River Engineering & Restoration
Marden Mill Chalk Stream Rehabilitation
Keywords: River Restoration/ Rehabilitation Chalk Stream, Flood Risk Assessment, Low Flow Assessment Fish Passage
Client, & Other Organisations
Cain Bio-Engineering
Environment Agency
Description
As part of our long-standing relationship with Cain Bio-Engineering, Edenvale Young Associates Ltd. was commissioned to provide hydraulic modelling support to a river restoration/ rehabilitation scheme for a mill leat system at Marden Mill, Wiltshire.
The mill leat system is part of a SSSI in the upper reaches of the River Avon catchment. The original chalk stream channels above the mill suffer from historical engineering work and consequent low geomorphological diversity with uniform planform and cross sections and extensive siltation.
The overall project aim was to re-establish flow through the leat system with a view to creating favourable condition, self-sustaining chalk stream habitat along the length of the restored leat - primarily by repairing a dilapidated control structure at the head of the offtake and re-engineering the channel to a more natural state. Edenvale Young carried out modelling of pre- and post-restoration/ rehabilitation scenarios using the iSIS 1D modelling package principally to ensure that the proposed work posed no additional flood risk to the residential property at the Mill.
Scope of Works
In collaboration with Cain Bio-Engineering, supplementary modelling design and modification work was incorporated into the project brief in order to accommodate: Flood failsafe design; verification; leat control structure design; Mill race bypass channel design verification; fish pass design and verification and examination of low discharge scenarios
This iterative process of testing design ideas using hydraulic modelling allowed the final design to be produced with a strong appreciation of the likely impact of the rehabilitation work on local habitats and with proposed flood mitigation measures tested and verified prior to final submission of the proposed works.
The need to ensure fish passage at low flows was tested within iSIS using and refining proposed fish pass structure designs according to model outputs. Modelling also allowed an estimate of maximum and minimum velocities to be made. This ensured that the proposed substrate at areas of high velocity could be sized appropriately and suitable installation techniques selected. The range of velocities modelled was also used to crudely infer probably sediment transport and deposition within the rehabilitated reach and to confirm the suitability of soft-engineered in-channel and bankside alterations. After the design of critical structures had been tested within the model, an assessment of whether or not there would be any increase in flood risk to the Mill property due to the proposed modifications was made.