Mapping Landslide Risk on the North Shore

Stephen Ohno Oscar Zimmerman Kawai Ma

April 11th 2017

Table of Contents Page Abstract 3 Description of Project, Study Area, and Data 4-5 Methodology of Analysis 5-6 Discussion of Results 6-8 Error and Uncertainty 8-9 Further Research and Recommendations 9-10 Works Cited 11 Appendices 12-14

Abstract The District of North Vancouver is situated at the base of the North Shore Mountains where mountainous terrain can give rise to landslides in the backcountry but also in residential areas. Two maps of landslide risk within the district boundary have been constructed using ArcMap. Three classes of risk have been determined from a model of risk based on threshold slopes that have the potential to trigger debris flow, rockslide, and rockfall-type landslides. Geographic analysis about this layer determined 552 residential buildings at risk as well as 24 of those buildings, which lie on weak surficial geology, to be at additional risk. A theoretical 25m buffer was applied around the buildings at risk to simulate an expanded hazard zone to simulate extreme events. Six neighborhoods were identified as being at risk, characterized by an abundance of residential buildings at risk. The slopes that give rise to risk in these neighborhoods have been oversteepened by hillslope, fluvial, glacial or coastal processes. In addition, 97.083km of public trails were determined to be at risk. While the model and subsequent analyses have their limitations, they allow for an understanding of the spatial distribution of landslide risk which can be used for determining sites for further investigation.

Description of Project, Study Area and Data According to the BC Ministry of Energy and Mines, landslides cause more deaths and property damage than other natural hazards like flooding and earthquakes in British Columbia. This project is a natural hazards analysis that attempts to identify areas at risk and populations that have potential to be impacted by these hazards. Analysis will involve determining the intersection of hazard areas with residential buildings and public trails, as well as a buffer analysis to determine an expanded hazard area for severe events. The study area is located on the North Shore of Vancouver, British Columbia and is bound by the District of North Vancouver municipal boundary. The study area extends from the waterfront, which overlooks Burrard Inlet, North into the North Shore Mountains, containing the Capilano and Seymour watersheds, allowing for examination of landslide hazards in industrial and developed (residential) land as well as in the backcountry. The North Shore Mountains range in elevation from sea level to 1500m (Jakob, & Weatherly, 2003). The annual rainfall is extremely variable, and increases in an upslope direction on the North Shore Mountains, ranging from about 1300 mm per year near the ocean to over 4000 mm per year at summit elevations because of a strong orographic effect (Jakob, & Weatherly, 2003). The coastal mountainous terrain is highly gullied and has very steep slopes that are mantled by loose sediment. This includes slopes that have been glacially oversteepened, overlain by shallow (< 2m thick) morainal and colluvial deposits (Jakob, & Weatherly, 2003). Also subject to high rates of rainfall as well as snowmelt, these climatic and geological characteristics of the North Shore Mountains create favourable conditions for landslide, increasing the potential loss of life and property damage in developed and backcountry areas. The data used for this project is sourced from the District of North Vancouver and Natural Resources Canada, as summarized in Figure 1. The data is in GCS North American 1983 and is projected in NAD 1983 UTM Zone 10, or has been converted or reprojected to match these GCS and PCS’s.

Data Source Year

DEM Natural Resources Canada 2015

Surficial geology Natural Resources Canada 2014

District of North Vancouver (DNV) DNV 2006 municipal boundary

Railway DNV 1997

Roads DNV 2003

Buildings DNV 2012 5

Landuse DNV 2009

Public trails DNV 2013 Figure 1: Summary of data used for the project.

Methodology of Analysis The hazard zone was determined upon a single variable of slope. Slope was interpolated from a DEM, clipped to the project area, using the “slope” spatial analyst tool. This was followed by a reclassification in which four classes of risk were determined, including a class of no risk. The breaks for the classification were based on thresholds of slope instability used in a GIS analysis of landslide risk along the Sea to Sky Corridor by Blais-Stevens, et al (2012). The classification combined thresholds determined for different frequencies of debris flow and rockslide/rockfall events (see Figure 2). Before the raster could be converted into a vector file of polygons, values were converted from floating-points to integer-points. Once converted, polygons were merged for simplicity and the area of no risk was removed from the shapefile.

