Dhrubajyoti Sen

Coupled 1D–quasi-2D flood inundation model with unstructured grids

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.

Flood inundation simulation in Ajoy River using MIKE-FLOOD

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) and the calibrated setup was validated for the same gauging station. The simulated values for the validation period were having good agreement with observed values. Mannings roughness coefficient, n, was the only calibrating parameter. The bathymetry (Digital Elevation Model in MIKE-compatible format) of the study area for the MIKE-21 hydrodynamic setup was the only important input for 2-D MIKE-21. The MIKE-FLOOD setup consisted of simulation files of both MIKE-11 and MIKE-21, where the river banks in MIKE-11 were connected with MIKE-21 cells with lateral links. The simulation of MIKE-FLOOD was carried out for two monsoon months of year 2000 as the flooding was severe for this year. This simulated flood inundation extent by MIKE-FLOOD is compared with Dartmouth Flood Observatorys flood inundation extent map.

Flood Hazards in India and Management Strategies

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.

Real-time rainfall monitoring and flood inundation forecasting for the city of Kolkata

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.

APPLICATION OF THE 1D-QUASI 2D MODEL TINFLOOD FOR FLOODPLAIN INUNDATION PREDICTION OF THE RIVER THAMES

ABSTRACT 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.

An experimental investigation on effect of drawdown rate and drawdown ratios on stability of cohesionless river bank and evaluation of factor of safety by total strength reduction method

ABSTRACT 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 influencing parameters controlling Factor of Safety (FoS) of a river bank under depletion of water level have been considered in the present study. A series of laboratory model studies have been carried out to investigate the cases of drawdown rate and drawdown ratios rendering the bank to maximum damage. Moreover, the effect of water level drawdown on the response variables namely pore water pressure, deformation of the bank profile, and shear strength of bank material have been observed and analysed. Stability analysis of the experimental model banks under drawdown conditions was carried out by evaluating FoS using the principle of reduced shear strength methodology. The in-situ total shear strength after each drawdown was measured using a laboratory vane shear apparatus. The minimum shear strength among these values has been identified. Now the FoS against each drawdown ratio has been computed from the ratio of total shear strength obtained after drawdown for that particular drawdown ratio to the minimum shear strength as obtained. This experimental programme examined the optimum combination of drawdown rate and drawdown ratio causing mass failure of the bank. It was revealed from the variation of pore pressure after drawdown that changes of pressures at points close to the toe of the bank slope is strongly controlled by the stress-state induced by drawdown. From the failure observations it was found that the drawdown rate is the dominating cause of maximum deformation of the bank than that of drawdown ratio. The findings of the present research work may help in predicting the actual failure scenario and stability condition of prototype river under similar boundary conditions.

A Numerical Simulation Model for Conjunctive Water Use in Basin Irrigated Canal Command Areas

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.

Real-time OGC compliant online data monitoring and acquisition network for management of hydro-meteorological hazards

Abstract Natural calamities are undesirable events which often cause significant damages to the society and environment. One of the prime drivers for such a situation is rainfall which, when in excess, is responsible for initiating hydro-meteorological hazards like flash flood, landslides. In this context, a suitably designed real-time, telemetred and open-source standard rainfall and water level data acquisition network for flood-prone areas may be indispensable in managing the likely hydro-meteorological hazards. This paper demonstrates the design of such a web-based online low-cost high sampling OGC compliant data acquisition and signal transmission system with multiple remote data logging points to capture information of rainfall distribution over a pilot scale catchment. The network also monitors the water level at the exit end of the basin to facilitate the estimation of catchment run off. A study is also made to compare the distributed rainfall observations of tipping bucket rain gauges with that of an ordinary rain gauge. Finally, the possible applications of the observed data in managing hydro-meteorological hazards are indicated.

Sediment Flushout from Pond of River Diversion Barrages by Gate Operation

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.