Simon Tait

Aquatic interfaces: a hydrodynamic and ecological perspective

ABSTRACT Ecologically-appropriate management of natural and constructed surface water bodies has become increasingly important given the growing anthropogenic pressures, statutory regulations, and climate-change impacts on environmental quality. The development of management strategies requires that a number of knowledge gaps be addressed through interdisciplinary research efforts particularly focusing on the water-biota and water-sediment interfaces where most critical biophysical processes occur. This paper discusses the current state of affairs in this field and highlights potential paths to resolve critical issues, such as hydrodynamically-driven mass transport processes at interfaces and associated responses of organisms through the development of traits. The roles of experimental methods, theoretical modelling, statistical tools, and conceptual upscaling methods in future research are discussed from both engineering and ecological perspectives. The aim is to attract the attention of experienced and emerging hydraulic and environmental researchers to this research area, which is likely to bring new and exciting discoveries at the discipline borders.

Sediment transport over a flat bed in a unidirectional flow: simulations and validation

A discrete particle model is described which simulates bedload transport over a flat bed of a unimodal mixed–sized distribution of particles. Simple physical rules are applied to large numbers of discrete sediment grains moving within a unidirectional flow. The modelling assumptions and main algorithms of the bedload transport model are presented and discussed. Sediment particles are represented by smooth spheres, which move under the drag forces of a simulated fluid flow. Bedload mass–transport rates calculated by the model exhibit a low sensitivity to chosen model parameters. Comparisons of the calculated mass–transport rates with well–established empirical relationships are good, strongly suggesting that the discrete particle model has captured the essential elements of the system physics. This performance provides strong justification for future interrogation of the model to investigate details of the small–scale constituent processes which have hitherto been outside the reach of previous experimental and modelling investigations.

Towards quantification of uncertainty in predicting water quality failures in integrated catchment model studies.

This paper describes the development and application of a method for estimating uncertainty in the prediction of sewer flow quantity and quality and how this may impact on the prediction of water quality failures in integrated catchment modelling (ICM) studies. The method is generic and readily adaptable for use with different flow quality prediction models that are used in ICM studies. Use is made of the elicitation concept, whereby expert knowledge combined with a limited amount of data are translated into probability distributions describing the level of uncertainty of various input and model variables. This type of approach can be used even if little or no site specific data is available. Integrated catchment modelling studies often use complex deterministic models. To apply the results of elicitation in a case study, a computational reduction method has been developed in order to determine levels of uncertainty in model outputs with a reasonably practical level of computational effort. This approach was applied to determine the level of uncertainty in the number of water quality failures predicted by an ICM study, due to uncertainty associated with input and model parameters of the urban drainage model component of the ICM. For a small case study catchment in the UK, it was shown that the predicted number of water quality failures in the receiving water could vary by around 45% of the number predicted without consideration of model uncertainty for dissolved oxygen and around 32% for unionised ammonia. It was concluded that the potential overall levels of uncertainty in the ICM outputs could be significant. Any solutions designed using modelling approaches that do not consider uncertainty associated with model input and model parameters may be significantly over-dimensioned or under-dimensioned. With changing external inputs, such as rainfall and river flows due to climate change, better accounting for uncertainty is required.

A Low Power Wireless Sensor Network for Gully Pot Monitoring in Urban Catchments

Sewer and gully blockages are the main cause of residential sewer flooding in the U.K. A low-cost and power efficient wireless sensor mesh networking communication system has been designed, developed and implemented to provide adequate warning on potential blockage incidents to prevent sewer failure. By monitoring the water level of the gully pot at each residential property, the water company will be proactively informed of the best course of actions to eliminate the causal problem, i.e., blockage and leakage within the sewer infrastructure. Hence, the number of residential sewer flooding and pollution incidents can be reduced. The prototype system consists of eight Zigbee based wireless sensor nodes and a GPRS enabled data gatherer. Each Zigbee sensor node comprises of a radio transceiver, a data acquisition board and an acoustic sensor probe. Field trials were carried out in an outdoor scenario to cross-validate the theoretical and practical performance of the prototype system. The results in terms of durability of sensors, sensor nodes and gateways and reliability of communication under real operational conditions and within a typical inner city urban environment are discussed. The problems encountered and solutions to tackle these problems were addressed.

Wall-Wake Flows Downstream of a Sphere Placed on a Plane Rough Wall

The time-averaged characteristics of turbulent wall-wake flows downstream of a sphere placed on a rough wall are studied. The profiles of the defect of streamwise velocity, Reynolds shear stress, and turbulence intensities exhibit some degree of similarities when they are scaled by their respective peak defect values. For the velocity defect profiles, the vertical distances are scaled by the height of the location of the half-peak velocity defect. However, for the defect profiles of the Reynolds shear stress and the turbulence intensities, the vertical distances are scaled by the height of the location of the half-peak Reynolds shear stress defect. The magnitudes of the peak defect of all the quantities diminish with the distance downstream of the sphere characterizing the recovery of their undisturbed profiles. Additionally, the theoretical similarity solution for the velocity defect profiles is obtained. The third-order correlations imply that in the inner layer of wall wakes, a streamwise acceleration ...

Probabilistic description of grain resistance from simultaneous flow field and grain motion measurements

Experiments were carried out using a mobile gravel bed placed in a tilting flume with a modified particle image velocimetry (PIV) system. Individual grain movements were surveyed using data from time series of images. Near-bed velocity flow field measurements were made simultaneously above the same area of the sediment surface by applying cross-correlation techniques to the collected plan view images. Statistics of grain motions were collected through a semiautomatic procedure. Significant changes in the flow field were observed in the proximity of the entrained or deposited particles. A strong correlation is shown between the changes in the local streamwise and lateral velocity and the movement of the grains. The theory of Grass is revisited and developed based on the experimental results. The probability distribution of individual grain resistance has been derived from the statistics of the near-bed velocity field and of the entrainment risk.

Sewer system operation into the 21st century, study of selected responses from a UK perspective

The present study examined the future of UK sewer systems at timescales, up to 2020 and to 2080. The most significant drivers for change were identified for different socio-economic and climate scenarios. A wide range of potential responses to these drivers were studied using the analysis of literature, expert workshops and limited modelling. The assessment of effectiveness of a number of these responses are presented and discussed. In the short to medium term it was shown that technological responses were most likely to be effective. In the longer-term controls on urbanisation and land use were seen to be more effective.

Sediment transport mechanics

The Editor of Acta Geophysica and the Guest Editors wish to dedicate this Topical Issue on Sediment Transport Mechanics to the memory of Stephen Coleman, who died recently. During his career, Stephen had made an outstanding scientific contribution to the topic of Sediment Transport. The level of his contribution is demonstrated in the paper by Aberle, Coleman, and Nikora included in this issue, on which he started working before becoming aware of the illness that led to his untimely death. For scholars and colleagues Stephen remains an example of intellectual honesty and scientific insight.

The first foul flush in combined sewers: an investigation of the causes

A previously developed numerical model, able to simulate the erosion from an organic in-pipe deposit, was used to investigate the physical parameters that control the first foul flush in combined sewers. Systematic adjustment of physical parameters indicated that the accurate characterisation of bed properties, i.e. the surface erosional strength and its variation with depth, was more important than the accurate description of the imposed hydraulic conditions.

Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages: 2. Experiments

Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub-processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near-bed flow and the particles having different exposure.