Is Montana at Risk?
Identified Hazards for the State of Montana
Basic Disaster Information:
Flooding:
Floods constitute one of the most destructive natural hazards facing Montana. Flooding occurs when abnormally high streamflow overtops the natural or artificial banks of a water course. There are three principle types of floods which may affect Montana: riverine floods, flash floods, and dam break floods. Riverine floods result from precipitation over large areas and/or from snowmelt. This type of flood occurs in river systems whose tributaries may drain large geographic areas and include many independent river basins. The duration of riverine floods may vary from a few hours to many days. Factors which directly affect the amount of flood runoff include precipitation amount, intensity and distribution, the amount of soil moisture, seasonal variation in vegetation, snow depth and water-resistance of the surface due to urbanization.
The term "flash flood" describes localized floods of great volume and short duration. In contrast to riverine flooding, this type of flood usually results from a torrential rain on a relatively small drainage area. Precipitation of this sort usually occurs in the summer. The sudden break-up of an ice jam or the failure of a dam may also result in flash flooding. Flash floods are a potential threat to life and property in areas characterized by steep terrain, high surface runoff rates, and narrow canyon streams and/or subject to severe thunderstorms.
Flooding may also result from the failure of a dam. Dam break floods are usually associated with intense rainfall or prolonged flood conditions. The greatest threat to people and property is normally in areas immediately below the dam since flood discharges decrease as the flood wave moves downstream. Dam failure may be caused by faulty design, construction and operational inadequacies, or a flood event larger than the design flood. The degree and extent of damage depend on the size of the dam and circumstances of failure. A small dam retaining water in a stock pond may break resulting in little more damage that the loss of the structure itself. In contrast, a dam break could result in the loss of irrigation water for a season causing extreme financial hardship to many farmers. An even larger dam failure might bring about considerable loss of property, destruction of cropland, roads and utilities and even loss of life. More far-reaching consequences can include loss of income, disruption of services and environmental devastation.
All three types of floods described have occurred in Montana's main river basins. Because many Montanans have chosen to locate in flood hazard areas, all three flood types have historically had damaging effects. As long as people continue to develop in these areas, whether knowingly or unknowingly, flood events will continue to take their toll.
Flood mitigation strategies should combine both structural and non-structural approaches to alleviating the hazard. Structural approaches include reservoir storage, channel modification, levees and flood walls, pumping stations and other engineering works designed to control floodwaters. Non-structural approaches include both preventive and corrective actions. Preventive actions involve comprehensive floodplain management techniques that prevent unwise and hazardous development of the floodplain. Corrective actions are directed mitigating flood damages and losses which result from unwise development of flood hazard areas.
In addition to the above mentioned techniques, dam failure flooding may be mitigated through dam-safety legislation, effective emergency warning or preparedness plans, group insurance plans for dam owners and increased public awareness of dam safety problems. The flood hazard in Montana will never be subject to complete control by humans but its damaging effects could be substantially mitigated.
