JERICHO: Recommendations for monitoring coastal wave climate
A variety of users have a requirement for offshore and coastal wave data. These
include the oil and gas industry, ship owners, coastal protection agencies,
those responsible for monitoring waste disposal, research institutes carrying
out ecological and sedimentological studies, and organisations responsible for
wave and tidal energy devices and flood warning. Thus, although JERICHO was
established to provide information to aid the Environment Agency in the development
of their strategy for the coastal defence, there is a wider community who may
find the results of interest.
A specific remit for JERICHO was to recommend a system to monitor coastal wave
climate over the medium to long term. Whilst an operational system could be
said to exist to monitor offshore climate, there is no similar arrangement for
the coastal regions.
Satellite Altimeter Data
Continuously recorded data are required for real time monitoring, determination
of long-term statistics and assimilation into operational models. Satellite
altimeters form such a data source and provide data to within 10-40 km of the
coast. The time and space scales do not, however, match model requirements.
At present, satellite altimeter data are sparse in time (TOPEX repeats each
track every ten days) and the tracks are widely separated so often do not coincide
with model boundaries.
In Situ measurements
Waves are traditionally measured by in situ instruments and a
large amount of historical data exist from surface-following buoys, bottom pressure
recorders, and others. Such in situ instruments can be vulnerable to
damage by the elements or human interference and only give a measurement at
a single location. The bottom pressure recorder can be used in depths less than
about 20m. Figure 1 shows the locations of satellite tracks and in situ
data sets used in JERICHO.
Surface and Satellite Radar
Recently, wave measurements from satellite radar (Synthetic Aperture
Radar) and surface based remote-sensing radar in the High Frequency and X-band
ranges have become available. Under favourable conditions HF radar wave measurements
from a coastal installation can reach up to 100 km offshore. X-band measurements
are more localised. Figure 2 shows an example of the significant wave height
observed by an HF radar deployed at Holderness.
Figure 1. Locations of in situ data (red triangles) and altimeter tracks . Magenta lines indicate TOPEX tracks (10 day repeat), blue lines ERS-2 tracks (35 day repeat).
Figure 2. HF radar wave data at Holderness. Arrows indicate wave direction. (Courtesy L. Wyatt, Sheffield University, EU MAST SCAWVEX project)
COMBINING DATA SETS:
The recent EU MAST projects SCAWVEX and EUROROSE have demonstrated
what may be achieved with a combination of in situ measurements, surface
radar data and wave models. SCAWVEX carried out an intercomparison of wave data
sources at Holderness, whereas EUROROSE tested an operational arrangement of
wave radar and wave models near to a harbour entrance on the Norwegian Coast.
Whilst neither experiment provided a long term deployment, or included the use
of satellite data, the complementarity of ground based data sources was usefully
tested. These experiments also demonstrated the significant logistical back-up
required for such operations. Figure 3 shows deployments at Holderness which
were supported by the SCAWVEX project and by MAFF (as part of the LOIS and CAMELOT
projects).
Figure 3: Wave measurements at Holderness (1994/95 and 1995/96) + indicates POL Measuring Platforms, with bottom pressure recorders, and directional wave measuring devices at nearshore stations). W is Waverider location, blue squares are OSCR HF radar shore stations (master and slave) and the red cross is the X-band radar shore station.
RECOMMENDATIONS:
The recommended arrangement for monitoring coastal wave climate is
a combination of satellite and in situ data, together with coastal wave
models. The exact configuration of a final system would of course depend on
the funds available, and the extent of the region that was to be monitored.
A minimal specification would be a limited network of offshore (up to10-40km
from the coast) directional buoys to be combined with satellite altimeter and
(if available) SAR data to derive boundary conditions for fine scale wave-transformation
models. A set of inshore wave monitoring stations possibly deployed for more
limited periods would also be required for model validation. For shallow water
of less than 10m depth directional spectra can be obtained from bottom pressure
recorder/ current meters and coastal X-band radar (which can also be used to
examine coastal bathymetry). Note that measurements over a period of 10 years
or more are required to give a full description of the wave climate. Measurements
made over a shorter period may be dominated by unusual weather patterns and
provide an unrepresentative picture of the climate.
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.