Specify the Rational Method design parameters.
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Option
|
Description |
|
Return Period (years) |
The return period or statistical frequency of the storm event. |
|
Mean Annual Precipitation (mm) |
The average annual precipitation in mm. |
|
Rainfall Region |
If you specify one of the following regions:
This is the traditional Rational Method, where the rainfall intensity (I) is interpolated from the depth-duration-frequency diagram documented in HRU Report 2/78 - A Depth-Duration-Frequency Diagram for Point Rainfall in Southern Africa (1978).
If you specify one of the following regions:
This is known as the Alternative Rational Method and is based on a modified and recalibrated Hershfield equation, as proposed by WJR Alexander (2001). |
|
Proportional Flow Depth (%) |
Specify the proportional flow depth to be used to calculate culvert capacity and select a culvert size. |
|
Time Step (min) |
The time increment in minutes used in the analysis of the storm and runoff model. |
|
Routing Method |
Specify which routing method to use:
|
|
Area Reduction Factor |
Specify which area reduction factor calculation method to use:
|
|
Probable Maximum Flood |
Select this checkbox to calculate the probable maximum flood run-off. The rural run-off factor will be set to the maximum value, and the urban run-off factor and the adjustment factor will be set to 1. |
|
Next button |
Click to display the next page. |
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Cancel button |
Click to cancel the function. |
The depth of precipitation is calculated from the time of concentration, the mean annual precipitation, the return period and the rainfall region according to Figure 3.8 of the SANRAL Drainage Manual.
The area reduction factor for small areas is calculated from the point intensity and catchment area values according to Figure 3.20 of SANRAL Drainage Manual. The area reduction factor for medium to large areas is calculated from the catchment area and storm duration values according to Figure 3.21 of SANRAL Drainage Manual.
See Also Network Analysis