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Numerical modeling of upstream propagation of a positive surge wave in an open-channel flow caused by a sudden rise of downstream water level, either by gate operation or by tidal influence, remains an important challenge from many aspects. A perusal of literature suggests that the Serre-Green-Naghdi (SGN) models have not been methodically explored for a positive surge flow modeling. In this study, the SGN equations for variable bed profiles are solved for simulating the positive surge waves. The governing equations are reformulated in the form of a conservation law, which is solved using a hybrid finite volume-finite difference method. The eddy-viscosity type wave breaking model is coupled with the governing equations for simulating a breaking surge. The performance of second- and fourth-order time stepping schemes and second- and fourth-order accurate schemes for determining the numerical flux is assessed. The results indicate that using an overall fourth-order accurate scheme is preferred for an accurate depiction of the positive surge waves. A comparison is also made with laboratory observations from a wide range of experiments conducted under different discharge, Froude number and bed slope conditions, which demonstrates that the proposed model can accurately reproduce the key free surface characteristics corresponding to positive surge waves. The importance of computing an accurate initial gradually varied flow profile before applying the transient downstream condition, such as a gate closure, is highlighted. Further, the significance of considering full non-linearity is demonstrated through numerical tests as the weakly non-linear Boussinesq-type equations are found to perform poorly in predicting the secondary undulations for an undular surge. The non-hydrostatic pressure field occurring during the propagation of an undular surge has rarely been reported from laboratory experiments. In this study, the bed pressure beneath a propagating undular surge is measured and compared with that computed according to the SGN equations. Finally, the results obtained from the proposed SGN model are compared with those obtained from Large Eddy Simulation results available in the literature.

In this study, a related and hitherto largely unexplored and unreported area of sedimentation in trapezoidal reservoirs is investigated, with the aid of a coupled numerical hydrodynamic and sediment transport simulation model. This research study is categorised into the following steps: identifying significant geometric parameters defining any typical storage reservoir, setting up and running a numerical model for simulating flow and sediment movement for a range of possible geometries, expressing characteristic parameters defining the extent of sedimentation (height of sediment mound, spatial spread and longitudinal extent) in terms of the geometric parameters, validating relations so obtained with field observations of sedimentation from the Hirakud Reservoir (Eastern India) within its two contributing rivers — Mahanadi and Ib. The study shows that the reservoir geometry and bathymetry significantly influence the flow field which, in turn, dictates the conditions of sediment transport and deposition within the reservoir. The developed relations are expected to help the reservoir managers in understanding the nature of bed elevation rise with time for a given reservoir and for planning desiltation strategies for conserving reservoir capacity.

A major unsolved problem of the Proterozoic is the genesis and tectonic evolution of the massif type anorthosites. The idea of large-scale floating of plagioclase crystals in a basaltic magma chamber eventually generating massif type anorthosite diapirs from the floatation cumulates is not supported by observations of the major layered basic complexes of Proterozoic to Eocene age. In this paper, we test and propose a new genetic process of anorthosite diapirism through Rayleigh-Taylor instability. We have carried out a numerical modeling study of parallel, horizontal, multiple layers of norite and anorthosite, in a model layered basic complex, behaving like Newtonian or non-Newtonian power law fluids in a jelly sandwich model of the continental lithosphere. We have shown that in this pressure-temperature-rheology configuration the model lithosphere generates Rayleigh-Taylor instability, which triggers diapirism of the anorthosite. In our model, the anorthosite diapirs buoyantly rise through stages of simple, symmetrical upwelling and pronounced bulbous growth to a full-blown mushroom-like form. This is the growth path of diapirs in nearly all analog and numerical previous studies on diapirism. Our anorthosite diapirs fully conform to this path. Furthermore, we demonstrate that the progressive diapirism brings in striking internal changes within the diapir itself. In the process, the lowermost anorthosite layer rises displacing the upper norite and anorthosite layers as progressively stretched and isolated segments driven to the margin of the rising diapir—a feature commonly seen in natural anorthosite massifs. We propose that a large plume-generated basaltic magma chamber may be ponded at the viscous lower crust or ductile-plastic upper mantle or further down in the weaker mantle of the jelly sandwich type continental lithosphere. The magma may cool and crystallize very slowly and resolve into a thick-layered basic complex with anorthosite layers. Rheologically behaving like Newtonian or non-Newtonian power law fluids, the layers of the basic complex with built-in density inversions would generate RT (Rayleigh-Taylor) instability. The RT instability would trigger a buoyant rise of the unstable anorthosite from the layered complex. The upward driven anorthosite, accumulated as anorthosite plutons, would gradually ascend across the lower and middle crust as anorthosite diapirs.

