Flood

Flooding is the most frequent and costly natural hazard in the United States—a hazard that causes more fatalities than any other natural hazard and averages nearly $10 billion in losses per year. Nearly 85 percent of federal disaster declarations result from natural events where flooding was a major factor (Implementing a Federal, 2015, p. 2).

Technically, a flood is a general and temporary condition of partial or complete inundation of normally dry land areas from: (1) the overflow of stream banks; (2) the unusual and rapid accumulation of runoff of surface waters from any source; or (3) mudflows or the sudden collapse of shoreline land. Flooding results when the flow of water is greater than the normal carrying capacity of the stream channel or accumulates faster than surface absorbency allows (Colorado Natural Hazards Mitigation Plan, 2013, p. 3-47). The severity of a flood event is typically determined by a combination of several factors, including but not limited to precipitation and weather patterns, stream and river basin topography and physiography, stormwater conveyance capacities, recent soil moisture conditions, and the degree of vegetative clearing and/or impervious surface coverage.

Floods in Colorado generally result from the accumulation of water from excessive precipitation and/or rapid snowmelt. They can be classified under two categories: general floods, resulting from heavy precipitation or snowmelt in a given watershed over an extended period of time; and flash floods, the product of heavy localized precipitation in a short time period.

General floods are typically long-term events that may last for multiple days, and over widespread areas. The primary type of general flooding in Colorado is associated with lands adjacent to riverine and lake areas, and is a function of excessive precipitation levels and the inability of natural systems to adequately absorb or convey the resulting volume of runoff. Urban/stormwater flooding occurs where development has obstructed the natural flow of water and decreased the ability of natural groundcover to absorb and retain surface water runoff.

More frequent in Colorado is flash flooding, most of which is caused by slow-moving thunderstorms with intense but isolated rainfall. Such events develop rapidly and are intensified by major elevation changes, steep slopes, and base alluvial fans that characterize mountain river canyons. Flash flooding events may also be caused by a sudden failure or release by a dam, levee, retention basin, or other stormwater control facility, or by the obstruction of natural flows by ice jams or other blockages that cause backflow and overtopping. Although flash flooding occurs most often along Colorado’s mountain streams, it is also common in urbanized areas where much of the natural landscape is covered by impervious surfaces.  

The periodic flooding of lands adjacent to rivers, streams, lakes, and other water bodies (land commonly known as “floodplain”) is a natural and inevitable occurrence that can be expected to take place based upon established recurrence intervals. The recurrence interval of a flood is typically defined as the average projected time (in years) between a particular magnitude flood event or annual percent chance of that flood occurring. For example, the “100-year” flood has a one percent chance of occurring in any given year, and the “500-year” flood has a 0.2 percent chance of occurring in any given year—and these two distinct magnitudes are the basis for the special flood hazard areas identified in FEMA Flood Insurance Rate Maps (FIRMs). The recurrence interval is an average; it does not necessarily mean that a flood of such a magnitude will happen exactly every 100 years or 500 years, and in some cases only a few years may pass between major flood events.  

It is important to note that flooding is not always confined to special flood hazard areas identified by FEMA. Therefore, evenhomes that are not in the mapped floodplain should exercise caution and diligence during flood events and should prepare themselves before flooding occurs. Some jurisdictions regulate based on their own, more stringent floodplain maps. Fluvial Hazard Zone Maps are being developed in some communities to capture additional flood risk. See Fluvial Hazards for more information on identifying and planning for these flood-related hazards.