Predicting Which Natural Dams Might Fail

Lakes held in place by a glacial moraine can potentially cause a devastating flash flood if the dam of rubble suddenly gives way. Hundreds of moraine-dammed lakes exist in remote reaches of BC's mountains. Now scientists have found a way to predict which of these lakes are most likely to drain catastrophically.

Unexpectedly, a lake's topographic setting has little to do with its vulnerability, as evidence from lakes that have already drained when their natural dams failed shows. Features of the glacier that feeds a lake, such as its potential to calve icebergs, are also not relevant.

The biggest factor determining how likely a lake will drain suddenly is the shape of the moraine containing it. High, narrow moraines take less erosion to open up a gap wide enough for quickly releasing a lake. What the moraine is built of is also important. Moraines most likely to fail lack a core of ice and comprise sand and gravel rather than large boulders. Larger lakes also hold more potential for damage, boosting their risk.

Of the 175 lakes assessed in BC's Coast Range south of the Klinaklini River, one-quarter have a moderate to very high likelihood of causing a rush of flooding through sudden failure of the moraine dam. The model uses data from aerial photos to provide a low-cost means of initially assessing the risk that a lake could rapidly let loose.

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