JERICHO: Overview
AIM: To provide improved information on coastal wave climate and inter-annual variability essential to the planning of Britain’s coastal defences and to make progress towards developing a predictive capability of long-term trends.
BACKGROUND:
Analysis of wave data from ships and buoys has shown that
winter wave heights in the North East Atlantic increased by over 50% between
the 1960’s and 1980’s. More recent analyses of satellite data confirmed
that this increase continued into the early 1990’s (Figure 1) and found
a link with the North Atlantic Oscillation Index (the anomaly in the pressure
gradient between Lisbon and Iceland). JERICHO aimed to develop techniques, combining
satellite data and models, to investigate the impact at the UK coast of a changing
offshore wave climate.
Figure 1. The percentage increase in mean winter significant wave height, 1985-89 to 1991-96.
METHODOLOGY:
Satellite and in situ data were used together
with coastal wave models to project measurements and predictions of offshore
variability to the UK coast at three locations. These JERICHO test sites were:
Holderness (East Yorkshire), Lyme Bay (Dorset) and Carmarthen Bay (S. W. Wales)
— Figure 2. Satellite data were used to identify large scale patterns of
variability and to derive boundary conditions for the wave models. In situ
data were used to verify the satellite measurements and to provide additional
wave information. Coastal wave models were used to provide estimates of wave
conditions close to the coastline. Various future scenarios were also modelled
to investigate the consequences of possible future changes in storminess and
sea level.
Figure 2. Locations of JERICHO in situ data, TOPEX satellite altimeter tracks, and the three JERICHO coastal sites at (right to left) Carmarthen Bay, Lyme Bay and Holderness
Wave Climate Variability
There is significant year to year variability in UK wave climate. The North Atlantic Oscillation Index has been identified as the major influence on offshore wave climate near the west and (to a lesser extent) south coast of the UK. The east coast is subject to different influences, and further investigation is needed.
Coastal Applications of Satellite Altimeter Data
JERICHO has shown that satellite data can be used to monitor UK offshore wave climate, but close to the coast (within 10-20 km) coastal wave models and in situ data are required.
New Applications of Coastal Wave Models
Two types of wave model were used in JERICHO: a "third generation" full spectral gridded wave model (SWAN), and a ray tracing model (STORM). Techniques have been developed to use these models to transform offshore extreme waves to the coast, using three coastal sites as test cases.
Investigation of Possible Future Trends
At present, predictions of changes in weather patterns are not sufficiently reliable to permit confident projections of future coastal wave climate. However, JERICHO has established techniques which will allow such projections should more reliable predictions become available. In the meantime, some "worst case scenarios" have been modelled. Coastal areas most likely to feel the biggest influence of future increases in wave climate are those with exposure to the largest waves (the South-West Coast), and areas with deep water close inshore.
Recommendations for Monitoring Wave Climate
Recommendations for monitoring future wave climate have been provided to the Environment Agency. A combined network of coastal in situ measuring sites (buoys, bottom pressure recorders, and others) and nested wave models which should be used together with satellite data was suggested. Occasional deployments of HF radar would complete the picture.
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.