Cefas response to Professor Robin Pingree
Cefas response to "Scientific appraisal of conditions found at
Rame Head and Whitsand Bay" by Professor Robin Pingree, 8 August
2005
Cefas wish to acknowledge and thank Professor Pingree for his
valuable comments. The appraisal document has undergone internal
review and discussion and here we respond to the specific and more
general comments raised by Professor Pingree, by addressing each of
the four numbered sections (1-4) and the general comments.
1. Wind Induced currents
- "Wind induced currents in Whitsand Bay resulting from wind
fields over the English Channel and Southwestern Approaches are
known but were not considered".
Cefas response - Although it is arguable whether a detailed
knowledge of winddriven currents is required for the purpose of
assessing potential environmental impacts in the area, we are happy
to comment. The quantification of wind-induced currents requires a
long time-series dataset from a current meter mooring, so that
semi-diurnal tides and spring-neap cycles can be removed from the
flow data. Once this has been accomplished, the residual current
can be compared with wind forcing and other potential non-tidal
forces e.g. currents generated by thermal or density
stratification, changes in atmospheric pressure etc. Data from the
NMA mooring at Scylla has shown a strong eastward residual flow,
and if appropriate, this data may be used to assess residual and
wind-induced currents. Resources for monitoring were used for a
detailed short-term spatial ADCP survey, focused on the
critical area between the disposal site and the coast, results of
which have been presented in the report and elsewhere.
2. Sea state, local wind, waves and swell
- "Sea state and swell are important for resuspension and sorting
and transport of material shoreward. These effects were not
considered."
Cefas response - To move to a mathematical assessment of
wave-associated sediment transport would require good estimates of
the bed shear stress generate by tides and waves during a variety
of storms. This was not possible due to resource constraints, but
is not necessary for the work to be fit for purpose. It is clearly
acknowledged in the report that some material will be moved
shoreward. The regional and local distribution of bed sediment
provides the evidence of the capacity for sediment sorting and
accumulation in the area.
3. Tidal Flow
- "Only the effects of tidal flow were considered or
presented."
Cefas response - The constraints of the modelling were clearly
presented in the report. However, in terms of tidal direction and
strength, the results are consistent with the ADCP measurements,
drifter data and models developed by local scientists, and thus are
considered fit for purpose.
- Is the disposal site optimally sited in view of our knowledge
of bays and headlands?
Cefas response - The location of the site is historical. If it
were decided to move disposal to another site, then the applicant
would have to undertake a new assessment of the site. If the
residual sediment movement is to seaward (perhaps related to flows
close to the headland), then this would indicate that perhaps the
site is already in a location with favourable physical
environmental characteristics. The issue of site location was not
discussed in the report, because it was not our remit. At present,
the results show little long-term sediment accumulation at the dump
site itself, and little biological impact outside it.
- "What do the measurements or observations show near Queener
Point or Rame Head?"
Cefas response - The work measuring currents was undertaken on
RV Cefas
Endeavour and thus getting in close to Queener Point
(where a gyre may be more evident) was impossible. However, sea-bed
imagery shows the presence of gravel waves in the area whose crest
orientation indicates fast flows to seawards.
- "What did the measurements from the survey show?" [in relation
to Polhawn Cove]
Cefas response - The first part of the discussion in the Cefas
report describes the wide range of evidence for the conclusion that
there is no long-term accumulation of sediment in Polhawn Cove. The
evidence includes basic bay geomorphology, sedimentary facies
distribution, sediment texture, sedimentary bedform type and
orientation, sediment budget, flow measurements and flow
models.
4. Stratification
- "Whitsand Bay is a region of weak tidal mixing in terms of
stratification, and stratification parameters indicate the waters
would stratify in calm weather conditions with net surface buoyancy
flux. The resulting 3D circulation and suppression of turbulence
will affect dispersion and concentration gradients."
Cefas response - CTD casts undertaken in Whitsand Bay show very
little stratification. Should stratified conditions occur in the
summer, it would have little impact upon sediment dispersal.
However, stratification in summer may be important in terms of the
generation of bacteria in the sea-bed sediments.
General comments from Professor Pingree
- Siltation evidence was not appraised
Cefas response - Cefas have seen the videos taken by a local
diver and were aware of these when drawing their conclusions.
Observations by divers tend to indicate that the 'silt' is soft to
the touch rather than gritty. Current data does not indicate that
accumulation of minerogenic silt is occurring.
- Local scientists were not consulted
Cefas response - regarding the physical processes, Cefas were in
contact with Reg Uncles from PML to discuss the findings of the
work.
- No relevant Literature search was undertaken
Cefas response - Cefas took account of all relevant and
available literature with the exception of an MoD-commissioned
tracer report, of which we are now aware and which accords with our
findings. Not all literature consulted was referenced in the
report.
General comments from Cefas
The best model we have available is the environment itself. Data
from the range of physical biological and chemical studies are
consistent in indicating the Cefas interpretation.
Cefas hope to examine a suite of disposal sites and their
physical characteristics in further detail later this year as part
of a research programme. The Rame Head disposal site will be
included in this review.
Cefas welcome further suggestions for targeted survey
work.
2nd March 2005