BinLiang Lin

Hydro-environmental modelling for bathing water compliance of an estuarine basin

In recent years, considerable investment has been committed to sewerage infrastructure and new sewage treatment plants in the catchment surrounding an estuarine basin along the north-west coast of England. Although this capital investment has resulted in a marked reduction in the input of bacterial loads, relatively high counts of faecal indicator organisms are still being encountered in the coastal receiving waters, and the local bathing waters continue to fail on occasions to comply with the European Community (EC) Bathing Water Directive (1976) mandatory standards. Details are given herein of a comprehensive modelling study aimed at quantifying the impact of various bacterial inputs into the estuary and surrounding coastal waters on the bathing water quality. The model domain includes the coastal area and the entire estuary (namely the Ribble) up to the tidal limits of its tributaries. Faecal coliforms have been used as the main water quality indicator organisms. The numerical model developed for this study combines a depth integrated two-dimensional coastal model and a cross-sectionally integrated one-dimensional river model, and is capable of predicting water surface elevations, velocity fields and faecal coliform concentration distributions across the entire model domain. The hydrodynamic model was calibrated using water level and velocity measurements from three surveys and then validated against measured data from three other surveys. In order to predict the faecal coliform concentration distributions, variable faecal coliform decay rates were used, i.e. different values of decay rates were applied to the coastal and riverine waters, for day- and nighttime, and for wet and dry weather conditions. The maximum and minimum decay rates used were 2.32/day and 0.71/day for the dry and wet weather surveys, respectively. The model was then applied to (i) assess the impact of previous discharge strategies and investigate the effectiveness of future capital investment works and (ii) predict the impact of a range of strategic options, including: the effects of adding UV treatment, constructing storm water storage tanks and incorporating various combined sewer overflow (CSO) discharge scenarios for different weather conditions.

An implicit numerical algorithm for solving non‐hydrostatic free‐surface flow problems

Details are given of the development of a two-dimensional vertical numerical model for simulating unsteady free-surface flows, using a non-hydrostatic pressure distribution. In this model, the Reynolds equations and the kinematic free-surface boundary condition are solved simultaneously, so that the water surface elevation can be integrated into the solution and solved for, together with the velocity and pressure fields. An efficient numerical algorithm has been developed, deploying implicit parameters similar to those used in the Crank–Nicholson method, and generating a block tri-diagonal algebraic system of equations. The model has been applied to simulate a range of unsteady flow problems involving relatively strong vertical accelerations. The results show that the numerical algorithm described is able to produce accurate predictions and is also easy to apply. Copyright

Numerical modelling of sediment-bacteria interaction processes in surface waters.

Faecal bacteria exist in both free-living and attached forms in surface waters. The deposition of sediments can take faecal bacteria out of the water column and to the bed. The sediments can subsequently be re-suspended into the water column, which can then lead to the re-suspension of the faecal bacteria of the attached form back into the water column, where it may desorb from the sediments. Therefore, the fate and transport of faecal bacteria is highly related to the governing sediment transport processes, particularly where these processes are significant. However, little attempt has been made to model such processes in terms of predicting the impact of the sediment fluxes on faecal bacteria levels. Details are given of the refinement of a numerical model of faecal bacteria transport, where the sediment transport processes are significant. This model is based on the model DIVAST (Depth Integrated Velocities And Solute Transport). Analytical solutions for steady and uniform flow conditions were derived and used to test the sediment-bacteria interaction model. After testing the sediment-bacteria interaction model favourably against known results, the model was then set up for idealized case studies to investigate the effects of sediment on bacteria concentrations in the water column. Finally the model was applied to a simplified artificial flooding study to investigate the impact of suspended sediment fluxes on the corresponding bacteria transport processes. The model predictions have proved to be encouraging, with the results being compared to field measurements.

