JERICHO: Modelling and analyses of coastal wave climate

INTRODUCTION:
Coastal wave climate refers to patterns of wave conditions which prevail from year to year and their inter-annual variability. Knowledge of wave climate allows predictions to be made of the frequency of occurrence of certain types of event which may impact on coastal defences or on the movement of "soft" coastal features (sandbanks etc.) Information on the waves at any particular coastal location is sparse. Usually coastal management plans are based on a limited number of short-term wave data sets carried out for a specific study. The longer time series that are available offshore and from satellite data are too far from the coast to be directly applicable. This is why it is necessary to use wave transformation models to convert the more prolific offshore data to inshore.

SWANand STORMWAVE MODELS:
JERICHO attempted to improve our knowledge of coastal wave climate by a combination of offshore wave data and nearshore wave transformation models. Two types of wave model were applied. The first was Halcrow’s STORM model, a ray tracing model which could be run economically with a long time series of input data, so allowing the generation of wave statistics. The other model was the SWAN 3^rd-generation spectral wave model, a state-of-the-art model which includes representation of all relevant physical processes. One of the benefits of SWAN is that it produces a 2-D map of wave heights over the whole model area rather than the predictions for a single location available from ray-tracing models. Figure 1 provides an example of SWAN output for Carmarthen Bay. It was not economical within the scale of the JERICHO project to run long time series with the SWAN model so a different approach was taken to model extreme wave conditions: transforming offshore extreme wave conditions to the nearshore.
Both types of model had to be carefully set up for each coastal location, and verified against in situ measurements. Figure 2 compares output from SWAN and STORM against measured data at Holderness.

Figure 1: Map of wave height contours for Carmarthen Bay with arrows showing mean wave direction.

Figure 2: Time series of SWAN and STORM versus observed data at the Holderness inshore site.

WAVE TRANSFORMATION PROCESSES:
The SWAN model was also used to test the characteristics of wave transformation over the nearshore zone. The most important factors controlling wave height at the coast were found to be offshore wave height and period, total water depth and the formulation of bottom friction. Secondary effects were the functional form of the boundary spectra and local wind conditions, The spatial variation in current should be included in an area of complex bathymetry such as Carmarthen Bay where current refraction is likely to be important. This requires simultaneous solution of a 2-D hydrodynamic model at the same resolution as the wave model.

COASTAL WAVE HEIGHT EXTREMES:
To generate estimates of inshore extreme wave heights (for instance the highest value that could be expected in 1 year or 100 years), different approaches were required for the two models. For the STORM model, a time series of data was supplied as input to the model offshore boundary, and transformed to an inshore time series. A statistical analysis then generated estimates of extreme wave heights. For SWAN, an offshore boundary condition representing the 1 in 10- or 1 in 100-year wave was generated and transformed to the coast. Tables 1-3 give the results for the three JERICHO sites. We can see that there are differences between the estimates derived from the two procedures. A more comprehensive comparison of the models using more inshore wave data would be required to establish the causes of all the differences between the results from the two models. Figure 3 shows cross-sections of wave energy dissipation against offshore distance from the SWAN model.

Location \ return period (yrs)	1	100
offshore model boundary	5.46	8.00
inshore (STORM)	4.02	6.22
inshore(SWAN)	3.92	5.04
+ sea level rise (83 cm)	4.04	5.29
Hs +20% or +10% (1y: 100y)	4.57	5.38

Table -1 Estimated return wave heights for Holderness

Location \ return period (yrs)	1	100
offshore model boundary	7.47	11.20
inshore (STORM)	3.50	5.35
inshore (SWAN)	5.49	6.68
+ sea level rise (80 cm)	5.66	6.98
Hs +20% or +10% (1y: 100y)	6.12	7.06

Table -2 Estimated return wave heights off Lyme Bay

Location\ return period (yrs)	1	100
offshore model boundary	8.54	12.95
inshore (STORM)	3.70	5.64
inshore (SWAN)	1.56	2.27
+ sea level rise (79 cm)	1.57	2.33
Hs +20% or +10% (1y: 100y)	1.80	2.45

Table -3 Estimated return Hs values in Carmarthen Bay

FUTURE WAVE CLIMATE:
Uncertainties in climate models make it difficult to predict reliably future wave climate. There are confident predictions of an increase in sea level, but the long term trends of storminess are not so well established, with different climate models giving different predictions. Because of these uncertainties, a "worst case scenario" was modelled, based on an extrapolation of recent (20 year) trends in wave height combined with an increase in sea level (bottom two rows of Tables 1-3). This allowed an initial study of which inshore sites may be more vulnerable to increases in offshore wave height. It was concluded that whilst the biggest waves can be expected off the south-west coast, the nearshore impact may not be the largest here. The morphology (including bathymetry) and geological character of the coastline are equally important factors in determining possible vulnerability. In particular the presence of deep water close inshore will allow larger waves to propagate into the coastline.

THE PARTNERSHIP (see Contact) : The Environment Agency was the customer for JERICHO, it wished to develop its long term strategy for the protection of the English and Welsh coastline. The Centre for Coastal and Marine Science — Proudman Laboratory, and Halcrow Maritime provided expertise in shallow water wave modelling. Southampton Oceanography Centre undertook analyses of large scale wave climate variability and provided computing support, and Satellite Observing Systems were project managers and carried out analyses of satellite and in situ data. JERICHO was supported by the British National Space Centre under the Earth Observation LINK programme.