Draft plans released for decommissioning, restoration of Coats Marsh weir

Rachelle Stein-Wotten

Local Journalism Initiative Reporter, Gabriola Sounder

Gabriolans can view the preliminary Coats Marsh weir decommissioning design and ask questions about the project at an information session on Jan. 20.

Regional District of Nanaimo parks staff will host the open house at the Gabriola Arts and Heritage Centre on South Road from 11 a.m. to 3 p.m. Design drawings will be on display.

In December, a preliminary plan was drafted by Northwest Hydraulic Consultants Ltd. to decommission the concrete weir in Coats Marsh Regional Park. With cracks and other deterioration discovered, earlier this year the RDN and The Nature Trust of BC (TNTBC), who co-own the property, opted to remove the weir, preserve the beaver dam and re-naturalize the marshland rather than build a new structure.

The design will be refined prior to tender and following the conclusion of the public information period.

The draft design involves reducing the weir’s height by two-thirds; a 1.2 metre-high portion would be retained with the centre notch filled with new concrete. That structure would control the grade and block the drainage channel that was historically used to drain the marsh, the plan says. Riprap and boulders would be placed upstream and downstream of it.

The berm adjacent to the weir, which has “several deficiencies of concern for its long-term integrity,” according to the plan, would be removed as would sediment and beaver dam debris accumulated on the upstream side of the weir. The existing footbridge, Clemson pond leveller and wood flashboards would also be dismantled.

A portion of the outlet channel would be widened to mitigate activity from the resident beaver and improve flood conveyance capacity should the beaver’s dam located 50 metres away from the current weir fail. The plan says the RDN is undertaking a risk assessment to identify possible outcomes of a beaver dam failure. The RDN and TNTBC would be liable for any downstream flooding from a dam failure, the plan says.

Decommissioning is planned to take place in late summer, during the driest conditions of the year, to minimize impacts to wildlife in the marsh, including the beaver, native frogs and waterfowl, according to the RDN. During construction, water will be diverted around the work site to keep the area dry and preserve downstream habitat, the plan says.

Registered professional biologists, EDI Environmental Dynamics Inc., have created a preliminary restoration and planting plan to restore the 1,188 m2 of existing marsh area that will be impacted “as a slightly drier wetland feature, with more seasonal variation,” EDI’s plan says. “The restoration plan will ensure a similar habitat area of emergent vegetation is available after decommissioning to ensure similar availability of habitat for northern red-legged frogs,” which are listed in the federal species at risk registry. Salvaging and relocating amphibians prior to construction is proposed.

The site would be monitored during construction, including for water quality, as well as for up to five years after for aspects including survival rate of replanting and control of invasive plant species, which are already present in the marsh.

“The majority of the existing reservoir area will not be impacted by decommissioning of the dam due to the presence of the beaver dam,” the decommissioning plan says. Most excavated soil will be re-used in higher portions of the existing weir pool.

“Given that the seasonality and amount of precipitation can vary substantially from year to year, it will be difficult to assess how [weir] removal affects water retention and seasonal water levels upstream of the beaver dam for several years after implementation of dam decommissioning,” the plan says.

Before construction can happen, the RDN and TNTBC will need authorization from the provincial dam safety authority as well as through the provincial Water Sustainability Act. Fisheries and Oceans Canada will also need to review the proposal.

The preliminary decommissioning and restoration plans are available to view at www.getinvolved.rdn.ca/coats-marsh-weir.

RDN staff said those who cannot attend the Jan. 20 open house can also submit questions on that website.