FLOOD07745

• County: Larimer
• Community: Fort Collins
• Stream Name: Spring Creek
• Basin: South Platte
• Title: Master Drainageway Planning Study
• Date: 3/1/1988
• Prepared For: Fort Collins
• Prepared By: Engineering Professionals
• Floodplain - Doc Type: Floodplain Report/Masterplan

Results

The dam breach outflow hydrograph for: eilch event was input as
an inflow hydrograph into the modified SWMM files to determine tha
peak flows throughout the reach below the BNRR e~Il.J"nk)Oent. The
assumption of embankment failure results J.n double-peaked
hydragraphs downstream of the embankment. 'I'he first hydrograph
peak is a result of runoff from the Spring Creek watershed below
the BNRR embankment. The second hydrogrilph peak, representing the
peak flow, is the result of the dam breach outflow. This peak is
essentially constant downstream of College Avunue with discharge
values of 2400 cfs and 3360 cfs for the 100- and 500-year events,
respectively.

4. S Floodway

The floodway is the stream channel ilnd any adjacent floodplain
area characterized by deep water and high flows during flooding
events. Encroachment within the flowway area creates an
unacceptable rise in the floodplain elevation upstream.

In accordan,",,, with city of J'"ort Collins regulations, the
f100dway determined by this study is based on an encroachment
resulting in a maximum O.S-foot floodplain elevation rise. Eq<.lal
conveyance reduction from e(loh sid,-,_ of the chunncl '..as used in
cillculating the Cloodway. Tn many areas the floodwilY boundary is
the same as the lOO-year floodplain boundary. In theSe! areaS only
the floodplain line is shown. The following Table 11 identifies
floodway width and elevation 'It ern"" ",pet-inns, "",it.h the l()()-Yl'ar
floodplain elevation also identified.

"

TABLE U
FLOODWAY DATA
Cross lOa-year Floodway width
Section Floodplain Elevation (feet)
No. Elevation
LO 4897.4 4897.4 201
2.0 4897.8 4897.8 H'
3.0 4898.3 4898.3 200
'.0 4898.8 4898.8 He
5.0 4899.0 4899.0 m
'.0 4899.2 4899.3 m
7.' 4899.9 4899.9 m
'.0 4899.9 4899.9 m
U 4899.9 4899.9 m
U 4902.4 4902.4 '"
U 4902.8 4902,8 '"
U 4903.8 4903.8 170
10.0 ~904. 3 4904.3 ;0
1l.0 4905.5 4905.5 "
11.5 4905."/ 49lJ5.7 "
12.0 490';.1 490".1 n
12.5 4906.5 4906.5 "
12.7 4907.2 4907.2 ""
1>.0 4907.5 4907.5 "
13.5 4907.6 4907.6 "
14.0 4907.8 4907.8 G.',
15.0 1908.4 4908.4 n
16.0 1910.1 4910.1 "
16.5 4916.1 4916.6 ,,,
18.0 1916.? 4916.7 He
19.0 4915.3 4916.8 H2
20.0 4916.1 4916.5 "'
21.0 4916.5 4<)16.8 '"
"