Academic literature on the topic 'Reactive transport in riverine sediment'

Abstract. Estuarine sediments are key sites for removal of phosphorus (P) from rivers and the open sea. Vivianite, an Fe(II)-P mineral, can act as a major sink for P in Fe-rich coastal sediments. In this study, we investigate the burial of P in the Öre Estuary in the northern Baltic Sea. We find much higher rates of P burial at our five study sites (up to ∼0.145 molm-2yr-1) when compared to more southern coastal areas in the Baltic Sea with similar rates of sedimentation. Detailed study of the sediment P forms at our site with the highest rate of sedimentation reveals a major role for P associated with Fe and the presence of vivianite crystals below the sulfate methane transition zone. By applying a reactive transport model to sediment and porewater profiles for this site, we show that vivianite may account for up to ∼40 % of total P burial. With the model, we demonstrate that vivianite formation is promoted in sediments with a low bottom water salinity and high rates of sedimentation and Fe oxide input. While high rates of organic matter input are also required, there is an optimum rate above which vivianite formation declines. Distinct enrichments in sediment Fe and sulfur at depth in the sediment are attributed to short periods of enhanced input of riverine Fe and organic matter. These periods of enhanced input are linked to variations in rainfall on land and follow dry periods. Most of the P associated with the Fe in the sediment is likely imported from the adjacent eutrophic Baltic Proper. Our work demonstrates that variations in land-to-sea transfer of Fe may act as a key control on burial of P in coastal sediments. Ongoing climate change is expected to lead to a decrease in bottom water salinity and contribute to continued high inputs of Fe oxides from land, further promoting P burial as vivianite in the coastal zone of the northern Baltic Sea. This may enhance the role of this oligotrophic area as a sink for P imported from eutrophic parts of the Baltic Sea.

Abstract. River deltas are particularly important in the marine carbon cycle as they represent the transition between terrestrial and marine carbon: linked to major burial zones, they are reprocessing zones where large carbon fluxes can be mineralized. In order to estimate this mineralization, sediment oxygen uptake rates were measured in continental shelf sediments and river prodelta over different seasons near the outlet of the Rhône River in the Mediterranean Sea. On a selected set of 10 stations in the river prodelta and nearby continental shelf, in situ diffusive oxygen uptake (DOU) and laboratory total oxygen uptake (TOU) measurements were performed in early spring and summer 2007 and late spring and winter 2008. In and ex situ DOU did not show any significant differences except for shallowest organic rich stations. Sediment DOU rates show highest values concentrated close to the river mouth (approx. 20 mmol O2 m−2 d−1) and decrease offshore to values around 4.5 mmol O2 m−2 d−1 with lowest gradients in a south west direction linked to the preferential transport of the finest riverine material. Core incubation TOU showed the same spatial pattern with an averaged TOU/DOU ratio of 1.2±0.4. Temporal variations of sediment DOU over different sampling periods, spring summer and late fall, were limited and benthic mineralization rates presented a stable spatial pattern. A flood of the Rhône River occurred in June 2008 and delivered up to 30 cm of new soft muddy deposit. Immediately after this flood, sediment DOU rates close to the river mouth dropped from around 15–20 mmol O2 m−2 d−1 to values close to 10 mmol O2 m−2 d−1, in response to the deposition near the river outlet of low reactivity organic matter associated to fine material. Six months later, the oxygen distribution had relaxed back to its initial stage: the initial spatial distribution was found again underlining the active microbial degradation rates involved and the role of further deposits. These results highlight the immediate response of the sediment oxygen system to flood deposit and the rapid relaxation of this system towards its initial state (6 months or less) potentially linked to further deposits of reactive material.

