Scarp evolution of fan terraces along the Lanyang River, Taiwan

An evolutionary model of scarp slopes has been constructed for the Lanyang River area of northeastern Taiwan. In this area, younger terraces have higher free-face height/scarp height ratios than older terraces, while the latter have higher debris-slope height/scarp height ratios than the former. The time necessary for a slope to evolve from the free-face to the debris-slope stage is estimated to be less than 1,000 years for the Mimodan fan and the Fubur fan. When a scarp slope forms, its debris-slope angle initially increases exponentially, and then flattens when the debris slope takes over the entire slope. On the other hands, its free-face angle decreases exponentially and then approaches a constant. The slope evolution observed here is close to the non-parallel retreat model. A general slope evolution model can be applied to terrace scarps of subtropical Taiwan as well as scarps in arid western American, in spite of climatic differences. Vegetation cover and weathering conditions on the scarps, however, may vary because of different climatic conditions. These different conditions can result in different rates of degradation and different repose angles. The angles of repose measured in the Lanyang River area for scarps without free-faces may be as large as 45°in contrast to those ranging from 33°to 38°proposed by previous works. The result may improve the resolution of morphological dating in river terraces.