Article

Morphodynamics and Sediment Tracers in 1-D (MAST-ID): 1-D sediment transport that includes exchange with an off-channel sediment reservoir

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Abstract

Bed material transported in geomorphically active gravel bed rivers often has a local source at nearby eroding banks and ends up sequestered in bars not far downstream. However, most 1-D numerical models for gravel transport assume that gravel originates from and deposits on the channel bed. In this paper, we present a 1-D framework for simulating morphodynamic evolution of bed elevation and size distribution in a gravel-bed river that actively exchanges sediment with its floodplain, which is represented as an off-channel sediment reservoir. The model is based on the idea that sediment enters the channel at eroding banks whose elevation depends on total floodplain sediment storage and on the average elevation of the floodplain relative to the channel bed. Lateral erosion of these banks occurs at a specified rate that can represent either net channel migration or channel widening. Transfer of material out of the channel depends on a typical bar thickness and a specified lateral exchange rate due either to net channel migration or narrowing. The model is implemented using an object oriented framework that allows users to explore relationships between bank supply, bed structure, and lateral change rates. It is applied to a ∼50-km reach of the Ain River, France, that experienced significant reduction in sediment supply due to dam construction during the 20th century. Results are strongly sensitive to lateral exchange rates, showing that in this reach, the supply of sand and gravel at eroding banks and the sequestration of gravel in point bars can have strong influence on overall reach-scale sediment budgets.

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... Indeed, aspects of sediment pulse propagation have been investigated and explored by many researchers, including topics such as the evolution of sediment pulses following landslides, debris flows (Cui et al., 2003a(Cui et al., , 2003b, and dam removal (Cui et al., 2006a;Cui et al., 2006b;Gilet et al., 2021), but they have only rarely been considered in the context of gravel augmentation (Sklar et al., 2009). Over recent decades, several numerical models have been developed for simulating and predicting the propagation of coarse-sediment pulses along river channels, including the Dam Removal Express Assessment Model (DREAM) (Cui et al., 2006a;Cui et al., 2006b), the Unified Gravel-Sand Model (TUGS) (Cui, 2007), the Morphodynamics and Sediment Tracers in 1D model (MAST 1D) (Lauer et al., 2016), the Lagrangian framework proposed by Czuba (2018) and Ahammad et al. (2021), or the recent numerical model proposed by Viparelli et al. (2022). Many of these morphodynamic models focus on the aggradation/degradational response of the riverbed and provide an estimate of bed level changes through time. ...
... However, they are not designed to track the movement of individual particles, which in the case of gravel augmentation is of particular interest given the need for information on the timeframe of sediment evacuation before arriving at potentially sensitive areas. This concern applies, for instance, to the DREAM (Cui et al., 2006a;Cui et al., 2006b), TUGS (Cui, 2007) or MAST 1D (Lauer et al., 2016) models. Some recent developments (e.g., Czuba, 2018;Ahammad et al., 2021;Viparelli et al., 2022) are promising in their ability to track the downstream propagation of sediment but, in our opinion, still remain focused on estimating bed response. ...
... In this regard, most of these models were derived from purely mathematical/theoretical lines of reasoning or were developed based on observations collected from flume experiments. Although it is true that many of them have been validated with field data (e.g., the Navarro River in Cui et al., 2006aCui et al., , 2006b; the Ain River in Lauer et al., 2016; the Buëch River in Viparelli et al., 2022), in almost none of these models is information derived directly from field observations collected in rivers used in the calibration or in the definition of model parameters, which represents a major limitation. To the best of our knowledge, this fact is likely related to the lack of robust and strong functional relations linking water discharge to the distances or velocities of sediment propagation observed in the field (Hassan and Bradley, 2017;Klösch and Habersack, 2018;Papangelakis et al., 2022), which is indeed an elusive (although longstanding) topic in river research. ...
... Indeed, aspects of sediment pulse propagation have been investigated and explored by many researchers, including topics such as the evolution of sediment pulses following landslides, debris flows (Cui et al., 2003a(Cui et al., , 2003b, and dam removal (Cui et al., 2006a;Cui et al., 2006b;Gilet et al., 2021), but they have only rarely been considered in the context of gravel augmentation (Sklar et al., 2009). Over recent decades, several numerical models have been developed for simulating and predicting the propagation of coarse-sediment pulses along river channels, including the Dam Removal Express Assessment Model (DREAM) (Cui et al., 2006a;Cui et al., 2006b), the Unified Gravel-Sand Model (TUGS) (Cui, 2007), the Morphodynamics and Sediment Tracers in 1D model (MAST 1D) (Lauer et al., 2016), the Lagrangian framework proposed by Czuba (2018) and Ahammad et al. (2021), or the recent numerical model proposed by Viparelli et al. (2022). Many of these morphodynamic models focus on the aggradation/degradational response of the riverbed and provide an estimate of bed level changes through time. ...
... However, they are not designed to track the movement of individual particles, which in the case of gravel augmentation is of particular interest given the need for information on the timeframe of sediment evacuation before arriving at potentially sensitive areas. This concern applies, for instance, to the DREAM (Cui et al., 2006a;Cui et al., 2006b), TUGS (Cui, 2007) or MAST 1D (Lauer et al., 2016) models. Some recent developments (e.g., Czuba, 2018;Ahammad et al., 2021;Viparelli et al., 2022) are promising in their ability to track the downstream propagation of sediment but, in our opinion, still remain focused on estimating bed response. ...
... In this regard, most of these models were derived from purely mathematical/theoretical lines of reasoning or were developed based on observations collected from flume experiments. Although it is true that many of them have been validated with field data (e.g., the Navarro River in Cui et al., 2006aCui et al., , 2006b; the Ain River in Lauer et al., 2016; the Buëch River in Viparelli et al., 2022), in almost none of these models is information derived directly from field observations collected in rivers used in the calibration or in the definition of model parameters, which represents a major limitation. To the best of our knowledge, this fact is likely related to the lack of robust and strong functional relations linking water discharge to the distances or velocities of sediment propagation observed in the field (Hassan and Bradley, 2017;Klösch and Habersack, 2018;Papangelakis et al., 2022), which is indeed an elusive (although longstanding) topic in river research. ...
... The model accounts for production and decay of radioisotopes in the floodplain and is applied to a generic river system (Viparelli et al., 2013). In another study, Lauer et al. (2016) developed a 1-D framework for simulating morphodynamic evolution of bed elevation and size distribution in a river that actively exchanges sediment with its floodplain. The program can track changes in radioisotopic concentration in particles in any size class and is applied to a catchment in France (Lauer et al., 2016). ...
... In another study, Lauer et al. (2016) developed a 1-D framework for simulating morphodynamic evolution of bed elevation and size distribution in a river that actively exchanges sediment with its floodplain. The program can track changes in radioisotopic concentration in particles in any size class and is applied to a catchment in France (Lauer et al., 2016). Such research advances reflect the strength and potential of the use of radiometric fingerprinting in management. ...
Article
Reliable quantitative information on sediment sources to rivers is critical to mitigate contamination and target conservation and restoration actions. However, for large-scale river basins, determination of the relative importance of sediment sources is complicated by spatiotemporal variability in erosional processes and sediment sources, heterogeneity in sediment transport and deposition, and a paucity of sediment monitoring data. Sediment source fingerprinting is an increasingly adopted field-based technique that identifies the nature and relative source contribution of sediment transported in waterways. Notably, sediment source fingerprinting provides information that is independent of other field, modeling, or remotely sensed techniques. However, the diversity in sampling, analytical, and interpretive methods for sediment fingerprinting has been recognized as a problem in terms of developing standardized procedures for its application at the scale of large river basins. Accordingly, this review focuses on sediment source fingerprinting studies conducted within the Mississippi River Basin (MRB), summarizes unique information provided by sediment source fingerprinting that is distinct from traditional monitoring techniques, evaluates consistency and reliability of methodological approaches among MRB studies, and provides prospects for the use of sediment source fingerprinting as an aid to large-scale landscape conservation and restoration under current management frameworks. Most MRB studies reported credible fingerprinting results and found near-channel sources to be the dominant sediment sources in most cases, and yet a lack of standardization in procedural steps makes results difficult to compare. Findings from MRB studies demonstrated that sediment source fingerprinting is a highly valuable and reliable sediment source assessment approach to assist land and water resource management under current management frameworks, but efforts are needed to make this technique applicable in large-scale landscape conservation and restoration efforts. We summarize research needs and discuss sediment fingerprinting use for basin-scale management efforts with the aim of encouraging that this technique is robust and reliable as it moves forward.
... Indeed, the bankfull discharge results from the balance between floodplain erosion and deposition (Naito & Parker, 2019. Building on previous work on the equilibrium of engineered rivers, that is, rivers with fixed banks and sinuosity (Arkesteijn et al., 2019;Blom et al., 2016Blom et al., , 2017, as well as formulations for floodplain morphodynamics Lauer et al., 2016;Viparelli et al., 2013) and bank migration (Davidson & Eaton, 2018;De Rego et al., 2020;Eke et al., 2014;Parker et al., 2011), we derive equilibrium solutions for channel geometry (width, depth, slope), floodplain sediment size distribution, bankfull discharge, channel migration and overbank deposition rates. The formulation has been published elsewhere: we provide it in the Supporting Information S1. ...
... where Q w,f represents the water discharge on the floodplain, C k is the average suspended sediment concentration in the channel above the floodplain, and Fl k is the floodplain number or trap efficiency factor (Lauer et al., 2016), here assumed to be the same for the sand and the mud fraction. Mud concentration is assumed to be uniform so that C Q Q m m w c  / , with Q w,c being the water discharge in the channel. ...
Article
Full-text available
Equilibrium geometry of single‐thread rivers with fixed width (engineered rivers) is determined with a flow resistance relation and a sediment transport relation, if characteristic discharge, sediment caliber and supply are specified. In self‐formed channels, however, channel width is not imposed, and one more relation is needed to predict equilibrium geometry. Specifying this relation remains an open problem. Here we present a new model that brings together a coherent train of research progress over 35 years to predict equilibrium geometry of single‐thread rivers from the conservation of channel and floodplain material. Predicted channel geometries are comparable with field observations. In response to increasing floodplain width, sand load and grain size, the equilibrium slope increases, bankfull depth and width decrease. As the volume fraction content of mud in the sediment load increases, bankfull width‐to‐depth ratio and slope decrease suggesting that mud load has a strong control on channel patterns and bankfull geometry.
... Net sediment contributions (+7 Mg yr À1 ) from streambank erosion (+43 Mg yr À1 , from migration and widening) and floodplain deposition (À36 Mg yr À1 ) are also a small component (3% of sediment) to the Le Sueur fine-sediment budget [Gran et al., 2011;. Representation of these exchange dynamics requires further developments of the model [e.g., Lauer and Willenbring, 2010;Viparelli et al., 2013;Lauer et al., 2016] and is beyond the scope of the present study. ...
... Thus, a logical next step is to implement a mechanism for sediment storage and release from floodplains. Developing a probabilistic approach to floodplain exchange is fairly straightforward given the channel migration rate and the sediment load for a given reach [e.g., Malmon et al., 2003;Lauer and Willenbring, 2010;Viparelli et al., 2013;Lauer et al., 2016]. The residence time of sediment in the floodplain is a key constraint for simulating channel-floodplain exchange. ...
