Source, storage and mobilisation of fine sediment in a chalk stream system

Concern for sediment problems in the Upper River Piddle, Dorset, including accumulation of fine sediment on the channel bed and elevated turbidity lev els during periods of stable flow, provided the stimulus for a detailed fie ld investigation of the sediment dynamics of this chalk stream system under taken during the period November 1991 to April 1993. Two monitoring station s were established at South Farm, Piddletrenthide and Lawrence Mede, Piddle hinton and estimates of suspended sediment load were obtained for these two sites. In addition, suspended sediment concentrations and bed sediment loa dings were regularly sampled at several other sites, sediment traps were de ployed at ten representative sites along the river channel, field observati ons of sediment sources and pathways were made during storm events and a se diment source fingerprinting study was undertaken. Estimates of annual susp ended sediment yield provided values of 9-12 t km(-2) year(-1), which must be seen as low by UK standards. Elevated suspended sediment concentrations were not restricted to storm events, but occurred throughout the monitoring period in response to remobilisation of sediment stored in the channel. Th e evidence provided by the sediment loads recorded at the two monitoring st ations, by the sediment traps and by the measurements of bed sediment loadi ng, indicated that substantial amounts of sediment accumulated in the chann el of the upper and middle reaches of the river during the winter months an d that this 'slug' of sediment was slowly transmitted downstream during the following summer. Fingerprinting of the transported sediment and of potent ial sources indicated that the dominant source of suspended sediment was li kely to be surface soil from cultivated areas. Field reconnaissance surveys undertaken during storm events indicated that substantial inputs of surfac e runoff and suspended sediment entered the stream. Much of this sediment i s, however, stored within the channel system and slowly transmitted downstr eam. Copyright (C) 1999 John Wiley & Sons, Ltd.