This study aims at evaluating the seasonal and spatial fluctuations in the quality of wastewater passing through the stormwater flow (SWF) channel located in Kolkata, India, using multivariate statistical methods. In this study, the primary quality data were collected from eight selected sampling stations on the SWF channel during three seasons (summer, monsoon, and winter of 2019) for 10 parameters (ammonia, biochemical oxygen demand, dissolved oxygen, nitrate, orthophosphate as phosphorous, pH, temperature, total coliform, total suspended solids, and discharge). Cluster analysis was used to group the eight sampling stations into three categories of similar water quality characteristics. This resulted in three water quality rating zones: bad, moderately bad, and good. Principal component analysis (PCA) was carried out to identify the significant parameters responsible for the wastewater quality fluctuations during all three seasons. The results of the PCA revealed that temperature and pH are the most important constituents contributing to wastewater quality fluctuations in the SWF channel for all seasons in a year. The PCA studies additionally led to the conclusion that a parameter that may be massive in contribution to wastewater quality fluctuations within the channel once of a year may not be substantial for every different season. The study revealed the current state of the wastewater quality in the SWF channel by using just 1 year of primary data. In addition, it is expected to provide help for future studies on the channel.

Urban water distribution networks (WDNs) in developing economies often refrain from investing in sensor-based leakage management technologies due to financial constraints and other techno-managerial issues. Thus, this study proposes a generalized decision support framework based on network sensitivity analysis (NSA) and multi-criteria decision-making (MCDM) to assess the prospect of effective leakage control through robust sensor placement in existing deficient WDNs. Four sensitivity parameters are formulated for NSA to ascertain the pressure response of the potential sensor positions for diverse hydraulic and leak scenarios. Subsequently, selecting the optimal number of sensors and their relative positions within the WDN is framed as an MCDM problem that entails the simultaneous maximization of Euclidean distances among the potential sensor positions and the leak-induced pressure residuals obtained at these sensors. The proposed methodology is developed on a numerical benchmark network assuming ideal conditions, and its applicability is verified on a sensor-equipped experimental network considering realistic system uncertainties. The outcome of this study aims to provide an insightful understanding of the system behavior that governs its leak localization potential and ascertain the practical challenges of sensor-based leakage monitoring in existing WDNs. Decision-makers of resource-strained utilities can beneficially utilize the proposed framework to assess the environmental and cost trade-offs of employing sensor-based technologies for leakage management and proactive decision-making before its actual implementation.

Wastewater is a complex mixture of organic and inorganic impurities, suspended solids, nutrients, saprotrophic and disease-causing bacteria and other microbes. When untreated wastewater is discharged into the rivers, seas, or oceans, it causes harm to human and aquatic life. Therefore, it is necessary to monitor the wastewater quality before diverting it into receiving water. The main aim of this study was the selection of an appropriate aggregation function to propose the wastewater quality index (WWQI) which is suitable for monitoring the spatial and seasonal variations of wastewater quality in the stormwater flow channel (SWF) located in Kolkata, India. Two different aggregation functions, i.e., weighted geometric mean function and weighted arithmetic mean function, were tested and analysed with primary data. Twelve wastewater quality parameters were selected. And, sampling was done seasonally for a 1-year period from December 2018 to December 2019 at eight selected locations along the SWF channel. The relative weights of each parameter were calculated based on their relative significance. Sub-index rating curves were generated to each parameter for WWQI calculations. The results revealed that weighted arithmetic mean function produced higher values of WWQI than geometric mean function. Furthermore, sensitivity analysis was carried out to evaluate the aggregation function that best suits the data of the SWF channel. The sensitivity analysis also revealed that weighted arithmetic mean function, being linear and free from ambiguity and less eclipsing, was more appropriate for measuring the WWQI for the SWF channel. Therefore, the study concluded that weighted arithmetic mean function was the most suitable aggregation function to calculate the WWQI for the SWF channel.

