Prof.T.Shivaji Rao,
Director, Centre for Environmental Studies, Gitam University, Visakhapatnam
http://www.youtube.com/watch?v=72_evCt8rpU&feature=related (Real live pictue of collapse of Polavaram dam that kills lakhs of people of Rajahmundry and Konaseema)
For Bio-data, http://www.bagchee.com/BookDisplay.aspx?Bkid=B34014
http://www.bagchee.com/books.php?id=26110
http://www.bagchee.com/books.php?id=26110
Browse all the 6 (0 to 5) websites on Polavaram dam
(See other chapters from 1 to 8 of the above website)
http://kfki.baw.de/fileadmin/conferences/ICHE/2002-Warsaw/ARTICLES/PDF/129C2.pdf : [ Asian Standards]
http://www.sscac.gov.in/p_maindam.html [Catchment :88,000Sq.km.Design flood is taken as 1 in 1000 yearsflood of 87,000 cumecs.Hence the Probable Maximum Flood, PMF has to be taken as 1.0 Cumec per 1.0 Sq.km.catchment area.]
www.icold-cigb.org/PDF/berga.PDF : [ Narmada historical record: peak flood of 71,000 cumecs in July 1994 ]
(Historical peak flood in Krishna is 10.61 lakh cusecs and 1000 years return flood is taken as 20.60 lakh cusecs)
PART-I
Andhra Pradesh Government submitted Polavaram project report before Bachawat Tribunal in May 1978. The head works of the project consist of an earth-cum-rock-fill dam across Godavari river with spillway on the right flank and power-cum-river sluices block on the left flank. The other details of the project are presented in the annexure.-I
1. INTRODUCTION: For the Polavaram dam construction with FRL at 150ft . Madhya Pradesh (now Chattisgarh) and Orissa states insisted that their tribal villages in Konta and Motu taluks should not get submerged due to floods and back waters upto more than 150ft ft level. The Central Water Commission (CWC) in its letter dated 3-7-1979 stated that the A.P. State engineers and officials do not perhaps know that even without Polavaram dam the Godavari floods in 1966 touched 153ft. elevation in Konta and Motu taluks and hence the agreement on Polavaram must be suitably revised. This clearly shows that the Engineers and the officials of the A.P. State Government are in the habit of making ill-considered and hasty decisions on major irrigation projects and mislead even the Chief Ministers to accept their unscientific proposals blindly. Ultimately the Bachawat Tribunal in their final report in July 1980 accepted the full reservoir level of FRL/ MWL of 150ft elevation for Polavaram dam subject to the condition that the ultimate decision on matters pertaining to design is left in the hands of the CWC and that the A.P. state Government has to accept the decisions of the CWC.
2.ARE RIVER WATER DISPUTES TRIBUNALS COMPENTENT TO DECIDE ON DESIGN OF DAMS ?: The Bachawat Tribunal in its report on sharing of Krishna river waters between Maharashtra, Karnataka and Andhra Pradesh has rightly considered the claims made by the states for allocation of the dependable river flows for various projects and they have never ventured to make technical, engineering decisions on the design of the projects. They merely allocated the share of river water quantities for each of the projects on the basis of the respective claims and the water availability. Unfortunately in the case of Polavaram project they have given their finding that the CWC may clear the project with the height of the dam at an elevation of 150ft above the mean sea level. Such a decision gives a wrong impression to the Chief Ministers and the judges of the High Court and Supreme Court as such unscientific and indiscriminate decisions are treated as scientific and legally valid .But the subsequent Environmental Protection Acts and the rules and regulations made by Parliament render such decisions of the tribunal are equivalent to the judges unknowingly passing death sentences against potential victims of people likely to be drowned for no fault of theirs due to a maximum credible accident to the dams resulting in collapse of the projects. In fact according to the latest laws on Dam safety in most of the countries including the Environmental Protection Act in India and the Dam safety guidelines formulated in 1986 by the Indian Government the decisions on heights of the dams and the design of the spillways have to be based upon several considerations including the number of people likely to be killed due to a potential dam failure. Hence the intention of the tribunal decision on the height of the Polavaram dam should not be followed blindly as it is highly detrimental to the interests of the public and environment.
