Using CFD and 3D salinity modelling to support the Nicomekl River Sea Dam replacement while protecting river ecology

 

The City of Surrey relies on the Nicomekl River Sea Dam to protect low‑lying agricultural lands from tidal flooding and saltwater intrusion. Built more than 100 years ago, the structure no longer meets modern structural, seismic or flood protection standards, particularly as sea levels continue to rise. To address this, the City began planning for a replacement water control structure capable of managing river flows, tides, storm surge and salinity intrusion under future climate conditions, while also improving ecological connectivity and meeting regulatory requirements. DHI supported this work through advanced Computational Fluid Dynamics (CFD) and 3D salinity modelling. The modelling provided the City with an evidence-based understanding of hydraulics, salinity intrusion and scour performance, supporting informed decision-making for a climate-resilient replacement of the Nicomekl River Sea Dam.

 

Challenge

The challenges related to the existing sea dam were driven by both naturally occurring and climate‑related factors, as well as infrastructure limitations. Tidal forcings and storm surge are naturally occurring, while rising sea levels are increasing flood risk over time. In addition, the ageing structure was not designed for current hydraulic, seismic or climate conditions, and did not include modern pumping strategies or optimal accommodations for fish passage.

If left unaddressed, these challenges were expected to result in an increased risk of:

 

• Flooding of agricultural and residential lands, threatening crops and livelihoods
• Saltwater intrusion into freshwater systems used for irrigation
• System‑wide disruption during seismic events, given the dam’s critical role in the drainage network
• Degraded river connectivity, limiting salmonid access upstream and negatively affecting the aquatic ecosystem

The City recognised that managing these risks would require a comprehensive understanding of hydraulic, salinity and operational performance to inform replacement planning.

 

Solution

The City of Surrey’s Coastal Flood Adaptation Strategy (CFAS) aims to reduce vulnerability to coastal flooding and sea‑level rise. As part of this programme, the Nicomekl Water Control Structure (WCS) project involves construction of a new structure upstream and adjacent to the existing facility to improve seismic resilience, enhance fish passage and strengthen flood protection for surrounding agricultural areas under future climate conditions.

Together with a multidisciplinary project team, DHI was appointed by the City of Surrey to deliver detailed design of the Nicomekl River Sea Dam replacement. DHI supported the project through advanced numerical modelling, including Computational Fluid Dynamics (CFD) and 3D salinity simulations.

The CFD modelling assessed hydrodynamic conditions and scour potential across multiple structural configurations and pump build-out scenarios during high-flow events. Using MIKE 3, a 3D salinity model of the lower Nicomekl River was developed to evaluate the effects of proposed fish passage designs on salinity levels upstream of the WCS.

Findings were reviewed with the client and design team to inform the need for scour protection, assess hazards associated with high flow velocities near the dam, and provide key input to the overall WCS design. They also supported refinements to fish passage design and WCS operations to reduce upstream saltwater intrusion.