ETL 1110-2-540
30 Jun 96
(g) The continuous monitoring of the system enables
condition, the typical measurements desired, and the general
the preparedness actions to be implemented in
hardware applicable for various types of streams.
predetermined phases. The initial phase may be that the
officials are alerted of a potential flood threat. The next
c. Hydraulic analysis. Hydraulic analysis is needed to
phase may require that they are assigned to key locations
determine water surface profiles, study reaches, and channel
in the filed as observers and readily available for specific
routing criteria.
EM 1110-2-1416 describes general
emergency actions. Subsequent phases might implement
requirements for flood damage reduction studies. Water
actions of emergency response for the general public and
surface profiles are developed for the historic events of
maintenance of vital services. The greater and more
interest and a range of hypothetical frequency events. The
reliable the warning time, the more effective the
analysis results in a series of rating curves at desired
emergency response actions become.
locations (and water surface profiles) that may be used to
determine discharge for various levels (stages) of flooding at
(h) Dotson and Peters (1990) provide an example of
key locations throughout the watershed. The extent of
how to estimate warning times and select the upstream
flooding is also used to develop flood inundation boundaries
threshold stage to trigger flood warnings. The first step is to
for selected events and information of depths and velocities
obtain stream flow data from historical records or simulated
throughout the study area.
data from a computer model such as HEC-1 for the location
where warnings are required. Stream flow data are also
(1) Flood inundation maps. The extent of flooding or area
needed for selected upstream locations that are under
inundated is part of the information required to define the flood
consideration for use as a trigger or index for downstream
hazard. This is determined by defining the boundaries of
flood warnings. Next, the stage and/or flow associated with
inundation for selected flood levels for the study reach. Flood
the point where flood damage begins at the downstream
inundation maps are important for developing appropriate
location is defined. Flood records are then analyzed, and the
warning dissemination procedures and response actions for
set of occurrences where flood flows are exceeded at the
various levels of flooding. Results of the water surface profile
downstream location are determined. Upstream records are
computations, described above, can be used to determine the
analyzed for threshold values of upstream flow that can be
longitudinal water surface profile along the reach under study.
used to forecast the occurrence of downstream flooding.
The areas inundated are identified on a topographical map using
Different potential upstream threshold values are tested to
the water surface profiles associated with a range of stages as
determine warning time provided and the percentage of
defined by the hydraulic analyses. Once these areas of inundation
accurate flood warnings provided.
are determined, then warning dissemination messages and
methods can be determined, threatened properties identified,
(i) Typically, there is an inverse relationship between
impacted vital services (traffic, power, gas, water, sanitary, etc.)
increased warning times and the percentage of correct
determined, egress routes identified, and locations for specific
forecasts. Selecting a lower upstream threshold generally
flood fighting efforts defined.
increases the warning time, but the percentage of correct
forecasts decreases. Conversely, selection of a higher
(2) Depth-velocity.
Combinations of depths and
threshold increases the percentage of correct forecasts but
velocities of water pose a serious physical rise to people during
decreases the warning time available.
In an example
flood events. Shallow flooding with high velocities often pose an
provided by Dotson and Peters (1990), selection of a
unrecognized but greater threat than deeper slower flowing
5.66 cms (200-cfs) upstream threshold yielded correct
streams. Table 3-2 shows the risk associated with various
forecasts 36 percent of the time and at least 30-min warning
combinations of flood depths and velocities.
times 69 percent of the time. Selection of a threshold value of
11.33 cms (400 cfs) increased the percentage of correct
Flood damages occur due to the depth of water and velocity of
forecasts to more than 80 percent, but the percentage of
the water. The duration may also be a factor. Some materials
forecasts with more than 30 min of warning dropped to about
(e.g., carpeting, food supplies, dry goods inventory, etc.) are
40 percent.
damaged with direct contact with the water. Other damage
(3) Observed storm patterns. The types of storms and
occurs due to hydrostatic pressures caused by high water
flood trigger mechanisms must be identified.
Types of
(e.g., roadways, basement walls, swimming pools, etc.).
storms include thunderstorms, frontal storms, winter storms,
Buoyant forces can cause cars, storage tanks, mobile
summer storms, snow-melting events, and hurricanes. The
homes, and any other unsecured objects to float away.
flood triggering event can be rainfall, snowmelt, ice jams,
Other damages are velocity related. High velocities also
dam failure, large or small rivers and can be involved. The
type of situation involved defines the flood warning -
preparedness required. Table 3-1 lists the type of monitoring
3-3
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