AppendixA - CivicWeb

Appendix A

Landslide Assessment ReportStittle Road Landslide Event

Prepared for:Ryan Thoms

Manager of Emergency ServicesPowell River Regional District

Prepared by:Marion Blank, P. Geo.

Planterra EnvironmentalPowell River BC

December 19, 2014

Landslide Assessment Report Stittle Road Landslide Event

1.0 Introduction Ryan Thoms, Manager of Emergency Services, Powell River Regional District requested that Marion Blank, P.Geo. of Planterra Environmental Consulting complete an immediate assessment of private properties at 9449 and 9459 Stittle Road, south of Powell River, where a landslide had occurred on December 10, 2014 (Sketch Map 1 Stittle Road). The purpose for the assessment was to determine if the property owners who had been evacuated could return to their homes. After the initial call out to determine if the citizen could return to there homes, the Regional District requested a detailed landslide assessment report be completed and submitted to their office. 1.1 Background On December 10, 2014 a landslide occurred at around 2 pm on private property at 9949 Skittle Road. The Powell River Regional District received a call shortly after the event. Ryan Thoms, Manager of Emergency Services for Powell River Regional District, and Clint Monson, Area Manager Ministry of Transportation and Infrastructure, arrived on site with only about a hour of day light left to assess the extent of the damage and risk to the residents. The Regional and Ministry managers recognized the need to retain a qualified professional to complete an assessment of the slide area. It was too late in the day to arrange for this type of assessment so a decision was made to evacuate the residence of 9449 and 9459 Stittle Road to ensure their safety. The rainfall estimates for the time leading up to this event were approximately 120 mm and it was continuing to rain through the night, so the safety of the people were given the highest priority. On the eve of December 10, Ryan Thoms contacted Marion Blank, P.Geo. to request a slide/risk assessment be completed to determine if the residents under an evacuation order could return to their homes. On December 11, Marion Blank, P.Geo. accompanied by Ryan Thoms (PRRD) arrived at the site around 9:30 am. At this time an initial assessment was completed and it was determined that the residents could return to their homes. On December 12, Marion Blank, P.Geo. wrote a letter to Ryan Thoms, outlining some immediate considerations and steps that could be taken to reduce the potential of further erosional events (see attached letter, dated December 12, 2014). On December 13, a landslide occurred above Atrevida Road, the slide blocked the road and shifted one of the homes off its foundation. Ministry of Transportation completed an assessment of this slide.


2.0 Detailed Slide Assessment On December 12, Marion Blank, P.Geo. of Planterra Environmental revisited the slide sited at Stittle Road to gather additional information needed to complete a detailed slide assessment report. Site information collected during this field review included geologic, topographic, surficial drainage, and vegetation. Meteorological statistics were obtained from http://climate.weather.gc.ca. 2.1 Location The landslide initiated within private property with a street address 9449 Stittle Road, Powell River Regional District (Sketch Map 2, Landslide at 9449 Stittle Road). It was estimated that approximately 1,100 cubic meters of material was displaced covering approximately 350 square metres of the slope (Photo 1). The material travelled downslope for a distance of about 65 metres and deposited material on the adjacent property to the north and the marine shoreline below (Photo2).

Photo 1 Slide Area Photo 2 Deposition Zone

2.2 Geologic Information Geologic information was gathered from the area surrounding the slide zone. Reave Road drainage to the end of Stittle Road drainage corridor(Sketch Map 2). The surficial materials underlying the area were identified as well as any present and past slope moments in the adjacent areas to the initiation zone of the present slide.


2.2.1 Surficial Geology Surficial geology was determined from observations made of exposed material in the slide zone. The material was identified as glaciofluvial with possible marine influences, characterized by fine-grained sands with a minor silt component. (Photo 3). This material type extends from the Stittle Road drainage to the Reave Road drainage corridor.

Photo 3 glaciofluvial sands

2.2.1 Present Slope Movement A sunken concave feature that extends from properties 9449 to 9441 was identified as a headwall scarp of a failure zone (see illustration below). This feature indicates the entire slope is moving downward. There is visual evidence the entire slope has dropped an estimated 0.5m. (Sketch Map 2, white symbol at top of slope traces the approximate location of the headscarp, (Photo 4).

Illustration Diagram of present slope dynamics


Photo 4 Headscarp area of an active slope failure zone

Active slope processes were also noted on the slopes within the Reave Road drainage corridor outfall, evidenced by a group of alders that were up-rooted and washed down the slope (Photo 5).

