MODELING OF BOULDER KINEMATICS IN A FLUVIAL SYSTEM: TWENTY MILE CREEK, ONTARIO, CANADA

Theories of fluvial dynamics in river systems have usually focused upon small particles, sand, alluvium or small gravel. Relatively little investigation of the dynamics of larger particles has been undertaken. This paper applies theoretical work on boulder kinematics to three boulders with b-axes on the order of one metre, which were monitored over the 2000/2001 flood season. A ten-centimetre mesh in three spatial axes was used to measure boulder dimensions and estimate dependent properties. Channel position and streambed properties of slope, bed roughness, and local morphology were recorded for each boulder. Velocity rating curves were available, or were estimated for each site. Throughout the winter the ice thickness encasing the boulders was measured at regular intervals from early December until the spring flood in late February, which took place in ice-free conditions within the channel. One of the three boulders moved, and several others, which moved nearby, were assessed in similar fashion post-hoc. A later, smaller flood in April failed to move any boulders. Theoretical analysis and the field results suggest that both sliding and overturning is possible and occurs in flows which were about 1.5 metres deep with ambient mean velocities on the order of 3.5 to 4 m/s.