FLOOD04455

• County: Statewide
• Community: State of Minnesota
• Basin: Statewide
• Title: Flood-Proofing Administrative Manual for Minnesota
• Date: 1/1/1997
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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D. ADMINISTRATIVE DATA

NEEDS

The following paragraphs provide information on the more important data
needs for the design and construction of flood-proofed structures.

Depth of Flooding. The depth of flooding at a site is the difference
between the water surface elevation and the ground elevation at that location.
Flood depths determine the hydrostatic forces which act on a structure.
These forces are usually resolved into the horizontal loads which tend to
cause lateral displacement or overturning and the vertical loads which tend to
cause downward forces on roofs or uplift pressures on basement floors.
Structures must be designed to resist these forces. Section 1401.2 of the
Flood-Proofing Regulations indicates that the maximum practical depth for
which flood-proofing measures are economically effective is 10 feet of free
water. However, for residential flood-proofing flood depths up to about 2 to
3 feet are a more reasonable upper limit. This is based upon economic consider-
ations and the need to minimize problems of access during times of flooding.

Velocity. The flow of water against and around a structure at moderate
or high velocities results in loads in addition to the forces caused by the
depths of the water. Velocities also provide a good indication of the magnitude
of impact loads from floating objects carried by the flood. Section 1401.3 of
the Flood-Proofing Regulations states that a velocity of 10 feet per second is
considered the upper limit for which flood-proofing measures are economically
effective. However. for residential flood-proofing, with the many problems
associated with flowing water such as scour and impact loads. an upper limit for
velocities of 2 or 3 feet per second is considered more realistic. High
velocities also create problems of access to elevated or otherwise flood-proofed
structures.

Duration. The duration of flooding is an important factor in determining the
saturation of soils and building materials. the amount and rate of seepage, and
other time-dependent effects. In addition, the flood duration has a significant
impact on the amount of repairs that will be required and also the economic loss
that may result if an industrial facility is forced to cease operations for an
extended period of time during the flood. These considerations will help to
determine whether flood-proofing is a viable alternative to other flood damage
reduction measures.

Rate of Rise and Fall. The rates of flood rise and recession affect the
design of intentional flooding systems and devices and individual drainage
systems which may be used. These rates also determine the practicality of using
closures which require human intervention. Further. since sloughing of levees
and river banks may occur when rapid stage reduction is experienced, knowledge of
these rates is of importance regarding slope stability.

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