DEVELOPMENT AND APPLICATION OF A CATCHMENT SCALE SEDIMENT ROUTING MODEL

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 sub-basins affected by large dams and wildfire. Model results highlight the importance of hydrologic conditions on post-wildfire sediment yield, and illustrate the spatial extent of dam-induced sediment deficit during a flood. We also combined SeRFE iv outputs for the mainstem Santa Clara River with a simple recruitment model of the invasive riparian plant Arundo donax, which is prevalent in the basin. This coupled modeling approach provided an ecogeomorphic framework for predicting source and sink areas of the plant. Results for these different scenarios highlight how 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

Sediment source fingerprinting as an aid to large-scale landscape conservation and restoration: A review for the Mississippi River Basin

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

Discharge-Suspended Sediment Relations: Near-channel Environment Controls Shape and Steepness, Land Use Controls Median and Low Flow Conditions

We analyzed recent total suspended solids (TSS) data from 45 gages on 36 rivers throughout the state of Minnesota. Watersheds range from 32 to 14,600 km2 and represent a variety of distinct settings in terms of topography, land cover, and geologic history. Our study rivers exhibited three distinct patterns in the relationship between discharge and TSS: simple power functions, threshold power functions, and peaked or negative power functions. Differentiating rising and falling limb samples, we generated sediment rating curves (SRC) of form TSS = aQb, Q being normalized discharge. Rating parameters a and b describe the vertical offset and steepness of the relationships. We also used the fitted SRCs to estimate TSS values at low flows and to quantify event-scale hysteresis. In addition to quantifying the watershed-average topographic, climatic/hydrologic, geologic, soil and land cover conditions, we used high-resolution lidar topography data to characterize the near-channel environment upstream of gages. We used Random Forest statistical models to analyze the relationship between basin and channel features and the rating parameters. The models enabled us to identify morphometric variables that provided the greatest explanatory power and examine the direction, form, and strength of the partial dependence of the response variables on individual predictor variables. The models explained between 43% and 60% of the variance in the rating curve parameters and determined that Q-TSS relation steepness (exponent) was most related to near-channel morphological characteristics including near-channel local relief, channel gradient, and proportion of lakes along the channel network. Land use within the watershed explained most variation in the vertical offset (coefficient) of the SRCs and in TSS concentrations at low flows

Modeling Particle Dispersal in Bedload Dominated Settings

Tracer particles are used to study bedload transport in gravel bed rivers. One of the advantages associated with their use is that they allow for direct measurements of particle entrainment rates in bedload transport and particle displacement. The main issue in field studies with tracer particles is the difference between tracer short term and long term behavior. This difference is due to the fact that particles undergo vertical mixing or move to less active locations such as bars or even floodplains. For these reasons the mean tracer velocity decreases over time. This phenomenon has been called tracer slowdown and it can have a significant impact in estimating the bedload transport or in modeling the dispersal of contaminated sand and gravels. The vast majority of the morphodynamic models that account for the non-uniformity of the bed material (tracer and not tracer, in this case) are based on a discrete description of the alluvial deposit. The deposit is divided in two different regions; the active layer and the substrate. The active layer is a thin layer in the topmost part of the deposit in which particles can interact with the bed material transport. The substrate is the part of the deposit below the active layer. Due to the discrete representation of the alluvial deposit, active layer models are not able to reproduce tracer slowdown. To overcome some of the limitations of layer-based models, Parker and co-authors introduced probabilistic, not layer-based morphodynamic framework. This framework is based on a probabilistic description of the temporal variation of bed surface elevation associated with sediment transport processes, and it is used herein to model the dispersal of tracer particles. Particle entrainment rates are computed as a function of the flow and sediment characteristics, and particle deposition is modeled with a step length formulation. Here we present one of the first implementation of the probabilistic framework at laboratory scale, validate it against laboratory data, and then we use the validated model to investigate some of the characteristics of tracer dispersal at laboratory and field scales

Identifying and Quantifying Sediment Sources and Sinks in the Root River, Southeastern Minnesota

Currently, our ability to predict the flux of fine sediment at the watershed scale is limited by the precision of erosion rate estimates for the many potential sources distributed throughout a landscape as well as our understanding of the connectivity of sediment pathways during transport. In absence of a robust predictive model which can be validated by measurements of sediment fluxes and use of geochemical tracers. Predicting fine sediment yield at the watershed scale requires multiple redundant lines of information. This thesis outlines the methods used, and the data sets collected in the Root River watershed in Southeastern Minnesota, all of which are multiple lines of evidence to the sediment dynamics in the Root River. The research indicates that the Root River is a very dynamic watershed. The hydrologic regime of the watershed has shifted over the last half century. Due to this shift sediment fluxes are very dependent of the magnitude and sequence of events. Geomorphic analysis of the landforms and the use of a developed tool, TerEx, indicate that many reaches of the river have easily accessible near-channel sources of sediment. Sediment fingerprinting results illustrate that source tracer concentrations are variable across the landscape, that as a whole, upland sources are still a major contributor to the suspended sediment load, and that in some sub-watersheds near-channel sources are dominate in the suspended load. Over all the channel-floodplain exchange exerts strong control on the flux of sediment through this river system

