I. Guymer

Dynamics of the Turbidity Maxima in the Upper Humber Estuary System, UK

Abstract Turbidity maxima in estuaries are important due to their influence on primary production, pollutant flushing, fish migration and dredging. There is a need for a better understanding of the dynamics of turbidity maxima. Results are given for the response of turbidity maxima and salinity in the tidal Trent and (Yorkshire) Ouse over time scales of a semidiurnal tide to an annual cycle with respect to tidal and fluvial influences. Under low fresh water flow conditions sediment moves landward and the turbidity maximum is observed upstream of the fresh–salt water interface. Longitudinal dispersion processes governing salinity appear to be dominated by transverse shear induced effects. Suspended solids concentrations within the turbidity maximum are strongly influenced by tidal asymmetry and suspended solids vertical concentration gradient effects and their associated influences. It is proposed that the higher concentrations of suspended solids in the Ouse estuary, compared with the tidal Trent, are due to the influence of the lower fluvial discharge in the appropriate reaches.

Removal of linear alkylbenzene sulfonate from a small Yorkshire stream: contribution to GREAT-ER project #7.

An in-stream removal experiment has been carried out in Red Beck, a small stream which receives effluent from Shibden Head Sewage Treatment Works. This trickling filter works serves a purely domestic population of 9408 but is scheduled to be closed, and the flows diverted to another works, as part of Yorkshire Waters continuing capital investment programme. An anionic detergent, linear alkylbenzene sulfonate (LAS), boron, and standard water quality parameters have been measured at seven sites downstream of the effluent discharge point. Time of travel has been measured by detection of a fluorescent dye added to the effluent sampling chamber, and the increase in flow as the river proceeds through the catchment has been determined from current flow measurements, and from boron dilution data. Assuming a first order removal mechanism, the overall half-life for LAS removal is just over 2 h (2 h 14 min). Faster removal takes place in the upper portion of the stream, and removal over the last five sampling points is somewhat slower, with a half-life of 2 h 40 min. This removal may comprise both primary biodegradation and the deposition of suspended matter to which the surfactant has been adsorbed. There was no significant difference in the removal half-lives of the individual alkyl chain length homologues.

Response to the slug injection of a tracer—a large-scale experiment in a natural river

Abstract A unique, large-scale tracer test performed along a 90-km reach of a natural river is presented. This method was crucial for evaluating the impact of a retention reservoir on protected areas of the river downstream, and to assess the threats due to potentially catastrophic releases of toxic substances into that river. The response to the slug injection of a soluble tracer is assumed to imitate the characteristics of a soluble pollutant, so an understanding of how tracers mix and disperse in a stream is essential to understanding the processes of pollution transport. The procedure applied during this experiment consisted of the instantaneous injection of a known quantity of Rhodamine WT into the stream and the determination of the temporal variation in concentration of the tracer at sites as it moved downstream. The results were analysed from the perspective of a transient storage model. Relevant model parameters were evaluated by fitting the computed breakthrough curves to the observed ones on a reach-by-reach basis.

Bulk Flow Resistance in Vegetated Channels: Analysis of Momentum Balance Approaches Based on Data Obtained in Aging Live Vegetation

Previous work has sought to investigate flow resistance caused by vegetation in open channels. However, many existing flow resistance models are based on data from artificial plant mimics in laboratory channels and are untested with live, aging vegetation and may neglect some of the key effects. This paper presents results from a study using two types of live vegetation grown within a laboratory channel. A series of flow resistance tests were conducted over a time period sufficient to observe changes caused by growth. The effects of the vegetation on bulk flow resistance are presented and discussed. The results are compared with predictions made by existing practical momentum balance based models for flow through emergent vegetation, and new empirical relationships are presented. The work shows that momentum balance methods can provide an accurate prediction of flow depth in live vegetation provided that the relationship between bulk drag coefficient and flow is known. This relationship has been shown to ...

Longitudinal Dispersion Coefficients Within Turbulent and Transitional Pipe Flow

The longitudinal dispersion coefficient is used to describe the change in characteristics of a solute cloud, as it travels along the longitudinal axis of a pipe. Taylor (1954) proposed a now classical expression to predict the longitudinal dispersion coefficient within turbulent pipe flow. However, experimental work has shown significant deviation from his prediction for \(Re <\) 20,000. This paper presents experimental results from tracer studies conducted within the range 2,000 \(< Re <\) 50,000, from which longitudinal dispersion coefficients have been determined. Initial results are also presented for a numerical model that aims to predict the longitudinal dispersion coefficient over the same range of Reynolds numbers.

Vertical variation of mixing within porous sediment beds below turbulent flows

Abstract River ecosystems are influenced by contaminants in the water column, in the pore water and adsorbed to sediment particles. When exchange across the sediment‐water interface (hyporheic exchange) is included in modeling, the mixing coefficient is often assumed to be constant with depth below the interface. Novel fiber‐optic fluorometers have been developed and combined with a modified EROSIMESS system to quantify the vertical variation in mixing coefficient with depth below the sediment‐water interface. The study considered a range of particle diameters and bed shear velocities, with the permeability Péclet number, PeK between 1000 and 77,000 and the shear Reynolds number, Re*, between 5 and 600. Different parameterization of both an interface exchange coefficient and a spatially variable in‐sediment mixing coefficient are explored. The variation of in‐sediment mixing is described by an exponential function applicable over the full range of parameter combinations tested. The empirical relationship enables estimates of the depth to which concentrations of pollutants will penetrate into the bed sediment, allowing the region where exchange will occur faster than molecular diffusion to be determined.