Barrage weirs, which are trapezoidal-sectioned sharp-crested weirs having finite crest length, are primarily used in the Indian subcontinent for diverting water for irrigation. This study p...

The 125 km2 East Kolkata Wetlands (EKW) is not only the world’s largest wastewater fed aquaculture site but also a Ramsar recognized eco-diversity zone. The EKW, comprising of multiple water-bodies, interspersed with cultivable land and human settlements, came into existence in the early twentieth century from an interaction between the then newly established wastewater disposal system of the city of Kolkata and the large salt lakes that once existed in the region. The wastewater solids being let into the natural salt-water marshes transformed them into captive water-bodies by cutting them off from the tidal creeks joining the ocean. Local traders converted some of these shallow wetlands by around 1930-s into fish farming ponds fed by wastewater after appropriate treatment. Since then these water-bodies have not only provided livelihood opportunities to several marginalized groups, it has helped to meet a bulk of the city’s fish requirement. The area has also developed into a unique ecosystem sheltering a wide variety of flora which acts as a green lung to the city. For the past couple of decades, however, land encroachment for real estate has threatened the biodiversity of this region. Further, the younger generation of the farming community has largely shifted to other professions, leading to a decline in the wastewater-fed fish farming activities. The paper thus attempts to evaluate the benefits of the wetlands vis-a-vis concerns like the possible uptake of harmful elements by the fish population. The benefits, nevertheless, are seen to outweigh the disadvantages and the EKW appears to emerge as a model for a low-cost, ecologically sustainable bio-treatment plant of wastewater in an era when low-energy, low-impact developments are being vigorously promoted to counter global warming and climate change adversities.

Numerical simulation of undular hydraulic jumps, which occur at an inflow Froude number slightly above unity, remain an important and enigmatic challenge in hydraulic engineering. In this s...

Two-dimensional shallow water models are widely used for flood risk assessment. Application of these models to large-scale urban areas significantly increases the computational cost as it requires high-resolution simulations to capture the complex hydrodynamic processes. Hence, simplified models that are based on local-inertial formulations have been proposed by different researchers. Although these models are successful in simulating floods, their applications are limited by the use of structured grids and instability issues. Structured grids do not have the flexibility of providing different mesh resolutions to model the computational domain. Importantly, in the case of structured grids, the use of a single grid or the combination of finer and coarser grids inevitably increases the overall computational time. To overcome all these problems, an unstructured grid-based local-inertial model is developed. The performance of the model is rigorously evaluated by solving analytical test cases and simulating an urban flood in Glasgow, UK. Finally, the model is applied to simulate a catastrophic flood event that occurred in Chennai, India. The simulated water depths and inundation extents are compared with observed data or full-2D model results. The investigations show that the developed model has the potential for simulating floods on a large-scale with high-resolution grids at a comparatively lower computational cost than its predecessors for a similar range of accuracy.

The earliest sewer networks of the city of Kolkata, or erstwhile Calcutta, covered a modest 19.1 km2 of the central district of the city and the first phase of the network-commenced operation in 18...

The study is focused on numerical simulation of one-dimensional unsteady flow in open channels using MacCormack Scheme. This scheme is an explicit numerical method for simulating one-dimensional and two-dimensional unsteady flow. This study is focused on depth and velocity variation in different sections of a rectangular flume at different time intervals. A series of experiments were performed on a multipurpose tilting flume on two different slopes viz., (a) nearly horizontal (zero slope) and (b) 3% slope. Experimental results are compared with the results of a computer program developed on the FORTRAN 90–95 programming language for simulating the MacCormack scheme. Results obtained from the developed code show comparable trends with the experimental result.

