for the effectiveness of groynes include an adequate supply of sediment and the presence of medium-strong longshore sediment movement.
A groyne’s primary job is to catch and hold back some of the sediment that is traveling through the surf zone, mostly in a longshore direction.
Experiments have shown that groynes partially diffuse the energy of water motion under weak and moderate wave conditions, which causes the updrift beach to accrete. However, the groynes’ protective function is reduced when storm waves come at the shore more or less perpendicularly, and a portion of the beach is wiped away (see Natural causes of coastal erosion).
Groynes are frequently utilized, however significant lee side erosion makes them a questionable choice when used as the only coast protection device. When used in conjunction with other (soft) shore protection techniques like artificial beach nourishment or shore nourishment, groyne solutions can be effective.
Effects of groynes on the shoreline: The impact on coast morphology is influenced by the groyne design (planform, length, height, cross-shore profile, and inclination); the impact also depends on sea level, wave climate, and sediment availability in the surf zone.
The deployment of a single groyne to protect the shore is frequently ineffective. As a result, groynes are typically planned as a group of a few to tens of distinct buildings that provide beach protection (see Groynes as shore protection). Figure 1 depicts a diagram of interacting groynes. Coastal erosion caused by a single groyne occurs on the lee side of the structure, whereas erosion caused by a collection of groynes occurs on the lee side of the entire system. Additionally, erosion is seen very next to the buildings, especially when the predominant wave direction is perpendicular to the shore. When water builds up between the groynes, compensatory flows are induced along the structures, which lead to localized seabed erosion and sand loss into deep water. The groynes are “short” in comparison to the surf zone width during strong storms, and erosion happens all around them. Groynes become “long” (length comparable to the surf zone width) under gentle wave conditions, which favors updrift sand accumulation and widening of the beach. A groyne’s loss of contact with the shore should be prevented. Longshore flows are created in this situation between the beach and the groyne root. The beach is washed away as a result of these flows.
Features of groynes: The efficiency of coastal protection depends on the proper selection of groyne shapes, sizes, and locations. The mean width of the surf zone and the longshore spacing in the groyne field are typically correlated with groyne length. With increasing wave incidence angle, the groyne’s active length grows. Groynes work best when they do not completely block the flow of longshore sediment. According to numerous studies and observations, groynes shouldn’t extend more seaward than 40 to 50 percent of the storm surf zone’s width. The permeability of groynes affects their efficiency as well. In comparison to impermeable or high groynes, those that are structurally permeable or submerged (permanently or during high water levels) allow more silt to travel alongshore.
The volume of longshore sediment movement that groynes trap depends on their height. Depending on changes in water level brought on by tides and storm surges, the same groyne can function as an emerging or submerged structure (Figure 2a). Groynes typically have an elevation of hs=0.5–1.0 m above mean sea level (MSL). Groynes that are excessively high reflect waves, which causes local scouring. The groynes can have a straight, bending, or curved shape in plan view, as well as an L-, T-, or Y-shaped shape. In Figure 2, the most common groyne forms and shapes are schematically depicted.
Types of groynes: In terms of structure, groynes come in a variety of forms, including wooden, sheet-pile, concrete, rubble-mound, formed of concrete blocks or stones, and geobags filled with sand.
- Wooden groynes: Most frequently, one- or two-row palisade structures are used for wooden groynes. In Figure 3, the impact of the T-shaped wooden pile groyne on the shore is shown (local erosion on the lee side and accretion on the updrift side). In general, one-row wooden groynes are partially permeable constructions; this eliminates unfavorable nearshore water circulations and lessens lee-side erosion. While inexpensive, wooden palisade groynes have a limited lifespan.
- Steel groynes: Steel groynes typically consist of single or double vertical sheet piles with a variety of profiles that are positioned perpendicular to the coastline. These buildings are impermeable. Due to material corrosion and abrasion from flowing sand, experiments have revealed that groynes constructed from single sheet pile walls are not long-lasting. In addition, ice loading is extremely hazardous, leading to instability and steel sheet piling failure. Mixed enormous buildings made of steel and concrete are far more dependable and long-lasting.
- Groynes of concrete elements: The most durable and solid coastal buildings are groynes made of reinforced concrete blocks. The components of such a groyne necessitate the presence of proper soil conditions and a sufficient foundation due to their enormous weight. Figure 4 depicts an illustration of a groyne made of reinforced concrete components.
- Rubble-mound groynes and groynes built of sand-filled geobags: It is common practice to use groynes made of rubble for coastal protection. They are constructed either as loose stone mounds or as mounds of different types of armor, like tetrapods. Often composite structures, these groynes are internally reinforced by sheet piling. They are substantial, resilient, and impermeable. In comparison to steel, concrete, and timber groynes, rubble-mound groynes are superior in dissipating wave and current energy.
- Groynes made of layered sand or ground-filled sacks are to be taken into consideration as a temporary measure of safety. Additional safety precautions are required, especially at the groyne head. To minimize settlement in soft bottoms, a specific filter cloth ought to be placed underneath the bags.
