The Variation of Riverbed Material due to Tropical Storms in Shi-Wen River, Taiwan

Abstract

Taiwan, because of its location, is a flood prone region and is characterised by typhoons which brings about two-thirds to three quarters of the annual rainfall amount. Consequently, enormous flows result in rivers and entrain some fractions of the grains that constitute the riverbed. Hence, the purpose of the study is to quantify the impacts of these enormous flows on the distribution of grain size in riverbeds. The characteristics of riverbed material prior to and after the typhoon season are compared in Shi-Wen River located at southern Taiwan. These include grain size variation, bimodality, and roughness coefficient. A decrease (65%) and increase (50%) in geometric mean size of grains were observed for subsurface and surface bed material, respectively. Geometric standard deviation decreased in all sites after typhoon. Subsurface material was bimodal prior to typhoons and polymodal after. For surface material, modal class is in the gravel class, while after typhoons it shifts towards cobble class. The reduction in geometric mean resulted to a decrease in roughness coefficient by up to 30%. Finally, the relationship of Shields and Froude numbers are studied and a change in the bed form to antidunes and transition form is observed, respectively.

Controls on the grain size distribution of landslides in Taiwan: the influence of drop height, scar depth and bedrock strength

“…1), we have to assume a value for D b and use point load strength values (σ c ) of 0.5 MPa for LS-16, 1.5 MPa for LS-10 and 5 MPa for the other landslides in metasediments. These point load measurements are consistent with the typical unconfined compressive strength of intact rock (Lin et al, 2008) after dividing them by 20 (Chau and Wong, 1996). With these strength values and assuming D b = 1900 mm, the ratio of the modeled D i to the measured D 50 agrees with Eq (1) (R 2 = 0.9), even though its coefficients (k 1 and k 2 ) were calibrated on rock avalanches with rock strength and median grain size that were orders of magnitude larger than the ones in this study (Table 1, Figs.…”

“…However, in the Nanchuang Formation, sandstones alternate with weak shale (strength below 10 MPa), with an equal proportion of each. In contrast, the Shipachungshi metasandstone is intercalated with less frequent slates, often stronger than the Nanchuang shales (Lin et al, 2008). These measurements clearly make the case for highly variable rock strength and are far from encompass-ing the potential diversity of the rock types sampled by the studied landslides.…”

“…In an effort to constrain the mechanical strength of these units, we refer to measurements reported for 128 samples from the Chenyoulan catchment, both for the Nanchuang/Nankang Formation, which extends to the southern tip of Taiwan and contains LS-10, and for the metasedimentary units of the Shipachungshi Formation, where LS-15 occurred (Lin et al, 2008). The unconfined compressive strength of Nanchuang sandstone ranged from 29 to 117 MPa (mean of 70 MPa), whereas the Shipachungshi metasandstone ranged from 45 to 179 MPa (mean of 100 MPa) (Lin et al, 2008). However, in the Nanchuang Formation, sandstones alternate with weak shale (strength below 10 MPa), with an equal proportion of each.…”

“…With these goals, and given the sparse amount of data on landslide GSDs, we performed detailed measurements on 17 recent landslide deposits in Taiwan. Taiwan is a prime example of an active mountain belt where landslides are the main supplier of sediment to rivers (Hovius et al, 2000) and where reports of river GSDs exist in the literature (Chung and Chang, 2013;Lin et al, 2014). Nevertheless, to our knowledge, comprehensive landslide GSD measurements are still lacking in Taiwan.…”

Controls on the grain size distribution of landslides in Taiwan: the influence of drop height, scar depth and bedrock strength

“…With these goals, and given the sparse amount of data on landslide GSDs, we performed detailed measurements on 17 recent landslide deposits in Taiwan. Taiwan is a prime example of an active mountain belt where landslides are the main supplier of sediment to rivers (Hovius et al, 2000) and where river GSDs exist in the literature (Chung and Chang, 2013;Lin et al, 2014). Still, to our knowledge, comprehensive landslide GSD measurements are still lacking in Taiwan.…”