Enhancement at Riverside Walk, Duke of Northumberland’s River, Isleworth

A river with few morphological features required a bioengineering solution to enhance its channel bed and banks to increase base flow and variation in its flow rate.

The Duke of Northumberland’s River in Isleworth had been previously straightened and had artificial hard banks with many sections lacking marginal and aquatic vegetation. Its flat bed was homogenous with some areas that were covered by silt. Due to the minimal hydro morphological features, there was low sinuosity, low flow and uniform water flow types.

Salix proposed a river enhancement design with the main aim of increasing base flow, reducing low flows and narrowing the channel using vegetated wet berms of varying widths, locally sourced coarse wood deflectors and vegetation to be used both marginally and within the channel itself.

Locally sourced materials used in bioengineering techniques to create habitats

It was also suggested that, subject to the river bed being found structurally sound after site investigations, optional shallow pools could also be excavated. The excess material removed during excavation of the pools would be used as backfill for the berms. Locally cut tree limbs would be staked upstream of the pools to encourage scour of the pool to remain free from infilling with fine silts.

Approximately 580m of channel reach was enhanced using the proposed bioengineering techniques. However, only 370m of berm was installed as this was the full extent that existing tree cover and existing marginal vegetation would permit.

Tree limbs were cut locally where the channel was over shaded by trees growing on both banks. This provided coarse wood deflectors which narrowed the channel locally. Due to the variability in shape and size of natural materials such as whole tree limbs the subsequent impact upon flow varied, which meant that adjacent features needed to be adapted and modified to allow for this natural variability.

The height of the existing hard bank varied from 0.9 to 1.2m. The widths of the vegetated wet berms used on the footpath side of the river to concentrate flow also needed to vary from between 0.5m and 1.5m wide.

The berms sway in and out from the side of the channel and include the installation of Coir Pallets pre-established with mature and diverse wetland plants to increase marginal vegetation. The seed mix Salix used includes Lesser Pond Sedge, Yellow Flag Iris, Purple Loosestrife and several other native species.

A brushwood fascine, staked into place with a chestnut post, secures any backfill used to raise the level of the berms in places where the water level is over 100mm.

Where water levels are very low (below 100mm), then pre-established Coir Pallets were laid directly onto the river bed and secured in place.