Fundamentals of Engineering Design
5.1.5.1 Project Hydrology Considerations
The hydrology of the watershed determines one of the fundamental driving forces of the system,
i.e., the amount of water flowing across the watershed and through the stream system. Flowing water
shapes the watershed and channel systems through erosion processes. The following section will present
concepts of hydrology essential to evaluating runoff and sediment transport potentials.
5.1.5.1.1 Gage Data
Historical and real time stream gaging data are available for many gaged streams. These data along
with a wide variety of useful information may be easily downloaded by those with Internet access at the
following USGS web site: http://waterdata.usgs.gov/.
5.1.5.1.2 Frequency Analysis
The recurrence interval for discharges at the ungaged sites can be determined from USGS
regionalized relationships using regionalized hydrology procedure, for example Landers and Wilson (1991)
was used for the DEC Project. Because of the relatively short period of record for the gaged sites, less
than ten years, this method was also utilized for the gaged sites. The relationship proposed by Landers and
Wilson (1991) is:
Qx ' aDA bS
c
(L d)
(5.1)
where Qx
=
discharge at recurrence interval x years;
DA
=
drainage area in square miles;
S
=
channel slope, ft/ft;
L
=
major drainage path length; and
a, b, c, d
=
coefficients and exponents are listed in Table 5.6
Of course, other more computationally sophisticated hydrology models are available. Some of these are
discussed in Section 5.1.5.2.
Table 5.6 Regional Discharge Recurrence Interval Coefficients and Exponents
Recurrence Interval
a
b
c
d
2 year
66.2
0.88
0.51
-0.11
5 year
94.7
0.93
0.51
-0.15
10 year
122
0.96
0.49
-0.19
25 year
164
0.99
0.47
-0.24
50 year
197
1.00
0.45
-0.26
100 year
230
1.00
0.44
-0.25
113
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