Abstract. Arctic rivers will be increasingly affected by the hydrological and biogeochemical consequences of thawing permafrost. During transport, permafrost-derived organic carbon (OC) can either accumulate in floodplain and shelf sediments or be degraded into greenhouse gases prior to final burial. Thus, the net impact of permafrost OC on climate will ultimately depend on the interplay of complex processes that occur along the source-to-sink system. Here, we focus on the Kolyma River, the largest watershed completely underlain by continuous permafrost, and marine sediments of the East Siberian Sea, as a transect to investigate the fate of permafrost OC along the land–ocean continuum. Three pools of riverine OC were investigated for the Kolyma main stem and five of its tributaries: dissolved OC (DOC), suspended particulate OC (POC), and riverbed sediment OC (SOC). They were compared with earlier findings in marine sediments. Carbon isotopes (δ13C, Δ14C), lignin phenol, and lipid biomarker proxies show a contrasting composition and degradation state of these different carbon pools. Dual C isotope source apportionment calculations imply that old permafrost-OC is mostly associated with sediments (SOC; contribution of 68±10 %), and less dominant in POC (38±8 %), whereas autochthonous primary production contributes around 44±10 % to POC in the main stem and up to 79±11 % in tributaries. Biomarker degradation indices suggest that Kolyma DOC might be relatively degraded, regardless of its generally young age shown by previous studies. In contrast, SOC shows the lowest Δ14C value (oldest OC), yet relatively fresh compositional signatures. Furthermore, decreasing mineral surface area-normalised OC- and biomarker loadings suggest that SOC might be reactive along the land–ocean continuum and almost all parameters were subjected to rapid change when moving from freshwater to the marine environment. This suggests that sedimentary dynamics play a crucial role when targeting permafrost-derived OC in aquatic systems and support earlier studies highlighting the fact that the land–ocean transition zone is an efficient reactor and a dynamic environment. The prevailing inconsistencies between freshwater and marine research (i.e. targeting predominantly DOC and SOC respectively) need to be better aligned in order to determine to what degree thawed permafrost OC may be destined for long-term burial, thereby attenuating further global warming.

The aim of this study was to investigate the sediment dynamics in the largest lagoon in Europe (Curonian Lagoon, Lithuania) through the analysis of in situ data and the application of a sediment transport model. This approach allowed to identify the propagation pathway of the riverine suspended sediments, to map erosion-accumulation zones in the lagoon and calculate the sediment budget over a 13-year-long simulation. Sampled suspended sediment concentration data are important for understanding the characteristics of the riverine and lagoon sediments, and show that the suspended organic matter plays a crucial role on the sediment dynamics for this coastal system. The numerical experiments carried out to study sediment dynamics gave satisfactory results and the possibility to get a holistic view of the system. The applied sediment transport model with a new formula for settling velocity was used to estimate the patterns of the suspended sediments and the seasonal and spatial sediment distribution in the whole river–lagoon–sea system. The numerical model also allowed understanding the sensitivity of the system to strong wind events and the presence of ice. The results reveal that during extreme storm events, more than 11.4 × 106 kg of sediments are washed out of the system. Scenarios without ice cover indicate that the lagoon would have much higher suspended sediment concentrations in the winter season comparing with the present situation with ice. The results of an analysis of a long-term (13 years) simulation demonstrate that on average, 62% of the riverine sediments are trapped inside the lagoon, with a marked spatially varying distribution of accumulation zones.

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (N r ) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic N r are on land (240 Tg N yr −1 ) within soils and vegetation where reduced N r contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer N r contribute to nitrate (NO 3 − ) in drainage waters from agricultural land and emissions of trace N r compounds to the atmosphere. Emissions, mainly of ammonia (NH 3 ) from land together with combustion related emissions of nitrogen oxides (NO x ), contribute 100 Tg N yr −1 to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH 4 NO 3 ) and ammonium sulfate (NH 4 ) 2 SO 4 . Leaching and riverine transport of NO 3 contribute 40–70 Tg N yr −1 to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr −1 ) to double the ocean processing of N r . Some of the marine N r is buried in sediments, the remainder being denitrified back to the atmosphere as N 2 or N 2 O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of N r in the atmosphere, with the exception of N 2 O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 10 2 –10 3 years), the lifetime is a few decades. In the ocean, the lifetime of N r is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N 2 O that will respond very slowly to control measures on the sources of N r from which it is produced.

Observations on 6 ship moorings during the spring and neap tides show that the suspended sediment concentration (SSC) changed according to the variation of the local tidal forcings under the normal weather conditions. During the neap tide, the measured concentration of suspended sediment is comparable to that of Jinjiang River, the only external sediment source in the course of observations; while during the spring tide, the observed SSC is one order higher than that of Jinjiang, meaning that relative to the riverine impact, the tidal current is more probably responsible for the sharp variation of SSC between the spring and neap cycle.

