With spring comes flooding

March is not the wettest month of the year in North Idaho. Five other months: November, December, January, February and May receive more precipitation on average. In the springtime, however, the combination of warmer temperatures and the resulting snowmelt, plus the nearly 2 inches of average rainfall for the month, mean that flooding becomes a potential hazard. This past week we’ve had to deal with minor flooding from the Coeur d’Alene River at Cataldo and the St. Joe River at St. Maries. Fortunately, no homes were damaged by the flooding of the Coeur d’Alene River, which mainly affected fields and agricultural land. The timing couldn’t be better for Flood Safety Awareness Week which runs from March 19 to 23. During the week, you can visit http://www.weather.gov/floodsafety/ to find detailed information on the many ways floods can occur, the hazards associated with floods, and what you can do to save life and property. It might surprise you to know that flooding causes more damage across the United States than any other severe weather-related event, an average of $4.6 billion a year since 1984. Even more alarming, on average floods kill more Americans than lightning, tornadoes and even hurricanes.

Not all flooding happens in the same way. Of course, all require a large amount of water. Some floods can happen in a flash and are hard to foresee, while others happen slowly and can be predicted days in advance. Flash flooding, as its name implies, happens very quickly (in a flash) and sometimes without warning. It is defined as a rapid rise in water along a stream or low lying urban area. It can occur in a matter of several seconds to several hours. Flash floods are usually the result of intense rainfall over a relatively small area, but can also be caused by a failure in a dam or levee or the sudden release of water from an ice or debris jam. In the Midwest, the main contributors to flash flooding are the intense slow-moving thunderstorms that occur during the spring and summer months. Here in the West, the mountainous topography can enhance the danger when steep slopes funnel rainfall into narrow canyons. Across the nation, urban areas can be especially susceptible to flash flooding because of the large areas covered with impervious surfaces such as parking lots, streets and roofs where runoff travels quickly.

River flooding, which can cause widespread damage, occurs when an area receives repeated instances of heavy rainfall or a combination of rainfall and snowmelt. Remnants of tropical storms often bring flooding conditions to the eastern half of the United States. Here across the Inland Northwest, we have to deal with the “Pineapple Express” during the cool season, which brings long stretches of heavy precipitation along with “snow melting” mild temperatures. The good news is, with careful monitoring by meteorologists and hydrologists, river rises can be forecast. Flood warnings can be sent out days in advance to warn of areas where the water will rise above flood stage and by how much.

During severe flooding events, sometimes the terms 100-year flood or 500-year flood are used. Don’t get the false impression that after going through one of these events you will be immune from future flood catastrophes for another century or more. All 100-year flood means is that there is a 1 percent chance of that level of flooding or worse occurring in any given year. It is possible to have two or even more 100-year floods occur within a year or even a week of each other.