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 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
