Yantao Cui

Numerical simulation of aggradation and downstream fining

divers typically exhibit a tendency for grain size to become finer in the downstream direction. Data for a set of large-scale experiments on the aggradation of heterogeneous gravel have recently become available. These experiments show substantial downstream fining over several tens of meters. Here a decoupled numerical Tiodel for bed aggradation and downstream fining is developed in an attempt to test an existing gravel ransport model against the experimental data. Generally good agreement is found between the predictions and he observations in the absence of all but trivial adjustments to the gravel transport model. The same transport elation does not perform as well for a corresponding case of uniform sediment. In all of the experiments the Froude number was close to unity, a condition which would suggest that a decoupled model might break iown. Coupled and decoupled models for uniform sediment are thus compared for a case with Froude number very close to unity. They are also compared for cases in whic...

Dam Removal Express Assessment Models (DREAM). Part 1: Model development and validation

Many dams have been removed in the recent decades in the U.S. for reasons including economics, safety, and ecological rehabilitation. More dams are under consideration for removal; some of them are medium to large-sized dams filled with millions of cubic meters of sediment. Reaching a decision to remove a dam and deciding as how the dam should be removed, however, are usually not easy, especially for medium to large-sized dams. One of the major reasons for the difficulty in decision-making is the lack of understanding of the consequences of the release of reservoir sediment downstream, or alternatively the large expense if the sediment is to be removed by dredging. This paper summarizes the Dam Removal Express Assessment Models (DREAM) developed at Stillwater Sciences, Berkeley, California for simulation of sediment transport following dam removal. There are two models in the package: DREAM-1 simulates sediment transport following the removal of a dam behind which the reservoir deposit is composed primarily of non-cohesive sand and silt, and DREAM-2 simulates sediment transport following the removal of a dam behind which the upper layer of the reservoir deposit is composed primarily of gravel. Both models are one-dimensional and simulate cross-sectionally and reach averaged sediment aggradation and degradation following dam removal. DREAM-1 is validated with a set of laboratory experiments; its reservoir erosion module is applied to the Lake Mills drawdown experiment. DREAM-2 is validated with the field data for a natural landslide. Sensitivity tests are conducted with a series of sample runs in the companion paper, Cui et al. (2006), to validate some of the assumptions in the model and to provide guidance in field data collection in actual dam removal projects.

Dam Removal Express Assessment Models (DREAM). Part 2: Sample runs/sensitivity tests

This paper presents sample runs of the Dam Removal Express Assessment Models (DREAM) presented in the companion paper, Cui et al. (2006): DREAM-1 for simulation of sediment transport following the removal of a dam behind which the reservoir deposit is composed primarily of noncohesive sand and silt, and DREAM-2 for simulation of sediment transport following the removal of a dam behind which the upper layer of the reservoir deposit is composed primarily of gravel. The primary purposes of the sample runs presented here are to validate some of the assumptions used in the model and to provide guidance as how accurately the field data should be collected. Sample runs indicate that grain size distribution of the reservoir sediment deposit is the most important piece of information needed during the field campaign. Other than the grain size distribution of the reservoir sediment deposit, errors within a reasonable range in other parameters do not result in significant variations in the predicted depositional patterns downstream of the dam, although different magnitudes of sediment deposition may result from such errors. Sample runs also indicate that when the reservoir deposit is composed primarily of gravel, sediment deposition downstream of the dam following dam removal may not propagate far downstream of the dam, and may be limited to isolated reaches where sediment transport capacity is low. Farther downstream sediment deposition becomes progressively smaller due to the attenuation of sediment transport and gravel abrasion. When the reservoir deposit is primarily fine sediment, however, there may be more extensive sediment deposition (both larger area and higher magnitude) downstream of the dam following dam removal. Dredging part of the sediment in advance reduces the downstream impact due to the reduced volume, and the extra distance provided by dredging allows for attenuation of sediment transport. Sample runs with staged dam removal indicate that it provides only limited benefit compared to a one-time removal in case the reservoir deposit is composed primarily of coarse sediment, but may provide significant benefits in case the reservoir deposit is composed primarily of fine sediment. The benefits of a staged removal for the latter case include reduced magnitude and area of deposition as well as reduced suspended sediment concentration downstream of the dam.

The arrested gravel front: stable gravel-sand transitions in rivers Part 2: General numerical solution

Many rivers have an abrupt transition from gravel-bed to sand-bed morphology. In many cases the point of transition is neither prograding nor retrograding, but is rather arrested in place. Two mechanisms are hypothesized as responsible for stabilizing the gravel-sand transition, basin subsidence (or alternatively base level rise) and abrasion of gravel. The companion paper offers a simplified analytical solution for the long profile of a river with such a transition. This treatment allows for direct insight into the relation between the morphology and the controlling mechanisms at the expense of several gross approximations. The present paper offers a rigorous complete numerical solution which takes such facts as the streamwise sorting of heterogeneous gravel into consideration.