Two-dimensional shallow water models have been widely used in forecasting, risk assessment and management of floods. Application of these models to large-scale floods with high-resolution terrain data significantly increases the computation cost. In order to reduce computation time, shallow water models are simplified by neglecting the inertial and/or convective acceleration terms in the momentum equations. The local-inertial models have proved to significantly improve the computational efficiency even for large scale flood forecasting. However, instability issues are encountered on smooth surfaces of urban areas having low friction values. This problem was resolved by de Almeida et al. (Water Resources Research 48: 1 - 14, 2012) by introducing limited artificial diffusion in the form of weighting factors for the neighboring fluxes. The arbitrary value of the weighting factor poses a practical limitation of being case specific and requiring calibration for accurate solutions. This study derives an explicit expression for the weighting factor, an adaptive formulation dependent on local velocity, flow depth, grid and time step size, that eliminates the need for trials and approximations. Comparisons between analytical, experimental and real-world applications confirm the accuracy and robustness of the proposed weighting factor. Implementation of adaptive weights results in less computation time compared to LISFLOOD-FP (~1.2 times) and hold a significant advantage over HEC-RAS (~25.9 times) as it allows the use of larger time step at higher CFL values. The contribution of the present study therefore resolves an important problem of current large scale flood simulations, especially those implemented in real-time.

The relatively rapid recession of glaciers in the Himalayas and formation of moraine dammed glacial lakes (MDGLs) in the recent past have increased the risk of glacier lake outburst floods (GLOF) in the countries of Nepal and Bhutan and in the mountainous territory of Sikkim in India. As a product of climate change and global warming, such a risk has not only raised the level of threats to the habitation and infrastructure of the region, but has also contributed to the worsening of the balance of the unique ecosystem that exists in this domain that sustains several of the highest mountain peaks of the world. This study attempts to present an up to date mapping of the MDGLs in the central and eastern Himalayan regions using remote sensing data, with an objective to analyse their surface area variations with time from 1990 through 2015, disaggregated over six episodes. The study also includes the evaluation for susceptibility of MDGLs to GLOF with the least criteria decision analysis (LCDA). Forty two major MDGLs, each having a lake surface area greater than 0.2 km2, that were identified in the Himalayan ranges of Nepal, Bhutan, and Sikkim, have been categorized according to their surface area expansion rates in space and time. The lakes have been identified as located within the elevation range of 3800 m and 6800 m above mean sea level (a msl). With a total surface area of 37.9 km2, these MDGLs as a whole were observed to have expanded by an astonishing 43.6% in area over the 25 year period of this study. A factor is introduced to numerically sort the lakes in terms of their relative yearly expansion rates, based on their interpretation of their surface area extents from satellite imageries. Verification of predicted GLOF events in the past using this factor with the limited field data as reported in literature indicates that the present analysis may be considered a sufficiently reliable and rapid technique for assessing the potential bursting susceptibility of the MDGLs. The analysis also indicates that, as of now, there are eight MDGLs in the region which appear to be in highly vulnerable states and have high chances in causing potential GLOF events anytime in the recent future.

This paper presents an experimental investigation on the stability of a model river bank composed of homogeneous cohesionless soil under rapid drawdown condition. The effects of major influ...

Summary The areas of glacial lakes in the Himalayas continue to increase alarmingly due to global warming with consequent risk of glacial lake outburst flood (GLOF). The outburst is most common for moraine-dammed lakes as a result of a failure of the dam by overtopping and erosion or piping due to seepage. In the present context, GLOF events, especially for the moraine dam failure process needs to be predicted to assess the drastic and detrimental impacts on the downstream river valley. This study focuses on validating an integrated model for simulating the failure of moraine dam by overtopping and erosion using a coupled numerical simulation model. The proposed numerical methodology is validated with the data from experiments in laboratory scale physical models. The results of the model are compared for both the erosional profiles during collapse of the dam and the resulting outflowing flood hydrograph. The primary finding is that the moraine dam height and the volume of the lake upstream of the dam are the most sensitive parameters influencing the GLOF peak. The model is also applied to the field problem of the Tangjiashan lake outburst and dam failure and shows reasonably good agreement with observations. The validated model is also run for the South Lhonak glacial lake in the Sikkim Himalayas, India to obtain the probable flood hydrograph in case of failure of the moraine dam.