Numerical assessment of flood hazard risk to people and vehicles in flash floods

Flash flooding often leads to extremely dangerous and sometimes catastrophic conditions in rivers due to characteristics such as: short timescales, the limited opportunity for issuing warnings, and the frequent high average mortality. Many past extreme flood events have been accompanied by flash floods, and they have also been one of the main sources of serious loss of human life among the worlds worst natural disasters. Flash floods can also cause large loss of property, such as the recent floods in Pakistan and the damage to vehicles in the 2004 Boscastle flood in the UK. It is therefore desirable to be able to assess the degree of safety of people and vehicles during flash floods using numerical models. In the current study, an algorithm for assessing the flood hazard risk to people and vehicles has been integrated into an existing two-dimensional hydrodynamic model capable of simulating flash floods. In the algorithm, empirical curves relating water depths and corresponding critical velocities for children and adults, developed by previous researchers, are used to assess the degree of people safety, and a new incipient velocity formula is used to evaluate the degree of vehicle safety. The developed model was then applied to three real case studies, including: the Glasgow and Boscastle floods in the UK, and the Malpasset dam-failure flood in France. According to the analysis of model predictions, the following conclusions have been obtained: (i) simulated results for the Glasgow flood showed that children would be in danger of standing in the flooded streets in a small urban area; (ii) simulations for the Boscastle flood indicated that vehicles in the car park would be flushed away by the flow with high velocity, which indirectly testified the predictive accuracy of the incipient formula for vehicles; and (iii) simulations for the Malpasset dam-failure flood showed that the adopted method for the assessment of people safety was applicable, and some local people living below the dam would have been swept away, which corresponded well with the report of casualties. Therefore, the developed integrated model can be used to evaluate the flood hazard risk to people and vehicles in flash floods, and these predictions can be used in flood risk management.

Development of an integrated model for assessing the impact of diffuse and point source pollution on coastal waters

Pollution loads from land sources and their impact on the receiving waters can be predicted by using land-use and surface water quality models, respectively. In this paper details are given of the development of an integrated modelling system for managing water quality in coastal basins. The system includes a model linking the pollution input loads to land-use and a model for simulating surface flow and chemical and biological processes. The land-use model consists of two main components: a database and a model base. It uses a GIS system, namely ArcGIS, as the model platform. The database, built in the Geo-database format, includes six data sets containing information on social, economical, bathymetrical and hydrological aspects of a study area. The model base includes a series of transfer functions that link the pollution loads to key sources, including those from: industrial, domestic and agricultural inputs, and fish and animal farming. The water quality model is a numerical model which solves the governing equations representing the physical, chemical and biological processes in coastal waters. The modelling system was set up for Bohai Bay in China, with the land-use model being used to provide nutrient loadings from individual inputs and the water quality model used to predict the impact of these inputs on the receiving waters. Model simulations were also undertaken to predict the water quality conditions for various development and management scenarios.

Predicting faecal indicator levels in estuarine receiving waters - An integrated hydrodynamic and ANN modelling approach

A new EU Bathing Water Directive was implemented in March 2006, which sets a series of stringent microbiological standards. One of the main requirements of the new Directive is to provide the public with information on conditions likely to lead to short-term coastal pollution. The paper describes how numerical models have been combined with Artificial Neural Networks (ANNs) to develop an accurate and rapid tool for assessing the bathing water status of the Ribble Estuary, UK. Faecal coliform was used as the water quality indicator. In order to provide enough data for training and testing the neural networks, a calibrated hydrodynamic and water quality model was run for various river flow and tidal conditions. In developing the neural network model a novel data analysis tool called WinGamma was used in the model identification process. WinGamma is capable of determining the data noise level, even with the underlying function unknown, and whether or not a smooth model can be developed. Model predictions based on this technique show a good generalisation ability of the neural networks. Details are given of a series of experiments being undertaken to test the ANN model performance for different numbers of input parameters. The main focus has been to quantify the impact of including time series inputs of faecal coliform on the neural network performance. The response time of the receiving water quality to the river boundary conditions, obtained from the hydrodynamic model, has been shown to provide valuable knowledge for developing accurate and efficient neural networks.