Studying the process of riverine sediment at mouths and continental shelves is a critical subject for many engineering applications, such as dredging, navigation, dispersal and remobilization of contaminants. Sediment deposits also determine seabed properties, coastal geomorphology, and the health of coastal habitat/ecology. During extreme conditions, episodic river discharge triggered by large rainfall due to tropical cyclones may contribute significant amount of riverine sediment into the ocean. In the past decade, evidence of severe seabed erosion (up to 1m/year) along the sandy coast of Yunlin County has raised concerns regarding the sustainability of coastal structures. The exposed riverine sediment from the Jhuoshuei River is considered as one of major sources for sediment supply in this region. Bottle samples collected from bridge station in the Jhuoshuei River during the passage of tropical cyclones suggest sediment concentration can exceed 40 g/l for the major duration of the storm (Milliman et al. 2007). To mitigate the damage caused by shoreline retreat, 600,000 cubic meters per month of sand has been placed in two specific locations near the offshore industry park. The overarching goal of this study is to clarify the contribution of exposed riverine sediment and beach nourishment to enhance our understanding on the observed sediment transport and morphological evolution.

Abstract. Bed-material sediment particle size data, particularly the median sediment particle size (D50), are critical for understanding and modeling riverine sediment transport. However, sediment particle size observations are primarily available at individual sites. Large-scale modeling and assessment of riverine sediment transport are limited by the lack of continuous regional maps of bed-material sediment particle size. We hence present a map of D50 over the contiguous US in a vector format that corresponds to approximately 2.7 million river segments (i.e., flowlines) in the National Hydrography Dataset Plus (NHDPlus) dataset. We develop the map in four steps: (1) collect and process the observed D50 data from 2577 U.S. Geological Survey stations or U.S. Army Corps of Engineers sampling locations; (2) collocate these data with the NHDPlus flowlines based on their geographic locations, resulting in 1691 flowlines with collocated D50 values; (3) develop a predictive model using the eXtreme Gradient Boosting (XGBoost) machine learning method based on the observed D50 data and the corresponding climate, hydrology, geology, and other attributes retrieved from the NHDPlus dataset; and (4) estimate the D50 values for flowlines without observations using the XGBoost predictive model. We expect this map to be useful for various purposes, such as research in large-scale river sediment transport using model- and data-driven approaches, teaching environmental and earth system sciences, planning and managing floodplain zones, etc. The map is available at https://doi.org/10.5281/zenodo.4921987 (Li et al., 2021a).

Abstract. Lateral carbon transport from soils to the ocean through rivers has been acknowledged as a key component of the global carbon cycle, but it is still neglected in most global land surface models (LSMs). Fluvial transport of dissolved organic carbon (DOC) and CO2 has been implemented in the ORCHIDEE LSM, while erosion-induced delivery of sediment and particulate organic carbon (POC) from land to river was implemented in another version of the model. Based on these two developments, we take the final step towards the full representation of biospheric carbon transport through the land–river continuum. The newly developed model, called ORCHIDEE-Clateral, simulates the complete lateral transport of water, sediment, POC, DOC, and CO2 from land to sea through the river network, the deposition of sediment and POC in the river channel and floodplains, and the decomposition of POC and DOC in transit. We parameterized and evaluated ORCHIDEE-Clateral using observation data in Europe. The model explains 94 %, 75 %, and 83 % of the spatial variations of observed riverine water discharges, bankfull water flows, and riverine sediment discharges in Europe, respectively. The simulated long-term average total organic carbon concentrations and DOC concentrations in river flows are comparable to the observations in major European rivers, although our model generally overestimates the seasonal variation of riverine organic carbon concentrations. Application of ORCHIDEE-Clateral for Europe reveals that the lateral carbon transfer affects land carbon dynamics in multiple ways, and omission of this process in LSMs may lead to an overestimation of 4.5 % in the simulated annual net terrestrial carbon uptake over Europe. Overall, this study presents a useful tool for simulating large-scale lateral carbon transfer and for predicting the feedbacks between lateral carbon transfer and future climate and land use changes.

