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31 pages, 7891 KiB

Open AccessArticle

A Study of Wave Dissipation Rate and Particles Velocity in Muddy Beds

by S. Hadi Shamsnia, Mohsen Soltanpour, Majid Bavandpour and Carlo Gualtieri

Geosciences 2019, 9(5), 212; https://doi.org/10.3390/geosciences9050212 - 10 May 2019

Cited by 5 | Viewed by 3043

The interactions between free surface waves and layers of cohesive sediments including wave height attenuation and mud movement are of great importance in coastal and marine engineering. In this study, the results from a new analytical model were compared with those from literature [...] Read more.

The interactions between free surface waves and layers of cohesive sediments including wave height attenuation and mud movement are of great importance in coastal and marine engineering. In this study, the results from a new analytical model were compared with those from literature experimental works and analytical models in terms of wave height dissipation rate and mud velocity. It was found that the new model provided good agreements in the case of coexisting waves and currents, while the literature model of Ng (explained in ^{Section 2} of the text) —assuming the mud layer as a highly viscous layer with high shear rates—matched well with the experimental data for high viscosity (mud viscosity, ν_m = O [0.01 m²/s]). In addition, it was found that the new model is able to successfully simulate particles velocity in the presence of co-current. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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10 pages, 2458 KiB

Open AccessArticle

Lateral Momentum Fluxes at the Confluence of the Negro and Solimões Rivers

by Adriano Coutinho de Lima

Geosciences 2019, 9(1), 7; https://doi.org/10.3390/geosciences9010007 - 23 Dec 2018

Viewed by 2598

Abstract

Hydrodynamic zones of river confluences are remarkable not only for the turbulent mixing induced by the shear layer at the center of the mixing interface but also for the lateral momentum fluxes associated with channel topography. Detailed characterizations of lateral momentum transfers in [...] Read more.

Hydrodynamic zones of river confluences are remarkable not only for the turbulent mixing induced by the shear layer at the center of the mixing interface but also for the lateral momentum fluxes associated with channel topography. Detailed characterizations of lateral momentum transfers in river confluences, however, are few. In this study, contributions to the lateral momentum fluxes in the confluence of the Negro and Solimões rivers in Brazil were calculated based on a comprehensive set of field data. Results show that the lateral fluxes by the mean flow exceed the turbulent fluxes by two orders of magnitude. Furthermore, the Reynolds stress along the far field of the Solimões side of the Amazon channel scales with or surpasses the Reynolds stress at the interface with the Negro side. The importance of the shear layer in the lateral mixing is thus overshadowed by the competing hydrodynamic processes. This configuration partially explains the long distance required to complete the mixing of the waters of the two tributary rivers. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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14 pages, 2872 KiB

Open AccessArticle

Developing an Approximation of a Natural, Rough Gravel Riverbed Both Physically and Numerically

by Alex Stubbs, Thorsten Stoesser and Bettina Bockelmann-Evans

Geosciences 2018, 8(12), 449; https://doi.org/10.3390/geosciences8120449 - 30 Nov 2018

Cited by 5 | Viewed by 3242

Abstract

Near-bed and pore space turbulent flows are beginning to be understood using new technologies and advances in direct numerical simulation (DNS) and large-eddy simulation (LES) techniques. However, the riverbed geometry that is used in many computational studies remains overly simplistic. Thus, this study [...] Read more.

Near-bed and pore space turbulent flows are beginning to be understood using new technologies and advances in direct numerical simulation (DNS) and large-eddy simulation (LES) techniques. However, the riverbed geometry that is used in many computational studies remains overly simplistic. Thus, this study presents the development of an artificial representation of a gravel riverbed matrix, and the assessment of how well it approximates a natural riverbed. A physical model of a gravel riverbed matrix that was 120 mm deep, 300 mm wide, and 2.048 m long was manufactured from cast acrylic. Additionally, a numerical approximation of the physical model was created and used for analysis. The pore matrix of the artificial riverbed was found to be comparable to that of a natural gravel riverbed in terms of its porosity and void ratio. The diameters of the artificial riverbed’s surface particles were found to vary less, with fewer irregularities, than those found for natural gravel riverbeds; yet, they were normally distributed similarly to natural riverbeds. A power spectral density function showed that the artificial riverbed exhibited a degree of roughness that was much lower than that found in nature. Thus, the hydraulic resistance and friction factor will both be lower than desired. These findings suggest that the novel methods that have been developed in this study can offer both the physical and numerical approximation of a gravel bed surface that is comparable to a natural gravel riverbed with low surface roughness, reduced particle size variance, and typical particle distribution and porosity. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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20 pages, 4427 KiB

Open AccessArticle

How to Define Priorities in Coastal Vulnerability Assessment

by Francesca De Serio, Elvira Armenio, Michele Mossa and Antonio Felice Petrillo

Geosciences 2018, 8(11), 415; https://doi.org/10.3390/geosciences8110415 - 12 Nov 2018

Cited by 46 | Viewed by 7070

Abstract

Awareness of coastal landscapes vulnerability to both natural and man-made hazards induce to monitor their evolution, adaptation, resilience and to develop appropriate defence strategies. The necessity to transform the monitoring results into useful information is the motivation of the present paper. Usually, to [...] Read more.

