Modelling the roles of magnitude and frequencyin the evolution of an upland catchment

Despite recent advances in numerical modelling of our environment, the relative roles of random as opposed to more deterministic factors are frequently ignored. These include the non linear behaviour of a river basin in response to stochastic events such as extreme weather and the effects of climate change. Existing modelling approaches tend to ask how a landform develops, but not why. Whilst we have a good understanding of the processes involved in a catchments evolution, a gulf exists between our knowledge of the cause and consequent effect. To investigate this, a cellular automaton (CA) type model is being used, to simulate the evolution of a small catchment (>3km2) over a variety of climatic and cultural scenarios.

This innovative model is applied to the catchment of Cam Gill Beck, above Starbotton, North Yorkshire. This is divided into 1 million, 2m by 2m 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 grainsize fractions. This provides a good representation of valley floor geometry whilst retaining a fully dynamic interaction with the surrounding valley sides.

The model has allowed for the first time the relative roles of flood magnitude, frequency and vegetation change to be examined in great detail over time scales from seconds to 100 years. Furthermore, the model produces detailed stratigraphies, grainsize distributions, erosion, deposition, and examples of avulsion and channel change. Results show geomorphological change resulting from deforestation and especially from larger flood magnitudes, with a non linear sediment discharge.