The integration of various scientific disciplines is becoming more and more important to define the complete evolution of an entire river system, both in time and in space. The aim of this thesis is to analyze the evolution of a river subjected to hydrological, morphological and biological interactions. This analysis is done at watershed scale, by using a numerical 1-D model plus a quasi 2-D sub-model. These models are able to integrate the available data, often scarce both qualitatively and quantitatively, especially in large unsurveyed rivers, typical of less developed countries. The models analysis allows to study the evolution of a river system at large space-scale and long-time scale, because the computational effort required by simplified models are quite low. The results of these models are indicative of a trend of the evolution of the river, in space and time, which can be useful in different studies, for example as input for other detailed models. In the first Chapter we analyze the principal mechanisms of formation and transport of sediments, making an overview of the various models that can be adopted to describe the morphological evolution of a river system. In addition to this description, in this chapter we study the different conformations of riverbeds, classified on the basis of their geometry and morphology. The lack of detailed data, from the point of view both of geometry and biology, implies the application of a non-detailed model, able to describe the hydrodynamic, morphodynamic and biological main processes of the river along its course. This model, essentially 1-D, is based on simplifications related to the hypothesis of the Local Uniform Flow. The 1-D model is associated to a quasi 2-D sub-model, able to provide a synthetic description of the river cross-section. This sub-model is useful for the analysis of large river systems, for which often we do not have bathymetric surveys. In the second Chapter and in the third Chapter is made a detailed analysis of these models, highlighting the advantages and disadvantages of the various simplifications. The growth of the riparian vegetation is strongly influenced by the forcing terms present in a river, related to hydrology and morphology. In Literature there are only a few site-specific studies or purely qualitative analysis, which study the interaction between the forcing terms and the riparian vegetation. The aim of this thesis is trying to model the influence of hydrology and morphology on the growth of the riparian vegetation, by a simplification of all the possible mechanisms involved. In this Chapter we have reported a detailed analysis of our studies, highlighting the limitations related to the lack of experimental data needed for a good calibration of our quasi 2-D sub-model. In the next two Chapters two different applications of the complete model are given: the first one about the Paranà River, and the second one about the Zambezi River. The application to the Paranà River has been made to highlight the goodness of the model for describing the evolution of the river, even compared to a commercial 1-D model as the Hec-Ras code. That application was also made to verify the general approach and the formulation used in the description of the synthetic river cross-section and the interaction between the river forcing terms and the growth of the riparian vegetation. The study of the Zambezi River is useful to see how the alteration of flow due to hydropower dams along the river strongly influences both morphology and biology of the river, with medium and long term effects. This analysis evaluates the planimetric and bathymetric changes subsequent to an alteration of the natural flow regime of a river due to the construction of hydroelectric reservoirs. In the last Chapter we have discussed the general results and the future developments, underlining, however, that the simplified models used in this work require further verifications, also against the analysis of more experimental data.
L’integrazione di varie discipline scientifiche sta diventando sempre più importante nel definire in maniera compiuta l’evoluzione di un intero sistema fluviale, sia dal punto di vista spaziale che temporale. Questo lavoro di tesi si prefigge lo scopo di analizzare l’evolversi di un fiume sottoposto ad interazioni idrologiche, morfologiche e biologiche. Tale analisi viene fatta a scala di bacino, adottando un modello numerico 1-D integrato con un sottomodello quasi 2-D. Entrambi i modelli sono capaci di utilizzare ed integrare i dati reperibili lungo un corso d’acqua, che spesso risultano carenti sia qualitativamente che quantitativamente, soprattutto nel caso di grandi fiumi non strumentati, tipici dei paesi in via di sviluppo. Tale approccio permette quindi di studiare l’evolversi di un complesso sistema fluviale a grande scala e a lungo termine, in quanto i tempi computazionali richiesti dal codice semplificato risultano piuttosto contenuti. I risultati ottenuti sono indicativi di una tendenza evolutiva del fiume a media risoluzione, che può essere utile in differenti studi, anche come dato di input per successive modellizzazioni di maggior dettaglio. Nel primo capitolo si evidenziano tutti i meccanismi di formazione e trasporto dei sedimenti, in modo tale da fornire una panoramica sui vari modelli che si possono adottare per descrivere l’evoluzione morfologica di un sistema fluviale. Oltre a tale descrizione, in questo capitolo vengono messe in evidenza anche le differenti conformazioni fluviali presenti in natura, suddivise sulla base della loro morfologia dominante. La mancanza di dati specifici, sia dal punto di vista dell’idrologia che della morfologia e della biologia, comporta l’applicazione di un modello, sia pure non di dettaglio, capace di descrivere i principali processi dell’idrodinamica, della morfodinamica e della crescita della vegetazione riparia lungo il corso del fiume. Questo modello, sostanzialmente 1-D, si basa sulle semplificazioni connesse all’imposizione del moto localmente uniforme. Al modello 1-D viene associato un sottomodello quasi 2-D, capace di fornire una descrizione sintetica della sezione trasversale del fiume. Tale sottomodello si rende necessario per l’analisi di grandi sistemi fluviali, dei quali spesso non si dispone di un rilievo batimetrico di dettaglio. Nel secondo e nel terzo capitolo viene quindi fatta un’analisi di tali modelli, evidenziando i pregi ed i difetti delle varie semplificazioni effettuate. Lo sviluppo della vegetazione riparia è fortemente influenzato dalle forzanti agenti su di essa, sia quelle idrologiche che quelle morfologiche. Allo stadio attuale, esistono solo alcuni studi sito-specifici o di carattere puramente qualitativo che analizzano compiutamente l’interazione tra le forzanti fluviali e la vegetazione riparia. In questa tesi si è quindi voluto provare a descrivere l’influenza dell’idrologia e della morfologia sullo sviluppo della vegetazione, semplificando il più possibile i meccanismi coinvolti. Viene quindi proposta tale analisi, evidenziandone i limiti legati all’assenza di dati sperimentali tali da permettere una buona taratura del modello. Nei successivi due capitoli vengono riportate due differenti applicazioni del modello completo: la prima al fiume Paranà e la seconda al fiume Zambezi. L’applicazione al fiume Paranà viene fatta per mettere in evidenza la bontà del modello nel descrivere l’evoluzione fluviale, anche mediante un raffronto con un modello commerciale unidimensionale quale Hec-Ras. Tale applicazione è stata fatta anche per verificare l’impostazione generale e le formulazioni adottate nella descrizione della sezione sintetica e dell’interazione tra le forzanti fluviali e lo sviluppo della vegetazione riparia. Lo studio del fiume Zambezi vuole invece verificare come l’alterazione delle portate a causa degli sbarramenti idroelettrici presenti lungo il corso d’acqua influenzi fortemente sia la morfologia che la biologia dell’ambiente fluviale, con effetti sia a medio che a lungo termine. Questa analisi è stata fatta con l’intento di analizzare le variazioni planimetriche e batimetriche susseguenti all’alterazione del regime idrologico naturale di un corso d’acqua causata dalla costruzione di sbarramenti idroelettrici. Nell’ultimo capitolo vengono discussi i risultati generali ed introdotti i possibili sviluppi futuri, sottolineando come tutti i modelli semplificati analizzati in questo lavoro di tesi necessitino di ulteriori verifiche e validazioni, anche attraverso l’analisi di ulteriori dati sperimentali.

