effects of a bedrock channel cascade with intermediary lakes
Glacial lake outburst floods have produced a distinctive and widespread Quaternary record both onshore and offshore via widespread and intense geomorphological impacts, yet these impacts remain poorly understood due to a lack of modern analogues. This study therefore makes a systematic quantification of the evolution of a bedrock-channelled outburst flood. Channel topography was obtained from digitised aerial photographs, a 5 m grid resolution DEM and bathymetric surveys. Flood inundation was measured in the field from dGPS measurements. Flood evolution was analysed with application of a numerical model. Novel flood metrics, including ‘rate-of-rise-to-peak’ and ‘persistence-above-a-value’ of hydraulics were calculated. The key findings were that outburst floods; (i) that enter intermediary lakes are dramatically attenuated in terms of propagation whilst basin filling proceeds to the outlet level, dissipated of energy due to considerable flow recirculation during the rising stage of the flood, and after overtopping of the outlet are moderated in terms of peak discharge; (ii) can have kinematic waves that are introduced to a flood via hydraulic ponding, (iii) may develop a partitioning of flow regimes for instantaneous peak discharge, bed shear stress and Froude number due to time-transgressive changes in channel cross-section; specifically between inner channel and overbank regions. Furthermore, normalised change in cross-sectional area and normalised change in channel slope can be used to suggest the relative magnitude of instantaneous peak discharge. Net erosion along a reach can be related to hydraulic persistence above a marker value and net deposition can be related to a ‘time to peak’ value. Reconstructions of Quaternary outburst floods should consider the importance of including intermediary lakes. Modern hazard mitigation studies could usefully note the potential use of reservoirs as an outburst flood alleviation resource.
Quaternary Science Reviews, 2013, Vol 67, p. 39-58