Groynes is a structure which protrudes into the river bed with the aim of moving the flow away from the side of the river on which the groynes are built. There are seveal types of groynes for upstream or downstream flow. They can appear as straight, T-head or a bayonet shape.
Groynes should not be used where the river is already narrow or where the alignment of the river banks cannot be modified or reduced. It is also not advisable to use groynes where the opposite bank is exposed to transverse flows, which create unacceptable erosion. In such cases continuous longitudinal protection is required. Groynes should never deflect the flow violently but should systematically train the flow smoothly and gently in the desired direction, resulting in deposition of material adjacent to the groyne structure.
Siltration depends on the spacing of the groyne relative to their length. The ratio on average is between 1:4 and 1:6 with a minimum value for the concave curves and a maximum for the convex ones, depending on the location of the main river flow.
The largest materials are deposited first in order of magnitude from the outer to the inner section of the groyne. As the depth of water decreases, the finer grained materials settle out. The final stratas are muds which constitute the surface deposits.
The disposition and shape of groynes have an influence on the intensity and location of siltration. The effect is most visible in mountain streams and valley regions. Generally speaking, deposits in mountainous streams are wider upstream of the structure and smaller downstream. Groynes having a T-head shape appear to be the most effective solution even if they are more expensive. Disposition and shapes become important in relation to the dimensions of the materials transported by the flow. Groynes facing downstream are preferable where large cobbles are carried in floods. Groynes facing upstream with a bayonet shape and protruding well in the river, can deflect small pebbles where the river has a wide bed and a small gradient.
It is advisable that the heads of the groynes are not built higher than the low water level. The root must always remain exposed during high level flows in order to avoid the water eroding the river bank and possibly affecting the stability of the groyne.
Gabion structures are suitable for every kind of groyne. They can be subject to strain without losing their effectiveness due to movement and they can adopt to all variations in river bed levels.
Groynes become an integral part of the river bank and of the riverbed. In cases where a combination of gabion defences and groynes are well built, the works are destined to disappear under the disposition of silts which become further stabilised by plant colonisation.
Gabion groyne structures can function in two ways:
- Constructed on a firm river bed where little settlement is expected.
- Constructed on a river bed where erosion at the outer limit of the groyne is expected, with the groyne flexing down to reach equilibrium in the scour hole.
Groynes without foundations can be usefully employed on mountain sections where rivers carry large amounts of silt. They can also be employed in small river training works or as a combination of complex groyne systems.
In valleys, it is advisable to adopt a regular pattern of concave curves. The geometry of the curves should be similar to those found in the same watercourse where the river bed and banks are in equilibrium.
If the curve is wide and the works are not too prominent, it is preferable to adopt a straight shape perpendicular to the flow. If on the contrary the river works are prominent, it is preferable to use the bayonet shaped groyne oriented upstream as it is the most suitable for encouraging siltation. In narrow curves, T-shaped groynes are normally the best option. In straight stretches it is preferable to use straight perpendicular groynes if they are not prominent, or bayonet groynes facing upstream if the works are very prominent.
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