Living with Dams: Extreme Rainfall Events | 2015
Earthfill dams and many concrete
dams are not typically designed
to withstand the erosive forces of
overtopping flows. As an extreme
rainfall event exceeds the capacity
of a dam’s spillways, water begins
to flow across the top and then
down the downstream slope
of the earthen embankment
dam or cascades down the face
of a concrete dam. As the flow
continues to the downstream
toe, velocities become so great
that erosion begins to cut away
the earthen embankment dam
or erodes the foundation material
of the concrete dam. This erosive
process progressively works its way
in an upstream direction through
the earthen embankment dam or
under the concrete dam and can
lead to a gradual partial failure of
the dam or, more catastrophically,
to a sudden complete breaching
or collapse of the dam with the
release of the entire reservoir to
impact downstream inhabited
Phreatic SurfaceWithin Earth
The Phreatic Surface is the line
between relatively dry soils and
saturated soils in the dam.
Click images above to see animations showing how piping (top) and overtopping
(bottom) cause complete breach and failure of the dam.
The reservoir upstream of an
earthfill dam seeps through
the embankment materials in
a downstream direction on a
The rate of water movement
through the earthfill dam is
dependent on the properties of
the embankment soils and the
compaction effort that was utilized
when the dam was built. A well
compacted dam built with proper
soils is relatively impervious to the
flow of water.
What are the most likely failure modes?
What else can fail a dam?
How much damage would be done?
Could there be loss of life?
Ask Yourself This: