Hydraulic guidelines for the re-introduction and management of large woody debris in lowland rivers

The volume of large woody debris in most of the world's lowland rivers has been depleted, either through persistent desnagging or clearance of the riparian vegetation from which it is naturally recruited. The now recognized important environmental role of debris in rivers, the established environmental value of vegetated riparian buffer strips and the movement towards rehabilitation of degraded riverine habitats demand more objective procedures for the management of woody debris in streams. In some instances it may be centuries before the process of natural recruitment of wood from rehabilitated riparian strips achieves an ecologically adequate volume and quality of instream debris. To accelerate this process, the re-introduction of debris is being considered. This paper presents the results of laboratory and field hydraulic investigations relevant to the problem of managing debris in lowland rivers. The laboratory experiments were used to develop a model, based on the momentum principle, of the effect of debris on afflux, or the increase in water surface elevation. Debris drag was found to be less affected by position and shape than by orientation to the dow and blockage ratio, or the proportion of the channel occupied by the debris. Debris aligned at 20-30 degrees to the flow produced an afflux one-third of that produced by debris which was perpendicular. Significant loss of conveyance occurs only for debris which is large relative to the channel dimensions (greater than about 10% of the channel area blocked by debris). Wake interference acts to reduce the hydraulic effect of debris so that if spaced within two diameters, multiple in-line items of debris produce an afflux no greater than that of a single item. The models of debris hydraulics presented here can be used to predict the effect of removing, lopping, rotating or re-introducing debris to rivers.