Catalyzing Climate and Disaster Resilience: Processes for Identifying

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Catalyzing Climate and Disaster Resilience: Processes for
Identifying Tangible and Economically Robust Strategies
BOX 1
The Kosi Embankment Breach
On August 18, 2008 a flood control embankment along the Kosi River in Nepal Tarai breached. The failure occurred when flow in that river was below
the long-term average flow for the month of August. Over the following weeks a disaster slowly unfolded as the Kosi River began flowing along one of
its old courses east of its previous one.
The Kosi drains an area of 60,000 km 2 in Tibet, Nepal and North Bihar (India). In one year the river transfers an estimated 95 million m 3 of sediment
derived from landslides and mass wasting to the Ganga. Much of the sediment is deposited in a huge fan where the river exits from the mountains to
the plains. This exceptionally high load of sediment is brought down to Chatara in the Tarai and is dumped on the riverbed as the river slope levels off.
In the past, as its main channel aggraded, the Kosi had naturally shifted its course. In the preceding 220 years the river had oscillated over a stretch of
115 kilometres. In 1959 this natural process was interrupted when the river was jacketed between two embankments following an agreement between
the governments of Nepal and India that had taken place in 1954.
Following completion of the Kosi Barrage in 1964, the river gradient changed and sediment deposition in the river section upstream of the barrage
increased rapidly. Over time, this raised the level of the riverbed above the surrounding land, a factor that contributed to the 2008 breach. When that
occurred, the main river discharge began flowing along a course that had been blocked by the eastern embankment. Instead of permanently
protecting the surrounding area from floods, the embankments had changed the morphology of the river, raising the jacketed channel above the level
of the surrounding land. Other factors leading to the breach include poor maintenance and institutional corruption and dysfunction in the aftermath of
Nepal-India treaty on the river. The resulting flood caused widespread inundation and concomitant adverse effects on the social and economic systems
dependent on the river.
Once the embankments were completed in 1959, the area to the east of the river was largely protected from major flooding and in the subsequent
four decades roads, irrigation channels, railways and other features were constructed. These developments blocked the natural drainage and divided
the region into a series of enclosed basins. When the Kosi embankment breached, the waters no longer flowed in one or a few clearly defined
channels, but instead spread out across a width of 30-40 kilometres seeking the path of least resistance and filling the enclosed basins, low-lying lands
and ponds. Low points were scoured and transformed into new main channels for the river, and sand and sediment were deposited across fields and in
irrigation channels, drainage ditches and other structures. In addition, approximately 50,000 people in Nepal and more than three million in India were
displaced and lives were lost.
| FIGURE 3 | Flood inundation map of part of North Bihar state
and part of Nepal Tarai, satellite image of
September 3, 2008
| FIGURE 4 | Changes in the course of the Kosi,
1700 - 2000
Source: Gole and Chitale, 1966
Adapted from: NRSA, Dept. of Space, Govt. of India
(Based on the analysis of Radarsat-2 data of September 3, 2008)