A groyne (in the U.S. groin) is a rigid hydraulic structure built perpendicularly from an ocean shore (in coastal engineering) or a riverbank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concrete, or stone. In the ocean, groynes create beaches, prevent beach erosion caused by longshore drift where this is the dominant process and facilitate beach nourishment. There is also often cross-shore movement which if longer than the groyne will limit its effectiveness. In a river, groynes slow down the process of erosion and prevent ice-jamming, which in turn aids navigation.
Groynes run generally perpendicular to the shore, extending from the upper foreshore or beach into the water. All of a groyne may be underwater, in which case it is a submerged groyne. They are often used in tandem with seawalls and other coastal engineering features. Groynes, however, may cause a shoreline to be perceived as unnatural. Groynes are generally straight but could be of various plan view shapes, permeable or impermeable, built from various materials such as wood, sand, stone rubble, gabion, etc.
What are Groynes??
A groyne is a rigid hydraulic structure built either from the shore (in case of seas) or bank (in case of rivers) in order to dissipate the wave energy or to protect the banks from erosion by trapping the sediments.
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In general, Groynes are perpendicular to the shoreline or river bank or sometimes slightly oblique.
Why Groynes are provided??
Groynes serve the following purposes:
- They protect the river bank by keeping the flow away from it.
- They create still pond along a particular bank with the aim of silting up the area in the vicinity.
- They train the river to flow along a desired course by attracting, deflecting or repelling the flow.
- They contract the wide river channel for improving the navigation depth.
Factors influencing the choice and design of Groynes
- Fall and velocity of flow in the river.
- Character of bed load carried by the river.
- Depth of waterway, maximum HFL and nature of flood hydrograph.
- Width of waterway, at high water, low water, and mean water.
- Availability of funds and construction materials.
Advantage & Disadvantage of Groyne
Prevents longshore drift from moving beach material along the coast. Allows the build-up of a beach. Beaches are a natural defence against erosion and an attraction for tourists
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They can be unattractive. Costly to build and maintain.
Classification of Groynes
Classification of groynes based on different factors is explained in the next few slides.
Groynes are classified into different types based on different factors which are as follows:
- Materials used for construction
- Permeability characteristics
- Height of groynes
- Function of groynes
Types of Groynes based on Materials Used for Construction
Based on the materials used in construction, the groynes are classified into:
- Wood groynes
- Rubble-mound groynes
- Rock groynes
- Sheet pile groynes
- Sandbag groynes
- Concrete groynes
1. Wood groynes
Wooden groynes are built with timber piles. Generally, single row or double row wooden groynes are constructed as they suffice the requirement.
The durability of wooden groynes is very low but they are economical and useful for short term purposes.
2. Rubble-mound Groynes
Rubble-mound groynes are widely used structures along the seashores. They are built using stones or specially made concrete units such as tetrapods. To attain more strength, sheet piling is provided inside the rubble-mound groynes. The durability and stability of rubble-mound groynes are very high.
3. Rock Groynes
Rock groynes, constructed using large-sized rocks have more durability compared to other materials. They absorb a good amount of wave energy and maintain good stability in any situation.
4. Sheet pile Groynes
Sheet pile groynes are constructed using steel sheet piles. The sheets pile groynes constructed are either single sheet pile or double sheet pile. However, double sheet pile walls are more durable and stable.
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5. Sandbag Groynes
Sandbag groynes are constructed using sand or earth-filled bags which are stacked in the form of a barrier.
They are used for temporary or short-term purposes. To prevent the sinking of sandbags into the ground, a special type of filter cloth is provided under the bags.
6. Concrete Groynes
Concrete groynes are constructed using reinforced concrete or prefabricated concrete blocks. These are the most stable and durable structures. Good foundation and appropriate soil conditions are required to construct concrete groynes.
Types of Groynes based on Permeability Characteristics
- Impermeable groynes
- Permeable groynes
1. Impermeable groynes
Impermeable groynes do not permit water to flow through them. They are made of rock, gavel, gabions etc. Since they are impermeable, the water may overflow during peak conditions hence, a thick protection layer is necessary for this type of groynes.
2. Permeable groynes
Permeable groynes permit water through it but with a reduced velocity of flow. Groynes built using wood, sandbags etc. come under this category. This type of groyne is suitable when a river is carrying a certain amount of sediment in suspension.
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Types of Groynes based on Height
- Submerged groynes
- Non-submerged groynes
1. Submerged groynes
Submerged groynes are constructed where river depth is very deep. Their submergence condition varies according to the water surface level in the river. Permeable materials are used to construct this type of groynes and they reduce flow velocity that results in preventing erosion of the top portion of the groyne.
2. Non-submerged groynes
Non-submerged groynes are constructed with a height greater than the maximum flood level. They are generally built using impermeable materials.
Types of Groynes based on functions
- Attracting groynes
- Repelling groynes
- Deflecting groynes
- Sediment groynes
1. Attracting groynes
Attracting groynes are constructed in such a way that their head is pointing towards the downstream side of the river as shown in the figure below. They are built with an angle of 45 to 60 degrees with the bank. Since it is inclined towards downstream, the water flow will attract towards the bank on which groyne is located.
The upstream side of attracting groynes undergoes severe attack by water flow hence, it should be constructed with proper protection. They provide safety to the opposite bank but adjacent banks may get affected by this type of groynes. Silting is also not possible in this type of groyne. Hence, these are not recommended.
2. Repelling groynes
Repelling groynes are built with their head towards upstream with an inclination of 60 to 80 degrees with the bank. They repel the water flow towards the bank on which it is located.
The head portion of repelling groynes is under the main attack by the flow. Hence, it should be built with strong protection. The sediments carried by the water gets deposited in the silt pocket which is formed at the upstream side of repelling groyne. These groynes are more advantageous than attracting groynes and are used widely for river training and bank protection.
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3. Deflecting groynes
Deflecting groynes are built perpendicular to the bank and they just deflect the water flow without repelling and provide local protection to the banks.
4. Sediment groynes
Sediment groynes are constructed when there is a considerable amount of sediments carried by the river water. These reduce the stream velocity and allow the sediments to deposit. They do not repel or deflect the flow. Generally, permeable groynes are used as sediment groynes.
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