Junqiang Xia

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.

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.

Dynamic channel adjustments in the Jingjiang Reach of the Middle Yangtze River

Significant channel adjustments have occurred in the Jingjiang Reach of the Middle Yangtze River, because of the operation of the Three Gorges Project (TGP). The Jingjiang Reach is selected as the study area, covering the Upper Jingjiang Reach (UJR) and Lower Jingjiang Reach (LJR). The reach-scale bankfull channel dimensions in the study reach were calculated annually from 2002 to 2013 by means of a reach-averaged approach and surveyed post-flood profiles at 171 sections. We find from the calculated results that: the reach-scale bankfull widths changed slightly in the UJR and LJR, with the corresponding depths increasing by 1.6u2009m and 1.0u2009m; the channel adjustments occurred mainly with respect to bankfull depth because of the construction of large-scale bank revetment works, although there were significant bank erosion processes in local regions without the bank protection engineering. The reach-scale bankfull dimensions in the UJR and LJR generally responded to the previous five-year average fluvial erosion intensity during flood seasons, with higher correlations being obtained for the depth and cross-sectional area. It is concluded that these dynamic adjustments of the channel geometry are a direct result of recent human activities such as the TGP operation.

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.

Modelling flood routing on initially dry beds with the refined treatment of wetting and drying

Details are given of the refinement of a two-dimensional hydrodynamic model, based on a total variation diminishing finite volume method, for predicting rapid flood flows on initially dry beds. A Roe approximate Riemann solver, with the monotone upstream scheme for conservation laws (MUSCL) scheme and the procedure of predictor–corrector in time stepping, has been used in this model. The scheme is second-order accurate in both time and space and is free from spurious oscillations. The model deploys unstructured triangular grids. A wetting and drying approach, originally developed for a regular grid finite difference model, has been refined to suit the triangular grid finite volume model. The model was first verified against analytical solutions and experimental data of dam-break flows on initially dry beds, with favourable agreement being obtained between the model predictions and measurements. The model was then employed to simulate flood propagations in a large flood detention basin in China and in an urban region in the UK. Numerical model tests were undertaken to investigate the sensitivity of model predictions to the value of a minimum water depth as required for treating the wetting and drying fronts. It was found that the selection of the minimum water depth can have a significant impact on the speed of the flood wave propagation on an initially dry bed. For a given time step, an excessively large value of the minimum water depth would result in inaccurate predictions of the wetting and drying wave fronts, but a very small value would lead to numerical instability.

Experimental studies on the interaction between vehicles and floodplain flows

Floodwater flows through urban floodplains with storm water systems are often inadequate during extreme storm events and/ or when the river flood inundation extent becomes extreme. Such flows may cause potential hazard risks to humans and their properties along the floodplains. Recently, flood hazards relating to vehicles have become more noticeable and it is vital to investigate the hydraulic behaviour of vehicles on urban floodplains. Therefore, this paper outlines a study of the theoretical and experimental aspects of the hydrodynamics of floodwater flows over urban floodplains with vehicles. A theoretical background study is discussed to establish an understanding of the hydrodynamics of floodwater flows over urban floodplains with vehicles; a condition which can be very important for extreme storm events, or even moderate storm events, when the storm water system is insufficient to drain away the surface runoff. Extensive investigations have been undertaken on stationary scaled die cast model vehicles in laboratory hydraulics flumes by conducting a series of physical experimental studies on: (i) the threshold of vehicle instability, (ii) the effects of vehicle orientation, (iii) the effects of ground surface gradient, (iv) the vehicle stability on urban floodplains, and (v) the influence of vehicles on floodwater flows. The results for all the test cases have been analysed to investigate the effects of vehicles on floodwater flow propagation over urban floodplains and, on the other hand, the influence of the floodwater flows on the stability of model vehicles. The two principal factors of hazards (i.e. the floodwater depth and flow velocity) that affect the stability of model vehicles in urban floodplains have been identified to confirm the significant impact of hydrodynamic processes in urban floodplains with vehicles. All experiments undertaken so far have only looked into the conditions under which the model vehicles begin to be moved. Observations have been made from the theory studied and experiments conducted to systematically look into the hydraulic behaviour of vehicles in urban floodplains. The main findings have highlighted that: (i) the model vehicles had a significant impact on the floodwater flow propagation and the hydrodynamic processes in the flooded area, (ii) if the incoming flow depth was less than the vehicle height, then the threshold velocity increased for a decease in the depth of flow; (iii) if the incoming flow depth was greater than the vehicle height, then the threshold velocity would rise with an increase in the depth of flow, and (iv) a flooded vehicle was more likely to move if the incoming depth just approached the vehicle chassis height due to the buoyancy effects. Based on these findings, an innovative approach of a straightforward three colour zone envelope curve has been developed, and first introduced herein, which has been defined as the Traffic Light of Hydraulic Stability (TLHS) system. This novel approach can be readily used to evaluate the degree of hydraulic stability for model vehicles, and it is also invaluable for assessing the vehicle hazard conditions in urban floodplains.

