Prefeasibility Access Road Design-Revised
25
Warren Peak Hydrology Data
Watershed
Area
(acres)
Longest Flow
Path (ft)
Average
WS Slope
Percent
Forest
Curve
Number
Lag Time
(min)
1
282
7796
23.9%
97%
66
30
2
529
11485
30.2%
97%
66
36
Watershed
25 yr Peak Flow (cfs)
Snow Melt Flow (cfs)
1
227
87
2
373
163
VIII HYDRAULICS
Culverts for stream crossings with drainage areas greater than 100 acres were sized
using Chart 2B from
Hydraulic Design of Highway Culverts Third Edition
. This is a
nomograph that is used to size culverts under inlet control conditions. Generally
inlet control is the governing condition for steep channel slopes (3% or greater) and
is a reasonable method for preliminary culvert sizing when channel geometry is not
available. The culverts were sized to keep the water surface (WS) elevation below
the crushed base. Due to the preliminary nature of the sizing the ratio of headwater
depth to culvert diameter was set at 1. Hydraulic data for each watershed is
summarized in the tables below. Note that these are only the major culverts. There
are several minor culverts (those with watersheds less than 100 acres) needed for
both roads.
Forest debris is likely to wash into culverts, so a 50% blockage rate was selected for
culverts under 4 ft in diameter. All of the culverts analyzed are larger than 4 ft so the
debris factor was not incorporated. This factor will be used for minor culverts.
All culverts are assumed to be corrugated metal pipe (CMP) that project from the
embankment fill. However, CMP may not be the best choice in Miller Creek
watershed 4 and both Warren Peak watersheds due to large road embankment fills
(over 40 ft) and large culvert drops (over 30 ft). These culverts may need to be
evaluated structurally for high fills and lined to avoid bed load damage or be a
different material altogether.
