UFC 3-210-10
25 October 2004
both annual accumulations and peak events. A comparison of the volume (depth) of
runoff from the pre- and post-development curve numbers for a 130 mm (5-in) rainfall
(P - I a )2
Q=
Equation 10-4
(P - I a ) + S
Where: Q = runoff depth (in)
P = rainfall depth (in)
Ia = initial abstraction (in)
Ia = 0.2S
Equation 10-5
Where: S = potential maximum retention after runoff begins (in)
1000
S=
- 10
Equation 10-6
CN
Table 10-4. Runoff Depth for Existing and Proposed Conditions (5-inch Rainfall)
Condition
Runoff (in)
Existing (CN = 63)
1.5
Proposed (CN = 80)
2.9
10-2.4
LID Site Planning Strategies. Several LID site design strategies will be
employed to reduce the CN for the proposed condition. A lower CN value will be
obtained by:
Reducing impervious cover
Disconnecting impervious areas
Reducing the grading footprint to retain more wooded area
Restoring the infiltration capacity of disturbed and compacted soils
Figure 10-8 shows the resulting site plan. A significant amount of disturbance
to the woods and wetlands has been avoided by eliminating the centralized stormwater
facility and distributing the stormwater management among LID components throughout
the site. (The elimination or reduction of impacts to wetlands and water bodies may
have a significant effect on permitting in many areas.) The condition of lawn areas will
be improved by ensuring that adequate topsoil and aeration are included in the final
grading and stabilization of the project. The road width has been reduced from 15 m
(48 ft) to 9.8 m (32 ft ). The parking areas have been maintained as head-in parking
and the green space in the central island is expanded. Additional reductions in
impervious area could be incorporated into the design, such as further reducing the
road width or sharing driveways. Remaining impervious areas should be disconnected
to the greatest possible extent. Table 10-5 summarizes the CN calculations for the
proposed conditions using the LID site planning approach.
42
Ibid.
70
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