Article
Understanding how sediment moves along source to sink pathways through watersheds – from hillslopes to channels and in and out of floodplains – is a fundamental problem in geomorphology. We contribute to advancing this understanding by modeling the transport and in-channel storage dynamics of bed-material sediment on a river network over a 600-year time period. Specifically, we present spatiotemporal changes in bed-sediment thickness along an entire river network to elucidate how river networks organize and process sediment supply. We apply our model to sand transport in the agricultural Greater Blue Earth River Basin in Minnesota. By casting the arrival of sediment to links of the network as a Poisson process, we derive analytically (under supply-limited conditions) the time-averaged probability distribution function of bed-sediment thickness for each link of the river network for any spatial distribution of inputs. Under transport-limited conditions, the analytical assumptions of the Poisson arrival process are violated (due to in-channel storage dynamics) where we find large fluctuations and periodicity in the timeseries of bed-sediment thickness. The timeseries of bed-sediment thickness is the result of dynamics on a network in propagating, altering, and amalgamating sediment inputs in sometimes unexpected ways. One key insight gleaned from the model is that there can be a small fraction of reaches with relatively low transport capacity within a non-equilibrium river network acting as “bottlenecks” that control sediment to downstream reaches, whereby fluctuations in bed elevation can dissociate from signals in sediment supply.
... The most widely modelling approach used is two-dimensional horizontal (2DH) [3,4,7,8,22] and in two cases one-dimensional [10,20] . Three papers [4,9,20] perform simulations considering sediment mixtures. ...
... Duró et al. [3] analyse the 2DH morphodynamic response of river channels to widening and narrowing. Lauer et al. [9] show applications of a simple 1D model quantifying river http://dx.doi.org/10.1016/j.advwatres.2016.01.005 0309-1708/© 2016 Elsevier Ltd. All rights reserved. ...
Article
Numericalmorphodynamicmodelsprovidescientificframe-worksforadvancingourunderstandingofriversystems.There-searchoninvolvedtopicsisanimportantandsociallyrelevantun-dertakingregardingourenvironment.Nowadaysnumericalmodelsareusedfordifferentpurposes,fromansweringquestionsaboutbasicmorphodynamicresearchtomanagingcomplexriveren-gineeringproblems.Duetoincreasingcomputerpowerandthedevelopmentofadvancednumericaltechniques,morphodynamicmodelsarenowmoreandmoreusedtopredictthebedpatternsevolutiontoabroadspectrumofspatialandtemporalscales.Thedevelopmentandthesuccessofapplicationofsuchmodelsarebaseduponawiderangeofdisciplinesfromappliedmathemat-icsforthenumericalsolutionoftheequationstogeomorphol-ogyforthephysicalinterpretationoftheresults.Inthislightweorganizedthisspecialissue(SI)solicitingmultidisciplinarycon-tributionswhichencompassanyaspectneededforthedevelop-mentandapplicationsofsuchmodels.MostofthepapersintheSIstemfromcontributionstosessionHS9.5/GM7.11onnumericalmodellingandexperimentsinrivermorphodynamicsattheEuro-peanGeosciencesUnion(EGU)GeneralAssemblyheldinVienna,April27thtoMay2nd2014.
... Numerical and mathematical models have been applied to simulate river-floodplain hydraulics (Anees et al., 2016;Bates et al., 2005;Horritt and Bates, 2002) and sediment transport and deposition processes (Asselman and Wijngaarden, 2002; Boechat Albernaz et al., 2020; Büttner et al., 2006;Hardy et al., 2000;Klein-hans et al., 2018;Nicholas et al., 2006) over a range of time scales and variable topography (Middelkoop and van der Perk, 1998;Walling, 1997, 1998;Nicholas and Mitchell, 2003;Ahilan et al., 2018;Benjankar and Yager, 2012). Most of these numerical modelling approaches have been limited by poor resolution of topographic data (Lauer and Parker, 2008;Lauer et al., 2016;Hardy et al., 2000;Viparelli et al., 2013) (prior to the widespread availability of lidar data) and considered the floodplain as a flat surface adjacent to the river (Karssenberg and Bridge, 2008). Lidar data with its ability to represent floodplain topography at meter-scale resolution (Hilldale and Raff, 2008) can reveal the complex nature of floodplain hydromorphodynamic processes (Rak et al., 2016;David et al., 2017David et al., , 2018Kupfer et al., 2015;Thayer and Ashmore, 2016;Xu et al., 2020). ...
Article
Full-text available
Variation in floodplain topography can lead to gradual flooding and increase river-floodplain connectivity. We show that incorporating flowpaths as an explicit measure of river-floodplain connectivity can improve estimates of floodplain sediment deposition. We focus on the floodplain of the South River, downstream of Waynesboro, Virginia, where measurements of mercury accumulation have been used to estimate decadal-scale sedimentation rates. We developed a two-dimensional Hydrologic Engineering Center's River Analysis System (2D HEC-RAS) hydrodynamic model and used simulated model results with sediment deposition data to create regression models describing sedimentation across the floodplain. All of our statistical models incorporated a flowpath length from the location on the floodplain downstream to the riverbank as an explicit measure of river-floodplain connectivity that improved our estimates of floodplain sediment deposition (r2 = 0.514). We applied our best regression model to our hydrodynamic model results to create a map of floodplain sedimentation rate and discuss differences of three separate sections of floodplain. We found that floodplains with variable topography had wider, bimodal probability distribution functions (PDFs) of sedimentation rate (aggregated spatially) than floodplains without this topographic relief (with narrower log-normal PDFs). Our work highlights how floodplain topography and river-floodplain connectivity affect sedimentation rates and can help inform the development of floodplain sediment budgets.
... For example, the simulated dam occurs closer to the toe of the landslide L2 than the remnants of the dam we observed in the eld, which occur ~ 100 m downstream (Fig. 3). However, these are to be expected considering challenges in replicating spatially explicit two-and three-dimensional landscape evolution in uvial morphodynamic models (e.g., Lauer et al., 2016) To further test the hypothesis that the topographic change of the channel was caused by the formation and then removal of the sediment dam, we performed an additional simulation (Fig. 4d) without landslide sediment input, to evaluate whether the ood ow alone and the sediment it mobilized from upstream to downstream (i.e., not including landslide sediment input) could have caused the channel change. In this scenario, only limited erosion (i.e., < 0.10 m) was observed in the main channel downstream from L2 and the extent was substantially smaller than the post-event observations. ...
Preprint
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Channel widening is a major hazard during floods, particularly in confined mountainous catchments where roads and buildings compete for space with river channels. Flood-induced channel widening is also an important process in eroding and shaping the landscape. However, channel widening during floods is not well understood and not always explained by hydraulic variables alone, with implications for flood risk management. Floods in mountainous regions often coincide with landslides triggered by heavy rainfall on steep valley sides. Whilst the long-term impact of increased sediment supply on channel widening is well established, landslide-channel interactions at the event timescale are not well known or documented. Here we demonstrate with an example from the Great Colorado Flood in 2013, a 1000-yr precipitation event that induced a 200-yr flood, and 100-yrs of erosion, how landslide-channel feedbacks can substantially amplify channel widening and flood risk. We use a combination of field analysis and multiphase flow modeling to document landslide-channel interaction during the flood event in which sediment delivered by landslides temporarily dammed the channel before failing and bulking the flow with sediment resulting in large channel widening. We propose that such landslide-flood interactions will become increasingly important to account for in flood hazard assessment as flooding and landsliding increase with extreme rainfall under climate change.
... The sediment budgeting approach aims at estimating sediment erosion, transfer, and deposition through a given river reach by considering three-dimensional morphological changes [38,39]. It can be based on different techniques that may be more or less advanced, such as use of a bedload transport formula at a station [40], morphodynamic modelling [41], field validations based on sediment tracking [42], and sediment flux estimates based on geophones or traps [43]. This sediment budgeting approach is usually combined with historical information (topographical and planimetric) and field campaigns (bank height estimates, overbank fine sediment volumes), and sometimes with LiDAR-derived digital elevation models (DEM) to detect geomorphic changes following gravel mining or other human pressures on sediment storage and floodplain-channel interactions. ...
Article
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River rehabilitation and ecological engineering are becoming critical issues for improving river status when ecological habitats and connectivity have been altered by human pressures. Amongst the range of existing rehabilitation options, some specifically focus on rebuilding fluvial forms and improving physical processes. The aim of this contribution is to illustrate how geomorphological expertise and process-based thinking contribute to river rehabilitation success. This semantic contribution is intended to feed the rehabilitation debate, particularly concerning the design of actions and the proposed references for monitoring target reaches and evaluating rehabilitation effects empirically. This article is also based on lessons learned from practical cases, mainly in gravel-bed rivers. Geomorphic understanding is needed at a local level to achieve an adequate diagnosis of river functioning, estimate human impacts and potential remnant river responsiveness, and to assess the gains and risks from rehabilitation, as well as to appraise success or failure through several pre- and post-project assessment strategies. Geomorphological studies can also be upscaled in a top-down manner (from high-order controls to small-scale processes, understanding detailed processes in their regional or basin-wide context), providing large-scale information at the regional, national, or even global level, information that can be used to diagnose the health of riverscapes in relation to local site-specific contexts. As such, geomorphological studies support strategic planning and prioritization of rehabilitation works according to specific contexts and river responsiveness, so as to move from opportunistic to objective-driven strategies.
... ados para simular los procesos morfodinámicos en ríos con lechos de sedimentos no-uniformes (Ribberink, 1987;Armanini y Di Silvio, 1988;Di Silvio y Peviani, 1991;Parker, 1992;Basile y Di Silvio, 1994;Cui y Parker, 1997;Tsujimoto, 1999;Basile, 2000aBasile, , 2002Wu y Wang, 2008;Parker, 2008;Fasolato et al., 2009;Chiari et al., 2010;Sun et al., 2015;Lauer et. al., 2016;Stecca et al., 2016). ...
Article
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En el presente trabajo se describe el desarrollo y posterior aplicación del modelo hidro-morfodinámico espacialmente semidistribuído a escala de cuenca, denominado TEDRI-1D7C (Transporte y Erosión-Deposición en RIos - 1 Dimensional 7 Clases granulométricas). El modelo simula integralmente, desde la transformación lluvia-caudal y la alimentación sólida, en forma espacialmente agregada a escala de subcuenca, hasta la propagación distribuida de caudales (líquidos y sólidos) y los procesos de erosión-deposición en el curso principal. En particular, a lo largo del río, el modelo simula la evolución espacial y temporal de: i) caudal líquido (y variables de flujo asociadas), ii) transporte de sedimentos (fondo, suspensión y total) por clases granulométricas, iii) procesos de erosión-sedimentación y iv) cambios de composición granulométrica del lecho, en ríos aluviales con lechos de sedimentos no-uniformes y pendientes superiores al 0.2 %. El modelo fue calibrado con experimentos realizados en canales de laboratorio, ya sea con lechos de sedimentos de grava como de arena. Asimismo, el modelo fue aplicado en la cuenca del Aº Marea (Chubut), para simular crecidas de diseño extraordinarias, a corto y largo plazo. Las comparaciones de los resultados del modelo con las mediciones disponibles muestran que el modelo reproduce adecuadamente la hidromorfodinámica en ambas aplicaciones.
