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Landslide hazards exist in mountainous areas throughout the world. Wherever there are steep slopes, there is a risk of material falling or sliding down the slopes. Heavy rains can often cause instability as soils become overly saturated. As climate change increases the intensity and frequency of rainstorms in the Cowichan region, the CVRD has been studying what this means to the region’s existing natural hazards including (including flooding and landslides). The steep slopes above Cowichan Lake are prone to landslides, so the CVRD has been investigating these hazards.
There are many types of landslides, varying by the amount of water mixed with the sliding material. In the Youbou area, debris slides, debris flows, and debris floods are all potential hazards. Click here for more information on types of landslides.
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It is the responsibility of local governments to identify hazards affecting or with potential to affect the community and to use that information to inform and guide land use decisions. Building inspectors must also ensure that all buildings under permit are safe for their intended use.
The CVRD has completed an initial study into landslide risk in the Youbou area. This study identified significant risks and led to follow-up studies.
Initial Study – based on existing air photos and provincial base mapping accurate at 1:15,000 scale
Ebbwater / Palmer (2019) analyzed historical air photos and provincial geological and topographical mapping to establish the landslide hazard on the slopes above Cowichan Lake from Cottonwood Creek to Hill 60. The landscape was divided into large polygons, each with hazard score based on the likelihood of a landslide impacting that polygon. This mapping is accurate at the 1:15,000 scale, but the hazard varies within each polygon; generally the hazard is greater closer to the base of the slope and within gullies. The mapping serves as a flag that further investigation at the site level may be required to determine the actual risk to an individual property. In cases where properties were found to be within the high hazard areas, notification letters were sent to the property owners. A public meeting was held in May, 2019 to share the results of the risk assessment with the community.
Follow-up: Debris Flow Modelling – based on new specially flown high-resolution data accurate at 1:5,000 scale
Palmer / Stantec (2020) conducted refined debris flow runout modelling for much of the same study area as the 2019 study. This project involved the acquisition of high-resolution LiDAR for the study area. Stantec then modelled over 68,000 hypothetical landslides and established a line beyond which no landslides travelled. This mapping is accurate at 1:5,000 scale and identifies areas where the hazard from a debris flow is very low. Previously-notified property owners will receive an update about the new mapping, and the results are being shared with the community.
Follow-up: Rock Slope Deformations – LiDAR and field investigations by CVRD & Mosaic
The initial study identified several rock slope deformation features on the slopes above Cowichan Lake. Both the CVRD and Mosaic Forests (the landowner) conducted additional investigations into these features and found that they were stable and presented an extremely low risk.
Based on the type of hazard and the level of risk identified in the first study, the CVRD committed to the community that they would try to do two things: 1) undertake further investigations to refine the information within the polygons; and, 2) try to reduce the impact on property owners having to do additional engineering work prior to development. The second study now provides this information at a resolution of 1:5,000 and does not require property owners to do landscape level hazard assessments beyond their property boundaries with regards to landslides.
No, the results of the two studies are consistent. The 2020 study is an update and refinement of a hazard identified in the 2019 study. The CVRD was able to obtain high-resolution LiDAR for the study area allowed for a firmer definition of hazard areas. The 2019 Ebbwater / Palmer study was based on provincial TRIM mapping (visually inferred 10 meter contours) and historical air photos. It identified the hazard at a 1:15,000 scale, dividing the landscape into large areas with a hazard score based on the likelihood of a landslide impacting that polygon. Within each hazard area, the hazard varies. The 2020 Palmer / Stantec study is based on the new LiDAR which allows mapping at a much finer scale than was previously possible (digital 10 cm accuracy). The modelling of debris flow runout enables us to map a line beyond which no modelled landslides travelled.
A comparison of the results shows the 2020 runout limit cuts across various 2019 hazard polygons. For example, a high hazard polygon was identified in the area of Creekside Drive with an Annual Encounter Probability equating to 1:3,000 years. It is understood that within this polygon the hazard varies, generally increasing closer to the base of the steep slopes and within gullies. Further geotechnical investigation is required to show where, within the polygon, the higher hazard areas are located and what the actual risk is at the site level. What the 2020 results do, is a first step in this further geotechnical investigation. The runout limit runs parallel to the slope, bisecting the hazard polygon. It defines parts of the polygon (below the line) in which the hazard is below the 1:10,000 year threshold. In the rest of the polygon, there may be a hazard due to landslides – in these areas, further geotechnical investigation would be required to define the risk.
At the individual property level, the results of this study are good news to many. The message to many residents from the 2019 study was that they may be living in a landslide hazard area and that additional, site-level geotechnical assessments may be required to determine the risk at the property level. By better defining the potentially hazardous areas at a higher resolution, the 2020 study effectively shrinks the hazard polygons, in many cases meaning that part or all of a property is no longer considered to be within a hazard zone. In cases where hazards still exist on a property, the 2020 mapping can provide additional information about the likelihood and extent of potential damage.
The likelihood of a hazard occurring in any year, expressed as a percentage. For example, a hazard with a 1% chance of occurring in a given year has an AEP of 0.01. This AEP is also commonly expressed as a 1 in 100 year chance of occurring.
If you already live in this area we will require you to do a further assessment if you are considering any additional work on your home or property to ensure that the proposed works do not increase the level of risk. If you are planning a new development we will want to ensure that it is done safely and risks are mitigated.
We don’t know what the vision you have for your property is, so additional site-based assessments would take this into consideration and help you ensure that whatever you are planning meets the safety requirements. The point is to make sure that you and future residents of the property know the risks and take them into consideration in development.
The level of risk identified in the 2019 study meant that the CVRD was obligated to notify property owners and the community as a whole about the risks from landslides in the Youbou area. The CVRD hosted a public meeting in Youbou in May, 2019, and had several follow-up meetings with local residents. In response to various hazards identified across the region, the CVRD adopted a Board approved Natural Hazard Risk Tolerance Policy to ensure development in hazardous areas is done in ways to mitigate the risks to acceptable levels.
We are informing property owners in areas where there is risk as well as informing property managers in adjacent areas so that the risks to the communities can be kept as low as possible. This means addressing either the hazard or the likelihood that an event will affect people. The CVRD continues to build out its regional mapping program and assessments of the multiple hazards our communities face.
Climate change projections for the Cowichan Valley include increases to both the amount and intensity of rainfall in the coming decades. Heavy rainfall can be a trigger of landslides, so there is a potential that climate change and/or human activities could increase the frequency of landslides in the future. The answer to how this affects the hazard identified is different above and below the extent of modeled runout. This line shows the furthest downslope any of the modelled landslides travelled regardless of whether the originating trigger was natural, climate induced or development based.
Your building permit or development permit process will help us identify this potential and ensure that that risk is mitigated.