Instability and Mixing in a Small Estuarine Plume Front

• Published in: Estuarine, coastal and shelf science Vol. 55; no. 2; pp. 275 - 285
• Main Authors: Pritchard, M., Huntley, D.A.
• Format: Journal Article
• Language: English
• Published: London Elsevier Ltd 01.08.2002; Elsevier
• Subjects: bulk-mixing coefficient; Coastal oceanography, estuaries. Regional oceanography; Earth, ocean, space; Exact sciences and technology; External geophysics; Froude number; gravity head current; Physics of the oceans; plume front; Teignmouth; United Kingdom; Europe; gravity head current; Froude number; plume front; bulk-mixing coefficient; Teignmouth; Brackish water environment; Forehead; Mixing; Gravity current; Hydraulics; Tidal current; Estuaries; Instability; Flow velocity; Plume
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1006/ecss.2001.0902

Detailed field observations at the front of a small tidally modulated radial river plume have been carried out to study the localized internal instabilities thought responsible for mixing. A novel ship-borne instrument array was used to resolve internal features inside the spreading plume down to horizontal length scales of less than 0.25 m within the top 4 m of the water column. Results from this study, that applied stringent correctional and filtering procedures to the field data, identify the presence of multiple internal hydraulic jumps on the plume interface some 30–40 m behind the main plume frontal discontinuity. This region of instability ( Fr i >1) corresponds to the rear of a gravity head current. Bulk-mixing characteristics in the frontal region of the plume were quantified through methods originally applied to laboratory models of gravity current mixing and also used to represent the extent of turbulent mass and momentum exchange in frontal boundary conditions of numerical models. Results from field experiments suggested that mixing in the plume front was modulated by the tidal outflow from the estuary and across-frontal velocity. The ebb tidal mean magnitude of the bulk frontal mixing coefficient, β, was more than double (0·37) that computed from previous analogous laboratory experiments but of the same order of magnitude as recent small-scale river plume frontal studies.