Slope Risk Implication (degrees)

0 – 15 No risk Flow events are uncommon and slide and falls do not occur

15 – 25 Risk Flow, slide, and fall events can occur

25 – 45 High risk Conditions are ideal for flow events and slide and fall events are common

45 – Very high Flow events can occur and conditions are ideal for slide and risk fall events Figure 2: Classification of landslide risk and implications on different types of landslide events. Based on analyses by Blais-Stevens, et al (2012).

Determining weak surficial geology that would decrease slope stability was also based on the analysis by Blais-Stevens, et al (2012). This layer was created by selecting for colluvial and glaciomarine deposits from a layer of surficial geology. This layer supplements the model in identifying areas that may be at additional risk to a landslide event. The use of the model by Blais-Stevens, et al (2012) in this analysis is valid due to similarities in physiography, between the two study sites. The North Shore Mountains and mountains that border the Sea to Sky Corridor are both belong to the Pacific Range – a subrange of the Coast Mountains. Upon determination of the risk levels according to elevation, as well as the areas of weak surficial geology, these layers were added to the base map which contained the District of North Vancouver and its land use distribution. Another layer which contained all buildings in the district was also 6

added, and was further narrowed to residential-only buildings by selecting for buildings with the residential-class attribute only, thereby eliminating commercial and industrial buildings from the map. Another selection by attributes was performed, this time selecting for buildings with the “single family” or “multi family” types, allowing for non-inhabited carport structures to be eliminated from the map. In order to select only houses that were in the landslide risk area, the buildings layer and risk area layer were intersected, which resulted in a new layer with a selection of houses meeting the criteria. To model the possibility of an expanded slope hazard area in case of a more severe landslide, a buffer of 25m, that extends further outwards in all directions, was created around the buildings that fell in the initial hazard area. After a dissolve which joined all the buffered zones together, this produced the large buffer zone which included a higher number of residential buildings that are located in the landslide risk area. The impact to public trails was determined by first isolating the trails in the district by clipping the trails layer to the district’s boundaries layer. Afterwards, clipping the public trails to the landslide hazard area would produce a layer with only the trails that fall inside the risk zone. To determine the area of the project zone, which encompassed the whole District of North Vancouver, each land use parcel from the district land use layer was added together, resulting in a total area of 164.98 square kilometers.

Discussion of Results Upon analysis, it was determined that 552 residences, out of a total of 21,823, lie within an area of landslide risk. Areas that showed an abundance of residences at risk were highlighted in the 6 insert maps. The communities that these areas enclose are described in Figure 3.

Insert Map Description of Community

1 Canyon Heights/Mosquito Creek

2 Upper Lynn

3 Riverside Drive/Seymour River

4 Upper Capilano/Capilano River

5 Deep Cove/Cove Cliff

6 Indian Arm Figure 3: Description of communities at risk of a landslide, determined by an abundance of residences at risk. Communities referenced according to their insert map number in Figure A1.

Physiographic Distribution of Communities at Risk The six identified communities share commonalities in their physiographic setting. Residences in Upper Lynn and Canyon Heights are subject to risk due to their proximity to the foot of the North Shore Mountains. Other residences at risk at Canyon Heights border Mosquito Creek; Proximity to waterways is the second commonality, where residences that lie near the banks to Capilano River and the Seymour River (along Riverside Drive) are also at risk. Specifically, these hazardous slopes are common in more upstream reaches where the river has incised into the channel, producing steep slopes at its margins. The hazardous slopes on the west side of Indian Arm have given rise from similar landscape processes, however, these slopes have been steepened by glacier rather than by river. The steepening at the North-most side of the cove at Deep Cove is likely due to similar processes. Cove Cliff, on the other hand, is a unique case where its slope is convex in profile, a product of coastal erosive processes.

