LAND SLIDES \ Factors contributing to wet flows:

FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS

Get Help To Apply for University and Loans

Factors contributing to wet flows:

i)                    High precipitation, snow melt, volcanic eruption, rise in ground water

ii)                  Presence of clay minerals is the soil

iii)                Presence of unconsolidated soil (alluvium) on rock slopes.

iv)                Presence of soil with open fabrics resulting from flocculation during deposition

v)                  Vibration/earthquakes in unconsolidated saturated material.

Dry flows: Common with sand and silt in quarries and also for desert sand dunes and loess.

vi) Snow avalanches

Entrain boulders, trees and other debris as well as destroying stands of trees. Accumulation tends to occur at the base of the slopes. Large snow avalanches occur on slopes of 30^o-50^o, small avalanches on slopes of 50^o-65^o and minor shedding small snow accumulation on steeper slopes where accumulation is minimal. Snow avalanches are not common on slope less than 30^o.

vii) Topples

Consist of the rapid rotation of a unit of rock or soil about pivot point. Toppling may not lead to falls, slides or flows

viii) Complex landslides

Involve more than one type of movement. One type of movement may initiate material to move and once underway the material takes on the character of another type of movement. Example starts as debris slide and end-up as debris flow. Complex movement is named in the order of occurrence: debris slide-debris flow.

Assessment of landslide hazard

Both large and small landslides are capable of causing significant damage and loss of life. The extent of the damage is assessed based on the probability of landslide occurrence within a certain time period and risk area that could be affected.  However, it is difficult to specify the time frame for the landslide occurrence. As a result landslide hazard is often represented by landslide susceptibility (Brabb, 1984). The susceptibility of an area to landslides is determined / depicted by using landslide zonation/map. Landslide hazard map identify areas of differing landslide potential. The natural changes as well as human induced changes can affect the susceptibility to landslides. Therefore, it must be understood when assessing the landslide potential. Once landslide susceptibility is identified, investment projects can be developed that avoid, prevent or substantially mitigate the hazard.

A)    The assessment starts by establishing the like hood of future landslide occurrences through understanding of the conditions and processes controlling past landslide in the area of interest. It is achieved by examining and mapping past landslides in the area. Geologic, topographic and hydrologic circumstances associated with past landslides if properly studies may indicate which natural or artificially created circumstances are likely to produce landslides in the future. Certain types of landslides may be associated with specific land uses, for the example road cut or excavation. Thorough field studies can provide insight into how different factors have contributed to the failures.

Landslide risk assessment

Landslide risk can be assessed qualitatively or quantitatively depending on the desired accuracy of the outcome and nature of the problem. It involves the following processes:

v  Identify elements at risk (E): These are like buildings, roads, farmlands, water sources, forest, etc. Determine their current market values (that is replacement value).

v  Determine the vulnerability of the elements at risk: Vulnerability (V) is the degree of damage caused by a landslide hazard to the elements at risk. It is usually expressed in relative terms, using words such as 'no damage', 'some damage', 'major damage', 'and total loss', or by a numerical scale between 0 (no damage) and 1 (total loss). Values are assigned based on the judgement of the nature, volume and down slope velocity of displaced material.

v  Estimate the probability of landslide occurrence in a particular period (in years) by judgement in combination with empirical evidence. Approaches used are variable. Can be by compilation of landslide inventory or heuristic approaches based on expert opinions or by deterministic approaches based on slope stability analyses or by using statistical models involving statistical determination of the combinations of variables that led to landslide occurrence in the past.

v  Calculate risk involved which is the product of the elements at risk, vulnerability and probability of landslide occurrence in a particular period.

Pata msaada kujisajili na AJIRA PORTAL, bofya hapa