By Rajiv Shah
The new state-sponsored report, “Environmental Impact and Risk Assessment for the Proposed Barrage across River Narmada near Bhadbhut, Dist. Bharuch”, by Government of India consultants National Environmental Engineering Research Institute (NEERI), has insisted on a “study of catastrophic flooding that may occur in the event of barrage … failure is of great concern and importance because of the risk of life and property in the down stream of the structure.” The report has been prepared for discussion at the environmental pubic hearing on the Rs 4,000 crore Bharbhut project, to take place on July 19 near Bharbhut.The report, which is environmental impact assessment study of the barrage to be built across Narmada river's month next to the Bay of Khambhat, says, “The statistics on dam/barrage failure during the last one century show about 200 major dam failures resulting in the death of more than 11,000 people. However, their distribution is uneven throughout the world. Some of these dam failures have occurred in India also and MachhuDam (Morvi, Gujarat) in 1979 is one of the glaring examples, which took a toll of 2,000 lives and caused enormous loss to property.”
Keeping in view the adverse effects of dam break on human population, loss of property and environment in and around the areas of impact of the dam break, the report wants the Gujarat government to “assess the possibility of such failures and formulate a plan of management. Apart from the inherent hazard potential of dam, the risk may get compounded due to the occurrence of natural events like an unprecedented runoff, earthquake, etc.”
The report says, “Records have shown that few dam failures are on account of earthquakes, whereas large number of partial or complete failure of dams is due to unprecedented floods. Therefore, there exists a need for an emergency evacuation plan to lessen or mitigate its impact upon human habitation and property.”
Coming specifically to Bharbhut, which is proposed to be built on the mouth of Narmada river, the report says, once the barrage is built, ”in normal situation, the river water will get stored up to 7.5 metres height, which is the full reservoir level (FRL) in the upstream of the barrage.” As for the downstream, “flow of the river water will be limited to environmental flow.”
Against this backdrop, the report says, “since the tidal water flow will be predominant in reverse to present regime of the predominant river water flow, the tidal flow dominated hydrodynamic condition will bring about morphological change in the downstream estuary from the uniformly shaped form to the irregularly shaped tidal estuarine form. Moreover, the tidal water on getting reflected from the barrage will rise to a higher level and spread over a larger area.”
The Aliabet island, which is in the mouth of Narmada river, at the spot where the river merges into the sea, “may face the problem of erosion”, the report says, adding, “In post barrage scenario, the increase in tidal water level concomitant with the increase in the spread of tidal water area will be accompanied with reduction in the flow velocity of tidal water. In consequence, sedimentation will increase and the tidal transmission will have increasingly irregular and aggravating characteristics.”
Of the two Narmada estuary channels, the report says, “the western channel is already silted up, and its siltation levels will get further aggravated.” The existing mud island of Aliabet having the elevation of El. 4.0 m to 7.0 m form the left bank of the estuary and is presently stabilized. “Any erosion of the Aliabet will not only destabilize it but will also contribute materials for increased sedimentation. As regard erosion of estuary banks, “the Ministry of Environment and Forests, Government of India, has launched final report on shoreline changes for Gujarat coast prepared by the Institute of Ocean Management (IOM), Anna University, Chennai, in January 2011.”
The study identifies shoreline changes in the coastal districts have been classified into four categories -- stable coast, low erosion coast, medium erosion coast, and high erosion coast. “The barrage location falls in Hansot taluka on left bank and Vagra taluka on right bank of Bharuch district. The area of Bharuch district comes under three of the four categories -- high erosion, medium erosion and low erosion categories. One village of Hansot taluka falls under medium erosion category and eight villages of Vagara taluka fall under low erosion category.”
The study recommends that to overcome the erosion in the downstream estuary will have to be appropriately trimmed and the channel bed will have to be properly graded to impart non-silting velocity. “By adopting this measure, estuary water will remain confined within the banks and flow smoothly without undue sedimentation”, the report states.
During the monsoon months, the accumulated flood would have be released by opening the gates. As the large flood volume will have concentrated flow, the increased flow velocity resulting thereof “will flush out the estuary bed sediments.” After sediment removal, “the western Aliabet channel, which is presently silted up, will get cleared and become functional.”
Further, it is also likely, the report states, “that the increased tidal motion will cause deepening and widening of the estuary channel which will help accommodate larger flood volume and also facilitate easier exit of flood water. Under the improved condition of channel flow, Aliabet will not be subjected to erosive forces.”
Pointing towards yet another risk, the report points out, “Narmada river flood is the most significant risk”. It says, “At the highest flood level of 8.31 metres observed at Bhadbhut in 1970, the maximum flood discharge of 27 lalj cusecs was estimated, which caused full to partial flooding in low lying areas of 17 villages on the left bank of the river. According to present estimate, the total flood discharge at the Bhadbhut barrage is considered as 1,02,364 cubic metres per second or cumecs, or 36.15 lakh cubic feet per second (cusecs). After barrage construction, “because of the flood discharge of 36.15 lakh cusecs and incidental afflux, the flood can be anticipated to cause damage in the left bank areas”.
While such upstream dams such as Indira Sagar, Omkareshwar and Maheshwar in Mandhya Pradesh, and the Narmada dam in Gujarat can absorb flood waters, reducing the “probability of occurrence of high flood is reduced considerably”, at the FRL of 7.5 mtres, the height of the protective embankment will have to be 1.0 mtres above the high flood level (HFL). The report adds, “Having the provision of river widening and sufficiently raised embankment, adequate space will be made available for accommodating the incoming flood within the confines of the river banks fortified with the embankment.”
Even then, the report underlines, “disposal of flood at Bhadbhut barrage site will be contingent upon the tidal situation. During the no tide period, on receipt of early flood release signal from the Narmada dam site, the barrage gates will be opened in advance so as to empty the upstream reservoir as per the requirement of storage space to accommodate the incoming flood. The barrage storage capacity is estimated as 450 Mm³. This quantity could be evacuated during no tide condition in 6.5 hours if the gates are opened for 20,000 cumecs.”
In an extreme situation, the report says, ”since the barrage gates will remain closed for this condition, the reflection of the tide will cause the further rise in tide level. In this situation, the flood water level in the upstream is likely to rise to above the FRL.” But considering the development of numerous water resource projects, particularly Indira Sagar, Omkareshwar, Maheshwar and the Narmada dam, the report claims, “in recent times, a considerable moderation of flood discharge is all likely and the probability of occurrence of the above stated extreme situation is least likely.”
The risks nevertheless do exist, one reason why one should set up alarms and warning systems, even as addressing “both hardware and software aspects” which ne to be “integrated into the design of emergency management.”A contingency plan or disaster management plan for dam failures should be kept ready. This plan would serve as a reference document consisting of salient information indicating the actions to be taken at the time of disaster, and hence, it has to be made as comprehensive as possible and it needs to be tested and updated periodically.”
Comments