The water flow and building behaviour of a small alluvial fan : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science at Lincoln University

Abstract

A small alluvial fan was built with a constant rate of input of water and sediment. The fan surface was measured at half hour intervals from the eleventh to the thirty fifth hour of accumulation and the area affected by flow during the previous half hour described and recorded. Flow behaviour was examined at two scales: the long term scale where flow is responsible for the production of the gross fan morphology and the short term scale associated with the formation of surface features on the fan. The long term hydraulic form of the fan is described quantitatively. The form is constant despite infiltration of flow into the fan surface. This consistency in long term hydraulic form allows a simple transformation of fans to a common volume, and hence common surface area, to enable records of flow occurrence on fans of different size to be compiled in to a description of long term flow behaviour. This long term flow behaviour is described quantitatively. It shows a division between behaviour associated with channelised flow and the surface flows which spread out from these channels. An empirical long tern flow model incorporating infiltration appears to provide an accurate description of long term flow behaviour but short term variation precluded statistical validation. Despite this imprecision it is inferred that the long term flow behaviour is not responsible for the constant form. Short term flow behaviour is summarised in a conceptual model which proposes that the two components of long term flow are linked through base level control of the channelised flows at the intersection points between these and surface flows. Short term behaviour arises from dynamic exchanges of channel dominance through threshold controlled adjustments to intersection point base levels and non-instantaneous transmission of these base level changes up delivery channels. It is further complicated by chance interactions between fan surface topography and shifting channels causing extensive reorganisations of the channel system. It is recommended that more quantitative description of short term flow on rapidly forming fans should be limited to those subject to variable flow and in which deposition near the intersection point is less predominant.