Basin irrigation is a common practice for growing water intensive crops like paddy. Irrigation water, when supplied through a network of canal, is often found to be inadequate to meet the crop water requirement uniformly throughout the irrigated command area. The most deprived are the cultivators of the lower end of the command, who resort to supplementing the crop water requirement by extractions from the ground. This practice is noticeable in irrigation system without a proper canal water distribution schedule and often result in water logging in the upper command regions contrasted with excessively depleted groundwater table in the lower commands. The present contribution attempts to model the conjunctive water use of such a canal irrigated command using physically based numerical sub-models for simulating surface flow, groundwater flow and the interlinking process of moisture movement through the unsaturated zone for a given quantum of supplied water and crop water demand. Individual models are validated to demonstrate their applicability in an integrated framework. Various plausible conjunctive water use scenarios are tested on a hypothetical command area practising basin irrigation to identify the best possible water distribution strategy under given constraints.

Hirakud, the longest earthen dam in Asia on the river Mahanadi, has been affected by sedimentation problems which affect reservoir storage capacities and different reservoir operations. This article presents the hydrological modeling of the Mahanadi catchment up to Hirakud, located in the states of Chhattisgarh and Odisha in India, using the modified version of Soil and Water Assessment Tool (SWAT) with an interface of ArcView Geographic Information System software to predict soil erosion and sediment transport to the reservoir for improving its useful life. The data sources used in the study were: terrain data 90 m resolution of Shuttle Radar Topography Mission, Land Use and Land Cover derived from Landsat Enhanced Thematic Mapper Plus, soil map published by National Bureau of Soil Survey and Land Use Planning, India and meteorological data collected from India Meteorological Department, India. The study involves the analysis of sensitive parameters of the simulated data (discharge 15, sediment load 6); 5-year calibration and 3-year validation on daily basis to predict the discharge and sediment load. The sensitivity analysis was carried out using the Latin Hypercube and One-factor-At-a-Time. The results also show that the prediction of sediment yield is highly sensitive to the sizes of different sub-basins due to the sensitivity of topographic factors used in the Modified Universal Soil Loss Equation. The graphical as well as statistical results achieved in this study indicate that the model is capable to identify critical erosion-prone areas within the watershed on the scale of sub-watershed which helps the hydrologists to introduce effective management practices at the lowest cost. The study shows that about 34% area of total catchment falls under high or above soil erosion zones with combination of coarse loamy type of soil and agricultural type of land use and land cover condition. The results of this study also identify regions of high sediment yield and sediment delivery ratio. To enhance the SWAT model performance, it is recommended to use sub-daily data and to prioritize soil erosion at the Hydrological Response Units level for improvement of watershed management.

Erosional failure of granular dams by an overtopping body of water is investigated using a depth-averaged morphodynamic model. The transport of sediment by the flow assumes the sediment flux to remain in equilibrium with the local bed shear stress. Accordingly, the shallow-water hydrodynamic equations are coupled with the Exner equation for mass conservation of the sediment. The system of equations is solved using a fully coupled well-balanced finite volume method, second-order accurate in time and space. The effect of the steep bed slope of a dam face is incorporated into both the hydrodynamics and sediment transport equations, leading to improved predictions. Comparison with results obtained from nonequilibrium sediment transport models indicates that such models perform poorly while predicting the bed evolution near the toe of an eroding dam. Observations from experimental studies demonstrate that the amount of sediment entrained by the flow is not significant, except during the initial moments of failure. This suggests that the vertical exchange of mass between the bed and the flow layer, as assumed by the nonequilibrium models, may not be completely valid during the failure. The equilibrium model results, reproducing the key flow features of the overtopping failure process, are validated by experimental measurements. The study provides fresh insights into the sediment transport processes associated with the erosion of a granular dam by overtopping, establishes the appropriateness of the equilibrium approach for its numerical modeling, and proposes a well-balanced second-order accurate solution technique for solving the resulting coupled equations of flow and sediment transport.

