Green Shores Case Study
Rathtrevor Beach Provincial Park Seawall Removal and Shoreline Restoration Project Rathtrevor Beach Provincial Park is located 3 km south of Parksville on Hwy 19a, on central Vancouver Island. The closest communities are Parksville and Qualicum Beach to the west and Lantzville and Nanaimo to the west (see Figure 1 - Location Map).
Figure 1: Location Map (NHC)
Established in 1967, this very popular park contains old growth forest adjoining a 2-km long sandy beach. The park is 374 ha in size and includes a large campground, extensive picnic areas and over 5 km of trails. As BC Parks states on its website: “Majestic old growth trees, beautiful ocean sunsets and an inviting, wide sandy beach with campsites nestled amongst mature Douglas fir trees makes Rathtrevor Beach Provincial Park one of the premier vacation destinations on Vancouver Island…. At low tide the ocean recedes almost a kilometer, providing an ideal place for visitors of all ages to play in the sand and explore the shoreline. When the tide rolls in over the sun-baked sand, the warmed water is unbeatable for swimming and salt water fun.”
Shoreline Issue A reinforced concrete seawall, about 1-1.5m high and 300 m long, was built 45 years ago along the southeast shoreline of the park (Figure 1). By 2015, the seawall was deteriorating and becoming a public safety hazard. BC Parks staff in the Ministry of Environment realized that seawalls often cause negative impacts, including loss of beach sand and vegetation, increased erosion of the surrounding area, and reduction of 1 Rathtrevor Beach Provincial Park Seawall Removal
Green Shores Case Study recreation opportunities. So, rather than rebuild the seawall, BC Parks decided to take a soft shore approach using large woody debris (LWD) and beach nourishment. For BC Parks, the benefits of this approach included: • • • •
A shoreline that is naturally resilient to erosion shorelines and better able to absorb wave energy. Increased public access to the beach – the seawall acted as a barrier for park users. Improved wildlife habitat and biodiversity. Beautification of the shoreline.
Design Solution The seawall removal and shoreline restoration plan was designed by Northwest Hydraulic Consultants (NHC) (Figures 2 and 3). Construction was carried out in Fall 2016 by Parksville Heavy Equipment (PHE). Major stages of the project were: • • • •
Preparing for construction and removing the seawall. Installing 17 LWD structures along the length of the original seawall. Laying a sand-gravel mix around and seaward of the LWD structures. Planting native vegetation behind the LWD structures.
Figure 2: Plan view of Rathtrevor Beach seawall removal (NHC)
Figure 3: Rathtrevor Beach seawall removal schematic (NHC)
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Green Shores Case Study Construction Preparation and Seawall Removal One of the parking lots close to the foreshore was blocked off to the public so that it could be used as a dedicated staging area. A single access route was provided to the foreshore work area. The construction zone was limited to a right of way extending seaward from a line about 3 m inland of the top of the seawall. Public safety and convenience was a primary consideration. The project was occurring in a heavily used foreshore area and there was going to be park users around the construction site at all times. PHE delineated the active work area with safety fencing, and signs were placed around the site explaining the project to the public (Figure 4). No trees were damaged and vegetation that had to be removed was lifted, preserved and replaced. Existing woody debris that had to be moved was stockpiled and replaced behind the LWD structures after the seawall was removed. Figure 4: Sign explaining the project PHE was also responsible for removing the concrete seawall with minimal surface disturbance and transporting it offsite to a disposal site satisfactory to BC Parks. The old concrete wall was crushed in the staging area parking lot and hauled away. The metal rebar went to a metal recycler and the ground concrete will be recycled into road surfacing material and other uses. LWD Structures The 17 LWD structures consisted of Douglas fir or Western red cedar logs cabled to ballast rock oriented with the long axes parallel to the shore (Figure 5 and 6). Each structure consisted of one 10-m log and two 7-m logs with root wads (minimum 0.75 m diameter) cabled to a series of 14 quarry stones (minimum 1 m diameter and 1200 kg each). The structures were embedded so that the logs were at least halfway into gravel/sand.
Figure 5: LWD structure detail (NHC)
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Green Shores Case Study
Figure 6: LWD structures waiting for installation
Sand/Gravel Nourishment The sand and gravel mix consisted of rounded rock of graded diameter (0.1 to 150 mm) similar to the existing beach material. Sand/gravel was added to meet the original height of the seawall. To do this, sand/gravel was added in increasing amounts from south to north along the shoreline, at slopes ranging from 1:5 in the south to 1:10 in the north end of the foreshore (Figure 7). An estimated 8000 cubic meters was required for the project.
Figure 7: Selected cross-sections from south and north ends of the project; see Figure 2 or 3 for Station references (NHC)
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Green Shores Case Study Native Planting Dunegrass (Elymus mollis) was planted upland of the LWD structures to provide short term stabilization and beach aesthetic (Figure 8). Silver burweed (Ambrosia chamissonis) seeds were harvested on site in late summer and early September, to be reseeded once the project is completed (this plant does not relocate well). Split rail fencing work will be started in the spring and be completed in the fall of 2018, along with replanting of dune grasses.
Accessibility An existing concrete wheelchair ramp at the north end of the beach was eroding at the toe where the ramp met the beach. The same beach nourishment gravel/sand mix used for the seawall removal was placed around the end of the ramp (Figure 9). This beach nourishment will be monitored to determine if more work is required on the wheel chair access.
Figure 8: Dunegrass planted upland of LWD structures
Monitoring Monitoring of the beach nourishment and revegetation will continue until 2019 when it is hoped that the vegetation will be established and the beach has stabilized. Figure 9: Beach material placement at end of wheelchair ramp
Acknowledgements Green Shores and the Stewardship Centre for BC thanks Drew Chapman, Rik Simmons, Ashley Sloat and Kirby Villeneuve of the BC Ministry of Environment for providing the background information, drawings and photos of this project and for reviewing this article.
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Green Shores Case Study
Photo Documentation: Pre-(original condition) and Post-Construction See Figure 2 or 3 for station references. All photos are courtesy of BC Ministry of Environment.
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