EP 1110-2-11
30 Nov 94
Chapter 2
Problem
2-1. Flooding in the United States
a. Flooding and flood-related events cause greater damage and more fatalities than any other natural disaster.
About 80 percent of all presidential disaster declarations are the result of flooding (Federal Emergency Management
Agency (FEMA) 1992a). Flood damages averaged .3 billion and flood-related fatalities averaged about 100 annually
over the past 10 years (USACE 1993, 1994). The most common type of flood occurs as a result of a major rainfall or
snowmelt. A second type of flood occurs suddenly, as in the case of dam failures or intense rainfall that generates a
flash flood. A third category of flood results from an ice or debris jam. Flood stages during an ice jam (Figure 2-1) can
increase more rapidly and attain higher levels than those associated with open water conditions. Ice jam flooding may
occur outside the regulatory floodplain, often when the river flow would not otherwise cause problems.
b. Many laws and regulations have been developed to reduce national vulnerability to flooding. Most American
communities have floodplain regulations designed to prevent future development in areas subject to conventional open
water flooding. Some communities are protected by structural controls such as dikes, levees, and flood control dams.
Mitigation measures specifically designed to protect against ice jam flooding are used less commonly.
2-2. Ice Jam Flooding
a. In many northern regions ice covers the rivers and lakes annually. The annual freezeup and breakup com-
monly occur without major flooding. However, some communities face serious ice jam threats every year, while others
experience ice-jam-induced flooding at random intervals. The former often have developed emergency plans to deal with
ice jam problems, but the latter are often ill-prepared to cope with a jam event when it occurs.
b. Ice jams take place in 30 states, primarily in the northern tier of the United States (Figure 2-2). Even moun-
tainous regions as far south as New Mexico and Arizona experience river ice. Ice jams affect the major navigable inland
waterways of the United States including the Great Lakes. A study conducted in Maine, New Hampshire, and Vermont
identified over 200 small towns and cities that reported ice jam flooding over a 10-year period (USACE 1980). In March
1992 alone, 62 towns in New Hampshire and Vermont reported ice jam flooding problems after two rainfall events.
Table 2-1 lists some of the major ice jams recently recorded.
c. In a l992 survey, USACE offices reported ice jam problems within 36 states. Of the 36 states, 63 percent
reported that ice jams occur frequently, and 75 percent rated ice jams as being serious to very serious (White 1992).
d. Because ice jam events are less common and more poorly documented than open water events, it is more
difficult to characterize these events than open water flooding. In addition, due to the complex processes involved in the
formation and progression of ice jams and the highly site-specific nature of these jams, these events are more difficult to
predict than open water flooding.
e. The rates of water level rise can vary from feet per minute to feet per hour during ice jam flooding. In some
instances, communities have many hours of lead time between the time an ice jam forms and the start of flooding. In
other cases, the lead time is a little as one hour. For example, in March 1992, an ice jam developed at 7:00 a.m. in
Montpelier, VT. By 8:00 a.m. the downtown area was flooded (Figure 2-3). During the next 11 hours, the business
district was covered with an average of 1.2 to 1.5 m (4 to 5 ft) of water. The event occurred so quickly that there was
not sufficient time to warn residents so they could protect their goods. Even after water levels dropped, damage related
to the flooding continued as cold temperatures caused freezeup of wet objects. Damages of less than one day were esti-
mated at million (FEMA 1992b).
2-1
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