Pulborough Brooks presented a classic wetland challenge: an ageing tilting weir system that limited water control and allowed nutrient‑rich backflow from the River Arun onto the Pulborough Brooks SSI wetland. Keystone replaced the failing system with three robust sluices and a pipe dam to give rangers simple, reliable control over floodplain water levels, retain fresh inland catchment water through dry months, and reduce nutrient ingress — improving habitat resilience and cutting ongoing maintenance.
Challenge
An extensive scope of works presented a challenging and interesting project brief:
Track upgrades
Upgrading the main wetland access route to improve access in the wetter months. The track needed to be capable of withstanding tractor and machinery access without becoming seasonally compromised by floodwater.
Pipe dam installation
Desilting, coffer dam construction, dewatering, clay foundations and pipework with control elbows; shaped and sealed to form a trapezoid dam that provides adjustable flow control.
Three pre‑cast concrete sluices
Ditch silt excavation and clay backfill, keyed wing walls, subbase bedding and levelling, lifting and alignment of sluice units, sealing and anchoring, installation of sluice boards and emergency release valve.
Site logistics and risk management
Repeated dewatering in tidal windows, temporary bunding, and mitigation for leaking authority flood defence sluices.
Finishing and safety
Handrails, tilting weir removal site tidying and ditch reinstatement and snagging activities worked through with the client to confirm water retention and operational performance.
We experienced tidal and leakage issues, including recurring flooding from defective authority sluices on the estuary wall, and unusually high tides that significantly restricted our working windows. We adapted by using staged cofferdams, rapid installation sequences, and continuous pumping to meet programme constraints.
Approach
Design brief
Each sluice required an eel pass to enable upstream migration of European eel while maintaining the primary water management function. The passes had to be durable, low‑maintenance and compatible with the concrete sluice geometry and local flow regimes.
Design solution
Keystone developed a compact, passive eel pass that integrates directly into the sluice. The pass features a stepped internal profile to create low‑velocity resting zones and a gentle gradient to assist upstream movement. Materials and fixings were specified for long service life in a tidal wetland environment.
Fabrication and installation
We partnered with local fabricator Cotswold Steel to produce stainless steel pass units to our drawings. Cotswold Steel manufactured the passes to tight tolerances then our workshop team fitted the eel brush beds and pre‑drilled the anchor points ready for fittings capable of withstanding abrasion and corrosion. On site, the units were set into prepared recesses in the sluice structures enabling them to be fully integrated with the sluice control system. Final checks confirmed free passage and robust anchorage against tidal surges and inevitable flooding events.
Outcomes
Operational success
After commissioning, the system held water as designed, provided straightforward level control for rangers, reduced backflow nutrient exchange and provided the capability to retain catchment water through drier periods. Eel passes were fully functional and fabricated to a high standard by one of Keystone’s trusted suppliers, Cotswold Steel.
Stakeholder benefit
The RSPB ranger team gained a resilient, low‑maintenance water management system that supports the overarching objective of raising the condition status of the wetland SSSI.