Awareness of coastal landscapes vulnerability to both natural and man-made hazards induce to monitor their evolution, adaptation, resilience and to develop appropriate defence strategies. The necessity to transform the monitoring results into useful information is the motivation of the present paper. Usually, to this scope, a coastal vulnerability index is deduced, by assigning ranking values to the different parameters governing the coastal processes. The principal limitation of this procedure is the individual discretion used in ranking. Moreover, physical parameters are generally considered, omitting socio-economic factors. The aim of the present study is to complement a geographical information system (GIS) with an analytical hierarchical process (AHP), thus allowing an objective prioritization of the key parameters. Furthermore, in the present case, socio-economic parameters have been added to physical ones. Employing them jointly, an integrated coastal vulnerability index (ICVI) has been estimated and its effectiveness has been investigated. To show how it works, the proposed method has been applied to a portion of the Adriatic coastline, along the Apulian region in southern Italy. It has permitted to identify and prioritize the most vulnerable areas, revealing its efficacy as a potential tool to support coastal planning and management. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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15 pages, 3097 KiB

Open AccessArticle

Re-Aeration on Stepped Spillways with Special Consideration of Entrained and Entrapped Air

by Daniel B. Bung and Daniel Valero

Geosciences 2018, 8(9), 333; https://doi.org/10.3390/geosciences8090333 - 5 Sep 2018

Cited by 11 | Viewed by 3745

Abstract

As with most high-velocity free-surface flows, stepped spillway flows become self-aerated when the drop height exceeds a critical value. Due to the step-induced macro-roughness, the flow field becomes more turbulent than on a similar smooth-invert chute. For this reason, cascades are oftentimes used [...] Read more.

As with most high-velocity free-surface flows, stepped spillway flows become self-aerated when the drop height exceeds a critical value. Due to the step-induced macro-roughness, the flow field becomes more turbulent than on a similar smooth-invert chute. For this reason, cascades are oftentimes used as re-aeration structures in wastewater treatment. However, for stepped spillways as flood release structures downstream of deoxygenated reservoirs, gas transfer is also of crucial significance to meet ecological requirements. Prediction of mass transfer velocities becomes challenging, as the flow regime differs from typical previously studied flow conditions. In this paper, detailed air-water flow measurements are conducted on stepped spillway models with different geometry, with the aim to estimate the specific air-water interface. Re-aeration performances are determined by applying the absorption method. In contrast to earlier studies, the aerated water body is considered a continuous mixture up to a level where 75% air concentration is reached. Above this level, a homogenous surface wave field is considered, which is found to significantly affect the total air-water interface available for mass transfer. Geometrical characteristics of these surface waves are obtained from high-speed camera investigations. The results show that both the mean air concentration and the mean flow velocity have influence on the mass transfer. Finally, an empirical relationship for the mass transfer on stepped spillway models is proposed. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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10 pages, 17504 KiB

Open AccessCommunication

Suspended Sediment Variability at the Solimões and Negro Confluence between May 2013 and February 2014

by Thiago Marinho, Naziano Filizola, Jean-Michel Martinez, Elisa Armijos and André Nascimento

Geosciences 2018, 8(7), 265; https://doi.org/10.3390/geosciences8070265 - 19 Jul 2018

Cited by 7 | Viewed by 4189

Abstract

This study focuses on the confluence of two major rivers of the world, the Solimões River (white waters) and Negro River (black waters). Surface suspended sediment samples (SSC) and spectroradiometer taken along transverse profiles at 500 m intervals over a distance of 10 [...] Read more.