Physical erosion can mobilise particulate organic carbon (POC) from vegetation and soil, representing an export of primary productivity from ecosystems, and a lateral transfer of carbon recently-derived from the atmosphere. These carbon transfers are thought to be enhanced in mountain forests where erosion rates are high. However, the rates and controls on POC transfer remain poorly constrained, as does the impact of POC export on carbon cycling at regional and global scales. This thesis takes an interdisciplinary approach to address this issue, using remote sensing, river geochemistry, river hydrology, and geomorphic mapping in the Kosñipata Valley, in the Central Andes of Peru. Its main aims are to: 1) estimate stream discharge throughout the year and to evaluate the water balance and sources; 2) quantify the source of riverine POC, accounting for POC derived from sedimentary rocks (POC_fossil) to examine the POC eroded from soils and vegetation (POC_non-fossil); 3) quantify river POC yields; 4) assess the hillslope processes that erode POC; and 5) assess how POC export impacts the carbon balance of mountain forest, and how fluvial transfer impacts the wider carbon cycle. Stream flow was monitored from January 2010 to February 2011 at two newly installed river gauging stations in the Kosñipata Valley at 2250 m (Wayqecha, 48.5 km²) and 1360 m (San Pedro, 164.4 km²). Then annual water balance for the San Pedro catchment was quantified. Rainfall inputs of 3028 mm and cloud water inputs of 308 ± 97 mm were balanced by outputs via stream runoff (2721 mm) and actual evapotranspiration (907 mm), leaving a residual of -294 ± 97 mm (< ~10 % of water inputs). The source of POC in river suspended sediment samples was quantified using radiocarbon (Δ¹⁴C, ‰), stable carbon isotopes, and the nitrogen to carbon ratio. This revealed that river POC_non-fossil was sourced from very young organic carbon in the valley (Δ¹⁴C ~50 ‰) and that POC_fossil comprised 43 % of total POC. Combining the hydrometric measurements with river samples, annual particulate load fluxes were quantified. The vast majority (73 % to 77 %) of the annual suspended sediment transfer and POC (both POC_fossil and POC_non-fossil) occurred in the wet season over a period of 4 months. The suspended sediment yield for the valley (960 – 1200 t km^-2 yr^-1) was consistent with those for the Andean portion of the Madre de Dios River into which the Kosñipata River drains. The river POC_non-fossil yield was 5.2 – 6.9 tC km^-2 yr^-1. Landslides are likely to have played an important role in the mobilisation of POC_non-fossil. A detailed landslide mapping using 25 years of remote sensing data revealed that on average 0.09 % of the valley per year is impacted by this mass-wasting process. These landslides mobilise ~28 tC km^-2 yr^-1 of soil and vegetation valley-wide. The discrepancy between the landslide erosional flux and fluvial POCnon-fossil export suggests an important fraction of the POCnon-fossil harvested by landslides is either exported as coarse debris (not quantified in the fluvial POC_non-fossil flux), remains buried onsite, or is degraded and respired onsite. Landslides also played an important ecosystem function, turning over some sections of the mountain forest within ~625 years, with a 1200 year valley-wide mean. On the basin scale, the Madre de Dios River drains ~ 6 % of the Amazonian Andes. This study enables estimation of the delivery of POC to the lowland Amazon Basin. Using the observation that POC_non-fossil and POC_fossil fluxes were closely linked with suspended sediment transfer, total yields of ~0.22 MtC yr^-1 and ~0.17 MtC yr^-1, respectively, were estimated from this section of the Andes. The export of POC_non-fossil from mountain forests by rivers represents 0.4 – 1.0 % yr^-1 of the net primary productivity of Andean forest and so even if only a small portion of this is buried in sedimentary deposits, it may promote the Andes as a carbon sink. These results demonstrate the long-term influence of erosional processes in the cycling of carbon in the Amazon Basin.

