The key stages of drainage design and Flood Risk Assessment

Drainage design is a vital component of construction and development, ensuring that surface water is effectively managed while mitigating flood risks. This process not only safeguards properties and infrastructure but also complies with regulatory standards. In this article, we explore the stages of drainage design, including the role of flood risk assessments (FRAs), topographical surveys, greenfield run-off rate calculations and surface water hydraulic design.

Stage 1: Flood Risk Assessment (FRA)

Before commencing any drainage design, a Flood Risk Assessment (FRA) is conducted to evaluate the site’s vulnerability to flooding and its potential impact on surrounding areas. FRAs are mandatory for developments in flood-prone zones or those larger than 1 hectare, as per UK planning policies.

Key elements of an FRA:

1. Site evaluation: Analysis of historical flood records, proximity to watercourses, and groundwater levels.
2. Flood zone classification: Determining if the site falls within a flood zone as defined by the Environment Agency (Zone 1: Low risk, Zone 2: Medium risk, Zone 3: High risk).
3. Impact assessment: Evaluating how the development might affect flood risks downstream.
4. Mitigation strategies: Proposing solutions such as elevated building platforms, flood storage areas, or sustainable drainage systems (SuDS).

Incorporating the FRA findings into the drainage design ensures the system is both effective and compliant with planning requirements.

Stage 2: Topographical Survey

Once the FRA is completed, a topographical survey is conducted to map the site’s physical features. The survey provides critical data on land elevation, slopes, and natural water flow patterns, which are essential for designing a drainage system that aligns with the site's characteristics.

Why is a topo survey necessary?

• Informed design: Accurate data ensures the drainage system follows natural gradients.
• Identifying risks: Low-lying areas prone to water pooling can be addressed in the design.
• Integration with mitigation measures: FRA recommendations, such as flood storage zones, can be effectively incorporated.

Stage 3: Calculating the Greenfield Run-Off Rate

The greenfield run-off rate represents the natural rate at which water would run off the site before development. This calculation forms the foundation for sustainable drainage design, particularly in controlling post-development surface water flow.

Why do Local Authorities use the greenfield run-off rate?

• Flood prevention: Ensures developments do not increase flood risks downstream.
• Environmental protection: Supports sustainable water management by mimicking natural conditions.

Using rainfall intensity data, soil permeability, and the site area, engineers calculate the greenfield rate. This figure helps determine the required size of attenuation systems, such as basins or underground tanks, to limit discharge to acceptable levels.

Stage 4: Surface Water Hydraulic Design

With the greenfield run-off rate as a baseline, the next step is the hydraulic design of the drainage infrastructure. This ensures the system can handle stormwater effectively while adhering to regulatory requirements.

Key considerations in hydraulic design:

1. Pipe Sizing:
2. Pipe Gradients:
3. Storage Requirements:
4. Compliance:

Sophisticated software tools are often used to simulate hydraulic performance under various conditions, ensuring the system is both robust and efficient.

Conclusion

A comprehensive approach to drainage design integrates flood risk assessments, topographical surveys, greenfield run-off calculations, and detailed hydraulic design. By identifying potential flood risks and designing systems that manage water sustainably, developers can create projects that are resilient, environmentally responsible, and compliant with planning regulations.

With flood risks increasing due to climate change, the importance of robust drainage design cannot be overstated. By following these steps, developers contribute to safer and more sustainable communities.