Surficial Geology It was determined that 24 residences lie on an area of weak surficial geology that also lie within a slope hazard area. These residences were determined to be at higher risk than those that fall only within the slope hazard area. While this number may seem low, other surficial material may increase in instability under conditions, such as after prolonged rainfall, increasing the number of especially unstable sites. This number is also not to be overlooked because residences that are in proximity to areas of weak surficial geology have not been identified in the analysis. Such an analysis is of significance because landslide events, especially flow-type movements, have the potential to travel a distance away from the zone of failure, therefore causing damage at this distance. Examining, specifically, the spatial distribution of colluvial deposits has significance in determining areas that have a legacy of landslide events. Colluvium, defined as gravity transported material, is the sedimentary product of landslide events themselves. One area of risk where the abundance of colluvium characterizes the recent history of the landscape is along Riverside Drive. In Inset Map 3, bands of colluvial deposits that transverse the zone of high risk are indicative of past landslides that have involved failure at the top of the hillslope and deposition of debris at the bottom, including in areas zoned as developed. One of these bands marks a landslide that resulted in a death in 2005. The area upslope and downslope from the event is currently zoned as park, likely a rezoning in response to this fatal event. Another example of a noticeable colluvium deposit can be found in Insert Map 2, at the fringe of a high risk area, before a lower slope area which around 10 houses are built on. While at both the Riverside Drive and Upper Lynn sites colluvium appears to be associated with higher grade slopes that may more commonly experience slide or fall-type landslides, it should be noted that flow-type landslides that can especially be initiated under wet conditions can travel across such lower grade slopes. Slide or fall-type landslides may also turn into flow-type landslides if conditions allow for the debris to channelize.

Expanded Hazard Area While only 552 total residents were located in the assessed landslide risk zone, this number was found to increase to 7,352 when the 25 meter buffer was created around buildings in the initial hazard area. This resulting jump is significant and represents a much greater portion of the residences that lie at the margins of slopes. Since slope aspect was not considered in the analysis, residences along the base of slopes as well on lower grade or flat upslope areas are included in this buffer. Residences along this buffered area at the base of slopes are susceptible to extreme events as well as flow-type landslides, as previously mentioned. Buffering for residences at the top of slopes can be valid for gentler slopes as slump-type events, or even creep over time, can cause lateral migration of the hillslope. Similarly, failure near the top of the hillslope can cause instability at regions above, producing consequences such as migration or even induced failure.

Public Trails at Risk In terms of the District of North Vancouver’s public trails network, there are 306,185 meters of walkable trail inside the district boundaries. After clipping the trails to the landslide danger area, 97,083 meters were found to fall inside the risk zone. A large proportion of these potentially impacted trails are located outside of residential areas and further into the park lands to the north of the city, however, some low-lying areas within the urban neighbourhoods contain trails at risk as well. Many of the public trails outside of the city extend into the slopes and mountainous terrain to the north, and roughly follow the paths of either roads or rivers. Both of these features can contribute towards the severity of landslide impacts. The construction of roads can be particularly destabilizing to the structure of hillslopes, and can leave the sides of the mountains more susceptible to failure due to weakened foundations. Rivers can also be concerning with regards to public risk due to the ability of rivers to conduct landslide flows and spill any accumulating landslide debris and water over river banks and onto adjacent trails.

Error and Uncertainty Error and uncertainties in the model of landslide risk and geographic analysis based upon this model limit the uses of the maps produced. Like other map layers that cover areas with strictly formed boundaries, the slope hazard area has an irregular shape across the whole map. When intersected with the buildings layer, it results in some residential buildings becoming partially inside and outside of the hazard area, although ArcMap still considers them, regardless of how much they are inside or outside, as part of the zone. This can potentially lead to an overestimate of the number of affected buildings, since some are even nearly completely outside of the zone, but not quite enough to be removed from the count. The creation of the 25 meter buffer zone is also not the most accurate representation of a potential expansion of the slope hazard area. The original base layer itself is a complex shape and does not 9

