The use of one- and two-dimensional hydraulic modelling to reconstruct a glacial outburst flood in a steep Alpine valley

Abstract

Summary The hydrologic characteristics of the 1943 outburst flood from the Glacier du Mont Miné, 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 m 3 s À1 as the hydraulic roughness ðk s Þ 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 twodimensional modelling were confined in a narrower, lower, range of 358-454 m 3 s À1 . This degree of sensitivity to bed roughness k s , 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 twodimensional modelling approach appears to be acceptable, even for floods in the very steep valley (0.1 m/m) that is subject of this study. ª

Paleohydraulic Reconstruction of Modern Large Floods at Subcritical Speed in a Confined Valley: Proof of Concept

“…It is important to find key control reaches with stable cross-sections and well-preserved high-water indicators. Subsequently, a channel with exposed slate substrate and bedrock gorge was selected in this study, allowing the use of two-dimensional shallow water equations over fixed bed in the hydraulic reconstruction [3,8].…”

“…Indirect estimation techniques of peak water discharges such as slope-conveyance, slope-area method, one-and two-dimensional hydraulic modelling are frequently used in paleoflood hydrology [1], reconstructions of glacial-lake outburst floods [2][3][4], mega-floods [5][6][7][8] and extreme floods with stream gauges damaged during the event [9]. Large flood at subcritical speed could develop in a bedrock stream down a low slope, a preferred configuration to apply the paleoflood reconstruction method [10].…”

“…Two-dimensional hydraulic modelling has become a standard in paleohydrology [2,3,7,[23][24][25]] similar to slope-conveyance, slope-area methods and step-backwater analysis [11,26,27]-perhaps due to recent advances in computational power, the availability of Digital Elevation Models (DEM) and open-source 2D codes, e.g., DassFlow-Shallow [28,29]. Guidelines in the development and application of computational methods to environmental fluid mechanics, including quantification of uncertainty and model testing, can be found in [30].…”

“…Key inputs for the schematisation of 2D models that use the Saint-Venant equations are the hydraulic roughness of the surface, the 2D channel geometry, and the boundary conditions (BCs). The first is determined using Manning's parameter [11] or, alternatively, by means of Darcy-Weisbach friction factor [3,31]; the second is based on high-resolution digital terrain models or surveys of channel depth [32]. Lastly, two types of BCs are required when solving numerically the shallow-water equations [33]: physically based (e.g., inlet water discharge and outlet flow depth) and numerical BCs (e.g., characteristic variable extrapolation).…”

Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment: Extreme events in the Himalaya

“…The advantages of using 2-D numerical models for hydrodynamic simulation of sudden-onset floods include their ability to simulate multi-directional and multi-channel flow, superelevation of flow around channel bends, hydraulic jumps, transcritical flow regimes, and flow recirculation (see for example, Bohorquez and Darby, 2008). A number of two-dimensional models include sediment transport functionality, making them ideally suited for quantifying the geomorphological impact of such flood flows.…”