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 + Clog_{10}P)

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 + log_{10} 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.