Effects of Holocene climate and sea-level changes on coastal gully evolution: insights from numerical modelling [In special issue: Gully Erosion Processes: Monitoring and Modelling]

Gullies are known to be sensitive to a wide range of environmental disturbances so their geomorphology can provide insight into the environmental history of the surrounding landscape. Coastal gullies are of particular interest in that they are strongly influenced by both terrestrial and marine processes. For example, the coastal gullies found on the Isle of Wight, UK, known locally as ‘Chines’, are highly dynamic, with episodes of sea cliff erosion frequently causing the rejuvenation of the channel network. Consequently a key factor in the long-term evolution of the Chines 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 the Chines using a numerical landscape evolution model that has been modified to include a cliff recession function. Knickpoint recession rates are simulated using a detachment-limited channel 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 12000 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 Chine and landscape forms match. The results suggest that the rate of sea-level rise is the key control on Chine formation and that it is only in this 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. Our interpretation that the Isle of Wight gullies are of relatively modern is in agreement with previous studies. Finally, the simulations also indicate that contemporary Chine gully systems are close to a critical threshold, suggesting that future gully evolution is likely to be sensitive to small changes in future rates of effective precipitation and/or sea-level rise.