FLOOD11733

• Designation Number: 257
• County: Boulder
• Community: Longmont
• Basin: South Platte
• Title: Proposed Amendments to the FBFMs for Dry Creek No. 1
• Date: 2/1/1985
• Designation Date: 9/4/1986
• Prepared For: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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~ldraulic Analysis

The hydraulic analysis for this amendment subnittal is based upon the
to-pography prepared by Parker and Associates in 1980, in conjunction
with subsequent field verification by TST in the surrnner of 1984. This
to-pographic data is represented on the profile sheets in Figures 3, 4
and 5, and on the "Dry Creek No. 1 Flooded Area" maps (Sheets 1, 2 and
3) included in the back coverlX)Cket of this report.

TI1e water surface profile eva]~ation was performed from a combination
of ham~-calculated backwater canputations through the concrete box
culvert section (lower 3318 IF) and the HEC-II canouter program for
the remai nder of the improved charmel.

TI1e water surface elevations of flocxl.s of the selected recurrence
intervals are initialized \-lith the starting flood elevation at the new
confluence with St. Vrain Creek. These starting elevations are
presented in Table III (from WRC, 1981).

Table III. Starting \"later Surface Elevations at St. Vrajn Creek

Recurrence Interval
(years)

10
50
100
500

vvater Surface
Elevation (feet)

4949.8
4953.7
4956.4
4964.2

The Manning's n-va1ues used for the analysi s are based upon the
guidelines presented in Table IV. These values can be verified by the
acccmoanyi ng photographs included in thi s report.

Table IV. Manning's n-Values for Hydrauli. cAnal ys i s

Charmel Section

rvlann i nq 's n-Value

Concr~te box- culverts
Concrete-lined open charu1el
Grass-lined open charmel
Riprap channel drops
Native grass overbank sections

.013
.016
.027
.035
.035

Floodway determinations for the channel section above Hover Road have
been made using Method 5 of the "Floodway Encroachment Calculations"
descri bed in the September, 1982 versi on of the "HEC-2 \'later Surface
Profiles - User's Manual (U.S. Anny Corns of Engineers, 1982). The
Ci ty of Longrront is request ing that the flood.way determi nat i on for the
reach below Hover Road (to the confluence with the St. Vrain Creek) be
designated as coincident ,vi th the lOa-year floodplai n limits and
elevations. Due to excessive backwater conditions in the vicinity of
Hewer Road, the minimum floodv~y width is constrained to the channel
bcmks set jn the program without an increase in water surface depth
until station 8155. These results are summarized by the "Floodway
Data" in Table V.