Laser diffraction methods measure particle sizes in the range of 0.4 – 2000μm. However, for practical reasons, the maximum diameter for marine sediment is 1000µm (1mm) when completing analysis using Malvern Mastersizer 2000.


If >5% gravel was present in the sample a full PSA was carried out using NMBAQC PSA Recommended Methodology (NMBAQC, in prep.). This included sieving sediment >1mm at 0.5 φ fractions and merging with laser data to produce a full particle size distribution (PSD).


Full details are available in “Standard Operating Procedure (EAOW_2011) (v1) – Particle Size Analysis (PSA) of EAOW Samples”, which has been provided to EAOW Ltd.


2.10 Optical Backscatter Sensors (QA and Calibration) OBS sensors emit light into the water. This light is scattered by suspended particles and some of the light is returned to the sensor. The Seapoint OBS sensors measure the intensity of this back‐ scattered light and output a voltage in proportion to this intensity. During the MiniLander deployments, this voltage was recorded by the AWAC as an ADC count (0‐65535 units) on Analog_in channel 1.


The intensity of the back‐scattered light depends on the concentration, size, shape and reflectivity of the particles suspended in the water. Calibration of an OBS sensor to derive a suspended sediment concentration (SSC) therefore requires sediments with similar properties to those encountered by the sensor in situ in order that the appropriate sensor response can be reproduced.


To calibrate the OBS sensors a sediment flume at the Cefas Laboratory was used to simulate the suspended sediment conditions present at the sensor’s deployment location. Freshwater was used in the flume rather than sea water. It was unlikely the use of sea water would have improved the calibration as the highly dynamic environment maintained in the flume should have been sufficient to prevent flocculation during the calibration exercise. The provision of the necessary volume of filtered sea water would be time consuming and costly.


Suitable sediment samples for this calibration were acquired using a Booner tube suspended sediment trap attached to each MiniLander. It was preferable to use suspended sediment samples rather than benthic sediment samples (obtained using a grab, for example) as they are likely to provide a calibration more representative of the conditions in which the sensor was deployed.


EAST ANGLIA THREE – CEFAS FINAL TECHNICAL REPORT Page 13 of 82


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