Landslide Distribution Assignment Final

Landslide Distribution Assignment Ame Alexandra Plant • 301024848 •Geog 318 Introduction The area of focus is the New Plymouth District in Taranaki on the west coast of New Zealand’s North Island. The New Plymouth district has two areas which have distinctly different landforms. The western section of the region is dominated by volcanic depositional landforms. Patuha, Pouakai and Mount Taranaki (the volcanoes are named successively from the coast inland) are composite volcanic cones and the broad flat expanse surrounding the mountains is the Taranaki ring plain(). The ring plain extends to the western fringes of the hill country which characterises the eastern region. The hill country is a product of regional subduction followed by uplift over time which has created a bedrock of marine tertiary sediments(). The underlying geology with soil type, slope and regional tectonics are the internal controls which influence the shape of the lands surface and the occurrence of mass movement processes(). Vegetation cover and local climate are external factors which also influence the occurrence of mass movement(). Mass wasting represents one of the most active processes in modifying the landscape in areas of significant relief(). The properties of the surface between the object and the slope (e.g. friction) and the physical properties of the sliding object itself all contribute to the potential for mass wasting. The object is more likely to move if friction between the object and the slope is reduced(). This essay is concerned with the interplay of these internal and external factors and how they govern the distribution of landslides across the study area.

How is the distribution of erosion types related to soil type? How are landslides distributed over the area? The area is characterized by many different types of erosion (Figure 1), for this essay we are only concerned with mass wasting processes. As the term mass wasting refers to all downhill movements of weathered material() the erosion type was categorized into four simple sub-types; debris avalanche, earth flow, earth slip, and soil slip. Visually evident in Figure 1, the mass movement processes are not equally dispersed across the study area, nor do the mass movement types occur in equal proportions. The western study area has a smaller percentage (<10%) of its

area altered by mass movement, and those areas that are characterized by erosion are dominated by the sub-types debris avalanche and soil slip. In contrast the hill country of the eastern study area and its western fringes have a dominant proportion of its area (>90%) affected by all of the sub-types of mass movement processes. In both the western and eastern study areas the mass movement processes are dominated by debris avalanche flows (40% of the hill country and 70% of volcanic cones) and soil slip (40% of the hill country and 30% of volcanic cones). The remainder of the hill country exhibits around ten percent earth flow processes and less than five percent localized earth slip erosion. The Taranaki ring plain has no erosion data, presumed to be because of the long term soil accumulation within this area augmented by the addition of tephra periodically from volcanic eruptions(). The spatial distributions of the mass movements indicate that some areas of the New Plymouth region are more susceptible to certain erosion types than others and therefore underlying factors must be responsible for the observed arrangement.

How is this distribution connected to soil type? The underlying substrate which has been detached from its original position and moved downhill either as a debris avalanche, earth flow, earth slip, or as soil slip also varies in spatial distribution (Figure 2). The area covered by different soil types correlates closely with the locations of mass movement. Within the eastern study area illustrated in Figure 2 the sandy loam on sandstone (blue) only occurs in the top north east corner between the 140 and 160 isohyets of rainfall. This soil type predominantly produces debris avalanches (Figure 1 and Figure 2). The other debris avalanche locations between sites ten and six are mostly a by-product of a silt loam on mudstone soil (orange) with a lesser occurrence upon silt loam on mud/sandstone (green). Similarly soil slip processes occur in the area of silt loam on mudstone and silt loam on mud/sandstone. Earth flow on the other hand correlates mostly with areas of no data and loam on ash (pink) soil. The localized occurrence of earth slip occurs only upon sand on ash (pale green). In the western area debris avalanches show a relationship with the clay loam on basalt soil (dark yellow), and the occurrence of soil slip is linked with sandy loam ash soils (pale yellow). The range of soil types can be divided into two broad categories based upon their dry density, those soils with a dry density of greater than one, and those with a density less than one illustrated below in Table 1. Silt loam on mudstone, silt loam on mud/sandstone and sandy loam on sandstone all have a density greater than one and are derived from tertiary marine beds(). The remaining soil types all have a

density less than one and are volcanic soils, developed from the pyroclastic and epiclastic deposits from the Taranaki volcanoes(). T ABLE 1:

