Flume Studies of the Effect of Perpendicular Log Obstructions on Flow Patterns and Bed Topography

It is known that woody debris in stream channels modifies morphology in many ways, ranging from scour of the bed to lateral migration of the channel over time. Since the occurrence of woody debris in streams results in a complex set of inter-related processes, it is useful to use controlled conditio ns in laborato ry experim ents to aid in the understan ding of thes e processe s. A flume study was undertaken to determine the spatial dimensions of influence on flow patterns and on bed topography of woody debris oriented normal to flow. Fluid depth was varied over three se ts of stage conditions (stage condition 1 = low stage; condition 2 = medium stage; condition 3 = high stage) to change the obstruction ratio (the diameter of the debris to the depth of unobstructed flow), and speed of the fluid was va ried to maintain a constant Frou de number. Bed profiling and flow velocity sampling w ere used to determine the effects of the obstruction ratio. Results show that as the percentage of obstruction increases, there is an increase in scour pool area, and a corresponding morphological effect on the shape of the influence zone (or zone of reverse circulation) immediately behind the obstructio n, which is attributable to fluid passing un der the obs truction. Ge ometric relationships of the length of this influence zone to obstruction diameter are presented and discussed in terms of aquatic habitat in natural channels.