Once removed – decommissioning Finlayson dam

C. Richard Donnelly, Larry King and Mike Phillips discuss the removal of Canada’s Finlayson dam

Figure 1

THE removal of the Finlayson dam from the Big East river represented the first documented case of a dam in Canada that had been removed following a planned and structured process. The goal of the project was the restoration of the Big East river to its natural state in order to enhance the brook trout fishery, while ensuring that accumulated sediments within the former reservoir were contained.

In the US, dam removal (or decommissioning) has become a relatively common and accepted alternative for dealing with ageing dams, with over 500 documented cases of dam removal to date. In many cases, real benefits to society and to the environment occur following a dam removal. However, as reported by Donnelly et. al. (2001) and Zhou and Donnelly (2004), removing a dam from a river is not always an appropriate solution. Indeed, there have been significant unexpected problems that have resulted from inadequately planned and executed removal projects. As well as this, there is insufficient data available in the literature to support (or refute) the notion that dam removal actually enhances the health of a river system (Stanley, 1999).

Dam removal in canada

Numerous dams have been removed in Canada. However, these removals have generally been implemented without a structured assessment process, often following a dam failure event. For this reason, most of the projects are not documented and any benefits (or detrimental effects) are not known.

The Finlayson dam was a 5m high concrete gravity control dam situated on the Big East river, west of Algonquin Park (Figure 1), that was originally built to serve the interests of the logging industry in north central Ontario. The dam was never intended to fulfil functions associated with flood control, hydroelectric power generation, water supply or recreation. As such, with the decline of logging in the region, the Finlayson dam appeared to have ceased to serve any useful purpose. It was for this reason that, in 1999, this dam was selected as a possible candidate for decommissioning by the Ontario Ministry of Natural Resources (the Ministry).

The decommissioning process in ontario

This process is described by Donnelly et al. (2000). In 1999, it consisted of a draft set of guidelines intended to allow alternatives to be evaluated in a rational manner and minimise the potential for future potential liabilities resulting from inappropriate decisions on decommissioning. At the time of the removal of the Finlayson dam, these guidelines had never been tested. However, as shown in Table 1, they were in general agreement with the 1997 American Society of Civil Engineers (ASCE) guidelines that had been used in the US with some success. For this reason, the Ministry had confidence that the use of the draft Ontario guidelines would result in an appropriate decision on the final disposition of the Finlayson dam.

As described by Donnelly et al. (2000) the merits of each of these options were evaluated accounting for technical, economic and environmental issues. This included a review of the potential impacts on land, water, fish, wildlife, biophysical processes, social concerns and cultural concerns involving an environmental checklist format that examined over 60 different environmental indicators. For each indicator, where a potential environmental impact was identified, the significance of the impact was qualified based on a high, medium or low rating which were then converted to a numeric value to obtain an overall ‘score’ to facilitate comparison. This approach was found to be quite useful in providing an objective overview of the merits of each option with each of the factors given equal weighting in the assessment. The results could then be viewed in a more subjective way to determine if any one of the issues provided a compelling enough argument in favour of a particular alternative without losing the clarity and transparency of the basic data being used to make the decision.

An important part of the assessment process involved the performance of field studies undertaken to define the present condition of the reservoir and allow for an extrapolation of what might happen if the dam were to be removed as described as follows.

The Finlayson dam had created a shallow, bowl-shaped reservoir approximately 3.2km long and up to 300m wide at its greatest extent, with a surface area of approximately 39ha with average water depths less than 3m. In 1997, the reservoir had been lowered approximately 1.2m in response to concerns over operator safety and dam integrity. This resulted in the exposure of previously flooded shoreline, which was in various stages of natural regeneration allowing for a good assessment of the potential for self-regeneration of the shoreline following dam removal.

Field investigations showed that the fish community within the Finlayson reservoir consisted of a variety of warm-water species: yellow perch, pumpkinseed, brown bullhead, white sucker, and six different minnow species. However, these species were considered to be of low quality and there were no upper-level predatory species (such as bass or pike). In contrast, most of the remainder of the Big East river and tributaries were populated with one main cold-water species (brook trout), a species that local sportsmen groups indicated was the most sought after fish in the general area.

As described by Donnelly et al. (2001) methods to mitigate the release of accumulated sediments represent the single most important dam removal issue. When inadequate, or no mitigation measures were implemented in earlier US attempts, some significant adverse downstream effects occurred for many years after the removal. At the Finlayson site, sediment sampling revealed that a considerable thickness of fine sediments (predominantly fine sand, silt and woody debris) had accumulated in the upper section of the reservoir. However, downstream of a natural constriction in the river, there was generally no significant sediment accumulation. At the dam itself, virtually no sediments were found.

