Flood: 100/200/500-year and Localized Flooding
The area adjacent to a channel is the floodplain. Floodplains are illustrated on inundation maps, which show areas of potential flooding and water depths. In its common usage, the floodplain most often refers to that area that is inundated by the 100-year flood, the flood that has a one percent chance in any given year of being equaled or exceeded. The 100-year flood is the national minimum standard to which communities regulate their floodplains through the National Flood Insurance Program (NFIP). The 500-year flood is the flood that has a 0.2 percent chance of being equaled or exceeded in any given year. The potential for flooding can change and increase through various land use changes and changes to land surface, which result in a change to the floodplain. A change in environment can create localized flooding problems inside and outside of natural floodplains by altering or confining natural drainage channels. These changes are most often created by human activity.
In addition to those 100- and 500-year floodplains regulated under the NFIP, recent California legislation resulting from Senate Bill 5 (2007) requires cities and counties within the Sacramento-San-Joaquin Valley to address new flood protection standards of the 1-in 200 year (0.5 percent chance of being equaled or exceeded in any given year) flood when considering new development. These standards are under development and will become effective over the next several years as ongoing technical studies are performed.
Historically, the Sacramento County planning area has always been vulnerable to flooding because of its relatively flat terrain and number of watercourses that traverse the County. Flooding frequently occurred before a flood control system existed. Early residents of downtown Sacramento were forced to build on top of the original town level to avoid floods.
According to the 2008 Flood Insurance Study for the County, flooding can occur in the Sacramento County planning area anytime from October through April. Flooding results from prolonged heavy rainfall and is characterized by high peak flows of moderate duration and by a large volume of runoff. Flooding is more severe when antecedent rainfall has resulted in saturated ground conditions.
Cloudburst storms, sometimes lasting as long as three hours, occur over Sacramento County anytime from late spring to early fall, and they may occur as an extremely severe sequence within a general winter rainstorm. Cloudbursts are high-intensity storms that can produce peak flow equal to or somewhat greater than those of general rainstorms in portions of the study area. Flooding from cloudbursts is characterized by high peak flow, short duration of flood flow, and small volume of runoff.
Dams are manmade structures built for a variety of uses including flood protection, power generation, agriculture, water supply, and recreation. When dams are constructed for flood protection, they are usually engineered to withstand a flood with a computed risk of occurrence. Dam Failure.
Approximately 150 years ago, the levees of the Sacramento-San Joaquin Delta were raised to prevent flooding on what remains some of the most fertile farmland in the nation. While the peat soils were excellent for agriculture, they were not the best choice to create strong foundations for levee barriers meant to contain a constant flow of river water. Nevertheless, it was these native soils that were primarily used to create the levee system. Levee Failure.
Severe Weather Heavy Rain and Storms
Storms in the Sacramento County planning area are generally characterized by heavy rain often accompanied by strong winds and sometimes lightning and hail. Approximately 10 percent of the thunderstorms that occur each year in the United States are classified as severe. A thunderstorm is classified as severe when it contains one or more of the following phenomena: hail that is three-quarters of an inch or greater, winds in excess of 50 knots (57.5 mph), or a tornado. Heavy precipitation in the Sacramento County area falls mainly in the fall, winter, and spring months.