The Influence of Aggradation Rate on Braided Alluvial Architecture: Field Study and Physical Scale-Modelling of the Ashburton River Gravels, Canterbury Plains, New Zealand

Theoretical process-based models of braided alluvial architecture suggest that aggradation rate is a primary control on the geometry, stacking and heterogeneity of sedimentary deposits. This hypothesis is tested at the scale of the channel and bar using a combined field and flume modelling study, which quantifies the impact of a change in aggradation rate on the frequency of occurrence and geometry of the key depositional units that dominate coarse-grained, braided alluvial architecture. Aggradation of a 1 : 50 scale model of the braided Ashburton River, New Zealand, produces realistic alluvial architecture that closely corresponds to 7 km of logged field prototype outcrop. A twofold change in aggradation rate in the flume model and an order-of-magnitude change in the field outcrop, have no influence on the geometry and vertical distribution of fine- and coarse-grained depositional niches. Braided alluvial architecture at the channel scale therefore is determined by the local 'instantaneous' aggradation rate, related to individual flood events, rather than the long-term, regional aggradation rate.