An empirical–conceptual gully evolution model for channelled sea cliffs

Abstract

Incised coastal channels are a specific form of incised channel that are found in locations where stream channels flowing to cliffed coasts have the excess energy required to cut down through the cliff to reach the outlet water body. The southern coast of the Isle of Wight, southern England, comprises soft cliffs that vary in height between 15 and 100 m and which are retreating at rates ≤ 1.5 m a − 1 , due to a combination of wave erosion and landslides. In several locations, river channels have cut through the cliffs to create deeply (≤45 m) incised gullies, known locally as 'Chines'. The Chines are unusual in that their formation is associated with dynamic shoreline encroachment during a period of rising sea-level, whereas existing models of incised channel evolution emphasise the significance of base level lowering. This paper develops a conceptual model of Chine evolution by applying space for time substitution methods using empirical data gathered from Chine channel surveys and remotely sensed data. The model identifies a sequence of evolutionary stages, which are classified based on a suite of morphometric indices and associated processes. The extent to which individual Chines are in a state of growth or decay is estimated by determining the relative rates of shoreline retreat and knickpoint recession, the former via analysis of historical aerial images and the latter through the use of a stream power erosion model.

Geology, landscape and human interactions: examples from the Isle of Wight

“…Hutchinson et al, 1981). At Blackgang Chine, the rate of erosion, although variable and sporadic, is in the order of 0.5 m a − 1 (Leyland and Darby, 2008).…”

“…Hutchinson et al, 1981). At Blackgang Chine, the rate of erosion, although variable and sporadic, is in the order of 0.5 m a − 1 (Leyland and Darby, 2008).Remains of old brick pits are also commonly seen across the Gault outcrop denoting the historical importance of this resource. For example around Rookley an old pit is still visible where brickworks once stood and clay was extracted.…”

Space-for-time substitution and the evolution of a submarine canyon–channel system in a passive progradational margin

“…Glock, 1931;Schumm et al, 1984;Simon and Hupp, 1986). More recently, the concept has been applied to determine how drainage basins evolve towards steady state (Stolar et al, 2007) to identify the erosional and topographic response of drainage basins to tectonic deformation (Hilley and Arrowsmith, 2011), explain the transition from fluvial to glacial landscapes (Kirkbride and Matthews, 1997), reconstruct the evolution of channelled sea cliffs (Leyland and Darby, 2008), infer rates of cliff retreat and talus development (Obanawa et al, 2009), and understand how slope geomorphology and soil change with elevation and precipitation (Huggett, 1998;Schmidt and Meitz, 2000). The application of space-for-time substitution in submarine geomorphology has been sparser and focused almost entirely on submarine canyons.…”

“…In geomorphology, ergodicity has not been applied in the original sense of time and space averages; instead, it is based on the use of space as a surrogate of time and the translation of a spatial morphological sequence into a temporal sequence of individual landform evolution and change (Kirkbride and Matthews, 1997;Leyland and Darby, 2008;Obanawa et al, 2009;Hilley and Arrowsmith, 2011). Distance, location, landform dimension and complexity are used as ergodic indicators of landform development to derive the spatial sequence, which was not intended by the original concept.…”

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

“…When stream channels have the excess energy required to cut down through the cliff to reach the outlet water body, an 'incised coastal channel' (Leyland and Darby, 2008) can form. Sea cliffs are ubiquitous, occurring along ~80% of the Earth's ocean coasts, with similar features bordering lakes and other large water bodies (Emery and Kuhn, 1982), and their associated gullies must therefore also be widespread.…”