Study on Headland-Bay Sandy Coast Stability in South China Coasts

Headland-bay beach equilibrium planform has been a crucial problem abroad to long-term sandy beach evolution and stabilization, extensively applied to forecast long-term coastal erosion evolvement and the influences of coastal engineering as well as long-term coastal management and protection. However, little concern focuses on this in China. The parabolic relationship is the most widely used empirical relationship for determining the static equilibrium shape of headland-bay beaches. This paper utilizes the relation to predict and classify 31 headland-bay beaches and concludes that these bays cannot achieve the ultimate static equilibrium planform in South China. The empirical bay equation can morphologically estimate beach stabilization state, but it is just a referential predictable means and is difficult to evaluate headland-bay shoreline movements in years and decades. By using Digital Shoreline Analysis System suggested by USGS, the rates of shoreline recession and accretion of these different headland-bay beaches are quantitatively calculated from 1990 to 2000. The conclusions of this paper include that （a） most of these 31 bays maintain relatively stable and the rates of erosion and accretion are relatively large with the impact of man-made constructions on estuarine within these bays from 1990 to 2000; （b） two bays, Haimen Bay and Hailingshan Bay, originally in the quasi-static equilibrium planform determined by the parabolic bay shape equation, have been unstable by the influence of coastal engineering; and （c） these 31 bays have different recession and accretion characters occuning in some bays and some segments. On the one hand, some bays totally exhibit accretion, but some bays show erosion on the whole. Shanwei Bay, Houmen Bay, Pinghai Bay and Yazhou Bay have the similar planforms, characterized by less accretion on the sheltering segment and bigger accretion on the transitional and tangential segments. On the other hand, different segments of some bays have two dissimilar evolvement characters. Dacheng Bay, Shenquan Bay, Hudong Bay, Wukan Bay, Fengjia Bay, Wuchang Bay, Lingshui Bay and Tufu Bay produce accretion on the tangential segment, erosion on the transitional segment and accretion on the sheltering segment. However, Guang＇ao Bay, Haimen Bay, Jinghai Bay, Sanya Bay（a）, Dajiao Bay, Hailingshan Bay, Hebei Bay, Fuhu Bay, Shuidong Bay, Wangcun Bay and Bomao Bay generate erosion on the tangential part, accretion on the transitional part and accretion on the sheltering part. It seems to imply some relations between headland-bay beach evolvement and controls on headland-bay beaches, which may possibly to classify headland-bay beach types and should be further studied.

The best hydraulic section of horizontal-bottomed parabolic channel section

The best hydraulic channel section makes the maximum flow capacity for the same flow cross-area, and the minimum cross-area and wetted perimeter for the same discharge. The construction cost can be reduced nearly to the minimum at the same time The horizontal bottom parabolic section （HBP section） is a composite section. It is important for design to find the best combination form of the horizontal bottom and the parabolic sides. This paper studies the best hydraulic section and its hydraulic characteristics. The explicit formulae are proposed to determine the dimensions and the best combination form of the horizontal bottom and the parabolic sides. These explicit formulae and the parameters make it easy to design the channel. It is shown that the ratios of the surface width to the depth and the bottom width to the depth are constant for the best hydraulic section. The comparisons with the classic parabolic, rectangular, trapezoid, triangular, semi-cubic and horizontal-bottomed semi-cubic sections show that the HBP section has the largest flow capacity and the shortest wetted perimeter for the same flow area, and has the smallest flow area for the same discharge. It is indicated that the parabolic side parts of the best hydraulic HBP section are different from those of the classic section. The results of the best hydraulic section of the classic parabolic channel cannot be applied directly to the HBC section.