Prefeasibility Access Road Design-Revised
24
To account for snowmelt runoff, a generalized equation from the U.S, Army Corps of
Engineers, and reprinted in
Introduction to Hydrology 5th Edition
by W. Viessman
and G.L. Lewis 2003, was used to estimate the maximum daily snowmelt. Inputs
used for the equation were an average wind speed of 30 mph, an air temperature of
70°F, a dew point temperature of 60°F, and a snow surface temperature of 30°F. The
daily maximum snowmelt found was 7.65 inches. This daily depth was then
distributed using the unit hydrograph method described in the
National Engineering
Handbook
, Part 630, Chapter 11. This method calculated hourly depths based on a
sine curve of the hourly temperature. This hourly snowmelt distribution was then
treated as hourly rainfall in HEC-HMS to calculate a runoff hydrograph and a peak
flow.
Hydrologic data for each watershed is summarized in the tables below. For all
watersheds, the 25 year storm was significantly larger than the snowmelt.
Therefore, the culverts were designed for the 25 year storm.
Miller Creek Hydrology Data
Watershed
Area
(acres)
Longest Flow
Path (ft)
Average
WS Slope
Percent
Forest
Curve
Number
Lag Time
(min)
1
363
8310
10.4%
22%
57
60
2
2038
21678
25.0%
66%
62
73
3
2729
20901
26.2%
77%
63
68
4
113
3063
21.8%
91%
65
15
5
376
5385
22.1%
68%
62
26
Watershed
25 yr Peak Flow (cfs)
Snow Melt Flow (cfs)
1
137
97
2
768
588
3
1122
801
4
136
35
5
300
110
