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Glacier recession and formation of glacial lakes along with outburst flood hazards are increasing in response to changing climate in the Himalayan region. This study describes evolution of Dalung and Padam glaciers and their associated lakes employing multi-temporal datasets of Landsat (MSS, TM, ETM+), Indian Remote Sensing LISS-III, Google EarthTM, ASTER GDEM and Survey of India topographic maps. The Dalung Glacier (DG) and Lake (DL) are present in Suru River catchment whereas the Padam Glacier (PG) and Lake (PL) are present in Tsarap River catchment of Zanskar basin, western Himalaya. From 1962 to 2015; the DG and PG have shrunk at different rates. The DG has shrunk by 2.6 ± 0.56 km2 (19.2 ± 4.1%) at a rate of 0.05 ± 0.01 km2 a−1 whereas, the PG has shrunk by 3.4 ± 0.65 km2 (12.7 ± 2.4%) at a rate of 0.06 ± 0.01 km2 a−1. The frontal recession was recorded as 1.240 ± 0.02 km (15.7 ± 2.0%) at a rate of 20 ± 0.4 m a−1 for DG and 2.690 ± 0.5 m (21.3 ± 2.1%) at a rate of 50.7 ± 0.1 m a−1 for PG. Consequently, the proglacial glacial lakes have formed indicating an expansion of 20 ± 3.4% (DL) and 58 ± 4.5% (PL) during 1990–2015. Using integrated criteria of standard indices for the identification of potentially dangerous glacial lakes (PDGLs), the lakes were found to be vulnerable and have a potential to cause breaching in the coming years. In order to provide inputs for the emergency action plan, one dimensional MIKE 11 Model was used to generate the peak flood hydrographs at the lake and other vulnerable sites downstream. Highest peak discharge of 3361.7 m3 s−1 (DL) and 4210.3 m3 s−1 (PL) were estimated at the lake sites which has reduced to 1520.9 m3 s−1 and 1722.4 m3 s−1 respectively at the outlet sites. The reduced discharge at downstream sites still bears the potential to create dangerous flood hazards.

Summary The areas of glacial lakes in the Himalayas continue to increase alarmingly due to global warming with consequent risk of glacial lake outburst flood (GLOF). The outburst is most common for moraine-dammed lakes as a result of a failure of the dam by overtopping and erosion or piping due to seepage. In the present context, GLOF events, especially for the moraine dam failure process needs to be predicted to assess the drastic and detrimental impacts on the downstream river valley. This study focuses on validating an integrated model for simulating the failure of moraine dam by overtopping and erosion using a coupled numerical simulation model. The proposed numerical methodology is validated with the data from experiments in laboratory scale physical models. The results of the model are compared for both the erosional profiles during collapse of the dam and the resulting outflowing flood hydrograph. The primary finding is that the moraine dam height and the volume of the lake upstream of the dam are the most sensitive parameters influencing the GLOF peak. The model is also applied to the field problem of the Tangjiashan lake outburst and dam failure and shows reasonably good agreement with observations. The validated model is also run for the South Lhonak glacial lake in the Sikkim Himalayas, India to obtain the probable flood hydrograph in case of failure of the moraine dam.

