Mixing in a small tidal estuarine plume

Features inside a radially spreading river plume discharge in the English Channel are compared to those predicted by a simple generic model that simulates radial spreading from a constant source. X-band radar imagery used to scale model output predicted the presence of an internal bore lagging the surface front of the plume by approximately 200m. High-resolution measurements of temperature, salinity and velocity recorded inside the plume outflow found no evidence of the internal bore in its predicted position. However, temperature contours showed an abrupt shallowing of the interfacial region some 40m behind the surface front. Super-critical Froude numbers present in this region of the plume suggest the bore may form closer to the leading front than predicted by the model. Field measurements show the initial value chosen for the mixing parameter (β) included in the frontal boundary condition is underestimated by about a factor of 2. The required increase in β reduced the lag length scale of bore formation to one where critical Froude numbers were detected inside the plume. Further estimates of entrainment velocity in the frontal region showed convergence and downwelling at the surface front that is followed by a region of upwelling. A 'caterpillar track' type circulation is thought to be responsible for mixing and entrainment in the head region of the plume. This head remains a reasonably constant size due to any increase in across frontal velocity over the ebb tidal cycle being matched by increased entrainment and mixing.