Geomorphic change implied by regolith — Slope relationships on steepland hillslopes, Taranaki, New Zealand

Abstract

Slope angle and regolith depth were measured along profile lines covering a range of hillslopes in several small neighbouring drainage basins in Taranaki hill country, New Zealand.

Slope measurements show a cyclic variation along the length of broadly rectilinear profiles. Variation in slope on the lower portion of profiles adjacent to stream channels indicates that hillslopes have undergone phases of accelerated downcutting and hillslope rejuvenation. A recent phase of increased erosion and hillslope rejuvenation is suggested for hillslopes steeper than 31° by the steep lowerslope and incised stream channel. Hillslope evolution occurs primarily by parallel retreat of steeper swale sites and slope decline during pauses in the rate of downcutting.

All slope profiles show a large range in regolith depth. Mean regolith depth of swale sites is highly correlated with mean profile slope angle. Mean regolith depth decreases and the frequency of landslides increase with increasing mean profile slope angle. Microtopographic variation in regolith depth is attributed mainly to vegetation effects, recurrent landsliding, and in-part to bedrock curvature, hillslope position, and slope angle. Shallowing of regolith is caused by recurrent landsliding on steep swale sites, fluvial erosion at the base of steep profiles, and surface erosion on extremely convex spur sites. The very shallow soil on the steep lowerslope portion of profiles suggests a phase of soil removal, consistent with hillslope rejuvenation.

Average depth of regolith is least at 97 cm, and variability greatest, on hillslopes with a profile mean exceeding 31°. Erosion rates are greater on these sites where soil is derived predominantly from underlying Tertiary sandstone. This is attributed to a greater frequency of landsliding compared to hillslopes with a profile slope less than 31° where a more regular and deeper average regolith of 153 cm infers a longer period of hillslope stability consistent with the high volcanic ash content of soils. Airfall andesitic volcanic ash, deposited throughout the Holocene, has largely been removed from steeper sites.

The work demonstrates the need to interpret the frequency and distribution of landslides in context with natural geomorphic changes in order to assess the effect that landuse changes has on erosion rates.