Sustainable Urban Drainage Systems (SuDS): Design Principles and Hydraulics

Rapid urbanization replaces natural vegetated landscapes with impermeable surfaces (roads, roofs, parking lots). This prevents rainwater infiltration, causing elevated peak runoff, shortened lag times, and flooding of urban water courses.

Sustainable Urban Drainage Systems (SuDS) Philosophy

SuDS mimics natural hydrology to manage surface runoff. It aims to reduce flow rates, filter pollutants, and provide ecological value using a “management train” (source control, site control, and regional control).

Hydraulic Design of Retention Facilities

Designing SuDS components requires estimation of storm runoff volumes. The peak runoff discharge $Q_p$ for small drainage basins is determined using the Rational Method:

$$Q_p = C \cdot I \cdot A$$

Where:

• $C$ is the runoff coefficient representing land permeability, from $\sim 0.05$ for lawns to $0.95$ for roofs.
• $I$ is the average rainfall intensity for a duration equal to the time of concentration.
• $A$ is the drainage basin area.

The water quality volume ($W_Q$) required for detention basins to settle suspended solids is calculated as:

$$W_Q = P \cdot C \cdot A$$

Where $P$ is the design rainfall depth (e.g. 90th percentile storm depth).

SuDS Components and Mechanics

1. Swales: Broad, shallow channels covered with vegetation that slow water and promote infiltration. Flow velocity is kept low $<0.5 \text{ m/s}$ to prevent erosion.
2. Retention Ponds: Permanent wet basins that store storm runoff and release it slowly via an orifice structure while promoting settlement of heavy metals and hydrocarbons.
3. Permeable Pavements: Porous concrete or blocks over a gravel sub-base that stores water temporarily during intense rainfalls.