Modelling geomorphic response to environmental change in an upland catchment

In the UK's upland catchments river terraces and alluvial features indicate a history of periodic aggradation and degradation linked to Holocene changes in land use (primarily deforestation) and climate change (altering flood frequency and magnitude). Although both factors are important, calculating their individual effects is complicated by the likelihood of their concurrent alteration. To investigate the relative impacts of land use and climate change, a cellular model is applied to the upland catchments of Cam Gill Beck, above Starbotton, North Yorkshire. This is divided into 1 million 2 m by 2 m grid cells, to which a range of process laws are applied. These include approximate expressions for mass movement rates, soil creep, the influence of vegetation and hillslope hydrology, as well as fluvial erosion and deposition in ten grain-size fractions. This provides a good representation of valley floor geometry while retaining a fully dynamic interaction with the surrounding valley sides. Previous applications of this model have shown the detailed formation of bars and berms as well as examples of braiding, avulsion and channel range. Running on a Silicon Graphics Origin 2000 computer, an ensemble of simulations were completed, bracketing a wide range of environmental scenarios involving changes in flood frequency, magnitude and vegetation cover. Over time-scales ranging from 10 to 100 years, these showed that decreasing tree cover and increasing rainfall magnitude individually produced similar 25% to 100% increases in sediment discharge, whereas in combination they generated a 1300% rise. Furthermore, channels formed by the model in response to increased rainfall magnitudes are located where relic channels are found in Cam Gill Beck, implying that these are the products of previous periods of high rainfall magnitudes. Copyright © 2000 John Wiley & Sons, Ltd.