the need to establish a corridor that
would be more resilient to future
flooding. A driving factor leading
to the flood damage was the high
velocities and depths of floodwater
caused by the steep, confined, and
constructed river.
“The velocities and hydraulic shear
stresses were extremely high, and
the destructive forces were severe,”
said John Hunt, a senior hydraulic
engineer and manager at Ayres
Associates. “Whenever you have a
highway running alongside a river in
a steep canyon environment like this,
the highway can be exposed to severe
damage during a flood.”
SOPHISTICATED
MODELING AIDS
IN EFFORTS
Using 2-dimensional hydraulic
modeling, Ayres’ hydraulic engineers
were able to successfully recreate the
conditions of the 2013 flood. Sections
16│ TRENDS
of the highway where the model
predicted a high erosion potential
proved to be consistent with the
sobering reality of what happened in
the flood. With this validation of the
modeling technique, Ayres completed
model simulations of future design
flood conditions for the proposed
highway configuration. The results
of the modeling were used in several
ways: to determine the correct size
of rock riprap for protecting the
highway embankment; to figure
out how deeply the embankment
protection needed to be embedded
into the ground; to assess the impacts
of access bridge design; and to
determine any potential negative
impact of the design on the opposite
bank.
Ayres’ engineers collaboratively
designed armoring and other
treatments to protect the highway
embankment from suffering the
same level of massive destruction
in a future 100-year flood as had
occurred in the 2013 flood. The
armoring, or embankment protection,
came in the form of conventional
loose rock riprap and, where the
required loose riprap size would
be excessively large, an innovative
technique called matrix riprap, which
provides increased hydraulic stability
over the conventional method. Matrix
riprap requires using much smaller-
sized rock than traditional riprap and
cementing the pieces together with
a mortar adhesive mixture, resulting
in a highly interlocked and erosion-
resistant armor layer.
Using 2-dimensional hydraulic
modeling instead of conventional
1-dimensional modeling was crucial
in this project because of its greater
level of accuracy and the wealth
of information it provides, Hunt
explained.
“Two-dimensional modeling
produces more reliable and useful
results,” he said. “You need an
accurate understanding of the velocity,