Distinguishing the Transition Reach between Torrent and River Using Longitudinal and Cross-Sectional Morphology in River Course

Abstract:

To distinguish a transition reach between river and torrent, this study designed a method to quantify the morphology of channel cross-section. The 5m DEM, which administered by central government, was used to obtain cross-sections from third and fourth order river of 10 main drainages in Taiwan with an interval of 140 to 150 m.

We designed a Cross-Sectional Complex-Index (Ics) to determine the morphological complexity and quantify the degrees of wide-shallow for each sections. This index can be applied to define the location of notch, because it can be knew form river regime that a notch is defined as a narrow-deep cross-section and narrow-deep and relatively large slope to adjacent reach. Therefore, this study defined notch index (Inotch) as the gradient divided by cross-sectional complex-index, and applied it to present the distribution of notch-type sections in study drainages, and the results of field investigation in 22 site showed that a section can be defined as notch while Inotch is lager then 60. The distribution of notch in a drainage shown that the notch which conform our quantitative definitions almost concentrated into several reaches intersecting with some broad-shallow section, and the gradient of these reaches are between 2% to 3%. Therefore, we can define that these reaches are the transition reaches between river and torrent.

There are 54 sites of transition reach in our study drainages, 80% of the sites had a watershed area within a range of 2000 to 6000 ha. It proof that our method can define transition reach effectively and consistently. In addition, because of the type of disaster is collapse and debris flow in torrent and flood damages in river. It can be found that type of disaster are derived from the river morphology. Therefore, our method and result can be applied to determine the disaster type and strategic planning of disaster prevention as a reference.