This dissertation presents the design of an integrated watershed model, WASH123D version 3.0, a first principle, physics-based watershed-scale model of integrated hydrology/hydraulics and water quality transport. This numerical model is comprised of three modules: (1) a one-dimensional (1-D) simulation module that is capable of simulating separated and coupled fluid flow, sediment transport and reaction-based water quality transport in river/stream/canal networks and through control structures; (2) a two-dimensional (2-D) simulation module, capable of simulating separated and coupled fluid flow, sediment transport, and reactive biogeochemical transport and transformation in two-dimensional overland flow systems; and (3) a three-dimensional (3-D) simulation module, capable of simulating separated and coupled fluid flow and reactive geochemical transport and transformation in three-dimensional variably saturated subsurface systems. The Saint Venant equation and its simplified versions, diffusion wave and kinematic wave forms, are employed for surface fluid flow simulations and the modified Richards equation is applied for subsurface flow simulation. The reaction-based advection-dispersion equation is used as the governing equation for water quality transport. Several physically and mathematically based numerical options are provided to solve these governing equations for different application purposes. The surface-subsurface water interactions are considered in the flow module and simulated on the basis of continuity of interface. In the transport simulations, fast/equilibrium reactions are decoupled from slow/kinetic reactions by the decomposition of reaction networks; this enables robust numerical integrations of the governing equation. Kinetic variables are adopted as primary dependent variables rather than biogeochemical species to reduce the number of transport equations and simplify the reaction terms. In each time step, hydrologic/hydraulic variables are solved in the flow module; kinetic variables are then solved in the transport module. This is followed by solving the reactive chemical system node by node to yield concentrations of all species. Application examples are presented to demonstrate the design capability of the model. This model may be of interest to environmental scientists, engineers and decision makers as a comprehensive assessment tool to reliably predict the fluid flow as well as sediment and contaminant transport on watershed scales so as to evaluate the efficacy and impact of alternative watershed management and remediation techniques prior to incurring expense in the field.
Ph.D.
Department of Civil and Environmental Engineering
Engineering and Computer Science
Civil Engineering PhD

