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<br />. <br /> <br />Memorandum/Issues/Rules <br />September 22. 1987 <br /> <br />d. station statistics used in a Log Pearson III frequency <br />analysis <br /> <br />e. adjusting the stream gage records for zero flow years <br /> <br />f. how to handle impact on hydrology from diversions by <br />irrigation structures <br /> <br />Under Hydraulic Analysis, proposed Rule 5.C.l. Water <br />Surface Profiles. requires development of multiple <br />discharge profiles. Since most hydraulic analyses utilize <br />a computer. this should not be a problem since it can be <br />done by simply changing a few variables in the input data <br />file. <br /> <br />A review of floodplain information reports designated and <br />approved by the Board over the last five years (94 studies) <br />indicated multiple water surface profiles were developed <br />and published in 86 percent of the previously designated <br />studies (see table). <br /> <br />. <br /> <br />Proposed Rule 5.C.2. Floodplain Boundaries. requires both <br />the 100- and 500-year floodplain boundaries shown on the <br />final map. The UD&FCD and the City of Westminster have <br />opposed showing two floodplains on the same map. Basically <br />their arguments are that 1) this will increase topographic <br />base mapping costs in order to show a wider floodplain. and <br />2) the pUblic will become confused. <br /> <br />The staff feels the 500-year floodplain is important <br />towards "the prevention of flood damages" and should be <br />shown. in addition to the lOO-year floodplain. on flooded <br />area maps for the fOllowing reasons: <br /> <br />1. Showing the 500-year floodplain will demonstrate to the <br />pUblic that floods larger than the lOO-year regulatory <br />flood can and do occur. This may be an important <br />factor in avoiding liability to a community if a larger <br />flood does in fact occur. <br /> <br />2. Drawing the 500-year floodplain will serve as a check <br />on the interpretation of the lOO-year floodplain <br />boundary. <br /> <br />3. Showing floods of different magnitude will illustrate <br />the sensitivity of the lOO-year flood boundaries. <br /> <br />. <br /> <br />4. The 500-year floodplain is important in siting critical <br />facilities such as water and sewer treatment plants. <br />solid waste disposal pits. and power plants. <br /> <br />-6- <br />