... To date, most network-scale sediment conveyance models do not directly incorporate between compartment interactions such as hillslope-river or channel-floodplain processes. Lauer et al. (2016), Gilbert and Wilcox (2020) and Beveridge et al. (2020) estimated sediment supply rates to the river network by integrating their river bedload sediment transport modeling framework with surface runoff and erosion models. These approaches are likely to be more appropriate for the finer fraction of the sediment distribution whereas other methods are required for gravel or larger grain size. ...
Chapter
Predictions of sediment flux and the evolutionary trajectory of river systems cannot be conducted effectively independent from quantitative understandings of sediment (dis)connectivity. This requires analysis of structural and functional interactions within and between landscape compartments, and the way these interactions play out at the catchment scale. Building upon a conceptualization of connected and disconnected landscapes, this chapter reviews recent modeling applications that quantify these cross-scalar relationships, highlighting applications in different settings. A summary of approaches to analysis of ecological (dis)connectivity relationships in river systems highlights significant prospects for future interdisciplinary research applications.
... Specifically applied to dam removal pulses, the Dam Removal Express Assessment Models (DREAMs) simulate aggradation and incision following dam removal for reservoir deposits composed primarily of noncohesive sand and silt (DREAM-1) and gravel (DREAM-2; Cui et al., 2006aCui et al., , 2006b. The Morphodynamics and Sediment Tracers in one-dimension (MAST-1D) model also incorporate particle-size distributions to simulate morphodynamic evolution of a river bed and lateral exchange of sediment between the channel and floodplain (Lauer et al., 2016;Viparelli et al., 2013). The Lagrangian framework presented by Czuba (2018) and Pfeiffer et al. (2020) also uses a surface-based bedload equation and considers mixed-size sediment transport. ...
Article
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Mountain rivers often receive sediment in the form of episodic, discrete pulses from a variety of natural and anthropogenic processes. Once emplaced in the river, the movement of this sediment depends on flow, grain size distribution, and channel and network geometry. Here, we simulate downstream bed elevation changes that result from discrete inputs of sediment (∼10,000 m³), differing in volume and grain size distribution, under medium and high flow conditions. We specifically focus on comparing bed responses between mixed and uniform grain size sediment pulses. This work builds on a Lagrangian, bed‐material sediment transport model and applies it to a 27 km reach of the mainstem Nisqually River, Washington, USA. We compare observed bed elevation change and accumulation rates in a downstream lake to simulation results. Then we investigate the magnitude, timing, and persistence of downstream changes due to the introduction of synthetic sediment pulses by comparing the results against a baseline condition (without pulse). Our findings suggest that bed response is primarily influenced by the sediment‐pulse grain size and distribution. Intermediate mixed‐size pulses (∼50% of the median bed gravel size) are likely to have the largest downstream impact because finer sizes translate quickly and coarser sizes (median bed gravel size and larger) disperse slowly. Furthermore, a mixed‐size pulse, with a smaller median grain size than the bed, increases bed mobility more than a uniform‐size pulse. This work has important implications for river management, as it allows us to better understand fluvial geomorphic responses to variations in sediment supply.
... For this reason, the integration of sediment connectivity indices with models focusing on geomorphic processes occurring on hillslopes and floodplains (Cavalli et al., 2013;Heckmann et al., 2014) represent an important next step for generating information on sediment supply from sources outside the channel (Beveridge et al., 2020;Gilbert and Wilcox, 2020). Further developments may also better integrate channel-floodplain interactions including river lateral mobility and bank erosion modelling (Gilbert and Wilcox, 2020;Lauer et al., 2016). However, for a network-scale sediment connectivity model to be efficient, it is important to simplify local-scale processes without losing the ability to represent the main driving forces and connectivity patterns. ...
Article
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In this paper we apply the CASCADE network-scale sediment connectivity model to the Vjosa River in Albania. The Vjosa is one of the last unimpaired braided rivers in Europe and, at the same time, a data scarce environment, which limits our ability to model how this pristine river might respond to future human disturbance. To initialize the model, we use remotely sensed data and modeled hydrology from a regional model. We perform a reach-by-reach optimization of surface grain size distribution (GSD) and bed load transport capacity to ensure equilibrium conditions throughout the network. In order to account for the various sources of uncertainty in the calculation of transport capacity, we performed a global sensitivity analysis. The modelled GSD distributions generated by the sensitivity analysis generally match the six GSDs measured at different locations within the network. The modeled bed load sediment fluxes increase systematically downstream, and annual fluxes at the outlet of the Vjosa are well within an order of magnitude of fluxes derived from previous estimates of the annual suspended sediment load. We then use the modeled sediment fluxes as input to a set of theoretically derived functions that successfully discriminate between multi-thread and single-thread channel patterns. This finding provides additional validation of the model results by showing a clear connection between modeled sediment concentrations and observed river morphology. Finally, we observe that a reduction in sediment flux of about 50% (e.g., due to dams) would likely cause existing braided reaches to shift toward single thread morphology. The proposed method is widely applicable and opens a new avenue for application of network-scale sediment models that aid in the exploration of river stability to changes in water and sediment fluxes.
... 3. When applied at the field scale, the model describes the long-term evolution of the river channel. It does not account for the exchange of sediment between the channel and the floodplain due to for example overbank deposition of suspended sediment, channel migration, widening, or narrowing (e.g., Lauer et al., 2016;Viparelli et al., 2011). 4. Base level is assumed constant. ...
Article
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Recent studies reveal that low‐slope bedrock reaches (bedrock surface slope milder than ~5 m/km) are more common than previously thought and can be found in engineered rivers and densely populated deltas. Here we present a novel formulation of alluvial morphodynamics of low‐slope bedrock rivers transporting nonuniform bed material that accounts for the nonuniformity of the sediment size and the presence of small scale bedforms such as dunes and can thus be of aid to solve management/restoration problems in low‐slope bedrock rivers. The formulation is implemented in a one‐dimensional morphodynamic model. Numerical results are compared with laboratory experiments on equilibrium bedrock reaches downstream of stable alluvial‐bedrock transitions. The differences between experimental and numerical results are comparable with those obtained in the alluvial case. Model applications simulate (1) bedrock reaches with a stable bedrock‐alluvial transitions, (2) an alluvial‐bedrock transition subject to sea level rise, and (3) steep bedrock reaches. Upstream of a stable bedrock‐alluvial transition the flow decelerates in the streamwise direction with the formation of a stable pattern of downstream coarsening of bed surface sediment. In response to sea level rise, alluvial‐bedrock transitions migrate downstream and bedrock‐alluvial transitions migrate upstream. Opposite migration directions are expected in the case of sea level fall. When applied to steep channels, the model predicts gradual alluviation, but it fails to reproduce runaway alluviation.
... A scientific framework is needed to assess this instability. Although morphodynamic modelling can be employed (Sanyal, 2017;Lauer et al., 2016) for simulating post-dam erosion in a channel, there is also value in using more traditional, datadriven analytical frameworks for characterizing likely geomorphic response to dams. Such approaches involve comparing the ratio of pre-to post-dam slope of the channel, index for bed erosion (Schmidt and Wilcock, 2008), carrying capacity and prevalent load (Brandt 2000), and most notably, the ratio of post-to pre-dam frequency of sediment transporting flow (T*) and the ratio of below to above-dam sediment supply (S*) ( Fig. 1) [Grant et al., 2003]. ...
Article
Long-term geomorphic evolution of rivers depends both on the amount of sediment supply and on the river’s ability to transport that sediment. Climate change is anticipated to change flow frequency distributions and sediment yield and could thus alter the geomorphic trajectory of many regulated rivers. We aim to examine the possible geomorphic response of the upper Godavari River, India, to climate change using two well-accepted metrics: the frequency of the sediment-transporting flows (T*) and the ratio of the sediment flux below and above a dam (S*). Soil and Water Assessment Tool (SWAT) was used to simulate streamflow and sediment yield in the basin. We calibrated and validated the model using SWAT-CUP for pre- (1971–75) and post-dam (1976–81) conditions downstream of Jayakwadi dam. The model performance was good (NS value > 0.6 for discharge and PBIAS < ± 10 for sediment) with low uncertainty (p-factor > 0.6 and r-factor <1) and it improved significantly by factoring the operation of the reservoirs into the simulation. We ran the model for 10 years consecutively in the pre and post-dam states for baseline (1971–81) and future (2090–99) climate conditions derived from NEX-GDDP climate projections (Emission scenario RCP 8.5 & 4.5). A linear relationship between T*/S* and observed erosion (m) below the dam provided the basis for estimating the amount of erosion associated with future climate states. Our simulations show substantially larger T* below the dam site in 40% of the 2090s scenarios relative to the baseline conditions. These could be associated with 4–6 times the erosion compared to the baseline. Severe erosion is indicated only under RCP 8.5. Simulations suggest that amplified peak releases from dams in the future could lead to greater downstream erosion potential.
... The level of detail in the parameters required to run sediment models varies with the spatial resolution of modeling. Reach-scale morphodynamic models can accurately predict channel behavior and morphologic change through time, but require higher-resolution input data (e.g., topography) and calibration of site-specific parameters, some of which are difficult to measure (e.g., Call et al., 2017;Lauer et al., 2016). Consequently, they are difficult to apply at larger spatial extents. ...
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Abstract Sediment regimes, i.e., the processes that recruit, transport, and store sediment, create the physical habitats that underpin river‐floodplain ecosystems. Natural and human‐induced disturbances that alter sediment regimes can have cascading effects on river and floodplain morphology, ecosystems, and a river's ability to provide ecosystem services, yet prediction of the response of sediment dynamics to disturbance is challenging. We developed the Sediment Routing and Floodplain Exchange (SeRFE) model, which is a network‐based, spatially explicit framework for modeling sediment recruitment to and subsequent transport through drainage networks. SeRFE additionally tracks the spatially and temporally variable balance between sediment supply and transport capacity. Simulations using SeRFE can account for various types of watershed disturbance and for channel‐floodplain sediment exchange. SeRFE is simple, adaptable, and can be run with widely available geospatial data and limited field data. The model is driven by real or user‐generated hydrographs, allowing the user to assess the combined effects of disturbance, channel‐floodplain interactions and particular flow scenarios on the propagation of disturbances throughout a drainage network, and the resulting impacts to reaches of interest. We tested the model in the Santa Clara River basin, Southern California, in subbasins affected by large dams and wildfire. Model results highlight the importance of hydrologic conditions on postwildfire sediment yield and illustrate the spatial extent of dam‐induced sediment deficit during a flood. SeRFE can provide contextual information on reach‐scale sediment balance conditions, sensitivity to altered sediment regimes, and potential for morphologic change for managers and practitioners working in disturbed watersheds.
... It is not trivial to include the effects of floodplains in the proposed method. There seem to be three possibilities: (1) to prescribe the difference in bed elevation between the main channel and the floodplains, (2) to prescribe the bed elevation of the floodplains, or (3) to account for the flux of water and sediment between the main channel and the floodplains (e.g., Lauer et al., 2016;Viparelli et al., 2013). ...
