Recent and past floods in the Alaknanda valley: causes and consequences

Uttarakhand Himalaya in general and Alaknanda and Bhagirathi valleys in particular have experienced one of the worst forms of disaster in recent times (Figure 1). Flash floods are common in the Himalaya, but the kind of destruction witnessed this time was unparalleled in recent history. Houses collapsed like a pack of cards and the roads and bridges swept away in the turbulent flood waters. Probably the worst causality of the century was the destruction of Kedarnath valley. According to the data published in various national dailies, nearly 4000 people were either killed or lost, 2232 houses were damaged, 1520 roads in different parts of Garhwal were badly damaged and about 170 bridges have been washed away. According to economists, the tourism industry in Uttarakhand will suffer a loss of ~12,000 crore rupees, which is around 30% of the state’s GDP. Over the years increased frequency and magnitude of flash floods in Uttarakhand Himalaya is worrying the inhabitants. Was it due to the commercial forest felling that was prevalent until around 1980s, or the recent rampant terrain tampering for hydropower projects? In order to appreciate the sensitivity of the terrain towards unusual weather events like cloudburst, let us look into the genesis of two major flash floods, viz. 26 August 1894 and 20 July 1970. These floods are reasonably well documented. The 1894 flood occurred well before the commercial forest felling extended into the inner catchments of the Alaknanda, whereas the 1970 event occurred when the commercial forest felling was at its peak in the Alaknanda valley. However, both floods owe their genesis to the breaching of dams created by landslides on the tributaries of the Alaknanda river, a common geomorphic expression during unusual rainfall events in the monsoon-dominated Himalaya. On 6 September 1893, a tributary of the Alaknanda river called Birahi Ganga (Figure 1) was blocked by ~5000 million tonnes of rock mass that rolled from 900 m high valley flank. The debris blocked the river forming a lake 270 m high, 3 km wide at the base and 600 m wide at the summit. It was estimated that the lake would have taken at least a year to fill. The dam would partially breach only after the water began to topple it, which would cause flash floods in the downstream till Haridwar. The untiring efforts of Pulford, the then superintending engineer and his team, particularly Pandit Hari Krishen Pant (district surveyor of Garhwal), helped to meticulously estimate the magnitude of downstream inundation. An excellent telegraph system was installed between Birahi Ganga and Haridwar for real-time monitoring and timely warning of the flood. Around May 1894, pilgrim traffic on the way to Kedarnath and Badrinath was diverted to the new pedestrian route