Coastal models generally use long crested waves (with each wave crest stretching all the way across the model) and focus on design and overload conditions with return periods of 1:100 years or greater. Ship models are often around a scale of 1:50 to 100 in order to fit the required extent of the model within the available test area. As well as the model scale being dictated by the need to model the extent of the harbour and surrounding areas, the capability of the wavemakers to generate the shortest wave period required by the study is a key consideration when setting the scale. Ship model studies often use short crested wave conditions (waves come from different directions within a defined sector and interact with each other) and focus on operational conditions with low return periods (10:1 through to 1:5 years), while also considering the generation of second order waves to ensure harbour resonance is modelled accurately.
These different scales for different model types mean that careful consideration of scale effects and other aspects of modelling are needed when considering combining the multiple types into a single model. For instance, while it may be possible to use a single model to test both coastal structures and ship motion, the tests cannot be run at the same time because they require different return periods and wave types – short and long crested. The tests would therefore have to be run one after the one. The alternative is to have two (or more) separate models so that we can run them at the same time in different facilities, potentially providing our clients with results sooner than could be obtained using a single combined model.
Combining other types of model brings similar challenges. Depending on the type of sediment model required (gravel, sand, or cohesive material), sediment models can be combined successfully with coastal models, for example looking at shore protection structures. The same issues of different required scales and model designs come into play but the use of lightweight alternative materials, such as anthracite coal as the beach material, can overcome many of these. As an example, we recently completed a 3D coastal model of Colwyn Bay, which was used to calculate the evolution of the beach profile for various beach re-charge schemes, as well as providing overtopping measurements at the promenade. For sediment models focusing on detailed scour measurements around structures, such as wind turbine foundations, the preferred scale is 1:25 which makes combining with 3D coastal models difficult, even in HR Wallingford’s largest 75m long basin.