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<br />TECHNICAL MEMORANDUM FOR: COTTONWOOD CREEK PHYSICAL MAP REVISION <br />ARAPAHOE ROAD TO EASTER AVENUE <br /> <br />Site Description <br /> <br />The portion of Cottonwood Creek for which a map revIsIon is requested consists <br />of 0.7 stream mile, from Arapahoe Road to East Easter Avenue, in Arapahoe County, <br />Co 1 orado. Thi s reach i ncl udes sect ions G, H and I of the exi st. i ng Flood <br />Insurance Rate Map (FIRM). The site location is presented in Figure 1. <br /> <br />Channel alterations were made in several locations of this reach of Cottonwood <br />Creek as part of street construction and improvement. projects for East Easter <br />Avenue, South Lima Street, and East Bri arwood Avenue. These improvementsi nc 1 ude <br />construction of a triple box culvert at the East Briarwood Avenue crossing of <br />Cottonwood Creek, and placement of bank stabil izing materials along the east bank <br />of the channel parallel to South Lima Street between East Briarwood Avenue and <br />East Easter Avenue. <br /> <br />Related Reports <br /> <br />The Federal Emergency Management Agency (FEMA) has recently updated the Flood <br />Insurance Study (FIS) and Flood Insurance Rate Maps (F I RM) for th i S rl~ach of <br />Cottonwood Creek. These documents include the FIS and FIRM Arapahoe County, <br />Colorado and Incorporated Areas (April 17, 1989). <br /> <br />tlY.draulic Analysis <br /> <br />Water surface profiles were generated for 10-, 50-, 100- and SOO-year recurrence <br />interval floods using the HEC-2 computer model (f"ile CWCASBLT.HC2). Cross <br />sections from Arapahoe Road through East Easter Avenue were based on post- <br />construct i on survey by Ki rkham, Mi chae 1 and Assoc i ates dated January 4, 1989. <br />Water surface profiles are tied into the current FIRM profiles downstream of the <br />Arapahoe Road box culvert. At both the Arapahoe Road and East Briarwood Avenue <br />crossings, water surface elevations were calculated based on the capacities of <br />the box culverts, using the HY8 culvert program. Manning's 'n' values are the <br />same as those used in a previous analysis done by Kirkham, Michael and Associates <br />duri ng the des i gn phase of the project. Flow rates for all return i nterva 1 <br />storms are the same as those used in the map revi 5i on submittal for the <br />Cottonwood Creek Regional Detention Pond prepared by Muller Engineering Company <br />(June 7, 1988). These peak flow rates are lower than the Current FIRM rates due <br />to attenuation in the detention pond upstream of Easter Avenue. Justification <br />for the lower flow rates is contained in the June 7, 1988 report. Water surface <br />profiles at Easter Avenue are tied into the upstream profiles shown in the June <br />7, 1988 report. <br /> <br />In addition to the floodplain water surface profiles, a laO-year floodway <br />analysis was performed (file CWCFLDWY.HC2). The criteria used in determining <br />the 100-year floodway are as specified in the National F.lood Insurance Program <br />regul at ions, "... the regul atory floodway must be des i gne!d to carry the waters <br />of the base flood, without increasing the water surface elevation of that flood <br />more than one foot at any poi nt. " Fl oodp 1 a in and floodway ca 1 cul at ions are <br />presented in the attached documentation. <br /> <br />The revised one-hundred and five-hundred year floodplains, and 100-year floodway <br />boundaries are shown on the enclosed mapping. Water surface profiles are shown <br />for 10-, 50-, 100-, and SOD-year return period floods. Record drawings showing <br />the construction of drainageway and street improvements are enclosed. <br />