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An engineered alluvial river (i.e., a fixed‐width channel) has constrained planform but is free to adjust channel slope and bed surface texture. These features are subject to controls: the hydrograph, sediment flux, and downstream base level. If the controls are sustained (or change slowly relative to the timescale of channel response), the channel ultimately achieves an equilibrium (or quasi‐equilibrium) state. For brevity, we use the term “quasi‐equilibrium” as a shorthand for both states. This quasi‐equilibrium state is characterized by quasi‐static and dynamic components, which define the characteristic timescale at which the dynamics of bed level average out. Although analytical models of quasi‐equilibrium channel geometry in quasi‐normal flow segments exist, rapid methods for determining the quasi‐equilibrium geometry in backwater‐dominated segments are still lacking. We show that, irrespective of its dynamics, the bed slope of a backwater or quasi‐normal flow segment can be approximated as quasi‐static (i.e., the static slope approximation). This approximation enables us to derive a rapid numerical space‐marching solution of the quasi‐static component for quasi‐equilibrium channel geometry in both backwater and quasi‐normal flow segments. A space‐marching method means that the solution is found by stepping through space without the necessity of computing the transient phase. An additional numerical time stepping model describes the dynamic component of the quasi‐equilibrium channel geometry. Tests of the two models against a backwater‐Exner model confirm their validity. Our analysis validates previous studies in showing that the flow duration curve determines the quasi‐static equilibrium profile, whereas the flow rate sequence governs the dynamic fluctuations.
... There has been a plethora of such mathematical models over the last few decades. Many 1D models were developed to simulate the size variations of bed sediment [30][31][32], suspended-load [33], and the hyper-concentration flow [34,35] in alluvial channels. Rudorff et al. [36] predicted the variation in channel-floodplain suspended sediment exchange along a 140 km reach of the lower Amazon River for two decades (1995-2014) with a 2D hydraulic model. ...
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The focus of this paper is on studying novel approaches to estimate sediment exchange between suspended-load and bed material in an unsteady sediment-laden flow with fine-grained sand. The erosion-deposition characteristics of the channel have close relation with the variation of size compositions of both suspended-load and bed material. These aims are addressed by deducing the sediment exchange equations from the mass conservation perspective and establishing a river-sediment mathematical model based on the theory. The model is applied in the middle and lower Yellow River, China, and calibrated and verified under both deposition and erosion conditions using a generalized channel and a large quantity of measured data in the Yellow River basin. The results indicate that the grading curves of suspended-load and bed material calculated by the mathematical model are close to those of the measured data. The temporal and spatial variations in the mean sizes of suspended-load and bed material, flow rate, sediment concentration and erosion or deposition volume estimates during the entire flood process can be accurately predicted. The model performance is considered acceptable for determining the sediment exchange process and the change in channel morphology for unsteady sediment-laden flow.
... The unmixing model presented by Belmont et al. (2014) was used to compute the proportion of sediment from each source area. Lauer et al. (2016) and Viparelli et al. (2013) used a portion of the sediment fingerprinting data to develop a geochemically tagged sediment routing model that accounts for production and decay of radioisotopes in the floodplain. The data are available through Belmont (2018b). ...
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Observatory-scale data collection efforts allow unprecedented opportunities for integrative, multidisciplinary investigations in large, complex watersheds, which can affect management decisions and policy. Through the National Science Foundation-funded REACH (REsilience under Accelerated CHange) project, in collaboration with the Intensively Managed Landscapes-Critical Zone Observatory, we have collected a series of multidisciplinary data sets throughout the Minnesota River Basin in south-central Minnesota, USA, a 43,400-km ² tributary to the Upper Mississippi River. Postglacial incision within the Minnesota River valley created an erosional landscape highly responsive to hydrologic change, allowing for transdisciplinary research into the complex cascade of environmental changes that occur due to hydrology and land use alterations from intensive agricultural management and climate change. Data sets collected include water chemistry and biogeochemical data, geochemical fingerprinting of major sediment sources, high-resolution monitoring of river bluff erosion, and repeat channel cross-sectional and bathymetry data following major floods. The data collection efforts led to development of a series of integrative reduced complexity models that provide deeper insight into how water, sediment, and nutrients route and transform through a large channel network and respond to change. These models represent the culmination of efforts to integrate interdisciplinary data sets and science to gain new insights into watershed-scale processes in order to advance management and decision making. The purpose of this paper is to present a synthesis of the data sets and models, disseminate them to the community for further research, and identify mechanisms used to expand the temporal and spatial extent of short-term observatory-scale data collection efforts.
... The Unified Gravel-Sand (TUGS) model incorporates transport dynamics of a sand-gravel grain size distribution via a surface-based bedload equation, three conceptual layers (bedload, surface, and subsurface), transfer functions between layers, and abrasion (Cui, 2007). The Morphodynamics and Sediment Tracers in 1D (MAST-1D) model also incorporates mixed-size transport, but its key contribution is in its handling of size-specific exchange between sediment in the channel and floodplain (Lauer et al., 2016). However, these 1D models cannot suitably explore additional complexities that arise from channel network structure. ...
Article
The movement of sediment from source to sink in a watershed is a complex process with multiple interactions and feedbacks across scales. At small scales, the size characteristics of a sediment mixture affect the transport of that sediment through hiding and through the nonlinear effect of sand content on gravel transport. At large scales, the channel network itself, through network geometry and the spatial pattern of transport capacity, adds additional complexity in organizing sediment moving through the system such that aggradational hotspots can emerge. The purpose of this paper is to present a Lagrangian framework for exploring complexities of mixed-size sediment transport in gravel-bedded river networks. The present model builds off of previous network-based, bed-material sediment transport models; but key advancements presented herein (i) allow for a mixture of sediment sizes in the river network, (ii) incorporate a mixed-size sediment transport equation, and (iii) utilize a daily flow hydrograph to drive intermittent transport. The model is applied to the roughly 4700 km² Methow River Basin in Washington State, USA, using simplified model inputs with the goal of illustrating the utility of the model for motivating future work.
... However, when an explicit FVM model is used, two key shortcomings remain, 8 one being the smaller time step imposed by the Courant-Friedrichs-Lewy (CFL) limiting condition (Delis et al., 2000;Stelling, 2003), and the difficulty in maintaining 10 robustness for complex looped and dendritic river networks (Jin et al., 2002). For long-term simulations, e.g. for up to 100 years, and for a series of scenario runs of the 12 hydrodynamic, sediment and mass transport processes, 1D models are extensively used because of their higher efficiency and even higher accuracy than 2D and 3D 14 models when dealing with large and complex river networks (Lauer et al., 2016;Wu et al., 2004;Zhou and Lin, 1998). Usually a 1-D model is used to link a catchment 16 hydrological model (Merkhali et al., 2015;Paiva et al., 2011) and a 2D or 3D estuarine and/or coastal model (Bladé et al., 2012;Twigt et al., 2009). ...
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Predicting the rate of Escherischia coli (E.coli) loss in a river network is one of the key conditions required in the management of bathing waters, with well verified numerical models being effective tools used to predict bathing water quality in regions with limited field data. In this study, a unique finite volume method (FVM) one-dimensional model is firstly developed to solve the mass transport process in river networks, with multiple moving stagnation points. The model is then applied to predict the concentration distribution of E.coli in the river Ribble network, UK, where the phenomena of multiple stagnation points and different flow directions appear extensively in a tidal sub-channel network. Validation of the model demonstrates that the proposed method gives reasonably accurate solution. The verification results show that the model predictions generally agree well with measured discharges, water levels and E.coli concentration values, with mass conservation of the solution reaching 99.0% within 12 days for the Ribble case. An analysis of 16 one-year scenario runs for the Ribble network shows that the main reduction in E.coli concentrations occurs in the riverine and estuarine regions due to the relatively large decay rate in the brackish riverine waters and the long retention time, due to the complex river discharge patterns and the tidal flows in the regions.
... Despite these advances, many challenges remain for morphological modeling. Examples include the description of physical processes ( Mosselman, 2012 ); computational costs, transport formula accuracy, and the inclusion of mixed-grain size sediment ( Hirano, 1971( Hirano, , 1972Williams et al., 2016b ); riparian vegetation and wood effects ( Bertoldi & Ruiz-Villanueva, 2015;Lauer et al., 2016 ); bank erosion ( Stecca et al., 2017 ); and spatial and temporal discretization and the influences of small boundary and initial condition errors ( Williams et al., 2016a ). ...
Article
New data collection techniques offer numerical modelers the ability to gather and utilize high quality data sets with high spatial and temporal resolution. Such data sets are currently needed for calibration, verification, and to fuel future model development, particularly morphological simulations. This study explores the use of high quality spatial and temporal data sets of observed bed load transport in braided river flume experiments to evaluate the ability of a two-dimensional model, Delft3D, to predict bed load transport. This study uses a fixed bed model configuration and examines the model's shear stress calculations, which are the foundation to predict the sediment fluxes necessary for morphological simulations. The evaluation is conducted for three flow rates, and model setup used highly accurate Structure-from-Motion (SfM) topography and discharge boundary conditions. The model was hydraulically calibrated using bed roughness, and performance was evaluated based on depth and inundation agreement. Model bed load performance was evaluated in terms of critical shear stress exceedance area compared to maps of observed bed mobility in a flume. Following the standard hydraulic calibration, bed load performance was tested for sensitivity to horizontal eddy viscosity parameterization and bed morphology updating. Simulations produced depth errors equal to the SfM inherent errors, inundation agreement of 77–85%, and critical shear stress exceedance in agreement with 49–68% of the observed active area. This study provides insight into the ability of physically based, two-dimensional simulations to accurately predict bed load as well as the effects of horizontal eddy viscosity and bed updating. Further, this study highlights how using high spatial and temporal data to capture the physical processes at work during flume experiments can help to improve morphological modeling.
... Reservoir sedimentation is an important research topic as it has direct application in dam construction, flow regulation, flood control, navigation, hydropower, water supply and many other issues related to environmental benefits. In recent years, a voluminous amount of research in predicting the reservoir sedimentation has been performed from empirical, semi-empirical and numerical viewpoints (Ching-Hseinm et al. 2012;Caputo and Carcione 2013;Haun et al. 2013;Tsai et al. 2014;Ahm and Yang 2015;Hosseinjanzadeh et al. 2015;Issa et al. 2016;ji et al. 2016;Lauer et al. 2016). Chen et al. (2015) developed and demonstrated a mathematical model for sediment deposition and erosion in a river during flood events. ...
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The present study attempts to predict the reservoir sedimentation in 32 km region of the Tenryu River between the Hiraoka and Sakuma Dams in Japan. For numerical simulations of the reservoir sedimentation, the one-dimensional model of the Hydrologic Engineering Centre-River Analysis System (HEC-RAS) is used together with the inclusion of channel geometry, bed gradation curve, Exner-5 bed sorting mechanisms, fall velocity of the particle, and flow and sediment boundary conditions pertaining to modeling region. The modeling region of the Tenryu River is divided into 48 river stations with 47 reaches in the numerical simulations. The numerical model is calibrated using the available data for 48 years from 1957 to 2004. The formulae of sediment transport function, Manning’s roughness coefficient, computational increment and fall velocity have been identified for getting the best estimation of the Sakuma Dam reservoir sedimentation. Combination of obtained sensitive parameters and erodible limits of 2 m gave the best comparison with the measured bed profile. The computed results follow the trend of measured data with a small underestimation. Although Manning’s roughness coefficient has an effect on the sedimentation, no direct relation is found between the Manning’s roughness coefficient and reservoir sedimentation. It is found that the temperature of water has no effect on the reservoir sedimentation.