3. POLAVARAM COLLAPSE DUE TO UNDER-DESIGN OF SPILLWAYS ?: Polavaram dam and its very long reservoir at the end of the long Godavari river near its most densely populated delta region is ultimately bound to fail due to maximum credible accident caused by extreme floods, earthquakes, construction of defects of the earth and rock-fill dam foundation failures, terrorist attacks, mal-functioning of the spillway gates leading to over-topping of the dam or abnormal man-made floods released from the dams in the upper states for protecting the safety of their own dams subjected to extreme peak floods caused by intense rainfalls of long duration during cyclones of the monsoon season or due to collapse of one or more dams in the catchment area.
Extreme floods due to cloud bursts:
70% of the dams in England were constructed before 1931 when there was not much scientific data from rain gauges and river gauges and hence spillway sizes were estimated on the basis of the largest flood upto that date at that location. For example the Woodhead dam built in 1847-1876 in England is the highest dam in cascade in the Longdendale Valley supplying Manchester, where the design floods have increased gradually over a number of years as shown in the following table and necessary remedial action was taken to ensure the safety of the dam and the people and properties in the downstream area.
4. PROGRESSIVE DEVELOPMENTS IN THE SPILLWAY DESIGN CRITERIA IN DIFFERENT COUNTRIES :
The weather and environmental conditions in UK are different from those in tropical countries where cyclones and intense rainfalls of longer duration during the monsoon are common.
Example of upgrading of spillway capacity with time for a UK dam (Chalmer, 1990)
Year | Inflow Design Flood (IDF) (m³/s) | Remarks |
1847 | 28.3 (1000 ft3/s) | Highest flood of record |
1848 | 42.5 (1500 ft3/s) | Large flood in nearby Blackburn |
1874-80 | ? | Intense rainfall showed inadequacy of spillway hence level of reservoir held down by 1.5m to provide for flood storage. |
1938 | 204 | Follow recommendations of ICE 1933 Flood Committee |
1986 | 550(PMF) | Further upgrading following FSR(Flood Studies Report) |
a)In order to build safe dams in USA upto 1912 the design spillway flood was estimated by considering the high water marks in the river bring dammed are in adjacent catchments and the design flood was calculated by applying a factor of safety. Since many dams failed because the American Engineers assumed that the previous historical peak floods were indicative of the maximum flood likely to be experienced by the dam during its design life period. Hence some spillways were designed by using a factor of safety to multiply the historically known maximum flood.
b) In the modern times the US engineers use the current state-of-the-art-criteria that use hydrologically safe design standards based on risk analysis and detailed impacts of the dam failure. Hence Inflow Design Flood (IDF) is related to height of the dam, volume of the reservoir storage and the hazard potential classification that depends upon the damage to human life and economic properties in the event of a hypothetical dam failure.
c) For high hazard dams the peak maximum flood (PMF) is used although for small dams it has reduced to 0.5 PMF while for low hazard dams the design spillway flood generally varies between 100years return flood and 0.25 PMF
d) The inflow design flood is defined as that level of flood flow above which any incremental increase in the downstream river water elevation caused by dam failure is no longer considered to present any unacceptable extra threat to lives of people and their properties.
e) It means that in the case of Godavari river the extreme floods that should be expected to enter into Polavaram reservoir must be of such high magnitude that while it may break all the previous historical flood records of 36 lakhs cusecs of August 1986 yet the storage of water in the Polavaram dam must be of such magnitude that even if it were to be converted into incremental flood flow increase its combination with the extreme Godavari river flood should not present an unacceptable additional threat to lives of animal and human populations and their properties in the downstream side of the Polavaram dam.(Fig)
f) After examining the gradual increases in the intensities of peak maximum precipitations and the consequential peak maximum flood flows in the catchments of several rivers in England since 1930 a hydrology expert, Kennard (1975) suggested that the ICE (Institute of Civil Engineers) “normal maximum flood” for a river was to be taken as a 150-year flood.
g) Another expert Seddon (1971) reported that the actual observed peak floods in rivers were found to be upto 3 times higher than the normal maximum flood specified in the interim British report on flood studies. These studies include a methodology to derive PMP (Peak Maximum Precipitation) , in Meteorological studies Volume II and PMF (Peak Maximum Flood), in hydrological studies in the United Kingdom.
h) ICE produced an engineering guide in 1978 and its specified dam design inflows for different kinds of reservoirs in terms of PMF as stated below
0.5 PMF : 10,000 year return period flood
0.3 PMF : 1,000 year return period flood
For details http://www.defra.gov.uk/environment/water/rs/pdf/defra_rs_flood-etc-18.pdf (Page-78)
i) Several states in USA have developed standards for inflow design flood for spillways.
