It’s been a few years, but the Town of Slave Lake has flooded. More recently, in both 2018 and 2019, the Hamlet of Marten Beach flooded.
In order to better plan for future floods, Alberta Environment and Parks (AEP) did a flood study on Sawridge Creek. It is still in draft form with a call out for public feedback.
There are 21 flood studies being finalized across Alberta, says the Alberta government’s website. Four started in 2020, including one focused on Marten Beach.
The Marten Beach study notice says, it started in Oct. 2020 and is expected to finish in spring 2022. It looks at two km of Marten Creek including where it flows through Marten Beach.
Regarding the Slave Lake study, one of the documents that the AEP seeks feedback on is a draft of ‘Slave Lake Flood Hazard Study: Study Summary Report’ written by Golder Associates.
“The study is conducted under the provincial Flood Hazard Identification Program (FHIP),” says the draft summary, “the goals of which include enhancement of public safety and reduction of future flood damages through the identification of flood hazards.”
“Flood studies are based on river conditions, floodplain topography, and flood mitigation infrastructure present at the time a study is conducted,” says the Alberta government’s information sheet on the Slave Lake flood study.
The last government study of this creek was done in 1993, says the draft summary. Once the study is finalized, it will replace the old one.
The Slave Lake study focuses on Sawridge Creek which flows through Slave Lake, the M.D. of Lesser Slave River, and Sawridge First Nation. This includes the Slave Lake flood control diversion canal.
Sawridge Creek headwaters are in the Grizzly Ridge Wildland in the hills southwest of town. It meanders through southeast Slave Lake and ends up in the Lesser Slave River in the second bend downstream from the weir.
“Flooding along Sawridge Creek typically occurs because of high river flows in spring or summer,” the Slave Lake information sheet says. “We are aware that there may be other sources of flooding in the area, including groundwater flooding or flooding caused by local drainage issues as well as debris or log jams, but the focus of this study is on riverine flooding caused by high river flows.”
The draft summary includes a brief history of Sawridge Creek floods.
In 1961, the major Sawridge Creek flood didn’t overflow the banks, says the draft study. However, others flooded the communities in the floodplain. The 1988 flood “was one of the largest floods, most destructive floods ever recorded in the Town of Slave Lake.” It damaged 800 homes and businesses, with over 2,500 evacuated.
The summary adds Sawridge Creek had major floods in 1961, 1968, 1983, 1988, 1996, 2004, 2011, and 2018.
The 1935 flood, which caused the relocation of Slave Lake, isn’t included, as this was of Lesser Slave Lake and Lesser Slave River (See below).
Hundred plus year flood
One of the terms thrown around when speaking about floods is a one-in-100-year flood. This has a specific scientific meaning.
A general flood study information sheet says, “a 1:100 flood is a flood that has a 1 per cent chance of occurring each year. The terms 1:100 flood, 100-year flood, and 1 per cent annual exceedance probability flood are all different ways of describing the same flood.”
The hydrology assessment within the flood study looked at various possible flood types in a range from 1:2 to 1:1,000.
Merriam-Webster Dictionary defines hydrology as “a science dealing with the properties, distribution, and circulation of water on and below the earth’s surface and in the atmosphere.”
“The diversion canal from the Sawridge Creek was constructed in 1971,” says the draft study. “Construction of the improved diversion canal was completed in 1992/1993, and its operation commenced in June 1993.”
The diversion canal is the deep ditch between Highway 2 and the ‘bear trails’ section of the Allarie Trails. It is crossed by Highway 88. After Hwy. 88, it turns and continues northeast. The canal is 3.1 km long, says the draft summary.
The study looked at 13.3 km of the Sawridge Creek and 8.2 km of Lesser Slave River, starting directly downstream of the weir. The study also looked at nine bridges. Of these bridges, eight were over the creek: two highway bridges, the 6th Ave. bridge, two pedestrian bridges, a railway bridge, and a private bridge. Also, one is over the canal.
There are three flood control structures on the creek, says the draft summary. Also, one on the diversion canal. These are berms.
People who walk the Allarie Trails would be familiar with these.
For example, one is “adjacent to the Slave Lake Healthcare Centre on the right floodplain.”
Technical expertise is not required to give feedback, says the Slave Lake study information sheet. The deadline is January 15, 2021. The draft study and feedback form are on alberta.ca.
This stage of engagement focuses on flood inundation maps, says the Slave Lake flood study engagement page on the Alberta government’s website. “The draft reports and flood maps have gone through several stages of review already, but we welcome your feedback regarding factual errors or omissions. We are particularly interested in feedback on hydraulic modeling and flood mapping components of the draft study, as well as the flood inundation maps themselves.”