Spatial organisation and physical characteristics of large peat blocks in an upland fluvial peatland ecosystem

Abstract

This paper assesses the size, shape and spatial organisation of organic, carbon-rich debris (peat blocks) in an upland fluvial peatland ecosystem. Peat block inventories collected in 2002 and 2012 at an alluvial reach of Trout Beck (North Pennines; United Kingdom) provide independent surveys for investigating the physical characteristics and spatial organisation of the organic debris. Peat blocks deposited along the 450 m reach represent a substantial volume of fluvially derived in-channel sediment and carbon flux at the macroscale (total peat volume 11 m3 (2002) and 17 m3 (2012)). Results show that inferred peat block transport distances depend on their size and shape. Smaller and more spherical equant shaped peat blocks are transported 1.62 and 1.72 times the distance of prolate and elongate shaped peat blocks. Downstream fining relationships provide a first-order approximation of peat block degradation rates. These degradation rates are high (up to 2 mm/m for the a-axis) and indicate considerable fine sediment release during transport. Hypsometric relations show that 73% of peat blocks are distributed within 1 channel width of the thalweg, indicating lateral organisation and a pattern of preferential deposition at the active channel margin. The local effects of obstructions from topography, roughness and slope promote peat block deposition, but given the low density of the blocks and close proximity to the flow the potential for re-entrainment is high.