The Study of Flow Hydrodynamics and Transport Processes Over Mixed Bedrock-Alluvial Reaches

Research efforts on mixed bedrock-alluvial rivers primary focused on the bedrock incision and very few studies investigated the alluvial morphodynamics of such systems. To the best of my knowledge none of these models have been considered the spatial variability of the sediment grain size of the bed surface in mixed bedrock-alluvial reaches and very few models focused on the spatial changes in alluvial cover within these reaches. Furthermore, a perusal of the literature on mixed bedrock-alluvial river morphodynamics reveals that very little information is available on 1) bedform geometry and flow resistances and 2) sediment sorting patterns in presence of a non-erodible bedrock surface. Understanding the interactions between flow and sediment transport processes in mixed bedrock-alluvial reaches is important to e.g. predict the long-term river response to engineering works, changes in climate and sediment supply; perform large scale sediment budgets; and determine the quality of the riparian habitat. I thus designed and performed laboratory experiments to investigate the effects of a model bedrock surface on flow hydraulics and sediment transport processes. I derived a novel mathematical formulation of mixed bedrock-alluvial morphodynamics that accounts for the non-uniformity of the bed material. I implemented this formulation in a one- dimensional model of river morphodynamics. The experiments revealed that equilibrium in mixed bedrock-alluvial reaches is characterized by flow acceleration in the streamwise direction when the slope of the bedrock surface is milder than the equilibrium slope of an alluvial reach transporting the same discharge and sediment load. The morphodynamic response to this spatial flow acceleration is characterized by 1) streamwise reduction in the alluvial cover, 2) streamwise reduction in bedform height, and 3) formation of a pattern of downstream fining of the bed surface sediment. The morphodynamic model was validated at laboratory scale against the experimental results. The validated model was then used to study the changes in flow hydraulics and sediment transport processes in mixed bedrock-alluvial reaches with a bedrock surface slope that was steeper than the alluvial equilibrium slope of a channel subjected to the same discharge and sediment supply of the mixed bedrock-alluvial reach of interest. The numerical results at equilibrium show that in this case flow velocity decreased on the mixed bedrock-alluvial reach in streamwise direction. The morphodynamic effects of this spatial flow deceleration were 1) a streamwise increase in alluvial cover, and 2) the formation of a pattern of downstream coarsening of the bed surface sediment. The morphodynamic formulation presented in this dissertation will be applied at field scale on the gravel bed Buech River, Southeastern France, to study the impacts of dam construction and gravel mining on a mixed bedrock-alluvial gravel bed river, and to identify possible restoration strategies to control the observed widespread erosion and the associated deterioration of the aquatic and riparian habitat

Numerical modeling of traces in gravel-bed rivers

2012 - 2013The erosion, transport and deposition of pebbles in rivers have often been studied by considering the motion of tracer particles. There are reports of bedload tracing programs in field and laboratory since the late 1930s. The theoretical basis for the study of the dispersal of sediment tracer particles was delineated for the first time in 1950 by Einstein, who formulated the problem in terms of a standard 1D random walk in which each particle moves in a series of steps punctuated by waiting times. Subsequent to Einstein’s original work on tracers, the study of random walks has been extended to the case of continuous time random walks (CTRW). CTRW, accompanied by appropriate probability distribution functions (PDFs) for walker step length and waiting time, yields asymptotically the standard advectiondiffusion equation (ADE) for thin-tailed PDFs, and the fractional advection-diffusion equation (fADE) for heavy-tailed PDFs, the latter allowing the possibilities of subdiffusion or superdiffusion of particles, which is often referred as non-local behavior or anomalous diffusion. In latest years, considerable emphasis has been placed on non-locality associated with heavy-tailed PDFs for particle step length. This appears to be in part motivated by the desire to construct fractional advective-diffusive equations for pebble tracer dispersion corresponding to the now-classical fADE model. Regardless of the thin tail of the PDF, the degree of non-locality nevertheless increases with increasing mean step length. In the thesis, we firstly consider the general case of 1D morphodynamics of an erodible bed subject to bedload transport analysing the effects of non-locality mediated by both heavyand thin-tailed PDFs for particle step length on transient aggradational- degradational bed profiles. Then, we focus on tracers. (i) We show that the CTRW Master Equation is inappropriate for river pebbles moving as bed material load and (ii) by using the Parker-Paola-Leclair (PPL) framework for the Exner equation of sediment conservation, which captures the vertical structure of bed elevation variation as particles erode and deposit, we develop a new ME for tracer transport and dispersion for alluvial morphodynamics. The new ME is derived from the Exner equation of sediment continuity and it yields asymptotic forms for ADE and fADE that differ significantly from CTRW. It allows a) vertical dispersion, as well as streamwise advection-diffusion, and b) mean waiting time to vary in the vertical. We also show that vertical dispersion is nonlocal (subdiffuive), but cannot be expressed with fractional derivatives. Vertical dispersion is the likely reason for the slowdown of streamwise advection of tracer pebbles observed in the field, which is the key result of our modeling when co-evolution of vertical and streamwise dispersion are considered. [edited by author]XII n.s

