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<br />SECTIOIIFOUR
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<br />In addition, there are several subdivision areas that are located directly adjacent to the estimated 100-
<br />year floodplain extents and, as such, minimal setback and freeboard exists for these structures.
<br />These subdivisions include the Cottonwood subdivision, Salisbury Park, and Stroh Ranch. Increased
<br />damages and/or flooding due to locally raised flood levels associated with debris blockage of bridges
<br />and floodplain areas could arise in these areas.
<br />
<br />Under the 0.2% probability (500-year) flood, significant flooding of both residential and commercial
<br />structures would occur on both side of the Cherry Creek, especially at the Cherry Creek Country
<br />Club, Cottonwood Subdivision, and Stroh Ranch. Major bridges across Cherry Creek, including
<br />Arapahoe Road, Broncos Parkway, Lincoln Avenue, Main Street, Stroh Ranch, and Scott Road -
<br />would also be overtopped during a 0.2% probability flood.
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<br />4.3.2 Obstructions to F1oodflows
<br />Several roadways cross the floodplain of Cherry Creek in the study reach. The physical
<br />characteristics of these roadways created conditions in the study that resulted in an increase in water
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<br />increase of water surface elevation immediately upstream from these roadways generally goes up
<br />from the 10-year to the 500-year event. Under the 1oo-year flow condition, this increase of water
<br />surface elevation immediately upstream from these roadways could reach 6 feet. In addition, the
<br />upstream water surface elevations would increase drastically if the bridge area were substantially
<br />reduced by debris especiaily at those roadways having high embankments. These higher water
<br />swface elevations would flood more area than shown in this report and increase velocities through
<br />the unobstructed portions of the bridges.
<br />
<br />The profiles on Plates 20 through 33 show the extent to which flood stages are increased at the
<br />bridges. For the 1oo-year frequency events or smaller, an appreciable stage increase occurs ouly
<br />upstream from Arapahoe Road and slight stage increases occur upstream from Lincoln Avenue, E-
<br />470, Cottonwood Drive, Main StreetlWest Parker Road, and Stroh Road. For the 5oo-year frequency
<br />event, an appreciable stage increase occurs upstream from all those major bridges as the flood
<br />elevations are higher than the low chords of the bridges.
<br />
<br />For the 1oo-year frequency event, only Arapahoe Road will be overtopped. The maximum
<br />overtopping of the road reaches 5.2 feet at its lowest elevation. The road would also be overtopped
<br />under the 50-year frequency flow condition but not under the lO-year frequency flow condition.
<br />
<br />Obstructions such as trees and buildings do exist on the floodplain in the study reach. The
<br />obstructive effect is, however, relatively insignificant because of the sparsity in number.
<br />
<br />The majority of development that has occurred within the corridor has been generally outside the
<br />estimated 1oo-year floodplain extents. As most of the development within the corridor is relatively
<br />recent, proper floodplain management has significantly reduced the potential for flooding related
<br />damages within the floodplain. The majority of damages that would occur from significant flooding
<br />events occurs within undeveloped agricultural areas, along the pedestrian path (including
<br />footbridges) and damages associated with the overtopping of Arapahoe Road. A number of other
<br />structures are identified to be inside the loo-year floodplain, however, they appear to be minor or
<br />temporarily structures that do not have high content values.
<br />
<br />FUTIlE ROODS
<br />
<br />As minimal freeboard has been allowed for in many of the developed residential areas, any blockages at roadway
<br />crossing could have an increased adverse impact locally. However, damages related to this type of flooding are
<br />difficult to estimate.
<br />
<br />4.3.3 Velocities of Flow
<br />Water velocities during floods depend upon such factors as size and shape of the cross section,conditions of the
<br />stream, and bed slope. All of these factors vary for different steams, different locations along the same stream, and
<br />different time of the year. On Cherry Creek in the study reach, the channel velocity for the 1oo-year flood, in
<br />general, ranges from 3.3 to 19.5 feet per second and averages about 10.4 feet per second The maximum channel
<br />velocity generally occurs at the entrance or outlet of a bridge where flow is contracted The flow velocities in the
<br />overbank areas are generally smaller. The overbank velocity for the loo-year flood ranges from 0.2 to 11 feet per
<br />second and averages 4.8 feet per second.
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<br />4.3.4 Rate of Rise and Duration of Flooding
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<br />range. A flood of rainfall originating on Cherry Creek would peak in approximately 2.5 hours at the upstream
<br />study limits and in approximately 3.5 hours at the downstream limits. The peak discharge at the upstream limits is
<br />due to floodwater from the upper watershed while that in the lower watershed is due mostly to floodwater from
<br />tributaries draining the intervening watershed between the upstream and downstream limits. The flood wave would
<br />have a duration of approximately 9 hours at the upstream limits and approximately 12 hours at the downstream
<br />limits. The average peaking time for the study reach is about 3 hours and the average duration of flooding is about
<br />10 hours.
<br />
<br />4.3.5 Floodway Delineation
<br />Table 5 summarizes the floodway characteristics of the two 1oo-year floodways deteIDlined by the hydraulic HEC-
<br />RAS models, based on encroachment resulting in maximum increase in the energy gradeline of 0.5 foot and 1.0
<br />foot. These floodways are defined for future floodplain management so that future developments in the floodplain
<br />are kept away from these hydraulically efficient waterways.
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<br />4-3
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