... Therefore UASbased SfM photogrammetry could seemingly provide high quality, spatially distributed roughness and morphology data that are needed by hydraulic and morphodynamics models (Tamminga et al., 2014). For example, some recent morphodynamic models (i.e., Lauer et al., 2016) considered lateral variations in grain size, and UAS-derived SfM terrain models may have the potential to feed them with the required data in this regard. This paper reports the testing of optical imagery acquired from UAS in connection with SfM and multiview stereo (MVS) photogrammetry to retrieve the GSD of surface bed sediment in a braided gravel-bed river (Vénéon River, French Alps). ...
Article
This paper explores the potential of unmanned aerial system (UAS) optical aerial imagery to characterize grain roughness and size distribution in a braided, gravel-bed river (Vénéon River, French Alps). With this aim in view, a Wolman field campaign (19 samples) and five UAS surveys were conducted over the Vénéon braided channel during summer 2015. The UAS consisted of a small quadcopter carrying a GoPro camera. Structure-from-Motion (SfM) photogrammetry was used to extract dense and accurate three-dimensional point clouds. Roughness descriptors (roughness heights, standard deviation of elevation) were computed from the SfM point clouds and were correlated with the median grain size of the Wolman samples. A strong relationship was found between UAS-SfM-derived grain roughness and Wolman grain size. The procedure employed has potential for the rapid and continuous characterization of grain size distribution in exposed bars of gravel-bed rivers. The workflow described in this paper has been successfully used to produce spatially continuous grain size information on exposed gravel bars and to explore textural changes following flow events.
Article
We propose a holistic approach to define a river corridor as the minimum space needed to sustain key river functions based on an understanding of the desired functions of that corridor and the processes governing channel and floodplain formation. Giving such space is a fundamental nature‐based solution to river management, as it allows the river to use its own energy to maintain flood conveyance and habitat function. The review of existing river corridor concepts shows that these often focus on one or two potential functions of a river corridor and may not be well suited as tools to optimize eco‐geomorphic river function. We argue that evaluating the effects of river corridor width on multiple processes can provide an objective means to optimize delineation in areas where development encroaches onto floodplains and channel migration zones. Key processes are linked to channel migration and include floodplain rejuvenation, emergence of a dynamic patch mosaic of riparian habitat that sustains a functioning large wood cycle, and effects of constriction and confinement on channel dynamics and morphology. Quantification of these processes for an example river shows the most rapid gains for habitat and flood protection values up to the threshold for planform‐controlled conditions and an asymptotically reducing rate of gain in function above that threshold. For the example river, the threshold width approaching that asymptote is substantially more than its constrained condition but much less than the width of the floodplain and channel migration zone, offering a compromise for managing rivers with developed floodplains. This article is categorized under: Engineering Water > Sustainable Engineering of Water Water and Life > Nature of Freshwater Ecosystems Water and Life > Conservation, Management, and Awareness
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This work proposes a modified form of two-dimensional shallow water (2D-SWD) equation with water surface gradient at the source term. This modification was carried out as the conventional 2D-SWD equations often require special treatment when applied to complex braided rivers. The modified model is then applied to two different flow domains of the Brahmaputra river viz. at the braided reach near Umananda Island and reach with a series of spurs dyke near Majuli island. The stability of the model is checked using Courant–Friedrichs–Lewy condition. The maximum flow depth in the braided reach is observed as 15–18 m, and near the spur dykes, it is found as 35–37 m. Model simulated flow depth and velocity are compared with the field measured data. The model’s computational accuracy near the flow-hydraulic structure interaction zone is evaluated with two different boundary conditions: no slip condition and reflecting boundary condition. Based on root mean square error (RMSE) and Nash–Sutcliffe Efficiency (NSE), the reflection boundary condition shows superior in simulating the velocity profile (RMSE = 0.11, NSE = 0.60) as compared to no slip condition (RMSE = 0.28, NSE = 0.22). These findings suggested that the water surface gradient in the modified 2D-SWD model gives more flexibility for application in the braided river. It also stresses the importance of considering reflecting boundary conditions while applying the model near the fluid–structure interaction region.
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Abstract A mountain watershed network model is presented for use in decadal to centurial estimation of source‐to‐sink sediment dynamics. The model requires limited input parameters and can be effectively applied over spatial scales relevant to management of reservoirs, lakes, streams, and watersheds (1–100 km2). The model operates over a connected stream network of Strahler‐ordered segments. The model is driven by streamflow from a physically based hydrology model and hillslope sediment supply from a stochastic mass wasting algorithm. For each daily time step, segment‐scale sediment mass balance is computed using bedload and suspended load transport equations. Sediment transport is partitioned between grain size fractions for bedload as gravel and sand, and for suspended load as sand and mud. Bedload and suspended load can deposit and re‐entrain at each segment. We demonstrated the model in the Elwha River Basin, upstream of the former Glines Canyon dam, over the dam's historic 84‐year lifespan. The model predicted the lifetime reservoir sedimentation volume within the uncertainty range of the measured volume (13.7–18.5 million m3) for 25 of 28 model instances. Gravel, sand, and mud fraction volumes were predicted within measurement uncertainty ranges for 18 model instances. The network model improved the prediction of sediment yields compared to at‐a‐station sediment transport capacity relations. The network model also provided spatially and temporally distributed information that allowed for inquiry and understanding of the physical system beyond the sediment yields at the outlet. This work advances cross‐disciplinary and application‐oriented watershed sediment yield modeling approaches.
Article
Alluvial rivers are composed of self‐formed channels which are sensitive to disturbances in their flow and sediment‐supply regimes. Regime changes commonly occur over decadal and longer timescales and can be caused by anthropogenic alterations such as dam construction and removal. Advances in numerical modeling have increased our ability to explore geomorphic adjustments over long timescales; however, many models designed to be run for decades or longer assume that banks are immovable or that channel width is constant. Since river channels often respond to disturbance by adjusting their geometry, this is a significant shortcoming. To investigate the impact of long‐term sediment supply alterations on channel geometry and stability, we have adapted MAST‐1D, a reach‐scale bed evolution model, to incorporate functions for bank erosion, vegetation encroachment, and local avulsions. The model is designed for medium‐large, coarse multithreaded rivers and can be run over long (decades‐centuries) timescales. Bank erosion is a function of the mobility and transport capacity for structurally‐important grains which protect the bank toe. Vegetation growth is proportional to point bar width and occurs during conditions of low shear stress. Local avulsions occur when aggradation causes channel depth to drop below a threshold. We apply the model to the Elwha River in Washington, USA with the goal of investigating if and when the river recovers from dam emplacement and removal. The Elwha was dammed for nearly 100 years, and then two dams were removed, releasing a large pulse of sediment. We have modeled the set of reaches between the two dams. Our simulations suggest that channel response to dam emplacement occurs gradually over several decades but that the channel recovers to near pre‐dam conditions within about a decade following the removal. The dams leave a lasting legacy on the floodplain, which does not completely recover, even after two centuries.
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In large watersheds, sediment storage imposes geologic timescales on the downstream movement of suspended sediment and associated contaminants. Modeling these processes requires temporal and spatial averaging, a task often accomplished using concepts derived from reservoir theory, where residence times and age and storage time distributions of stored sediment are treated as constants, even though all of these quantities should be time-variable under conditions of unsteady sediment transport. Here, concepts from reservoir theory are replaced by deposition and erosion laws to develop a parsimonious quantitative framework for watershed-scale routing of sediment and contaminants that explicitly accounts for time-dependent alluvial storage. Solution of the new equations, combined with appropriate initial and boundary conditions, predicts the age and storage time distributions of stored sediment, the sediment residence time, the mass of stored sediment, and the variations in all of these quantities through time. Model calibration is illustrated by reinterpreting previously published data from the Little Missouri River in North Dakota to create a 3-dimensional chronostratigraphic model of stored sediment, supplemented by observations of contemporary patterns of erosion. Model parameters from the Little Missouri River are then used to demonstrate that approximately 750 yr will be required to remove arsenic-contaminated alluvium supplied by a century of mine tailings discharges to a 112-km reach of the Belle Fourche River in South Dakota. The new approach described here requires very modest computing resources to simulate long timescales across large watersheds, and the fidelity of storage parameters, which ultimately control the timing of sediment delivery, is supported by site-specific stratigraphic observations and sediment dating.
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When the water discharge, sediment supply, and base level vary around stable values, an alluvial river evolves toward a mean equilibrium or graded state with small fluctuations around this mean state (i.e. a dynamic or statistical equilibrium state). Here we present analytical relations describing the mean equilibrium geometry of an alluvial river under variable flow by linking channel slope, width, and bed surface texture. The solution holds in river normal flow zones (or outside the hydrograph boundary layer and the backwater zone) and accounts for grain size selective transport and particle abrasion. We consider the variable flow rate as a series of continuously changing yet steady water discharges (here termed an alternating steady discharge). The analysis also provides a solution to the channel-forming water discharge, which is here defined as the steady water discharge that, given the mean sediment supply, provides the same equilibrium channel slope as the natural long-term hydrograph. The channel-forming water discharge for the gravel load is larger than the one associated with the sand load. The analysis illustrates how the load is distributed over the range of water discharge in the river normal flow zone, which we term the ‘normal flow load distribution’. The fact that the distribution of the (imposed) sediment supply spatially adapts to this normal flow load distribution is the origin of the hydrograph boundary layer. The results quantify the findings by Wolman and Miller [1960] regarding the relevance of both magnitude and frequency of the flow rate with respect to channel geometry.
