Channel geometry in lower stream courses of Tanshui River drainage system (Taiwan)

Abstract

This study uses air photo maps on the scale of 1:5000 published in 1984 and channel geometry data measured from 1970 to 1985 to analyze the channel forms and dimensions in the lower courses of the Thanshui River drainage system. The study shows that channel gradient is not one of the dominant factors which influence channel sinuosity. The channel gradient is in low negative relationship with sinuosity. The smaller the gradient is, the more the sinuosity becomes, It is found that meander wavelength (L) and bend amplitude (A) are related to channel width (W) and their regression equations are L = 2W1254and W = 0.298 + 0.172A, respectively. The channel width of the reach not controlled by hard rock and with less sinuosity normally gets wider downstream. As a result, the channel width got narrower and the channel depth became deeper during the past 15 years, after stored by dams and quarried on channel beds. At present, the deepened rate on stream beds has reached 20 cm per year in the Tuyng and the Tachi reaches of the Tahan River as well as in the Taipei reach of Thanshui River. The ratio of width to depth is higher in the braided channel but lower in the meandering reach. The maximum ratio is above 500 and the minimum is below 20 in the study area. Usually, cross section areas are supposed to be controlled mainly by flowing discharge, but in this study they are obviously influenced by quarrying bed loads in many reaches. The variation of section asymmetry, positive or negative skewness, is well concordant with meander bend direction and is influenced by the tributary junction. On the whole, section asymmetry is related to bend amplitude. The bend amplitude with a wider rank gets a larger skewness in section asymmetry.