Yatenga, Burkina Faso. Traditional selective clearance. Fire is an indispensable tool in traditional systems for disposing of woody vegetation progressively and selectively. In the foreground note the soil, which remains covered and retains its root network, and in the background, the forest fallow, which regenerates the soil in the course of 12 years, under a continuously harvested natural palm stand. Lakou, agroforestry "Garden A. The positive interaction between trees, livestock and crops is optimal here.
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Forage trees planted along the risers keep the soil in place and provide large amounts of forage prized for its nutritional qualities. These nitrogen-rich foods are essential to the digestion of dry coarse forage during the dry season. Gulmi District, Nepal. Alley cropping between Leucaena hedges.
Ibadan, Nigeria. Cropping under cover of Cedrella or Grevillea trees. By removing trees of different ages and pruning low branches and surface roots, cereals and other foodcrops with a staggered growth cycle can be produced under their cover. In the Sahelian zone of Niger, only the valleys are covered with trees Acacia albida , which send their roots down to tap the groundwater. Their protection is vital to reducing the ill-effects of drying winds on crops. Tahoua Valley, Niger. Many varieties of fruit tree are grown here, together with some bananas.
Contour channels on calcareous bluffs. The hills are covered with shallow, grey rendzinas. The small plots are rented out to "rich city folk," and their edges are marked by contour channels, which quickly fill with sediment, so that they are now of use only as paths on these very steep slopes. Since the whole approach has never increased yields, the local small farmers do not maintain it unless paid to do so.
Clearance is general, except around houses. Bouchereau, Jacmel. Individual cistern. Rural development started in the Jacmel region with the building of cisterns to catch rainwater from roofs or small cemented areas. The water has allowed improvements in family hygiene and reductions in the time and labour entailed in fetching water, and is also used to water livestock and a small vegetable and fruit garden.
Gully garden. Once it becomes "worn-out land, " the slopes are scoured down to the rotten rock and left to fallow RAK grazed by livestock. Only the bottom lands are still productive where dry stone walls have been built to trap water and sediment. Stabilized tracks and communal cisterns. One of the first development activities in mountain areas is the creation of tracks.
Landslides Mass Wasting Soil And Mineral Hazards The Hazardous Earth
However, these tracks are often the cause of serious gullying, and they have therefore been paved so as to collect runoff from the slope in a sand trap, from which it flows into a large communal cistern. The water is used for livestock watering, household purposes, and irrigation of a small off-season household vegetable and fruit garden. Water management on the Salagnac toposequence. A little below the track, the soil is deeper, and cassava is grown on mounds in combination with beans and maize.
Lower still, on the shelf where the houses are located, the red soil is much deeper and is intensely cropped multi-storey gardens. Such plots are in danger of gullying by runoff from the scoured hilltops, so that it is important to catch this runoff on the track. Plot-bordering hedges. In the foreground, large cuttings, which act as hedges to protect the cultivated plot on the left from passers-by. The project has tried to improve these hedges by introducing forage and fruit species. In the background, the stony surfaces or "worn-out land" where runoff concentrates.
In the Nippe area, the weathering of basaltic soils has given birth to an undulating landscape covered with fertile vertisols. Traditionally sorghum is sod seeded, and vetiver, which is very resistant to overgrazing but not to gullying of the valley bottoms, is planted on the edges of the fields. Mangoes produce masses of fruit, and are used to feed pigs. Since the ravages of swine fever, the dried foliage has been used as forage, but many mango trees have been sold for timber.
Petite River, Nippe. Cayambe Basin, altitude m. Mojanda, altitude m. Watershed management by the local rural community. High yields on the experimental plots encourage the farmers to invest in land husbandry; if they sign a contract undertaking to maintain the works, they can receive a loan enabling them to purchase sufficient inputs to double yields. When they repay the loan after a year, another family is granted the same loan, so that a small amount of aid eventually benefits a whole community.
