Needle ice processes and sediment mobilization on river banks: the River Ilston, West Glamorgan, UK

Abstract

This paper examines the role of needle ice processes in the bank erosion system of a site on the River Ilston, West Glamorgan, South Wales, UK. Bank retreat rates were monitored with erosion pins over a 2.25 year period. Concurrent detailed data were also obtained on river flow levels and ambient air and stream temperatures. Unique data on river bank thermal regimes are presented, along with rare information on the morphological properties and sediment yield characteristics of needle ice growths. Needle ice occurred even with mild freezing temperatures if sufficient bank moisture was present, with average growth rates of approximately 1 mm h −1. Bank material incorporation and mobilization within needle ice was significant: sediment yields of up to 4.02 kg m −2 of bank face were observed, and represented between 32% and 43% of total bank erosion recorded. Most sediment was incorporated within needle ice when growth became stressed (because of overrapid cooling and/or limited moisture supplies), which supports the models of Outcalt and Fukuda. Four mechanisms of direct needle ice sediment transport on river banks were identified: direct particle fall, sediment-laden rivulets, sliding failures, and toppling failures. However, the indirect, preparatory role of needle ice and frost disturbance of bank surfaces is also emphasized, and the enhanced fluvial erosion which takes place when bank materials have first been conditioned by frost-related processes is noted. Both indirect and direct effects are expressed in a simple conceptual model and a number of the important system linkages examined. Combinations of needle ice action and subsequent fluid entrainment of weakened bank material appear to be the dominant mechanism for bank erosion here. Pronounced and persistent erosional notches in the lower parts of the bank profile were observed, showing that, in winter, even single, low-magnitude, flow events can effect significant channel width adjustment here.