Application of timber groynes in coastal engineering

Abstract

Timber groynes have been used for centuries to retain and control beaches. They are found throughout Europe but little information is available about the design or detailing of the structures. During recent years alternatives such as beach renourishment, detached breakwaters and rock groynes have been used increasingly, however timber groynes can still offer advantages in terms of sustainability, cost and appearance. These include the relative ease with which the level and profile of the groyne may be adapted (by adding or removing planks) or maintained. Their appearance and small footprint are particularly advantageous on amenity beaches. Beaches constantly change as they respond to natural processes including waves, currents and the wind. These processes can result in material being transported from the beach, which if not replaced by material from adjacent areas, will result in erosion. Groynes are constructed more or less perpendicular to the shoreline to restrict the movement of sediment along the shore. They usually work in two ways, by providing a physical barrier to the movement of the beach material, which enables the beach to re-orientate approximately perpendicular to the incident wave direction, and by diverting longshore currents (which may be caused by tides or waves) away from the beach. In the past, groyne design was often limited to the slightly improved replacement of existing structures. As most local authorities in the United Kingdom are responsible for their own coastal protection, groyne design differs largely along the coastline. The performance and durability of timber groynes is highly dependent on the design and detailing of the structure. By comparing several schemes, various advantages and disadvantages of groyne design have been identified. Keeping a structure and its connections as simple as possible will benefit the ease of construction and maintenance. Design details such as attaching softwood or recycled timber rubbing pieces to the piles at critical levels, thus protecting them from abrasion, can considerably increase the lifetime of a groyne. In order to assure the sustainable use of timber, certified timber should be specified. Additionally, reuse and recycling of timber can substantially reduce the need for new timber, and should be incorporated in the design and maintenance of structures. If designed properly, timber groynes can be an effective and sustainable solution to retain a beach where the main erosion problem is caused by longshore sediment transport.