Geomorphic response to gravel augmentation and high-flow dam release in the Trinity River, California

The geomorphic effect of introducing a gravel augmentation totaling 520m(3) into a gravel-bed stream during a dam-controlled flood in May of 2015 was monitored with bedload transport measurements, an array of seismometers, and repeated topographic surveys. Half of the augmented gravel was injected into the flow with front-end loaders on the rising limb of the flood and the other half was injected on the first day of the peak. Virtually all of the gravel transported past the injection point was deposited within about 7 to 10 channel widths of the injection point. Most of the injected gravel deposited along the left bank of the river whereas the right half of the channel bed was dominated by scour. The downstream third of the depositional area consisted of a small dune field that developed prior to the second gravel injection and subsequently migrated about one channel width downstream. A second depositional front was observed upstream from the gravel injection point, where a delta-like wedge of bed material developed in the first hours of the flow release and changed little over the remainder of the release. These two depositional areas represent small-scale bed-material storage reservoirs with the potential to accumulate and periodically release packets of bed material. Interactions with such storage reservoirs are hypothesized to cause large bed-material pulses to disperse by fragmenting into multiple smaller pulses. As a refinement to the conceptual model that views sediment pulse evolution in terms of dispersion and translation, the concept of pulse fragmentation has practical implications for gravel management. It implies that gravel augmentations can produce morphologic changes at locations that are separated from the augmentation point by arbitrarily long reaches, and it highlights the dependence of pulse propagation rates on the nature and distribution of the bed-material storage reservoirs in the channel system. Copyright (c) 2017 John Wiley & Sons, Ltd.