The hydraulic geometry of narrow and deep channels; evidence for flow optimisation and controlled peatland growth

Abstract

At-a-station and bankfull hydraulic geometry analyses of peatland channels at Barrington Tops, New South Wales, Australia, reveal adjustments in self-forming channels in the absence of sediment load. Using Rhodes ternary diagram, comparisons are made with hydraulic geometry data from self-forming channels carrying bedload in alluvial settings elsewhere. Despite constraints on channel depths caused at some locations by the restricted thickness of peat, most stations have cohesive, near-vertical, well-vegetated banks, and width/depth (w/d) ratios of ∼2 that are optimal for sediment-free flow. Because banks are strong, resist erosion and can stand nearly vertical, and depth is sometimes constrained, adjustments to discharge are accommodated largely by changes in velocity. These findings are consistent with the model of maximum flow efficiency and the overarching least action principle in open channels. The bankfull depth of freely adjusting laterally active channels in clastic alluvium is well known to be related to the thickness of floodplain alluvium and a similar condition appears to apply to these swamps that grow in situ and are formed almost entirely of organic matter. The thickness of peat in these swamps rarely exceeds that required to form a bankfull channel of optimum w/d ratio for the transport of sediment-free water. Swamp vegetation is highly dependent on proximity to the water table. To maintain a swamp-channel and associated floodplain system, the channels must flow with sufficient water much of the time; they not only offer an efficient morphology for flow but do so in a way that enables bankfull conditions to occur many times a year. They also prevent the swamp from growing above a level linked to the depth of the channel. Once the channel attains the most efficient cross section, further growth of the swamp vertically is restricted by enhanced flow velocities and limited flow depths. This means that the volume of peat in such swamps is determined by the hydraulic efficiency of their channels. The development and maintenance of the hydraulic geometry of these swamp channels is biogeomorphic and biohydraulic in nature and yet accords to the same optimising principles that govern the formation of self-adjusting channels and floodplains in clastic alluvium.