Water-bed Interface Research Articles

Measuring Bed Exchange Properties of Cohesive Sediments Using Tripod Data

The Krone–Partheniades (K-P) framework has been used for decades to quantify and analyze the sediment exchange at a water–bed interface. Measuring the erosion and deposition parameters that are part of this framework requires time-consuming field observations. Additionally, the erosion parameters are measured independently of deposition parameters, while in reality they are coupled. In numerical models applying the K-P framework these parameters are often assumed to be constant in time and mutually independent. In this study, we develop a relatively simple methodology to determine the erosion and deposition parameters, using conventional near-bed observations of bed level, sediment concentration and flow velocity. This methodology is subsequently applied to tripod observations collected in the Changjiang estuary, China, to compute continuous time-varying erosion and settling parameters. We propose a diagram to visualize the interdependency and accuracy of erosion and deposition parameters, which is the input for K-P framework models requiring this interdependency.

GAIA - a unified framework for sediment transport and bed evolution in rivers, coastal seas and transitional waters in the TELEMAC-MASCARET modelling system

In rivers, coastal seas and transitional waters, sediment transport processes involve a variety of interacting factors that dynamically vary over time and space. The flow dynamics within these highly heterogeneous natural systems influences the spatial patterns of erosion and deposition of the bed sediments, which in turn shapes and conditions the bottom morphology. By taking advantage of the established modelling framework in both two- and three dimensions for unstructured meshes proposed within the Telemac-Mascaret system, the new module Gaia provides a code structure for solving sediment transport and morphological evolution problems. By a clear treatment of sedimentary processes that happen in the water column, in the bed structure, and at the water–bed interface, Gaia efficiently manages the spatial and temporal variability of sediment size classes, properties and transport modes for two- and three dimensions. In addition, this module can easily be expanded and customised to particular requirements by modifying user-friendly, easy-to-read, and well-documented FORTRAN-90 subroutines.

Quantification of electrical resistance to estimate NaCl behavior in a column under controlled conditions

Accurate prediction of solute transport processes in surface water and its underlying bed is an important task not only for proper management of the surface water but also for pollution control in these water bodies. Key issue in this task is an estimation of parameters as diffusion coefficient and velocity for solute transport both in water body and in the underlying bed. This estimation would greatly help us to understand the deposition and release mechanism of solute across the water-bed interface. In this study, a column experiment was conducted in laboratory to estimate the velocity and diffusion coefficient of sodium chloride (NaCl) in water body and underlying sand layer (bed). The column used with a diameter of 30 cm and a height of 100 cm, was filled with sand at the lower half part and water at the upper half part. Total 64 stainless steel electrodes were installed on its surface around. The sodium chloride solution was injected from the top of the column, and electrical resistance between electrodes was monitored for 71 h. Then the dimensionless resistance breakthrough curve was fitted with one dimensional analytic solution for solute transport and the related diffusion coefficient and velocity parameters were estimated. The results show that the NaCl transport velocity was high in the water body but extremely low in the underlying sand layer (bed). The diffusion coefficient estimated in sand layer coincides with those reported well. This indicates that the electrical resistance based solute transport parameter estimation method is not only effective but also has an advantage of multipoints monitoring. This is useful both in mapping solute transport parameter for solute transport process analysis and in providing parameter input for solute transport numerical modeling.

Discussion of “Combined Effects of Wind and Streamflow on Gas-Liquid Transfer Rate” by Zhiyong Duan, James L. Martin, William H. McAnally, and Richard L. Stockstill

In their interesting and timely contribution, the authors proposed atheoretical model to predict the wind-streamflow–driven gas-liquidtransfer rate. When wind blows over the stream surface, turbulenceis generated at both the air-water interface and the water-bed inter-face, which is the drivingforce of the surface renewal movement ofwater parcels. The total surface renewal rate is considered as thesum of the effective surface renewal rates driven by the turbulencegenerated at these two interfaces. This model correlates the gas-liquid transfer rate with the dynamic fluid parameters such as windspeed and stream velocity.The discussers would like to highlight some points raised in thepaper. First, the authors considered at the air-water interface a tur-bulent boundary layer formed by a viscous sublayer (VBL) and anunderlying turbulent layer (TL). This structure of layers is assumedto control the gas-transfer process. Also, this structure controls themomentum transfer.However, a more detailed analysis is needed to distinguish be-tween momentum transfer and mass transfer (Gualtieri and PulciDoria2008).Farfromtheinterface,intheturbulentboundarylayer,both momentum and mass transport are dominated by turbulentmotions, which provide full vertical mixing. Thus, the main bodyof the liquid phase is assumed to bewell mixed with the gas profilepracticallyuniformatthebulkconcentration.IntheTL,momentumand mass-transport processes can be related to the turbulent eddyviscosity ν

