John K. Wooster

Experimental evidence for the effect of hydrographs on sediment pulse dynamics in gravel‐bedded rivers

[1] Gravel augmentation is a river restoration technique applied to channels downstream of dams where size-selective transport and lack of gravel resupply have created armored, relatively immobile channel beds. Augmentation sediment pulses rely on flow releases to move the material downstream and create conditions conducive to salmon spawning and rearing. Yet how sediment pulses respond to flow releases is often unknown. Here we explore how three types of dam releases (constant flow, small hydrograph, and large hydrograph) impact sediment transport and pulse behavior (translation and dispersion) in a channel with forced bar-pool morphology. We use the term sediment ‘‘pulse’’ generically to refer to the sediment introduced to the channel, the zone of pronounced bed material transport that it causes, and the sediment wave that may form in the channel from the additional sediment supply, which can include input sediment and bed material. In our experiments, we held the volume of water released constant, which is equivalent to holding the cost of purchasing a water volume constant in a stream restoration project. The sediment pulses had the same grain size as the bed material in the channel. We found that a constant flow 60% greater than the discharge required to initiate sediment motion caused a mixture of translation and dispersion of the sediment pulse. A broad crested hydrograph with a peak flow 2.5 times the discharge required for entrainment caused pulse dispersion, while a more peaked hydrograph >3 times the entrainment threshold discharge caused pulse dispersion with some translation. The hydrographs produced a well-defined clockwise hysteresis effecting sediment transport, as is often observed for fine-sediment transport and transportlimited gravel bed rivers. The results imply a rational basis for design of water releases associated with gravel augmentation that is directly linked to the desired sediment behavior.

Managing reservoir sediment release in dam removal projects: An approach informed by physical and numerical modelling of non‐cohesive sediment

Abstract Sediment management is frequently the most challenging concern in dam removal but there is as yet little guidance available to resource managers. For those rivers with beds composed primarily of non‐cohesive sediments, we document recent numerical and physical modelling of two processes critical to evaluating the effects of dam removal: the morphologic response to a sediment pulse, and the infiltration of fine sediment into coarser bed material. We demonstrate that (1) one‐dimensional numerical modelling of sediment pulses can simulate reach‐averaged transport and deposition over tens of kilometres, with sufficient certainty for managers to make informed decisions; (2) physical modelling of a coarse sediment pulse moving through an armoured pool‐bar complex shows deposition in pool tails and along bar margins while maintaining channel complexity and pool depth similar to pre‐pulse conditions; (3) physical modelling and theoretical analysis show that fine sediment will infiltrate into an immobile coarse channel bed to only a few median bed material particle diameters. We develop a generic approach to sediment management during dam removal using our experimental understanding to guide baseline data requirements, likely environmental constraints, and alternative removal strategies. In uncontaminated, non‐cohesive reservoir sediments we conclude that the management impacts of rapid sediment release may be of limited magnitude in many situations, and so the choice of dam removal strategy merits site‐specific evaluation of the environmental impacts associated with a full range of alternatives.

Optimal reproduction in salmon spawning substrates linked to grain size and fish length

Millions of dollars are spent annually on revitalizing salmon spawning in riverbeds where redd building by female salmon is inhibited by sediment that is too big for fish to move. Yet the conditions necessary for productive spawning remain unclear. There is no gauge for quantifying how grain size influences the reproductive potential of coarse-bedded rivers. Hence, managers lack a quantitative basis for optimizing spawning habitat restoration for reproductive value. To overcome this limitation, we studied spawning by Chinook, sockeye, and pink salmon (Oncorhynchus tshawytscha, O. nerka, and O. gorbuscha) in creeks and rivers of California and the Pacific Northwest. Our analysis shows that coarse substrates have been substantially undervalued as spawning habitat in previous work. We present a field-calibrated approach for estimating the number of redds and eggs a substrate can accommodate from measurements of grain size and fish length. Bigger fish can move larger sediment and thus use more riverbed area for spawning. They also tend to have higher fecundity, and so can deposit more eggs per redd. However, because redd area increases with fish length, the number of eggs a substrate can accommodate is maximized for moderate-sized fish. This previously unrecognized tradeoff raises the possibility that differences in grain size help regulate river-to-river differences in salmon size. Thus, population diversity and species resilience may be linked to lithologic, geomorphic, and climatic factors that determine grain size in rivers. Our approach provides a tool for managing grain-size distributions in support of optimal reproductive potential and species resilience.

Lessons Learned from Sediment Transport Model Predictions and Long-Term Postremoval Monitoring: Marmot Dam Removal Project on the Sandy River in Oregon

AbstractThe 14-m-tall Marmot Dam was removed during the summer of 2007, and the cofferdam protecting the working area was breached during a storm on October 19, 2007, allowing approximately 750,000  m3 of reservoir deposit to be eroded freely and released downstream to the Sandy River. Prior to the Marmot Dam removal, sediment transport models were developed to predict the transport dynamics of both gravel and sand, providing key pieces of information for stakeholders and regulatory agencies to select the most appropriate dam removal alternative. A monitoring program was implemented following dam removal that was designed to examine model predictions and assess when potential fish passage issues related to dam removal were no longer of concern. Comparisons of model predictions with field observations indicate that the model successfully predicted the erosion of the impoundment deposit, the deposition of sediment in a short reach downstream of the dam, and the lack of deposition in the majority of the Sand...

Comparing 1-D sediment transport modeling with field observations: Simkins Dam removal case study

ABSTRACT We present a sediment transport modelling study for the 2010 removal of the 3.3-m tall Simkins Dam on the Patapsco River, MD that released more than 56,000 m3 of sediment downstream. Our objectives are to validate the pre-removal model forecasts with detailed post-removal monitoring data, and through hindcast modelling, examine the effects of using approximate channel geometry data or more accurate data on model results. Comparisons of DREAM-1 model predictions using approximate data and field observations indicate that reach-scale model predictions were generally accurate, but some discrepancies between predicted and observed magnitudes of sediment deposition at specific locations occurred. A refined model, developed post-dam removal with more accurate channel geometry as model input, produced slightly improved results in reaches where input data were significantly improved. However, more accurate input data did not change the general conclusions nor substantially improve the model performance for the entire study reach. In conjunction with two previous studies, our results support a simplified data collection approach that enables timely predictions for decision making and minimizes study costs.