Laserfiche WebLink
<br />ENGINEERING METHODS <br /> <br />Hydrololrlc Analysis <br />Hydrologic information used for the streams studied by detailed methods in Logan County's PIS <br />were extracted from the first three studies listed under "other studies" above, Hydrology <br />information for the streams with approximate floodplain delineations in Logan County's PIS can be <br />estimated in accordance with the CWCB's Guidelines for Determining lOO-Year Flood Flowsfor <br />Approximate Floodplains in Colorado (Guidelines). The lOO-year flow for each of the streams <br />with approximate floodplains (in all of Logan County) can be estimated by utilizing the regression <br />equation for South Platte River Basin, subregion SPL-l. The equation is QUNI = 707.9(A)o.6S4 , <br />where Ql00 is the lOO-year peak flow in cubic feet per second, and A is the upstream <br />(contributing) drainage area in square miles, The regression curve for subregion SPL-I is presented <br />in the Appendix, The regression equation is only valid for tributary streams having drainage areas <br />between 2 square miles and 1090 square miles. and is not valid for the South Platte mainstem, <br /> <br />Users should be cautioned that the regression equation may significantly overestimate peak flows <br />for drainage basins that are primarily within "sand hills" areas. The regression equations may not <br />give reasonable results for basins having significant diversions, reservoirs, or other features that <br />would affect flood flows. The lOO-year flow for approximate floodplains of streams that have <br />drainage areas between 0,5 and 2 square miles can be estimated using the latest available USGS <br />publication containing regional regression equations for eastern Colorado, USGS publications are <br />available from the USGS publication center in Denver, Colorado, <br /> <br />~ <br /> <br />A detailed study or other hydrologic analysis must be performed for projects involving streams with <br />drainage areas that fall outside the applicable range of the two methodologies discussed above, <br /> <br />The Guidelines methodology is not appropriate for development involving residential <br />subdivisions greater than 50 lots or occurring on sites greater than 5 acres, commercial <br />construction, industrial construction, construction of public facilities, or construction of flood <br />control facilities. The hydrologic methodology developed in the Guidelines is appropriate for <br />designation and approval of approximate floodplain information and for use in simple floodplain <br />analyses for development such as open space, agriculture, farm bridges, pole bams, and individual <br />residential structures, A procedure for determining hydrology information for approximate <br />floodplains of lakes, ponds, local drainages, and overflows from irrigation canals and ditches has <br />not been established, <br /> <br />Hydraulic Analysis <br />The approximate floodplain delineations in the PIS for the South Platte River are based on the <br />information contained in the 1977 Corps study, The floodplain delineations in the PIS for the <br />streams studied by detailed methods are based on the information contained in studies 2) and 3) <br />listed under the "other studies" section above, Hydraulic methods used for the approximate <br />floodplain delineations of other streams studied in the Logan County PIS are not available or have <br />not been documented by FEMA. For future studies, an approximate lOO-year flood elevation at a <br />particular location on a stream of interest may be obtained by using an appropriate lOO-year peak <br /> <br />'"-' <br /> <br />4 <br />