SEDIMENTATION IN THE ESTUARY OF THE RIVER CROUCH, ESSEX, ENGLAND

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Abstract
The coast of Essex is bordered by marshland and is subsiding. Under natural conditions sedimentation on the marshes keeps pace with subsidence, but where the land has been reclaimed and embanked, subsidence has continued with no compensating sedimentation.Material in suspension is flocculated and is mainly inorganic but may contain up to 25% organic material. Particle diameters of the flocculi range from less than 1 µ to about 80 µ with a mean at about 10–15 µ. The concentration varies with state of tide, depth, and location. There is more material near the bottom than at the surface, and greater concentrations occur near the mouth than near the head of the estuary.Bottom deposits are made up of two distinct particle populations. There is a fine sand fraction which is believed to be transported near the bottom, and a silt fraction derived from suspension. Offshore, in areas exposed to wave action, there arc fine sands with very little silt. The proportion of silt increases and the modal diameter of the fine sand fraction decreases towards the head of the estuary.The sediments may be derived from offshore deposits of boulder clay, and evidence was found to suggest that these deposits are being eroded by tidal currents. Material eroded from an unsorted boulder clay could produce the distribution of sedimentary properties found in the Essex estuaries.

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