Effects of Holocene climate and sea-level changes on coastal gully evolution: insights from numerical modelling

Gullies are sensitive to a range of environmental disturbances so they can provide insight into the environmental history of surrounding landscapes. Coastal gullies are of particular interest as they are influenced by terrestrial and marine processes. For example, the coastal gullies found on the Isle of Wight, UK, are highly dynamic, with episodes of sea cliff erosion causing rejuvenation of the channel network. Consequently a key factor in their long-term evolution is the relative balance between rates of cliff retreat (driven primarily by Holocene sea-level rise) and headwards incision caused by knickpoint migration (driven primarily by Holocene climate via its impact on runoff). In this paper we explore the Holocene erosional history of these gullies using a numerical landscape evolution model modified to include a cliff recession function. Knickpoint recession rates are simulated using a detachment-limited erosion law wherein erosion rate is a power function of drainage area and stream gradient with model parameters defined using empirically-derived data. Hindcast simulations, from 12 000 cal. years BP to present, are undertaken for a range of scenarios of Holocene climate change and sea-level rise. Plausible erosional histories are extracted from scenarios in which simulated and observed gully forms match. The results suggest that rate of sea-level rise is the key control on gully formation and that it is only in the late Holocene period, and specifically in the last 2000 years, that sea-level rise has slowed sufficiently for knickpoint recession rates to exceed cliff recession rates and create sustainable gully networks. The simulations also indicate that the contemporary gully systems are close to a critical threshold, suggesting that future gully evolution is likely to be sensitive to small changes in rates of effective precipitation and/or sea-levei rise. Copyright (C) 2009 John Wiley & Sons, Ltd.