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.

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.