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THE NEED FOR A FLOOD PLAIN MANAGEMENT PROGRAM AND FLOOD PLAIN ZONING FLOOD PLAIN An excessive amount of rainfall causes stream channels to overload thus inundating the surrounding areas with the excess runoff. The causes for excess runoff are varied in circumstance. An extreme overabundance of precipitation in a short period of time will 'not allow the normal amQunt of flow or infiltration to take place, thus creating a flood c~ndition. Low temperatures and subsequent frozen ground conditions existing during a time of rainfall or snow melt will also cause a great loss of infiltration and, in many instances, add to the flood level. Conversely, the same amount of precipitation under normal, non-freezing ground conditions may not result in flooded conditions. The loss of vegetative cover resulting from development along the flood plain area, development in the flood plain, and other encroachments produces more direct surface runoff and decreases infiltration, thereby causing greater fluc- tuation in stream discharges, including higher and more frequent floods. Flood plain is divided into three basic parts: 1. The channel or that portion of the flood plain which conveys the normal flow. 2. The floodway or that portion of the flood plain which is necessary to convey the intermediate regional or one hundred year flood, having an average - frequency of occurrence of once in one hundred years, at a designated location, although it may occur in any year and possibly even in successive years. 3. The floodway fringe or that portion between the floodway and the standard project flood (most severe storm) where adequate data is available (Figure II). The distinction between a floodway and a floodway fringe is useful in discussing flood plain development (Figure III). FLOOD PROTECTION There are several methods for protection against floods which are conveniently divided into two groups: Structural and corrective or non-structural and preventive. Structural or protective measures include reservoir construction and operation, local protection works such as levees and bulk- heads, and channel alteration. During periods of high run- off, these structural measures tend to withhold flood waters within their limits or increase the hydraulic efficiency of flood waters to get them downstream quickly. These structures tend to protect their designed limits, but have also had the effect of creating new areas of flood-prone land downstream due to faster runoff. An example is flood proofing which is any structural feature or adjustment to a structure designed for the reduction or elimination of flood damage. - 2 -