Construction of dams causes reduced flow velocities, inducing gradual deposition of sediments carried by the inflowing stream, and resulting in sedimentation and ultimately diminishing reservoir storage capacity. This study focuses on sedimentation of Hirakud Reservoir in Odisha, India, using available reservoir capacity and numerical simulation data. Reduced trap efficiency, observed and projected capacity curves, rising reservoir bed level and the capacities of the different storage zones for various projected years are analysed. The area-reduction method indicates the loss in the live, gross and dead storage will be 58%, 63% and 100%, respectively, of their original capacities by 2057, which represents 100 years of impounding of water in the reservoir. If the present sediment inflow rate continues without regular flushing of the deposited sediment, it is predicted the reservoir bed level will rise to the full reservoir level of 192.02 m by the year 2110. Brune's trap efficiency and step method indicate the gross storage zone of Hirakud Reservoir will be completely depleted by the end of 2110, with the trap efficiency reduced to zero. The empirical area-reduction method is found to be more suitable for determining the storage capacities of Hirakud Reservoir in the absence of sedimentation survey data. An attempt was also made to solve the combined hydrodynamic and sediment transport equations numerically to predict morphological changes in Hirakud Reservoir. The finite-element code TELEMAC-2D and finite-volume code for SISYPHE, respectively, were applied to solve the above set of equations in order to predict the bed profiles at different reservoir cross sections for the period of 1958–2008. Analysis of the simulated results demonstrates that, considering the model inputs, the model performs well in simulating the morphology and dynamic characteristics of a reservoir. Projection of the numerical results indicates a complete loss of reservoir operational life due to sedimentation by around 2150.

Construction of river diversion barrages produces a shallow reservoir, called the pond, which is used more often for flow balancing between the inflows of the river and the outflows of the off-taking canal. However, deposition of sediment in the pond due to the relatively low velocities reduces the pond capacity. The current study investigates the effectiveness of gate operation and variations of other parameters in flushing out these sediment mounds, or shoals, from the barrage pond. Data from laboratory experiments on a scaled model of a prototype barrage is used to train and test different Artificial Neural Network (ANN) models of the system. The models map the relationship between flushing efficiency of a sediment shoal from the upstream of a barrage, and the parameters river discharge, barrage pond depth and area of gate opening, position of the sediment shoal with respect to the barrage. The ANN models are then used to study the effect of different parameters on the sediment flushing efficiency. Apart from river discharge and net area of gate opening, upstream pond depth is also found to have a significant effect on flushing efficiency, with a general trend of decrease in efficiency with increase in pond depth becoming apparent. Efficacy of different gate opening pattern is also tested, with the ‘inverted arch’ gate opening pattern proving to be the most efficient when compared with ‘arch’ and ‘uniform’ gate opening.

Kolkata, the capital city of the Indian state of West Bengal, is prone to occasional flooding at times of moderate to heavy rainfall owing to relatively flat terrain and tide-dominated drainage outfalls. This article describes an instrumentation network that attempts to capture the distributed rainfall information in real time over the city with the help of an array of automatic rain gauges. Digital water level sensors, which monitor the elevations of the drainage channels, provide information about the water level in real time. Both rainfall and water level data, transmitted through GSM technology as SMS to a server at the Indian Institute of Technology (IIT) Kharagpur, get uploaded to a website continuously. Future plans include linking the rainfall and channel water level data with an overland flow and inundation numerical simulation model for predicting possible inundation scenarios in the city. Plans also propose to utilise the real-time data of the Kolkata Doppler Weather Radar for supplementing the ground rainfall data for producing longer lead-time forecasts.

A two-dimensional cell-centred finite volume model for quadrilateral grids is presented. The solution methodology of the depth-averaged shallow water equations is based upon a Godunov-type upwind finite volume formulation, whereby the inviscid fluxes of the system of equations are obtained using the HLL Riemann solver. A simple yet precise analytical expression is presented to compute hydrostatic flux through an interface of a quadrilateral cell in order to achieve exact balance between flux gradient and bed slope source terms under still water condition. A multidimensional gradient reconstruction procedure and a continuously differentiable multidimensional slope limiter based on a wide computational stencil are proposed to maintain second-order spatial accuracy. The proposed second-order scheme is shown to be more accurate even when distorted grids are used and is therefore more suitable for practical applications. The presented model is verified and validated by solving a wide variety of test cases having analytical solutions and laboratory measurements.

