The Science behind the development of the Cowichan Estuary Restoration Project

Considering this is the largest estuary restoration project to occur on Vancouver Island, The Nature Trust of BC and partners in the West Coast Conservation Land Management Program (ECCC, DFO, Province of BC, DUC, HCTF) and Cowichan Tribes along with supporting consultants (Northwest Hydraulic Consultants Ltd., SmartShores, MC Wright and Associates, Thurber Engineering Ltd, Kerr Wood Leidal Associates Ltd., Millennia Research Ltd., SFU Salmon Watersheds Lab, Biologica Environmental Sciences) want to provide everyone with an inside look at the science involved in developing, implementing and monitoring a project of this scale.

Background

Filling, draining and diking for conversion to other uses has resulted in 70% loss of the original wetlands in the Fraser River delta and 60% of estuarine marsh habitat in the Salish Sea. Given that estuaries make up only 2.3% of BC’s coast, it is essential we conserve and restore these important ecosystems whenever possible.

Monitoring data collected over the last six years have indicated that the Cowichan Estuary will not be resilient to sea-level rise and will ‘drown’ or suffer from ‘coastal squeeze’ if this project is not undertaken. If we do not proceed with the restoration project, approximately 60% of the marsh habitat in the estuary will be lost under current scenarios by 2100 and will greatly impact fish, wildlife and the local community. The Cowichan Estuary Restoration Project will build in resilience for these crucial estuarine ecosystems.

To find out more about the background of the project visit here. We also have a video series entitled “The Living Estuary” which provides an in-depth look at the importance of estuaries to Pacific salmon, shorebirds and waterfowl, bears, harbour seals, river otters, benthic Invertebrates, crabs, Lyngbye’s sedge and sandhill cranes.  

What professionals have been involved in this project?

To date, this project has involved Professional Biologists (RP Bio)(RB Tech), Engineers (P. Eng), Geoscientists (P. Geo) and Applied Science Technologists (ASct) all of which are governed by the Professional Governance Act 2021. The project is also working with Registered Professional Consulting Archaeologists (RCPA) as well as trained archaeological monitors and knowledge keepers from Cowichan Tribes. We are supported by academic researchers within Canada and the US with expertise on salmon, estuary restoration, climate change, carbon sequestration and resilience. The management team for the project has extensive experience in estuary restoration and monitoring projects and is also guided by a management committee that includes representatives from ECCC-Canadian Wildlife Service, Fisheries and Oceans Canada, Province of BC, Ducks Unlimited Canada, The Nature Trust of BC and the Habitat Conservation Trust Foundation.

What methodology was used to collect the data to determine marsh resilience?

This link will take you to a detailed presentation of the science and methodology used to assess estuary resilience not only at the Cowichan Estuary but at 15 other sites throughout British Columbia. Additional information regarding the development of the Marsh Resilience to Sea-Level Rise (MARS) tool can be found here. The detailed scientific article explaining the MARS tool can be found by following this link to the scientific journal Biological Conservation. We have also incorporated recommendations and methodologies in the detailed scientific review of “Understanding tidal marsh trajectories: evaluation of multiple indicators of marsh persistence”.

What is the relative sea-level rise calculation for the Cowichan Estuary? How was it calculated?

Our project worked with Smart Shores to assess relative sea-level rise for all of our 15 monitoring sites. 

Overview

The land surface is constantly moving. The elevation of a given plot of land or point on the earth’s surface is influenced by short-term, intermediate and long-term geological processes. These affect the function and appearance of ecosystems, as well as estimations of sea-level rise.

Short-term processes include sediment erosion and accretion. These phenomena occur at the earth’s surface, at very fine spatial scales, are often observable with the naked eye and can be easily measured. For example, we used sediment cores and radiometric dating to determine rates of long-term sediment accretion,  surface elevation tables (SET) readings, marker horizon plots, and survey equipment are all used to quantify short-term processes.

Intermediate and long-term processes are equally important in estimating relative sea-level rise. These processes – referred to as land velocity – include vertical and horizontal shifts in bedrock influenced by changes in ice mass and the movement of tectonic plates. Land velocity can lead to functional ecosystem change over intermediate (decadal) and long-term time scales (millennia) by exposing land to, or removing it from, the reach of the sea. These processes are imperceptible to the naked eye and are much more difficult to measure, requiring years of data from precise instruments over a large geographic range. For this reason, the baseline land velocities for this project were extracted from the NAD83v70VG National Crustal Velocity Model.

