THE STABILITY OF BENTHIC HOMES VARIES BETWEEN RIVERS AND REGIONS: FLOW REGIME AND SEDIMENT SUPPLY AS CONTROLS ON GRAVEL BED MOBILITY AND ECOLOGICAL DISTURBANCE

Contourite deposits are generated by the interplay between deepwater bottom-currents, sediment supply and seafloor topography. The Gulf of Cadiz, in the Southwest Iberian margin, is a famous example of extensive contourite deposition driven by the Mediterranean Outflow Water (MOW), which exits the Strait of Gibraltar, flows northward following the coastline and distributes the sediments coming from the Guadalquivir and Guadiana rivers. The MOW and related contourite deposits affect the stability of the SW Iberian margin in several ways: on one hand it increases the sedimentation rate, favoring the development of excess pore pressure, while on the other hand, by depositing sand it allows pore water pressure to dissipate, potentially increasing the stability of the slope.In the Gulf of Cadiz, grain size distribution of contourite deposits is influenced by the seafloor morphology, which splits the MOW in different branches, and by the alternation of glacial and interglacial periods that affected the MOW hydrodynamic regimes. Fine clay packages alternates with clean sand formations according to the capacity of transport of the bottom-current in a specific area. Generally speaking, coarser deposits are found in the areas of higher MOW flow energy, such as in the shallower part of the slope or in the area closer to the Strait of Gibraltar, while at higher water depths the sedimentation shifts to progressively finer grain sizes as the MOW gets weaker. Previous works show that at present-day the MOW flows at a maximum depth of 1400 m, while during glacial periods the bottom-current could have reached higher depths.In this study we derived the different maximum depths at which the MOW flowed by analyzing the distribution of sands at different depths along the Alentejo basin slope, in the Northern sector of the Gulf of Cadiz.Here we show how changes in sand distribution along slope, within the stratigraphic units deposited between the Neogene and the present day, are driven by glacial &#8211; interglacial period alternation that influenced the hydrodynamic regime of the MOW.By deriving the depositional history of sand in the Alentejo basin, we are able to correlate directly the influence that climatic cycles had on the MOW activity. Furthermore, by interpreting new multi-channel seismic profiles we have been able to derive a detailed facies characterization of the uppermost part of the Gulf of Cadiz.An accurate definition of sand distribution along slope plays an important role in evaluating the stability of the slope itself, e.g. to understand if the sediments may be subjected to excess pore pressure generation. As sand distribution is a direct function of the bottom-current transport capacity, the ultimate goal of this study is to understand how climate variations can affect the stability of submarine slope by depositing contourite-related sand.

Download Full-text

Slit Wall Effect on the Stability of Wavy Vortex Flow

Advances in Mechanical Engineering ◽

10.1155/2014/853069 ◽

2014 ◽

Vol 6 ◽

pp. 853069 ◽

Cited By ~ 2

Author(s):

Dong Liu ◽

Ying-ze Wang ◽

Hyoung-Bum Kim ◽

Fang-neng Zhu ◽

Chun-lin Wang

Keyword(s):

Numerical Simulation ◽

Flow Field ◽

Radial Velocity ◽

Flow Regime ◽

Experimental Measurement ◽

Vortex Flow ◽

Wall Effect ◽

Taylor Vortices ◽

Simulation Results ◽

The Stability

The wavy vortex flow in the plain model was studied by experimental measurement; the preliminary feature of wavy vortex flow was obtained. This flow field in the plain model was also studied by numerical simulation. The reliability of numerical simulation was verified by comparing with the experimental and numerical simulation results. To study the slit wall effect on the wavy vortex flow regime, another two models with different slit number were considered; the slit number was 6 and 12. By comparing the wavy vortex flow field in different models, the axial fluctuation of Taylor vortices was found to be different, which was increased with the increasing of slit number. The maximum radial velocity from the inner cylinder to the outer one in the 6-slit number was increased by 12.7% compared to that of plain model. From the results of different circumferential position in the same slit model, it can be found that the maximum radial velocity in slit plane is significantly greater than that in other planes. The size of Taylor vortices in different models was also calculated, which was found to be increased in the 6-slit model but was not changed as the slit number increased further.

