Investigating radionuclide bearing suspended sediment transport mechanisms in the Ribble estuary using airborne remote sensing - PhDData

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Investigating radionuclide bearing suspended sediment transport mechanisms in the Ribble estuary using airborne remote sensing

The thesis was published by Atkin, Paula Ann, in September 2022, University of Stirling.


BNFL Sellafield has been authorised to discharge radionuclides to the Irish Sea since
1952. In the aquatic environment the radionuclides are adsorbed by sediments and are
thus redistributed by sediment transport mechanisms. This sediment is known to
accumulate in the estuaries of the Irish Sea. BNFL Springfields is also licensed to
discharge isotopically different radionuclides directly to the Ribble estuary. Thus there
is a need to understand the sediment dynamics of the Ribble estuary in order to
understand the fate of these radionuclides within the Ribble estuary. Estuaries are
highly dynamic environments that are difficult to monitor using the conventional
sampling techniques. However, remote sensing provides a potentially powerful tool for
monitoring the hydrodynamics of the estuarine environment by providing data that are
both spatially and temporally representative.
This research develops a methodology for mapping suspended sediment concentration
(SSC) in the Ribble estuary using airborne remote sensing. The first hypothesis, that
there is a relationship between SSC and l37Cs concentration is proven in-situ (R2=O.94),
thus remotely sensed SSC can act as a surrogate for \37Cs concentration. Initial in-situ
characterisation of the suspended sediments was investigated to identify spatial and
temporal variability in grain size distributions and reflectance characteristics for the
Ribble estuary. Laboratory experiments were then perfonned to clearly define the SSC
reflectance relationship, identify the optimum CASI wavelengths for quantifying SSC
and to demonstrate the effects on reflectance of the environmental variables of salinity
and clay content. Images were corrected for variation in solar elevation and angle to
give a ground truth calibration for SSC, with an R2=O.76. The remaining scatter in this
relationship was attributed to the differences in spatial and temporal representation
between sampling techniques and remote sensing.
The second hypothesis assumes that a series of images over a flood tide can be animated
to provide infonnation on the hydrodynamic regime, erosion, and deposition. Spatial
and temporal data demonstrated the complex controls on sediment transport. The data
also showed the importance of microphytobenthos in the stabilisation of intertidal
sediments, highlighting their importance in defining sources and sinks of radionuclides
in intertidal areas. Water volume data from the VERSE model were combined with
SSC from the imagery to calculate the total sediment in suspension for each flight line.
This provided the figures used to detennine total erosion and deposition, which were
then used to derive net suspended sediment and l37Cs influxes of 2.01xl06kg and
604MBq per flood tide.

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