Enhancement of habitat heterogeneity by placement of boulders in streams

Boulder placement provides an effective way of enhancing hydraulic diversity in stream restoration projects. The effects of boulder number and arrangement on the extent of their influence and the variance of the velocity distribution induced have been investigated using laboratory experiments and 2D simulations. The 2D model was tested against laboratory observations and then applied to different conditions. The dimensions of the zone of influence within which the velocity deviates by a specified amount from the undisturbed value and the velocity variance within this zone are related to flow conditions and boulder and channel characteristics for single and multiple boulder arrangements. The zone of influence was found to be increased considerably by the placement of multiple boulders in line transverse to the flow direction, especially if placed close enough to induce local critical flow; nonlinear arrangements were less effective. Angular boulders are shown to be more effective than rounded ones.