b'Overbank Floods Flash FloodsOverbank ooding of rivers and streams is theFlash oods are a rapid and extreme ow of high most common type of ood event. The oodplain,water into a normally dry area, or a rapid water level an area of land adjacent to a stream or river,rise in a stream or creek above a predetermined ood often oods during periods of high-water ows.level, typically beginning within six hours of the Floodplains range from narrow, conned channelscausative event (e.g., intense rainfall, dam failure, ice to wide, at areas depending on topography.jam). The two major factors in ash ooding are the The volume of water in the oodplain, and theintensity and duration of rainfall and the steepness speed at which it moves through the oodplain,of watershed and stream gradients; other important depends on the size of the contributing watershed,factors include the amount of watershed vegetation, topographic characteristics such as watershedthe availability of natural and articial ood storage shape and slope, and climatic and land-useareas, and the conguration of the stream bed characteristics. For example, in steep, narrowand oodplain. Flash ooding in urban areas is valleys, ooding usually occurs quickly, for aincreasingly dangerous due to the replacement of short duration, and oodwaters are likely to bevegetation with impermeable surfaces that increase rapid and deep. In relatively at oodplains, areasruno and the construction of drainage systems may remain inundated for days or weeks, butthat increase the speed of runo. Flash oods can oodwaters are typically slow moving, relativelyroll cars and boulders, tear down trees, undermine shallow, and may accumulate over long periods ofinfrastructure, and scour new channels.2time. Flood stage, which is established by historicalAlluvial Fan Floodsood measurements, delineates the river height or ow volume at which water begins to overowAlluvial fans are made of sediments that are banks and is used to communicate short termdeposited where a stream or river leaves a dened ood potential resulting from current weatherchannel and enters a broader and atter oodplain, conditions. Flood stage poses a hazard risk to life,dropping sediment as it slows down. As sediments property, and critical infrastructure. Furthermore,are deposited, the ow path becomes unpredictable oods can also be classied by elevation. The Basebecause of random deposition patterns, resulting in a Flood Elevation is the elevation of a ood with afan-shaped deposit of alluvium. Due to unpredictable one percent annual chance of occurring, oftenow patterns in these areas, high water ows can referred to as the 100 year ood, while a ood withcause rapid changes and form new channels. Human a 0.2 percent chance of being equaled or exceededalterations to these environments can further each year is called a 500-year ood (see Figure 6).2 exacerbate ooding and erosion problems.2102 I D A H O I N F R A S T R U C T U R E G U I D E B O O K'