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According to GB 50014-2006 “Code for Outdoor Drainage Design”, the rainwater drainage capacity is calculated using the following formula:
Q — Design rainfall runoff flow rate (L/s)
F — Catchment area (L×W) m²
q — Design storm intensity L/(s·hm²)
ψ — runoff coefficient, which can be referenced in the table below.
Note:
Q represents the stormwater drainage capacity that a channel designed for a specific catchment area is required to handle. To determine the drainage capacity per meter of the channel, this value must be divided by the total length of the channel as designed.
F. Refers to the catchment area of the section requiring drainage (length L × width W).
q. The intensity of local heavy rainfall shall be determined in accordance with the design codes; refer to sources such as the “Building Water Supply and Drainage Design Manual” to obtain rainfall intensity tables for different regions and return periods.
ψ: runoff coefficient. The runoff coefficient varies depending on the type of surface material; specific values can be found in the table below.
| Ground Type | Runoff coefficient |
| Green space | 0.15–0.3 |
| Various types of roofs, concrete or asphalt pavements, and plazas | 0.85–0.95 |
| Paving of road surfaces and plazas with large stone slabs | 0.55–0.70 |
| Gravel Pavements and Plazas with Asphalt Surface Treatment | 0.55–0.65 |
| Graded Crushed Stone Pavement and Plaza | 0.40–0.50 |
| Dry-laid brick, stone, or crushed-stone pavement and plaza | 0.35–0.40 |
| Unpaved dirt road | 0.25–0.35 |
Friendly Reminder:
When the terrain is relatively complex—such as when there are buildings surrounding the site, with roof runoff discharged directly to the ground via pipes, or when large slopes surround the drainage area—it is necessary to delineate additional drainage zones or install ditches to intercept and divert runoff.
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