Peak discharge estimates of glacial-lake outburst floods and "normal" climatic floods in the Mount Everest region, Nepal

Glacial-lake outburst floods (GLOFs) in the Mount Everest region of Nepal on 3 September 1977 and 4 August 1985 dramatically modified channels and valleys in the region by eroding, transporting, and depositing large quantities of sediment for tens of kilometers along their flood routes. Prior to this research, the GLOF discharges had not been determined and the hydrology of "normal" climatic floods (SHFFs: seasonal high flow floods) was not known. A one-dimensional step-backwater flow model was utilized, in conjunction with paleostage indicators, to estimate the peak discharges of the GLOFs and SHFFs and to reconstruct the hydrology and hydraulic conditions of the GLOFs at 10 reaches and SHFFs at 18 reaches. The most reliable GLOF and SHFF peak discharge estimates were upstream from constrictions where there was critical-depth control. The peak discharge of the 1977 GLOF at 8.6 km from the breached moraine was approximately 1900 m(3)/s. At 7.1 km downstream from the breached moraine, the 1985 GLOF discharge was estimated at 2350 m(3)/s. At 27 km downstream from the breached moraine, the 1985 GLOF attenuated to an estimated discharge of 1375 m(3)/s. The peak discharges of SHFFs ranged from 7 to 205 m(3)/s and were positively correlated with increasing drainage area. The GLOF discharges were 7 to 60 times greater than the SHFF discharges with the greatest ratios occurring near the breached moraines. The downstream decline in the ratio between the GLOF discharge and SHFF discharge is the result of the downstream attenuation of the GLOF and the increased discharge of the SHFF because of increased contributing drainage area and the increased effects of monsoonal precipitation at lower elevations. (C) 2001 Elsevier Science B.V. All rights reserved.