Glacier lake outburst floods (GLOFs), especially those in the Arctic, can deliver exceptionally high volumes of sediment and solutes to fjords and shallow-marine settings, in comparison to typical seasonal river flows. These sediments and solutes strongly affect coastal geomorphology and aquatic ecosystems, yet are rarely observed. In this study, we have quantified the short-term geomorphological response of the most distal part of the Zackenberg River (northeast Greenland), where it enters Young Sund, to a glacier lake outburst flood that occurred on August 6th 2017. The main aims were to: (1) quantify riverbank and floodplain geomorphology changes that occurred as a consequence of the flood; (2) analyse the spatial patterns of those geomorphological changes and suggest the key controls on them. We used a time-series of very high-resolution UAV-generated images taken on the 5th, 6th, and 8th of August 2017, which enabled us to compare pre- and post-flood fluvial geomorphology. The GLOF induced intense and widespread geomorphological changes, which was surprising because several floods of a similar magnitude have occurred along this river. Approximately 30% of the area of interest experienced changes that were larger than the minimum level of detection (0.15 m). Lateral erosion reached almost 10 m in some places. The total volume loss from bank erosion was at least 26,561 m3 (+/− 14%), whereas the deposition was at least 7745 m3 (+/− 39%). Such an intensive geomorphological response resulted from a combination of factors; namely: (1) bank geometry; (2) composition of bank material; (3) time of occurrence of the event; (4) presence of permafrost; (6) channel geometry; and (7) multitude and diversity of geomorphological processes. We speculate the severity of the geomorphological impact relative to that from previous floods could have been due to warming air temperatures that provided sediment from thawed permafrost, and to an aggrading delta that raised the river base level. Overall, we contend that climate warming will not only make outburst floods from glaciers more likely but that those floods will achieve more geomorphological work with mechanical erosion of permafrost. Erosion and gravitational failures during future flood events will perhaps become even more widespread and intense.

The hydrologic characteristics of the 1943 outburst flood from the Glacier du Mont Mine, Switzerland, are herein reconstructed using field evidence (palaeostage indicators) in conjunction with shallow water modelling techniques. These techniques rely on accurately characterising the hydraulic roughness of the channel, the water height established as boundary conditions, and the main flow path during the former flood, but the selection of appropriate parameter values can be problematic and hence there is uncertainty in the estimated discharge. In this study, minimal flow discharge estimates derived from one-dimensional modelling were found to vary between 429 and 557 m3 s−1 as the hydraulic roughness (ks) and water height at the inlet were varied over a realistic range of values (0.8–1.4 m and 3.31–6 m, respectively), whereas flow rates derived via two-dimensional modelling were confined in a narrower, lower, range of 358–454 m3 s−1. This degree of sensitivity to bed roughness ks, boundary conditions and the spatial dimensions of the modelling approach is, for the one-dimensional modelling, higher than reported in previous studies, but the precision of flow discharge values reconstructed using the two-dimensional modelling approach appears to be acceptable, even for floods in the very steep valley (0.1 m/m) that is subject of this study.

Summary Dam break outburst floods are sudden, short-lived and high-magnitude discharges of water and sediment propagating over an initially ‘dry’ terrain. Both field observations by geoscientists and experimental measurements by engineers have produced understanding of outburst floods but neither approach has satisfactorily supplied data that is directly of use to hazard managers. This study draws together those two literature groups and provides new experimental data to test hypotheses concerning the effect of a series of external controls on resultant longitudinal flow propagation and transitory hydraulics. These external controls form experimental treatments and are; (i) the height of the lock gate raise as a surrogate for dam breach evolution and initial hydrograph, (ii) bed roughness, and (iii) suspended sediment concentration. These control both horizontal and vertical fluid motion, the latter of which is a key property for controlling aeration, bed shear stress and turbulence intensity. The experimental treatments show that outburst floods on horizontal beds can be identified to possess three longitudinal flow regimes. Firstly, there is a short acceleration due to the reservoir pressure level; i.e. the depth of impounded water. Secondly, channel flow quickly converges to an inertial regime. The third flow regime is viscous and dominated by channel bed friction. Flow depth increases in the inertial regime, then decreases in the viscous regime. Experiments herein show that the timing of the peaks at-a-point down channel of each hydraulic quantity varies; most notably peak bed shear stress precedes flow velocity, which precedes peak flow depth. A point of inflexion occurs with distance down channel that is a manifestation of the transition from accelerating to decelerating flows. These external controls, and the effects of them on longitudinal and vertical motion are analysed statistically and quantitative relationships are derived to underpin the resultant conceptual model. This conceptual model should serve as a platform for future probabilistic and process-based modelling of outburst floods.