Testing according to the Ontario Ministry of Environment Sediment Quality Guidelines (1993) indicated that concentrations of a number of metals in certain samples fell within the lowest effect level (LEL) concentration range. However, the measured levels were in accordance with background concentrations reported for Great Lakes sediments and Ontario surface soils (Table 2).

Highest concentrations were found within the central ‘deposition zone’ portion of the reservoir. As no potential sources of industrial contamination were present within the watershed, the elevated sediment concentrations were considered to be naturally occurring. However, uncontrolled release of the accumulated sediments could have significant negative effects on the downstream fish populations for many years. Therefore, the original plans for removal of the Finlayson dam included an allowance for removal of the potentially affected sediments.

Extensive consultation with the public, government agencies and non-governmental organisations (NGOs) was performed, something that is enshrined in both the Ontario and ASCE guidelines. In the case of the Finlayson dam, delaying public consultation until step 4 of the process (as opposed to Stage 1 in the ASCE guidelines) proved to be quite beneficial. At this stage, the scientific and base data had been gathered and assessed, allowing the merits of each of the options to be laid out in a clear and concise manner. Public input then concentrated on areas that may not have been completely accounted for, providing more focused and useful input than what may have been obtained had the input been solicited earlier in the process. In fact, the consultation process in this project led to a decision to retain another dam on the Big East river that had also been thought to be a candidate for removal (Donnelly et al., 2000).

The results of these consultations, combined with the detailed environmental impact assessments indicated that complete removal of the Finlayson dam was the preferred solution, provided that issues associated with sediment release and habitat restoration could be dealt with. It had the advantage that the river system would be returned to a cold water environment conducive to brook trout, eliminated safety concerns associated with the existing dam and eliminated future operations and maintenance costs. Although it was not originally evaluated to be the cheapest option in terms of direct cost (reference Table 3), it was deemed to provide the greatest overall benefit when intangible issues such as the restoration of the natural river habitat were accounted for.

Mitigation measures

Silt mitigation

As detailed in Table 3, the costs associated with mitigating the potential for post removal sediment transport accounted for up to 42% of the total cost. For this reason, studies were undertaken to attempt to predict the potential for excessive sediment transport following dam removal. The studies commenced with a review of historic air photos that clearly showed that the natural constriction and ‘pond’ had existed before dam construction. Hydraulic modelling clearly demonstrated that, following removal, this same small pond would remain. It was judged therefore that this pond had in the past and would continue in the future to act as a depositional zone, explaining the absence of the silt at the dam itself. To test this premise, a trial draw down of the reservoir was performed during the spring of 2000 that confirmed the results of the hydrotechnical model. Therefore, it was concluded that intrusive silt mitigation measures, such as dredging or low flow channel stabilisation, were not required. The elimination of this requirement significantly reduced the costs of the project.

Shoreline restoration activities consisted of the installation of live stakes at select areas adjacent to the new river channel, installation of a reinforced, under-seeded straw mat in one area considered to be highly prone to erosion, and the seeding of specific disturbed areas with a native seed mix. The goal of this programme was to stabilise the previously inundated sediment as quickly as possible after reservoir draining, by initiating vegetative growth which would quickly stabilise surface and sub-surface soil conditions (i.e. with root growth and surface vegetation cover).

Fish habitat enhancement

Fish habitat enhancement measures consisted of the installation of 37 sweeper trees (also used for energy dissipation/erosion control at selected areas), eight stump root/fans (large white pine stumps relocated from previously flooded side slopes) and three boulder/log clusters. (Figure 2). The structures were designed to provide short-term habitat for fish along the former low flow channel until such time that natural river processes, such as sediment transport, large woody debris movement and bank regeneration, could take over these habitat functions. It was anticipated that the structures would also collect and retain some of the mobile woody debris within the river system.

Restoration activities were also undertaken at the dam site once dam removal was complete. These included the installation of large boulders (from existing side slopes) over the remnant concrete dam sill, and the stabilisation and seeding of adjacent side slopes.

Removal of the dam

The Finlayson dam removal process was undertaken during the summer of 2000, commencing on 2 July and ending on 15 September. In fact, the actual removal process was completed within about a one-week period with a hoe ram being used to break up the concrete dam (Figure 3). Clean concrete debris was buried at the site. Reinforced concrete and metals were hauled off site to approved disposal areas.