The Unified Gravel-Sand (TUGS) Model: Simulating Sediment Transport and Gravel/Sand Grain Size Distributions in Gravel-Bedded Rivers

Received 9 July 2006; revised 7 August 2007; accepted 14 August 2007; published 30 October 2007. [1] This paper presents The Unified Gravel-Sand (TUGS) model that simulates the transport, erosion, and deposition of both gravel and sand. TUGS model employs the surface-based bed load equation of Wilcock and Crowe (2003) and links grain size distributions in the bed load, surface layer, and subsurface with the gravel transfer function of Hoey and Ferguson (1994) and Toro-Escobar et al. (1996), a hypothetical sand transfer function, and hypothetical functions for sand entrainment/infiltration from/into the subsurface. The model is capable of exploring the dynamics of grain size distributions, including the fractions of sand in sediment deposits and on the channel bed surface, and is potentially useful in exploring gravel-sand transitions and reservoir sedimentation processes. Simulation of three sets of large-scale flume experiments indicates that the model, with minor adjustment to the Wilcock-Crowe equation, excellently reproduced bed profile and grain size distributions of the sediment deposits, including the fractions of sand within the deposits. Simulation of a flushing flow experiment indicated that the sand entrainment function is potentially capable of simulating the short-term processes such as flushing flow events.

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.

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...

Gravel-bed river evolution in earthquake-prone regions subject to cycled hydrographs and repeated sediment pulses

Sediment often enters rivers in the form of sediment pulses associated with landslides and debris flows. This is particularly so in gravel-bed rivers in earthquake-prone mountain regions, such as Southwest China. Under such circumstances, sediment pulses can rapidly change river topography and leave the river in repeated states of gradual recovery. In this paper, we implement a one-dimensional morphodynamic model of river response to pulsed sediment supply. The model is validated using data from flume experiments, so demonstrating that it can successfully reproduce the overall morphodynamics of experimental pulses. The model is then used to explore the evolution of a gravel-bed river subject to cycled hydrographs and repeated sediment pulses. These pulses are fed into the channel in a fixed region centered at a point halfway down the calculational domain. The pulsed sediment supply is in addition to a constant sediment supply at the upstream end. Results indicate that the river can reach a mobile-bed equilibrium in which two regions exist within which bed elevation and surface grain size distribution vary periodically in time. One of these is at the upstream end, where a periodic discharge hydrograph and constant sediment supply are imposed, and the other is in a region about halfway down the channel where periodic sediment pulses are introduced. Outside these two regions, bed elevation and surface grain size distribution reach a mobile-bed equilibrium that is invariant in time. The zone of fluctuation-free mobile-bed equilibrium upstream of the pulse region is not affected by repeated sediment pulses under the scenarios tested, but downstream of the pulse region, the channel reaches different fluctuation-free mobile-bed equilibriums under different sediment pulse scenarios. The vertical bed structure predicted by the simulations indicates that the cyclic variation associated with the hydrograph and sediment pulses can affect the substrate stratigraphy to some depth. Copyright

Analyses of the erosion of fine sediment deposit for a large dam-removal project: an empirical approach

ABSTRACT Large quantities of fine sediment can be accumulated in reservoirs, and the potential impact of their downstream release is often a great concern if the dams are to be removed. Currently, there are no reliable numerical models to simulate the dynamics of the release of these fine sediments, mostly because their release following dam removal is often driven by a rapid erosional process not addressed by traditional sediment transport theory. However, precise quantification of fine sediment transport is rarely necessary to evaluate potential environmental impacts of alternative scenarios. Using the removal of Matilija Dam in southern California, USA, as an example, we quantify the likely magnitude of suspended sediment concentration and the duration of associated downstream impacts, two necessary (and most likely adequate) parameters for assessing alternatives. The analyses first estimate the general magnitude of suspended sediment concentration and duration of impacts based on field and experimental data; they then quantify the duration of impacts under both worst-case and reasonable assumptions according to the underlying physics and common sense. For rapid sediment release with fine-grained impoundment deposits, initial suspended sediment concentrations are likely to approach 106 mg/L, persisting for a few hours to no more than a couple of days. Suspended sediment concentrations are expected to decline approximately exponentially after the initial peak, reaching background levels within a few hours to a few days, provided that sufficient flow is available. The general method presented in the paper should be useful for stakeholders choosing amongst dam-removal alternatives for implementation under similar conditions.

Preliminary Assessment of Sediment Transport Dynamics Following Dam Removal: A Case Study

Prediction of sediment transport dynamics following dam removal usually requires extensive field data, the collection of which requires both resources and time. Certain management decisions regarding dam removal, however, may be required before resources may be allocated to provide sufficient data to evaluate whether dam removal is a viable option. Here, we present a case study and demonstrate that some aspects of the sediment transport characteristics following dam removal can be evaluated with very limited information. The case study in question involves J.C. Boyle, Copco, and Iron Gate Dams on the Klamath River, California which together have an estimated 12 million m of sediment deposited in their reservoirs. With a reconnaissance field observation and upon examination of the very limited existing field data, we determined that it is possible to evaluate the potential for sediment deposition downstream of the dam following the removal of the dams under the worst-case-scenario assumptions. The evaluation was carried out with the help of DREAM-1, one of the Dam Removal Express Assessment Models developed at Stillwater Sciences. Results of the assessment indicate that potential sediment deposition would occur only for a brief period within approximately 10 km downstream of the dam with a maximum thickness of sediment deposition no more than 1.2 m.