Control and risk management of floods using non-structural measures such as flood forecasting and flood warning, flood hazard mapping and flood risk zoning are quite effective. Of these, preparation of flood hazard maps and flood plain zoning require flood inundation simulation, for which various numerical models are available, for example, one-dimensional (1D), two-dimensional (2D) and 1D-2D-coupled models. In the present paper, the last model was used for hydrodynamic simulation for Ajoy River, in West Bengal, from the Ilambazar road bridge to the confluence of river Ajoy with the river Bhagirathi. The flood inundation simulation for the Ajoy River flood plain was carried out using modelling tool MIKE-FLOOD, which integrates the 1-D MIKE-11 model with the 2-D MIKE-21 model. River network layout, cross sections, boundary conditions and hydrodynamic and simulation parameters were the inputs for 1-D MIKE-11. The MIKE-11 hydrodynamic model was calibrated by using the gauge data at a single station (Nutanhat...

TINFLOOD, a simplified numerical model developed for simulation of floodplain inundation, is applied to predict the extent of flooding at a small reach of the River Thames, UK. River flow is computed by solving the de Saint Venant equations with a one-dimensional finite volume approach. Over-bank flood water spillage from the river onto the floodplains is computed considering mass exchange only between the one-dimensional river cells and the adjacent floodplain cells. Flow exchange between the river and floodplain cells or that between adjoining floodplain cells is represented by a weir type equation. The model employs a linearized form of the flow equation for avoiding instability, common to such coupled models. The rigorously tested model is applied in this study for predicting the extent of inundation for a flood event to a stretch of the River Thames, United Kingdom and compared with the corresponding observed imagery of the flood extents.

A simplified numerical model for simulation of floodplain inundation resulting from naturally occurring floods in rivers is presented. Flow through the river is computed by solving the de Saint Venant equations with a one-dimensional (1D) finite volume approach. Spread of excess flood water spilling overbank from the river onto the floodplains is computed using a storage cell model discretized into an unstructured triangular grid. Flow exchange between the one-dimensional river cells and the adjacent floodplain cells or that between adjoining floodplain cells is represented by diffusive-wave approximated equation. A common problem related to the stability of such coupled models is discussed and a solution by way of linearization offered. The accuracy of the computed flow depths by the proposed model is estimated with respect to those predicted by a two-dimensional (2D) finite volume model on hypothetical river-floodplain domains. Finally, the predicted extent of inundation for a flood event on a stretch of River Severn, United Kingdom, by the model is compared to those of two proven two-dimensional flow simulation models and with observed imagery of the flood extents.

This paper reports on experiments carried out on a physical model of a typical river diversion barrage (weir with gated control) to study local scour characteristics downstream of its apron under various hydraulic conditions. Relations for maximum scour depth and its location from the apron edge are derived using non-linear regression and back-propagation artificial neural network techniques from the experimentally observed data. Variations of scour depth with discharge, tail water and gate opening are examined to determine the possible critical local scour condition downstream of a barrage. Finally, the case of an actual prototype scour for a barrage due to uncontrolled flow through one of its damaged gates is investigated. The scour profile for this study is predicted by numerical simulation carried out with a finite-volume-based computational fluid dynamics software package (Fluent) and compared with field measurements.

River cross-sections are the prime input to any river hydraulic model for simulation of water level and discharge. Field measurements of river cross-sections are labour intensive and expensive activities. Availability of measured river cross-sections is scanty in most of the developing countries, thereby making it difficult to simulate the water level and discharge using hydraulic models. A methodology for extracting river cross-sections from Shuttle Radar Topographic Mission digital elevation model (SRTM DEM) of 3-arc second has been proposed in the reported study. The extracted river cross-sections were used to simulate the magnitude of flood in the deltaic reaches of Brahmani river basin located in the eastern India. Forty cross-sections along the reaches of the rivers were extracted from the DEM and were used in the MIKE 11 hydrodynamic (MIKE 11HD) model. Prior to using the DEM-extracted river cross-sections in the model, the cross-sections were modified based on the results of the DEM error analysis. Four available measured river cross-sections were compared with the DEM-extracted modified cross-sections to examine their geometric and hydraulic similarity. By changing Manning’s roughness coefficient (n), same stage-discharge relationship could be obtained in both types of cross-sections. Subsequently, the DEM-extracted cross-sections were used in the MIKE 11HD model for the simulation of discharge and water levels at various sections of the rivers. The model was calibrated for the period of June 15–October 31 of the year 1999 and validated for the year 2003. The model validation results showed a close agreement between the simulated and observed stage hydrographs. The calibrated values of Manning’s n were found to vary within the range of 0.02 to 0.033. The study revealed that freely available SRTM DEM-extracted river cross-sections could be used in hydraulic models to simulate stage and discharge hydrographs with considerable accuracy under the scarcity of measured cross-section data.