THIS TABLE ILLUSTRATES SOME OF THE PHYSICAL PROPERTIES OF THE DIFFERENT SOIL TYPES . THE ROWS HAVE BEEN

COLOUR CODED TO CLOSELY MATCH THE EQUIVAL ENT SOIL TYPES ON THE SOIL MAP

Dry Densit y

Sand

t m-3

Silt

Clay

%

6 6.1 7 5 7.5 4 6 3.6 9 6 3.1 2 3 9.5 6 1 6.5 8 2 1.0 1

Site 1

Loam on Ash

0.78

Site 2

Stony Clay Loam on Basalt

0.64

Site 3

Sandy Loam on Ash

0.51

Site 5

Sand on Ash

0.60

Site 7

Silt Loam on Mudstone

0.83

Site 8

Silt Loam on Mudstone

1.04

Site 9

Silt Loam on Mudstone/Ash

1.21

Site 10

Sandy Loam on Sandstone

1.55

7 .12

Site 11

Clay/Silt Loam on Sandstone/Ash

0.60

5 9.2 1

2 0.3 3 2 8.6 0 2 1.7 2 2 3.6 4 3 3.0 1 6 2.4 1 5 8.0 5 6 2.5 3 2 3.7 0

(P RESTON

AND BOY S ,

Plastic Limit

Liquid Limit

%

%

2006)

Plasticity Index

Hydr. Conductivit y

cm s-1

7 .66

94.1 6

78.02

116.5 2

38.51

6.08E06

8 .17

94.3 1

83.22

123.3 4

40.12

1.65E04

6 .89

92.3 0

147.7 4

189.7 2

41.98

6.71E04

7 .88

94.6 4

109.1 8

158.1 1

48.93

9.89E05

96.7 8

71.19

103.9 7

32.78

9.25E04

105. 09

32.47

41.16

8.69

8.32E03

106. 65

34.62

44.97

10.35

4.43E04

108. 09

28.21

40.08

11.87

9.81E06

90.2 0

111.2 5

242.5 1

131.27

1.65E06

2 4.2 2 2 6.1 1 2 7.5 8 3 8.4 4 7 .29

The relative proportions of sand, silt, and clay vary between the mudstone derived soils and the volcanic soils and are better illustrated in Figure 3. The mudstone derived soils have a high percentage of silt (62.4, 58 and 62.5 respectively), a high clay percentage (26, 27.6, and 38.4) and a low percentage of sand (16.5, 21 and 7). In contrast the volcanic soils have a high percentage of sand (ranging from 57.5 to 66), a low silt percentage (20 to 28) and a low percentage of clay (6.8 to 8).

The different textures of the soils determine their permeability and water holding capacity. The finer grained mudstone-derived soils have numerous micropores which can retain more water than the coarser grained volcanic soils(). The higher water holding capacity of the mudstone derived soils can restrict infiltration rates and increase the likelihood of the soil becoming saturated. In contrast the volcanic soils have fewer but larger macropores which permit water to pass through more

quickly having a rapid infiltration rate but a lower water holding capacity(). Therefore the more porous volcanic soils are less likely to become saturated with the addition of water. In such non saturated soils the surface tension of the water tends to draw particles together. This increases cohesion and reduces soil movement. In a saturated soil the pore water pressure forces the particles apart, reducing friction and causing soil movement(). In addition saturated soil is heavier than unsaturated soil. This reduces the shear strength of the soil. With progressive saturation the shear strength will be reduced until it is smaller than the shear stress acting upon the soil resulting in the failure of the layer(). The mudstone derived soils have a very low plastic limit (32.5%, 34.6% and 28.2% respectively) and low liquid limits (41.1%, 44.9% and 40). The volcanic soils have much higher plastic limits (>71%) and liquid limits (>103%) as illustrated in Table 1. The low plastic and liquid values for the mudstone derived soils indicate that these soil types require little moisture before they begin to act plastically illustrated in Figure 4 below. These soils also have very low plasticity index values (8.7%, 10.4% and 11.8%) over a narrow range. The index values show that little additional moisture is needed for a soil behaving plastically to begin to behave like a liquid. The soils within this sub group will therefore change from a solid to a liquid with little change in the water content. The volcanic soils have a high plastic limit (ranging from 71.2% to 147.7%), a high liquid limit (103.9% to 242.5%) and have high plasticity values (32.7% to 131%) over a broad range and therefore require large quantities of water before they change from a solid to a semi solid and finally a liquid. The plasticity index of the soils can be used to access the likelihood of a soil to swell .Table 2 implies that the mudstone derived soils are less likely to swell whereas the volcanic soils are more likely to have shrink and swell tendencies().