Photos 5 Active erosion Reave Road drainage

2.2.2 Past Slope Movement On the slopes facing property 9433 an old slide track was located, the slide appears to have occurred over 30 years ago (from personal comments from the property owner the slide had occurred prior to her buying the property in the mid 1980s). This old slide track is denoted on Sketch Map 2 with a white arrow with a large S. The area below the old


slide track is completely overgrown with vegetation and was not assessed for this report. Minor erosional processes were noted in the old headscarp area and the property owners appeared to have built some small retaining structure to retain this material but there was evidence of active downslope movement. At the mid-slope just above the developed guesthouse, the property owner appears to have built a pond, this structure was holding water and it appears that its purpose was to collect/hold drainage that is migrating through the slope above. 2.3 Topographic The slopes have a rounded appearance with lobate bulges, irregular-shaped mounds and hummocks. These types of topographic feature are often associated with fine textured overburden materials. The slope gradient was estimated to be approximately 50% and would be considered moderate under most terrain rating criteria. 2.4 Surficial drainage 2.4.1 Road Ditches/Right of Ways There were two major drainage corridors in the area both associated with road development, Reave Road ditch/culvert and Stittle Road ditch/culvert. These drainages are shown on Sketch Map 2 . The drainage corridors are shown in dashed blue line and full ditches by solid blue lines. Surface drainage facilities such as ditches and culverts are constructed to move water from one area to another to prevent damage to public infrastructure like roads and also to protect private property. Often these facilities are not well maintained or constructed to a standard that allows the surface drainage to be carried down slope to an area or structure that can deal with the flow to prevent erosion. If these facilities do not function well, they can actually cause large erosional events during intense weather either by overflowing due to capacity/ adverse grades and/or blockage or from directing drainage onto highly erodible materials. This entire ditch system above the slide zone was full of water during the initial site visit on December 11, 2014 (Photos 6 and 7). This provided evidence the ditches were probably overflowing during the more intense rainfall period. The property owner at 9449 Stittle Road stated that during the most intense rainfall the ditches were overflowing and the water was running down his neighbour’s driveway, disappearing into the ground at some point before the slope break.


Photo 6 Stittle Road Ditch Photo 7 Stittle Road ditch blockage 2.4.2 Private Property Drainage Management Private property owners often build drainage systems that may not perform well during significant rain events. Private domestic drainage systems need to be designed for the site conditions. Well-designed drainage systems such as perimeter drains and proper placement of septic systems can certainly help to reduce or prevent damaging erosional event. It is important to insure that these systems are functioning in a way that allow for drainage to be removed from their building site without impacting the slope stability further. These systems were not inspected during this assessment but mentioned because these systems can influence long-term slope stability.


2.4.3 Seepage Zones between soil horizon/sand There were several shallow drainage/seepage zones noted at an elevation of approximately 29m near the top of the slope where the soil horizon and glaciofluvial sands interface (Photo 8).

Photo 8 Seepage zone at Landslide headscarp

There were several seepage zones were noted at around 22-24 meters at mid-slope. There were numerous water wells located at different elevations on the slope from mid slope to near the toe of the slope. A pond was also noted mid-slope on property at 9433 Stittle Road. 2.5 Vegetation The area directly above the slide scarp was well-maintained lawn, the adjacent slopes appeared to be covered by shrubs of both native and invasive species. Wet-site indicator plants such as horsetail and devil’s club were abundant on the lower slopes both north and south of the slide area.


2.6 Meteorological Conditions Information regarding rainfall and intensity of rainfall was gathered from Environment Canada Site and Goat Island Weather Station (Western Forest Products). The estimated rainfall for the three days prior to the slide event was 120-140 mm. The average monthly December rainfalls for the past 30 years have been approximately 140 mm, therefore over the days leading up to the slide the area received as much rain as the three days as the normal average for the month of December. This meteorological event was classified as a “Pineapple Express”, a non-technical term for a meteorological phenomenon characterized by a strong and persistent flow of atmospheric moisture from the waters adjacent to the Hawaiian Island and associated with heavy precipitation. 3.0 Discussion The evidence suggests that the area in and around the present landslide is inherently unstable. The presence of active downward slope movement is dynamically displayed in the landscape and the evidence of mass movement of the slope in one block is quite evident. The evidence of past landslides and the ongoing active erosional processes indicate that there will be more landslide/slumps in an around the immediate area. The most recent landslide, though somewhat dramatic, was a relatively small event considering the large headscarp mapped out during this field assessment (Photo 4).