Methodology

Rates of mean sea-level rise are based on the range presented in the International Panel on Climate Change (IPCC) 5th Assessment Report. RCP8.5 was selected because it represents the upper bound of the set of likely emissions scenarios and is broadly aligned with recent global emissions. Given that sea-level rise will continue beyond the project horizon of 2100, an increase in sea-level beyond the RCP scenarios is possible due to either a partial collapse of the west Antarctic ice sheet, an acceleration in melting permafrost or a megathrust earthquake may cause sudden subsidence, this upper bound of expected sea-level rise is a justifiable scenario. 

The rate of relative sea-level rise at each site was assessed by comparing local land velocities using the new Natural Resources Canada (NRCan) NAD83v70VG National Crustal Velocity Model rasters to adjust RCP8.5 sea-level rise estimates relative to the land movement. To account for the uncertainty associated with projecting sea-level rise 80 years in the future we identified potential future sea-levels for three scenarios and three planning horizons. The three scenarios are the median, low and upper confidence intervals (90% confidence range) of the RCP8.5 pathway. The three panning horizons are relative sea-level mid-century (circa 2050), at the year 2100, and at the year 2100 including the partial collapse of the west Antarctic ice sheet. For this assessment, the contribution to sea-level from the collapse of this ice sheet was set at 65 cm.

All sea-level rise estimates are relative to a baseline time. For the purposes of this project, our baseline is the year 2020. Therefore, for the purpose of this document, total sea-level rise at all sites in the year 2020 is 0.0 m. All projections presented here are for changes relative to the 2020 baseline.

How did you determine that 60% of the marsh vegetation would be lost?

Utilizing the relative sea-level rise estimates along with ecosystem mapping work completed by MC Wright and Associates, Smart Shores developed a model to assess the relative changes in Estuary Marsh and Estuary Meadow distribution in response to rising seas. 

Based on the monitoring information and reports, what are the results for the Cowichan?

Sea-level Rise

• Relative sea-level rise was estimated to be 28 cm by 2050 (9 mm/year) and 78 cm by 2100 (9.5 mm/year 2050-2100). 
• In a scenario combined with Antarctic ice sheet collapse – 141 cm by 2100.

Sediment Accretion

• Short-term sediment accretion measured at 4.1 mm/year. – half the rate of relative sea-level rise
• Long-term sediment accretion (radiometric dating) measured at 1.1 mm/year. – 8 times slower than relative sea-level rise

Estuary Vegetation Change

• The habitable intertidal zone for marsh vegetation will decrease at a rate of 0.6% per year from now to 2050, increasing to 1% per year from 2050 to 2100.
• High marsh vegetation will decrease by approximately 47% by 2050, being either drowned out or replaced with low marsh vegetation (which will expand by ~1% from its current extent).
• By 2100, high marsh vegetation will decrease by approximately 92%, being either drowned out or replaced with low marsh vegetation.
• By 2100, the overall estuary marsh area will decrease in area by 59% without intervention.

What is the MARS resilience score for the Cowichan Estuary?

MARS resilience scores were calculated using our monitoring data and the open-source data analysis platform provided by the National Estuary Research Reserve. Following designated criteria in the analysis, scores are assigned 1-5, where 1 represents the lowest resilience to sea-level rise and 5 is the highest.

RISK: Is overall resilience score and identifies estuaries that are at risk

AVERAGE: Overall average across the five resilience metric categories

RATIO: Scores <1 indicate that marshes are not gaining elevation at rates commensurate with sea-level rise

The MARS resilience scores are summarized below; the Cowichan estuary is at risk.

How did you determine what restoration projects to do using this information?

In 2021 our project team worked with Kerr Wood Leidal on an initial Estuary Resilience Restoration Scoping study that undertook an initial review of all of the monitoring sites to identify high-level opportunities for restoration. A key component of this included the development of an Evaluation Matrix to assess potential projects based on several project categories including: Resilience, Certainty of Success, Land Status, Project Risk, Liability Concerns, Constraints, Timing, Capital Cost, Cost per Hectare, Operations and Maintenance Effort, and Regulatory Effort. These categories were provided with a score and then reviewed by experts (biologists, engineers) for final assessment/adjustment.

This initial assessment identified 4 high-priority projects for the Cowichan Estuary (Dinsdale dike removal, Koksilah remnant berm removal, hydraulic reconnections and sediment augmentation).

The monitoring data collected and the scores produced for each site (identified above) were compared to the KWL report to validate the identification of the projects and the impact they would have on enhancing estuary resilience. For the Cowichan, the resilience scores re-confirmed the need to implement restoration activities.

With that validation about the importance of the projects, what happened next?

With the identification of the projects and the validating information, the project then proceeded to conduct detailed analysis and assessments of the component projects. This work not only included detailed physical process studies but we also began work with archaeologists and developed a detailed biological monitoring plan to assess baseline biological indicators pre/post restoration.