Incipient velocity for partially submerged vehicles in floodwaters

Vehicles parking in urban areas can often cause various hazards to people and buildings when they are swept away by flash floods. Therefore, it is necessary to investigate the appropriate criteria of vehicle stability in floodwaters. Different forces acting on partially submerged vehicles have been analysed, with a mechanics-based formula of incipient velocity being derived. Experiments were conducted to obtain the conditions of water depth and corresponding velocity at the threshold of vehicle instability for three typical types of die-cast model vehicles. The data were used to determine two key parameters. Incipient velocities for partially submerged prototype vehicles in floodwaters were estimated using two different approaches, including the predictions using the scale ratios and computations based on the formula. These critical conditions using the scale ratios compare well with the calculations using the derived formula, and the derived formula was also validated by the visually-observed data of swept vehicles in flash floods.

Predicting near-field dam-break flow and impact force using a 3D model

A three-dimensional (3D) numerical model based on the unsteady Reynolds equations was used to simulate near-field dam-break flows and estimate the impact force on obstacles. The model employs a projection method to solve the governing equations and the method of volume of fluid (VOF) to capture the water surface movement. The model is first applied to simulate two physical model experiments of dam-break flows. Model-predicted pressure, water depth and velocity distributions are compared with laboratory measurements. For the second case, the 3D-VOF model predictions are also compared with predictions made by a two-dimensional model. The 3D-VOF model is then used to calculate the impact force of dam-break flow on a steady obstacle. A physical model experiment is set up to assist the numerical model study. The model-predicted impact force on the obstacle and the critical condition for it to move are compared with the measurements from the experiment.

Flow and solute fluxes in integrated wetland and coastal systems

Details are given of an experimental study of the flow and tracer transport processes in an integrated wetland and coastal basin. A novel physical model was constructed to enable observations and measurements to be made of groundwater transport through a sand embankment between a wetland and coastal area. This was an idealised scale model of the West Fleet Lagoon, Chesil Beach and the adjacent coastal waters, located in Dorset, UK. An extensive set of data of the seepage fluxes through the embankment was collected by monitoring the varying water level and velocity distributions on both sides of the embankment. The transport behaviour of a conservative tracer was also studied for a constant water level on the wetland side of the embankment, while running a continuous tide on the coastal side. Time series pictures of the concentration distributions of the tracer were filmed using a digital camcorder. An integrated surface and groundwater numerical model was also used in this study to assist in the analysis, with the numerical model predictions being compared with the images recorded from the experiments. Details of the physical model, experimental procedures and the equipment used in the study are reported in this paper.

Numerical Model Assessment of Tidal Stream Energy Resources in the Severn Estuary, UK

Tidal stream systems make use of the kinetic energy of tidal movement to power turbines, in a similar manner to the way in which windmills extract energy from the wind. This method of renewable energy generation is gaining in popularity because of the high predictability of tides, the lower investment needed for tidal turbines, and the lower ecological impact, as compared with other schemes involving barrages or lagoons. It is generally considered that a mean spring peak tidal current of at least 2 m/s is required for tidal stream power to be worth exploiting. In the Severn Estuary, the peak tidal current exceeds 2 m/s, with a corresponding minimum depth of greater than 20 m, and it is thereby a potential location for tidal stream power. Previous studies cannot provide the detailed and precise distribution of tidal stream power in this estuary, and it has therefore been deemed appropriate to undertake a numerical model assessment of tidal stream energy resources in the Severn Estuary. In the present study, an existing finite volume numerical model has been refined, with the inclusion of an algorithm for computing the power density and its mean value across the estuary. The refined model has also been validated against (a) measured tidal currents at four sites, using the method of harmonic analysis; (b) measured tide level hydrographs at five tide gauging stations, taken over a 15-day period and covering the full spring—neap cycle; and (c) in situ velocity measurements at two sites. Finally, the validated model has been used to assess the potential tidal stream energy resources without and with the Severn Barrage, including the distributions of the mean power density over a spring—neap cycle in the Severn Estuary and a detailed assessment of the tidal stream energy resources at two sites near the coast of South Wales. The model predictions indicate that at two sites, the annual power output with the presence of barrage could be reduced by 70—80 per cent, as compared with the value without any structure.