Methods used for:

1. Monitoring amounts of deposition/erosion (i.e. extent of sinks) of sediment
2. Monitoring quantities and movement of suspended sediment

1. Monitoring deposition/sinks

1. Short term change
2. Long term change

1. Short Term Change - see current fieldwork: sinks

   (a) Accretion plates

   Aluminium plates, of approximately A4 size are buried underneath the mudflats at various intervals up and down the estuary. The depth of the sediment overlying these plates will be monitored monthly to examine sediment accretion/erosion over time (monthly, seasonally, annually).

   (b) Accretion Poles/Surveying

   Bamboo canes are inserted into the mudflat as transects running from the bank down to the mean low tide mark. A total station/tachometer set up on a known benchmark is then used to survey remotely the angle of the intersection of the pole and the mud surface. This will be repeated every two months and using trigonometry, the difference in elevation can be calculated.

2. Long-term (decadal) Change

   (a) Sediment cores

   Detailed stratigraphic investigations will be made as well as ²¹⁰Pb dating of sediment cores to provide a chronology of sediment deposition on the mudflats/salt marsh over the last 120 years.

   (b) Aerial photographs

   Aerial photographs from as far back as the 1940s (including those taken by the Luftwaffe) will be used to monitor changes in the extent of salt marsh development and channel position.

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2. For Monitoring Suspended Sediment:

1. Short-distance, localised measurement
2. Long-distance measurement: sediment tracing

1. Short-distance, localised measurement:

   (a) Short-term, highly accurate and intensive sampling: acoustic/optical methods (see current fieldwork).

   i. Acoustic Doppler Current Profiling (ADCP) surveys (NEW application!)

   The principles that the ADCP works on are essentially very simple: sound, which is emitted from the device, rebounds off the sediment particles and other objects in the river (known as back scattering). The device monitors this 'echo' and measures the shift in the acoustic wavelength due to the speed of movement of that particular particle (known as the Doppler effect). This also allows the direction of movement of the particle to be known (i.e. depending which direction the wavelength shifts in). The varying lengths of time for the backscattering to return allow a complete 3D picture of the sediment to be built up. In addition, to suspended sediment concentration, the ADCP also provides the user with data on discharge, channel depth and profile.

   Although widely used at sea by the oil industry, ADCPs have only recently started to be used in the fluvial and estuarine environment, where they are either moored in one location over a period of time or attached to the back of a boat as it traverses the body of water (boat velocity is taken into account). Most recent examples include the Humber and the Crouch but no such surveys have been carried out on the Ouse.

   Similarly, ADCP surveys will be carried out in the summer and winter seasons of 2002. At the wider locations on the estuary, such as in Newhaven town itself, a small boat will be used to pull the device across the width of the channel. Further upstream, from Piddinghoe northwards, the device will be pulled across the river on a rope.

   All ADCP surveying is overseen by Dr Jon Taylor of Compass Hydrographic Services/Compass Data Systems.

   (ii) Transmissometers

   Transmissometers, also known as beam transmittance meters, use light to sense the suspended sediment. They consist of two parts: a light source, at a known distance from the receiving photocell. The photocell measures the decrease in the intensity of the light having passed through the water: the darker it is, the greater the amount of suspended sediment.

   (iii) Traditional water sampling

   Filtration of the suspended sediment to calibrate the ADCP and transmissometer and carry out grain size analysis.

   (b) Long term sampling with inferences: Booner tubes (new and experimental!) (see current fieldwork)

   Plastic tubes, approximately 30cm long and with either end sealed are placed upright on the estuary bed, either in intertidal areas (mudflats) for ease of access or below the low tide mark. A short distance down from the top is a horizontal row of small holes. As the water flow passes through the tubes (via the holes) it eddy's round inside and slows down, forcing it to deposit a proportion of it's load at the base of the tube. In essence, they collect the sediment that would otherwise be impossible within the normal sampling program. They will provide a continuous record of suspended sediment over time, especially at times of peak discharge following storm events.

2. Long-distance Movement: Sediment Tracing

   One of the later phases of project will be to assess sediment sources and fine sediment transport pathways. Currently, there are no readily applicable methods or tracing fine, cohesive sediment transport in estuaries. Two methods will be tested:

   (a) "Geochemical fingerprinting"

   This procedure allows basic tracking of sediments by utilising the tracers found naturally in the environment, such as certain radionuclides and trace elements as well the anthropogenic ones, which were released accidentally as pollutants. Suspended and bottom sediments are sampled from the fluvial and from the marine environment and analysed so that it is known which trace elements/radio nuclides originate from which environment. Thus when future samples are taken, the ratio of fluvial to marine sources can be estimated for each location (known as a 2 end-member mixing model).

   (b) Labelled Sediments - NEW method!

   Typical upstream sediments i.e. clays will be labelled with a geochemical tracer, such as lanthanum (which is known to bind extremely well with clays, can be detected in samples in the lab easily and has no environmental impact). These samples are then released into the river and locally tracked by surface sediment sampling and analysis.

   Geochemically obtained data can then be compared with that of the ADCP survey data in order to validate these results.