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

Abstract

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 infl uenced 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 modifi ed 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 defi ned 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 specifi cally in the last 2000 years, that sea-level rise has slowed suffi ciently 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-level rise.

Knickpoint formation, rapid propagation, and landscape response following coastal cliff retreat at the last interglacial sea-level highstand: Kaua'i, Hawai'i

“…For example, if the rate of cliff erosion is faster than streams are able to incise vertically, and the channel is steeper than the shore platform generated by cliff erosion, then a knickpoint will be developed at the coast (e.g., Snyder et al, 2002). As an example, small coastal gullies in southern England are thought to have formed during rising sea level resulting in cliff erosion during the early Holocene (Leyland and Darby, 2008;Leyland and Darby, 2009), and a similar mechanism was proposed for Holocene cliffs in Tahiti (Ye et al, 2013). Similarly, Leckie (1994) reported that Holocene coastal erosion on New Zealand's eastern South Island caused the rivers to steepen and incise near the coast.…”

“…Indeed Stearns (1985) inferred that large waterfalls prevalent in canyons on the Hawai'ian islands resulted from upstream-propagating knickpoints generated at wave-eroded coastal cliffs, and such a mechanism has been proposed for other coastal landscapes (Leyland and Darby, 2009;Ye et al, 2013). Other workers pointed to groundwater seepage erosion as a driver for knickpoint formation and retreat on Hawai'ian islands (Hinds, 1925;Wentworth, 1928;Kochel and Piper, 1986).…”

21st century climate change: where has all the geomorphology gone?

“…For example, Leyland and Darby (2009) consider coastal gully response to Holocene sea level rise using a landscape evolution model. Predicting the non-linear behaviour described by many geomorphologists will be difficult.…”

Landscapes on the edge: examining the role of climatic interactions in shaping coastal watersheds using a coastal–terrestrial landscape evolution model

“…Terrestrial Landscape Evolution Models (LEMs) are valuable tools for visualizing and quantifying the response of landscapes to changing process dynamics in terrestrial environments (Tucker and Hancock, ). LEMs have often been used to unpick historical landscape evolution (Tucker and Slingerland, , ; Coulthard et al ., ; Coulthard and Macklin, ; Hancock, ; Leyland and Darby, ; Temme et al ., ). Recently, growing attention has been paid to modelling the response of landscapes to short‐term projections of future (~100 year) climate change (Temme et al ., ; Coulthard et al ., ), an area of geomorphological research which is in relative infancy but is potentially important in terms of identifying landscapes sensitive to climate change and in projecting the trajectory of future landscape response (Lane, ).…”