Pedro Moncayo, altitude m. Marls and soft rocks are very susceptible to water erosion. Following clearance of steep slopes, extensive cereal cropping and centuries of overgrazing, the hill has lost 1 metre of soil, and sheet and rill erosion are clearly visible. The form of the tree trunk also indicates mass movement.
On the nearby marry hill, the effects of sheet erosion can be seen at the top, and those of rill and gully erosion on the steep slopes, while the wadi eats away at the foot of the hill, causing the banks to slide. Sheet erosion carries only a few tonnes of sediment down to the bottom of the hill, whereas gullying and wadi streambed displacement carry hundreds or thousands of tonnes of sediment right down to the dam. This should influence the choice of sites and strategies for erosion control intervention at the watershed level.
With a view to developing mountain farming, international aid projects introduced fruit tree crops, which considerably increase small farmers' income. However, apricot trees lose their leaves in winter, so that these orchards provide very poor protection to the soil during the rainy season. Ouzera, Algeria. With a view to reducing runoff and erosion risks and improving income still further, the INRF-ORSTOM research team has established grass buffer strips under the trees, combining this with rotations beans and cereals that cover the soil during the rainy season and complete their cycle before summer starts.
Without reducing fruit yields, this system assured an additional crop of grain, produced straw useful for animal husbandry, and cut erosion risks. It aroused considerable interest among neighbouring peasant farmers. Half-orange landform in the gneissic regions of Vietnam is perfectly developed in terms of management of water, biomass and fertilizing elements. The top and the steep slopes are protected by a crop of tea. Runoff irrigates sugar cane and a rice field before flowing into a pond that is surrounded by a collection of useful trees.
Tilapia provides food for people, hens and pigs, and the latter recycle banana and sugar-cane residues, so that their dung fertilizes both rice field and pond. In this way, nutrients can be recycled several times per year. Bac Thai, Viet Nam. On these fields in the Sudano-Sahelian zone of Burkina Faso, there are stone lines to curb the velocity of water, a stand of acacia, and heaps of dung which will be dug into the soil: a mineral supplement is indispensable.
The interaction of all these ways of managing water, biomass and nutrients allows hopes of a relatively productive and sustainable agriculture. Burkina Faso. Land husbandry in Nepal. The case of the foothills of Nepal illustrates the complexity of traditional production systems which combine sophisticated water management on irrigated terraces on the slopes or in the valley bottoms, agroforestry and animal husbandry in order to propagate fertility on cultivated gradual terraces.
Multi-storey gully garden. Runoff on the basaltic slopes causes gullies, which can easily be controlled with sills of earth protected by plastic bags. Shallow landslides are rooted in the soil layer and often form slumps along roadways or fast-moving debris flows down valleys. These types of landslides are often called 'mudslides' by the news media. Shallow landslides also occur as flows, slides, or rockfalls and topples. Deep-seated landslides are those that fail below the rooting depth of trees and vegetation. They are often slow moving, but can also move rapidly. Deep-seated landslides can cover large areas and devastate infrastructure and housing developments.
These landslides usually occur as translational slides, rotational slides, or large block slides. Deep-seated landslides are typically much larger than shallow landslides, in terms of both surface area and volume. A deep-seated landslide may appear stable for years, decades, or even centuries.
These long-lived features can be partially or entirely reactivated for a variety of reasons. Flows are generally a slurry mixture of water, soil, rock and or debris that moves rapidly downslope. Flows may or may not be confined to a channel. Earthflows may have a characteristic 'hourglass' shape. The slope material loses strength and runs out, often forming a bowl or depression at the head. Flows usually occur in fine-grained material on moderate, water-saturated slopes.
Debris flows usually occur in steep gullies, move very rapidly, and can travel for many miles. They may contain more coarse material than a mudflow when channelized. Slopes where vegetation has been removed by fire or humans are at greater risk for debris flows and many other types of landslides. Debris avalanches are unchannelized debris flows that move very rapidly. They typically do not mobilize far and sometimes move like a snow avalanche.