Water quality was monitored throughout the dam demolition and reservoir restoration process to ensure that activities were occurring as planned. Shoreline restoration and fish habitat enhancement measures were initiated concurrently with the dam removal process (i.e. as water levels declined and shorelines became stable) and completed after dam removal was finished.

With the elimination of the requirement for dredging from the project, actual decommissioning costs reduced by almost 50%. However, as detailed in Table 4, the costs were still significant.

Post project monitoring

The post-project environmental monitoring programme was designed to evaluate the following parameters during years one, two and five after construction. The results of the first two years of monitoring are discussed below.

A total of 1300 live stakes were installed at a number of locations throughout the reservoir. Survival after the first year was estimated at only 1%. This overall lack of success may have been due to the very dry summer immediately after dam removal
and/or improper shrub selection (speckled alder was substituted in place of dogwood due to limited supply). A portion of the former reservoir that had been protected by an erosion control blanket (200 m2) was found to be in good condition, although it contained little emergent vegetation. Natural growth was evident but overall the blanket appeared to be inhibiting growth. In the remainder of the area, natural regeneration was well established.

Generally, the fish habitat structures were found to be in good condition and functioning as desired after the first year of use. However up to half had failed or were not functioning as intended by the end of the second season.

Continued down-cutting of the new river channel and transport and redeposition of sediment within the river channel were main factors in the failure of the fish habitat structures during their second year of service.

Fish community sampling was conducted around habitat structures, within tributary creeks and at downstream and upstream control points. Generally, several differences were found between the pre and post removal fish communities. Species such as yellow perch, white sucker and others that had dominated the warm reservoir fish community were not observed or were observed only in small numbers following dam removal. Following removal, the primary fish species collected included brook trout (in a cold-water side channel tributary stream), small mouth bass, common shiner, creek chub, and other cool/cold water species indicative of a return of the area to a more riverine environment. A slight increase in overall species diversity was also observed from year one to year two, with an increase from seven species to nine species, respectively. Finally, total abundance increases from year one (85 individuals) to year two (131 individuals) was observed. Post removal surface water temperatures decreased an average of 2 to 4ºC following dam removal, evidence of a return to a cold water system, which should continue to improve as shoreline vegetation communities develop and provide shade for the river. Of the habitat enhancement measures used, sweeper trees appeared to be the most effective, although some species were also observed at the log/boulder clusters and stump/root fans.

In summary, the monitoring programme showed that the area appears to be evolving into a cool/cold water environment conducive to flowing water species. Brook trout were seen to be returning to the area and small mouth bass, whose access was previously restricted by the dam, had moved upstream into the former reservoir area.

Excellent natural regeneration of the former reservoir bottom lands has occurred throughout the length of the reservoir over the two years since the removal. New grasses, sedges and annuals have provided almost 100% vegetative cover of the previously inundated soils. Changes from year one to year two include appearance of cattails in some of the wetter areas, as well as the development of trembling aspen saplings on more elevated portions of the shoreline.

Summary and conclusions

The removal of the Finlayson dam has proven to be a complete success. The environmental monitoring programme undertaken to assess the success of the project indicates that the predicted results of dam removal have materialised or are materialising. A riverine system has been restored, and the environment is continuing to adapt to the changes brought about by dam removal. Sediment transport has occurred, but evidence indicates that it has remained within acceptable limits. Fish habitat structures are providing the intended habitat features while the river system matures, and forage fish species (i.e. small cyprinids) are utilising the cover provided by those structures. The warm water lacustrine fish community that existed when the dam was in place has been replaced by one more adapted to cooler flowing water. Finally, natural regeneration has stabilised the reservoir sediments, and will continue to progress toward those conditions that occurred prior to reservoir formation. The legacy of this project lies in the fact that it is no longer possible to discern that construction had ever been undertaken at the site; aside from a small remnant of the former structure, nestled in the forest, left behind as a reminder of the role that logging once played in the region.

Author Info:

C. Richard Donnelly, Division Manager – Hydro, Central Canada, P. Eng., Acres International, 4342 Queen Street, P.O. Box 1001, Niagara Falls, Ontario, Canada, L2E 6W1. Email: rdonnelly@acres.com

Larry King, Senior Environmental Scientist, Acres International, 4342 Queen Street, P.O. Box 1001, Niagara Falls, Ontario, Canada, L2E 6W1. Email: lking@acres.com

Mike Philips, Project Manager, Ontario Ministry of Natural Resources, RR2, Highway 11 and High Falls Road, Bracebridge, Ontario, Canada, P1L 1W9. Email: mike.phillips@mnr.gov.on.ca

Tables

Table 2
Table 3
Table 4
Table 1