Dams Do Mitigate Floodsby Dharmendra Kapri | 06-01-2014 16:57 |
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 The Himalayan state of Uttarakhand is vulnerable to multiple natural hazards of which flood and flash flood are important ones. Floods and flash floods induced by cloudbursts or intense rainfall events as also by the breach of temporary lakes formed by landslides have been a common phenomenon in Uttarakhand. These have adverse impact on the life support strategy of the people of Uttarakhand together with those living in the adjacent plains of Uttar Pradesh. It is important to note that the habitations around the rivers originating from the state contribute significantly towards total agricultural and industrial product value of the country and any disruption in these areas is sure to be reflected adversely on national growth and development statistics. Thus, flood control and management in this area are of vital importance for social and economic development of the nation as a whole. Alaknanda and Bhagirathi, two major rivers originating from Uttarakhand, meet at Devprayag to form Ganga, the holy river of Hindus. These rivers contribute two third and one third respectively towards normal discharge of Ganga. Rishikesh and Haridwar are located at a distance of approximately 60 and 84 kilometers downstream of Devprayag along the course of the river. Tehri dam is 260.50 meters high earth fill dam constructed at the confluence of Bagirathi and Bhilangana rivers at Tehri that is located about 40 kms upstream of Devprayag. The reservoir of this dam extends for around 42 square kilometers upstream of the dam. One of the envisaged roles of Tehri dam is to store floodwaters in the reservoir so as to safeguard downstream habitations from floods and thereafter pass off the stored water in a regulated manner, based upon downstream demand. It is not out of context to mention here that during the floods of 2010, 2011 and 2013 Tehri dam played a crucial role in averting floods of higher order in Ganga. The dam actually mitigated havoc that would have been caused, had it not stored high flood discharge of Bhagirathi. Around 19 and 20 September 2010 when all major rivers of the state had crossed their peak discharge levels, the discharge of Bhagirathi into Tehri reservoir was well above 3,500 Cumecs (1,22,000 Cusecs). Of this only 800 - 900 Cumecs (28,000 - 31,500 Cusecs) was released from Tehri dam at the time of peak discharge. The dam thus reduced the discharge of Ganga by 2,600 – 2,700 Cumecs (90,500 – 94,000 Cusecs) that would have added to the devastation caused on those days around Haridrwar and downstream areas. Same was the scenario in the year 2011 on 16 August when the discharge of Bhagirathi had crossed 3,600 Cumecs (1,26,000 Cusecs) but of this only 900 Cumecs (31,500 Cusecs) was released from the dam. Tehri dam thus successfully mitigated the impact of floods at Rishikesh and Haridwar, as also in the downstream areas that were already facing havoc caused by floodwaters of Ganga. In the year 2013 all the rivers of the state were flowing well above danger levels between 16 and 18 June and in this period the discharge of river Ganga at Haridwar went up to 15,000 Cumecs (5,25,000 Cusecs) and water level reached 295.90 meters in the evening of 17 June. It was 1.90 meters above the danger level of 294.00 meters. It is worth noting that this flood was the creation only of the discharge of Alknanda and tributaries of Ganga in-between Devprayag and Haridwar. It is worth noting that in this period the flood of the order of about 7,500 Cumecs (2,62,500 Cusecs) of peak discharge in Bhagirathi was totally accommodated by Tehri reservoir and the discharge of Bhagirathi downstream of the dam was regulated around 500 Cumecs (17,500 Cusecs). Had the Bhagirathi flood not been stored in Tehri dam, the peak discharge at Haridwar would have been around 22,000 Cumecs (7,70,000 Cusecs) and devastation by this flood, not only at Rishikesh and Haridwar but also in the downstream and upstream of these habitations, would have been beyond our imagination. The scenario can well be imagined from then anticipated rise of then observed highest flood level by another 2.5 to 3.0 meters. It is gathered from the available records that 2013 floods in Ganges would have in that case, been of the order of or even higher that experienced in the year 1924. This can be easily understood with Tehri dam and non - Tehri dam scenarios. Actual observed discharge data of Bhagirathi at Tehri and Ganga at Haridwar together with outflow from the dam between 16 and 18 June, 2013 have been fully analyzed to understand these scenarios. As velocity of water is a direct function its volume, the velocity of water increases with the increase in discharge. The travel time of water from Tehri dam to Haridwar via Devprayag and Rishikesh is between 10 and 16 hours depending upon the volume of discharge. In the case of June 2013 floods, the discharges in the rivers were very high and thus the travel time from Tehri to Haridwar would have been about 12 hours. The accompanying graph depicts probable impact of Bhgirathi flood on the flood of river Ganga at Haridwar together with inflow and outflow at Tehri, which clearly shows the volume of water stored in the Tehri reservoir at any given point of time. The observed discharge at Haridwar (shown in blue colour in the graph) and anticipated discharge at Haridwar (shown in red colour in the graph), after superimposing the differential Bhagirathi discharges at Tehri (discharge stored by Tehri dam) over the observed discharges of Ganga at Haridwar, with a time lag of 12 hours that is the travel time of water for reaching Haridwar, is enough to exhibit that in the absence of Tehri dam flood at Haridwar would have been immensely devastating. On the aftermath of June 2013 flash floods and ensuing disaster incidences in Uttarakhand a number of environmentalists and activists have raised many questions on the utility of hydro-electric projects and dams in environmentally sensitive Himalayan terrain. Some have even gone a step further to out rightly hold dams responsible for the devastation. Authors neither have access to relevant data nor the required intellectual acumen and technical capabilities to either negate or verify these assertions. This article is thus an attempt to put forth simple assertion before the learned readers, with the help of available data and facts, that dams can and do mitigate floods. Tehri dam has actually shown this more than once. As is generally accepted, frequency of extreme climate events is expected to increase in near future due to climate change. Since 2010 Uttarakhand has been experiencing extreme rainfall events every year during monsoon. The probability of increased incidences of floods and flash floods calls for putting in place more of measures that cushion or mitigate their impact. Dams can well be considered as being one such measure. |