Chemical weathering of silicate rock consumes atmospheric CO2 and supplies the oceans with cations, thereby controlling both seawater chemistry and climate. The rate of CO2 consumption is closely linked to the rate of CO2 outgassing from the planetary interior, providing a negative feedback loop essential to maintaining an equable climate on Earth. Reconstruction of past global temperatures indicates that a pronounced episode of global cooling began ~50 million years ago, coincident with the collision of India and Asia, and the subsequent exhumation of the Himalayas and Tibet. This has drawn attention to the possible links between exhumation, erosion, changes in silicate weathering rates, and climate. However, many of the present-day weathering processes operating on the continents remain debated and poorly constrained, hampering our interpretations of marine geochemical archives and past climatic shifts. To constrain the controls on silicate weathering, this thesis investigates the lithium (Li) isotopic composition of river waters, suspended sediments and bed load sediments in the Alaknanda river basin, forming the headwaters of the Ganges. Due to the large fractionation of Li isotopes in the Earth’s surface environment, Li is sensitive to small changes in silicate weathering processes. As a consequence of the pronounced gradients in climate (rainfall and temperature) and erosion across the basin, the river waters show large variations in their Li isotopic composition (δ7Li), ranging from +7.4 to +35.4‰, covering much of the observed global variation. This allows a detailed investigation of the controls on Li isotope fractionation, and by extension silicate weathering. The Li isotopic composition is modelled using a one-dimensional reactive transport model. The model incorporates the continuous input of Li from rock dissolution, removal due to secondary mineral formation, and hydrology along subsurface flow paths. Modelling shows that the Li isotopic variations can be described by two dimensionless variables; (1) the Damköhler number, ND, which relates the silicate dissolution rate to the fluid transit time, and (2) the net partition coefficient of Li during weathering, kp, describing the partitioning of Li between secondary clay minerals and water, which is primarily controlled by the stoichiometry of the weathering reactions. The derived values of the controlling parameters ND and kp, are investigated over a range of climatic conditions and on a seasonal basis, shedding light onto variations in the silicate weathering cycle. In a kinetically limited weathering regime such as the Himalayan Mountains, both climate and erosion exert critical controls the weathering intensity (the fraction of eroded rock which is dissolved) and the weathering progression (which minerals that are being weathered), and consequently the fractionation of Li isotopes and silicate weathering in general. Modelling of the Li isotopic composition provides an independent estimate of the parameters which control silicate weathering. These estimates are then used to constrain variables such as subsurface fluid flux, silicate dissolution rates, fluid transit times and the fraction of rock which is weathered to form secondary clay minerals. The simple one-dimensional reactive transport model therefore provides a powerful tool to investigate the minimum controls on silicate weathering on the continents.

Accurate models to reliably predict sediment and chemical transport in watershed water systems enhance the ability of environmental scientists, engineers and decision makers to analyze the impact of contamination problems and to evaluate the efficacy of alternative remediation techniques and management strategies prior to incurring expense in the field. This dissertation presents the conceptual and mathematical development of a general numerical model simulating (1) sediment and reactive chemical transport in river/stream networks of watershed systems; (2) sediment and reactive chemical transport in overland shallow water of watershed systems; and (3) reactive chemical transport in three-dimensional subsurface systems. Through the decomposition of the system of species transport equations via Gauss-Jordan column reduction of the reaction network, fast reactions and slow reactions are decoupled, which enables robust numerical integrations. Species reactive transport equations are transformed into two sets: nonlinear algebraic equations representing equilibrium reactions and transport equations of kinetic-variables in terms of kinetically controlled reaction rates. As a result, the model uses kinetic-variables instead of biogeochemical species as primary dependent variables, which reduces the number of transport equations and simplifies reaction terms in these equations. For each time step, we first solve the advective-dispersive transport of kinetic-variables. We then solve the reactive chemical system node by node to yield concentrations of all species. In order to obtain accurate, efficient and robust computations, five numerical options are provided to solve the advective-dispersive transport equations; and three coupling strategies are given to deal with the reactive chemistry. Verification examples are compared with analytical solutions to demonstrate the numerical accuracy of the code and to emphasize the need of implementing various numerical options and coupling strategies to deal with different types of problems for different application circumstances. Validation examples are presented to evaluate the ability of the model to replicate behavior observed in real systems. Hypothetical examples with complex reaction networks are employed to demonstrate the design capability of the model to handle field-scale problems involving both kinetic and equilibrium reactions. The deficiency of current practices in the water quality modeling is discussed and potential improvements over current practices using this model are addressed.
Ph.D.
Department of Civil and Environmental Engineering
Engineering and Computer Science
Civil Engineering

