Assemblée générale 2002 - Québec, Province de Québec - Canada, 28 - 30 mai 2002
2002 INBO's General Assembly - Quebec City - Quebec - Canada, May 28-29-30, 2002
Asamblea General de la Red 2002 - Ciudad de Quebec, QUEBEC - CANADA, 28 al 30 de Mayo de 2002

Lessons from the Extreme Floods in South Central Nepal in 1993

Jagat K. Bhusal,

Department of Hydrology and Meteorology, Nepal

Email: jagat@dhm.gov.np

Kamal Prasad Bhattarai,

CEMAT Consultants, Nepal

Email: jagatbhusal@hotmail.com

Abstract The Kingdom of Nepal lies in the southern lap of the great Himalayas. The topographical variation within the kingdom is extremely high as it ranges from about 70 meters to 8850 meters altitude from mean sea level. The area above 5000 meters is covered by snow round the year. Nepal is suffered every year by floods and landslides. In 1993, floods and debris torrents caused to a catastrophic disaster, which appeared to be the worst in the disaster history of Nepal in damaging lives and properties in 20^th century,

Rainfall in the past Rainfall intensities about 40-50 mmh^-1 are common in lower Mahabharat and Siwalik regions of Nepal. The 24-hour rainfall depth of more than 400 mm were occasional events in the country. Some of the recorded events (DHM, 1990) are 431 mm rainfall at Bajura in far-western region in August 12, 1980; 446 mm at Beluwa, in western region in September 29, 1981; 500 mm at Ghumtang in central region in August 25, 1968 and 473 mm at Anarmani in eastern region in October 10, 1959. The maximum rainfall intensity of 88 mm h^-1 was recorded in 1989 at Pokhara in western region. A rain intensity of 45 mm h^-1 was an observed fact (Bhusal et al, 1993) on a steep slopping watershed that had initiated landslides and debris torrent in September 29, 1991.

Floods in the past Catastrophic floods and landslides have been occurring in Nepal since past. There had a similar flood occurred in 1902/03 in Bagmati river. Floods of 1964, 1981 and 1984 in Sunkosi river were some events in eastern region. Doming Xu (1985) had estimated a peak flood of 15,920 m³s^-1 near the breached site of moraine dammed glacial lake and 2316 m³s^-1 at about 50 km downstream in Sunkoshi. Debris flow disaster in Nakhu on September 30, 1981, which took lives of about 72, was an another example of debris flow. The Butwal landslide that occurred in 1978 destroyed completely a highway bridge. Flood in Tinau in 1981 had washed hectares of fertile lands and several lives. More than 30 lives were killed by debris flow disaster of 1951 in Bhemphedi area (ICIMOD, 1993).

LESSONS

The important question that a hydrologist have to answer was that whether the 1993 flood disaster was exceptionally larger or above normal. Unfortunately, there are not sufficient records of floods and landslides. However, it can be assumed that there might had similar floods and landslides in scale of flood volumes and mass wasting. But, if scaled in loss of human lives, livestock, and infrastructure, 1993 disaster is more deadly than the past.

People were unaware of the floods and debris flows. Most of the villagers remained in their house hoping that flood would recede as in earlier years. It was already late by the time they had realized that the flood would washout them. If the situation had had been forecasted (warned), human losses would have been lesser.

Debris flows and landslide disaster on the steep sloping terraces of hilly region are so challenging that even a real time hydro-meteorological forecast system would fail to save lives. Because these events do localize in short ranges with respect to time and distance in Nepal. Hence, unless people are aware themselves and abide residing and building their houses in the risky areas, the mitigation of disaster resulting from landslides, debris flows, and flash floods would be costly and difficult.

The destruction of infrastructures due to July flood has left another lesson that the statistical evaluation of design discharges also needed to be supplemented and justified from watershed environments, river bed changes and debris flows from high intensities of rain preceded by light but long duration of rainfall.

CONCLUSIONS

The rainfall intensities of around 60 to 80 mmh^-1 and 400-550 mm in 24 hours are to be expected during unusual storms elsewhere in the country in lower Mahabharat and in Siwalik region of Nepal.

The estimated peak floods of July 1993, in streams having drainage areas of different sizes (100 to 2700 km²) indicated that the specific flows range from 4 to 15 m³s^-1km^-2. The streams having smaller drainage areas (less than about 100 km².) are prone to severe flooding with debris torrents. Specific discharges have been found relatively high for smaller catchments.

The physiographic feature of a basin in combination with the rainfall duration and intensity is directly related to the scale of flood. Hence, flood assessment under such combination need to be made to avoid losses and damages to lives and properties..

Acknowledgements. The author would like to express sincere gratitude to the Department of Hydrology and Meteorology and Water Induced Disaster Prevention Technical Center.

REFERENCES

Fig. 1 Location map of Nepal(left). Severely affected districts of Nepal in July19-21, 1993 flood (right)

Fig. 2 Observed rainfall hyetograph at Tistung and Nibuwatar and flood hydrograph of Bagmati river at an outlet of 2720 km² drainage area.

Table 1 Rainfall records at five places of Nepal in July 19 – 22, 1993.

[Note : mm. = millimeter; H = hour] [Source : DHM, DOSC and NEA, 1993]