DEVELOPMENT OF EXPERIMENTAL LANDFORM BY ARTIFICIAL RAINFALL ON A SQUARE MOUND RISING FROM A FLAT SURFACE

Artificial rainfall of about 40mm/hour was applied on a flat surface, which has a square (60 x 60cm) sand mound of a mixture of fine sand and kaolinite buried in the ground at the center. The sand mound was uplifted by the uplift-generating device set beneath it. The uplift rate was 0.2mm/20min for 270 hours in the first run, and it was increased to 0.4mm/20min in the second run, which is still in process. The apparent erosion of the uplifted area started from its periphery after around 6 hours of rainfall and uplift (3.4mm of actual uplift) with a series of shallow and fine grooves, while the development of valley system by head-ward erosion of valleys from the mound edges was clear in the previous experiments, which started with a square mound having initial height of about 12cm. Valley development into the uplifted area became clear only after around 40 hours (about 25mm of uplift) in the first run. The valleys, however, were relatively wide and flat-bottomed and the development of valley system was not clearly observed. The maximum height increased as much as the uplift in both runs. The average height of the uplifted area increased at lower rate and the rate of increase declined with time. After 222 hours in the first run, the average height seemed to stop its constant increase. The minimum height of the uplifted area increased slowly but continuously with the development of alluvial fans around the uplifted area. The relief also increased continuously throughout the experiment, but the equilibrium between uplift and erosion apparently did not appear in the experiment. Miniature mountains rose as much as the uplift. This is probably due to the short duration of experiment, but ridge tops may have become less sensitive to erosion without the rapid valley incision (or without cliffs of certain height around the mound). The rapid valley incision with the development of valley system was not the main process of erosion on the mound rising from a flat surface, and this indicates that some of the terrace- or plateau-like topography with deep valleys may have been initiated by the extremely rapid uplift.