Preventing Silt Pollution Whilst Working in Rivers

Bank protection on the Afon Gwili in Carmarthenshire

Bank protection on the Afon Gwili in Carmarthenshire

frog environmental silt control measures take suspended solids from >2000 mg/l down to 12 mg/l downstream of in channel work and boost oxygen levels in the water.

Preventing pollution whilst working in rivers is a serious challenge for all contractors who work in and around water.

Working on the bed or banks of a river whether installing bank protection, de-shoaling or creating a river crossing can result in high levels of suspended solids being passed downstream.

Silt plumes can have a negative environmental impact and can draw the attention of the regulator.

The Afon Gwili in Carmarthenshire is a tributary of the River Loughor.

It is an important river for migratory fish including the sea trout and salmon, the catchment management plan focusses on improving water quality and protecting the river corridor habitats.

River bank stabilisation works had to be scheduled on the Afon Gwili to protect the adjacent highway and reduce high levels of erosion.

Constraints on site included the close vicinity of the road, an ancient woodland on the opposite bank, and the presence of Japanese Knotweed.

These constrictions meant that heavy plant entering the river channel was unavoidable and pollution prevention plans would be essential to minimise disturbance to the bed and banks of the river.

Works were required to control erosion and prevent damage to critical highway infrastructure.

Works were required to control erosion and prevent damage to critical highway infrastructure.


frog environmental are silt control and water quality specialists, working with contractors to deliver site specific silt control measures that manage the risk of causing a pollution incident.

frog environmental attended site and worked closely with the contractor to assess risk of causing an incident.

Given the sensitivity of working in a river environment and the need to work without the use of any chemicals it was decided that a series of measures would be needed to improve water quality and prevent downstream silt pollution on the Afon Gwili.

The first action was to create a dry working area whilst facilitating the need to move machinery in and out the steep sided channel.

A flow diversion channel was created using an impermeable liner covered with Silt Net 150, a natural fabric to increase friction and support the entrapment of solids.

The channel became naturalised with cobbles following a flood event.

We used a range of products including: FlocMat™, SiltNet 150, Rock Checks, Bubble Tubing®, SiltMat and Silt Wattles.

Diverting-the-river-flow-within-the-impression-of-the-original-channel-to-create-a-dry-working-area Silt-Net-150-used-to-line-the-impermeable-membrane-in-the-flow-diversion-channel SiltMat-positioned-immediately-below-work-area-entrapped-muddy-water,-filtered-solids-and-slowed-the-flow-Each-SiltMat-can-capture-over-40kg-of-silt Silt-Wattle-and-untreated-Floc-Mat Four lines of Bubble Tubing® were deployed to create a bubble curtain boosting oxygen levels downstream of works and providing a further barrier to silt movement A-final-rock-check-and-SiltMat-defence-provided-further-filtration-and-sediment-capture-whilst-enabling-fish-migration
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Silt Wattle and untreated FlocMat™ were used to separate the runoff from the work area and the main river channel, creating a low flow environment in which further sediment could be removed from suspension and filtration as it passes from this area.

Results

frog environmental undertook water quality testing to support compliance and ensure the interventions were working as planned. Total suspended solids (mg/l) and turbidity (NTU) were measured at the point of works, then following each of the silt control interventions.

Total suspended solids were recorded at over 2000 mg/l at the point of works, following the silt control interventions these levels dropped to 12 mg/l.

Turbidity as a measure of the discolouration of the water reflected the same reduction, lowering incrementally from 161.43 NTU at the point of works to 85.76 NTU, 17.53 NTU and finally 9.63 NTU after the final control measure.

This complex erosion control scheme on the Afon Gwili was able to continue on schedule without concern over works being delayed by a pollution incident.

frog environmental provide the following services to contractors:

  • Provision of cost-effective products to control silt pollution
  • Advice on mitigating silt pollution
  • Surface water management planning
  • On-site support
  • Monitoring

Silt control measures are essential for nearly all construction projects near rivers or in channel.

Our aim is to protect sensitive environments and help contractors to manage environmental, legal and financial risks associated their projects.

frog environmental always encourage early involvement when supporting our clients in their project delivery.