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Maintenance of coastal structures – Phase 1: Timber groynes

The following study is being incorporated into the CIRIA ‘Groynes in Coastal Engineering‘ Manual

Timber groynes are widely used across the SCOPAC region and have been for many years at some locations. Some operating authorities report that maintenance of these structures occupies a significant proportion of their revenue coast protection expenditure, on an annual basis. Others carry out minimal maintenance of structures.

In view of the frequent use of timber groynes, it is quite remarkable that there are no publications that deal specifically with practical aspects of groyne management. Although the initial design of groynes is provided in industry standard publications, maintenance merits minimal discussion and practical issues are never addressed. The SCOPAC study on Maintenance of Timber Groynes seeks to cover this gap.

The cost of capital groyne replacement is very high and the effective extension of life of structures, through efficient maintenance, may make the difference between financially sustainable systems and those which are not cost effective as a management solution.

Although the current grant aid structure requires applications to be supported by full life cycle assessment of costs, the funding system does not make full provision for funding of life cycle maintenance works through grant in aid; this consequently encourages poor or no maintenance at some locations. Further to this the grant aid calculations discriminate between EA and local authority schemes, on the basis of the varied basis for the revenue funding for the two types of organisations. Assessments do not reflect recent changes to the funding of local authorities, which no longer identify coast protection as a discrete line within the budget. Examination of structures within the region suggests that maintenance is conducted very infrequently at many sites. In fact it appears that some structures may be unmaintained throughout their life. There is clear evidence that structures can degrade quickly and fail to provide the function for which they were originally designed. This has a serious implication for life-cycle costs. In many instances improved maintenance could provide cost effective extension of life of these structures.

As with many maintenance activities, procedures and processes have been developed at various locations over many years, often on a trial and error basis. Some of these processes have resulted in increasingly efficient management. Regrettably these experiences have hitherto not generally been documented, although internal procedures are often logged within operating authorities’ internal management systems. Changing approaches to management within coastal operating authorities have meant that there are now fewer skilled engineers working with these structures on a day to day basis, and some considerable practical knowledge is gradually being lost. The SCOPAC report seeks to capture the experiences of local engineers and to share these experiences with others. The document captures both techniques that have been considered to be successful, and those which have worked less well. In an attempt to capture the experiences of less successful approaches, these have been documented but not attributed to specific sites, thereby providing assessments of techniques that some may be considering.

The objective of the document is not to provide a prescriptive solution to maintenance, but to offer a range of alternative approaches, and risks together with experiences of their success. Some of the techniques have generic applicability and others may be site specific. The document is intended for use by those wishing to share these experiences.

The initial design of structures and plan shape layout of groyne systems is dealt with extremely well elsewhere and the focus of this document is on maintenance of structures that have already been constructed. Some of the procedures discussed here should be considered at the design stage however, since they may allow for more efficient maintenance if integrated into the design process. Since maintenance is not considered in any detail in these documents, designers of new structures may benefit from the experiences of structure performance however; this may allow modification of structural detailing to improve long term performance and life cycle costs.

The document provides details on the following key areas.

  • A general introduction to groyne structures provides a brief overview of key structure components.
  • Life cycle deterioration of structural elements and the range of degradation causes are considered.
  • Larger scale structural failures are discussed in context with initial construction, structure degradation and beach evolution
  • Approaches to recording inspections and analysis of life cycle costs are examined, in conjunction with applications of beach monitoring data.
  • The relative performance and costs of alternative materials for construction and repairs are discussed.
  • Tools and maintenance techniques are discussed and considered for each structural element of groynes.
  • Methods of structure modification are addressed to deal with changing site conditions, which particularly include changes to sediment availability.
  • The diversity of timber groyne systems within the SCOPAC region is demonstrated with illustrated examples of the layout and detail of many timber groyne systems across the region.