Client: Wessex Rivers Trust
Categories: Fish Pass Design, Hydraulic Modelling, River Restoration
When: Jan 2021 - June 2021
Location: Chesapeake Mill, Wickham, Hampshire, UK
Edenvale Young Associates Limited (EVY) was commissioned by Wessex Rivers Trust Ltd to undertake hydraulic modelling of the River Meon, Wickham. The objective of the modelling was to inform a new mill offtake structure, designed by Cain Bio Engineering, for the bypass channel which forms part of the historic mill infrastructure for Chesapeake Mill. The offtake structure to the bypass channel had fallen into disrepair and excess flow was routed down the bypass channel causing low water levels at the mill. This reduced the performance of the existing fish pass and caused damage to the historic mill building.
The design requirements for the new bypass structure included:
• Providing flow to the bypass channel to maintain existing stream habitats; ensuring sufficient flow down the mill leat
• Maintaining a water level in the mill pond suitable for fish passage at the Alaskan fish pass (minimum depths of 330mm)
• Maintaining water levels at the mill building to within historic operational water levels (21.6mAOD at low flows and 21.8mAOD at high flows)
• Ensure that velocities at the bypass structure are suitable for salmon, sea trout and eel which are the key migratory species found on the River Meon
• Assess flood risk to ensure that there is no increase between the baseline and post development scenario.
Existing water level and flow splits were assessed by hydraulic modelling for low flows from Q95 to Q5, the levels from this modelling were validated against the mill owner’s observations of low flows at the Mill gauge. A number of offtake structure designs were tested in the model to ensure all design requirements of the new offtake structure were met.
The hydraulic modelling showed that the new bypass structure met all the design requirements:
• Flow to the bypass channel in all return periods has been achieved with 11% in Q95 to 15% in Q5; ensuring sufficient flow down the mill leat was also achieved
• Water levels in the mill pond have been maintained at a minimum depth of 400mm with the design requirement at 330mm
• Water levels at the mill building have been maintained within historic operational water levels (21.61mAOD at low flows and 21.84mAOD at high flows)
• Velocities at the bypass structure are suitable for salmon, sea trout and eel with maximum velocities of 1.3m/s in Q5
• There is no increase in flood risk as a result of the offtake structure design at low flows.
• A hydraulic model of the River Meon was constructed
• Good agreement was achieved between reported levels at the Mill gauge and modelled levels for low flows
• The model was internally reviewed to ensure that best practice was followed.
• All design requirements for the offtake structure were met with several options tested.