According to the International Standards for spillways it is clearly stated under the dam safety rules 1989 of Montana state in USA that the spillway flood magnitude will have to be based on estimated loss of life downstream due to a dam collapse. Under Montana state standards the inflow design flood for an estimated loss of life of 0.5 or less shall be the 500 year recurrence interval flood and if the loss of life is equal to 5 the design flood is estimated by multiplying the estimated loss of life by 1000. If the loss of life is between 5 and 1000 the design flood is estimated from the design precipitation derived by a formula. Ultimately if the estimated loss of life is greater than 1000 the minimum inflow deign flood shall be the Probable Maximum Flood (PMF). The historical flood is taken as 100 to 150 year return period flood and it is taken as 0.5 to 0.25 fraction of the peak maximum flood and is generally used for small dams of low hazard potential. For intermediate dams of modern hazard potential the design flood is taken as 0.5 PMF and for high dams of high hazard potential, the design flood is taken as 1.0 PMF
j) In some of the European countries like Switzerland while the design flood for embankment dams is taken as the return period flood for 1000 years the safety check flood is taken as 1.5 times the design flood and a free board upto 3 meters has to be provided. More details furnished in the tables in annexure. Similar standards are followed for dam safety in several states in USA including New York as can be seen from the website.
5. VIEWS OF EXPERTS OF THE INTERNATIONAL COMMITTEE ON LARGE DAMS (ICOLD) :
a) In his latest article on design flood for dams F.Lemperiere, Chairman of the one of the expert committees of the ICOLD(International Commission on Large Dams) published in The International Journal on Hydropower & Dam, Issue2, 2005 stated that the discharge of extreme floods (such as the probable maximum flood) is in the range of 3 times the likely maximum discharge during the dams life. He stated that the failure of the dams by floods is caused by a small overtopping of the embankment dams and a huge overtopping of high concrete dams. The yearly probability of the design flood used for dams usually lies between 1/500 and 1/5000. He advocated from a realistic approach a “safety check flood” of very low probability (often chosen as the PMF), for which are accepted a reservoir level close to the crest of the dam and also some limited damages. He questions whether for answering the yearly probability of the maximum flood for ensuring safety of dam should be 1/1000 or 1/1,00,000 or quite nil. He states that the criteria for answering this question and the design methods are often the same as 50 years ago and have not been adopted to the present knowledge and conditions. He emphasizes that today there is much more data on extreme rains and floods which were considerably underestimated 30 years ago. According to him one of the most critical design criteria is that the volume and flow of an extreme flood (PMF) lie in the range of 2 to 5 or an average of 3 times the flow and volume of the maximum flood likely to happen during the life of the dam, i.e over 100 years. He says that the true return period of an estimated “1000 years flood” used as “design flood” may well be 200 years or 5000 years. He emphasizes that the design of mot existing dams are those under construction are based on a “design flood” which can be spilled (and possible partly stored) without damage. This International expert says that many small ungated embankment dams may withstand the peak maximum flood; but very large gated dams with a design flood of yearly probability of 1/1000 may fail for a 1/10000 flood with all gates open or for an yearly flood incase of all gates jamming. He further states that the evaluation methods are not the same for safety check flood which is close to the extreme floods and for the “operational flood” which is close to the 100 years flood. According to the reports on maximum reported flood collected from all over the world based upon their catchment areas (ICOLD bulletin 125, page 75) the peak floods are close to the following:
Extreme flows reported worldwide
Catchment area, S(km)2 | 1 | 10 | 100 | 1000 | 10,000 | 1,00,000 |
Flow (m3/sec) | 100 | 600 | 4,000 | 15,000 | 40,000 | 1,00,000 |
Flow(m3/sec) per (km)2 | 100 | 60 | 40 | 15 | 4 | 1 |
About 50 | About 2 | |||||
And may be roughly represented by 2 formulae: Q = peak flood discharge in cumecs
For S<300>2, Q = 10,000(S/300)0.8
S>300 km2 , Q= 10,000(S/300)0.4
Comparison between dams of the same or similar region is reliable because the impact of the different soil and vegetation conditions is very similar including its shape and slopes.
