Swales over-sprayed with TerrAffix-G and HydraCX week 5 grasses starting to establish

Green Swales Trap Pollutants

Due to the success of our green drainage channel solution on the A11 Salix were once again employed by Balfour Beatty to implement the SuDS on the Norwich Northern Distributor Road (NDR).

This large roadside project involved creating 28km of green engineered roadside swales on 14km of dual carriageway.

Six weeks after treatment we had significant plant growth on Type-1 aggregate. Zero soil. No imported topsoil or subsoil. Simply Type 1 fill material.

Our approach was to treat the swales using a combination of our advanced hydraulically applied HydraCX and TerrAffix  over compacted type-1 aggregate

The subsoils on the roadside had a high proportion of silt and clay. The compacted aggregate had low porosity, zero soil and extremely difficult conditions for vegetation establishment.

The TerrAffix biochar serves several functions including encouraging healthy plant growth, carbon sequestration and reducing the mobilisation of pollutants into the vegetation, preventing them from entering wider watercourses.

We treated 88,000m² of swales with this high performing solution and within six weeks had vegetation establishing.

Water Quality Improvement

The water quality benefits of grass lined swale channels compared to all concrete lined channels is obvious in this CIRIA chart showing the control of pollution from highway drainage discharges.

Plant growth in these road side swales will trap a variety of salts, metals and other pollutants harmful to our rivers and streams.

Our HydraCX mixed with TerrAffix created a high surface area, water retentive layer.

This mix was specifically tailored to the ground conditions together with the correct seed species. The mix also contained slow release nutrients, which resulted in rapid establishment of grasses.

Carbon Sequestration

We have treated to-date 88,000m² of swales. On completion we will have locked in ~2T of fixed carbon and treated 120,000m².

Water Quality Benefits Grass lined swale channels compared to all concrete lined channels CIRIA 142

C0² in context

TerrAffix has sequestered 2 Tonnes* of fixed carbon within the ground which is equal to 7.32 Tonnes of atmospheric CO2 which is the equivalent of:

  • 6.59 tonnes of CO² removed from the atmosphere for millennia by the sequestration of refractory carbon within the construction
  • or offsetting the CO² produced by an average family sized car (1.5 – 2.0 litre engine) travelling 27,416km
  • or offsetting the CO² produced by 1958 single journeys along the NDR (14km)

*Based on 120,000m2 of treated area.