Abstract
The Teesta Barrage project is a large multipurpose water resources project in the state of West Bengal, India. The project covers aspects of irrigation, flood control and power generation. Currently an improved management system is being developed for efficient operation of the scheme. The information technology-based improved management system covers the aspects of annual and seasonal planning, integrated operation of the barrages, scheduling of the operation of one of the main canals and the corresponding distribution network and improved manual control of the barrage and canal operation. The IT-based system will use the real-time data of inflow into the barrages, canal flow and rainfall from selected remote locations. The management system, which includes both a radio-based and an electronic data communication network covering the office of the controlling authority, the field office, barrage locations and identified field locations, is scheduled to be commissioned before the monsoon of 1999.
Introduction
The Teesta Barrage project comprises multiple interconnected barrages and corresponding main canals branching off them and covering a very large area. The target area on completion of all the stages of the planned development will be of a little more than 0.9 million ha, covering the entire northern part of the state of West Bengal. A main conveyance system of about 200 km in length with a distribution network of a few thousand kilometres is planned to cover the entire command area.
Characteristics and representation of the system
The barrages
The Teesta barrage across the river of the same name is one of the many barrages of the interconnected barrage system of the Teesta Multipurpose scheme.
The Mahananda barrage across the Mahananda river receives diverted flow from the river through the Teesta-Mahananda Link canal. The Mahananda Main canal branching off the right bank of the barrage conveys the flow diverted through the link canal to irrigate a limited command area (of about 50 000 ha) and generate hydroelectric power through an in-line power plant. The Mahananda Main canal in turn links the barrage with the Dauk barrage. Construction of the main canal system to cover areas beyond the Tangon river is in progress.
The conveyance system and distribution network
The conveyance system comprises the following main canals:
A diagram of the main canals is presented on the next page. The distribution network planned for each of the main canals comprises distributary, minor, sub-minor, outlets and other watercourses in the outlet commands.
The Teesta-Mahananda Link canal
The Teesta-Mahananda link canal is meant to divert the flow from the Teesta river to the Mahananda. This flow is further conveyed through the Mahananda Main canal on the right bank of the Mahananda barrage. The link canal also irrigates an area of about 40 000 ha through ten distributaries and augments supplies to an established run-of-the-river irrigation scheme. The length of the Teesta Mahananda Main canal is about 25 km.
Characteristics of the general command area
The annual rainfall in the Teesta command varies from about 3 000 mm in the head reach to 700 mm in the tail end of the southern part of the command, with an average of 1 650 mm. The general slope of the command area is gradual and oriented south. The soil throughout the command area is predominantly either sandy or clayey loam. The effective depth of soil varies from 0.6 m to 2.4 m. The predominant crop during kharif (monsoon) is paddy, and wheat is the major crop grown in the rabi (winter) season.
Figure 1. Diagram of the main canals in the Teesta Barrage project
Operation of the present system
The scheme has been operated in the last few years to provide irrigation to the developed portion of the Teesta-Mahananda Link Canal command area. The diversion of flow to the Mahananda barrage for further conveyance through the Mahananda Main canal for hydropower generation started at the end of the 1998 monsoon.
The scheme was designed to use the available flow at the Teesta barrage for catering to the requirements of irrigation in the project command and for hydropower generation. The use of the Mahananda river flow is limited to fulfilling the agreed share of the state of Bihar. The barrages are also used to regulate flood flows in a limited manner. The conveyance and distribution systems were designed on the basis of peak irrigation requirements in the immediate post-monsoon period and rotational canal operation was envisaged. The state policy assigns first priority to irrigation supplies, and fulfilment of the requirements of hydropower generation is limited to the adequacy of inflow at the Teesta barrage (only during the monsoon season).
Flow availability at the Teesta barrage site
Teesta is a perennial river. The catchment area up to the barrage site across the Teesta is of about 8 500 km2. Preliminary estimates of the availability of flow indicate a lean season of six months duration (November to April). The variation of flow availability at the Teesta barrage site between the lean season and the monsoon months is significant. However, there is not much variation in flow from December to March.
Barrage operation
At present, the barrages are operated according to the method formulated by the Irrigation & Waterways Directorate of the government of West Bengal. The method focuses on the ability to divert maximum flows through the main canals for fulfilling irrigation and hydropower requirements, keeping in view the structural safety of the barrage and feasible flood regulation, based on data, information or warnings received from the upstream gauging station at short notice (about one hour).
Irrigation requirements
About 150-percent cropping intensity has been considered throughout the entire command area in the design of the scheme. Traditionally farmers grow paddy in the vast tract of land covered by the scheme command area. Indicative figures suggest that paddy is cultivated in about two thirds of the command area during the kharif season and pulses are cultivated in about 10 percent of the command area during the rabi season. The present cropping intensity in the rabi season is much lower than that in the kharif season. Perennial and hot-weather crops have an insignificant coverage. The estimation of demand for irrigation is made on the basis of duty prescribed according to accepted departmental norms.
Hydropower generation
Three in-line hydropower generation stations, in series, have been planned on the Mahananda Main canal. One hydropower station has so far been commissioned. The remaining two power stations are at different stages of construction. Each station has three turbines with a capacity to generate 22.5 MW and a corresponding flow requirement of 330 cumecs (three turbines with a discharge capacity of 110 cumecs each). Initial analysis indicates that the flow requirement for hydropower generation can be fulfilled only during the monsoon months, i.e. June to September, and that there is a reasonable probability of meeting the requirement in October.
