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Title: Temporal variability of suspended particulate matter in a tidal estuary
Author: Todd, David
ISNI:       0000 0004 5349 7831
Awarding Body: Prifysgol Bangor University
Current Institution: Bangor University
Date of Award: 2014
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Observations are presented of interactions between hydrodynamics and suspended particulate matter (SPM) on tidal, spring-neap and seasonal timescales from data collected in the Hilbre Channel at the mouth of the Dee Estuary (N.W. United Kingdom). Emphasis is placed upon the relationships between SPM and hydrodynamic controls, and the impact of specific biological components. Moored ADCP (Acoustic Doppler Current Profiler), temperature, salinity and LISST (Laser InSitu Scattering Transmissometer) data were collected during February-March and May-June 2009, supplemented by ship-deployed 25h vertical profiles of transmissometer, LISST, CTD, and fluorescence data taken at the beginning and end of each mooring deployment. The Dee Estuary is a hypertidal estuary with a mean spring tidal range of 7.7 m and strong, near-rectilinear currents dominated by the M2 and S2 tidal components. During May-June, flood-dominant current velocities resulted in flood-dominant SPM flux (63% by volume, 39% by mass). During February-March, a strong horizontal SPM concentration gradient reduced the flood dominant SPM flux to 16 % by volume and 39 % by mass. It is proposed that low biological activity (measured using low fluorescence as a proxy) enabled erosion of fine sediments from mud flats and saltmarsh within the estuary, thus causing a large horizontal concentration gradient of the fine particle component of SPM. By contrast, during MayJune, it is proposed that biological activity and atmospheric forcing increased the bed sediment erosion threshold, thus suppressing the fine particle concentration gradient. Furthermore, it is proposed that this biological activity mediated the observed increases in the strength, size, density and settling velocity of the flocs that made up SPM. In May-June, SPM composition changed due to a contribution of particles of uniform density but variable size, suspected to be either diatoms or aggregates ripped from the bed sediment of the intertidal areas. During February-March, flocs ruptured under the high turbulence conditions of flood and ebb tides, and reformed at high and low slack waters. By contrast, during May-June under comparable turbulence conditions, flocs resuspended on flood and ebb tides without significant breakup due to their increased strength. Seasonal changes to SPM properties potentially influence SPM flux via two different mechanisms: 1. Biological and atmospheric suppression of resuspension in May-June reduces the horizontal concentration gradient that is observed in February-March. Consequently, flood dominance of SPM flux is greater in May-June. This must increase net upestuary transport in May-June. 2. SPM undergoes rupture and flocculation due to reduced floc strength in FebruaryMarch. Flocculation and enhanced settling are greatest over an extended low water, so that flocs are deposited further up the estuary than their original starting point when they were resuspended by the flood. This should increase net up-estuary transport in February-March. These observations suggest that seasonal variations of SPM properties are important for net transport within estuaries characterized by large intertidal flats.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available