LIQUID LIMIT

PLASTICITY INDEX

EXPANSIONAL POTENTIAL

<50

<25

LOW

50-60

25-35

MODERATE

>60

> 35

HIGH

Therefore mass movement processes are more likely to occur within the mudstone derived soils due to their low porosity, low infiltration rates and low plastic and liquid limits. The physical properties of these soils (in part) result in the high natural erosion rates of the hill country. The resistivity of the volcanic soils to weathering and erosion is a product of the soils high infiltration rates and high plastic and liquid limits. The physical properties of these soils results in the relatively low natural erosion rates of the Taranaki ring plain and the lower flanks of the volcanoes. Soil

type is not the only factor responsible for the arrangement of mass movement processes. The debris avalanches atop the volcanoes and the earth slip and earth flow processes occur on volcanic soils which are inferred to be the more resistant of the two soil TABLE 2: ILLUSTRATES THE RANGE OF LIQUID AND types. It is therefore likely that other factors PLASTICITY INDEX VALUES AND THEIR EXPANSIONAL

(in addition to those assessed above) are having an influence POTENTIAL

upon the distribution

of landslides.

What is of greater importance for landslide occurrence: internal controls or external factors?

Internal Controls on landslide occurrence As the height of the land increases so to do the amounts of precipitation (rainfall isohyets are present on all maps), cloud cover and wind, while temperature decreases(). Aspect is of an important local factor with north-facing slopes of the area generally being warmer and drier than those facing south(). The slope angle is another important factor and is illustrated in Figure 5. The slope angle affects the drainage and depth of soils(). On gentler slopes water moves more slowly through the soil. This increases the likelihood of water logging and peat formation(). Peat formation occurs on areas of the Taranaki ring plain (visible at sites six and four) which has a slope angle of zero to four point eight degrees. The steeper the slopes are the faster the rate of throughflow and surface run-off which can accelerate mass movement. Soils on steep slopes are likely to be thin, poorly developed and dry(). The hill country of the eastern study area has slopes which are predominantly greater than twenty three degrees and are characterised by the four mass movement processes. Debris avalanches occur on slopes with an angle between thirty

two and eighty seven. Soil slip occurs over a much greater range of hill slope angles between fourteen and eighty seven degrees. Earth slide processes occur on relatively shallow slope angles of four to twenty three, and earth MAP ILLUSTRATING THE PRESENCE

FIGURE 6: A

flow

processes occur between zero to fourteen degrees (Figure 5). FAULTING WITHIN THE REGION() It is apparent that the debris avalanches are constrained to areas of high slope angle whereas earth flow and slip are constrained to low slope angles. On top of the Taranaki volcanoes debris avalanches occur over a much wider range of slope angle and soil slip occurs over slope angles of four to fourteen. The slopes of the region are influenced by the endogenic processes of tectonics. The area is experiencing continued uplift at a rate of around 0.3 m/ka(). Seismicity influences the regions landslide locations as mass movement processes can be triggered by earthquakes. The majority of mass movements are aligned parallel to the major faults of the region(). The volcanic activity of Mt Taranaki has produceded a miniumum of five major eruptions which have been caused by cone collapse(). The vast volume of material produced during such events is reflected in the extensive low lying (0-4) ring plain.

External factors influencing landslide occurrence The nature of the land cover changes dramatically across the study area as illustrated in Figure 6. The eastern high lands and western mountains are generally covered by scrub and indigenous vegetation. With increasing altitude the presence of indigenous forest subsides and a greater percentage of the land is covered by scrub. Low lying areas of land are covered dominantly by horticultural and pastoral vegetation. The type of land cover has a weak correlation with the types and occurrence of landslides. The small cap of debris avalanche material atop Patuha is covered by approximately fifty percent indigenous forest and fifty percent scrub land. The debris avalanche material on top of Pouakai closely correlates with the presence of scrub vegetation. The debris avalanche on the north eastern flank of Mt Taranaki is also closely correlated with scrub land cover. The soil slip on the other hand occurs in areas of

