Calculation of Low-Frequency Waves in Shallow Water and Comparison to Common Practice in Diffraction Methods

The increasing interest in LNG terminals at nearshore locations has led to the recent Joint Industry Project (JIP) HAWAI (sHallow WAter Initiative, 2005–2008), a collaboration between MARIN, Single Buoy Moorings, Bureau Veritas, and Deltares (formerly Delft Hydraulics). In this JIP shallow-water wave conditions and their effect on moored vessels were studied. Part of the JIP involved the evaluation of knowledge and numerical models from coastal engineering with respect to their potential for determining shallow-water wave conditions. Operational Boussinesq-type wave models and a long-wave shallow-water model with primary-wave forcing were considered in detail. The former were found to be limited in application due to the limited depth range that can be covered accurately by such models. The latter is considered the most practical and versatile modelling approach that is presently available in coastal engineering for the application considered here. It was used for numerical simulations of a measurement site near the city of Duck (NC, USA). Computed low-frequency wave parameters compare favourably to measured values for three typical primary wave conditions. A comparison of these numerical results with low-frequency wave parameters obtained from the standard approach used in diffraction methods, originally developed for deep water applications, indicates that such a standard approach is not expected to be applicable at shallow-water locations. Detailed local measurements, site-specific numerical calculations and scale-model tests that include the local geometry and the effects of directional spreading, are required to determine the low-frequency wave conditions for design and workability limits at such locations with sufficient detail and accuracy.