This study focuses on the confluence of two major rivers of the world, the Solimões River (white waters) and Negro River (black waters). Surface suspended sediment samples (SSC) and spectroradiometer taken along transverse profiles at 500 m intervals over a distance of 10 km, as well as satellite images (MODIS) during the hydrological year, were used to follow suspended sediment variability. In January and February, the confluence is dominated by white waters from the Solimões River in the two banks, and in June and July in the right bank by black waters from the Negro River and in the left bank by clear waters from the Solimões River. We found that indirect tools, such as reflectance obtained by spectrometer or MODIS images, can be used to determine surface suspended sediments in a contrasting zone. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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16 pages, 3772 KiB

Open AccessArticle

Vegetated Channel Flows: Turbulence Anisotropy at Flow–Rigid Canopy Interface

by Gerardo Caroppi, Paola Gualtieri, Nicola Fontana and Maurizio Giugni

Geosciences 2018, 8(7), 259; https://doi.org/10.3390/geosciences8070259 - 11 Jul 2018

Cited by 23 | Viewed by 4523

Abstract

This laboratory study aimed at investigating the mean and turbulent characteristics of a densely vegetated flow by testing four different submergence ratios. The channel bed was covered by a uniform array of aligned metallic cylinders modeling rigid submerged vegetation. Instantaneous velocities, acquired with [...] Read more.

This laboratory study aimed at investigating the mean and turbulent characteristics of a densely vegetated flow by testing four different submergence ratios. The channel bed was covered by a uniform array of aligned metallic cylinders modeling rigid submerged vegetation. Instantaneous velocities, acquired with a three-component acoustic Doppler velocimeter (ADV), were used to analyze the mean and turbulent flow structure. The heterogeneity of the flow field was described by the distributions of mean velocities, turbulent intensities, skewness, kurtosis, Reynolds stresses, and Eulerian integral scales. The exchange processes at the flow–vegetation interface were explored by applying the turbulence triangle technique, a far less common technique for vegetated flows based on the invariant maps of the anisotropic Reynolds stress tensor. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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22 pages, 8373 KiB

Open AccessArticle

Morphodynamic Trends of the Ribb River, Ethiopia, Prior to Dam Construction

by Chalachew A. Mulatu, Alessandra Crosato, Michael M. Moges, Eddy J. Langendoen and Michael McClain

Geosciences 2018, 8(7), 255; https://doi.org/10.3390/geosciences8070255 - 9 Jul 2018

Cited by 22 | Viewed by 6491

Abstract

The meandering Ribb River flows in northwest Ethiopia to Lake Tana, the source of the Blue Nile River. The river has already undergone changes due to several human interventions, such as embanking, sand mining, water extraction and lake level regulation for hydropower. At [...] Read more.

The meandering Ribb River flows in northwest Ethiopia to Lake Tana, the source of the Blue Nile River. The river has already undergone changes due to several human interventions, such as embanking, sand mining, water extraction and lake level regulation for hydropower. At present, a dam and a weir are under construction to store and divert water for irrigation. This will strongly alter both water and sediment discharges to the downstream river reaches, causing adjustments to the morphology. Assessing the current morphodynamic trends is the first necessary step to study the future effects and find ways to mitigate them. This paper presents an analysis of the current and past river based on newly collected data, aerial photographs, SPOT and Google Earth images. The riverbed changes are derived from historical staff gauge height analysis. The effects of sediment mining and water extraction are assessed using the theory of morphodynamic equilibrium. The findings of the analysis show a reduction of sediment transport capacity in the downstream direction, which has resulted in intense sediment deposition, resulting in blockage of the Lower River reach and subsequent channel avulsion. The effects of Lake Tana level regulation on the observed processes appear to be minor. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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18 pages, 6821 KiB

Open AccessArticle

Numerical Simulation of Flow and Scour in a Laboratory Junction

by Javad Ahadiyan, Atefeh Adeli, Farhad Bahmanpouri and Carlo Gualtieri

Geosciences 2018, 8(5), 162; https://doi.org/10.3390/geosciences8050162 - 3 May 2018

Cited by 7 | Viewed by 3984

Abstract

Confluences are a common feature of riverine systems; the area of converging flow streamlines and potential mixing of separate flows. The hydrodynamics about confluences have a highly complex three-dimensional flow structure. This paper presents the results of a numerical study using the CCHE2D [...] Read more.

Confluences are a common feature of riverine systems; the area of converging flow streamlines and potential mixing of separate flows. The hydrodynamics about confluences have a highly complex three-dimensional flow structure. This paper presents the results of a numerical study using the CCHE2D code to investigate the influence of junction angle and discharge ratio on the flow and erosion patterns. The hydraulic and geometric parameters which affect the maximum relative scouring depth are analyzed. The model is first calibrated and validated. Then three discharge ratios, seven junction angles and five width ratios are considered and compared. Results generally agree with experimental data and show that the process of scouring depends on all these parameters. Numerical results demonstrate that a decrease in the ratio of the tributary width to the main channel width results in an increase in the size of the separation zone. Furthermore, the increase in the width ratio leads to a decrease in the maximum depth of bed erosion. Finally, the maximum depth of bed erosion at the confluence increases with the increasing angle of the junction. Full article

(This article belongs to the Special Issue Beyond the Channel—Investigating Processes at Crucial Fluvial Interfaces)

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