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THE NORTH ISLAND SLIDES OF 2006 Headlines in November 2006 shouted about the power outages, flooding and road closures on Vancouver Island stemming from a cluster of 'November Gales' blowing through the region. "The latest winter storm hammered B.C.'s South Coast Wednesday, with extensive ferry cancellations, road closures and massive power outages." In the northern half of the island, access via road to the north island was cut off by two mudslides that closed a large section of Highway 19 from the Sayward Road Junction to just south of Woss. This is the only highway access to the north island communities of Port Hardy and Port McNeill, and other smaller communities. Travellers were caught away from home when the slides occured, and had to 'make due' for several days for conditions to stabilize, and while crews worked to clear a path for vehicles to pass. As frequent travellers to the north island, we luckily were 'caught' at home vs away from home. Many others were not. Over the next 2 years, significant construction occurred to modify conditions at both slide areas,so that there would not be a re-occurence of this disruption to this only north island route. This cache focusses on the larger of the landslides and repaired areas. WHAT IS A LANDSLIDE? Landslide is a general term used to describe the down-slope movement of soil, rock and organic materials under the influence of gravity. It also describes the landform that results. British Columbia's steep, mountainous terrain, its complex geology, its high precipitation, both as rain and snow, its abundance of unconsolidated (loose sediment that has not been cemented or otherwise converted to solid rock) glacial sediments (sediments which form as a result of the actions of ice or glaciers), and its geographic position astride the earthquake zone that surrounds the Pacific Ocean, all combine to make our province particularly susceptible to landslide activity. In fact, in British Columbia the loss of life and damage to property caused by landslides is greater than losses caused by other natural hazards such as earthquakes and flooding. However, by understanding the answers to the following questions, we may be able to lessen the effects of landslides. WHAT CAUSES LANDSLIDES? Some slopes are susceptible to landslides whereas others are more stable. Many factors contribute to the instability of slopes, but the main controlling factors are the nature of the underlying bedrock and soil, the configuration of the slope, the geometry of the slope, and ground-water conditions. Three distinct physical events occur during a landslide: the initial slope failure, the subsequent transport, and the final deposition of the slide materials. Landslides can be triggered by gradual processes such as weathering, or by external mechanisms including: - Undercutting of a slope by stream erosion, wave action, glaciers, or human activity such as road building, - Intense or prolonged rainfall, rapid snowmelt, or sharp fluctuations in ground-water levels, • Shocks or vibrations caused by earthquakes or construction activity, - Loading on upper slopes, or - A combination of these and other factors. Once a landslide is triggered, material is transported by various mechanisms including sliding, flowing and falling. Landslides often occur along planes of weakness that may parallel the hill slope. In bedrock, planes of weakness are usually beds, joints or fractures. Soils such as silt and clay are weaker than rock and commonly have complex or multiple planes of weakness. Types of weakness planes and associated landslides: (1) slope failure in glacial sediment resulting in slumps; (2) parallel bedding in rock causing slides; and (3) fracturing of rock promoting falls. EFFECTS OF HUMAN ACTIVITY Landslides may result directly or indirectly from the activities of people. Slope failures can be triggered by construction activity that undercuts or overloads dangerous slopes, or that redirects the flow of surface or ground-water, for example: - Poor road construction - Poorly planned forest clearing may increase rates of surface water run-off or ground-water infiltration. Inefficient irrigation or sewage effluent disposal practices may result in increased ground-water pressures, which in turn can reduce the stability of rock and sediment. - People increase the risk of landslides by modifying the landscape, for example, by building on unstable slopes or in the path of potential landslides. Unfortunately, many people are unaware of their exposure to landslide risks. As our cities, towns, roads and highways steadily encroach onto steeper slopes and mountainsides, landslide hazards become an increasingly serious threat to life and property. WHAT ARE THE DIFFERENT TYPES OF LANDSLIDES? Landslides or slope movements can be classified in many ways. There are many attributes used as criteria for identification and classification including: - Rate of movement: This ranges from very slow creep (millimetres/year) to extremely rapid (metres/second). - Type of material: Landslides are composed of bedrock, unconsolidated sediment and/or organic debris. - Nature of movement: The moving debris can slide, slump, flow or fall. Common landslide types are: - Slide: movement parallel to planes of weakness and occasionally parallel to slope. - Creep: gradual movement of slope materials - Slump: complex movement of materials on a slope; includes rotational slump. - Topple: the end-over-end motion of rock down a slope. - Fall: material free falls. - Flow: viscous to fluid-like motion of debris. - Torrent::a sporadic and sudden channelized discharge of water and debris. WHERE DO LANDSLIDES OCCUR? Some areas of British Columbia are more susceptible to landslides than others because of their unique geological conditions: - The sedimentary rocks of the Skeena Mountains are the most prone to large slumps and shallow slides in the province. - Debris flows and torrents often occur in the humid west coast of Vancouver Island and the Queen Charlotte Islands within the Insular Mountains. • Dormant volcanic centres in the Coast Mountains (e.g., Mt. Garibaldi) are susceptible to rock and debris avalanches and flows. - Thick clay and silt-rich glacial sediments in the Northeastern Plateau are prone to soil creep and are frequently undercut by rivers, which results in slumps. - The steeply dipping beds of sedimentary rocks in the Rocky Mountains are prone to rock slides, topples, falls and avalanches. TO LOG THIS CACHE To log this cache, please email us with your answers to the following questions (do not place answers in your logs, or they will have to be deleted): 1. From your observations of the site, and the information above, what do you believe were the likely causes of the North Island landslides? 2. What materials were used in the reconstruction, and why were these materials utilized? 3. What human activity may have increased the risk of landslide in this area?
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