Physical Experiments on the Development of an Ice Tunnel from an Upstream Water Reservoir through Simulated Glacier Dam
Abstract. The hydraulic and glaciological conditions that control the timing and mode of floods from proglacial-lakes impounded by glacial-ice fronts are not well known. Yet, flooding due to the failure of such lakes is increasing due to climate change, posing increased risk to people and infrastructure downstream. Many floods occur due to tunnels developing within the ice, allowing impounded water to discharge to the glacier margin rapidly. However, the manner in which ice tunnels develop through time is understood poorly; current understanding being conditioned by referral to physical theory of ice melt in the presence of flowing water and limited field observations. In this study, the basic principles of the development of a simple linear ice tunnel are explored in a laboratory flume. A sudden flux of water from an upstream water reservoir passes through an open circular tube formed within an ice block. Growth in the shape of the tube simulates the development of an ice tunnel within a glacial dam. The velocity within the entrance to the tunnel and the discharge were recorded, as the head within the reservoir reduces. The data are used to determine the temporal development of the energy slope, and the roughness, size and shape of the tunnel for different water temperatures and flood durations. An increase in the water temperature was a significant positive control on the rate of rise of hydrographs. The behaviour of the energy slope can be defined using the pipeflow equation proposed by Barr in 1981, with the Nikuradse equivalent roughness value increasing from c. 10-9 to 10-4 m as the hydrograph progresses. For any given temperature and for skin roughness conditions, the surcharged wetted tunnel cross-sectional area increases as a logarithmic function of time whilst velocity also increased on the rising hydrograph. As frictional melt induces form roughness, velocity declines and the surcharged tunnel cross-sectional area increases to accommodate the discharge. Once a free surface occurs within the tunnel on the falling hydrograph limb, the decline in the open-channel wetted area with time is linear. The initial circular tunnel section enlarges as an ovoid as the surcharged discharge increases. Once a free surface develops, downcutting is pronounced, leading to a final key-hole shape to the tunnel. Incorporating a time-varying Manning roughness coefficient, the simplified Nye model for discharge through an ice tunnel reproduced well the observed surcharged-tunnel discharge data.
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