Innovative flood mitigation and water quality solutions at Botley

Client: Hampshire County Council
Duration: August 2025- ongoing

Situation

Our M Group civil engineering team were working to deliver substantial engineering works as part of the Botley Bypass project, partnering with Hampshire County Council.

During the bridge foundation works, we identified a heightened risk that site activities taking place within a designated flood zone and during periods of heavy rainfall could cause silt-laden runoff entering local streams and sensitive ecological areas. With the River Hamble and adjacent key fish located within the site our team introduced enhanced protective measures to safeguard ecological health and maintain regulatory best practice. 

Challenge

Traditional sit and flood management methods can sometime struggle to prevent fine sediment and pollutants from flowing into surrounding watercourses, especially during periods of heavy rainfall. With Environment Agency (EA) guidance, our team needed a robust, innovative solution that could deliver beyond compliance, safeguarding water quality and local biodiversity.

Solution

• Our team created and introduced a multi-stage filtration and flood mitigation system, including:
• Grips (ditches) to channel and settle runoff.
• Gravel fill to promote sedimentation and filter silt.
• Hay bales as an additional filtration barrier.
• Silt fencing to capture fine suspended solids and direct water flows.
• Silt mats to reduce erosion and contain sediment
• Bund construction to divert floodwater and protect woodlands. 

The innovative approach of grips, which was introduced by one of our foremen on site, was complimented by the Environment Agency for best practice when visiting the site.

This approach involved daily site monitoring, the strategic placement of each barrier and quick adaptation when site conditions changed.

Result

The innovative system effectively filtered sediment-heavy runoff, and remained resilient during periods of intense rainfall preventing impacts on the fish pass and adjacent River Hamble.

We continue to monitor water and runoff on site, supporting the Environment Agency’s regulatory objectives.