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Living with Dams: Extreme Rainfall Events | 2015

| 13


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

continuous basis


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:

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