Rainwater Downpipe Diameter and Flow Rate Calculation
According to EN 12056-3 roof drainage design standard, the flow rate of a rainwater downpipe is calculated, or the required pipe diameter is selected based on the design discharge.
The Wyly–Eaton equation is used for hydraulic capacity determination.
Where;
\( Q_{RWP} \) : Rainwater downpipe capacity (L/s)
\( k_b \) : Pipe roughness (mm) — typically taken as 0.25 mm
\( d_i \) : Internal pipe diameter (mm)
\( f \) : Pipe filling ratio (dimensionless)
These are the governing parameters.
Wyly–Eaton equation (Equation 1):
\begin{equation}
Q_{RWP} = 2.5 \times 10^{-4} \;
\frac{d_i^{8/3} \, f^{5/3}}{k_b^{1/6}}
\end{equation}