Wetland, Stream Bank, and Shoreline Restoration
Product Case Study
Combining AquaRock Bag & Soil Bioengineering Revetments in Deeper Water Scour Environments
Year Weir is located on a reach of the River Derwent in Barepot, approximately 0.75 km east of Workington, Cumbria, UK
Following recent major flooding (Winters of 2009 & 2015) increased erosion of the right bank of the River Derwent had impacted upon key assets:
The 160m length of bank in between the United Utilities (UU) assets (weir and pumping station) underwent rapid and accelerated bank erosion after two major flood events (firstly 2009 and then again in 2015). It was considered that further erosion here could potentially compromise the integrity of the weir further downstream. In addition, during the 2009 flood event, part of the access road to the weir (from the pumping station) was washed away. It was necessary to provide an alternative access to the weir, needing some form of protection against future erosion.
Ground investigation and geomorphological survey concluded that fine week alluvial soils that were highly erodible with the alluvium found to comprise ‘dark grey’ and ‘brown’ fine to coarse sands. Vegetation structure on the adjacent floodplain was heavily grazed agricultural, shallow rooting grassland species and the eroding banks were devoid of any vegetation cover at all.
Erosion processes during flood events were twofold, firstly severe bank erosion caused by toe scour, bank failures and associated bank retreat and secondly the stripping back of the turf (sod) layer on the flood plain, exposing further floodplain soils to erosion. The river also began to cut a new channel on a shorter course across the floodplain, bypassing a large weir structure, a process known as avulsion (abrupt change in course of a river). After the 2013 flood the bank had retreated over 6 metres and large scour pools have formed at the toe of the riverbank, leaving the weak alluvial banks primed for further erosion in smaller flood events.
A detailed bathometric survey was undertaken in late 2017, followed by hydraulic modelling of the existing channel and flood plain to build up a map of bed, bank and floodplain flow velocities and map shear stress for 1:200-year flood return events. As an ecologically sensitive site the use of traditional hard engineered bank protection was not an option and bioengineered designs were favoured. The scour pools provided a particular challenge, as bioengineering techniques would not control the rate of erosion in 2-3 metres of water. Large rip rap stone was also ruled out due to the geomorphological impact downstream and the adverse ecological impact. The solution to controlling the erosion in the scour pools came in the form of Aqua Rock Bags. Aqua Rock bags are a mesh bag that is filled with graded, angular stone that can be stacked in scour pools in deeper water. These were installed to average water level and above this level more traditional bioengineering solution could be used. The smaller stone in the Aqua Rock bags provides high interstitial space for benthic invertebrates, fish fry as well as supporting submerged and marginal macrophyte vegetation communities. The smaller stone size also negates the issue of the downstream morphological impacts that larger rip rap has on channel processes.
From average low flow water level, a continuous run of liner toe logs, rootwads and live willow staking were installed. Additional tree planting on the reprofiled river banks and the adjacent floodplain will slow flood flows and reduce erosive force in the future. The banks were also protected with a high performance turf reinforcement system, using a combination of woven HP-TRM and Gripple ground anchors.