Transport and dispersal of organic debris (Peat blocks) in upland fluvial systems

This paper assesses the mechanisms and pathways by which peat blocks are eroded and transported in upland fluvial systems. Observations and experiments from the north Pennines (UK) have been carried out on two contrasting river systems. Mapping of peat block distributions and appraisal of reach-based sediment budgets clearly demonstrates that macro-size peat is an important stream load component. In small streams block sizes can approximate the channel width and much of the peat is transported overbank. Local 'peat jams' and associated mineral deposition may provide an important component of channel storage. In larger systems peat blocks rapidly move down-channel and undergo frequent exchanges between bed and bank storage. Results of peat block tracing using painted blocks indicate that once submerged, blocks of all sizes are easily transported and blocks break down rapidly by abrasion. Vegetation and bars play an important role in trapping mobile peat. In smaller streams large block transport is limited by channel jams. Smaller blocks are transported overbank but exhibit little evidence of downstream fining. In larger rivers peat blocks are more actively sorted and show downstream reduction in size from source. A simple model relating peat block diameter (D p) to average flow depth (d) suggests three limiting transport conditions: Flotation (D p <d), rolling (d <D p > d/2) and deposition (D p > d/2). Experiments demonstrate that peat block transport occurs largely by rolling and floating and the transport mechanism is probably controlled by relative flow depth (d/D p ratio). Transport velocity varies with transport mechanism (rolling is the slowest mode) and transport lengths increase as flow depth increases. Abrasion rates vary with the transport mechanism. Rolling produces greater abrasion rates and more rounded blocks. Abrasion rates vary from O to 10 g m -1 for blocks ranging in mass from 10 to 6000 g. Copyright © 2001 John Wiley and Sons, Ltd.