EFFECTS OF DIFFERENT CONTROLLING FACTORS ON THE EXPERIMENTAL LANDFORM DEVELOPMENT

I have been conducting a series of geomorphic experiments with uplift and rainfall erosion, expecting to provide any ideas to help interpreting the real landform development from scarce and sporadic evidence. A mixture of fine sand, kaolinite and water (ca. 10 : 1 : 0.7 by weight) compacted in a square prism-shaped stainless container (ca. 60 × 60 × 40 cm) is pushed up by an uplift-generating device set under the bottom plate. Mist-type rainfall is generated by a pump through spray nozzles. The duration of experiments was rather long (ca. 200 - 1800 hours) to accommodate slow uplift. The results of experiments suggested the existence of two threshold uplift rates. Below the lower threshold (ca. 0.05 mm/h), fluvial erosion alone can offset the uplift, and gentle landforms reflecting the mound erodibility appear (Characteristic relief phase). Above the upper threshold (ca. 5.0 mm/h), uplift overwhelms and a massive mountain grows until it hits the limit determined by the experimental setting (Mountain building phase). When the uplift rate is between these thresholds, average surface height stops increasing after valleys fully develop and slopes grow enough to cause landslides, and then changes around a certain height depending on the uplift rate (Steady state phase). This phase is the most common condition throughout the series of experiments. Rainfall intensity is another important factor, controlling the rate of fluvial erosion. Low rainfall generates less surface flow, and reduces fluvial erosion making the surface rise higher. However, in runs with higher permeability, less rainfall caused more intensive erosion. The higher permeability of the same material was acquired by the lower degree of compaction, and it generally associates with lower shear strength, which is considered to promote slope failures by infiltrated water. More rainfall generates more overland flow, and accelerates the development of valley system in the early stage. Efficient water drainage through valleys of high density may have reduced the water infiltration and retarded slope failures, namely erosion. This possibly resulted in the higher surface under heavier rainfall. Simple controlling factors apparently exert complex effects on the experimental landform. Untangling this complexity is the key to make geomorphic experiments meaningful.