PERMFILE134266

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 70 REFUSE PILE EXPANSION
• Media Type: D
• Archive: No

REFUSE P/LE EXPANSION
control maximum runoff resulting from a 100-year,
24-hour Type fI precipitation event; the estimated
maximum runoff was obtained from the Phase 4
scenario, using and SCS runoff curve number of 88
for the reclaimed sideslopes. Refer to Section 6.2
for peak runoff calculations.
Sediment ditch design computations were
performed using the Chezy-Manning equation for
open channel flow. A Manning roughness constant
of n=0.03 was used for the ditch surface, and a
minimum freeboard of 0.3 feet was required for the
design. All ditches are V-notch shaped, with 2H:1 V
side slopes. Recommended maximum flow
velocities were obtained from Table 3, page 54 of
The Design of Roadside Drainage Channels,
Department of Commerce, Bureau of Public Roads,
1965.
The sediment ditches were divided into segments
and designed according to slope grade and the
proportion of the peak runoff to be conveyed.
Design criteria and calculations are presented in
Appendix L. Scouring will be prevented by limiting
flow velocity with appropriately sized riprap and
lining the ditch floor with geotextile or equivalent
~,~ material as necessary. Ditch design parameters are
presented in Table 13. Sediment ditches will
convey all runoff from the disturbed area to the
sedimentation pond located at the base of the RPE.
6.5 Clear Water Diversion
Clear water diversion ditch/berths will be
constructed along the accessible area upgradient
from the upper perimeter of the RPE as shown in
Appendix B. All water below [he diversion berms
will Flow into the RPE sediment pond. The berms
are essentially V-notch ditches with cut soil bermed
on the downgradient side to ensure clear water is
diverted. The berms are designed to control
maximum runoff possible from a Type 11 100-yr,
24-hr precipitation event; refer to Section 6:L and
Appendix L for peak runoff calculations.
Clear water runoff from the area above the RPE will
be diverted into Sylvester Gulch along the western
edge, and toward the unnamed drainage between
Sylvester Gulch and Box Canyon, east of the RPE
area. Runoff from the land upgradient from the
RPE was computed using an SCS Curve Number of
60 for undisturbed land. Runoff diversion ditches
and/or berms will be built for each construction
phase, and are designed according to slope
conditions. In addition, an upper clear water
diversion ditch berm will be constructed for the life
of the pile. Riprap and geotextile or equivalent
material were specified as needed to prevent
scouring. The diversion ditch/berm for each phase
has been divided into segments. Design parameters
for each seemen[ are presented in Table 14.
Clear water diverted toward the unnamed drainage
between Sylvester Gulch and Box Canyon shall pass
through an existing 220-ft long, 42-inch diameter
corrugated metal pipe (CMP) culvert and outlet into
the North Fork of the Gunnison river. Based upon
an SCS CN of 60 and a 100-year, 24-hour
precipitation event of 2.8-inches, the peak runoff
flow in the unnamed drainage is estimated to be
37.4 cfs. Using these same parameters, the peak
runoff for the flow from above the RPE to be
diverted into the unnamed drainage is estimated to
be 14.6 cfs. The combined peak flow will be 52 cfs
for the 100-year, 24-hour event. The existing
culvert and the drainage channel were examined to
see if the capacity could accommodated the
additional runoff influx. Results of the analysis are
presented in Appendix L. Assuming a 1% slope,
full flow with outlet control, the existing 220-feet
long. 42-inch diameter CMP has the apparent
capacity to convey 55-cfs without head above the
inlet, which is adequate for the expected peak Flow.
Clear water diverted into Sylvester Gulch will pass
through the existing Sylvester Gulch culvert beneath
Highway 133. No additional Flow through the
Svlves[er Gulch culvert will result from this
diversion because [he runoff proposed to be
diverted curently routes through the culvert at a
downgradient location.
Clear water ditch design computations were 6.6 Sedimentation Pond Design
performed using the same procedures and
!;~ parameters described in Section 6.4. A sediment settling pond was designed to collect the
sediment runoff from each phase of the RPE and
Harding Lawson Associates 13