The effect of land use on slope failure and sediment generation

The Coromandel region in New Zealand was subjected to a severe storm in 1995. Analysis of the relevant data by Land Care Research has provided a valuable opportunity to assess the type, extent, distribution and sediment generation rates by slope failures associated with steep-land forests and harvest practice.

The storm-initiated slope failures generated ~0.5 Mt of sediment, predominantly by debris avalanche. Most were located within indigenous forest and secondary regrowth and generated ~78% of the total sediment mass. 

Sediment generation rates were greater from areas of exotic forest clear-felled three years before the storm and these were 2.5 times greater than from cutover clear-felled just before the storm. This result is explained by the progressive loss of strength from decaying tree roots that had not yet been countered by an effective root system under a new tree crop

The erosion response was primarily controlled by rainfall variation and slope, which overrode the influence of vegetation cover. Time since clear-felling had a secondary influence. 

Click here for source (New Zealand Journal of Forestry Science)

Photo: FWPA