Dimensionless Method to Characterize the Mixing Effects of Surcharged Manholes

Solute transport processes affect the performance of a wide range of water engineering structures. In the context of urban drainage, the effects of dispersion may act to reduce peak concentrations associated with intermittent discharges or cause pollutants to be retained for longer or shorter durations than mean travel times would predict. With respect to surcharged manholes, previous research employed laboratory experiments to identify best-fit parameter values for the first-order advection-dispersion equation (ADE) and aggregated dead zone (ADZ) routing models. This paper presents data from a new set of smaller-scale laboratory measurements and demonstrates that the threshold depth separating two distinct hydraulic regimes can be identified independently of scale. However, the fitted ADE and ADZ routing model parameters are not generally amenable to conventional hydraulic scaling, because the models do not provide good fits to the observed data. An alternative approach is proposed based on the cumulative residence time distribution (CRTD). This approach is shown to be scalable and practical. The solute transport characteristics of a specific configuration of a surcharged manhole are shown to be characterized by just two dimensionless CRTDs corresponding to prethreshold and postthreshold surcharge depths.

The use of deconvolution techniques to identify the fundamental mixing characteristics of urban drainage structures

Mixing and dispersion processes affect the timing and concentration of contaminants transported within urban drainage systems. Hence, methods of characterising the mixing effects of specific hydraulic structures are of interest to drainage network modellers. Previous research, focusing on surcharged manholes, utilised the first-order Advection-Dispersion Equation (ADE) and Aggregated Dead Zone (ADZ) models to characterise dispersion. However, although systematic variations in travel time as a function of discharge and surcharge depth have been identified, the first order ADE and ADZ models do not provide particularly good fits to observed manhole data, which means that the derived parameter values are not independent of the upstream temporal concentration profile. An alternative, more robust, approach utilises the systems Cumulative Residence Time Distribution (CRTD), and the solute transport characteristics of a surcharged manhole have been shown to be characterised by just two dimensionless CRTDs, one for pre- and the other for post-threshold surcharge depths. Although CRTDs corresponding to instantaneous upstream injections can easily be generated using Computational Fluid Dynamics (CFD) models, the identification of CRTD characteristics from non-instantaneous and noisy laboratory data sets has been hampered by practical difficulties. This paper shows how a deconvolution approach derived from systems theory may be applied to identify the CRTDs associated with urban drainage structures.

One-Dimensional Mixing Model for Surcharged Manholes

Mixing and dispersion processes affect the timing and concentration of contaminants transported within urban drainage systems. Hence, methods of characterizing the mixing effects of specific hydraulic structures are of interest to drainage network modelers. Previous research, focusing on surcharged manholes, used the first-order advection-dispersion equation (ADE) and aggregated dead zone (ADZ) models to characterize dispersion. However, although systematic variations in travel time as a function of discharge and surcharge depth were identified, the ADE and ADZ models did not provide particularly good fits to observed manhole mixing data, which meant that the derived parameter values were not independent of the upstream temporal concentration profile, and no rules for predicting parameter values based on manhole size and configuration were provided. An alternative, more robust, method is described by using the system’s cumulative residence time distribution (CRTD). This paper shows how a deconvolution approach derived from systems theory may be applied to identify, from laboratory data, the CRTDs associated with surcharged manholes. Archive laboratory data are reanalyzed to demonstrate that the solute transport characteristics of a surcharged manhole with straight-through inflow and outlet pipes over a range of flow rates and surcharge depths may be modeled using just two dimensionless CRTDs, one for prethreshold and the other for postthreshold surcharge depths. The model combines the derived manhole CRTDs with a standard (Gaussian) pipe dispersion model to provide temporal solute concentration profiles that are independent of both scale and the ratio of the pipe and manhole diameters.

TRANSVERSE MIXING IN A TRAPEZOIDAL COMPOUND OPEN CHANNEL

Transverse mixing characteristics of solute in the open channel flow can provide useful information for river environmental management. The lateral mixing coefficient is a crucial parameter for reproducing the transverse mixing either by numerical simulation or by analytical prediction. Since the solute mixing can be greatly affected by the lateral variations in water depth, mixing coefficient should be determined in each sub-section (i.e., the main channel, side slope and flood plain) separately. In this article, the transverse mixing in a symmetric trapezoidal compound channel was studied based on laboratory measurement of longitudinal and transverse velocity components and lateral distribution of solute concentration. The lateral mixing coefficient was estimated by adopting different Schmidt numbers in different sub-sections divided according to the developing trend of the eddy viscosity, and finally a piecewise linear profile of mixing coefficient was adopted to analytically predict the transverse solute concentration. The comparison between the analytically predicted data and the measuring solute concentration proved that this is an effective way to estimate the lateral mixing in the open channel flow with lateral variations in water depth.