ABSTRACT
On October 29th 1927 a small moraine-dam on the eastern flank of the Helvellyn range in the English Lake District failed after heavy rainfall. The moraine had impounded a small mine-reservoir at Keppel Cove. The discharge from the breach, totalling some 1.24 × 105 m3, swept down the course of the Glenridding Beck devastating the village of Glenridding 2.7 km to the east.
In excess of 13000 m3 of material was eroded from the dam-breach and some of this debris exacerbated property damage in Glenridding. However most of the moraine debris was deposited as a series of boulder lobes and berms in expanding reaches throughout the stream course.
The event at the time was poorly documented, there being, for example, no estimate of flood velocity or peak discharge. However limited published information coupled with detailed field survey and a number of unpublished contemporary photographs have provided sufficient data to establish maximum water depths at sections throughout the valley. These data form the limiting conditions in an hydraulic reconstruction of the flood-wave dynamics using a published dam-break model.
The propagation of flood waves from breached dams clearly is of interest to the engineer and hydrologist. In addition, the activity of the flood wave is manifest today chiefly in the form and location of the berm structures. The location of these in respect of channel slope, cross-sectional geometry and flood dynamics is discussed. The value of the dam-break to the geomorphologist is that the unfortunate event represents a “controlled experiment” whereby a known volume of water is routed182 through a catchment. As such, it forms a prescribed model of catastrophic natural flooding which in certain circumstances may be related to a local magnitude-frequency model of flood occurrence and geomorphic effectiveness.