Understanding of complex flood-riverbed interaction processes in large-scale (field) outburst floods remains incomplete, not least due to a lack of well-constrained field data on hydraulics and sediment transport, but also because consensus on an appropriate model framework has yet to be agreed. This study presents a novel full 2D hydro-morphodynamic model containing both bedload and suspended load capability. Firstly, the model design is presented with an emphasis on its design to simulate rapidly-varied sediment-laden outburst floods and also the associated geomorphological impacts. Secondly, the model is applied to a large-scale (field) glacier outburst flood or ‘jokulhlaup’ at Solheimajokull, Iceland. For this real-world event, model scenarios with only water and with inclusion of sediment with different parameter setups were performed. Results indicated that grain size specifications affected resultant geomorphological changes, but that the sensitivity of the simulated riverbed changes to the empirical bedload transport formulae were insignificant. Notably, a positive feedback occurred whereby the jokulhlaup led to significant net erosion of the riverbed, producing an increase in flow conveyance capacity of the river channel. Furthermore, bulking effects of sediment entrainment raised the peak discharge progressively downstream, as well as the flood volume. Effects of geomorphological changes on flood water level and flow velocity were significant. Overall, despite the increased computational effort required with inclusion of sediment transport processes, this study shows that river morphological changes cannot be ignored for events with significant in-channel erosion and deposition, such as during outburst floods.

Summary Hydraulic behavior and unsteady sediment transport within dam break outburst floods are inherently inter-dependent, yet poorly constrained. This study experimentally determined how the mobility and clast size of channel bed sediments affected the peak flow depth and flow velocities, wave propagation and attenuation, sediment entrainment and duration of transport, flood power, energy transfer and geomorphological impact of outburst floods. Due to our choice of clast size and of a unimodal size distribution the predominant sediment transport mechanism was bedload. We found that frontal wave propagation was slower and attenuated faster for flows over a mobile bed versus those over a fixed bed. Peak flow depth was greater and was attained earlier for flows over a mobile bed and with increased clast size. Geomorphological power declined logarithmically with time after an initial period where gravitational forces exceeded frictional forces. Both peak and time-to-peak geomorphological power varied inversely with clast size. Peak deposition rates were greatest with coarser bed sediment but time to peak deposition rate was inversely proportional to clast size. Volume changes down channel were described by an exponential curve, which we suggest indicates that sediment transport proceeded in waves via both translation and dispersion mechanisms. Peak suspended load occurred coincident with peak flow velocity, peak bedload occurred coincident with peak flow depth, and peak sediment discharge lagged behind peak water discharge. These descriptions of kinematic transformation, multi-phase flow and of geomorphological work due to outburst floods help to provide data for numerical modelling, for mitigating outburst flood hazards and for informing on the immediate and longer-term geological legacy of outburst floods.

summary Lahars, and other sudden onset floods, are highly dynamic with temporally and spatially evolving sediment loads. Numerical models of sudden onset flood behaviour, particularly those applied to real world events, have tended to neglect sediment transport processes. This is a serious though perhaps understandable shortcoming given a lack of field measurements on flow-bed interactions, the extra computational complexity required, and the problem of iteratively updating ‘plastic’ channel boundaries. This paper applies a fluid dynamics model with suspended sediment and bedload transport to calculate flow conditions and rapid landscape change due to an initially dilute lahar from Mt. Ruapehu, New Zealand. We take advantage of an unprecedented dataset of net topographic changes determined from pre- and post-lahar airborne LiDAR surveys and time-series of hydrological measurements to run and validate the model. A sensitivity analysis of key controls on water surface elevation and final sediment thickness illustrates how the flow was supply-limited in the upper reaches. It was capable of transporting a greater volume and calibre of material than that available. Modelled patterns of net erosion and deposition can replicate those observed (r 2 = 0.79) and are dominantly controlled by channel gradient and topographic