EFFECT OF ADDITIONAL SPILLWAY OF HIRAKUD DAM ON DOWNSTREAM AREAS

Abstract

Hirakud Dam, built in 1957, is the longest major dam of India across the River Mahanadi in the State of Odisha. During non-monsoon seasons, the inflow into the reservoir reduces due to the construction of several dams and barrages in the State of Chhattisgarh. As a result, the area downstream of Hirakud gets lesser water during non-monsoon, which results in drought like situation. At the same time, the reservoir capacity is also gradually reducing due to sedimentation. The drought severity analysis has been carried out in the present thesis using various drought indices like Standardized Precipitation Index, China Z Index, Rainfall Departure and Z-Score, etc., and projected under RCP 4.5 and RCP 8.5 scenarios for different time scales for this area. The results of the analysis and inference drawn have been presented. In the monsoon season, under extreme flooding situations, the reservoir gets more water as the upstream dams release more to ensure the safety of their dams. To address these issues, Hirakud dam authorities have taken the steps for the revision of rule curves, planning of additional spillway for the Hirakud dam to ensure its safety during the monsoon season, and the evaluation of the impact of this additional spillway on the downstream areas in monsoon season and the evaluation of the impacts of reduced flow in the reservoir during the non-monsoon season on the downstream areas and the possible reduction in rainfall under climate change and increase in the water demands due to growing population and industrialization. The details of the flood study are summarised in the following paragraphs. The total discharging capacity of Hirakud Dam is 42,450 cumecs through the entire 34 nos. of spillway gates and 64 nos. of under sluices. The highest flood observed at the dam site was the order of 43,183 cumecs in July 1959 and 37,718 cumecs in Sept 1980, threatening the safety of the dam. The floods during 1982, 1999, 2005, 2008, 2009, 2011, 2013, 2014, and 2019, etc. also frequently affected various districts of Odisha covering the Mahanadi River Basin (MRB) and deltaic region. Further, when the design flood is routed through the reservoir and using the available spillways it is assessed that the dam can get overtopped which can result in failure of the dam resulting in large scale loss to life and properties in the downstream cities/towns of Hirakud, Sambalpur, Cuttack, Burla, Jagatpur, Bhubaneswar, etc. Hence additional spillway of capacity 27,182 cumecs, is being planned which would make the total discharging capacity as 69,632 cumecs (42450 original plus 27,182 additional). The latest estimate of PMF of Hirakud stands at 69, 632 cumecs as per CWC (2018). The spillway of same capacity as that of PMF without iii attenuation, is being planned as the PMF in future may be even more because of the land use land cover changes and climate changes, and during the operation, some of the gates may not function and in future further addition of the capacity may not be possible because of the site constraints. HEC-RAS (Hydrological Engineering Centre-River Analysis System) 2D Modelling has been carried out to assess the impact of 69,632 cumecs release (worst possible scenario as on today) at the downstream areas to provide relief to the downstream areas and prepare emergency action plans. For this purpose, computed probable maximum flood, discharge at Mundali, global land use land cover, Real-Time Data Monitoring (RTDM), Radar (RAD) tide gauge data of Indian National Centre for Ocean Information Services (INCOIS) at Paradeep and Dhamra port are used to obtain the maximum flood inundated area so that emergency action plans and mitigation plans and flood forecasting and dissemination systems for the area could be developed in future. Hirakud Dam, built in 1957, is the longest major dam of India across the River Mahanadi in the State of Odisha. During non-monsoon seasons, the inflow into the reservoir reduces due to the construction of several dams and barrages in the State of Chhattisgarh. As a result, the area downstream of Hirakud gets lesser water during non-monsoon, which results in drought like situation. At the same time, the reservoir capacity is also gradually reducing due to sedimentation. The drought severity analysis has been carried out in the present thesis using various drought indices like Standardized Precipitation Index, China Z Index, Rainfall Departure and Z-Score, etc., and projected under RCP 4.5 and RCP 8.5 scenarios for different time scales for this area. The results of the analysis and inference drawn have been presented. In the monsoon season, under extreme flooding situations, the reservoir gets more water as the upstream dams release more to ensure the safety of their dams. To address these issues, Hirakud dam authorities have taken the steps for the revision of rule curves, planning of additional spillway for the Hirakud dam to ensure its safety during the monsoon season, and the evaluation of the impact of this additional spillway on the downstream areas in monsoon season and the evaluation of the impacts of reduced flow in the reservoir during the non-monsoon season on the downstream areas and the possible reduction in rainfall under climate change and increase in the water demands due to growing population and industrialization. The details of the flood study are summarised in the following paragraphs. The total discharging capacity of Hirakud Dam is 42,450 cumecs through the entire 34 nos. of spillway gates and 64 nos. of under sluices. The highest flood observed at the dam site was the order of 43,183 cumecs in July 1959 and 37,718 cumecs in Sept 1980, threatening the safety of the dam. The floods during 1982, 1999, 2005, 2008, 2009, 2011, 2013, 2014, and 2019, etc. also frequently affected various districts of Odisha covering the Mahanadi River Basin (MRB) and deltaic region. Further, when the design flood is routed through the reservoir and using the available spillways it is assessed that the dam can get overtopped which can result in failure of the dam resulting in large scale loss to life and properties in the downstream cities/towns of Hirakud, Sambalpur, Cuttack, Burla, Jagatpur, Bhubaneswar, etc. Hence additional spillway of capacity 27,182 cumecs, is being planned which would make the total discharging capacity as 69,632 cumecs (42450 original plus 27,182 additional). The latest estimate of PMF of Hirakud stands at 69, 632 cumecs as per CWC (2018). The spillway of same capacity as that of PMF without iii attenuation, is being planned as the PMF in future may be even more because of the land use land cover changes and climate changes, and during the operation, some of the gates may not function and in future further addition of the capacity may not be possible because of the site constraints. HEC-RAS (Hydrological Engineering Centre-River Analysis System) 2D Modelling has been carried out to assess the impact of 69,632 cumecs release (worst possible scenario as on today) at the downstream areas to provide relief to the downstream areas and prepare emergency action plans. For this purpose, computed probable maximum flood, discharge at Mundali, global land use land cover, Real-Time Data Monitoring (RTDM), Radar (RAD) tide gauge data of Indian National Centre for Ocean Information Services (INCOIS) at Paradeep and Dhamra port are used to obtain the maximum flood inundated area so that emergency action plans and mitigation plans and flood forecasting and dissemination systems for the area could be developed in future.