Reservoir for Flood Moderation and Water Utilization: Some Critical Issues

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1 Reservoir for Flood Moderation and Water Utilization: Some Critical Issues Dr. Arup Kumar Sarma Associate Professor, Civil Enginering Department Indian Institute of Technology Guwahati INTRODUCTION Water is generally accepted as the most important of the renewable natural resources that will affect the development of developing countries in the next 50 years. Economic development of a region depends to a great extent on its development in the industrial and agricultural sectors. Water is a vital natural resource essential for development of both the industrial and the agricultural sectors. Need of drinking water cannot be overemphasized. These are the reasons as to why the terms like Water Sharing, Inter-basin Transfer, Linking of Rivers, and Riparian Rights are becoming more and more relevant in the context of economic development. Considering the above, apparently it appears that a region endowed with water resources should achieve significant development by utilizing them to their full capacity. However, the actual situation is not as straightforward as stated above. Northeastern part of India is the best example of the reality that merely having huge amount of water cannot help and can rather hamper the development. More important is the amount of utilizable water in a year and not the available water in a year. Temporal and spatial variations of available water make the problem of harnessing water resources quite complex and call for strategic planning and management. Construction of reservoir is one well-known technology that can significantly convert the water available in a year to utilizable water by augmenting its spatial and temporal distribution. The reservoir, if planned and operated properly, can also moderate the flood peak to a great extent. On the other hand, a reservoir constructed without considering flood mitigation as one of its objectives, may not be of any help so far reduction of flood hazard is concerned. Therefore, we should keep the aspect of flood moderation always in our mind during both planning and operation of any reservoir in the North-eastern part of India. NEED OF STORAGE TO UTILIZE SURFACE WATER OF NORTH EAST Proper assessment of surface-water resources is possible only through a continuous observation of the stream flow for several years. As per the basin-wise latest estimates, the water resources potential of the country, which occurs as natural run off in the rivers is about 1878 km3. However, all this flow cannot be utilized owing to the highly variable character of the flow and other limitations imposed by the physiographic factors. The temporal distribution of flow also calls for construction of storage reservoir for utilizing water of flood period. Considering all these factors Ministry of Water Resources (MOWR, 1999; Gupta and Deshpande, 2004) has given a basin wise distribution of available and utilizable water as given in the table 1. Table 1: Basin-wise Surface Water Potential Of INDIA in Km3/Year

2 Sl. Name of the River Basin Average annual Average Estimated No. potential in Monsoon Utilizable river runoff surface water 1. Indus (up to Border) a) Ganga b) Brahmaputra Barak & Others Godavari Krishna Cauvery Pennar East Flowing Rivers Between Mahanadi & Pennar 8. East Flowing Rivers Between Pennar and Kanyakumari 9. Mahanadi Brahmani & Baitarni Subernarekha Sabarmati Mahi West Flowing Rivers of Kutch, Sabarmati including Luni 15. Narmada Tapi West Flowing Rivers from Tapi to Tadri West Flowing Rivers from Tadri to Kanyakumari 19. Area of Inland drainage in Rajasthan desert NEG Minor River Basins Drainage into Bangladesh & Burma Total From the above table it is clear that utilizable flow of Brahmaputra river basin has been assessed as only about 4% of its available flow. The basic reason of such low utilizable percentage is the temporal distribution of water in the Brahmaputra and its tributaries. Therefore, to achieve benefit from the available water and to minimize the flood havoc of this region we must make provision to store the monsoon flow to utilize the same in nonmonsoon period. Improving ground water storage, increasing surface retention in the form of ponds and construction of reservoir are some of the options to achieve the above objective. Scope of improving ground water storage during monsoon is, however, very limited in the Northeastern part, as the water table during monsoon period, otherwise also, remains very near to the surface. Watershed management practices in the form of bund, pond, vegetative treatment etc. can minimize the surface erosion and surface runoff to some extent and can therefore contribute towards flood moderation at watershed level. Large-scale benefit in the form of power generation, irrigation, navigation, recreation etc. can be achieved by construction of storage reservoir. Construction of reservoir being a very costly affair, multipurpose reservoir is generally constructed for economic justification. To utilize the available water as well as to mitigate the flood hazard, a number of dam projects such as Pagladia, Tipaimukh, Dibang, Lohit, Kynsi have been proposed by Govt. of India in this region of the country. Proper care must be taken in the planning and operation of these multipurpose reservoirs to achieve flood mitigation, as erroneous operation of these reservoirs may lead to catastrophic artificial flood. Following are some important aspects that need to be considered in case of the reservoir of this region. FLOOD CONTROL STORAGE A multipurpose reservoir, as stated above, is generally constructed to achieve benefits like power generation, irrigation, flood mitigation, and recreation. Ten daily or monthly operating schedules are generally adopted for operation of these reservoirs. Adoption of daily operating schedule is difficult due to many other practical constraints such as gate

