Sediment Transfer From Gravel-Bed Rivers to Beaches

Geomorphology, Sedimentary geology

Gemmell, Sally L. G

University:University of Glasgow

关键词: Geomorphology, Sedimentary geology;   

Others  :  http://theses.gla.ac.uk/76156/1/13818968.pdf

来源: University of Glasgow

【 摘 要 】

A morphological approach was used to quantify the transfer of gravel-sized (> 2mm) sediment from one river to the coast and its subsequent redistribution within the coastal zone. The study investigates sediment transport in wandering gravel-bed rivers, river mouth processes and sediment transport on gravel beaches. The research was largely field based and used results from repeat morphologic surveys of the lower, wandering gravel-bed reach of the River Spey, the Speymouth delta and gravel beaches of Spey Bay, north-east Scotland. The supply of gravel to the river mouth was episodic, with transport rates varying from 41 000 +/- 6 000 m3a-1 (1993-1994) to 6 000 +/- 4 000 m3a-1 (1997-1999). Transport rates were not directly related to flow conditions because the availability of sediment for transport was critical. Sediment was mobilised according to the recent local history of erosion, deposition and channel adjustment and not only the magnitude of the flood. Delivery to the coast of this episodic sediment supply was made more variable depending on the operation of the delta. A gravel spit complex extended westwards across the river mouth at a mean rate of 150 ma' between 1997 and 1999. This resulted in temporary storage of sediment in the extending spit which had implications for the downdrift coast. Cycles of accretion and erosion were created in the lee of the spit which were most significant at the river mouth before being propagated downdrift. Volumetric information obtained from successive beach profile surveys indicated that zones of accretion and erosion were spatially and temporally variable along the 16km coastline of Spey Bay. This variability was caused by the passage of pulses (or slugs) of sediment which moved alongshore in response to variations in sediment supply (e.g. episodic delivery of fluvial sediment, river mouth processes, beach feeding and storms). It is argued that gravel sediment moves alongshore as slugs by a given distance depending on the magnitude of storm events. Travel distances of ca. 2-3km were recorded following a major storm event in March 1998. This has important implications for the management of gravel beaches, as erosional zones (or "problems") are not static and vary in space and time. . Fluvial, deltaic and coastal volume changes were combined to estimate a short-term (3 year) sediment budget for the system. A medium-term budget (100 year) was also compiled from map analysis and river-modelling studies. Both budgets showed a net loss of sediment from the system, indicating a system that has been erosional for at least the last century.