Waves

• Wave measurement at nearshore buoys and platforms
• Synthetic offshore and modelled nearshore wave data

The DEFRA WAVENET national wave measurement programme makes provision for measurement of waves in some 20-30m of water but makes no provision for nearshore wave data. As waves travel towards the shoreline, complex wave transformations occur as a result of the waves “feeling the sea bed”; this results in large changes in wave height, direction, and period as waves approach the shoreline. Initially processes such as shoaling, refraction and diffraction are important. As the waves reach shallow water the transformations become more complex, due to wave breaking and bed friction. Wave data collected within WAVENET cannot be used directly in any shoreline management applications, as a reliable nearshore wave climate is required; wave transformation modelling is required before the data is usable. Synthesis of a nearshore wave climate is far more complex to model than offshore. Representation of linear processes such as refraction and shoaling can be handled reasonably well by many numerical models, unless the bathymetry is complex. Non-linear processes such as friction and breaking are more difficult to model and numerical wave climates are unreliable at some locations. Many coastal schemes are faced with poor and inadequate wave data for design and management purposes.

There is a pressing need for more nearshore wave measurement. The WAVENET programme makes specific reference to the need for a complementary programme of nearshore wave measurement and provides support for the adopted approach. A few wave measurement sites have already been established within the region that demonstrate extremely concerning results, when compared with numerical approaches to derivation of nearshore wave climate. Validation of numerical approaches at some sites show modelling methods to be inadequate.

The usual basis for determination of extreme conditions within short fetch areas is the JONSWAP formulation; this has recently been demonstrated to be unreliable for high wind speeds, yet no recent validation has been conducted to verify the formulation or modifications to these predictive formulae for extreme conditions. Significant under-design of all coastal structures, in areas such as the Solent, could result. Provision has been made for wave recorders to be located at two appropriate locations to provide data in solution of this problem.

Proposed measurement locations have been carefully selected to provide coverage at difficult, and high-risk, sites where large investments are expected or have recently been made. A rolling programme makes provision for introduction and replacement of 1-2 wave recorders per year. The system will also provide links to the EA real-time flood forecasting network and consideration has been made for these requirements.