Development of an Ecohydraulic Model for Riparian Mangrove Expansion

• Main Authors: Sheng-Chi Yang, 楊勝崎
• Other Authors: 李鴻源
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/59909411414395529269

博士 === 國立臺灣大學 === 土木工程學研究所 === 100 === This study presents a new ecohydraulic model of riparian mangroves expension to quantify the relationship between mangrove habitats and its hydrological requirements. Because ecologists and engineers ignore the realities of mangrove hydrology, most mangrove often fail completely or fail to achieve the stated goals. A case study showed process model analysis and proposed a suggestion for mangrove management of the Tanshui River system. On spatial analysis, the model sieves mangrove habitats out from a tidal reach based on three indices: water salinity concentration, river secondary flow intensity-R/W (the ratio of radius of curvature to river width), and submerged probability. On temporal analysis, the model used flow regime analysis results to identify impacts of hydrolocial floods and water resource management on mangrove expansion. The results showed that the optimum conditions for K. obovata growth and dispersal exist in waters with mean annualsalinity levels that are higher than 5 ppt (parts per thousand) and the R/W lower than 3 on reach scale. K. obovata growth and dispersal is controlled by submerged probability on wetland scale: 0.5-23.2% for Watzuwei wetland, 4.9-28.0% for Zhuwei wetland, 11.2-42.9% for Guandu wetland, and 11.7-42.6% for Sherzi wetland, respectively, on wetland scale. There were two reasons why mangroves cannot invade Guandu wetland before 1978. First, instream sand mining caused instable and lower land elevation. Second, typhoon Ora on October 1978 created bare mud flats and thus K. obovata was able to grow on Guandu wetland. Discharge decreasing and small flood vanish due to past infrastructures of water resource advantaged mangrove growth and expansion. And flood (peak flow less than 600 cms) caused reed and other original vegetations died because of erision or prolonged inundation, and thus K. obovatawas able to occupy these bare mud flats. These two reasons were why K. obovata expanded so fast on Guandu wetland. However, if peak flow large than 800 cms, floods will damage mangroves and slow down mangrove expansion next few years. The model is able to distinguishmangrove areas, potential mangrove areas and non-mangrove areas. Different strategies for mangrove management shall be adopted with different categories. Mangroves are very tough species in the mangrove areas. Over-protection is not necessary. The potential mangrove areas can be rehabilitated by removing other environmental factors. Mangrove rehabilitation is not suggested within the non-mangrove areas. For mangrove management in the Tabshui River system, because mangroves on Guandu wetland and Sherzi wetland have higher expansion potential to upstream and midstream and may be invaded by terrestrial flora, they would cause more serious problem in flood-prevention.The mangrove density and area must be monitored and controlled. Plus, mangroves on northern Zhuwei wetland and eastern Guandu wetland may be replaced withterrestrial flora, becauseland elevation was close to the upper limitation for mangrove growth. The vegetation change needs to be monitored.