The grassed area entry time is the time taken for a drop of water to travel from the hydraulically most distant point of the contributing grassed area on to the contributing paved area or the entrance to the stormwater drainage system if there is no paved area. The entry time calculation is based on the Kinematic equation for overland flow:
Where:
t = Travel time in minutes.
n = Manning's retardance coefficient for overland flow.
L = Flow length in m.
S = Slope in %.
i = Excess rainfall intensity (mm/h).
The Grassed Area Entry Time Calculator allows you to specify the following parameters:
|
Option
|
Description |
|
Length |
The length of the grassed portion of the catchment area. |
|
Slope |
The slope of the grassed area expressed as a percentage. |
|
Manning Factor |
The roughness coefficient of the grassed area. |
The excess rainfall intensity multiplied by the width ratio ( Wi ) is assumed to be 25 mm/h, as recommended by Watson.
Woolhiser recommended the following Manning's retardance coefficients for overland flow.
|
Surface
|
Range |
|
Concrete or asphalt |
0.010 - 0.013 |
|
Bare sand |
0.010 - 0.016 |
|
Gravelled surface |
0.012 - 0.030 |
|
Bare clay - loam soil (eroded) |
0.012 - 0.033 |
|
Sparse vegetation |
0.053 - 0.130 |
|
Veld |
0.100 - 0.200 |
|
Lawns |
0.170 - 0.480 |
The calculator updates the grass entry time control when you click OK.