b) Another most reputed International expert Prof.L.Berga , Chairman of the committee on Dams and Floods of the International Commission on Large Dams{ICOLD} has considered the maximum flood flows of most of the rivers in the world that cover 90% of the existing large dams and found that floods are the most important natural hazard in 65% of the countries and that they constitute 90% of the cases of the first or second most natural hazard. Floods present the high recurrence of 7.2 years and in several cases the number of years between crucial floods ranges from 5 to 10 years. The majority of the victims are 200 per year in Bangladesh, 250 in South Korea, 1500 in India and more than 2500 per year in China. The extreme floods in the world have been analysed and he developed envelop curves and equations and presented specific flows in cumecs for sq.km based upon catchment areas extending upto a million sq.km. Even this data from the website http://www.icold-cigb.org/PDF/berga.PDF indicates that the extreme flood flow is in the order of about 1 m3/sec for every sq.km area of the river catchment.
According to Prof. Berga “the hydrologic criteria for the design of flood mitigation dams is based on two design floods:
1. An “ Inflow Design Flood” or “Safety Check Flood” to assure the hydrologic dam safety.
2. The protection design flood, which is the flood that the dam is capable of routing without producing damages downstream.
In general, and apart from the specific analysis in each case, the protection design flood
recommended are :
· In rural areas return periods of between 20 and 50 years.
· In urban areas return periods of between 50 and 200 years. In cases of protection of important cities, and if the economic, social and environments aspects are favourable, return periods of 500 years or even 1,000 years may be considered.
In the real cases studied by ICOLD the design flood protection varied between 35 and 200 years of return period, being able to reach in singular situations, in which there exists an important occupation of the flood plains and large cities downstream, values as high as 500 or 1,000 years”
The above views of the experts clearly show that in the case of the Polavaram project which is having a catchment area of about 3 lakh sq.km the extreme flood based on several relevant equations puts the maximum inflow design flood to lie between 2lakh to 3 lakh cumecs which is equivalent to about 70 lakh cubic ft. per second to 100 lakh cubic ft. per second. But unfortunately the A.P. State Government engineers who are always in a great hurry do not find time to update their knowledge on the design criteria to be used for such an important project like the Polavaram dam which posses a question of life and death to millions of people in the Godavari delta. If the peak maximum flood is scientifically estimated as per national and international standards the sizing of the reservoir and the spillways can be properly estimated and necessary remedial measures can be taken to make the dam safe along with providing safety to the people and their properties in Godavari delta.
6. CONSIDERATIONS FOR DESIGN OF SPILLWAYS:
According to the-latest-state-of-art design for spillways the international experts according to website: http://www.hydrocoop.org/spillways1.htm emphasize that if dam is to be safe its spillway must be capable of passing extreme floods without jeopardizing the dam itself. Correct design, proper construction, and reliable operation of the spillway are critical to the safety of the dam and downstream residents. The first crucial question faced by all the international experts of ICOLD was “Methods for determining the maximum discharge that should be expected at a dam and for which it should be designed. Selection of type, capacity and general arrangements temporary or permanent spillways”. According to International experts inadequate spillways and improper operation of spillway gates are the main causes for failure for 30% of the reported dam incidents. In the case of earth and rock-fill dams over-topping of the dams by extreme floods was the most important cause of failure in case of about 50% of the dam collapses while over-topping was again the cause of failure in 43% of the cases of masonry dam failures. Problems relating to spillways like insufficient spillway capacity, improper operation during flood passage, design deficiencies or impacts of shaking caused by medium earthquakes could become significant causes for the over-topping of dams leading to their failure. ICOLD bulletin 82 (2) stated that more than 40% of the dam failures had been caused by insufficient spillway capacity and it clearly shows the high level of uncertainty in the determination of a proper design flood. Hence the design of a spillway must be sufficiently moderate to allow for uncertainty in calculating the magnitude of the design flood without adopting the either too low a level or too high a level of extreme flood . If spillways are designed for extremely low levels of peak floods the dams will inevitably collapse and if one adopts an extremely high level of flood it becomes too costly and the project becomes unviable and hence the storage capacity in the reservoir must be correspondingly increased to a high level as had been followed in the construction of high Aswan dam which has been provided with adequate spillway discharge capacity to provide safety for the dam and the lakhs of people living down-stream of the dam in cities like Cairo.