Technical Report
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A study of the geomorphology of rivers draining Mount Rainier, Washington, was completed to identify sources of sediment to the river network; to identify important processes in the sediment delivery system; to assess current sediment loads in rivers draining Mount Rainier; to evaluate if there were trends in streamflow or sediment load since the early 20th century; and to assess how rates of sedimentation might continue into the future using published climate-change scenarios. Rivers draining Mount Rainier carry heavy sediment loads sourced primarily from the volcano that cause acute aggradation in deposition reaches as far away as the Puget Lowland. Calculated yields ranged from 2,000 tonnes per square kilometer per year [(tonnes/km2)/yr] on the upper Nisqually River to 350 (tonnes/km2)/yr on the lower Puyallup River, notably larger than sediment yields of 50–200 (tonnes/km2)/yr typical for other Cascade Range rivers. These rivers can be assumed to be in a general state of sediment surplus. As a result, future aggradation rates will be largely influenced by the underlying hydrology carrying sediment downstream. The active-channel width of rivers directly draining Mount Rainier in 2009, used as a proxy for sediment released from Mount Rainier, changed little between 1965 and 1994 reflecting a climatic period that was relatively quiet hydrogeomorphically. From 1994 to 2009, a marked increase in geomorphic disturbance caused the active channels in many river reaches to widen. Comparing active-channel widths of glacier-draining rivers in 2009 to the distance of glacier retreat between 1913 and 1994 showed no correlation, suggesting that geomorphic disturbance in river reaches directly downstream of glaciers is not strongly governed by the degree of glacial retreat. In contrast, there was a correlation between active-channel width and the percentage of superglacier debris mantling the glacier, as measured in 1971. A conceptual model of sediment delivery processes from the mountain indicates that rockfalls, glaciers, debris flows, and main-stem flooding act sequentially to deliver sediment from Mount Rainier to river reaches in the Puget Lowland over decadal time scales. Greater-than-normal runoff was associated with cool phases of the Pacific Decadal Oscillation. Streamflow-gaging station data from four unregulated rivers directly draining Mount Rainier indicated no statistically significant trends of increasing peak flows over the course of the 20th century. The total sediment load of the upper Nisqually River from 1945 to 2011 was determined to be 1,200,000±180,000 tonnes/yr. The suspended-sediment load in the lower Puyallup River at Puyallup, Washington, was 860,000±300,000 tonnes/yr between 1978 and 1994, but the long-term load for the Puyallup River likely is about 1,000,000±400,000 tonnes/yr. Using a coarse-resolution bedload transport relation, the long-term average bedload was estimated to be about 30,000 tonnes/yr in the lower White River near Auburn, Washington, which was four times greater than bedload in the Puyallup River and an order of magnitude greater than bedload in the Carbon River. Analyses indicate a general increase in the sediment loads in Mount Rainier rivers in the 1990s and 2000s relative to the time period from the 1960s to 1980s. Data are insufficient, however, to determine definitively if post-1990 increases in sediment production and transport from Mount Rainier represent a statistically significant increase relative to sediment-load values typical from Mount Rainier during the entire 20th century. One-dimensional river-hydraulic and sediment-transport models simulated the entrainment, transport, attrition, and deposition of bed material. Simulations showed that bed-material loads were largest for the Nisqually River and smallest for the Carbon River. The models were used to simulate how increases in sediment supply to rivers transport through the river systems and affect lowland reaches. For each simulation, the input sediment pulse evolved through a combination of translation, dispersion, and attrition as it moved downstream. The characteristic transport times for the median sediment-size pulse to arrive downstream for the Nisqually, Carbon, Puyallup, and White Rivers were approximately 70, 300, 80, and 60 years, respectively.
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Disparity between watershed erosion rates and downstream sediment delivery has remained an important theme in geomorphology for many decades, with the role of floodplains in sediment storage as a common focus. In the Piedmont Province of the eastern USA, upland deforestation and agricultural land use following European settlement led to accumulation of thick packages of overbank sediment in valley bottoms, commonly referred to as legacy deposits. Previous authors have argued that legacy deposits represent a potentially important source of modern sediment loads following remobilization by lateral migration and progressive channel widening. This paper seeks to quantify (1) rates of sediment remobilization from Baltimore County floodplains by channel migration and bank erosion, (2) proportions of streambank sediment derived from legacy deposits, and (3) potential contribution of net streambank erosion and legacy sediments to downstream sediment yield within the Mid-Atlantic Piedmont.
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The role of externally imposed sediment supplies on the evolution of meandering rivers and their floodplains is poorly understood, despite analytical advances in our physical understanding of river meandering. The Amazon river basin hosts tributaries that are largely unaffected by engineering controls and hold a range of sediment loads, allowing us to explore the influence that sediment supply has on river evolution. Here we calculate average annual rates of meander migration within 20 reaches in the Amazon Basin from Landsat imagery spanning 1985-2013. We find that rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads. Meander cutoff also occurs more frequently along rivers with higher sediment loads. Differences in meander migration and cutoff rates between the study reaches are not explained by differences in channel slope or river discharge. Because faster meander migration and higher cutoff rates lead to increased sediment-storage space in the resulting oxbows, we suggest that sediment supply modulates the reshaping of floodplain environments by meandering rivers. We conclude that imposed sediment loads influence planform changes in lowland rivers across the Amazon.
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Sediments of different size are transported in rivers under the action of flow. The first and still most popular sediment continuity model able to deal with mixed sediment is the so called active layer model proposed by Hirano [1971, 1972]. In this paper we consider the one-dimensional hydro-morpohodynamic model given by the Saint-Venant equations for free-surface flow coupled with the active layer model. We perform a mathematical analysis of this model, extending the previous analysis by Ribberink [1987], including full unsteadiness and grainsize-selectivity of the transported load by explicitly considering multiple sediment fractions. The presence of multiple fractions gives rise to distinct waves traveling in the downstream direction, for which we provide an analytical approximation of propagation velocity under any Froude regime. We finally investigate the role of different waves in advecting morphodynamic changes through the domain. To this aim, we implement an analytical linearized solver to analyze the propagation of small-amplitude perturbations of the bed elevation and grainsize distribution of the active layer as described by the system of governing equations. We find that initial gradients in the grainsize distribution of the active layer are able to trigger significant bed variations, which propagate in the downstream direction at faster pace than the “bed” wave arising from the uniform-sediment Saint-Venant-Exner model. We also verify that multiple “sorting” waves carry multiple associated bed perturbations, traveling at different speeds.
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We present a transport model for mixed sand/gravel sediments. Fractional transport rates are referenced to the size distribution of the bed surface, rather than subsurface, making the model completely explicit and capable of predicting transient conditions. The model is developed using a new data set of 48 coupled observations of flow, transport, and bed surface grain size using five different sediments. The model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions. The model uses the full size distribution of the bed surface, including sand, and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models. The model shares some common elements with two previous surface-based transport models, but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow, transport rate, and sediments.
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A model study evaluates sediment transport in a geomorphic channel proposed for restoration and flood damage reduction of an 11-km tidally influenced reach of the Napa River located in California. The model study employs the unsteady quasi-2D hydrodynamic and sediment transport model MIKE 11, the simplified marsh plain accretion model MARSH 98, and the Rouse equation to predict annual average morphological changes of the geomorphic channel. The adopted modeling approach allows for the simulation of salient sediment transport processes in a river estuary, including lateral and vertical sorting of sediments, and local flushing of fine cohesive and noncohesive sediments during flooding. Accretion rates, particularly within the marsh plain terrace of the multistage channel, are found to be within acceptable limits for project maintenance and ecosystem restoration purposes. This enhanced 1D modeling approach may offer a viable and cost-effective alternative compared to fully 2D and 3D models, with relatively less model set-up and run-time requirements.
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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. RÉSUMÉ
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Major changes in the morphology of the Trinity River in California, such as narrowing of the cross section and sedimentation of fine sediment in pools, occurred after the closure of a system of dams. These changes caused a dramatic reduction in the salmonid population and a resulting decline of the fishery. Gravel augmentation, regulated flood releases, and mechanical channel rehabilitation are currently being implemented to help restore the aquatic habitat of the river. The present paper describes a tool, named the Spawning Gravel Refresher, for designing and predicting the effects of gravel augmentation in gravel bed rivers. The tool assumes an imposed, cycled hydrograph. The model is calibrated and applied to the regulated reach of the Trinity River in four steps: (1) zeroing runs to reproduce conditions of mobile bed equilibrium as best can be estimated for the predam Trinity River, (2) runs to compare the predictions with the results of previous studies, (3) runs at an engineering time scale to reproduce the effects of the dams, and (4) runs to design gravel augmentation schemes. In the fourth group of runs, the combined effects of engineered flood flow releases and gravel augmentation are predicted. At an engineering time scale, the model indicates that the fraction of fine sediment in the surface layer and in the topmost part of the substrate should decrease when subjected to these two restoration measures, with a consequent improvement of the quality of the spawning gravel.
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Many existing numerical models of river channel morphology are limited in applicability because they neglect time-dependent changes in channel width. In this paper, the status of held-based and numerical modeling approaches for assessing river channel width adjustments is reviewed. This review complements the review of fluvial hydraulics and bank mechanics presented in the companion paper. In addition to describing a field-based approach for assessing channel width adjustments, the quantitative time-dependent models of width adjustment that are currently available are described. Relatively few numerical models of width adjustment have been developed to date, and many processes of width adjustment have never been successfully quantified. Existing models are research tools and are not yet ready to be adopted in widespread engineering practice. An interdisciplinary approach to the analysis and modeling of river width adjustments is presented. The hierarchical approach to analysis of width adjustment is based on field, analytical, and numerical modeling techniques. The principal limitations of existing field-based and numerical modeling approaches are listed. A List of recommendations for further research is provided.
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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 non-cohesive 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. © 2006 International Association of Hydraulic Engineering and Research.
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In gravel bed rivers, bed topography and the bed surface grain size distribution evolve simultaneously, but it is not clear how feedbacks between topography and grain sorting affect channel morphology. In this, the second of a pair of papers examining interactions between bed topography and bed surface sorting in gravel bed rivers, we use a two-dimensional morphodynamic model to perform numerical experiments designed to explore the coevolution of both free and forced bars and bed surface patches. Model runs were carried out on a computational grid simulating a 200 m long, 2.75 m wide, straight, rectangular channel, with an initially flat bed at a slope of 0.0137. Over five numerical experiments, we varied (a) whether an obstruction was present, (b) whether the sediment was a gravel mixture or a single size, and (c) whether the bed surface grain size feeds back on the hydraulic roughness field. Experiments with channel obstructions developed a train of alternate bars that became stationary and were connected to the obstruction. Freely migrating alternate bars formed in the experiments without channel obstructions. Simulations incorporating roughness feedbacks between the bed surface and flow field produced flatter, broader, and longer bars than simulations using constant roughness or uniform sediment. Our findings suggest that patches are not simply a by-product of bed topography, but they interact with the evolving bed and influence morphologic evolution.
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The morphology of an alluvial river channel affects the movement of water and sediment along it, but in the longer run is shaped by those processes. This interplay has mostly been investigated empirically within the paradigm of Newtonian mechanics. In rivers this has created an emphasis on equilibrium configurations with simple morphology and uniform steady flow. But transient adjustment, whether between equilibrium states or indefinitely, is to be expected in a world in which hydrology, sediment supply, and base level are not fixed. More fundamentally, water flows and all the phenomena that accompany them are inherently unsteady and flows in natural channels are characteristically non-uniform. The morphodynamics of alluvial river channels is the striking consequence. In this paper we develop the essential connection between the episodic nature of bed material transport and the production of river morphology, emphasizing the fundamental problems of sediment transport, the role of bar evolution in determining channel form, the role of riparian vegetation, and the wide range of timescales for change. As the key integrative exercise, we emphasize the importance of physics-based modeling of morphodynamics. We note consequences that can be of benefit to society if properly understood. These include the possibility to better be able to model how varying flows drive morphodynamic change, to understand the influence of the sediments themselves on morphodynamics, and to recognize the inherent necessity for rivers that transport bed material to deform laterally. We acknowledge pioneering contributions in WRR and elsewhere that have introduced some of these themes. This article is protected by copyright. All rights reserved.