The physical processes study and feasibility assessments

In order for the project to advance from the conceptual stage we hired Northwest Hydraulic Consultants Ltd. (NHC) to undertake detailed studies of the potential projects. This was a requirement not only to determine project feasibility but to also meet the requirements for the “Decommissioning of a Regulated Dike” under the Dike Maintenance Act (DMA).

NHC completed a report entitled: “Cowichan Estuary Marsh Restoration Project: Assessment of Physical Processes and Restoration Opportunities”.  This assessment involved 14 experts in their field ranging from geomorphologists, coastal engineers, hydrologists, hydraulic modellers, hydrometric monitors, river engineers, diking specialists, GIS specialists and involved biologists from the Nature Trust and Cowichan Tribes.

The work included the completion of an assessment of the Physical Setting of the estuary and restoration area and included: Anthropogenic History;  Existing Land Use and Infrastructure; Geological Setting; Hydrology (watershed overview, hydrometric data, seasonal flow regime, flood hydrology, distribution of freshwater flows to estuary); Hydraulic Conditions (hydraulic model development; simulations of flood events); Ocean and Coastal Setting (tides, storm surge, sea-level rise, winds, wind-waves); Geomorphology (channel characteristics, sediment input and mobility, sediment composition, accretion rates); and Ecological Context. 

In addition to the assessment of the Physical Setting an assessment of the Existing Level of Protection in the estuary was also completed and included inferred flood and erosion protection along with hydraulic and wave modelling. From this background work, feasibility assessments were completed for the Koksilah Marsh old agricultural berm removals, sediment augmentation and finally the removal of Dinsdale Dike. Each of these feasibility assessments included specific hydraulic and wave models for each project component. A part of the assessment of the project components also identified any mitigation works to make sure adjacent properties are still provided the same level of protection or better as a result of this project. Mitigation works identified for this project include constructing a setback dike structure on the old farm access road, leaving small spurs at the Koksilah Marsh and raising Lochmanetz Road.

This report concluded that not only were the projects feasible in terms of enhancing estuary resilience but that they would improve the flood conditions in the project area and surrounding land through the increased conveyance of riverine flood waters. 

The figure below provides an overview of some of the modelling results with the full Dinsdale dike removal and the complete removal of the Koksilah marsh agricultural berms (note: Phase 1 has been completed and the berms were not completely removed as some spurs were left in place).

The third image shows the change with the project being completed in terms of flood depths. Green represents a positive change in flood depths in that the depth of the water is reduced. Gray indicates no change and red indicates an increase in water. It is important to note that the flood depths are reduced for the majority of the properties in the area or there is no change from current conditions.

From feasibility assessments to implementation

With the detailed physical processes/feasibility assessments completed and with approval from the project partners, we moved to a preliminary design phase. This design phase was led by NHC with support from Thurber Engineering Ltd., and focused on the preliminary design of each of the project components (design criteria/design basis). This report was subsequently submitted as part of the permitting process under the DMA. This design work focused on geotechnical investigations, soil sampling and refinement of project components to meet the criteria/basis.

In addition to the preliminary design basis our project team also worked on meeting the requirements of the decommissioning guidelines in terms of transference of flood risk and identified all properties/infrastructure that was afforded protection by Dinsdale Dike and completed assessments of these properties. 

With the reports completed and following an extensive review from the Inspector of Dikes we received a permit under the DMA to proceed with the works.

Archaeology

This site has significant cultural importance to Cowichan Tribes. As such we have been working with Cowichan Tribes cultural monitors and two archaeological consultants on this project. To date, we have completed an Archaeological Overview Assessment, Preliminary Field Reconnaissance, onsite monitoring of geotechnical works, and restoration works at the Koksilah Marsh. Millennia Research Ltd. is leading the archaeological work and ensures our project is proceeding with the proper permits under the Heritage Conservation Act. 

Baseline environmental monitoring – channel geometry, carbon sequestration, fish, birds, invertebrates, water quality, invasive species

To determine the success of any restoration project in terms of ecosystem response, it is important to establish strong metrics to determine the effectiveness of restoration actions. Equally important is to collect data prior to the restoration action and to continue collecting the data in the future. For the Cowichan River Estuary Restoration project, a detailed Effectiveness Monitoring Plan was developed. The monitoring includes: hydrology and water quality, carbon sequestration rates (analysis and assessment being completed by Western Washington University and UBC), fish and fish habitat (being undertaken by the SFU Salmon Watersheds Lab), vegetation survivorship and invasive species and birds (migratory, breeding and shorebirds), and marsh elevation change. This is in addition to ongoing engineering assessments that will be completed as part of the work.

