Environmental Engineering Reference
In-Depth Information
Distributary flow systems, which are common on alluvial fans or in shallow ephemeral
channels, offer a third type of flood hazard.* This hazard is severe because channel avul-
sion, or abrupt shifting of channel location,
†
on relatively flat, unchannelized surfaces of
alluvial fans frequently occurs during floods.11,12
11,12
Avulsions combine the damaging effects
of overbank flooding and lateral channel change because channels shift abruptly into
positions either previously unoccupied or abandoned, and they have the added damaging
effect of sediment deposition.
13
Online resources are available to determine hazard mitiga-
tion for this type of hazard.
‡
Flood frequency, which is the regulatory standard for the National Flood Insurance
Program, can be estimated in two ways. The most common method uses the annual flood
series, which is a subset of gaging records.
§
The annual flood series represents the largest
instantaneous discharge in the water years of the gaging record (Figure 5.2); the water
100,000
80,000
Winter
Summer
Fall
60,000
40,000
20,000
0
(a)
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
60,000
50,000
Winter
Summer
Fall
40,000
30,000
20,000
10,000
0
(b)
1910
1920
1930
1940
1950 960 970 980 990 000 010
FIGURE 5.2
Annual flood series for two rivers in southern Arizona. (a) The San Pedro River at Charleston, Arizona. (b) The
Santa Cruz River at Tucson, Arizona.
*
For Arizona, some preliminary analyses of distributary flood hazard are in Hjalmarson and Kemna.
9
†
For general definitions of hydrologic or geologic terms, the reader should see Osterkamp.
10
‡
http://www.fema.gov/plan/prevent/loodplain/nipkeywords/alluvial_fan_looding.shtm (accessed June 24,
2010).
§
Most gaging records in the United States created and maintained by the U.S. Geological Survey can be
accessed on-line using http://waterdata.usgs.gov/nwis/sw (accessed June 24, 2009).
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