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Information about the grain-size distribution of the surface layer of sediment exposed on riverbeds is often critical in studies of fluvial hydraulics, geomorphology, and ecology. A variety of sampling and analysis techniques are in common usage that produce grain-size distributions that are not directly comparable. This paper seeks to explore the appropriate conversions between different types of surface grain-size sampling methods. This is particularly timely in the light of increasingly widespread use of automatic and semiautomatic image-based measurement methods, the comparability of which with conventional measurement methods is relatively poorly constrained. For conversions between area-by-number (paint-and-pick) and grid-by-number (pebble-count) samples, the empirically derived conversion factor (+/- 2.2) was found to be greater than that predicted by the Kellerhals and Bray model (+/- 2), but the errors associated with using the value predicted by the model were small (3.8% in mm). For conversions between areal samples recorded by count and weight, the empirically derived conversion factor was approximately +/- 2.9, but the use of the value predicted by the Kellerhals and Bray model (+/- 3) resulted in only small errors (5.2% in mm). Similarly, for conversions between image-based grain-size distributions recorded in area-by-number and grid-by number form, the emipirically derived conversion factor was +/- 1.9, but using the model value of +/- 2 resulted in only small errors (4.1% in mm). Although these results are specific to the data sets analyzed, the variety of sedimentary conditions included gives the authors of this study confidence that the results are representative. DOI: 10.1061/(ASCE)HY.1943-7900.0000595. (C) 2012 American Society of Civil Engineers.
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The predictions from a numerical sediment transport model inevitably include uncertainty because of assumptions in the model's mathematical structure, the values of parameters, and various other sources. In this paper, the writers aim to develop a method that quantifies the degree to which parameter values are constrained by calibration data and the impacts of the remaining parameter uncertainty on model forecasts. The method uses a new multiobjective version of generalized likelihood uncertainty estimation. The likelihoods of parameter values are assessed using a function that weights different output variables on the basis of their first-order global sensitivities, which are obtained from the Fourier amplitude sensitivity test. The method is applied to Sedimentation and River Hydraulics-One Dimension (SRH-1D) models of two flume experiments: an erosional case and a depositional case. Overall, the results suggest that the sensitivities of the model outputs to the parameters can be rather different for erosional and depositional cases and that the outputs in the depositional case can be sensitive to more parameters. The results also suggest that the form of the likelihood function can have a significant impact on the assessment of parameter uncertainty and its implications for the uncertainty of model forecasts. DOI: 10.1061/(ASCE)HY.1943-7900.0000343. (C) 2011 American Society of Civil Engineers.
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The Mississippi River delta is undergoing a catastrophic drowning, whereby 5,000 km2 of low-lying wetlands have converted to open water over at least the past eight decades, as a result of many anthropogenic and natural factors. Continued net land loss has been thought inevitable due to a decline in the load of total suspended sediment--both sand and mud--carried by the river. However, sand--which accounts for ~50-70% of modern and ancient Mississippi delta deposits but comprises only ~20% of the sampled portion of the total load--could be more important than mud for subaerial delta growth. Historically, half of the Mississippi River sediment load is supplied by the Missouri River. Here we analyse suspended sediment load data from two locations downstream from the lowest Missouri River dam to show that the measured sand load in the lower 1,100 km of the Mississippi River has not significantly diminished since dam construction. A one-dimensional numerical model of river morphodynamics predicts that the sand load feeding the delta will decrease only gradually over the next several centuries, with an estimated decline from current values of no more than about 17% within the coming six centuries. We conclude that the lower Mississippi River channel holds a significant reservoir of sand that is available to replenish diminished loads via bed scour and substantially mitigate land loss.
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Meandering rivers display active communication between bank erosion and bar deposition processes. How does this occur? How does the river select its width? To answer these questions, we implement a model for meander migration where both bank processes (erosion and deposition) are considered independently. Bank erosion is modeled as erosion of purely non-cohesive bank material damped by natural slump block armoring; channel deposition is modeled via flow-retarded vegetal encroachment. Both processes are tied to a slope-dependent channel forming Shields number; banks with near-bank Shields number below this value undergo deposition, and those above it undergo erosion. Channel-forming Shields number must increase with slope, as dictated by available data and model performance. Straight channel modeling shows that a channel arrives at an equilibrium width from any initial condition. For the channel bend, the river always approaches an asymptotic state where width reduces slowly in time and where bank erosion and deposition occur at nearly equal rates. Before this state is reached, however, the river follows a phase-plane trajectory with four possible regimes: a) both banks erode, b) both banks deposit, c) both banks migrate outward, but with a faster depositing bank (bar push), and d) both banks migrate outward, but with a faster eroding bank (bank pull). The trajectory of migration on the phase plane depends on initial conditions and input parameters controlling the rate of depositional and erosional migration. All input parameters have specific physical meaning, and the potential to be measured in the field.
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As regards river restoration, it is fundamental to better link human pressures and environmental responses and to take into consideration not only target species or habitat but diverse ecological elements. This permits to assess sustainable restoration plan, especially concerning sediment augmentation below dams. The use of a hierarchical multicriteria approach on the Ain River permits us to assess a diagnosis of sediment deficit impact integrating several morphological (channel shifting, river bed degradation and river bed coarsening) and ecological components (Riparian and floodplain lake and fish communities). Our diagnosis also integrates a temporal and spatial approach better to link human pressures and environmental responses and to identify the dam effects amongst other drivers (e.g. grazing decline and channel regulation). The results confirm causality links between sediment deficit and slight channel bed degradation (0.01 m.year−1) or channel bed paving and thus highlight the impact of the dam on the drying of the riparian forest and on former channel community. However, the relationship between incision and reduction in active channel lateral mobility is more difficult to establish. The role of sediment deficit in the current variability of the riparian regeneration capacity and, thereby, landscape diversity along the lower valley remains unclear. This study also confirms the relevance of using different ecological indicators, notably because all components present different adjustment time scales, whereas some of them are more sensitive to other impacts. Copyright © 2013 John Wiley & Sons, Ltd.
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Many gravel bed streams have a typical bed morphology consisting of pool-riffle sequences, which provides important habitat diversity both in terms of flow and substrate. A complete explanation of pool-riffle genesis and self-maintenance remains elusive and, despite advances in understanding the effects of flow spatial and temporal variability, the key sediment processes have been only marginally explored. Here we use a 1D unsteady multi-fraction morphodynamic model to explain the formation and degradation of pool-riffle sequences. Using a 1-year time series of measured flows below bankfull on a stream in which we have removed initial bedforms and sediment sorting our model spontaneously generates pools with finer substrate at narrow sections and riffles with coarser sediment at wider sections, closely resembling the natural bed morphology. Additional experiments show that under our modelling assumptions a variable flow regime is fundamental for development and self-maintenance of the longitudinal grain sorting characteristic of pool-riffle sequences, which could not be obtained or maintained with discharges held constant over relatively long periods.
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Distributions of distance of bedload particle movement were examined in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles. The compound Poisson model of Einstein-Hubbell-Sayre and a simple gamma function model were compared with observed distributions of moved particles, and of all particles. Both models fit the data reasonably well for small mean displacements, but notable misfits occurred in an event with large mean displacement. When mean particle travel distance approaches the scale of bar spacing, trapping in the bars interrupts particle progress and the dispersion process. The data remain very noisy, so definitive discrimination of suitable models will require trials with more than 103 particles.
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Developing accurate long-term, basin-scale sediment budgets using isotopic sediment fingerprints requires a sediment routing model that not only accounts for a range of sediment source terms (e.g. tributaries, surface erosion and erosion of bluffs and terraces) but also considers the variation in time of volume and tracer concentration for the sediment stored in the floodplain. This is accomplished here using a tracer routing model that accounts for production and decay of radioisotopes in the floodplain. The numerical model focuses on the average (i.e. across many hydrographs or years) budget of sediment and tracers at reach scale. To account for storage and remobilization of bulk sediment and/or tracer material, the model represents the floodplain as a system that can gain or lose mass depending on overbank deposition and net bank erosion rates. Isotopic tracers within the floodplain reservoir can be produced as a function of cosmic ray bombardment or atmospheric fallout, and can decay according to a first-order rate equation. Governing equations are derived using a simplified geometry that treats rivers at reach scale: channel sinuosity and migration rates are user-specified parameters, exchange of sediment and tracers between the river and floodplain is modeled at each cross section, and governing equations are derived in a 1D, width-averaged formulation. When the system reaches mobile equilibrium, the sediment deposited on the floodplain through overbank deposition is balanced by the sediment eroded from the floodplain through channel migration and by sediment contributed from external sources. The model is applied to a generic river system and is shown to converge over time to an equilibrium condition that is consistent with an independent analytical solution.
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A numerical model has been developed for the routing of gravel-sized sediment along a river channel which is free to adjust both its long profile and surface texture. Hydraulic calculations use a step-backwater approach, and sediment transport is predicted with the method of Parker (1990a), which uses a low degree of size selectivity. Exchange of sediment between the surface and subsurface is described using the modified Exner equation of Parker and Sutherland (1990). The model is applied to an idealized channel based on the highly concave Allt Dubhaig, Scotland, in which fining by particle wear is minor. The rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to
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A theory for the movement of suspendible sediment through the floodplain of a simple actively migrating river is developed. The floodplain is represented as a series of sediment storage reservoirs, each incorporating multiple bends of a river. Independent models for floodplain sediment deposition and for subsequent removal of sediment by lateral channel migration allow the volume of sediment stored in the floodplain (and the associated average floodplain elevation) to change over time and down the valley. At a given quasi-steady discharge, deposition depends on a simple one-dimensional partition between channel and floodplain discharge and on suspended sediment concentration (one size class) which varies along the reach depending on lateral supply and floodplain history. Sediment supply is specified at the upstream end of the reach using a suspended sediment rating curve. Long-term average overbank deposition rates are approximated by weighting multiple steady state solutions using a flow duration curve. The channel bed elevation of the system is assumed to be fixed, and tributaries are handled using simple lateral source terms. The interaction between the submodels is such that under constant boundary conditions, the channel evolves toward a state of geomorphic equilibrium where the volume of sediment added to the floodplain by overbank deposition just balances the volume removed by channel migration. The model is easily adapted to track conservative suspended sediment tracer material concentration as such material moves through the channel/floodplain system.
Article
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.
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Bed load samples from four locations in the Trinity River of northern California are analyzed to evaluate the performance of the Wilcock-Crowe bed load transport equations for predicting fractional bed load transport rates. Bed surface particles become smaller and the fraction of sand on the bed increases with distance downstream from Lewiston Dam. The dimensionless reference shear stress for the mean bed particle size (τ*rm) is largest near the dam, but varies relatively little between the more downstream locations. The relation between τ*rm and the reference shear stresses for other size fractions is constant across all locations. Total bed load transport rates predicted with the Wilcock-Crowe equations are within a factor of 2 of sampled transport rates for 68% of all samples. The Wilcock-Crowe equations nonetheless consistently under-predict the transport of particles larger than 128 mm, frequently by more than an order of magnitude. Accurate prediction of the transport rates of the largest particles is important for models in which the evolution of the surface grain size distribution determines subsequent bed load transport rates. Values of τ*rm estimated from bed load samples are up to 50% larger than those predicted with the Wilcock-Crowe equations, and sampled bed load transport approximates equal mobility across a wider range of grain sizes than is implied by the equations. Modifications to the Wilcock-Crowe equation for determining τ*rm and the hiding function used to scale τ*rm to other grain size fractions are proposed to achieve the best fit to observed bed load transport in the Trinity River.