Lahars are debris flows that originate on volcanoes. A volcanic eruption can rapidly melt snow and ice, causing a deluge of rock, soil, ash, and water that accelerates down the slopes of a volcano, devastating anything in its path. They can travel great distances and damage structures in flat areas far from their source.
Communities near rivers draining Mount Rainier and Glacier Peak are at greatest risk. Lateral spreads occur on very low-angle slopes toward a free face such as a cliff or embankment. Movement is accompanied by cracking of the ground. Failure is often caused by liquefaction when soil is transformed from a solid to a liquid , usually because of an earthquake.
Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges. Slides are the downslope movements of soil or rock along a discrete or narrow failure surface and can be deep-seated or shallow. The initiation of slides, like flows or rockfalls, is sensitive to steep slopes, the additional weight of water or other loads, and friction along their base. Translational slides usually fail along geologic discontinuities such as faults, joints, bedding surfaces, or the contact between two rock types.
They move out or down along a planar surface with little tilting, and can travel great distances. Translational slides can contain loose sediments or large slabs of bedrock. Block slides are a type of translational slide that occur when large and relatively intact slabs of rock or earth are rapidly transported downslope.
These type of landslides can be large and damaging and occur where alternating layers of strong and weak rock slope downhill.
Rotational slides slumps are landslides that occur along a curved or spoon-shaped surface. Back-tilting may occur near the scarp of the landslide and there is often a toe of displaced material. Rotational slides often occur because the internal strength of the material is overcome by its own weight. They are usually composed of relatively loose or unconsolidated material. Falls and topples are usually rapid, downward movement of large pieces of rock or debris. Sometimes this is enough rock to cover a road or block a stream or river.
Landslide figures in the "Types of Landslides" section are modified from U. Rainy-season storms can produce landslides that block roadways, damage homes, and threaten life and safety. The map below shows landslides reported within the last few weeks. A more comprehensive list of significant landslides in Washington is available here.
Skip to main content. Public Land Survey Office. Community Forest Program Development. Jobs Start a Career Internships. Search form Search. Read this page. Landslide Inventory Mapping Following the landslide inventory methods first produced by the Oregon Department of Geology and Mineral Industries, the Washington Geological Survey has developed their own landslide inventory protocol for Washington State. Warning signs of a landslide Common landslide triggers Warning Signs of an Impending Landslide Landslides are dangerous and unpredictable.
Cracks growing in the ground; downslope movement of rock, soil, or vegetation. Sudden changes in creek water levels, sometimes with increased sediment, especially during or right after large or protracted storm events. Sounds of cracking wood, knocking boulders, groaning of the ground, or other unusual sounds, especially if the sound increases.
If you notice these signs or observe a landslide in progress Warning Signs of a Potential Landslide A hillside that has increased spring and or seep activity, or newly saturated ground, especially if it was previously dry. New or developing cracks, mounds, or bulges in the ground. The force of gravity pulls all things on the planet toward the center of the Earth. Without gravity, mass wasting would not occur. But unlike many of the other factors, humans have no influence or control on gravity.
Climate is based on temperature and precipitation. Mass wasting is prone in the spring-time when snowmelt, water saturation, and runoff is greatest. Also the type of climate will help determine the type of mass wasting. Humid climates tend to have slides , where water-saturated slopes fail and fall. Drier climates tend to have rocks that fall; especially early spring.
Canyons and places prone to wildfires tend to have debris flows. When you build a sand castle, water is needed to build the walls and towers. That is because water has surface tension and is attracted to each other. This allows you to build towers greater than the angle of repose. So a little water can actually prevent slopes from sliding. But too much water lubricates the individual grains of sediment decreasing friction between each grain, so the possibility of mass wasting increases.