7. INADEQUATE SPILLWAY LEADS TO FAILURE OF DAMS:
Thus Polavaram project made of Earth and Rock-fill dam may be subjected to a maximum credible accident for various reasons. Moreover like so many dams which collapsed due to inadequate spillway capacities, Polavaram dam also has been designed about 30 years ago with highly inadequate spillway for discharging the peak floods. About 20 irrigation dams in India have collapsed. (See Annexure-II) Even in Gujarat state several dams failed due to mistakes committed by the civil engineers in the design of the spillways as can be seen from the following table.
Design Floods, Observed Highest Floods And Revised Spillways For Some Projects, Gujarat
River Valley Projects in Gujarat | Total Catchment Area (sq.km) | Spillway Design Flood as per Project Report (cumecs) | Highest observed flood (Cumecs) | Revised Spillway (Cumes) |
Dharoi | 5485.84 | 11213.00 | 14150.00 | 21662.00 |
Dantiwada | 2862.00 | 6654.00 | 11950.00 | 18123.00 |
Machhu-I | 735.00 | 3313.00 | 9340.00 | 5947.00 |
Machhu-II | 1928.71 | 5663.00 | 16307.00 | 20925.00 |
Damanganga | 1813.00 | 11100.00 | 12900.00 | 12854.00 |
Source: Narmada, Water Resources & Water Supply Dept., Govt. of Gujarat http://www.sardarsarovardam.org/faq/answers.htm (old website) http://www.sardarsarovardam.org/faqs/answers.htm#1 (revised website) | ||||
From the above table it can be seen that Machhu-II dam which is one of the 20 dams that collapsed in India, the spillway capacity was provided for 5663 m3/sec (cumecs). But the actual observed flood during intense rainfall became more than thrice the designed flood on 11-8-1979 amounting to 16307 cumecs that caused the collapse of the dam. Within 20 minutes the floods of 12 to 30ft height inundated the low-lying areas of Morvi industrial town located 5kms below the dam and atleast 2000 people were killed. During reconstruction of the dam the capacity of the spillway was increased by 4 times and fixed at about 21,000 cumecs. These dam failures clearly show that for economizing the costs, the Indian Engineering experts are in the habit of under designing spillway capacities of dams that lead to their collapse. In order to save the lives of lakhs of people of Cairo city located on the downstream of the Aswan High dam, the reservoir capacity was designed to hold about 6000 TMC of water while the annual flow in the Nile river was estimated at 3000 TMC. Since major metropolitan cities like Rajahmundry and industrial townships like Kovvuru are located about 30km below the Polavaram dam if similar action taken in the case of High Aswan dam is to be implemented at Polavaram Dam, the reservoir has to be designed with 40 times higher storage capacity which results in abnormal submersion of extensive tribal villages in the catchment area of the river.Such action leads to rehabilitation and resettlements costs that it will be several times higher than the cost of the Polavaram dam itself.
8. PEAK FLOODS IN GODAVARI AND DESIGN FLOODS FOR DAMS IN FOREIGN COUNTRIES: The project authorities considered the magnitude of Godvari floods for recurring periods of 25, 50, 100, 200 and 500 years and selected for this spillway 500 year return peak flood of about 1 lakh cumecs, equivalent to 36 lakh cusecs (cubic ft/sec). But a peak flood of 36 lakh cusecs has occurred in 1953, 1966, 1970, 1976, 1983, 1990 and 2005.
Even the dam break analysis report for Polavaram dam prepared by the experts of the Union Ministry of Water Resources organisation named National Institute of Hydrology located at Roorkee, the peak flood flows due to a hypothetical failure of Polavaram dam varies from 1.5 to 3 times the historical flood of 36 lakhs cusecs of August 1986. Thus the present spillway design flood for Polavaram dam is highly under estimated and it is definitely going to cause the collapse of the dam due to a maximum credible accident including peak maximum flood caused by extreme flood events due to intensified cyclonic rainfall of increased duration or due to sudden releases of heavy flood waters from the dams in the upper states or floods due to collapse of one or more dams in the catchment area of the river.