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We investigate how well a width-averaged morphodynamic model can simulate gravel transport and aggradation along a highly irregular 38-km reach of lower Fraser River and discuss critical issues in this type of modeling. Bed load equations with plausible parameter values predict a gravel input consistent with direct measurements and a sediment budget. Simulations using spatially varying channel width, and forced by dominant discharge or a 20-year hydrograph, match the observed downstream fining well. They reproduce major qualitative features of a 47 year sediment budget and match maximum local rates of aggradation and degradation to well within a factor of 2. Locations and rates of aggradation and degradation are influenced by channel constrictions. Runs for single years of unusually high or low peak discharge suggest that the main transfers of gravel occur in different parts of the reach in different years. One-dimensional morphodynamic modeling of highly nonuniform rivers has serious conceptual limitations but may be a valuable complement to empirical approaches.
Article
A theory for the evolution of a river floodplain and the associated movement of tracer sediment is applied to the case of the upper Clark Fork River, Montana, whose floodplain is contaminated with tailings derived from historical mining activities. Deposition of mine tailings raised the elevation of the floodplain, which now contributes contaminated sediment to the channel. The theory is capable of reproducing the basic aspects of the system's evolution, including aggradation of the floodplain during the mining era and subsequent net supply of sediment from floodplain to channel after the cessation of mine input. Results indicate that the natural removal of tailings from the floodplain could take thousands of years, a period longer than is required for the channel to simply rework the tailings. The self-stabilizing nature of the model results in an evolution toward a new postmine steady state, with tailings eventually almost entirely removed from the system.
Article
This study describes changes in mean channel-bed elevation, channel width, bed-material sizes, vegetation, water discharges, and sediment loads downstream from 21 dams constructed on alluvial rivers. Most of the studied channels are in the semiarid western US. Flood peaks generally were decreased by the dams, but in other respects the post-dam water-discharge characteristics varied from river to river. Sediment concentrations and suspended loads were decreased markedly for hundreds of kilometers downstream from dams; post-dam annual sediment loads on some rivers did not equal pre-dam loads anywhere downstream from a dam. Bed degradation varied from negligible to about 7.5 meters in the 287 cross sections studied. In general, most degradation occurred during the first decade or two after dam closure. Bed material initially coarsened as degradation proceeded, but this pattern may change during later years. Channel width can increase, decrease, or remain constant in the reach downstream from a dam. Despite major variation, changes at a cross section in stream bed elevation and in channel width with time often can be described by simple hyperbolic equations. Equation coefficients need to be determined empirically. Riparian vegetation commonly increased in the reach downstream from the dams, probably because of the decrease in peak flows. 120 references, 49 figures, 14 tables.
Chapter
This book offers a comprehensive overview of progress in the general area of fluvial remote sensing with a specific focus on its potential contribution to river management. The book highlights a range of challenging issues by considering a range of spatial and temporal scales with perspectives from a variety of disciplines. The book starts with an overview of the technical progress leading to new management applications for a range of field contexts and spatial scales. Topics include colour imagery, multi-spectral and hyper-spectral imagery, video, photogrammetry and LiDAR. The book then discusses management applications such as targeted, network scale, planning, land-use change modelling at catchment scales, characterisation of channel reaches (riparian vegetation, geomorphic features) in both spatial and temporal dimensions, fish habitat assessment, flow measurement, monitoring river restoration and maintenance and, the appraisal of human perceptions of riverscapes. Key Features: A specific focus on management applications in a period of increasing demands on managers to characterize river features and their evolution at different spatial scales. An integration across all scales of imagery with a clear discussion of both ground based and airborne images. Includes a wide-range of environmental problems. Coverage of cutting-edge technology. Contributions from leading researchers in the field.
Article
Welcome to the Hydrologic Engineering Center's River Analysis System (HEC-RAS). This software allows you to perform one-dimensional steady flow, unsteady flow, and sediment transport calculations. The current version of HEC-RAS only supports one-dimensional, steady flow, water surface profile calculations. This manual specifically documents the hydraulic capabilities of the Steady flow portion of HEC-RAS. Documentation for unsteady flow and sediment transport calculations will be made available as these features are added to the HEC-RAS. This chapter discusses the general philosophy of HEC-RAS and gives you a brief overview of the hydraulic capabilities of the modeling system. Documentation for HEC-RAS is discussed, as well as an overview of this manual.
Article
As a river-carrying sediment mixture aggrade, it creates a stratigraphic signature that records this evolution. This stratigraphy is characterized by the vertical/horizontal variation of substrate grain size distribution. If a river degrades, it mines this stratigraphy, and transfers the sediment so accessed farther downstream. Although several numerical models tracking the creation/consumption of stratigraphy are available, none has been tested against experiments under plane bed regime conditions. Here nine physical experiments modelling the creation/consumption of stratigraphy are described. These are compared with nine corresponding numerical experiments using a model that tracks stratigraphy. The results justify the numerical model, and in particular the scheme to track stratigraphy. This scheme can be used at field scale to characterize e.g. the response of a river to an increased/decreased sediment supply. The numerical model shows a discrepancy with the experiments, however, whenever a distinct delta front forms, because the model does not describe sediment sorting across an avalanche face.
Article
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.
Article
Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed-levels by as much as 6·1 m. Following degradation, reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0·12 m per year with the greatest rates occurring near the stream mouths. The adjustment of channel geometry and phases of channel evolution are characterized by six process-oriented stages of morphologic development—premodified, constructed, degradation, threshold, aggradation, and restabilization. Down-cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the ‘slough line’ develops as an initial site of lower-bank stability. The bank profile develops three dynamic elements (1) vertical face (70° to 90°), (2) upper bank (25° to 50°), and (3) slough line (20° to 25°). Alternate channel bars form during the restabilization stage and represent incipient meandering of the channel.
Article
The path length (downstream displacement over a given time period) of individual bed particles in gravel-bed rivers is central to morphological methods for measuring bed load transport rate and is also fundamental to understanding the bed load transport process and the development of channel morphology. Previous studies of particle movement using tracers report predominantly strongly positively skewed frequency distributions of path length with modes close to the point of entrainment. However, gravel-bed rivers often have regularly spaced erosion (scour pools) and deposition (channel bars) sites that are several channel widths apart and it is reasonable to expect that particle path length would reflect this morphological scale, at least during flows large enough to create and modify the morphology. Here, we synthesize and re-analyze results from published bed load tracing experiments in gravel-bed rivers to identify the variety of possible path length distributions for differing channel morphology, channel dimensions, bed particle size, and particle mobility (i.e. flow magnitude) and to look for occurrences of path length coinciding with the length scale of the morphology. The results show that path length distributions may be positively skewed, symmetrical, and uni-, bi-, or multi-modal and may include modes that coincide with known or expected pool–bar spacing. Primary path length modes equivalent to possible pool–bar spacing are more probable at higher non-dimensional bed shear stress, from which it is inferred that both particle mobility and channel morphology exert an influence on particle path lengths and that particle movement is unlikely to be stochastic except at relatively low particle mobility. Existing data are inadequate for more than a preliminary analysis of this problem consequently there is a need for new data collected explicitly and systematically to confirm these preliminary results, isolate the effect of the several variables that influence the characteristics of path length frequency distributions and identify the conditions under which path length coincides with the length scale of the dominant morphology.
Article
Near-future climate change will affect the discharge and base level of rivers and thus cause channel changes. The nature and pace of such changes can be simulated using morphodynamic models. As part of an investigation of how the changing hydrology of the St-Lawrence River, Quebec, Canada, will affect its tributaries we have made additions and modifications to a one-dimensional morphodynamic model developed for gravel-bed rivers (SEDROUT). The changes allow simulation of sand-bed rivers, variable discharge, downstream water level fluctuations, and flow and sediment routing in channels with islands. A revised formulation for calculating the grain size distributions of the surface and subsurface material is presented to allow for alternating sedimentation and erosion. We test the enhanced model using small-scale simulations and present-day conditions in four tributaries of the St-Lawrence River. The model is calibrated and validated for the tributaries and the capability to simulate river morphology over a 100-year period is tested. Good validation agreement on water level, cross-sectional mean velocity, and sediment transport rate is obtained for the four tributaries of the St-Lawrence River. With these modifications, modelling a very wide range of river morphodynamic problems is now possible.
Article
Erosion from the banks of meandering rivers causes a local influx of sediment to the river channel. Most of the eroded volume is usually replaced in nearby point bars. However, the typical geometry of river bends precludes the local replacement of all eroded material because (i) point bars tend to be built to a lower elevation than cutbanks and (ii) point bars tend to be shorter than the eroding portion of cutbanks because of channel curvature. In a floodplain that is in equilibrium (i.e., neither increasing nor decreasing in volume), sediment eroded from cut banks must be replaced elsewhere on the floodplain. The local imbalance caused by differences in bank height should be balanced primarily by overbank deposition, while the local imbalance caused by curvature should be balanced primarily by deposition in abandoned stream courses or oxbow lakes. Estimates of these local imbalances based on remotely sensed measurements of bank geometry and channel migration have been made on four systems in the United States: a 91-km reach of the Pearl River in Louisiana and Mississippi, a 62-km reach of the Bogue Chitto River in Louisiana, a 35-km reach of the Neuse River in North Carolina, and a 2.7-km reach of the Vermillion River in Minnesota. For these systems, the total local imbalance, integrated over many bends, ranged from 0.36 to 2.27 m3/yr/m of valley length (0.15 to 1.32 m3/yr/m of channel length), or 7 to 45% of gross cutbank erosion, with a typical value of about 17%. When compared with gauged suspended sediment data, the measurements provide estimates of the relative importance of floodplains for storing material transported by a river system. The data suggest that even if the studied systems are near long-term mass balance equilibrium (as opposed to undergoing net deposition or erosion), almost all of the sand and in some cases much of the silt and clay in transit through these systems is likely to have spent some time stored in an upstream floodplain.
Article
Livingston Dam on the Trinity River in SE Texas, USA disrupts the transport of sediment to the lower Trinity River and the Trinity Bay/Galveston Bay estuary. However, a sediment budget of the lower basin shows that the effects of this disruption are undetectable in the lower river. Sediment trapped in Lake Livingston is partly offset by channel erosion downstream of the dam and by inputs from the lower basin. Most importantly, however, the lower coastal plain reaches of the Trinity are characterized by extensive alluvial storage and are a bottleneck that buffers the bay from effects of upstream changes in sediment flux. Storage is so extensive that the upper Trinity basin and the lowermost river reaches were essentially decoupled (in the sense that very little upper-basin sediment reached the lower river) long before the dam was constructed. Whereas sediment storage in Lake Livingston is extensive, alluvial storage on the Trinity flood plain is even more extensive. Dam-related sediment starvation effects are noted for about 52 km downstream, and the sediment budget suggests that a majority of the sediment in this reach is likely derived from channel scour and bank erosion. The capacious alluvial storage in the lower Trinity not only limits flux to the bay, but the large amount of remobilizable alluvium also allows the system to adjust to localized sediment shortages, as illustrated in the dam-to-Romayor reach. Internal adjustments within the lower Trinity River valley thus buffer the bay from changes in sediment supply upstream.