FLOOD07708

• Designation Number: 317
• County: Boulder
• Community: Unincorporated Boulder County
• Stream Name: Bullhead Gulch and Tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Bullhead Gulch and Tributaries
• Date: 7/1/1991
• Designation Date: 11/1/1991
• Floodplain - Doc Type: Floodplain Report/Masterplan

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At the confluence of Bullhead Gulch and Prince Tributary, the peak lOO-year flow is 5,820 cfs
for future conditions, At this point, the runoff flows north through an existing sand and gravel
quarry, The topography in this area has been greatly altered due to past and current mining
operations. A reclamation plan (RMC, 1982) has been prepared for this quarry, which will
channel the runoff to Boulder Creek after the mining operation has ceased,

Table 7
Summary of Hydrologic Analysis for Future Conditions
Flood Flow, of Varying Reoccurring Inletvals Drainage
Design IO-YR 50-YR 100-YR Area
Stream Location!) Point" (cf,) (cf,) (cf,) (ac)
BULLHEAD GoodhuelHighline
GULCH Ditches 101 172 301 359 170,2
Upstream
of BNRR 106 335 574 689 327,0
DownslIeam
of BNRR 107 86 135 158 410,2
Baseline Road 108 532 804 924 650.9
Downstream PL-
of Golf Course 112 677 1153 1367 1093.1
Golf Tributary
at Highway 42 117 261 461 553 311.0
Arapahoe Road 121 1222 2155 2572 1841.9
E 114 Comer of
Section 28 124 1368 2459 2954 2177,9
Leyner Cotton.
wood Ditch 131 1622 2950 3560 2650.9
Isabelle Road 136 1726 3165 3846 3038.1
South line.
Section 15 142 1887 3472 4209 3409.3
Union Pacific RR 145 1757 3231 3952 3745.3
Jasper Road
(Confluence) 165 2631 4787 5824 5520,0
Boulder Creek 170 2636 4797 5831 5742,7
PRINCE North Line.
TRlB UT AR Y Section 26 158 1028 1741 2073 1021.4
Isabelle Road 160 1172 2002 2387 1317.8
Union Pacific RR 164 1189 2083 2505 1668,5
INDIAN Burlington N RR 15 118 209 249 113,9
PEAKS
TRlBUT ARY Highway 42 117 261 461 553 311.0
Bullhead Gulch
(Confluence) 119 330 596 718 435,2
I) See Figure 6.
n See Figure, 7 and 8,
FINAL
7131191
10

3.4 RESULTS

The results of the hydrologic analysis are summarized in Table 7. The Stream Discharge-
Probability Profiles are presented in Figure 9.

The flow paths are fairly well to well defined in the upper half of the Bullhead Gulch watershed.
Irrigation ditches effectively capture all of the low flow runoff in the lower half of this watershed
and recent quarry operations have altered historic drainage paths north of Isabelle Road.

The Burlington Northern railroad tracks, located in the upper parts of the watershed, create a
physical barrier to the surface runoff, Several small diameter culverts have been installed through
the railroad embankment, which act like release structures for detention areas on the upstream
side of the tracks. Approximately 24 acre-feet of detention storage volume has been accounted
for in this hydrologic analysis, The railroad embankment would be overtopped by the future
conditions lO-year frequency storm.

In the vicinity of Isabelle Road and U,S. Highway 287, several flow paths converge while the
defined drainage paths disappear, This results in much uncertainty in predicting the existing flow
paths in this area. Using the topography as a guide, it has been judged that a majority of the
flow will arrive at design point 142 (Figure 6). This location has a lOO-year flow of 4,210 cfs
for future conditions. The capacity of the roadside drainageway is approximately 150 cfs
resulting in a flow diversion from the west side of U.S. Highway 287 (routing element 42) to the
east side of U.S, Highway 287 (routing element 41).

FLOOD HAZARD AREA OELINEATION
BUUlIEAD GULCH AND lRIBUTARlES