A simple yet precise relation between the flux gradient and the bed slope source term is presented, which produces a net force within the cell with an inclined water surface, but ensures still water condition when there is no flow across the boundaries. The proposed method consists of calculating the pressure term based on the water depths at the cell vertices, which may be computed by a higher order scheme and the bed slope source term by a centered discretization technique. The methodology is demonstrated with a Godunov-type upwind finite volume formulation. The inviscid fluxes are calculated using Roe’s approximate Riemann solver and a second-order spatial accuracy is obtained by implementing multidimensional gradient reconstruction and slope limiting techniques. The accuracy and applicability of the numerical model is verified with a couple of test problems and a real flow example of tidal water movement in a stretch of River Hooghly in India.

This paper presents a comparison of two algorithms—the forward-elimination and branch-segment transformation equations— for separating out end-node variables for each branch to model both steady and unsteady flows in branched and looped canal networks. In addition, the performance of the recursive forward-elimination method is compared with the standard forward-elimination method. The Saint-Venant equations are discretized using the four-point implicit Preissmann scheme, and the resulting nonlinear system of equations is solved using the Newton-Raphson method. The algorithm using branch-segment transformation equations is found to be at least five times faster than the algorithm using the forward-elimination method. Further, the algorithm using branch-segment transformation equations requires less computer storage than the algorithm using the forward-elimination method, particularly when only nonzero elements of the global matrix are stored. Comparison between the Gauss-elimination method and the sparse matrix solution technique for the solution of the global matrix revealed that the sparse matrix solution technique takes less computational time than the Gauss- elimination method.

This paper presents an efficient solution technique for one-dimensional unsteady flow routing through a general channel network system—dendritic, looped, divergent, or any combination of such networks. The finite difference method is used to solve the de St. Venant equations in all the branches of the network simultaneously. The number of equations to be solved at a time during any iteration is reduced to only four times the number of branches of the network. This results in a significant reduction in storage requirements and solution time. Importantly, the algorithm does not require any special node numbering schemes and the nodes can be numbered independently for each branch. The algorithm is also suitable for programming on a parallel-processing computer.

India suffers from a large variability of precipitation both spatially and temporally. It is generally known that though the average rainfall for the country is about 1160 mm, the highest anywhere in the world of a comparable size (Kumar et al., 2005), the spatial variability ranges from an average of 2800 mm for most of the north-eastern states, Andaman and Nicobar Islands and northern areas of West Bengal to about 300 mm in the western part of Rajasthan (http://www.rainwaterharvesting.org/urban/Rainfall.htm). Again, except for the States of Assam, Jammu and Kashmir and the southern peninsula, more than 75% of India’s annual rainfall is received during the southwest monsoon season, i.e., June through September (Jagannathan and Bhalme, 1973). An effect of this skewed distribution of rainfall is excess water for a region received during a short interval of time, leading to flooding of the surroundings, if not drained off suitably. The region may be small as an urban space, like cities or towns, for which an intense rainfall of even half a day may cause flooding, or larger areas like the over bank and floodplain areas of a river where the state of flooding may extend for several days due to continuous rainfall of a couple of days in the upper catchment. Examples of the former include flooding events in the cities of Kolkata and Mumbai, which occasionally get flooded due to drainage congestion aggravated primarily because of insufficient slopes of drainage channels and tidal influences at the outfalls. Examples of floodplain inundation are common for the rivers of the eastern and north-eastern states of the country, though a few others from the other parts of the country are in the news sometimes. In addition, rivers flowing through the hills often suffer from flash floods due to occasional cloud bursts and the coastal regions, especially in the eastern part of the country, is prone to flooding due to cyclonic storms either by the associated intense rainfall or of storm surge waves, or both.