Download Full-text

Can gravel augmentation restore thermal functions in gravel-bed rivers? A need to assess success within a trajectory-based BACI framework

10.22541/au.160898418.84619343/v1 ◽

2020 ◽

Author(s):

Baptiste Marteau ◽

Kristell Michel ◽

Hervé Piégay

Keyword(s):

River System ◽

Sediment Supply ◽

Global Changes ◽

Fundamental Parameter ◽

Positive Feedbacks ◽

Gravel Bed ◽

Channel Adjustments ◽

Gravel Bed Rivers ◽

Term Monitoring

Gravel augmentation has become common practice to mitigate the effects of decline in upstream sediment supply in gravel-bed rivers. The success of such rehabilitation schemes relies partly on the monitoring strategy and efforts. When long-term monitoring is lacking, some aspects of rehabilitation initiatives suffer more than others, such as insights into functions and functionalities of the river system. Despite temperature being a fundamental parameter determining the general health of river ecosystems, a limited number of studies have tested whether gravel-augmentation can aid restoring thermal functions. With the help of airborne thermal infrared (TIR) imagery, this paper explores the potential positive feedbacks through the monitoring of gravel augmentation actions, of different magnitude, taken on 3 rivers of the Rhône basin in France. A specific trajectory-based Before-After-Control-Impact (BACI) framework using simple indicators, combined with a TIR-based Control-Impact strategy, was designed to assess the success of thermal function restorations based on dynamic fuzzy references. Results indicate that restoring forms is not sufficient to restore thermal functions. The control-impact strategy shows limitations in the sense that two neighbouring reaches can display similar planform characteristics but different thermal functions; what is observed in a control reach should not necessarily be expected following rehabilitation. When assessing thermal processes, a before-after strategy is needed to either serve as a target or help define an adequate target in accordance with changes in the catchment and channel adjustments and responsiveness. We therefore recommend a trajectory-based BACI assessment to identify current biogeophysical conditions within which rehabilitation can be assessed. From a technical perspective, airborne TIR proved to be useful to rapidly map surface temperature over dozens of kilometres at high resolution, and can be advocated as a powerful tool to monitor and diagnose thermal functions of gravel-bed rivers. With an increasing number of rehabilitation schemes, and increasing pressure of global changes on rivers, we suggest that monitoring of water temperature, even with simple but well-designed sampling strategies, becomes a routine part of river rehabilitation projects.

Download Full-text

Bedforms, Structures, Patches, and Sediment Supply in Gravel-Bed Rivers

Gravel-Bed Rivers ◽

10.1002/9781118971437.ch16 ◽

2017 ◽

pp. 439-466 ◽

Cited By ~ 14

Author(s):

Jeremy G. Venditti ◽

Peter A. Nelson ◽

Ryan W. Bradley ◽

Dan Haught ◽

Alessandro B. Gitto

Keyword(s):

Sediment Supply ◽

Gravel Bed ◽

Gravel Bed Rivers

Download Full-text

A New Mass-Conservative, Two-Dimensional, Semi-Implicit Numerical Scheme for the Solution of the Navier-Stokes Equations in Gravel Bed Rivers with Erodible Fine Sediments

Water ◽

10.3390/w12030690 ◽

2020 ◽

Vol 12 (3) ◽

pp. 690

Author(s):

Maurizio Tavelli ◽

Sebastiano Piccolroaz ◽

Giulia Stradiotti ◽

Giuseppe Roberto Pisaturo ◽

Maurizio Righetti

Keyword(s):

Numerical Scheme ◽

Stokes Equations ◽

Transport Processes ◽

Sediment Supply ◽

Navier Stokes ◽

Computational Domain ◽

Two Dimensional ◽

Navier Stokes Equations ◽

Gravel Bed ◽

Gravel Bed Rivers

The selective trapping and erosion of fine particles that occur in a gravel bed river have important consequences for its stream ecology, water quality, and overall sediment budgeting. This is particularly relevant in water bodies that experience periodic alternation between sediment supply-limited conditions and high sediment loads, such as downstream from a dam. While experimental efforts have been spent to investigate fine sediment erosion and transport in gravel bed rivers, a comprehensive overview of the leading processes is hampered by the difficulties in performing flow field measurements below the gravel crest level. In this work, a new two-dimensional, semi-implicit numerical scheme for the solution of the Navier-Stokes equations in the presence of deposited and erodible sediment is presented, and tested against analytical solutions and performing numerical tests. The scheme is mass-conservative, computationally efficient, and allows for a fine discretization of the computational domain. Overall, this makes the model suitable to appreciate small-scales phenomena such as inter-grain circulation cells, thus offering a valid alternative to evaluate the shear stress distribution, on which erosion and transport processes depend, compared to traditional experimental approaches. In this work, we present proof-of-concept of the proposed model, while future research will focus on its extension to a three-dimensional and parallelized version, and on its application to real case studies.

