Effects of hydraulic roughness and sediment supply on surface textures of gravel-bedded rivers

Buffington, John

Textural fining due to sediment supply and hydraulic roughness elements signficantly increases spawning habitat availability. Textural response to sediment supply and other hydraulic roughness elements in gravel-bedded rivers is examined by a comparing reach-average median surface grain sizes (D50S) with those predicted from a simple threshold channel model. An extensive literature review of bedload transport studies indicates that sediment transport in gravel-bedded channels is to a first approximation bankfull threshold. Using Shields' equation D50S is predicted from bankfull depth and slope, providing a theoretical reference point for examining textural response. The model, however, requires specification of a dimensionless critical shear stress parameter, which is made uncertain by the current state of the incipient motion literature. A detailed review of this literature demonstrates distinct methodological biases in previously reported results (commonly assumed, but poorly documented) and it is concluded that of the critical shear stress parameter values currently available 0.032 is the most appropriate for natural channels. Previously documented textural response to sediment supply is examined within the bankfull-threshold framework, showing that increased sediment loading causes a systematic textural fining and deviation from model predictions. Textural response to bedform and large woody debris (LWD) roughness is investigated through a field study of gravel-bedded channels in forest environments of western Washington and southeast Alaska. Surface textures are examined in three distinctly different channel morphologies representing an increasing complexity of channel roughness: plane-bed, LWD-poor pool-riffle, and LWD-rich pool-riffle. Results show that textural fining in response to hydraulic roughness features occurs at both reach and local scales and that the number and spatial complexity of textural patches increases with greater complexity of channel roughness. Within the bankfull-threshold framework, reach-average median surface grain sizes segregate by channel type into distinct zones of textural response, with plane-bed channels showing the least deviation from model predictions and LWD-rich pool-riffle channels the most. Median values of textural response distributions reveal that roughness caused by bedforms and minimal LWD result in a two-and-half fold decrease in D50S relative to model predictions, while pool-riffle channels with abundant LWD exhibit a four-and-a-half-fold decrease. In the channels studied, non-catastrophic levels of sediment loading are apparently subordinate to the controls on surface texture caused by bedform and LWD roughness. Although there is the potential for severe sediment impacts (and hence sediment-induced textural fining) in the study areas, such occurences tend to be infrequent pulsed events compared to the pervasive influence of stable bedforms and LWD. Consequently, bedforms and LWD provide long term influences that dominate surface textures of the study sites, while sediment supply effects are either relatively less significant or transient. Coupled with subsurface grain size sampling, the bankfull-threshold model allows evaluation of the degree of textural fining within theoretical grain size limits and is proposed as a management tool for assessing channel textural-response condition. Naturally large degrees of textural fining in LWD-rich pool-riffle channels generally correspond with small capacities for further surface fining. The bankfull-threshold model and empirically determined response ranges also offer an opportunity to examine geomorphic controls on the availability of salmonid spawning gravels. Textural fining due to sediment supply and hydraulic roughness elements significantly increases spawning habitat availability.