9:20 AM - 9:35 AM
[HGM04-02] Effects of permeability on the development of experimental landform
Keywords:development of experimental erosion landform, uplift, permeability, shear strength, surface runoff, slope failure
Uplift rate and rainfall intensity are the main factors controlling the development of experimental erosion landform. However, characteristics of sand mound to be eroded (mainly permeability) are also an important factor determining the way of experimental landform development. This time I would like to discuss the development of experimental landform with uplift and rainfall erosion from the view point of effects of permeability of sand mound, especially based on the results of runs with the same uplift rate (0.36mm/h), runs 26, 27, 32 and 38.
run-------permeability------------precipitation---------width of deposition area
26--------2.57x10-4cm/s---------40-50mm/h----------------100mm
27--------3.23x10-4cm/s---------80-90mm/h----------------100mm
32--------1.84x10-4cm/s---------80-90mm/h----------------200mm
38--------1.53x10-3cm/s---------80-90mm/h----------------200mm
When a square sand mound is uplifted from a flat surface under the mist type artificial rainfall, fluvial erosion starts from the edge of uplifted area and this erosion soon develop into valley systems. The advance of valley erosion as the mound elevation increases by uplift results in the development of slopes, and slope failures occur frequently. Stream channels become relatively stable and become paths of transport for the material yielded by slope failures. Sediments are discharged from the system effectively by this fluvial process. Large slope failures or landslides tend to occur concentratedly with a certain cycle, and the average mound height change around a certain height, decreases in the periods of landslide concentration and increases with uplift between these periods. This height seems to be determined by the rate of uplift except in the case of extremely high uplift rate.
Permeability and strength of sand mound is considered to be determined by the degree of compaction as far as the same material (a mixture of fine sand and kaolinite 10:1 by weight) is used. While density, which is considered to represent the degree of compaction, have a clear negative relation topermeability, shear strength of saturated material does not show clear relation to density. The degree of compaction apparently controls permeability but not shear strength at least in this series of experiments. Runs 26 and 27, the deposition area of which is 100 mm wide, are different in rainfall intensity, while runs 32 and 38, both of which have 200 mm wide deposition area, different in permeability. However, difference in the development of experimental landform within each pair of runs shows a certain similarity. Relatively low and flat surface with sporadic steep small hills developed in runs 27 and 32, while relatively high and massive mountains appeared in runs 26 and 38. Rainfall intensity is lower in run 26 than in run 27, and permeability is higher in run 38 than in run 32. Assuming that permeability controls the amount of surface runoff, high permeability in run 38 can be considered to have effects similar to the low rainfall intensity in run 26. The estimated amounts of surface runoff, calculated by subtracting the value of permeability (cm/s) from precipitation (mm/h), are 8.4-11.4x10-4, 6.4-9.7x10-4 cm/s in runs 26, 38, and 2.0-2.3x10-3, 1.9-2.3x10-3 cm/s in runs 27, 32, respectively. Runs 26 and 28 have the amount of surface runoff a digit larger than runs 27 and 32. Large amount of surface runoff promotes faster valley erosion longitudinally and laterally, and the development of low and flat surface in runs 27 and 32. In runs 26 and 38, on the other hand, valley erosion was not so active as in runs 27 and 32, and high and massive mountains formed with frequent landslides as a result. Effects of shear strength are uncertain this time; however, observation on the series of experiments including other runs revealed that the strength of sand mound has some effects on the way of slope failure.
run-------permeability------------precipitation---------width of deposition area
26--------2.57x10-4cm/s---------40-50mm/h----------------100mm
27--------3.23x10-4cm/s---------80-90mm/h----------------100mm
32--------1.84x10-4cm/s---------80-90mm/h----------------200mm
38--------1.53x10-3cm/s---------80-90mm/h----------------200mm
When a square sand mound is uplifted from a flat surface under the mist type artificial rainfall, fluvial erosion starts from the edge of uplifted area and this erosion soon develop into valley systems. The advance of valley erosion as the mound elevation increases by uplift results in the development of slopes, and slope failures occur frequently. Stream channels become relatively stable and become paths of transport for the material yielded by slope failures. Sediments are discharged from the system effectively by this fluvial process. Large slope failures or landslides tend to occur concentratedly with a certain cycle, and the average mound height change around a certain height, decreases in the periods of landslide concentration and increases with uplift between these periods. This height seems to be determined by the rate of uplift except in the case of extremely high uplift rate.
Permeability and strength of sand mound is considered to be determined by the degree of compaction as far as the same material (a mixture of fine sand and kaolinite 10:1 by weight) is used. While density, which is considered to represent the degree of compaction, have a clear negative relation topermeability, shear strength of saturated material does not show clear relation to density. The degree of compaction apparently controls permeability but not shear strength at least in this series of experiments. Runs 26 and 27, the deposition area of which is 100 mm wide, are different in rainfall intensity, while runs 32 and 38, both of which have 200 mm wide deposition area, different in permeability. However, difference in the development of experimental landform within each pair of runs shows a certain similarity. Relatively low and flat surface with sporadic steep small hills developed in runs 27 and 32, while relatively high and massive mountains appeared in runs 26 and 38. Rainfall intensity is lower in run 26 than in run 27, and permeability is higher in run 38 than in run 32. Assuming that permeability controls the amount of surface runoff, high permeability in run 38 can be considered to have effects similar to the low rainfall intensity in run 26. The estimated amounts of surface runoff, calculated by subtracting the value of permeability (cm/s) from precipitation (mm/h), are 8.4-11.4x10-4, 6.4-9.7x10-4 cm/s in runs 26, 38, and 2.0-2.3x10-3, 1.9-2.3x10-3 cm/s in runs 27, 32, respectively. Runs 26 and 28 have the amount of surface runoff a digit larger than runs 27 and 32. Large amount of surface runoff promotes faster valley erosion longitudinally and laterally, and the development of low and flat surface in runs 27 and 32. In runs 26 and 38, on the other hand, valley erosion was not so active as in runs 27 and 32, and high and massive mountains formed with frequent landslides as a result. Effects of shear strength are uncertain this time; however, observation on the series of experiments including other runs revealed that the strength of sand mound has some effects on the way of slope failure.