<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 />
|