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<br />11-1 <br />CHAPTER 11. <br />FLOOD <br /> <br />11.1. GENERAL BACKGROUND <br />A floodplain is the area adjacent to a river, creek or lake <br />that becomes inundated during a flood. Floodplains <br />may be broad, as when a river crosses an extensive flat <br />landscape, or narrow, as when a river is confined in a <br />canyon. <br />When floodwaters recede after a flood event, they leave <br />behind layers of rock and mud. These gradually build <br />up to create a new floor of the floodplain. Floodplains <br />generally contain unconsolidated sediments <br />(accumulations of sand, gravel, loam, silt, and/or clay), <br />often extending below the bed of the stream. These <br />sediments provide a natural filtering system, with water <br />percolating back into the ground and replenishing <br />groundwater. These are often important aquifers, the <br />water drawn from them being filtered compared to the <br />water in the stream. Fertile, flat reclaimed floodplain <br />lands are commonly used for agriculture, commerce and <br />residential development. <br />Connections between a river and its floodplain are most <br />apparent during and after major flood events. These areas form a complex physical and biological system <br />that not only supports a variety of natural resources but also provides natural flood and erosion control. <br />When a river is separated from its floodplain with levees and other flood control facilities, natural, built-in <br />benefits can be lost, altered, or significantly reduced. <br />11.1.1 Measuring Floods and Floodplains <br />The frequency and severity of flooding are measured using a discharge probability, which is a statistical <br />tool used to define the probability that a certain river discharge (flow) level will be equaled or exceeded <br />within a given year. Flood studies use historical records to determine the probability of occurrence for the <br />different discharge levels. The flood frequency equals 100 divided by the discharge probability. For <br />example, the 100-year discharge has a 1-percent chance of being equaled or exceeded in any given year. <br />The “annual flood” is the greatest flood event expected to occur in a typical year. These measurements <br />reflect statistical averages only; it is possible for two or more floods with a 100-year or higher recurrence <br />interval to occur in a short time period. The same flood can have different recurrence intervals at different <br />points on a river. <br />The extent of flooding associated with a 1-percent annual probability of occurrence (the base flood or 100- <br />year flood) is used as the regulatory boundary by many agencies. Also referred to as the special flood hazard <br />area (SFHA), this boundary is a convenient tool for assessing vulnerability and risk in flood-prone <br />communities. Many communities have maps that show the extent and likely depth of flooding for the base <br />•DEFINITIONS <br />•Flood—The inundation of normally dry land <br />resulting from the rising and overflowing of a <br />body of water. <br />•Floodplain—The land area along the sides of <br />a river that becomes inundated with water <br />during a flood. <br />•100-Year Floodplain—The area flooded by a <br />flood that has a 1-percent chance of being <br />equaled or exceeded each year. This is a <br />statistical average only; a 100-year flood can <br />occur more than once in a short period of time. <br />The 1-percent annual chance flood is the <br />standard used by most federal and state <br />agencies. <br />•Return Period—The average number of years <br />between occurrences of a hazard (equal to the <br />inverse of the annual likelihood of occurrence). <br />•Riparian Zone—The area along the banks of <br />a natural watercourse.