EROSION ET TRANSFERTS HYDROSEDIMENTAIRES DANS LES BASSINS VERSANTS : apports de la caractérisation physique des particules pour la compréhension et la modélisation des processus

Alors que la formation d’un sol se compte en milliers d’années, sa disparition partielle ou totale peut survenir localement en quelques heures en réponse à un épisode pluvieux. Cette balance de temps largement déséquilibrée place l’érosion hydrique comme une des principales menaces pesant sur les sols à l’échelle mondiale. Et si les sols disparaissaient progressivement? Outre les pertes locales qui limitent les capacités ultérieures de production alimentaire, le transfert des particules fine vers l’aval peut entraîner l’envasement progressif des ouvrages, hydroélectriques par exemple, ou générer une dégradation de la qualité des masses d’eau par apport de nutriments (Carbone, Azote, Phosphore,...) ou de contaminants (pesticides, métaux,…) adsorbés. A l’échelle des bassins versants de petite et moyenne échelle (quelques centaines de km²) où ces problématiques peuvent être très marquées, les modèles numériques à base physique capables de simuler de manière spatialement distribuée des flux particulaires sont des outils très attendus par les décideurs. Malheureusement, les capacités prédictives de ces modèles sont encore très limitées. Parmi les différentes raisons expliquant la mise en défaut de ces modèles, la mauvaise prise en compte de la dynamique spatio-temporelle de la production et du transfert des flux solides à l’intérieur des bassins versants constitue un verrou majeur. Avancer dans la compréhension des flux particulaires implique donc une analyse des mécanismes basée sur d’autres grandeurs que les débits liquides et solides mesurés classiquement aux exutoires. De la même manière que la caractérisation de la fraction dissoute a permis ces dernières décennies d’améliorer la compréhension des fonctionnements hydrologiques des bassins versants en précisant l’origine, les chemins et les vitesses de l’eau, l’essentiel de notre approche consiste à revisiter les mécanismes contrôlant les transferts hydrosédimentaires en s’intéressant principalement aux caractéristiques physiques de la fraction particulaire en suspension. Par caractéristiques physiques des particules nous entendons à la fois les propriétés géométriques (tailles, formes, densités, vitesses de chute) qui vont conditionner les temps de transfert et les propriétés physico-chimiques (résistances à la désagrégation, propensions à la floculation, signatures spectrales) qui renseignent sur l’origine spatiale. Dans ce mémoire nous présentons pourquoi la mise en place de cette démarche nous a amené à développer des méthodologies ou des instruments originaux, que ce soit au laboratoire ou in-situ. Nous illustrons ensuite comment cette démarche peut nous amener par exemple à mieux comprendre le rôle de la variabilité spatio-temporelle des pluies dans la réponse hydrosédimentaire d’un bassin versant ou encore à identifier des processus clés de l’érosion non pris en compte à ce jour dans les modèles numériques

Geomorphic History of the Grand Staircase Region of the Colorado Plateau: Understanding Arroyo Cut-Fill Dynamics, Erosion Rates, and Wildfire

Most streams in the southwestern United States do not flow all year, and given their delicate balance of sediment and water flow, they are sensitive to climate change. At the turn of the 20th century, many streams in the Southwest rapidly incised into their floodplains, forming arroyos with a channel entrenched into near-vertical channel banks mostly composed of sand and mud. This dissertation investigates past changes in watersheds draining the Grand Staircase region in southern Utah with the goal of understanding how changes in climate and sediment influence these types of streams. Results show sediment supply is highly variable across the study area because of different rock types and slope, with hotspots of erosion located along the White and Pink Cliffs. This conversely leads to sediment storage on low-relief benches and valleys. Fast rates of erosion and large amounts of sediment cause channels to be overloaded with sediment and unstable over time. This dissertation supports the hypothesis that channel entrenchment is caused by a combination of climate changes and internal-thresholds that control the stability of the channel. The role of climate and fire in this landscape is investigated, and results support the idea that the frequency and intensity of drought and its effect on vegetation have influenced fire activity over the past millennia