Q: How serious are ice jams?
In many northern regions ice covers the rivers and lakes
annually. The annual freeze up and breakup commonly 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.
Q: How much of the United States is affected
by ice jams?
The Cold Regions Research and Engineering Laboratory Ice
Jam Database (IJDB) contains data for ice events in 43 states. Ice jams
have been reported most frequently in Montana and New York, each with more
than 1400 ice events. Two additional states have reported more than 1000
ice events (Pennsylvania and Minnesota), and 24 states have reported more
than 100 ice events. Even mountainous 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.
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).
Q: How quickly does flooding occur from an ice
jam?
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. 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 freeze up of wet objects. Damages of less than one day were estimated
at $5 million (FEMA 1992b).
Although the actual time period of flooding may be short
compared to open water flood events lasting days to weeks, significant
damage can result. The winter weather conditions often prevalent when ice
jams occur also add to the risks and damages associated with ice jam flooding.
Q: What kind of problems do ice jams cause?
Ice jam flooding is responsible for loss of life, although
the number of fatalities in the United States is considerably less than
non-ice jam flooding. In the last 30 years at least seven people have died
as a result of ice jam flooding. Six of the deaths were attributed to rescue
attempts; the other death occurred from injuries sustained when a basement
wall collapsed due to pressure from flood waters and ice.
Ice jams in the United States cause approximately $125
million in damages annually, including an estimated $50 million in personal
property damage and $25 million in operation and maintenance costs to USACE
navigation, flood control, and channel stabilization structures.
Ice jams suspend or delay commercial navigation causing
adverse economic impacts. Although navigational delays are commonly
short, they may result in shortages of critical supplies, such as coal
and industrial feedstocks and large costs from the operation of idle vessels
(USACE 1981). Ice jams sometimes cause damage to navigation lock gates.
Ice jams also affect hydropower operations, causing suspension
of hydropower generation due to intake blockage, high tailwater, the necessity
to reduce discharge, or damage to intake works. Lost power revenue
due to such shutdowns can be substantial.
The presence of an ice jam can result in scouring and
river bed and bank erosion that may lead to bridge or river bank failure. Ice
jams can damage stream channels and improvements so that overall vulnerability
to flooding is increased. Riprap can be undermined or moved out of place. Ice-jam-elated
damage to river training structures costs millions of dollars each year.
Indirect
costs associated with ice jams include loss of fish and wildlife and their
habitat. Scour and erosion associated with ice jams may destroy habitat,
such as eagle roosting trees, and mobilize toxic materials buried in sediment. Some
scouring may, however, be beneficial to wildlife habitat as well. Shallow,
vegetation-choked wetlands may become open, allowing for fish and waterfowl
spawning and brood habitat