Estimating estuarine turbidity: An application to estuaries of the Isle of Man and northeast Irish Sea

A simple estimation method for estuarine turbidity that may be helpful for management purposes is used to predict SPM concentrations for the strongly tidal estuaries of the northeast Irish Sea. It demonstrates a systematic tendency for the longer, more slowly flushed estuaries to have greater ‘intrinsic’ SPM concentrations within their high turbidity regions than the shorter estuaries. Predicted maximum SPM concentrations for the mainland estuaries vary from ∼10 mg l−1 (Bladnoch) to ∼10 g l−1 (Solway, Mersey and Dee). The Isle of Man estuaries are predicted to have maximum SPM concentrations that are less than 10 mg l−1. These small predicted concentrations are largely a consequence of their short lengths and short residence times. Geomorphology is the ultimate reason for this difference in Isle of Man and mainland estuarine turbidity. The Isle of Man has steep slopes and small rivers that flow rapidly to sea via short estuaries, whereas the mainland has a number of large, long, coastal plain estuaries that allow strong tidal currents to develop and provide storage for large quantities of fine sediment. Survey data for the Solway Esk and the Forth and Clyde Estuaries are used to illustrate the effects of uncertainty in the definition of an estuaryʹs tidal length, which is a problem both for estuarine science and estuarine management. Despite the dearth of published, good quality field data for the estuaries described herein, there is nevertheless a good qualitative agreement between the predicted values of SPM and the majority of these observations.