follow any kind of recognized form, and naturally an expansion of something as unpredictable as slope hazard should not be uniform across the entire project either. However, the produced buffer creates a generalized expansion of risk area across the project area, which may not be entirely representative of the potential for increased risk in certain places; some locations may have higher potential for expansion of risk while others may have less. In addition, similar to other natural disasters, landslides in general tend to be unpredictable in terms of the direction and course that they take. While it is possible to predict the path of a slide’s impact by considering factors such as slope stability, surficial geology, terrain shapes, vegetation, and ground materials among other things, the possibility that a slide will deviate from a projected path is always present. Our investigation highlighted the risk and potential impact in a situation when certain geographical and geological data are considered, but the circumstances and effects surrounding a landslide are always fluid and can never be predicted perfectly. The model is limited in determining landslide risk in complex topography, which persists especially in the backcountry. The first limitation is the generalization of slope at a point as its maximized rate of change from that point, when interpolating slope from the DEM. Along with the negation of slope aspect, this makes it difficult to determine how debris will move in mountainous terrain where there are depressions, ridges, etc. in the landscape. The exclusion of slope aspect creates uncertainty even in relatively simple topography that can be found in residential neighborhoods. For example, if residences lie uphill and downhill from a slope, without knowing the aspect of the slope, whether they lie upslope or downslope is undeterminable. This may have implications where residences at the bottom of a slope may be more at risk than residences at the top. In another case, if two slope aspects were to be converging, creating a gully, residences at the base of the slope may be at high risk due to terrain that is ideal for a flow-type event. Even the data we acquired from trustworthy sources like Natural Resources Canada and DNV, is still at risk of having errors in source data for GIS. According to the text, “survey data can contain errors due to mistakes made by people operating equipment or recording observations, or due to technical problems with equipment” (Heywood, et al., 2006). For example, errors in attribute data could occur if features like buildings and public trails were recorded incorrectly by the operator.

Further Research and Recommendations Due to the significant difference between the number of residents affected in the initial hazard zone and the theoretical expanded buffer area, it is clear that an increase of even 25 meters to the projected assessment of the hazard can cause significant impact to a great number of citizens. For a municipality like the District of North Vancouver which is bordered by water bodies and slopes, new developments are constrained in terms of the physical land they can expand, which has led to interest in moving development further up along the mountains. This change, along with associated increased traffic and activity levels, has the potential to accelerate the erosion of the slope stability, which can in turn lead to future landslides that will extend beyond any previously predicted impact zones in residential areas at the foot of the mountains. The District would do well to investigate 10

further into the possibilities of landslide impacts that would go beyond the bounds of their previously assessed extents. These impacts may occur due to a variety of factors such as higher than average rainfall, to weakened ground geology as a result of vegetation cutting and construction along the slopes. While our work only extended the reach of the slope hazard by a buffer of 25 meters, an investigation into a potential expansion of 50 to 100 meters would be warranted, as a landslide on such a scale is not out of the question, provided the environmental and geological factors are present to trigger it. Landslides can be triggered by rainfall or other factors. In this project, the map includes information on factors like risk of slopes and surficial geology which trigger landslides, but the map does not include information on landslide triggering events like rainstorms and other factors. Therefore, we would analyze different factors which influencing occurrence of landslides in the North Shore, and include them on the map in the further research. That is, climate factors, soil saturation and soil physical properties are main factors which trigger landslides. We would gather antecedent rainfall and streamflow data from weather stations, associated with the number of high elevation to estimate the susceptibility to landsliding in the future. Also, the Factor of Safety would be included on the map. We would calculate the Factor of Safety by gathering data on several soil parameters such as cohesion and friction angle and using Infinite Slope Equation. Factor of Safety (FS) either calculated by resisting forces/driving forces or:  by the ratio of the frictional stress and the shear stress if a soil has no cohesion of any sort or  the cohesive component is added to the frictional stress if the soil has measurable cohesion FS < 1 means the slope is at risk of failure, a landslides can be expected to occur. FS > 1 means the slope should be stable. Through the Factor of Safety, we would be able to analyze the stability of the slopes and focus on the FS<1 in determining the susceptibility to landsliding in the future.

Works Cited B.C. Ministry of Energy, Mines and Petroleum Resources,. (1993). Landslides in BC. British Columbia. Blais-Stevens, A., Behnia, P., Kremer, M., Page, A., Kung, R., & Bonham-Carter, G. (2012). Landslide susceptibility mapping of the sea to sky transportation corridor, british columbia, canada: Comparison of two methods. Bulletin of Engineering Geology and the Environment, 71(3), 447-466. Heywood, I., Cornelius, S., & Carver, S. (2006). An introduction to geographical information systems (3rd ed.). Pearson Education. Jakob, M., & Weatherly, H. (2003). A hydroclimatic threshold for landslide initiation on the North Shore Mountains of Vancouver, British Columbia. Geomorphology, 54(3-4), 137-156.

Appendices

Figure A1: Map of landslide risk on the North Shore, residential buildings that lie within the hazard zone, and residential buildings that lie within an extended hazard zone, buffered 25m.

Figure A2: Map of landslide risk on the North Shore and public trails that lie within the hazard zone.

Figure A3: Flowchart describing process of geographic analysis.