FLOOD00852

• County: Arapahoe
• Community: Arapahoe County
• Stream Name: Cherry Creek
• Title: Cherry Creek River Run Improvements
• Date: 9/1/1986
• Prepared For: Arapahoe
• Prepared By: Greiner Engineering Sciences, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

100 _YEAR FLOC'O PLAIN

~

F LOODWA Y
FRINGE

I'LOOOWAY'

LOODWAY
FRINGE

_STREAM_
CHANNEl.

FLOOD ELEVATION WHEN
eNCR~~::I=::TWITH1N rOODWAY

.. c

SURCHARGE.

AREA OF FLOOD PLAIN THAT COULD ~
BE USED FOR DEVELOPMENT BY ..
RAISING GROUND ;
LINE AB IS THE FLOOD ELEVAltON BEFORE ENCROACHMENT '
LINE CO IS THE I' LOOO ELEVAltON AFTER ENCROACHMENT
'SURCHARGE IS NOT TO EXCEED 1 0 FOOT IFIA REOUIREMEr-. 1101'1 LESSE fl AMOUNT IF SPECIF IEO BY STATE

f-LOOOHEVA110N
BEFORE ENCROACfiMENl
ON FLOOD PLAIN

FIGURE 1

Flc.odway Schematic

The conditional letter of map revision, "hich was given approval by n:MA on
May l6, 1986, provided information on the proposed improvements throu!91 the
River Run site and all other improvemeats along Cherry Creek from Arapahoe
road to the Arapahoe-Douglas County line. These improvements "ere outlined in
the previously referenced Feasibility Study for the Cherry Creek Basin Drain-
ageway.

The HEC-2 model developed for the feasiblity study showed a new proposed 100-
year floodplain which would occur if all improvements to this reach of Cherry
Creek occurred simultaneously. This new proposed lOO'.year floodplain was
based on improvements which would result in no more than a 0.5 foot rise in
water surface over natural conditions.

Since the improvements to the River Run site are occurring before any other
improvements along the reach, it was necessary to run an independent HEC-2
analysis with the River Run improvements but .-ith natural existing conditions
both upstream and downstream of the sit". This analysis was performed in an
effort to encroach to the proposed flood?lain limits as outlined in the feas-
ibility study, while at the same time maJ.ntainJ.ng no more than a 0.5 foot rise
in water surface elevation throu!91 the s He and on the adjacent property up-
stream and downstream. After a number cf attempts with various cross sect ion
modifications, a HEC-2 model was develor,ed "ith the desIred results and some
necessary compromise. This final model J:eflects the majority of site improve-
ments that were outlined in the feasibility study. Ho"ever, the amount of
encroachment was reduced at the north ilnd south property lines in order to
properly blend in with the natural floodway alld floodplain since the offsite
improvements have not been constructed.

The enclosed floodplain map shows the new lOO-year floodplain for the River
Run site and the 0.5 foot floodway and floodplain upstre,am and downstream of
the site. Because of the improvements, ':he O.~; foot floodway and new lOO-year
floodplain are one in the same throu!91 the site. The results of the HEC-2
analysis can be seen in the following table which shows a comparison between
the existing floodway/floodplain throu!91 the ,;ubject reach and the new flood-
plain that occurs as a result of the Riv"r Run improvem,mts.

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