Logjam

Abstract: This chapter provides an overview of wood in rivers, focusing on wood stability in rivers and design considerations for the reintroduction of wood to larger alluvial channels.Wood debris is a common component of the particulate matter in streams and rivers and has been recognized throughout most forested portions of the globe as an important factor influencing stream geomorphology and ecology. The stability and preservation of wood in large channels is primarily a function of its embedment in the streambed. The ecological benefits of wood are evident at several scales ranging from the wood surface to the complex interstitial space of wood accumulations (logjams), to the role of wood on altering bed textures and bed forms, to the influence of wood on channel planform, particularly creating multichannel systems. A logjam can increase available surface area for invertebrates and cover for fish by more than four orders of magnitude. A logjam can split flow and increase edge habitat severalfold. Logjams create pools and bars and raise water elevations to increase floodplain connectivity and have been placed in rivers with basal shear stress values of 166 Pa. Regardless of whether wood is included in a restoration design, as long as riparian trees grow along a stream, wood will end up in the channel; hence, it is also important to understand how naturally recruited wood behaves in rivers. Reintroducing wood to rivers brings up many other issues, from flood conveyance to public safety, all of which should be considered in the design process.

Abstract: Large wood in rivers and logjams are linked to the presence of varied riverine morphologies and increased abundance and diversity of aquatic biota. Current research into the ecohydrological, morphological and geochemical effects of logjams is restricted by difficulties in comparing findings between river systems. The problem is exacerbated by a lack of standardised metrics for recording and reporting logjams and a scale dependence of logjam effects with river size. In this paper a new method for analysing logjams is presented based on a set of seven dimensionless metrics of structure and morphological effects. These metrics are used along with a cluster analysis to identify key logjam types within a study river. The analysis framework is applied to data from a small forest river in the UK and identifies that 73% of logjams in this system can be grouped into eight distinct classes. Of these classes, two are logjams which only partly fill the channel and six are channel spanning. The individual classes are differentiated from each other principally by the degree of lateral and vertical erosion found in association with the logjams. The dimensionless metrics are also applied qualitatively to a range of logjams described in the literature and show the potential for the method to be applied as a standard survey and analysis framework for logjams across diverse river environments. The potential to link logjam form with function, and therefore provision of specific habitats, has wide applicability to the design and monitoring of river restoration schemes.