Flood damage: How prepared are we?
R. Rangachari Former, member (Floods and River Management), Central Water Commission, and visiting Research Professor, CPR Humanity has lived with floods for centuries but the impact of floods was not felt to the same extent in the in the past as is experienced now. Rapid increase in population and urbanization coupled with increased economic and developmental activities together with the nonregularisable river bed colonization have resulted in the hazardous occupation of the floods plains to an ever increasing extent. India is subject to monsoon, which accounts for the rainfall. In our region the monsoon period is between July and September and three-fourth of the monsoon rain in any year is concentrated within rain in any year is concentrated within about 100 hours of intense downpour. Floods are common occurrences, particularly in the Brahmaputra and Ganga (including the Yamuna) basins. However, the actual intensity and the extent of flooding as also the flood damages vary from year to year. Delhi lies within the Yamuna sub basin of the Ganga sytem, which itself is a part of the greater Ganga Brahmaputra Mega basin. River Yamuna rises in the Himalaya at Yamunotri in UP. The uamuna receives many tributaries before it debouches into the plains near Tajewala. It then travels Generally along the UP - Haryana border before it reaches Delhi state. It traverses some 40km. in Delhi and has an average bed width varying from two to three km. thereafter the river runs again through UP and joins the Ganga near Allahabad. Delhi state has flat land, however there is a big depression in the southwest known as the Najafgarh jheel area, which receives the drainage from the adjoining states of Haryana and Rajasthan. The only outlet for these waters is the Yamuna. The Sahib river that flows through Rajasthan and Haryana used to be absorbed in the sandy areas of Haryana and has no definite course downstream of Masani in Haryana. The waters of the Sahibi flow into Delhi through the Dhansa bund. The waters find a final outlet in to the Yamuna (throught eh Najafgarh nala) below wazirabad. There are barrages across the Yamuna in Delhi at Wazirabad, Indraprastha and Okhla. There are also some 18 drains that join the Yamuna in Delhi. Delhi's flooding is due to
Between 1953 and 1990, the period for which published damages data are available, there were high floods in Delhi 1in 1956, 1958, 1962, 1964, 1966, 1967, 1971, 1975, 1976, 1977, 1978 and 1988. Prior to 1953 there were such floods in 1924 and 1947. Since 1990 the floods of 1995 were high. The highest flood level on record so far was in 1978, through the Yamuna. The 1924 floods, which comprised successive peaks of lesser order, had generated the largest volume of flood flows so far. The 1977 flood was the highest through the Sahibi nadi. Flood damage figures are reported by the Delhi state administration and these have been compiled by Central Water Commission. Taking these as reflecting the true position some inferences could be drain. The maximum area that was affected in Delhi was 70,000 ha of which 40,000 ha were stated to be under crops and this was in 1978. A million people were reportedly affected. Some 400 heads of cattle and 18 human lives were lost. The total damage to crops, houses and public utilities was put at around 18 crores rupees. Int eh 1977 floods, which was next in order of magnitude, the total damage was put at over Rupees five crores. A damage of comparable order earlier was in 1967. The expenditure incurred in planned flood control works in Delhi between 1951 and 1955 is around Rs 185crores. It is reported that under th Plans, 83 km of embankments were built and 453 km. of drainage channels made, benefiting come 78,000 ha. of land. The expenditure figures relating to unplanned relief are not available. Experience ahs shown that the loss of human life and property could be reduced considerably by giving reliable advance information about the oncoming floods. The Government of India's involvement in flood forecasting and warning activiti4es as initiated with the Nation's capital. A beginning with scientific flood forecasting was made by the Central Water Commission in late 1958 and the first forecast relating to the water level at old Railway bridge at Delhi across the Yamuna was issued on 25th July 1959. After the Sahibi Floods of 1964 a second site was started for the Dhansa Regulator on the Sahibi. This network now covers some 157 flood forecasting stations in India. As a part of modernization of flood forecast systems under CWC, considerable improvements were made in the forecasting set up for the Delhi Railway Bridge in the eighties with automatic data connection, telemetering and INSAT satellite system. The danger level at Delhi Railway bridge was indicated to be 204.83 meters and warnings area issued for likely water levels of over 204 meters. What is this magic number 204.83 m? it would appear to be no real magic. It was considered by Delhi administration in the 1950s that water level in the Yamuna higher than 672 feet above mean sea level could lead to damages. So 672 feet it was. As India switched to the metric system, some one meticulously converted the figure in FPS units to 204.83m. strictly speaking, a periodic examination of the changes in the river configuration and flow pattern is required on the basis of upto date records. This should take note of the embanking and other changes made in the upstream reaches as also the estent of instruction into the flood plains by structures, economically important installations etc. Thereafter, a fresh determination of the danger level at various locations should be done. We are not sure that this had been done. Every indication is that this has not been done. We know that much embanking upstream has been made and many gaps closed, greater intrusion into the flood plains has occurred and vast urbanization has taken place on the eastern side the Yamunapar area. The vast changes in the ground situation since the last high flood of 1978 have many serious adverse implications for Delhi. It has been observed in recent years that the water levels at Delhi for lesser magnitudes of flow are higher than in the past. The nest "high flood" in the Yamuna could have very serious consequences for the nations capital area. In 1978, a discharge of 15,690 cumec (5.54 lakh cusec) was estimated to have crossed Tajewala on 3rd September morning. However soon thereafter no more observations were possible as the observation post was abandoned and staff evacuated on considerations of safety. There were son many breaches lower down and spills occurred. Estimates by various experts of the peak discharge then attained at Tajewala varied from 18,000 to 26,400 cumec. One does not know that the likely position would be next time. A workshop sponsored by CWC and organized by CBIP in January, 1986 to consider the design discharge for structures and embankments on the Yamuna in Delhi noted the "grave risk involved in any underestimation of the stakes to the life and property of nearly 2 million people who were then living in trans - Yamuna area which was solely dependent on the safety of the left embankments". There are many who consider that the magnitude and water level of the next 'high flood' could be of an order higher than that recorded in 1978. We can sum up the flooding possibilities and related problems:
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