An investigation into aspect related erosion forms in a valley in the Port Hills, Banks Peninsula

Abstract

The study region is in an asymmetrical valley about 100 m a.s.l., within the Port Hills, Banks Peninsula. Mass movement of the loess colluvium regolith mantling the southwest facing backslopes is common but has rarely occurred on the northeast facing backslopes and footslopes. Tunnel gully erosion is common within the regolith of the northeast facing backslopes and footslopes and is less common within the regolith of the southwest facing backslopes.

Valley asymmetry has been caused by stream downcutting along lava beds which dip to the SW at 27°±1°. Mass movement of the loess colluviums occurs over the exposed flows because its natural angle of repose is between 25° and 29°. Mass movement is less frequent in slopes with the same angle that face northeast because of the absence of a smooth failure plane and because the regolith there has a higher interlocking basalt component.

From pinhole test results and from soil extensibility measurements, tunnel gullies are not caused by soil fissuring. Regolith susceptibility to tunnel gullying depends on the degree of soil dispersibility and erodibility on the sides of natural water bearing fissures and channels. Pinhole tests showed that the moist dense fragipan soil (p = 1.6 g/ml) of the northeast facing backs lope is more erodible than the soil above it (p = 1.4 g/ml) and is more erodible than soil at the same depth (p = 1.3 g/ml) on the other side of the valley. This soil did not contain a fragipan. Total clay content within the fragipan varied between 18% and 32%. Zones with the least clay were very erodible. Such clay variation was not encountered within soil of the SW facing backslope.

Pinhole test results showed that some soil from within the fragipan of the NE facing backslope was dispersed. These results did not agree with the porewater cation dispersion criteria used. Modifications to the method for establishing such criteria have been given.

Soil characteristics present within the SW facing backslope which would inhibit tunnel gully initiation were found to be; roots growing to depth between gleyed fissures, enhanced soil flocculation caused by a higher organic carbon content and a slightly lower pH.

An appraisal of the pinhole test for locating dispersible soils using in situ soil cores has been given.