Environmental Engineering Reference
In-Depth Information
15
Post development
10
5
Predevelopment
0
10
20
Time interval (hr)
30
40
Figure 1-17
Hydrograph of increase downstream of development.
8.0
7.0
Runoff values for the 1
storms generated
using the small storm hydrology methodology (Pitt,
1994) and runoff values for the storm generated
using the SCS runoff curve number method (CN = 98
for impervious and CN = 60 for woods, B soils, fair
condition).
″
and 1.5
″
6.0
5.0
4.30
4.04
4.0
3.37
3.04
3.0
2.60
2.0
1.26
0.95
1.0
0.0
1 inch
rainfall
1.5 inch
rainfall
2-yr storm
(3.27
″
)
5-yr storm
(4.09
″
)
10-yr storm
(4.78
″
)
50-yr storm
(5.51
″
)
100-yr storm
(7.5
″
)
Figure 1-18
Runoff volume increase from impervious cover on HSG B soil.
For a specific site, the net increase in runoff volume during a given storm
depends on both the predevelopment permeability of the natural soil and the
vegetative cover. Poorly drained soils result in a smaller increase of runoff volume
because the volume of predevelopment runoff is already high. Therefore, the
amount of runoff resulting from development does not represent as large a net
increase. Using the same rainfall values, Figure 1-19 illustrates that the 2-year
rainfall of 3.27 in. in 24 hours produces an increase of only 2.01 in. on a HSG C
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