Earle Street, Yeovil - Flood Risk Assessment & Drainage Strategy

According to a document from 2021–2024, the Flood Risk Assessment and Drainage Strategy (FRADS) was prepared to support the redevelopment of a block of flats at a site off Earle Street, Yeovil, Somerset.

The report evaluates fluvial, surface water, sewer, and overland flow risks and provides a comprehensive drainage strategy to ensure the scheme will not increase flood risk locally or downstream.

Project Overview

The FRADS confirms:

• Development type: Block of flats
• Location: Construction Site, Earle Street, Yeovil, BA20 1JW
• Purpose: To demonstrate that the development can be safely designed in respect of flooding and that runoff can be managed effectively.
• Objective: Mitigate all identified flood risks and provide a compliant drainage strategy under NPPF requirements.
  

Assessment of Flood Sources

The FRADS evaluates all flood risk types.

Fluvial Flooding

• The River Yeo is ~330 m south of the site.
• Assessed risk: Low.

Surface Water Flooding (Pluvial)

• Surface water flooding will not affect the development based on modelling and capacity checks.

Sewer Flooding

• Sewer flooding does not affect the development site.

Reservoir, Canals & Tidal Flooding

• All assessed as no risk or low risk.

Overall: All sources of flooding to the proposed development are Low or None.

Overland Flow & Culvert Capacity Analysis

A major element of the FRADS is the hydraulic assessment of upstream catchment flows and the capacity of the culverted system.

Key Findings

• CCTV survey (Appendix I) identified sedimentation and deposits, so a 20% reduction in hydraulic capacity was applied for conservatism.
• Even with this reduction:
• Northern catchment flow: 3136.1 l/s
• Eastern catchment flow: 154.6 l/s
• Culvert beneath site: 1449.2 l/s
• Culvert under Wyndham Street: 4187.4 l/s
• Free capacity available: 2253.1 l/s
• The off-site combined sewer system contributes to historical flooding elsewhere in Yeovil, but this is independent of the development and not attributable to flows from the site.
• The hydraulic assessment confirms that surface water flooding need not be considered further because the culverted system is sufficiently oversized.
  

Impact of Development on Catchment Flood Risk

• The development introduces only a small increase in impermeable area.
• The strategy incorporates appropriate mitigation, ensuring no worsening of flood risk downstream.
  

Drainage Strategy

The drainage design emphasises:

Surface Water Management

• Removal of surface water discharge from combined sewers → reduced CSO discharges into the River Yeo.

Water Quality Protection

• Treatment train includes permeable paving and a bypass separator.
• Assessed using the CIRIA C753 Simple Index Approach.

Overall Performance

• Significant betterment in discharge rates over the existing situation.
  

Overall Conclusion

The FRADS concludes:

• The development is not at significant flood risk for its design life.
• Surface water can be safely managed using the proposed drainage strategy.
• Downstream flooding and water quality impacts are improved, not worsened, by the development.