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Flash Floods in Urban Environments

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					Flash Floods in Urban Environments
Flash floods present many difficulties regarding the identification of hazard areas,
preparation for floods in those areas, and forecasting the onset of the flash flood event.
Urban environments can create some of the most complicated flash flood processes due
to the presence of small, very fast-response basins. Alterations to the landscape result in
complex and often more rapid runoff relative to the natural watershed. To distinguish a
flash flood from a main stem river flood, a flash flood can be defined qualitatively as:
a sudden and dangerous increase in water momentum that has the same scales of forcing
as the causative precipitation. Its impact is related to the sudden increase in water
momentum rather than the final depth, velocity, extent, or duration of floodwater.

Thus, flash floods almost always occur when intense precipitation occurs over small, fast-
response basins. Urbanized watersheds present ideal conditions for converting bursts of
precipitation into rapid runoff, often with little or no absorption of water into the ground.
However, the rapid runoff alone is not the only process working to enhance flash flood
dangers in our population centers. Drainages are often poorly defined and natural
streams may be altered to accommodate the development of commercial and residential
centers. Poorly drained, impermeable areas that frequently experience floodwater can
rapidly collect water to dangerous depths during particularly intense rainfall. Excessive
runoff can occur in areas where no visible channel exists. Flood control is often used to
minimize the impacts of flash floods, but can add complexity and a false sense of
security. Flood control structures and practices are based on the statistical likelihood of
historic rainfalls and floods (i.e. the 100- and 500-year floods). The accuracy with
predicting the recurrence of these events is questionable, and flood control can be
inadequate during particularly large events. Culverts, detention reservoirs, and
channelization are all among the flood control tools used to help minimize the occurrence
and severity of flash floods in urban areas. Although these practices do reduce the
occurrence of flash floods, they may not prevent some of the bigger ones, and may
actually displace, in both space and time, where the flood occurs.

The following is a sequence of events that could take place in a gently-sloped urban area
experiencing excessive rainfall through a rapid series of particularly intense bursts.

I. Pre-flood:

1. Along streams, culverts are installed under highways and railroads to accommodate
the estimated 100-year flow. In some cases pedestrian and bicycle underpasses are
designed to accommodate the 500-year flow. Smaller drainages, particularly those that
may be occupied by intermittent streams, may have little or no flood control.
2. Some streams may be re-directed to accommodate development. Channelization may
be performed to increase the efficiency of the stream to move the water volume
downstream. Streams with a recent flood history may get priority for channelization.
3. Onsite water retention reservoirs may be constructed in new developments to
minimize adverse impact those developments may have on the floodplain. In other areas,
detention reservoirs may be constructed to detain excess floodwater during major events.
These detention areas also serve as streamside parks and open space areas.
II. Exceptionally intense rainstorm:

1. Rainfall occurs with unusually great intensity in a series of successive bursts over a
period of less than 4 hours.
2. Initially, severe weather may require emergency response activity to be focused in
non-flood hazard areas.
3. Real-time rainfall reports are difficult to obtain.

III. Historic runoff:

1. Initially the intense rainfall brings small streams and drainages nearly to bankfull. A
few of these urbanized basins are less than 15 square miles in area, and are responding
very rapidly to the rainfall. Normal trouble spots are experiencing the typical nuisance
flooding. Channelized streams are running high and fast, but still well within their banks.
2. As the intense precipitation continues to impact a number of small drainages, small
streams overflow in a few spots, normal flood hazard areas accumulate water to
dangerous depths, and significant amounts of debris begin entering the flow and clogging
culverts and storm sewers. Significant volumes of water begin flowing in areas without
well-defined channels, such as roads and across fields, parking lots and yards. Onsite
retention areas are filling up, and the city’s planned water detention areas are
accumulation water now as well.
3. As the final burst of intense precipitation fall, the excess runoff contributes to severe
flooding along small streams resulting in bank scour and damage to road and properties.
Some of the larger, channelized streams have developed debris blockages at culverts
causing rapid rises in the water levels and flash flooding. Overland flooding and flooding
along roadways is widespread as water moves downstream along the path of least
resistance. Onsite retention facilities overflow and contribute to the volume of water
heading downstream. Detention reservoirs are now filling rapidly, some overflowing into
adjacent properties as the precipitation ends. Numerous areas with severe flash flooding
are not in designated 100-year floodplains. Redirected streams take back their original
courses cause severe damage to properties.

IV. Post-flood:

1. Officials try to piece together when and how the typical nuisance flooding turned
violent. Flood control structures worked as planned in most locations, but some were
inadequate to accommodate the volume of water, and the resulting flash flood impacted
places not considered within the high-risk areas.
2. Some argue that it was a 1000-year rainfall, but the flood control was only designed
for 100- and 500-year events. Thus, it probably will never happen again. Others argue
that the relatively short period of record make the estimates of probable maximum
rainfall very uncertain. Still others point out that rapid and widespread urban
development have seriously changes how the local watershed will respond to what may
have been considered a nuisance storm at one time in the past.

				
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posted:12/1/2011
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