HABITAT RESTORATION AT WEIR – A CASE STUDY IN SAVING KING HENRY III’S FAVOURITE FISH

Dr Peter Brunner Royal HaskoningDHV April / May 2019

RRC 20th Annual Conference, Liverpool Background

• So what was King Henry III`s favourite fish?

• The Shad (Twaite & Allis) was the staple food in the 13th- century court of King Henry III, and was preferred over Atlantic salmon (Shad the fish of Kings and Queens).

• Prior to mid-19th Century, Twaite shad (Alosa Fallax) was an economically important in the Severn River Basin.

• Up to two thirds of all fish caught in the Severn Estuary and were Twaite shad.

• However, Twaite shad populations in the River Severn and its major tributary the River Teme, have drastically reduced since the mid-1800`s, in response to the construction of weirs and navigation locks preventing access to upstream spawning habitat. Background

• The Channel (Severn Estuary) holds the only viable breeding population of Twaite shad in the UK.

• The River Severn is one of only four rivers in the UK known to support spawning Twaite shad.

• Because of the decline in Twaite shad, they are a notified species within the Severn Estuary SAC and River Teme SSSI. Objectives • Unlocking the Severn Project being funded by HLF and LIFE UK, aims to significantly improve access to quality spawning and nursery habitat for the Twaite shad; and • Re-establish 253 km of its former natural range in the River Severn and River Teme. • The overall project is being delivered by the Environment Agency in partnership with the Severn Rivers Trust (SRT), Canal and Rivers Trust (CaRT), Natural (NE).

• As part of the Unlocking the Severn Project, Royal HaskoningDHV were initially commissioned to undertake a feasibility study to assess fish passage and habitat improvements at Powick Weir along the River Teme SSSI. Objectives

Physical Barrier to Fish Migration

• Spawning of Twaite shad is believed to be limited to downstream of Powick Weir, despite the weir having an existing fish pass which appears ineffective for twaite shad and, other migratory fish species.

• However, several environmental constraints needed to be considered during the feasibility study in order to provide a WIN-WIN fish passage solution. Powick Weir and Constraints • Continued flows to the River Teme and Mill Leat, both SSSI.

• The integrity of Powick Historic Bridge and the A4440.

• Channel erosion and/or deposition both upstream and downstream of the existing weir.

• Fish passage requirement for Twaite shad.

• Habitat valued by recreational anglers, such as flow impoundment, versus preferred by coarse fish, such as Barbell (Barbus barbus). Approach

The key overall approach undertaken for the feasibility/appraisal included:

 Phase I – High level screening and options appraisal of proposed habitat restoration / fish passage solutions for Powick Weir.

 Phase II – Detailed 1D-hydraulic and 2D-hydrodynamic modelling (using ISIS / TUFLOW and MIKE21,) to further assess the proposed habitat restoration / fish passage solutions for Powick Weir on water levels and sediment transport. Fish Passage Options Appraisal

 Several habitat restoration / fish passage solutions were identified (e.g. upgrading the existing fish pass; replacement of the existing weir with a rock ramp; bypass channel) during Phase I.

 However, in the context of achieving “Favourable Condition” and “Good Ecological Status” in the River Teme SSSI, full and partial removal of Powick Weir was further appraised during Phase II.

Full Weir Partial Weir Removal Removal Q95  2D-hydrodynamic modelling (confirmed the 1D-hydraulic modelling), that full weir removal will direct low flows (Q95) along the main channel of the River Teme, reducing flows along the Mill (and Partly Dry Mill Leat (SSSI) increasing sedimentation).

Q95

QMED  Fully removing Powick Weir will reinstate natural fluvial processes, although increases in flow velocities (and bed shear stress) during bank flow conditions (QMED) could QMED potentially cause localised bank erosion along the River Teme; for Bank Erosion example upstream of the A4440 bridge and the Old Powick Bridge, a Grade II listed building.  Geomorphological site visits revealed the following: A. Existing concrete structure at Powick Weir (comprising the existing fish pass and ramp) was acting as flow deflector, syphoning flows of the River Teme towards the Mill Leat.

B. Primary flow path (thalweg), was concentrated along the left bank upstream of the Mill Leat i.e. flows were not ‘equally’ distributed across the entire length of the existing weir.  As such, the development of the partial weir removal solution was based on fluvial processes observed during the geomorphological site visits. • Partial weir removal would have less impacts upon overall water levels along the River Teme and Mill Leat compared to full weir removal during low flow conditions.

Q95 Partial Weir Removal

• 1D-hydraulic modelling further validates the importance of the existing concrete structure at Powick Weir for flow and sediment transport during low flows. Q95 Partial Weir Removal • Historically morphological adjustments along the lower reaches of the River Teme have been slow over the past 125 years.

• Although there is potential for erosion at some locations along the River Teme in response to full or partial weir removal, the natural adjustment of the channel to these fluvial processes may take a long time. Stakeholder Consultation

Fletching Mill, East Sussex

It was through detailed consultation, that partial removal was chosen over full removal of Powick Weir as the preferred habitat restoration solution and taken forward to detailed design. Detailed Design of Partial Removal of Powick Weir Low flow channel - sufficient depth of water for Shad passage during low flows when they typically migrate upstream

Existing structure retained

Historic sheet piles Will benefit all migratory and coarse retained fish, including Shad and Barbell Consents, Implementation and Monitoring

February 2019 Take Home Message

• Overall importance of “good quality feasibility studies” setting the scene for the selection of the most appropriate habitat restoration or fish passage solution!!

• Importance of site visits by fisheries and geomorphologist specialists in directing the detail design for fish passage projects

• Including the validation of hydrological and sediment modelling

• Striving for WIN-WIN fish passage projects

• For example, ensuring the wider environmental aspects, such as designated sites, heritage and recreational angling, are fully considered through SMART fish passage solutions Thank you

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