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<br />I <br /> <br />I <br /> <br /> <br />5-1 <br /> <br />I' <br /> <br />The floodplain for SJCD (South) downstream of South Platte Canyon Road is shown on the flood <br />hazard drawings to spill a PMF channel that was constructed in 2005 to convey flows around the future <br />South Platte Reservoir. The PMF channel will fully contain the 1 DO-year flows for this reach, A Letter <br />of Map Revision will be needed to reflect the new floodplain <br /> <br />" <br /> <br />,I' <br /> <br />The flood profiles were determined using the U.S, Army Corps of Engineers HEC-RAS program <br />(Reference 10), The 0,5- and 1.0-foot rise f100dways were determined using encroachment method 5 <br />and method 1 of the HEC-RAS program. In those locations where the regulatory floodway would <br />result in only a few feet of allowable encroachment, the f100dway was set equal to the flOOdplain. <br /> <br />I' <br /> <br />The 1 DO-year floodplain, and the 10- and 1 DO-year flood profiles are shown on drawings 1 through 38 <br />Tables 6 and 7 present the floodplain and floodway data for Massey Draw and SJCD (South), <br />respectively. <br /> <br />I' <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />5,3 <br /> <br />The most significant differences are upstream of the road crossings, It appears that the culverts were <br />not modeled in the original FHAD and that the headwater depth upstream of the culverts was not <br />considered. <br /> <br />A comparison of the 1 DO-year water surface elevations from the original FHAD was made and is <br />presented in Table 5. The 1978 water surface elevations were adjusted to the NAVD 1988 datum, <br />Water surface elevations are affected by road crossing improvements, channel improvements, and <br />channel aggradation and degradation that have occurred along the study reaches since the origina <br />FHAD was prepared. Therefore, the comparison was made at select locations where the cross <br />sections appeared to be consistent with the original FHAD <br /> <br />l!,P,~,t,~~9~"C?f"g?r~i~C?~, <br />North Trib, Massey <br />Draw <br />Upstream of <br />!Wadsworth <br />IQp~t~~~,~:~i,g6~~i~l~ <br /> <br />l!P~!~~,9,f!l_c:>f,g~~~i~C?~ <br />SJCD (South) <br />Upstream of S. Platte <br />g?lI'lY<?QBcj~ mm_'.'" _,_ <br />Upstream of Pierce <br />SJCD (South) <br />'Tributary No, 1 <br />Upstream of <br />Wadsworth <br /> <br />** <br /> <br />It appears that the culverts were not modeled in <br />upstream of the culverts was not considered. <br /> <br />740: <br /> <br />the origlna <br /> <br />5534,2 <br /> <br />FHAD and <br /> <br />10970! <br /> <br />that the headwater depth <br /> <br />6: <br /> <br /> <br />1 <br /> <br />1 <br /> <br />250 <br /> <br />. . <br />. , <br />. . <br />, , <br />..-.......-.--..------.-". .".--..-..----..........-----. <br />200.1 2441L._t5_484,Oj_.___~:!1 5487.1; 19129) <br />5531 <br /> <br />1 <br /> <br />3 <br /> <br />1 <br /> <br />491 <br /> <br />2281 <br />1961 <br /> <br />5541 <br /> <br />0.0 <br /> <br />- , <br />I"548:'-~7r--'---'O. 6 <br />:~_'___';__M__ <br /> <br />5426.4! <br /> <br />7.4** <br /> <br />Floodplam and Floodway Data <br /> <br />9191 <br /> <br /> <br />,_......-............- <br /> <br />),----X.~QL_ 7 4~! <br />L--~~9L__~!lj <br /> <br />1090: <br /> <br />2441 <br /> <br />5423.3' <br /> <br />5601 <br />5635 <br /> <br />?l <br />1: <br /> <br />3 <br /> <br />3 <br />3 <br /> <br />1 <br /> <br />1 <br />1 <br /> <br />!........ ....... ..........,..._..........~..... ..... ....... ,.......... ........, <br />L.._?~.94.~L 151791 <br />;__,_~?_~~_?L,_1730~ <br /> <br />, , <br />5426.4: 13907' <br /> <br />,I <br /> <br />I <br /> <br />Massey Draw was studied by approximate methods in the effective Flood Insurance (FIS) Study for <br />Jefferson County (Reference 7), and is shown as an approximate study area on the effective Flood <br />Insurance Rate Map, Profiles for the North Tributary of Massey Draw, SJCD (South), and SJCD <br />(South) North Tributary are included in the current FIS, however f100dway data is not included <br />Tributary NO.1 and No, 2 of SJCD (South) are not included in the FIS. <br /> <br />Massey Draw <br />Upstream of <br />lW~dswQr:!h, " <br />Approx. midway <br />between Wadsworth & <br />Carr <br /> <br />1181 <br /> <br />29381 <br />282~[ <br /> <br />6101 <br /> <br />........., .......... <br />~ 2401 <br />",--....