Criterion of vehicle stability in floodwaters based on theoretical and experimental studies

Vehicles parking on streets or roads can cause various hazards to people and property when they are swept away by urban floods. It is therefore appropriate to investigate the criterion of vehicle stability for such flood conditions, especially for different scenarios and where the criterion of vehicle stability is usually represented by the incipient velocity for the vehicle. In the current study, different forces acting on a partially submerged vehicle are outlined, together with the corresponding expressions of these forces, and a mechanics-based formula of incipient velocity is given for partially submerged vehicles under different orientation angles. About 200 runs of flume experiments were conducted to obtain the conditions of water depth and corresponding velocity at the threshold of vehicle instability for three orientation angles, using two types of die-cast model vehicles at two model scales. Experimental data obtained from the large-scale model vehicles were then used to determine two parameters in the derived formula. Finally, incipient velocities for three vehicle orientation angles were estimated using two different approaches, including predictions using the scale ratios from the small-scale model vehicles and computations based on derived formula using the prototype vehicle parameters. These critical conditions for the prototype conditions, based on the scale ratios, compared well with the calculations obtained using the derived formula, which guaranteed the predicative accuracy of the formula. In addition, the effect of different ground slopes on the vehicle incipient motion was also investigated, using similar experiments and based on the theory of similarity, which indicated that the incipient velocity for a small passenger vehicle on a ground slope of 1:50 was about 25xa0% lower than the value on a flat ground for an incoming depth of 0.25xa0m.

Study on tidal resonance in Severn Estuary and Bristol Channel

A horizontal hydrodynamic model was applied to predict the response characteristics of the Severn Estuary and Bristol Channel to regular long waves, in an effort to gain insight into the tidal behavior of this area. A boundary-fitted curvilinear mesh of high resolution was generated, covering the downstream reach of the River Severn, the Severn Estuary and the Bristol Channel, with the seaward boundary set from Milford Haven to Hartland Point to the west and the riverine boundary at Gloucester towards the east. The simulations were first calibrated against the observed tidal levels and currents at various sites, for typical spring and neap tides. Subsequently, water surface oscillations inside the domain were excited by sinusoidal long waves of different periods at the open boundary to find the fundamental mode of oscillation. The amplitude-frequency relationships were calculated at numerous sites. It was found that the primary resonant mode of oscillation in the Severn Estuary occurred at the tidal period of around 8 h. Although not exactly coinciding with this resonant mode, the M2 tide still observed a relatively high amplification factor, which helps explain why this water body experiences one of the largest tidal ranges in the world.

ON THE REFINEMENT OF A BOUNDARY-FITTED SHALLOW WATER MODEL

The two-dimensional shallow water equations were formulated and numerically solved in an arbitrary curvilinear coordinate system, which offers a relatively high degree of flexibility in representing the natural flow domains with structured meshes. The model employs an efficient TVD-MacCormack scheme, which has second-order accuracy in both time and space. Refinements were made to enhance the models accuracy and stability in computing the shallow wave dynamics in real-world scenarios, with irregular boundaries and uneven beds. In particular, advanced open boundary conditions have been proposed according to the method of characteristics, and rigorous mass conservation has been enforced during the computation at both the inner-domain and the boundaries. These refinements are necessary when modeling the flood inundation over a large area and the tidal oscillation in a macro-tidal estuary. The effectiveness of the refinements was verified by simulating the forced tidal resonance in an idealized condition and the Malpasset dam-break flood in the Reyran river valley. The application of the refined model in the study of tidal oscillations in the Severn Estuary and Bristol Channel can be found in the companion paper.

Hydrodynamic experiments on the impacts of vehicle blockages at bridges

Most urban rivers carry much natural debris, such as vehicles and trees, during extreme flood events, and these large debris, particularly vehicles, can block a local hydraulic structure such as a bridge. Such blockages usually increase the upstream water levels and cause more water to be diverted into adjacent urban areas. A scaled physical river model was constructed in a laboratory flume, consisting of a sketched urban reach with a compound cross-sectional geometry, and experiments were conducted on three model bridges, blocked by vehicles, with the corresponding hydrodynamic impacts of these vehicles blocking bridges being investigated. The main findings obtained from the investigation show that (i) the upstream water depths in the urban river increased significantly due to the vehicle blockage, for the single opening arch bridge and the straight deck bridge, while the upstream depths increased slightly for the three-opening straight deck bridge; (ii) the non-uniformity in the velocity profiles varied along the channel centreline, with the exponents in the power-law velocity distribution reaching their maximum values around the first two blocked model bridges, while the exponent values changed slightly along the channel for the third model bridge due to the middle opening not being blocked; and (iii) the downstream mean velocity was slightly greater than the upstream velocity for the first two cases, with the minimum value occurring just downstream of the bridge. However, the depth-averaged velocities just downstream of the bridge were relatively higher for the third case. Therefore, the obstruction caused by vehicle blockage at bridges has significant impacts on the hydrodynamic characteristics in an urban reach, and these impacts depend on the specified bridge type and the blockage mode.