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Storage & Treatment CIRIA C753 §3.4

Exceedance Flow & Overflow Routing

Standard 4 compliance: exceedance event routing and safe overland flow design.

1
Design Storm & Site
2
SuDS Capacity
3
Spillway Design
4
Flood Routing
5
Results

Design Storm & Site Parameters

Define the site characteristics and design storm conditions for SuDS and exceedance analysis.

Project Information

📍 Site Characteristics

Total contributing catchment area
Total paved/roof area
Proportion of rainfall as runoff (0-1)

💧 SuDS Design Parameters

Storage provided by existing SuDS system
Storm SuDS system is designed for
Intensity for design return period

⚠️ Exceedance Storm Parameters

Storm exceeding SuDS capacity
Percentage increase in rainfall intensity
Intensity for exceedance return period

⏱️ Storm Duration

Critical storm duration (typically 2-6 hours)

SuDS System Capacity

Define the SuDS storage and discharge characteristics, and identify overflow points.

🏗️ SuDS Storage & Discharge

Total attenuation volume available
Greenfield runoff rate or restricted outlet
Primary SuDS configuration

🚰 Overflow Points

Spillway Design

Design emergency spillways for each overflow point using weir equations.

Flood Routing

Define how exceedance flows are routed and assess flood hazard.

🌊 Exceedance Volume

📍 Flood Route Options

Select all applicable overflow routes for your design:

Results & Compliance

Review design storm capacity, exceedance provisions, and regulatory compliance.

✓ Design Storm (1:30 or 1:100)

⚠️ Exceedance Storm Analysis

🛡️ Regulatory Compliance

References: Exceedance design per CIRIA C753 (The SuDS Manual), §3.4: Managing exceedance flows