Land degradation does not involve a reduction of the forest area, but rather a quality decrease in its condition, this being related to one or a number of different forest ecosystem components (vegetation layer, fauna, soil, ...), to the interactions between these components, and more generally to its functioning. The estimation of degradation can be hampered by a number of difficulties that are notably caused by:
- The different choices of the initial state of reference: "climax" or its numerous substitutes, the forest condition which corresponds to the silvicultural model that was adopted,
- The criteria (with their indicators) one favours: health and vitality, species diversity, the production capacity of market or non-market goods and services;
- Depending upon whether we limit ourselves to the present date, or whether we consider that the present state is only transitory and leads to a satisfactory, or even improved later state.
The ambiguities of the term degradation, and the difficulties of estimating it, are additional reasons for clearly differentiating between deforestation and degradation.
Factors leading to erosion:
There are four principle factors leading to soil erosion. These are climate, soil characteristics, topography and ground cover.
The direct impact of rainfall is the driving force of erosion. Raindrop impact dislodges soil and overland flow of water washes these particles away. A high intensity storm of short duration is far more erosive than a low intensity storm of long duration. Larger raindrops are far more erosive than small drops.
- Soil characteristics:
The four main characteristics of soil that affect the erodibilty of soil are texture, organic matter content, structure and permeability.
- Texture: This refers to the size and proportions of the particles that comprise the soil. Sand, silt and clay are the three classes of particles, in decreasing size. Soil high in silt and clay are the most erodible and rocky and sandy soils are the least erodible.
- Organic matter: Plant and animal matter in various stages of decomposition that are in soil, can improve the soil structure and permeability. It also increases soil fertility.
- Structure: Compacted or crusted soil increases runoff and reduces infiltration of water. A granular structure increases infiltration and promotes plant growth.
- Permeability: Soil with high permeability allow more air and water to move through the soil. This increases the capacity of the soil to absorb water, which reduces erosion and promotes plant growth.
Slope length and steepness are critical factors in determining the velocity of runoff. Long, continuous slopes allow water to build up high momentum, so planners should try to break up slopes with flatter areas. North facing slopes are cooler and moister, but receive less sun exposure, which results in slow plant growth. South facing slopes are hotter and drier and results in sparse plant growth.
- Ground cover:
It refers to vegetation, wood chips, mulches, crushed rock, and netting. Non-vegetative ground cover shields the soil surface from raindrop impact and slows the velocity of runoff. Vegetation holds soil particles in place and maintains the infiltration capacity.
Types of landslide:
Slope material that becomes saturated with water may develop into a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses and cars, thus blocking bridgestributaries causing flooding along its path Debris flow is often mistaken for flash flood, but they are entirely different processes. and
Earthflows are down slope, viscous flows of saturated, fine-grained materials, which move at any speed from slow to fast. Typically, they can move at speeds from 0.17 to 20 km/h. The velocity of the earth flow is all dependent on how much water content is in the flow itself: if there is more water content in the flow, the higher the velocity will be. These flows usually begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to significantly decrease the internal shearing strength of the material. This thereby creates a bulging lobe which advances with a slow, rolling motion. Earthflows occur much more during periods of high precipitation, which saturates the ground and adds water to the slope content. Fissures develop during the movement of clay-like material creates the intrusion of water into the earthflows. Water then increases the pore-water pressure and reduces the shearing strength of the material.
A debris avalanche is a type of slide characterized by the chaotic movement of rocks soil and debris mixed with water or ice (or both). They are usually triggered by the saturation of thickly vegetated slopes which results in an incoherent mixture of broken timber, smaller vegetation and other debris. Debris avalanches differ from debris slides because their movement is much more rapid. This is usually a cause of lower cohesion or higher water content and commonly steeper slopes.
Debris slides generally begin with large blocks that slump at the head of the slide and then break apart as they move towards the toe. This process is much slower than that of a debris avalanche. In a debris avalanche this progressive failure is very rapid and the entire mass seems to somewhat liquefy as it moves down the slope. This is caused by the combination of the excessive saturation of the material, and very steep slopes. As the mass moves down the slope it generally follows stream channels leaving behind a V-shaped scar that spreads out downhill. This differs from the more U-shaped scar of a slump. Debris avalanches can also travel well past the foot of the slope due to their tremendous speed.
A sturzstrom is a rare, poorly understood type of landslide, typically with a long run-out. Often very large, these slides are unusually mobile, flowing very far over a low angle, flat, or even slightly uphill terrain.
Block glide at Mile Marker 23 along I-99 in Blair County, Pennsylvania. Part of a hill of Devonian shale was removed to make the road, forming a dip-slope. The upper block detached along a bedding plane and is sliding down the hill, forming a jumbled pile of rock at the toe of the slide.
Landslide in which the sliding surface is located within the soilweathered bedrock (typically to a depth from few decimetres to some metres). They usually include debris slides, debris flow, and failures of road cut-slopes. Landslides occurring as single large blocks of rock moving slowly down slope are sometimes called block glides.
Shallow landslides can often happen in areas that have slopes with high permeable soils on top of low permeable bottom soils. The low permeable, bottom soils trap the water in the shallower, high permeable soils creating high water pressure in the top soils. As the top soils are filled with water and become heavy, slopes can become very unstable and slide over the low permeable bottom soils. Say there is a slope with silt and sand as its top soil and bedrock as its bottom soil. During an intense rainstorm, the bedrock will keep the rain trapped in the top soils of silt and sand. As the topsoil becomes saturated and heavy, it can start to slide over the bedrock and become a shallow landslide. R. H. Campbell did a study on shallow landslides on Santa Cruz Island California. He notes that if permeability decreases with depth, a perched water table may develop in soils at intense precipitation. When pore water pressures are sufficient to reduce effective normal stress to a critical level, failure occurs. mantle or
Landslides in which the sliding surface is mostly deeply located below the maximum rooting depth of trees (typically to depths greater than ten meters). Deep-seated landslides usually involve deep regolith, weathered rock, and/or bedrock and include large slope failure associated with translational, rotational, or complex movement.