Flood: Flash, Riverine, Urban *

Descriptions:

Flood: A temporary overflow of water onto lands not normally covered by water and that are used or usable by man, producing measurable property damage/destruction or forcing evacuation of people and vital resources.

Flash: Quickly rising small streams after heavy rains, ice jams, or rapid snow melt.

Riverine/Riparian/Ice Jam: Periodic overbank flow of rivers and streams.

Urban: Overflow of storm sewer systems, usually due to poor drainage, following heavy rain or rapid snow melt.

Secondary Effects:

Loss of life; property damage and destruction; damage and disruption of communications, transportation, electric service and community services; crop and livestock damage and loss and interruption of business. Hazards of fire, health and transportation accidents; and contamination of water supplies are likely effects of flooding situations.

Vulnerability:

There are eight major rivers, more than 5,000 streams and brooks, 6,000 ponds and lakes, and 3,500 miles of coastline which are vulnerable to the effects of flooding. While minor to moderate flooding may occur at any time, major flooding has frequently occurred when heavy rains and rapidly melting snow and ice have caused flooding; when ice jams have restricted the natural run-off of water; or where high winds, heavy rain or snow, and higher than normal wind-driven tides have combined to cause heavy coastal damage. Residential and commercial development within the flood plain areas or near the ocean have contributed to the increased losses from flooding situations. Historically, minor to moderate flooding occurs annually and those areas affected are prepared to deal with it. Major flooding, while less frequent, has occurred and major property damage has resulted.

Flash Flooding: Flash flooding in Maine is directly related to riparian flooding. Flash flooding occurs when rivers and streams overflow because of ice jams. In Maine, flash flooding, unrelated to riparian flooding, does not present the imminent and frequent threat to lives and safety that many other geographic areas face.

Riverine/Riparian/Ice Jams: Maine has thousands of miles of rivers and streams, many of which flow steeply from the mountains eastward toward the sea. Some of the rivers, especially in mountainous regions, tend to rise very quickly after heavy rainfall because of the gradient of the beds and the drainage areas. Abundant precipitation, about 40 inches a year, generates a recurring flood threat. The low-pressure system over the seaboard and the tendency of some storms to follow one another rapidly provide heavy, combined moisture. As a result of the geographic and weather conditions, some rivers flood almost every spring. The danger is usually most severe in the early spring if, after heavy precipitation, ice jamming (an accumulation of floating ice fragments that blocks the flow of a river) causes an overflow onto frozen or snow-covered ground. Rainfall runs off frozen ground, and snow pack at first absorbs the moisture but then releases the water rapidly after temperature rise. If the water course is already full, flooding occurs.

The susceptibility of the major river basins to flooding is assessed by:

The extent of the drainage area.
The fall of the river.
The extent of development on the flood plain.

The most vulnerable of Maine’s rivers are the Kennebec and Androscoggin. Storage dams such as Wyman Dam control the upper part of the Kennebec River Basin, which drains about one-fifth of the State. The basin below the dam is largely uncontrolled. The Androscoggin River Basin drains less area than the Kennebec, but has a more rapid fall—1,245 feet from its source, an average slope of almost eight feet per mile. Like the Kennebec, the upper part of the basin is controlled by dams, but floods have been severe at downstream locations where sharp drops attracted industries and town development (e.g., at Rumford, Livermore Falls, and Lisbon Falls). Skowhegan, Waterville, Augusta, Auburn, Lewiston, Hallowell, and other communities have considerable development on the flood plains of the rivers and tributaries.

The Penobscot River Basin drains almost as large an area as the Kennebec and Androscoggin Rivers combined. The system of upstream dams, the relatively gradual fall of the river—averaging only three feet per mile—and the presence of extensive wetlands in the eastern part of the basin has in the past prevented massive floods.

The St. John River Basin drains a vast area in Canada and northern Maine and has a considerable drop in elevation in the upper section. Because of the wide channel and steep banks, the main stem of the St. John River has relatively moderate flooding. Coastal rivers, however, like the Presumpscot, do not drain extensive areas. Some tributaries of the St. John, such as the Aroostook River, are prone to flooding.

The Saco has approximately a quarter of the drainage area of the Kennebec River but no upstream storage dams. The St. Croix has as much drainage area but is controlled by upstream storage dams. The Saco, St. Crook, and St. John rivers do not have the extensive flood plain development of the Kennebec and Androscoggin Rivers.

Riverine/riparian/ice jam flooding is a significant disaster risk in Maine. The flooding caused by ice jams is similar to flash flooding. The formation of a jam results in a rapid rise of water at the point of the jam and upstream. Failure of the jam results in sudden flooding downstream. Huge ice masses moving downstream can shear off trees and destroy buildings and bridges above the level of the flood waters. Massive blocks of ice can cause significant damage to homes, roads, and bridges when an ice jam releases. Rapid increase in discharge from snow melt and rainfall can rapidly break up a thick ice cover and carry it downstream as an ice run. Ice runs can jam in river bends or against the sheet ice covering flatter reaches. The resulting ice jams can block flow so thoroughly that serious flooding may result within an hour of their formation. Failure of the jam can result in sudden downstream flooding because of higher than predicted flood elevations, rapid increase in water levels upstream and downstream, and the physical damage caused by the impacts.

Damages from the ice jam flooding usually exceed those of clear water flooding because of higher than predicted flood elevations, rapid increase in water levels upstream and downstream, and the physical damage caused by ice chunks.

Some of Maine’s rivers have overflowed many times, but recent flooding has caused much more damage because of the extensive development and denser population of the flood plains. For example, in 1896 the flood was more severe but much less destructive hat the flood of 1987. Changes in natural drainage patterns induced flooding in surprising areas that were not prepared for emergencies. A much larger population was living and working in the flooded areas and more people were affected.

Unfortunately, Maine does not require realtors to disclose to potential buyers that structures are in flood hazard areas where flood insurance is required.

Urban Flooding: Because of the sparse population in Maine urban flooding is directly related to riparian or coastal flooding.

As urban development increases, improvements in urban storm water systems are demanded. The combined sanitary and storm water systems that some urban areas installed years ago cause flooding of sanitary sewerage when riparian or coastal floods occur. Separate storm water systems are required.

Mitigation:

The damaging effects of flooding can be minimized when development in flood ways is restricted and when existing structures are flood proofed. Adequate warning of flooding can enable residents and businesses to evacuate and/or institute whatever procedures are necessary to protect themselves and their property. Adequate emergency plans for communities, businesses and individuals located in vulnerable areas can also minimize impact. Multi-government management of watersheds and multi-jurisdictional cooperation can improve controls, using regulations and structural systems (i.e., dikes, levees, dams) to control flooding. The State PLanning Office, Office of Floodplain Management provides assistance to communities, reviews proposed flood plain projects, monitors local administration and enforcement of shore land zoning and administers the National Flood Insurance Program. A copy of current Flood Hazard Maps is maintained by MEMA.

* An occurrence of this hazard could possibly be initiated by a deliberate act.

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MAINE PREPARES

Flood: Flash, Riverine, Urban *

Descriptions:

Secondary Effects:

Vulnerability:

Mitigation:

MAINE HAZARDS