How do we know this project will benefit fish?

Salmon are an iconic species in British Columbia and are an indicator species for ecological health. Recognizing the importance of this, our project has collaborated with Dr. Jonathan Moore and several graduate students of the Salmon Watersheds Lab at Simon Fraser University who have extensive research and field experience working in British Columbia estuaries. Dr. Moore’s team has worked alongside biologists from the Pacific Salmon Foundation, BC Conservation Foundation and Cowichan Tribes in conducting extensive surveys in the Cowichan Estuary assessing juvenile salmonid abundance, habitat preference and growth rates. 

The following is an excerpt from a scientific article published by Harris, Gottesfeld and Moore (2015).

 “Estuaries provide juvenile salmon with habitats where feeding and growth opportunities are relatively high [1,2] and predation pressure is relatively low. For example, Chinook salmon (O. tshawytscha) fry grew over 5% a day in the Nanaimo River, BC [1] and restored Puyallup, WA [12] estuaries. Estuary-rearing steelhead (O. mykiss) grew more rapidly in a seasonally-closed tidal lagoon and exhibited less size-selective mortality than their counterparts that reared in freshwater and went directly to sea [13]. Active feeding and growth in estuaries has been observed even in salmon populations that migrate rapidly seaward [11]. Estuaries can provide cover to juvenile salmonids from predators due to higher turbidity [14], estuarine vegetation, such as seagrass and algae beds [15], and riparian vegetation [16], and rates of predation on juvenile salmon may be lower in estuaries than other habitats. For instance, juvenile Chinook salmon released at estuarine sites were exposed to fewer fish and avian predators than those released to marine sites near Campbell River, BC [17]. Furthermore, while juvenile salmonids were an important food item for common mergansers (Mergus merganser) in freshwater habitats they were rarely consumed by mergansers in estuaries [18]. Given the potential importance of estuary food webs and habitats to juvenile salmon, it is perhaps not surprising that juvenile salmon survival rates have been found to be higher in estuaries with less degraded habitat [19].”

“Large-scale restoration projects such as this take an immense amount of work and investment, but have proven to be game-changers for thriving ecosystems. Large-scale restoration is needed to get ahead of climate change. Time and time again, science has found that restoring lost connections in ecosystems works. Salmon find new habitats, and flourish. Estuary marsh habitats provide important nursery habitat for young salmon. These nursery habitats provide opportunities for young salmon to grow quickly and thus boost the productivity of salmon populations for rebuilding their abundance and associated fisheries.” Dr Jonathan Moore on the Cowichan Estuary Restoration Project (2023)

Our project is committed to continuing this collaborative work with the Salmon Watersheds Lab and has incorporated this work into our extensive baseline monitoring and post-restoration monitoring moving forward.

For an in-depth look at Dr. Moore’s ongoing work please visit this link for a recording of a presentation he provided to the Climate Adaptation for Salmon Workshop.

How do we know that this project will not impact birds that currently use the farm land?

We have been monitoring migratory bird use on Dinsdale Farm and the surrounding natural estuarine marsh areas since 1992/93. Data have been collected during peak winter waterfowl periods during 1992-1997, 2001, 2002, 2015-2019 and 2022/23. Data collected from these surveys is utilized to determine bird use days in the area. 

Bird-use days (BUD), a statistic that represents one bird present in an area for one day, can be used to estimate the number of bird-use days for each species or species group (Monty, Merilees, and Dawe 2000). BUD is calculated by taking the average count from two consecutive surveys and multiplying by the difference in days between the surveys. This is then summed to produce the BUD for the season (Aagaard, Lyons, and Thogmartin 2017). BUD can then be used to determine the density/ha of an area to show habitat preferences and trends based on land use and other environmental variables (e.g. condition of land).

The data collected shows that the BUD for swans, Canada geese and waterfowl have significantly declined since the late 1990s.

From 2015-2023, our monitoring data has shown that there is higher species diversity and abundance in terms of waterfowl, shorebirds, herons and raptors utilizing the estuarine marsh and intertidal habitat than on the farm fields. By restoring natural estuarine habitat over time we anticipate a greater diversity and abundance of birds will utilize the restored area, which includes the majority of priority waterfowl species identified by the Pacific Birds Habitat Joint Venture.

To learn about the importance of estuaries to birds a great summary resource is found on eBird Northwest (Estuaries).

What is still on-going?

With this scientific framework in place, we have successfully completed the first Phase of the restoration works. Please visit the project update page for more details. Detailed design work for the next phase of the project is on-going and is focused on the final design of the mitigation works. All monitoring activities will continue throughout the project.