For Chinese conduit design, the regulation recommends the rational method, so every city has a rainfall intensity formula with same form :

Keifer & Chu (1957) design rainfall (also called as Chicago Rainfall) describes how to generate a design rainfall using that intensity formula.

This method may not be available for your region. For more information, refer to the Regionalisation topic.

The fields specific to Chinese Rainfall generation are listed below:

Return Period

Period, in years, between events of the same intensity or of a greater intensity than this storm.

Calculation of A

Select method of specifying value of A:

  • Calculated - A is calculated from a, C and return period values
  • User - A is user defined

A (Calculated)

A is calculated as:

a(1 + Clog10P)

Where P is return period.

A (User)

User defined equation constant.

a

Equation constant, used in calculation of A when calculation of A is set to Calculated

C

Equation constant, used in calculation of A when calculation of A is set to Calculated

r

The portion of the duration of the event occurring before the peak

r Value Examples

place

r(times)

area

r(times)

Chicago

0.375(83)

Beijing

0.355(57)

Soviet

0.35(—)

Shanghai

0.367(80)

Japan

0.50(1050)

North china

0.3-0.4(483)

Most Chinese city

0.3-0.4(—)

Wuhan

0.3-0.4(—)

b

User defined equation constant.

n

User defined equation constant.


Intensity and Rainfall Calculation

The duration for the design rainfall is 

Instantaneous intensity before peak is , corresponding duration is , and cumulative depth is .

Instantaneous Intensity after peak is , corresponding duration is , and cumulative depth is .


Total rainfall depth 


Set , the peak time ratio is r (which has a range of 0-1),  then .


Assume the density formula is 


In China, A = a(1 + log10 P) where P is the return period and a is a constant.


the instantaneous intensity before  and after peak  are:


So  and are calculated as follows:




From the above two equations, the cumulative rainfall depth is produced ad shown below:


The cumulative rainfall is calculated by following equations:



where

Reference on Chicago rainfall

C.J. Keifer, H.H. Chu, synthetic storm pattern for drainage design, proc paper 1332. ASCE. Aug. 1957.