- 5th June 2018
- Posted by: Sophie Smith
- Category: Blog
River West Allen upper catchment, located in the North Pennines, has been historically mined for lead and zinc, with an extensive legacy of mining remaining across the upper catchment. In partnership with JBA Bentley, our Geomorphology team were commissioned by the Coal Authority to undertake a geomorphological assessment within the River West Allen.
Its aim? To develop appropriate mitigation options for reducing the input of contaminated sediment into the river system and/or control the amount of sediment passing to lower reaches.
Sediment and preventative measures
During the geomorphological walkover, sediment sources were identified throughout the reach, with all sediment inputs within the upper reach representing contaminated (spoil) sediment sources. Sediment sampling of a range of sediment sources and sinks revealed the highest concentrations of both lead and zinc within the spoil erosion sites and in-channel sediment deposits. Natural bank erosion areas recorded the lowest concentrations.
A long list of mitigation options were developed including a combination of both hard-engineered options and more natural approaches. A collaborative options appraisal was undertaken, involving a range of disciplines (geomorphology, ecology, fisheries, archaeology and engineering), in order to assess the costs and benefits of each potential scheme.
Preventative measures were shown as favourable compared to reactive measures of sediment capture and removal (i.e. sediment traps).
Final short-listed options included preventive measures in the upper catchment (corridor widening and bank stabilisation), combined with floodplain lowering and a pilot sediment trap scheme further downstream. Floodplain lowering and connectivity options represent methods of Natural Flood Management (NFM) and will therefore help to attenuate flood peaks through the catchment.
The final options are now currently gaining planning permission, prior to the start of the construction phase.