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<br />
<br />SECTIONTWO
<br />
<br />2.1 SOURCES OF FLOOD DATA AND RECORDS
<br />
<br />Flow in Cherry Creek has been monitored by the USGS since 1939 at a gauging station near
<br />Franktown (USGS gage number 06712000), in Castlewood Canyon, and more recently, at a gauging
<br />station at Parker (USGS gage number 06712300). Historic gauged flow of Cherry Creek near
<br />Melvin (USGS gage number 06712500) averaged approximately 5,900 acre-feet annually for the
<br />period 1969-1993. Cherry Creek at the Parker gage averaged approximately 5,300 acre-feet
<br />annually. Flows near Franktown are approximately 6,900 acre-feet per year, while the average
<br />annual inflow into the Reservoir averages over 7,000 acre-feet. '
<br />
<br />More recently, since 1994, flow and water quality have been monitored at eight surface water
<br />monitoring stations and nine groundwater stations located from Castlewood Canyon to Cherry Creek
<br />Reservoir as part of the baseline water quality study being conducted for the Cherry Creek Basin
<br />Water Quality Authority (CCBWQA) (Halepaska and Associates, 2000).
<br />
<br />The stream gauging records from the USGS gages at Franktown and Melvin on Cherry Creek were
<br />used to perform a hydrologic analysis (statistical analysis) to determine the peak flow discharges
<br />along the creek in the previous Floodplain Information study performed by the U.S. Army Corps of
<br />Engineer (USACE, 1976) for Urban Drainage and Flood Control District. The results from the
<br />USACE study were subsequently used by FEMA to establish floodplain information for the Cherry
<br />Creek Conidor (FEMA, 1996). The peak flow discharges from that particular study were again used
<br />in this study to determine the flood elevations and flood characteristics of the design frequency
<br />floods. These peak flow discharges were verified by URS Corporation in an accompanying
<br />hydrologic study that included a statistical analysis and a hydrologic model of the watershed (URS,
<br />2002).
<br />
<br />The gage locations and pertinent data pertaining to them are shown in Table 2.
<br />
<br />Table 2
<br />Stream Gage Data
<br />
<br />
<br />Cherry Creek
<br />
<br />06712000
<br />
<br />2.5 miles south of
<br />Franktown
<br />
<br />169
<br />
<br />Continuous since 1939 except for
<br />periods between 1942 and 1943
<br />and between 1953 and 1954.
<br />
<br />Continuous since 1991.
<br />
<br />Continuous from 1933 to 1984
<br />except for periods between 1933
<br />and 1940 and between 1960 and
<br />1961.
<br />
<br />Cherry Creek
<br />Cherry Creek
<br />
<br />06712300
<br />06712500
<br />
<br />287
<br />360
<br />
<br />Parker
<br />
<br />6 miles northwest
<br />of Parker
<br />
<br />Information on past floods was obtained from U.S. Army Corps of Engineers flood records.
<br />Photographs used in the report are from the URS project team.
<br />
<br />flOOD CHARACTERISTICS
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<br />Urban Drainage and Flood Control District provided aerial photography taken during a January 2002 survey and the
<br />photos provided an overall view of the creek system from Cherry Creek Reservoir to Scott Road. Topographic data
<br />was produced from photogrammatric flights made in April 2002 by Aspen Survey, Inc. This survey provided
<br />topographic mapping having a contour interval of 2 feet and a scale of 1" = 400 feet. The data was used to collect
<br />channel geometry data for geomorphic analysis and hydraulic modeling.
<br />
<br />In addition, U.S. Geological Survey 7.5-minute quadrangle maps with 10- and 20-foot contour intervals at a scale
<br />of I-inch equals 2,000 feet were also used to verify conditions and for FHAD preparation where detailed survey is
<br />not available.
<br />
<br />Bridge and roadway data were also provided by TransVision, Inc. during a July 2002 survey.
<br />
<br />Several site visits to the conidor were conducted throughout the evaluation stages of this study. The site visits were
<br />conducted to visually inspect areas of flow obstruction and other important floodplain features within the conidor
<br />and to visually verify the results from the computer model.
<br />
<br />2.2 FLOOD SEASON AND FLOOD CHARACTERISTICS
<br />
<br />The Cherry Creek watershed is located within an area that is prone to receiving very intense rainfall sometimes of
<br />high magnitude. Floods generally occur in the period from May through August. Annual peak discharges in the
<br />study reach, however, have been recorded from February through September.
<br />
<br />The upland areas in the Cherry Creek watershed contain dissected topographic relief with steep stream slopes.
<br />Floods are characterized by rapid rises, high maximum discharges, short durations, and relatively low volumes of
<br />total runoff. Major flood crests generally pass from the upstream to the downstream areas within a period of
<br />between 6 to 8 hours. Flood peaks from thunderstorms covering small areas are generally rapidly attenuated by
<br />valley storage.
<br />
<br />Historically, the principal cause of flooding within the Cherry Creek watershed has been intense thunderstonns
<br />during the late spring and summer months. The earliest record flood occurred in May 1864. Other floods occurred
<br />in May 1876, May 1878, July 1885, July 1912, July 1927, August 1933,July 1946, June 1965, and May 1973. The
<br />peak flow during the June 16, 1965 flood was estimated to be 39,900 cfs at Arapahoe Road by the U.S. Geological
<br />Survey (USGS) and was the largest recorded flood this century.
<br />
<br />2.3 FACTORS AFFECTING FLOODING AND ITS IMPACT
<br />
<br />2.3.1 Obstructions to Floodflows
<br />There are seven roadways that cross the Cherry Creek floodplain within the study conidor: Arapahoe Road,
<br />Cottonwood Drive, E-470 Tollway, Lincoln Avenue, Main StreetlWest Parker Road, Stroh Road, Scott Road, and
<br />the soon to be built Broncos Parkway near the confluence with Happy Canyon Creek. In addition, there are nine (9)
<br />minor pedestrian bridges within the conidor. Arapahoe Road presents the largest potential for obstructing
<br />floodflows because of its small bridge opening, low chord and high road embankment. The minor pedestrian
<br />bridges pose negligible impact to the floodplain and thus were not investigated in detail as part of this study.
<br />
<br />Manmade objects, such as buildings, cars, and fences, as well as trees, brushes, and other vegetation located on the
<br />floodplain can also cause obstruction to floodflows. These items, while remaining in place, not only tend to create
<br />higher stages on the floodplain because of reduced flow area and flow blockage but may also create higher
<br />localized velocities as the flood waters flow around them. If the floodflows are of sufficient magnitude to dislodge
<br />these flow obstructions, flood stages may lower in the localized area. The debris may then lodge at locations
<br />
<br />2-1
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