---. <br />; <br />, <br /> <br />23751 <br /> <br />2375: <br /> <br />5518.7' <br /> <br />Water <br />Surface <br />Elev, <br /> <br />1847i <br /> <br />i <br /> <br />5521 <br /> <br />5614,31 <br /> <br />; <br />6' <br /> <br />5536.21 <br /> <br />3 <br /> <br />I Adjusted I <br />Datum Water Section <br />Adjust Surface No. <br />Elev <br /> <br />3 <br /> <br />3 <br /> <br />3 <br /> <br />1 <br /> <br />5521.8, <br />. . <br />. , <br />._....,.._._---_...~.._-.._-- <br />. , <br />, , <br />. . <br />. . <br />, . <br />. . <br />. , <br />, . <br />. . <br />, , <br />, , <br />. . <br />, . <br />, . <br />. ~ <br />; 5539.3/2438,2, <br /> <br />......--....-...............-..-.--........- ..................,.-...... <br />i 5617.41 7194! <br />.................................'.....m.....~ ......................,...........,. <br />. , <br />! ; ; <br />, , , <br />' , <br />. ; <br />, . <br />, , <br /> <br />i 5524, T 10968! <br /> <br />! i <br /> <br />681\ <br />549 <br /> <br />mm........................t,_ <br /> <br />?~Q~:Q! <br />5638,91 <br />................................ <br /> <br />-0,3 <br />0.7 <br /> <br />1 <br /> <br />1 <br /> <br />1 <br /> <br />5531 <br /> <br />5538,1 <br /> <br /> <br />m......................................__...__..........___.. <br />5617.91 <br />..-...-......-...........- .......,~. <br />, <br /> <br />1 <br /> <br />6.4** <br /> <br />.n .._ <br />0,5 <br /> <br />-0.6 <br /> <br />917.24' <br /> <br />3023\ <br /> <br />5528.3 <br /> <br />Water <br />Surface <br />Elev. <br /> <br />6,5** <br /> <br />I <br /> <br />I <br /> <br />5.2 <br /> <br />I' <br /> <br />The floodplain identifies those areas and structures that are expected to be flooded during a 1 DO-year <br />flood event. The f100dway identifies that area of the 1 DO-year floodplain that must be kept free of <br />obstructions in order for the 1 DO-year flood to be carried without significantly increasing the energy <br />grade line, The f100dway is defined by an encroachment limit into the 1 DO-year floodplain where the <br />increase in the energy grade line will not exceed a specified limit. A 1,O-foot rise floodway, which is <br />used by FEMA, and a O,5-foot rise floodway, which is used by the local entities for floodplain <br />regulation, were determined and are presented herein <br /> <br />the Effective Flood Insurance Study <br /> <br />Location <br /> <br />Section <br />No <br /> <br />Flow <br /> <br />1 DO-year <br />Floodplain Data <br /> <br />i 130, , <br /> <br />, , . <br />~------_...._,------' <br />, ' <br />" , <br />, . <br />. ' <br />- ' <br />, ' <br />, . <br />. ' <br />. . <br />, ' <br />, ' <br />. . <br />. ' <br />j 150' <br />i ~ <br /> <br />Flow <br /> <br />1 DO-year <br />Floodplain Data <br /> <br />Water <br />Surface <br />Elev. <br />Diff. <br /> <br />Comparison with <br /> <br />Table 5 - Comparison of 1978 and 2005 FHAD Water <br />1978 FHAD <br /> <br />Surface Elevations <br />2005 FHAD Update <br /> <br />On Massey Draw, upstream of Garrison Street, flows from the floodplain spill into the C470 right-of- <br />way. It appears that this is the result of grading that occurred with development of the parcel on the <br />south bank of Massey Draw, <br /> <br />I' <br /> <br />5 <br /> <br />1 <br /> <br />Genera <br /> <br />Between sections 26214 and 28497 on Massey Draw there are several houses that could be impacted <br />by a rise in water surface and energy grade line elevation. Therefore, along this reach the f100dway <br />was set at the floodplain limits. <br /> <br />I, <br /> <br />I <br /> <br />I <br /> <br />5, Flood Hazard Area Delineation <br /> <br /> <br />City 0/' Littleton <br /> <br /> <br />Flood Hazard Area Delineation <br />Massey Draw and SJCD (South) <br />December 2005 <br />