Although there is a larger stability issue than the present landslide, there is still a need to present some clear information of how this present landslide was triggered. The landslide was triggered by a series of contributing factors that relate to drainage, intensive rainfall and type of surficial materials identified in the slopes of this area. Glaciofluvial sands are considered non-cohesive material and in the presence of water can become unstable regardless of the slope profile. Any development in and around this type of overburden material can create temporary perched water table greatly increasing the probability of failure of such slopes . During the time leading up to the landslide there was an estimated 120 mm of rain that fell in the area creating saturated conditions. There were road ditches overflowing, drainage migrating through established seepage zones, water from the roof drains and septic fields migrating down slope. As well, there was an established zone of weakness / failure surface associated with a larger slope failure block where there was evidence that the entire slope had moved downward. It appears that the combination of overflow drainage and seepage in a particular location along the larger failure zone triggered the slide event.


4.0 Recommendations Public Notice It is recommended that the property owners in the area identified as a failure block retain the services of a Professional Civil/Geotechnical Engineer with slope rehabilitation experience immediately so that a remedial plan can be put into place to reduce further erosion and damage to their properties. They can find engineering services on line or contact the Association of Professional Engineers. It is important at this time to advise the property owners that this is a zone of ongoing slope movement and that any activities below the slopes should be limited especially during this high rainfall season. People living below these slopes should consider relocation until detailed assessments are completed. Planning The regional districts in the province of British Columbia are responsible for zoning and establishing development permit areas. Many of the regional districts in the province have not established any zoning due to the lack of capacity and/or resources. It has been an ongoing issue within the province. As more development occurs in unorganized territory the lack of zoning and designated development permits areas will increase the likelihood of more adverse impacts to private property owners and put the government agencies responsible for land management and zoning at risk of increased cost of mitigation and/or litigation. The recent landslide events in the area have shown that there is a need to develop criteria to assist the regional district in situations involving landslides and their associated impacts. To establish criteria the region should complete a regional hazard study that identifies high-risk terrain and drainages. The Ministry of Transportation and Infrastructure is the lead agency in subdivision approval outside established municipalities. Under the subdivision procedure high risk terrain is identified and a geotechnical assessment is a requirement for approval of subdivisions. It is recommended that all information regarding geotechnical assessments and associated covenants related to past and present development be shared between the Ministry and the Region. Operational The Ministry of Transportation and Infrastructure, Ministry of Forest, Forest Companies (Western Forest Product Limited) all have some established criteria regarding working in and around high-risk terrain. There are also established monitoring levels and procedures. A review of these already established systems could assist in developing criteria for the regional district staff. Relevant information collected by various agencies should be integrated to develop hazard alerts. This could be used to develop a hazard alert call out protocol between the land-based agencies that have a role in managing emergencies.


Weather station data Rainfall and other data is somewhat difficult to obtain in real time, the local Ministry of Forest office and forest Industry has online data stations. These station are not publicly available but it might be of interest to the region to have access to this information. It would be useful to develop an information sharing protocol with Western Forest Product Ltd. who have access to weather / rainfall data on a hourly basis. Access to this data could provide timely information to assess risk/emergency situation. Highway Maintenance The Ministry of transportation infrastructure under the road maintenance agreements have developed procedure for monitoring drainage along the highway right of ways during high volume rain events, the criteria for this is found in the provincial road maintenance agreement. Under these maintenance contracts, highway drainages are also maintained and upgraded. These contracts have been developed to ensure that highways maintenance is completed to an established standard. Under these contracts there are provisions for routine and annual maintenance, and some emergency projects with approval from the ministry. The local Area Manager for the Ministry plays a role in the administration of these contracts, this manager oversee the quality assurance program developed by the ministry to monitor the performance of the maintenance contractor. The right of way road drainage along Stittle and Reave Road should be reviewed and any upgrades should be completed as soon as possible to prevent the ditches in the area from over flowing onto the private properties. The outfall areas of these drainages should be assessed when the weather improves and all necessary measures should be taken to reduce the erosional impact at the outfall by dissipating the erosional force of the water. A protocol agreement should be established between the Ministry of Transportation and Infrastructure and the Regional District on at a local level, this agreement should reflect the needs of the operational and planning staffs and should be developed in such a way to allow easy communication and sharing of information between the various agencies staff. Local Expertise /Education There are several key people in the local community that have many years of experience with dealing with terrain management. It is recommended that these people be contacted and asked to assist by providing their knowledge in someway to assist with the ongoing management of these issues on community level. It is important that property owners take responsibility for site planning on their properties and that they understand the impacts that improper drainage management has on their properties as well as others properties adjacent or down slope. There are many ways this could be carried out. The most effective method could be a series of


workshops that directly involved local experts from agencies such as the Ministry of Transportation and Highways, the City, and the District. Marion Blank, P. Geo. Planterra Environmental Consulting 5464 Laburnum Ave. Powell River BC V8A 4M8

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AppendixA - CivicWeb

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