“DESIGN FLOODS” FOR DAMS
S.No. | Country | Design Flood Qd (years) | Safety check flood | Remarks |
1. | Norway | 1000 | PMF | -- |
2. | Spain Cat-A High Hazard Cat-B Significant Hazard Cat-C Low Hazard | 1000years 500 years 100years | 5000-10000years 1000-5000years 100-500 years | No over topping of embankment dams |
3. | Switzerland Concrete dams Fill dams | 1000years 1000years | 1.5 Qd 1.5 Qd | Freeboard upto 3 meters for fill dams. Rettemier shows safety flood as PMF |
4. | United Kingdom Cat – A Cat – B Cat – C Cat – D | PMF 10,000yrs 1,000 yrs 150 yrs | Overtopping of fill dams not completely excluded. |
RELATION BETWEEN “Q” FACTOR AND “RETURN PERIOD” OF FLOODS
S.No. | Q (Cumecs, m3/s) | Factor | Return period |
1. | 36.3 | 1 (PMF) | 1,000,000 years |
2. | 18.2 | 0.5(PMF) | 10,000 years |
3. | 10.9 | 0.3 (PMF) | 1,000 years |
4. | 7.3 | 0.2 (PMF) | 150 years |
5. | 6.2 | 0.17 (PMF) | 100 years |
USA 1985 STANDARDS – DESIGN FLOODS - HAZARDS
S.No. | Dam | Hazard | PMF Value | Remarks |
1. | High | High | 1.0 PMF | |
2. | Intermediate | Moderate | 0.5 PMF | |
3. | Small | Low | 0.25 PMF | 100 years flood |
Note: Before 1900 design flow was based upon collection of data on high water marks on buildings and structures for calculating peak flood and spillways were designed by using a multiple of this known maximum flood as a factor of safety. But some dams failed because engineers used for spillway design the previous floods that are indicating of the maximum flood likely to experienced by the dam during its design life.
UK ( 1975) DESIGN FLOODS
S.No. | Flood | Remarks |
1. | Normal Maximum Flood | Rainfall of 75mm in 24 hours, bulk of the rain falling in a few hours. Kennard (1975) treats this as 150 year flood |
2. | Acute catastrophic flood | Cloud burst with 127 to 178 mm rainfall in time of concentration. It is 2 times normal maximum run-off. |
3. | Prolonged Catastrophic Flood | Extreme rain of 244mm in 24 hours including 25 mm in one hour. |
Note: Sedden (1971) observed peak floods 3 times higher than the Normal Maximum Flood.
UK (1978) ICE GUIDELINES ON DESIGN FLOODS FOR DAMS IN TERMS OF PMF
S.No. | Flood | Return period |
1. | 0.3 PMF | 1,000 year Return period flood |
2. | 0.5 PMF | 10,000 year Return period flood |
3. | 1.0 PMF | Category-A high dams with high hazard potential |
Note: Britain is over safe with its guidelines based on local conditions including PMF for high dams.
A large number of dams failed in China resulting in the death of several lakhs of people and the details of the dams are furnished under the websites.
9.DAM SAFETY AND REMEDIAL MEASURES:
A Large number of dams/reservoirs are constructed to meet the demand for irrigation and power generation and flood control in addition to augmenting water supplies for industries and municipal corporations. About 78.5% of the total number of Dams in the world are Earthen dams (CBIP 1996). Earthen dams are very much vulnerable to breach due to overtopping of water. In the eventuality of any dam failure, the disaster would be catastrophic as had been witnessed in the case of Morvi disaster (1979) in Gujarat. Inspite of taking all the necessary preventive and control measures for ensuring the safety of dams as per the guidelines issued by the Government still,the dam failure cases are frequently occurring due to maximum credible accidents for several reasons like unforeseen earthquakes unusual rainfall due to abnormal weather conditions, foundation failures and human failures. There have been more than 2,000 dam failures around the world since 12th century, causing damage worth millions of dollars and loss of thousands of human lives.
According to a world bank dam safety report, out of 25 dams surveyed in India, 2 of the major dams namely Hirakud and Gandhi Sagar are likely to be hit by flood, 7 times larger than these dams were built to contain. According to Mr William price of the World Bank Asia Technical Division, the consequences of a failure of these dams during major floods would be highly disastrous. It is well known that the collapse of the Machchu-2 dam (4 Thousand Million Cubic ft.,TMC capacity) in August 1979 killed atleast 2000 people, due to a flood which was more than twice as strong as that for which the dam was designed.