The increase of water within the soils can come from over watering, pipe or swimming pool leaks, or prolonged stormy weather. In Utah and many mountainous regions, spring runoff of snow melt increases the water content within the soil. Notice how well it flows down the mountainside. There are two reasons why this landslide occurred. First, this slide occurred on the same slope as a previous landslide in But the slide was also influenced by the fact that above is an orchard that was over-watering the vineyards and over-saturated the soil.
Finally, gravity is the driving force of mass wasting. In the daytime, temperatures in the spring and fall tend to be above freezing, which allows liquid water to enter cracks within rocks. At night, the temperatures cool below freezing and the water within the rocks freezes and expands which causes the rock to break more. The following morning, the ice will melt and go deeper within the crack to refreeze later that night. This freeze-thaw action over time can cause rocks to break off and fall to the ground.
But rock falls can also occur when heavy precipitation is falling on a steep slope, causing the rocks to lose friction and fall. T he YouTube video on the right is a rock fall captured in Taiwan in late August , following heavy precipitation in the region. When this type of slide occurs, the upper surface of the slide tilts backwards toward the original slope and the lower surface moves away from the slope.
They are common when the soil tends to be deep in clay or soft sediment deposits. The video on the right is a large landslide again in Taiwan in early September following every rainfall. Needless to say, they were having a bad few days in the region. Often times the slope failure occurs on soil composed of clay or shale, or along old fault lines, or previous slide areas.
What makes translational slides dangerous is that they tend to flow faster and travel farther than rotational slides.
The most expensive translational slide in U. Debris flows tend to be a mixture of rock and water with two to three times the density of flooding streams. That density allows debris flows strip away the land and pick up objects as large as school buses. Debris flows are most common at the mouth of canyons along alluvial fans. Lets first explain an alluvial fan.
When floods occur within the mouth of a canyon, either because of intense thunderstorms or snow melt, the erosive power of the water can pick up sediment and boulders - a debris flow. Now once the debris flow reaches the mouth of a canyon, the sediment gets deposited in a fan-shaped delta called an alluvial fan. The problem is that people like to live along alluvial fans because of their scenic view on the canyon. Another influence of debris flows is wildfires. When a wildfire strips an area of its vegetation, the bare soil is easily eroded away in either a thunderstorm or snow melt creating these debris flows.
Because of Utah's topography and tendency to wildfires, debris flows are quite common. Image source: This image is in the public domain because it contains materials that originally came from the United States Geological Survey, an agency of the United States Department of the Interior. Recall that volcanoes eject pyroclastic material ranging is size from ash to boulders.
Now there tends to be two ways lahars occur. One is if a thunderstorm precipitates large amounts of moisture on the pyroclastic material and the pyroclastics flow downslope. The other option is if a volcano is snow-capped and the heat from the volcano causes some of the snow to melt and mix with the pyroclastic material.
What makes lahars so dangerous is that they have the consistency of concrete and can travel hundreds of miles. One component in landslide mitigation is basic drainage control of water. Recall that water can cause slopes to lose their friction as water lubricates individual grains of soil.
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And if you cut a slope and put a retaining wall for support, you may be preventing the water from filtering through. Thus you will often find drains at the base of retaining walls that allow underground water to within the slopes to drain out. This is particularly dangerous when the underlying rock layers slope towards the area. Ultimately preventing landslides is impossible because gravity will always exist. But smarter development can help minimize the risk and hazards created by landslides. Slope support is one of most common types of mitigation for potential mass wasting.
As mentioned above, a retaining wall can be built to support a steep slope.
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Next, the retaining wall must be anchored to the bedrock within the slope to hold the wall to the slope. Another type of slope support is simply planting vegetation. The roots of vegetation tend to grab and hold soil in place, so by planting various types of plants and trees can be a simple and cheap way to stabilize a slope. Now there are two types of subsidence. There are several examples of slow subsidence, but the best one is Venice, Italy. Venice was built at sea level on the now submerged delta of the Brenta River. The city is sinking because of the overlying weight of the city and pumping of ground water.
The problem now is that sea levels are rising as glaciers melt and water expands due to global warming.