Civil Engineering Reference
In-Depth Information
these may be some of the reasons for the increased severity of Midwestern
floods in recent decades.
As we saw after the Katrina disaster, coastal floods from hurricanes
can cause huge destruction. On water bodies near New Orleans, floodwaters
reached all the way up to the bridge decks. Bridges are designed to resist
water levels reaching part way up the piers. When the water extends up the
full height of the bridge, the structure has to resist not just the full brunt
of the moving water, but also the water's upward buoyant forces, tending
to force the span upward. Then floating logs, boats, shipping containers,
vehicles, and large appliances smash against the bridge. (Not least, the
moving water may scour away the soils at footings and foundations, but we
leave that topic for later.)
As with earthquakes, no single metric sufficiently measures flood
intensity. A fairly good single measure would be wave force and velocity.
AASHTO has only in recent years begun to establish threshold wave force
and velocity criteria for bridges.
An option unavailable for earthquakes may now be feasible for floods:
to lessen the likelihood of a flood occurring in the first place. Through proper
regional planning that manages the placing of impervious surfaces on land,
authorities can regulate water flows over land so as to reduce flood risk.
Where floods cannot be stopped, engineers can opt to strengthen the
bridge: design it with deeper and thicker foundations, strengthen the piers
and pier-to-superstructure connections, build the superstructure at higher
elevation, or shelter bridge piers with rock piles and artificial islands.
Once again, the operative question is how much to invest in counter-
measures. To answer, engineers have to agree on (their professional associa-
tions must set standards for) the flood threshold that the bridge must be
designed to resist. It may be, as earthquakes are, expressed as the largest
event likely to occur in 100 or 500 years.
HYDRAULIC FORCES FROM SCOUR
Scour
refers to the process that washes away sand and rock from the stream-
bed, leaving spaces or gaps that can destabilize the supports on which a
bridge rests. Scour can be dangerous even if it happens gradually over the
years, as normal river flow stirs up and suspends sediment and transports it
downstream, while the process is hidden from view. It is more dangerous
in raging streams, because swift water has more energy by which to lift and
carry the sediment. Scour can also be uneven. Rivers that meander may
erode soils at higher rates on one side of the river than the other, exposing
some piers to more scouring than others. The very existence of a bridge
can add to the problem, because the piers and abutments reduce the area
through which the water flows, increasing water velocity (figure 6.3).