Variation of demand, spatial and temporal
The large command area implies spatial variation in soil characteristics, topography and rainfall pattern. It is likely that there would be variation in soil characteristics both at local level (outlet level) and at the subcommand level (individual main canal commands) contributing to spatial variation in demand. The variation of demand during the monsoon can be significant, owing to varying rainfall across the command, especially as the tail end of the command receives significantly less rain than the head reach.
In the case of the rabi season, demand is unlikely to fluctuate because of the rainfall situation in the command area. Most of the rainfall in this region occurs during the monsoon months. The demand variation in rabi would be attributable to actual cropping patterns and the period of sowing.
The demand of water for hydropower generation would be a constant component of the total demand for water. The state policy assigns first priority to the fulfilment of irrigation requirements. Supplies for hydropower generation are made on the basis of the actual availability of flow and demand for hydropower generation is most likely to be fulfilled during the period of high availability, i.e. the monsoon months of June to September.
Allocation and distribution of flow
With the focus on construction of the main canals and the distribution network, the operation is limited to feasible flood regulation at the Teesta and Mahananda barrages and operation of the Teesta-Mahananda Link canal to provide irrigation supplies to the area commanded by the link canal and to the diversion of the flow to the Mahananda for further conveyance through the Mahananda Main canal for hydropower generation.
As part of the ongoing efforts to develop an information technology-based system, a decision framework is being evolved to effectively operate the scheme to cater to likely situations, especially that of low availability at the Teesta barrage, keeping in mind the aspects of equity and sustainability. The decision framework would also help devise suitable ways for responding to changes in the climatic conditions, depending on whether or not rain falls in certain parts of the command.
The communication system
The present communication facilities available are limited to Department of Telecommunications telephones at the field offices and in the offices of the project and subproject administrators. The communication facilities available at the barrage sites are inadequate for continuous monitoring and control of operation by the project authority. Feedback about operation problems, emergency maintenance or repair needs, or required changes in canal operation because of rainfall in a particular subcommand, cannot be received in time from the field, making it impossible to intervene or guide the field operators in further action.
System operation: improvements proposed
System-wide planning, long term and short term
For the main canals, it is possible to plan specific allocations for individual seasons on the basis of expected availability and most effective deliveries for irrigation. Two distinctly different approaches are being worked on for the winter and monsoon seasons. Planning of operation of the scheme in the kharif season would be based on the requirement of meeting varying irrigation demand for paddy, the dominant kharif crop, and fulfilment of hydropower generation requirements and other statutory obligations and commitments. Short-term adjustments would be possible by taking into account the occurrence of rainfall in the command area. Rabi or hot weather season planning would take into account the limited availability of the Teesta river throughout the season and would help decide on the number of turns and the suitable dates for starting the individual turns of canal operation and their duration. Planning for both the rabi and the kharif seasons would be the most effective use of available river flows, keeping in mind equity and other system-specific constraints.
Integrated operation of barrages
The operation of the barrages in the kharif season would focus on feasible flood regulation, keeping in mind structural safety and the diversion of the required flow through the main canals. A system of integrated operation of the Teesta and Mahananda barrages during the lean season is being designed with the objective of managing the available flows to maximize power generation with a high degree of assurance after assigning first priority to the fulfilment of irrigation requirements.
Scheduling of canal operations
The approach toward improved scheduling of canal operations focuses on their effectiveness in fulfilling the irrigation requirements across the command area and supplies for hydropower generation. The proposed system will also ensure operation rescheduling in the shortest possible time to respond to climatic changes in the command area or to variations of inflow at the barrages. A decision framework for the allocation and distribution of flow in case of deficit availability will be evolved. This will be used to manage flows effectively.
Monitoring and control operation - barrages and canal system
The improved system used for monitoring the operation of the scheme would rely on the radio-based voice and data communication networks. The voice communication network would cover the barrage locations, remote sites along the Teesta-Mahananda Link canal, field offices and the central control station located in the administrative office of the project. The data communication network would cover the barrage locations, identified sites along the Teesta-Mahananda Link canal and a few rain-gauge stations in that canal command. While the telemetry system would provide barrage pond level, canal flow level and rainfall data, the voice communication network would be used to obtain information on gate positions and other queries.
The barrages will continue to be operated by manual control on the basis of details worked out by the Directorate of Irrigation and Waterways. The control strategy for the operation of the Teesta-Mahananda Link canal is being worked out. All necessary instructions for the control of canal and barrage operations and changes required from time to time will be communicated over the voice communication network as required.
Summary and conclusion
The information technology-based improved management system for the Teesta-Mahananda link canal being developed for the Teesta Multipurpose Water Resources project in West Bengal by CMC Ltd is a comprehensive management system. The IT-based system covers integrated operation of the barrages, long-term and short-term plans of operation, scheduling of the operation of the main system and of the distribution network, and improved monitoring and control for proper implementation of planned operations. The improved monitoring and control system will also help in effecting required changes in response to changes in river flow availability or the rainfall situation in the command area. The IT-based system under development will rely on state-of-the-art technology. It would be easy to customize for implementation in any other project with similar components, and could be integrated in regional or river basin-level management systems. The present project includes components of user training and limited implementation support.