3 operation and flow forecasting. Flood is an extremely uncertain event that comes for a short period, say 3 to 7 days. While a monthly operating policy prepared on the basis of average monthly inflow can definitely reduce the monthly outflow from the reservoir it may fail to reduce the flood peak on the downstream of the reservoir. This is because, if power generation is kept as the prime objective, the operating policy will be adopted to keep the reservoir head as high as possible. This may lead to a situation that when flood is coming the reservoir is already full. This situation will compel the authority to release the inflowing flood to the downstream. It is worth mentioning that the flood movement through a reservoir may become faster than the movement of flood through a channel depending on shape, slope and resistance characteristics. Therefore, providing a storage space particularly to absorb flood is essential if one wants to have flood moderation as one of the objectives of the multipurpose reservoir. Flood moderation and power generation always become conflicting objectives. Policy-maker should give due consideration to both the objectives. NEED OF REAL TIME FLOOD FORECASTING Better operation of a reservoir for avoiding artificial flood situation will be possible if reasonably accurate flow forecasting can be made at the appropriate time. A mathematical model developed for simulating flood movement throughs the basin and the channel due to rainfall event occurring in the upper catchment areas can help a lot in this regard. Having adequate data about the catchments characteristic and having a network of rain gauges already installed for the purpose, such model will be able to forecast the inflowing flood at the reservoir site to facilitate prior release from the reservoir, if necessary, to make room for absorbing the inflowing flood. NEED OF COORDINATED OPERATION OF DIFFERENT RESERVOIRS Several reservoirs have been proposed or may come up in future in different tributaries of the Brahmaputra River. While a good operating policy for a single reservoir can ensure flood benefit in the downstream of that reservoir, a coordination in the operation of all the reservoirs located in different tributaries of a river is also necessary to avoid artificial flood due to adverse combination of releases made from different reservoirs. For example, as seen in the MODIS data of figure 2, if large releases made from the reservoirs located on Lohit and Dibang reache the river Brahmaputra simultaneously, the flood level of the Brahmaputra may rise significantly. This high discharge of the river Brahmaputra flowing downstream if again meets with a large release made from the reservoir located on Subansiri, an artificial flood situation may occur. Therefore, while making a large release from a reservoir, the releases made from the other reservoirs and the time of travel of those releases must be taken into consideration for avoiding adverse flood situation. Figure 2: MODIS data showing different tributaries of Brahmaputra NEED OF USING MOST EFFICIENT OPERATING POLICY Benefit from a reservoir can be increased significantly by using an efficient operating policy. Different optimization techniques can be applied for deriving an optimal operating policy for reservoir operation. Experiences have shown that use of long series of past flow record leads to development of a better operating policy. Use of synthetic data is therefore gaining popularity. New technique like Artificial Neural Network (ANN) has been used successfully for developing synthetic streamflow (Ahmed and Sarma, 2004) required for development of efficient operating policy. New heuristic optimization techniques like Genetic Algorithm have also been found to give better policy in respect of

4 several performance criteria as compared to the conventional techniques (Ahmed and Sarma, 2005). CONCLUSION A critical analysis on the need of multipurpose reservoir for better management of the available water of Northeast India has been presented at the beginning of this paper. To achieve optimal benefit from the available water and to minimize damages due to flood some of the important issues as discussed in this paper should be considered both during planning and operation stage of the proposed multipurpose reservoirs. This is particularly important for the reservoirs of Northeastern part, as erroneous planning and operation of reservoirs can even worsen the flood situation in some localities. Due consideration should be given to the development of optimal operating policies. Potential of the latest available techniques should be explored in this regard. REFERENCES 1. Ahmed J.A and Sarma A.K., Neural Network-Based Stochastic Dynamic Programming for Reservoir Operation, Proc. of International Conference on Advanced Modeling Techniques for Sustainable Management of Water Resources, January 2004, NIT Warangal, pp

5 2. Ahmed J.A and Sarma A.K., Genetic Algorithm for Optimal Operating Policy of a Multipur pose Reservoir, Journal of Water Resources Management, Kluwer, Vol.19, Number 2, April 2005, pp Gupta S. K. and Deshpande R.D., Water for India in 2050: first-order assessment of avail able options, Current Science, Vol. 86, No. 9, 10 May 2004, pp MOWR, Integrated water resource development - a plan for action. Report of the National Commission for Integrated Water Resources Development Plan, Ministry of Water Re sources, Govt. of India, 1999, Vol. 1, p. 515.