Since hundreds and thousands of people according to World Bank report are at risk downstream of the 25 dams surveyed in India, it is necessary that inundation maps are published and evacuation plans prepared for all the existing major dams and for all the future dams to ensure the safety of the dams and also thousands of people living downstream of some of the hazardous major dams.
10. NOTABLE DAM DISASTERS:
Even today it is to note that 68 dams collapse every day in China, killing lakhs of people as can be seen from the website. http://www.theepochtimes.com/news/6-6-11/42574.html
It may be interesting to recollect that the world’s worst dam disaster occurred in August 1975 in China due to the failure of two dams with a combined capacity 600 million cubic meters, (about 21 TMC) In August 1975 an unusual weather pattern resulted in a cyclone which caused torrential rains for three days giving 1m depth of rainfall during August 5th-7th 1975. But the dams were designed to handle a maximum of floods caused by a half a meter depth of rainfall of August 8th the flood water in Shimantan dam on Hong river rose by 40 cms above the crest of the dams and the dam collapsed and the reservoir emptied more than 4 TMC of water within in 5 hours after half an hour the Banquio dam also failed when it created a wall of moving water of 6km height and 12km width. Behind this moving wall of water was more than 20 TMC of additional flood of water. All together 62 dams collapse affecting 11 million people in the region. The wall of water traveling at 50km /hr killed more than 84,000 people. This disaster caused the death about 86,000 to 2,30,000 people because the dams burst due to an exceptionally heavy flood caused by abnormal 3 day rainfall of intensity twice the value used for design purposes.
Even in the case of Machchu dam failure the Morvi town was ill prepared to meet the 2 storey high wall of water that burst fourth from the dam 5 km upstream of the town and swept away the town on 11-8-1979 within a matter of 9 minutes. The waters receded after 4 hours and there were no emergency evacuation and disaster management schemes prepared as is the practice in USA for ensuring dam safety under the dam safety act that requires dam break analysis risk assessment and disaster management. Instead of the state Government officials at Rajkot the first news of the tragedy were known by the Americans who learnt about the dam collapse through the orbiting weather satellite much earlier than the Indian officials. The state officials admitted that the rainfall in the previous 24 hours was about 23 inches while the dam was designed to accommodate a maximum of 44 inches rainfall during the whole year.
11. REMEDIAL MEASURES:
several states in North America have passed Dam Safety Acts and made the owners of dams responsible for ensuring the safety of the dam. The dams are operated and maintained in such a way that they do not constitute a hazard to life, health or property. It means that the owner of the dam is leagally bound to maintain the safety of the dam. Since a dam that holds back or has the potential to hold back huge quantity of water, it obviously poses a foreseeable risk to human and animal population, crops and properties on the dam is responsible for taking precautionary measures.
In western countries since people are more aware of the importance of dam safety since three decades. Most of the dam owners have prepared dam break analysis, risk assessment and environmental management plans so that emergency action plans are to be taken in case of a dam failure are placed before the public and discussed for the necessary feed back from the concerned public. As a failure of dam can take only a few minutes or hours, it is necessary to have a detailed plan of action to rescue people and rehabilitate them in time in case of dam, failure due to a maximum credible accident. If dam break analysis is done for all the exiting and proposed dams. Early identification of a hazardous situation will provide sufficient time to give an early warning about an impending flood hazard down stream people and to implement measures to prevent or delay a dam failure due to rapid and unexpected failures of dams and their consequential devastating impacts.
At present dam safety analysis is conducted at regular intervals for all the dams in the United States for taking timely remedial action to prevent dam failures and the consequential economic losses to the nation. The American experts make a review of the dam safety by making the dam break analysis and risk assessment. In case a dam is found to be unsafe either the dam is abandoned or its storage capacity is reduced to such an extent that even if the dam were to break the consequential flood inundation would not cause undue economic damage and loss of life of human and animal population. Sometimes a dam is also strengthened by increasing the width of the berms for ensuring safety. Wherever necessary the developmental activities such as housing and construction of other public works in the inundation areas downstream of dam can be controlled and emergency evacuation action plans can be formulated to minimize the damaging impacts of the disaster due to a catastrophic failure of the dam.
http://wrmin.nic.in/publication/ar2000/ar00ch10.html For information on cloud seeding see the following websites
http://shivajirao32.googlepages.com/warmcloudseeding
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