Studying chemical exchange across the water-bed interface in Golubaya Bay, the Black Sea

The results of first application of automated bottom station for studying chemical exchange across the water-bed interface (IO RAS lander) were obtained for the case of sediments in the northwestern Black Sea near Gelendzhik (Golubaya Bay). The lander is equipped with bottom chambers, sensors of environmental characteristics, and samplers, allowing fluxes of chemical elements through the water-bed interface to be determined directly. Bottom sediments were represented by organogenous silts containing hydrogen sulfide. The high activity of diagenetic processes resulted in a higher consumption of O2 by sediments (the flux from the bottom water was 130 mM/(m2/day), high fluxes of biogenic elements (P, Si,) and metals (Mn, Fe). At the same time, fluxes across the water-bed interface were evaluated based on the difference between concentrations of these elements in the near-bed and silt water (Fick’s law). The obtained data demonstrate the priority of direct measurements of fluxes (lander) over the calculation method. The first application of the lander suggests the need for methodological improvements both of engineering character and in chemical-analytical support.

Combined Effects of Wind and Streamflow on Gas-Liquid Transfer Rate

A theoretical model is developed to predict the wind-streamflow-driven gas-liquid transfer rate. When wind blows over the stream surface, turbulence is generated at both the air-water interface and the water-bed interface, which is the driving force of the surface renewal movement of water parcels. The total surface renewal rate is considered as the sum of the effective surface renewal rates driven by the turbulence generated at these two interfaces. The gas-liquid transfer rate is formulated as a function of the total surface renewal rate based on the surface renewal theory. The concept of serial resistance, shear velocity, and effective viscous layer play significant roles in the model development. This model correlates the gas-liquid transfer rate with the dynamic fluid parameters such as wind speed and stream velocity. The predictions of this model show good agreement with existing experimental data. This model can be applied to predict the gas-liquid transfer rate in wind-streamflow-driven systems.

Partition of suspended and riverbed sediments related to the salt-wedge in the lower reaches of a small mountainous river

A month-long comprehensive field experiment during the beginning of the flood season of a small mountainous river in southern Taiwan was carried out in 2004. The major goal of the study was to investigate the effect of hydrodynamic sorting related to the salt-water intrusion on the spatial variability of suspended and riverbed sediments and their coupling of different sizes. The experiment included the deployment of an instrumented tetrapod near the river mouth with an upward-looking ADCP and two CTDs mounted at 50 and 100 cm above the bed (cmab), respectively. On three different days along the river, turbidity, salinity and temperature of the water column were profiled; and water samples were taken from the surface and near the bed at different stations. Additionally, one sediment sample was also taken from the riverbed. Suspended sediment concentrations (SSC) were analyzed for five different sizes, i.e. > 500, 250–500, 63–250, 10–63 and 1.2–10 μm by filtration. For each riverbed sample the size-composition was analyzed for the subsample that contained lithogenic and nonlithogenic components; and the subsample having major nonlithogenic components (organic matter, carbonates, and biogenic opal) was removed. The grain-size frequency distributions of the riverbed samples were analyzed using a laser particle analyzer. The results were grouped into the following size classes: > 473, 249–473, 62–249, 10–62, and < 10 μm for comparison with those of the suspended sediments near the riverbed. Some suspended sediment samples were analyzed for POC and PON. Some riverbed samples were analyzed for TOC. Statistical methods of linear regression and Empirical Orthogonal (Eigen) Function (EOF) were used in the data analysis. Two-layered estuarine circulation pattern was observed at the tetrapod site. The tidally-driven salt-water intrusion is the major factor influencing the hydrodynamics of the Gaoping River, which in turn, affect the longitudinal and vertical distribution of the suspended sediments and the longitudinal distribution of the riverbed sediments. During the flood, the intrusion front of the salt-wedge creates a dynamic barrier. Upriver from this barrier, the riverbed substrate is coarser, composed of sediments mostly coarser than 249 μm. Within the salt-wedge the riverbed substrate is finer, consisting of mostly mud (< 62 μm). The barrier creates a trap on the riverbed immediately seaward from the intrusion front, retaining higher percentages of clay-sized sediments and TOC. The barrier also creates partition in the terrestrial and marine sources of organic matter in the suspended and riverbed sediments. Within the salt-wedge the major contributor of riverbed TOC is the clay-sized marine sediment transport upriver by the intruding seawater. The terrestrial POC is a minor contributor to the riverbed TOC. The riverbed and suspended sediments are coupled. Most size-classes in corresponding suspended and riverbed sediments have a reciprocal relationship (negative feedback) through resuspension and deposition on the water–bed interface. The only size-class of 62–473 μm on the riverbed and 63–250 μm in the suspension are co-varying (positive feedback). This size class contains largely a transient floc population that is formed and disintegrated in-situ both on the riverbed and in the suspension in the course of a tidal cycle.

