4.1 Status and changes of glacial lake
There were 95 moraine-dammed glacial lakes with a total area of 11.38 km2 in the Yi’ong Zangbo River basin in 2000 and 105 moraine-dammed glacial lakes with a total area of 16.87 km2 in the basin in 2019, with increasing ratio in number and area of 10.52% and 48.24% (Table 4). In 2000, there were three glacial lakes with area larger than 1.00 km2, corresponding to the total area of 4.58 km2. In 2019, the number of glacial lakes of larger than 1.00 km2 remained three, however the total area has increased to 8.38 km2 with the ratio of 82.97%. The number and total area of glacial lakes in area intervals of 0.10 ~ 1.00 km2 and 0.01 ~ 0.10 km2 were both increased. However, the number and total area of glacial lakes smaller than 0.01 km2 both decreased during the past decade, which does not indicate that the growth of glacial lakes has stagnated or regressed, but rather that the smaller lakes have been subsumed into other larger size intervals due to their expansion. As the largest moraine-dammed glacial lake, the Jionglaco’s area has increased from 2.47 km2 in 2000 to 5.69 km2 in 2020 with the average rate of 0.16 km2/a.
There were three historical GLOFs in Yi’ong Zangbo River basin, they are Coga (GLOF date: 20090729) (Yao et al. 2014), Ranzeriaco (GLOF date: 20130705) (Sun et al. 2014), and Jiwuco (GLOF date: 20200626) (Liu et al. 2021). The areas of the above three lakes before and after the outburst were 0.42 km2, 0.58 km2 and 0.58 km2 as well as 0.29 km2, 0.25 km2 and 0.27 km2, respectively. At present, the areas for three glacial lakes are 0.36 km2, 0.28 km2 and 0.27 km2. The area of the Coga increase rapidly (24.14%), reaching 85.71% of the area before the GLOF event. In contrast, the Ranzeriaco has a little increase (12%). Jiwuco has a stable area due to the relatively recent time of the GLOF event.
Table 4
Number and area of moraine-dammed glacial lakes in 2000 and 2019
Year/change ratio | ≥ 1.00 km2 | 0.1 ~ 1.00 km2 | 0.01 ~ 0.10 km2 | < 0.01 km2 | Total |
Number | Area | Number | Area | Number | Area | Number | Area | Number | Area |
2000 | 3 | 4.58 | 19 | 4.45 | 59 | 2.22 | 14 | 0.12 | 95 | 11.38 |
2019 | 3 | 8.38 | 23 | 6.02 | 71 | 2.40 | 8 | 0.07 | 105 | 16.87 |
Change ration (%) | 0 | + 82.97 | + 21.05 | 35.28 | + 20.34 | + 8.11 | -42.86 | -41.67 | + 10.52 | + 48.24 |
4.2 GLOFs simulation
A lot of bridges and villages are distributed in the downstream valley of the Jionglaco. Based on Google Earth imagery and ArcGIS Earth imagery, a total of 15 bridge sites and 20 villages were identified in the study river channel. We only make statistic for GLOFs hydrological processes at the 15 bridge sites, including peak flow, flooding of breach, water level and flood propagation time. Depending on the height of the dam and the width of the outlet of the Jionglaco, five different scenarios with different combinations of breach width (80 m and 120 m), depth (2.5 m and 5 m) and flood peak time (1.5 h and 3 h) were simulated (Table 3). Each scenario produces the different magnitude of flood peak at breach outlet (Fig. 3). The results show that scenario 5 produces the maximum GLOF peak discharge of 1570.61 m3/s at the conditions of flood peak time of 3 h, breach width and depth of 120 m and 5 m. Scenario 2 produces the second maximum GLOF peak discharge of 1327.43 m3/s, it has the same breach depth of 5 m however the flood peak time and breach width are 1.5 h and 80 m, respectively. Scenario 3 produces the minimum GLOF peak discharge of 444.32 m3/s with condition of the flood peak time of 3 h, breach width and depth of 80 m and 2.5 m, respectively.
