Geomorphic changes in a complex gully system measured from sequential digital elevation models, and implications for management

High-resolution digital elevation models (DEMs) were derived from sequential aerial photography of an active fluvio-mass movement (gully) complex in New Zealand's North Island East Coast region, to measure geomorphic changes over approximately one year. The gully showed a complex behaviour, combining fluvial and mass movement erosion, deposition, and reworking of materials stored in an active debris fan. During the measurement period 5200 +/- 1700 m(3) of material were eroded from the 8.7 ha gully complex and 670 +/- 180 m(3) from the 0.8 ha depositional fan, giving a total of 5870 +/- 1710 m(3) for the entire gully complex-fan system. The results provide a high-resolution description of gully behaviour over a short time period, and also demonstrate that mass movement (slumping and debris flows) accounted for almost 90 per cent (4660 +/- 200 m(3)) of the sediment generated. This erosional response is described in terms of gully evolution by comparing the gully complex to other systems in the region in various stages of development. The effect of gully evolution on geomorphic coupling between the gully complex and channel system is described, and coupling is also shown to vary with the magnitude and frequency of rainfall events. From a land management perspective the success of strategies, such as tree planting, to mitigate against gully erosion depends on the stage of gully development - particularly on whether or not mass movement erosion has begun. In contrast to gully rehabilitation efforts elsewhere, basin-wide afforestation in the early stages of gully incision is favoured over riparian planting, given that mass movement assisted by excessive groundwater pressure is the main process leading to uncontrollable gully expansion. To protect land effectively against continuing gully erosion of headwater catchments and resulting downstream aggradation, it is necessary for land managers to understand the spatial and temporal variability of gully development fully so that mitigation efforts can be targeted appropriately. Copyright (C) 2003 John Wiley Sons, Ltd.