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<br />8 <br /> <br />tributaries of the main stream, probably 1000 acres or more in size, <br />The reaches should be selected according to change in land use or <br />jurisdiction, For example, a shopping center in the city and an <br />adjacent residential area in the county should not be included in <br />the same reach, The number of decision units should be kept as small <br />as possible without losing valuable decision information, <br />2, Development of Damage-Frequency Information. If the damage- <br />frequency information is gathered at this point, it can greatly aid <br />the formulation and screening of alternatives later. The basic <br />problem is to determine for each decision unit the variation in <br />potential flood damage with frequency of flooding under existing <br />floodplain conditions, If expected future tributary basin hydrology <br />is considerably different from eXisting conditions, it should be <br />used, It is a policy decision whether existing or future f1ood- <br />frequency relationships will be used, <br />The hydrological and hydraulic analyses will yield stage- <br />frequency curves for each channel reach or decision unit, These must <br />be converted to damage-frequency curves through use of depth-damage <br />curves as explained in detail in Appendix A, The information needed <br />is summarized on Figure 11-1, <br />3, Formulation of Alternatives. Experienced UDFC engineers <br />should formulate the most promising alternatives for each Decision <br />Unit. They should include structural and nonstructura1 strategies <br />in their array of alternatives to be considered, Normally, combina- <br />tions of the following basic conceptual approaches should be considered: <br />