Effects of sediment pulses on bed relief in bar-pool channels

摘要

To further develop prediction of the range of morphological adjustments associated with sediment pulses in bar-pool channels, we analyze channel bed topographic data collected prior to and following the removal of two dams in Oregon: Marmot Dam on the Sandy River and Brownsville Dam on the Calapooia River. We hypothesize that, in gravel-bed, bar-pool channels, the response of bed relief to sand and gravel sediment pulses is a function of initial relief and pulse magnitude. Modest increases in sediment supply to initially low-relief, sediment-poor cross-sections will increase bed relief and variance of bed relief via bar deposition. Modest increases in sediment supply to initially high-relief cross-sections, characteristic of alternate bar morphology, will result in decreased bed relief and variance of relief via deposition in bar-adjacent pools. These hypothesized adjustments are measured in terms of bed relief, which we define as the difference in elevation between the pool-bottom and bar-top. We evaluate how relief varies with sediment thickness, where both relief and mean sediment thickness at a cross-section are normalized by the 90th percentile of observed relief values within a reach prior to a sediment pulse. Field measurements generally supported the stated hypotheses, demonstrating how introduction of a sediment pulse to low-relief reaches can increase mean and variance of relief, while introduction to high-relief reaches can decrease the mean and variance of bed relief, at least temporarily. In general, at both sites, the degree of impact increased with the thickness of sediment delivered to the cross-section. Results thus suggest that the analysis is a useful step for understanding the morphological effects of sediment pulses introduced to gravel-bed, bar-pool channels. Copyright (c) 2015 John Wiley & Sons, Ltd.