EFFECT OF RAINFALL INTENSITY ON THE DEVELOPMENT OF EXPERIMENTAL LANDFORM

The experiments of landform development with rainfall-erosion and uplift of various rates revealed the existence of two thresholds in uplift rates, across which the experimental landform show different aspects of development. When the uplift rate is below the lower threshold, erosion is almost exclusively fluvial and a certain characteristic low relief reflecting the material erodibility and rainfall intensity dominates (characteristic relief level). The uplift above the lower threshold makes the areas dividing drainage basins grow into hills. Slope processes become active and increase sediment supply. The resultant increase in stream gradients with uplift helps streams to carry more sediment. Uplift and erosion then become balanced to keep average height around a certain height corresponding to the uplift rate. The similar landform lasts for a long time, although slope failures change the landform in detail temporally and repeatedly (steady state level). When the uplift rate becomes even higher and crosses the upper threshold, uplift overwhelms erosion. Hills grow into high mountains (mountain building level), until the mountain hits the limit of growth determined by the mound forming material, rainfall intensity and the width of deposition area.

Rainfall intensity is another important factor controlling the development of experimental erosion landform. Two runs (runs 26 and 27) with the same uplift rate in the steady state level (0.36 mm/h) but different rainfall intensity (50 and 90 mm/h, respectively) show the considerable difference in fluvial processes of erosion. During the period of fluvial erosion domination (from the beginning to the time when relief reaches about 60 mm), distinctive canyon-like gullies develop in run 26, while drainage basins having wide and relatively shallow valleys develop in run 27. Relief (maximum height – minimum height) starts to repeat decrease and increase after increasing straightly with the uplift while a piece of original plane surface remains on the top in both runs. The original surface remains longer and the surface rises higher in run 26. After hills start to grow and slope processes become significant, some separated hills appear in run 26, but a higher and massive hill develops in run 27. The increase in rainfall intensifies fluvial erosion and helps valleys grow wider.