Download Full-text

Impacts of gravel-bed rivers transformation on fluvial ecosystems and human society: Examples from the Czech flysch Carpathians

E3S Web of Conferences ◽

10.1051/e3sconf/20184002005 ◽

2018 ◽

Vol 40 ◽

pp. 02005 ◽

Cited By ~ 1

Author(s):

Václav Škarpich ◽

Miroslav Kubín ◽

Tomáš Galia ◽

Stanislav Ruman ◽

Jan Hradecký

Keyword(s):

Hydrological Regime ◽

Developed Countries ◽

Sediment Supply ◽

Human Society ◽

Large Set ◽

Aquatic Communities ◽

Negative Effects ◽

Gravel Bed ◽

Research Activities ◽

Gravel Bed Rivers

In the last centuries, gravel-bed rivers in developed countries have undergone rapid changes in channel morphology. The most serious problems include channel transformation related to progressive channel narrowing, incision or bed sediment coarsening. The main reasons for transformations were connected to the human interventions, which affected water and sediment fluxes in the basins. This paper summarizes contemporary research activities focused on these negative effects of channel transformations in the Czech flysch Carpathian rivers (the Morávka, the Olše and the Ostravice draining the highest mountainous areas of the Beskydy Mts). As the result of channel transformations, progressive changes in fluvial ecosystem were observed. The initial phytosociological survey demonstrates a higher biodiversity in the floodplain along the preserved multi-thread river channel than along the deeply incised channel in the Morávka River basin. Our observations of aquatic communities demonstrated that the channel transformation connected with incision and coarsening of bed sediments negatively affected fish or lamprey populations in the studied rivers. Regulation, damming and incision of channels caused changes of hydrological regime linked with gradual drying of floodplain. Additionally, a large set of hydraulic structures, bridges or weirs were affected by undercutting and progressive destruction in the Ostravice, Olše and Morávka River basins, which is assigned to increased transport capacity of regulated channels together with decreased sediment supply from mountainous parts.

Download Full-text

Responses of gravel-bed rivers to periodic climate change

10.5194/egusphere-egu21-7904 ◽

2021 ◽

Author(s):

Fergus McNab ◽

Taylor Schildgen ◽

Jens Turowski ◽

Andrew Wickert

Keyword(s):

Transport Processes ◽

Sediment Supply ◽

Sediment Flux ◽

Alluvial Channels ◽

Gravel Bed ◽

Fluvial Terraces ◽

Wide Range ◽

Gravel Bed Rivers ◽

Lag Times ◽

Climatic Signals

Periodic variation in Earth's orbit leads to variation in temperature and precipitation at its surface that are expected to exert a profound influence on landscape evolution. Indeed, cyclical fluctuations in sediment yield and grain size are a ubiquitous feature of the geological record, and recurrence times of geomorphological features such as fluvial terraces and alluvial fans often appear to reflect orbital periodicities. However, making quantitative interpretations of these records requires a detailed understanding of the ways in which sediment is transported from mountainous source regions along alluvial channels to depositional sinks. Sediment transport processes may dampen (i.e. buffer, 'shred') or amplify climate signals, such as changes in channel elevation or sediment flux, and may introduce a lag between them and the responsible external forcing. Recent modelling studies, mostly focused on the potential transmission of climatic signals to sedimentary archives, have predicted a wide range of behaviour and have proven challenging to test in the field. Here, we aim to clarify this discussion and also consider the potential preservation of climatic signals by fluvial terraces along alluvial channels. Our starting point is a recently developed model describing the long-profile evolution of gravel-bed rivers. This model is the first of its kind to be derived from first principles using physical relationships that have been extensively tested in laboratory settings, and takes a non-linear diffusive form. We employ perturbation theory to obtain approximate analytical solutions to the relevant equations that describe how channel elevation and sediment flux vary in response to periodic fluctuations in discharge and sediment supply. Our solutions contain expressions for response amplitudes and lag times as functions of downstream distance, system 'diffusivity' and forcing frequency. Lag times can be a significant fraction of the forcing period, implying that care is required when interpreting the timings of terrace formation in terms of changes in discharge or sediment supply. We also show that at the onset of periodic forcing, or a change in the dominant forcing period, alluvial channels undergo a transient response as they adjust to a new quasi-steady state. Importantly, this result implies that suites of fluvial terraces can be preserved without the need for significant local base-level fall. Since the expressions presented here are defined in terms of fundamental properties of alluvial channels, they should be readily applicable to real settings.

Download Full-text