Modeling Streamflow-Driven Gas-Liquid Transfer Rate

A streamflow-driven gas-liquid transfer rate model was developed. This model was developed in terms of total surface renewal rate and gas diffusion coefficient. The total surface renewal rate was considered to be the sum of the surface renewal rates caused by the turbulence from the air-water interface and that from the water-bed interface. A general mixing length formula and a vertical fluctuation velocity formula was developed to formulate the surface renewal rate. Predictions of this model show good agreements with the measurements in previous studies and the predictions of Churchill's, O'Connor-Dobbins', and Owens-Gibbs' formulae. It was indicated in some previous studies that the existing streamflow-driven gas-liquid transfer rate formulae have limited application ranges. The model developed in this study can be applied for all the normal ranges of flow velocity and water depth in natural rivers, which generalizes and simplifies the applications.

Modeling Lake Ladoga Zoobenthos and Studying Its Role in Phosphorus Exchange at the Water–Bed Interface

A zoobenthos submodel is included in an ecosystem model. The new model is used to assess the role of zoobenthos in the phosphorus exchange at the water–bed interface, in particular, the phosphorus cycle in the water body ecosystem is described more accurately. The calculated zoobenthos distribution over the bed of a lake can be used in modeling the lake ichthyocenoses.

Identification of In-Sewer Sources of Organic Solids Contributing to Combined Sewer Overflows

Previous research has shown that combined sewer systems are the main source of particle and organic pollution during rainfall events contributing to combined sewer overflow. The aim of this article is to identify in an urban catchment area called “Le Marais”, in the center of Paris, the types of sediments that are eroded and contribute to the pollution of combined sewer overflow. Three sediment types are considered : granular material found in the inverts of pipes, organic biofilms and organic sediment at the water bed interface, identified as an immobile layer in the “Le Marais” catchment area. The method used consists, firstly, of sampling and assessing the organic pollutant loads and metallic loads of the particles in each type of sediment. Then, the mass of each type of sediment is assessed. The mass and the characteristics of each type of sediment is finally compared to the mass and characteristics of the particles eroded in the catchment area, estimated by mass balances, in order to find the source of eroded particles. The only identified type of deposit that can contribute to combined sewer overflows is the organic layer. Indeed, the solids of this layer have mean and metallic loads that are of the same order of magnitude as the eroded particles. Moreover, the mass of the organic layer considered over different time scales is of the same order of magnitude as the eroded masses during rainfall events and an erosion experiment showed that the organic layer is actually eroded.

Les dépôts en réseau d'assainissement unitaire, une importante source de pollution des rejets urbains de temps de pluie

Previous research has shown that combined sewer systems are an important source of particle and organic pollution during rainfall events contributing to combined sewer overflow. The aim of this articles is to identify in an urban catchment area called Le Marais, in the center of Paris, the types of sediments that are eroded and contribute to the pollution of combined sewer overflow. Three sediment types are considered : granular material found in the inverts of pipes, organic biofilms and organic sediment at the water bed interface, identified as an immobile layer in the Le Marais catchment area.

Sources and erosion of organic solids in a combined sewer

Sewer deposits play a significant role as a source of pollution in combined sewer overflows. The objective of this paper is to identify, using a direct approach on a sewer length, the main source of organic solids among the different deposits and their erosion processes. The different deposits studied were large inorganic solids at the bottom of the sewer, biofilms on the walls of the sewer and organic solids at the water–bed interface. The method applied consisted firstly of analysing very precisely (mass and pollutant loads) the solids in the three types of deposits along a sewer line with a length of 150 m. Then, the deposits were eroded by in situ flushing experiments using tap water. Finally the nature and the mass of particles eroded along the length of the sewer were compared with the nature and the mass of solids in each type of sewer sediment in order to identify the main source of organic solids in the sewers. This comparison indicates that the organic layer at water–bed interface is the main source of eroded solids. An erosion test carried out under continuous observation confirms that the organic layer is actually eroded.

Mass transfer mechanism in a porous riverbed

The mass transfer mechanism in a porous riverbed was investigated in two experimental open channels which have porous beds composed of 1.9 and 4.08 cm diameter ceramic balls, respectively. Vertical diffusion coefficients in the porous bed were determined by analyzing the diffusional pattern of tracer (NaCl) from the overlying water into the deeper region of the porous bed. Velocity profiles both over and under the water-bed interface and turbulence at the interface was measured. The diffusion coefficients in the porous bed were expressed by the product of velocity component and mixing length along every depth of the porous bed. Near the interface, the diffusion coefficient was expressed by the product of the turbulent intensity and the void scale of the porous layer. In the deeper region of the porous bed, the diffusion coefficient was expressed by the product of the time-averaged velocity and the void scale.