Sediment erosion dynamics of a gullied debris slide: a medium-term record

Abstract

Medium-term post-event sediment flux investigations are rare for headwater catchments and particularly sparse for gullied hillslope failures. Repeat field observation, ground photography and cross section measurements of a debris slide scar at the Wet Swine Gill headwater catchment (0.65 km2) in the English Lake District (UK), provide evidence of erosion and deposition dynamics over the medium-term (2002–2014). These data are compared to site topographic and meteorological conditions, to evaluate potential process–response linkages. Rill and gully erosion networks establish soon after the slide failure (1 February 2002); thereafter gully enlargement proceeds rapidly, first by vertical downcutting, prior to lateral expansion and gully wall angle decline. Changes in cross sectional width, depth and area (2002–2013) are characterised by statistically significant (P = < 0.05) negative exponential growth models (R2 = width: 0.88–0.97; depth: 0.71–0.86; area: 0.87–0.93). Gully walls were dominated by erosion but the gully bed was characterised by episodic sediment production, storage and transfer often leading to temporary deposition. Specific erosion rates on the gully wall exceeded those on the adjacent slide scar by up to 764% (maximum values = wall: − 0.0084; scar:− 0.0011 m2 m− 1 d− 1). Upslope contributing (runoff) area and slope gradient are generally important for erosion; although linear regression analysis demonstrates weak or insignificant relationships between meteorological conditions and gully/scar sediment flux. A general conceptual model of slide scar evolution, integrating gully growth and capture, summarises activity at this site. However transferability to locations with terrain characteristics, land management practices and climate conditions different to those existing in the UK uplands remains to be tested. This investigation adds to growing appreciation of the complexities of sediment dynamics in headwater catchments and provides clear evidence for the potential of early management intervention to counter detrimental post-failure sediment erosion; which at this site would have been most effective up to 3–4 years following gully initiation.