Estate Dam Failure Assessment

A water authority asked what would happen downstream if an existing dam upstream of a residential estate failed — raised after a berm was removed from the creek on the authority’s own pre-development advice, and after the estate changed the flows reaching it.

The dam was assumed full to spillway level and failing at that point, which is the worst case rather than the likely one. Downstream catchment runoff was modelled as coincident with the breach, on the reasoning that a dam is most likely to fail during a storm — so the flood the estate sees is both at once, not either alone.

A request for further information late in an approval is usually asking one thing: show us you thought about the case we're worried about. The useful answer maps it rather than argues about its likelihood.