[MUSIC] Another major hazard in mountains is landslides, the downslope movement of rock and debris. Just like with snow, landslides occur when sheer stress within a slope outweighs the sheer strength of a slope’s rock or sediment layers, ultimately causing the slope to fail. The sheer strength of any given slope is usually increased with vegetation. Alpine vegetation can act as a barrier to slow down slope movement, but can also act as a natural anchor for the soils. Slopes with smooth surfaces, like rock for example, conversely have very low frictional strength, making them slippery. This decreases the sheer strength of a slope. Add a lubricant like water to the mix and the frictional strength of slopes decreases further. Streams and rivers are particularly effective at under cutting slopes, making them steeper and more prone to failure. [SOUND] In addition to natural factors, human activities also influence the likelihood of landslides. Clear cutting forests, for example, the mass removal of vegetation radically decreases slope stability. Other human activities like mining, road construction, and home building may also undercut or overload slopes. There are various different types of landslides. We often classify them based on their material composition, their water content, and how they move down a slope. We're going to distinguish six types of landslides. Rockfalls, topples, translational slides, rotational slides, earth flows and debris flows. Let's start with the first two, rockfalls and topples. They're the most basic type of landslides, occurring when rocks suddenly detach from slopes. Rock falls occur when a rock detaches and falls freely, bounces or roles down slope. Topples occur when a large piece of bedrock falls off a slope and rotates end over end. Rock falls and topples tend to occur on very steep slopes with exposed bare rock. The extreme steepness of a slope, which may be vertical or even undercut, will cause the rock to eventually fail. This failure may be the result of weathering or by faulting of the rock and is often triggered by rain or freeze and thaw cycles. Freeze and thaw cycles can eventually cause expansion and cracking and failure of rocks. The resulting masses of broken rock may travel for some distance by sliding or rolling down slope. One of the largest rock falls in Canadian history happened in the spring of 1903 when Turtle Mountain, in the Southern Alberta Rockies, partially collapsed. Around 82 million tons of limestone fell into the valley below, burying most of the town of Frank and killing around 80 people. Multiple factors led to the rock slide, but Turtle Mountain's unstable geology was the primary cause. Tectonic shifting during the creation of the Rocky Mountains cause structurally stronger rock layers to sit on top of weaker ones. Water seeped into the mountain through surface cracks, eroding the limestone. When it froze and thawed, those cracks widened, breaking apart the rock from the inside. In addition, the structure of the mountain was inherently weak because the base of the mountain had been undercut for thousands of years by glaciers and then the Crowsnest River. Translational and rotational slides occur when a plane of weakness or a failure surface in the rock or sediment causes an overlying consolidated mass to move downslope along the surface of the rupture. These landslides tend to happen in unconsolidated sediment, stuff like clay or sand or silt. And are often triggered by either increased moisture within the rock, or sediments, or by an earthquake. A distinguishing characteristic of translational and rotational slides is a steep head scarp. A scarp is a steep, nearly vertical region of exposed soil and rock at the head of the landslide, where the failure surface ruptures the ground surface. Whether a slide is rotational or translational depends on how it moves down the slope. If the failure surface runs parallel to the slope, the slide is translational. If however it's curved or concaved upwards The moving material will rotate as its moves, causing a rotational slide, which is also called a slump. Rotational slides are easily differentiated from translational slides, because the material involved in the rotational slide often remains as an intact block that will preserve its original internal structure. Therefore, the vegetation and trees growing on top of a transitional slide will retain their original orientation. However, the vegetation and trees on top of a rotational slide tend to tilt backwards, towards the slope due to the rotation of the entire block. Earthflows involve the fluid like movement of fine sediments down slope. They occur when slopes made of unconsolidated sediments become water saturated. This hazard is present when unconsolidated sediments overlie an impenetrable layer which prevents water drainage. When water saturates the sediment, it forces the grains apart, decreasing friction and allowing them to flow downslope. Unlike the other landslides that we've talked about, earth flows don't necessarily move along a well defined failure plane. Depending on what the slope is made of and how much water is in the sediment, earth flows can happen very quickly, sometimes over hours, or slowly over years. Very gradual downslope movements when little water is present are often referred to as earth creeps rather than earth flows. Now debris flows, which we discussed in an earlier lesson, are similar to earth flows in that they involve the fluid like movement down slope. But while earth flows are mostly comprised of fine sediments, debris flows are composed of larger sediments. Stuff like rocks and boulders, making them the most dangerous type of landslides. They are usually triggered by a large influx of water into the system. They often follow a heavy or long lasting precipitation events. Debris flows happen quickly. They're fast moving and can travel far. They often flow downhill following courses of mountain streams and rivers, but can initiate anywhere on the slope where the sediment is sufficiently water saturated. Landslides can be very dangerous and cause enormous amounts of damage leading to high economic cost. Several approaches are used to mitigate damage caused by landslides. To protect roads and railways rockfall shelters and tunnels are often constructed in areas of highest risk. Another option is to place drape nets across vertical rock cliffs and to build catchment fences to, as the name suggests, catch rockfalls. Levees are built along streams that are prone to debris flows to prevent them from overflowing onto the land. Diversion structure may also be constructed to deflect landslides and redirect them away from communities and infrastructure. Other approaches are used to prevent landslides from happening in the first place. Metal anchors can be inserted into mountainsides to reinforce and stabilize rock masses. Ditches, culverts, and drains are built to facilitate drainage and prevent water from accumulating in high risk areas. Tree planting is also used to help stabilize slopes.
