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<br />I <br /> <br />12 <br /> <br />frequency analysis in determing elevations. The lOO-year and other <br />frequency floods are determined by a flood frequency analysis, when <br />flood data are available, as described for riverine flooding in the <br />Water Resources Council's Bulletin 15, "A Uniform Tec:hrlique for <br />Determining Flood Flow Frequencies". The procedure has been <br />approved and adopted by the Council for use by all Federal agencies <br />in all riverine planning involving water and relat ed land resources. <br />When flood data are inadequate or unavailable, the regional analysis <br />or hydrome"teorological method of determining flood magnitude for a <br />given frequency is estilnated using flood records from nearby areas. <br />Details of typical methods of regional analysis are given in "Generalization <br />of Streamflow Characteristics", by Thomas and Benson; U. S. <br />Geological Survey, Water Supply Paper 1975; and in "Statistical <br />Methods in Hydrology", by L. R. Beard; U.S. Corps of Engineers. <br />Either the Geological Surveyor the Corps of Engineers can furnish <br />information as to whether regional analyses have already been made <br />in Drder tD prevent unnecessary duplication of effort. The Soil CDnser- <br />vation Service can alsD furnish information on the hydrometeorological <br />method. <br /> <br />Regulatory Principle. In the cDnsideration of the area required tD <br />convey the basic flood utilize the Regulatory Principle that all parts <br />of the riverine flDodplain are nDt alike in conveying floDd flows, and <br />that the effect of SDme rnarginal Dr other encroachments Dn the flDDd- <br />plain may be insignificant. <br /> <br />As used fDr regulatory purposes in the F Ian ApprDach and where <br />floodways have been delineated, the floDdway is that portion Df the <br />floodplain required to pass a regulatDry flDod, which equates to a <br />basic flood, with no significant increase in the profile due tD marginal <br />cDnfinement or impedance of flDw. It is delineated within a framework <br />of asswnptions that include the passage of a large flood, such as the <br />basic flood, with the constraint that asswned land filling from the <br />margins of the floodplain would not increase flood heights at any point <br />on the flDod prDfile by a significant amormt. A significant amormt is <br />generally taken as falling within the range of zero to 1 foot. <br /> <br />In the Case Approach and in the absence of a delineated floodway, <br />which will be the usual case in the near future, the Regulatory <br />Principle should be utilized. <br />