The GLOFs peak discharge, flood propagation time, water depth and flow hydrograph at the downstream bridge sites were generated for five scenarios (Table 5 and Fig. 4). Considering only the peak discharge caused by GLOFs, it decreases the further away from the lake due to surface frictional resistance and head loss along river path. From breach outlet to bridge site 15, the peak discharge of five scenarios were decreased from 489.00 m3/s, 1327.43 m3/s, 444.32 m3/s, 617.47 m3/s, 1570.61 m3/s to 403.39 m3/s, 1086.5 m3/s, 387.35 m3/s, 524.75 m3/s, 1353.02 m3/s, with the reduction ratios of 17.50%, 18.15%, 12.82%,15.02% and 13.85% (Table 5). However, different bridge sites from upstream to downstream show different change trends due to the dual effects of lateral runoff and flooding. Form bridge site 1 to bridge site 4, the peak discharge of five scenarios were reduced from 473.37 m3/s, 1285.58 m3/s, 433.78 m3/s, 600.72 m3/s and 1532.71 m3/s to 455.17 m3/s, 1227.7 m3/s, 421.72 m3/s, 580.25 m3/s and 1479.28 m3/s respectively (Table 5 and Fig. 4). However, because of a lateral runoff between bridge site 4 and bridge site 5, the peak discharge at the bridge site 5 of five scenarios were respectively increased to 521.35 m3/s, 1272.44 m3/s, 491.72 m3/s, 644.69 m3/s, 1523.35 m3/s. From bridge site 5 to bridge site 9, the peak discharge gradually decreased from upstream to downstream and they were 512.68 m3/s, 1228.08 m3/s, 488.44 m3/s, 632.22 m3/s, 1470.96 m3/s at bridge site 9 for five scenarios (Table 5 and Fig. 4). From bridge site 9 to bridge site 15, the peak discharge flow increases continuously due to the addition of lateral runoff, with peak discharges of 1040.89 m3/s, 1724.00 m3/s, 1024.85 m3/s, 1162.25 m3/s and 1990.25 m3/s for five scenarios, respectively (Table 5 and Fig. 4).
Bridge sites 3, 5, 9, 11, 14 and 15 are the locations with a high density of settlements (Fig. 1(c)). In the scenario with the maximum peak discharge (1570.61 m3/s) at the breach outlet, the peak discharges at above bridge sites are 1505.31 m3/s, 1523.35 m3/s, 1470.96 m3/s, 1874.61 m3/s, 1924.24 m3/s and 1990.52 m3/s, respectively (Table 5). The peak discharge decreases between bridge site 5 and bridge site 9, after which all show an increasing trend. The flood propagation times for above bridge sites are 23 minutes, 1 hour and 8 minutes, 2 hour and 8 minutes, 3 hour and 20 minutes, 4 hour and 11 minutes, 5 hour and 23 minutes, respectively (Table 5). Although the peak discharge in the upstream area is smaller than the downstream area, flood propagation time is short, leaving insufficient time for people’s transfer. However, the longer flood propagation time in downstream areas provides sufficient time for people to move to safe areas if timely warnings are received from upstream areas.
The modelled water depth is prone to uncertainty due to variations in datum between observed water level measured from mean sea level and modelled water level based on WGS-84 datum (Thakur et al. 2016). The DEM data used to extract the cross-sections also affect the accuracy of the water depth, and if the DEM data is of high accuracy, the error will be relatively small. The deepest water depth is at bridge site 15 and the shallowest is at bridge site 3, which is influence both by the shape of cross-section and the depth of baseflow. Deeper and narrower cross-section as well as more discharge can lead to deeper water depths. In this study, the 6.73 m increase of water level at bridge 15 in the extreme scenario (Table 5), whereas at bridge 3 the rise is only 2.55 m.