indigenous vegetation cover. Horticultural and pastoral land cover predominates over the Taranki ring plain upon which there is no erosion (at this scale of analysis). The hill country to the east is covered by around forty five percent indigenous forest, forty percent horticultural and pastoral land, around ten percent scrub land and five percent is covered by planted forests. Broadly speaking the debris avalanches are correlated with indigenous forest cover (Figure 1 and Figure 6). The arrangement of soil slip correlates with a combination of indigenous forest cover and horticultural and pastoral land. Earth flow and earth slip occur in areas covered by horticultural and pastoral land and scrub. The slip and flow mass movements occur dominantly within land that is being used for horticultural and pastoral purposes indicating that anthropogenic land use changes are affecting the geomorphological processes of the area (further detail of which is beyond the scope of this essay). The areas in which landslides are dominantly occurring are covered in part by indigenous forests. If land cover had a strong controlling effect upon the FIGURE 7: FOREST COVER CAN INFLUENCES HILLSOPE STABILITY occurrence of landslides it would be safe to assume hat THROUGH HYDOLOGICAL (1) INTERCEPTION AND EVAPORATION, (the presence of indigenous forest would reduce the 2) ATTENUATION OF DELIVERY, 3) INCREASED ROUGHNESS, occurrence of mass movement processes (Figure 4) MACROPORE INFILTRATION, 5) EVAPOTRANSPIRATION) AND 7).The canopy of a forest can intercept and hold AND MECHANICAL (5) ROOT REINFORCEMENT-COHESION, 6) ROOT considerable volumes of water from where it may be 7) WEIGHT, 8) TRANSMISSION OF SHEAR SURFACE 9) TURF MAT) evaporated. The rainfall that does penetrate the PROCESSES (). canopy has a decreased velocity reducing the intensity f of the fall. The roots and stems of the forest also decrease the velocity of over land flow by creating a rough ground surface which also promotes infiltration. Roots of the forest can provide a tensile strength when they bind and anchor regolith to the underlying strata and also act as buttress to down slope movement(). The distribution of landslide events in this region contradicts the above, indicating that the internal factors (mentioned previously) dominate over the external controlling factor of vegetation. The addition of excess water will destabilise slopes by the addition of weight, destruction of cohesion between grains and reduction in friction. Water will generally enter a hillslope system through rainfall, the magnitude and frequency of which will influence the distribution of landslides. The largest amount of rainfall, with an isohyet of three hundred and twenty precipitates at the summit of Mount Taranaki. The amount of precipitation decreases the further east from this location. The hill country of the eastern study area experiences less than half of the rainfall that falls on top of Mount Taranaki with isohyets between one hundred and forty and one hundred and sixty. As the mudstone derived soils are less well drained than the volcanic soils they are more susceptible to failure when saturated by a period of rainfall.

The largest quantity of rainfall falls upon the more resistant volcanic soils and the least amount of rainfall on top of the less resistant mudstone soils. Mass movement processes are greatest in the eastern study area which experiences the least amount of rainfall. As with vegetation cover it appears that the internal factors have more control on the distribution of landslides than the five year return period of rainfall.

Conclusion The study area is divided into two halves, the eastern area which is hill country and the western area which is a succession of volcanoes and their ring plain. Landslides cover more than ninety percent of the hill country and only ten percent of the land in the western area. The highly susceptible to erosion hill country is underlain by mudstone derived soils. The more resistant western area on the other hand is underlain by volcanic soils. These volcanic soils are more resistant to erosion due to their texture which allows effective infiltration producing soils that are well drained. In contrast the mudstone derived soils have high silt and clay percentages creating numerous micropores that are quickly filled with a relatively small amount of water making the soils more susceptible to mass movement than the volcanic soils. The distribution of landslides also correlates with slope angel. Those slopes with an angel greater than twenty three degrees are prone to hill slope failure in contrast to the more gentle slopes which have limited erosion. High slope angels are characterised by debris avalanche flows and low slope angles are typified by earth slips and flows. The highest slopes around Mount Taranaki receive the highest amount of precipitation and the hill country in the eastern study area receives the least. The occurrence of landslides therefore is not strongly correlated with the distribution of rainfall. The areas of high landslide occurrence are covered mainly by horticultural and pastoral land and indigenous forest. The occurrence of mass movement processes upon anthropogenically altered land (deforestation) is self explanatory but the occurrence of landslides in areas covered by indigenous forest indicates that internal factors have a greater control upon the underlying substrate. To conclude the distribution of erosion types is related to the texture and structure of the underlying soil. The mudstone derived soils are more likely to generate landslides than the volcanic soils. Internal factors are of greater importance for the distribution of landslides as the soil type and slope angel closely correlate with the location and nature of landslides. In contrast the external factors of rainfall and vegetation influence the location of landslides but are of lesser importance in comparison with internal factors as illustrated in the Figure 8.

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