No Squamish resident wants to think about her house or business being damaged or destroyed by rising water, but it’s a fact that much of Squamish is located on a floodplain.
The district hosted a second public open house, attended by about 40 concerned residents, at the Squamish Adventure Centre on Feb. 24 regarding flood mitigation plans for the Squamish, Mamquam, Cheakamus and Stawamus rivers.
The plans outline steps to reduce the probability and consequences of a severe flood. To reduce the probability of flood, plans call for increasing dike heights in some places, addressing dike deficiencies in others, build berms and instilling erosion protection, among other measures.
To reduce consequences, avoiding development in high-risk areas, establishing sufficient flood construction levels, directing growth to low-risk areas and developing an emergency response are recommended.
The area of most concern is within the Squamish/Mamquam floodplain including Brackendale and western portions of Garibaldi Estates, David Roulston, municipal engineer, said at the Feb. 16 committee of the whole council meeting.
In considering flood risks, district staff used a one-in-200-year flood event as a marker, according to Roulston. The total estimated economic loss due to a flood in the Squamish/Mamquam floodplain would be approximately $450 million, according to a report submitted to council.
“The further west you go, the flood depth and the hazard gets worse,” Roulston said, adding the financial estimates only include the costs of buildings and contents, not indirect costs such as downtime for businesses and the like.
Brackendale west of Cottonwood Road is at risk of a flood that could see water rising over three metres, Roulston warned.
In the northern, upper floodplain – north of the Mamquam River – up to 7,000 people could be displaced, he said. In the lower Squamish/Mamquam floodplain –south of the Mamquam River – up to 3,400 people could be displaced.
“This is a very bad situation,” Roulston said. “All of our grocery stores, gas stations, our emergency operations centre, are all located within the floodplain.”
Sewers, roads and the wastewater treatment plant could all be damaged or destroyed and recovery could take years, Roulston said, pointing to what happened in High River, Alberta, which is still recovering from a severe flood that occurred three years ago.
A district staff report recommends mitigation strategies for the Squamish/Mamquam floodplain that include improving dikes, raising buildings as they develop and limiting densification in the highest-hazard areas.
The district has already spent $4 million since 2012 on diking, according to Roulston, but that is a “drop in the bucket” compared to what is required to reduce flood risk in this area. He estimates fortifying diking in this area would total in the tens of millions of dollars. A $40-million “super dike” that is higher and wider than provincial standards is recommended for along the Squamish River. This plan would take decades to complete and should be started sooner rather than later, Roulston said.
In the Judd Slough and Eagle Run area, land tenure issues make deficiencies in diking hard to correct, Roulston said. He recommended a specific master plan for the eagle viewing area.
“This is going to be a very challenging area and it is going to require time to fix,” he said.
Mitigation of the downtown has unique challenges. According to the report submitted to council, the proposed construction of a sea dike around downtown to deal with sea level rise could create a “bathtub” effect and potentially fill the downtown with floodwater over 2.5 metres. However, by planning to intentionally breach the future sea dike to relieve floodwaters as an emergency response measure the potential floodwater depth would be reduced, according to the report. Therefore, mitigation for downtown would not need to include limiting density, Roulston said.
It is also recommended to raise all new development to one-in-200 year flood construction levels.
The floodplain mitigation strategy recommends growth in several places: Garibaldi Highlands area around Quest University and the areas south of Mamquam and east of Brennan Park.
With mitigation, lower-risk areas such as downtown, Valleycliffe and Dentville can also accommodate densification, Roulston said.
The river flood mitigation strategy is the third phase of the overarching Integrated Flood Hazard Management Plan (IFHMP).
The full cost of proposed mitigation programs will be summarized in the final phase of the IFHMP, district staff stated.
The first phase was doing a gap analysis; the second phase was developing a coastal flood mitigation strategy.
The fourth phase will be developing an implementation plan that encompasses all components.