| Summary: | The physical characteristics of the Loughor Estuary have been studied by field data collection on selected tidal cycles along the Central Channel of the Estuary, and monthly sampling programmes at its periphery. Data has been collected on water levels, salinity of the water mass and current speed and direction over a range of tides and low to medium freshwater inputs. The tidal wave is symmetrical at the mouth of the Estuary (eastern Carmarthen Bay), becoming asymmetrical as it propagates up the Estuary. Within the Estuary, the tidal response is a combination of a standing (symmetrical) wave and progressive (asymmetrical) wave, with the progressive component increasing towards the head of the Estuary. The flushing time of the Estuary is only of the order of a single tidal cycle; therefore, this estuarine area is one of high energy dissipation and turbulence. The general pattern of tidally-induced surface water circulation is directed along the main longitudinal axis of the Channel. In addition, the orientation of the current system in the main Channel, local variations in the current direction can occur due to the effect of abrupt coastal discontinuities and shallow bathymetry. Salinity variation across the Estuary is very small, with salinities along the southern shoreline being slightly higher than the corresponding northern values. The general pattern of the longitudinal (and vertical) salinity distribution at both High and Low Water, at least under low to medium freshwater discharge, demonstrates that, the lower portion of the Estuary is almost vertically homogeneous, with stratification increasing slightly towards the head of the Estuary. The correlation between salinity and salinity gradients along the Estuary and the total freshwater discharge (based on observations of the River Loughor and extrapolation to the remaining rivers) and tidal range, have demonstrated that there is a strong negative correlation between the salinity at each of the Stations and river discharge. Mixing is influenced slightly more by river discharge than by tidal range. Based on Ilansen and Rattray classification, the Estuary is "laterally homogeneous and well mixed". Net flow is seaward of all depths in the water column; consequently, upstream salt transport must be by diffusion. The net transport of salt upstream, across any of the sections, at the time of the observations, is indicative of flood waters of higher salinity than those on the ebb. The nature of the tidal current system at the mouth of the Loughor Estuary, based on tile analysis of self-recording current meter data is rectilinear immediately opposite to the main Channel axis; rotatory current systems predominate at the peripheral (shallower water) stations. Residual currents demonstrate that waters from the mouth of the Loughor Estuary take about 3 to 4 days to reach the Ilelwick Sands, a linear sandbank adjacent to the northern Bristol Channel coastline; presumably, they then mix with the water masses of the northern Bristol Channel.
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