Downstream fining in a rapidly aggrading gravel bed river

Abstract

Abstract. Downstream changes in particle size that occur in the Waipaoa River, a 104-km-long gravel bed river in which rapid aggradation in the historic (post-1800) period was triggered by the conversion of native forest to pasture, are summarized in this paper. The textural data presented are unique for a field situation, not only because of the spatial resolution and extent of the sampling program but also because they provide information about the pattern of fining at different points in time. They are supported by equally comprehensive topographic survey data from which local rates of aggradation can be derived. Despite variability induced by lateral sediment inputs, there is an essentially continuous pattern of fining along the entire length of the river. Fining occurs in both the fine and coarse size fractions of the bed material. The highest rates of fining occur in the larger percentiles of the subsurface bed material and in the surface bed material. Downstream fining in the Waipaoa River appears to be a response to changes in flow hydraulics that are regulated by the concave configuration of the long profile. The fining gradient developed rapidly (in <45 years). It does not appear to be influenced by the rate of aggradation (nor the overall rate of sediment supply to the channel system), because, in the short term, aggradation has a negligible impact on the inherited form of the long profile.

The potential rate of bed-load transport

“…2 data from three other gravel-bed rivers and a flume experiment in which high bed-load transport rates also have been observed but where the transport rate either was reported to have been supply-limited or constrained by the characteristics of the bed surface. These data are averages of stream-wide measurements made in the Elbow River (Alberta, Canada), estimates made in the River Jordan (Israel), at-a-point measurements made in the Waipaoa River (New Zealand), and runs made in the 4-m flume at the Environmental Research Centre, University of Tsukuba (Tsukuba, Japan) (22)(23)(24)(25). The bed-load transport regimes in these channels do not represent conditions to which Eq.…”

Grain Size Analysis of Surface Fluvial Sediments in Rivers in Kelantan, Malaysia

“…Both fine and coarse size fractions affect the downstream fining process. Within the downstream direction, sorting progressively was improved and the larger percentiles may caused the higher rates of fining occurred (Gomez et al 2001). …”

On integrated sediment transport modelling for flash events in mountain environments

“…Second, the sediment transport rates for a given water discharge may be different if the sediments are uniform or not (e.g., Cohen et al 2010). Finally, non-uniform size of sediments leads to downstream fining (e.g., Paola et al 1992, Ferguson et al 1996, Seal et al 1997, Gomez et al 2001. It is however likely that such process should not be particularly relevant in the short river reach modelled here.…”