The immense discharge of the Amazon River causes river/ocean mixing to take place out on the continental shelf instead of within a drowned river valley, as in many smaller dispersal systems (Nittrouer and DeMaster, 1996). The magnitude of this discharge can be appreciated by recognizing that the Amazon River supplies approximately 20% (6 x 1015 L yr−1) of the freshwater reaching the oceans via fluvial transport and roughly 6% (1.2 x 1015 g yr−1, Meade et al. 1985) of the global riverine sediment discharge. Chemical, physical, and biological processes occurring in the river/ocean mixing zone control the fates of these riverine materials, as well as the fates of substances brought onto the shelf from offshore as a result of the estuarine-like circulation. Depending on balances between transport, reaction rates, and sedimentation, the mixing zone may act as a net source, sink, or bypass conduit for chemical species in the coastal environment. For example, if Amazon River nutrients such as silicate, phosphate, or nitrate are simply removed from solution and buried as particulate biogenic debris on the adjacent shelf, the river would have little influence on global ocean nutrient budgets. In contrast, if nutrients coming down the river are not efficiently buried nearshore (as a result of minimal biological uptake or efficient recycling), then they may contribute to larger scale oceanic or atmospheric budgets of Si, P, and N (Treguer et al. 1995; Delaney, 1998). The Amazon River transports ~1015 moles yr−1 of particulate organic carbon from the terrestrial environment to the ocean (Degens et al. 1991). The fate of this material (some of it from leaf litter and some of it from older, more refractory soils) is important to understand because the Amazon River/ocean mixing zone comprises a significant fraction of all deltaic depositional environments, where ~50% of the marine burial of organic matter occurs (Berner 1982, 1989, Hedges and Keil 1995; Devol et al. this volume). The Amazon River also discharges an equivalent amount of dissolved organic carbon (~1012 moles yr−1), much of which is in the form of high molecular weight organic compounds (Sholkovitz et al. 1978, Degens et al. 1991).

This paper provides an approach for assessing and classifying riverine pipeline crossings to prioritize effective mitigation and monitoring. These processes require understanding of and accounting for channel processes, river dynamics, geomorphic principals and soil mechanics to estimate bed scour and bank erosion degradation mechanisms at water crossings and their potential effects on the pipeline. The intent of this paper is to share generic experiences in ranking water crossings based on their susceptibility to and identification of integrity threats under multiple existing and future hydrologic scenarios causing potential for pipeline exposure, spanning or damage. The intent is not to present or provide an analysis or review of the various methods for estimating channel bed or bank erosion. The details of such calculations are highly site specific and a variety of both qualitative and quantitative methods can be applied depending upon available site data, and as such, are outside the scope of the work presented here. Pipelines are static features within a dynamic environment with rivers and floodplains representing some of the most active areas within a landscape. Rivers can change course, migrate, deepen, and widen slowly over time or suddenly during large flood events. These hydrologic effects can impact existing pipelines thereby putting pipelines at risk for damage or failure. Understanding how rivers alter the landscape and transport water and sediment from the mountains to the sea provides a framework for realizing the potential toll that riverine changes can have on pipeline infrastructure. Further, integrating analysis of how rivers at specific pipeline crossing locations are likely to change can increase the effectiveness in protecting the environment during the design, construction, operation and integrity management of pipelines at river crossings. The paper provides an approach comprised of five (5) stages: 1. WC Inventory and Desktop Data Gathering 2. Screening Process: Preliminary WC Classification 3. Detailed Assessment 4. Final WC Classification, Prioritization, and Risk Assessment 5. Development of Mitigation and Monitoring Strategies This paper also presents two (2) case studies illustrating how assessing the geomorphic condition and processes of the river system being crossed by pipelines provides for a better understanding of susceptibility to existing hydro-geotechnical threats to the pipeline as well as the susceptibility for flood-related forces in the future. The first case study illustrates how changes to a river’s cross section as a result of construction activities upstream of a pipeline water crossing can cause significant and potentially damaging impacts, downstream. The second case study reinforces the importance of understanding the history of watershed and channel changes over time, both at the specific water crossing location, but also both upstream and downstream from the crossing itself to be able to identify and understand all potential threats to pipelines located within rivers and floodplains. A method for assessing and classifying the magnitude and probability of flood related risk at each case study is discussed. These cases are presented as generic examples for educational purposes only as every pipeline has its own specific characteristics conditions with jurisdiction-specific regulatory requirements requiring process customization and enhancements.

The new Preliminary Design Guidance is an updated version of the original PDG introduced in 2009. It incorporates not only what the MRC Member Countries have learnt from their own experience with hydropower, but also from examples and best practices around the world. It also includes the most current knowledge regarding design criteria, science and technology. While the older PDG spanned this range of construction and operation elements (hydraulics; sediment transport; geomorphology; water quality; aquatic ecology; fish and fisheries; dam safety; and navigation), the new PDG now includes hydrology and socio-economic impact to reflect the greater attention paid today to riparian communities and riverine livelihoods.