This is the classic SCS velocity equation for sheet flow.
Where
-
L is the distance to the hydraulically most remote point
-
V is an overland flow velocity for specific type of ground surface
The following equations are taken from the SCS velocity chart.
Where s is average slope in direction of flow and a & b are coefficients. The coefficients are:
|
Description |
a |
b |
|
Forest and fallow |
2.5 |
0.5 |
|
Minimum tillage cultivation |
4.7 |
0.5 |
|
Short pasture and lawn |
7.0 |
0.5 |
|
nearly bare ground |
10.1 |
0.5 |
|
Grassed waterway |
15.0 |
0.5 |
|
Paved surface |
20.0 |
0.5 |
The following figure is representative of the equations given above. The Figure was taken from Modern Sewer Design, 1995 and includes additional land use types.
Figure 4: Average Velocities for Estimating Travel Time for Overland Flow.