Behaviour of Turbidity Maxima in the Tamar (U.K.) and Weser (F.R.G.) Estuaries

The Tamar (England) and Weser (Germany) Estuaries exhibit strong turbidity maxima in their low-salinity reaches. Whereas their morphologies and freshwater inflows are different, the estuaries have similar mean tidal ranges and mean tidal current speeds. Despite the differences, the turbidity maxima show similar intratidal, spring–neap and seasonal behaviour for the two estuaries. On an intratidal time scale, strong suspended particulate matter (SPM) variability occurs in both turbidity maxima regions due to deposition over slack-water periods, subsequent re-suspension and depletion of easily erodible bed-source sediment during the following flood or ebb, and transport by tidal currents. Over a spring–neap cycle, the spring-tide SPM levels are greater than those during neap tides owing to enhanced re-suspension in the stronger spring-tide currents. Both estuaries exhibit a hysteresis in SPM during the falling and rising spring–neap cycle. On a seasonal basis, the occurrences of river spates in both the Tamar and the Weser move both the limits of saline intrusion and the turbidity maxima further down-estuary. Following the return to normal freshwater inflows, the saline intrusion moves rapidly up-estuary, whereas the turbidity maximum exhibits a time lag.