圖書典藏及影音數位平台

檢討海岸生態潛堤環境整體營造成效及擬訂改善策略

Considering of the erosive coast, which leads to the special conditions of Hualien coast, the protection measures are needed. The 9th Management Office, WRA has developed plans for several years, including new construction method research, ecological survey, association between structures and habitat diversity, coastal environment erection, as well as seawall improvements, aim to protect the coast. A large-scale amount of reef was found at the Nanbin and Huaren coast after the submerged breakwater was built, covering the tetrapod blocks as a natural barrier, which is also a protection as well. It is controversial that construction of structures would lead to the negative impact to the ecology. Besides the reef ecological survey, the special ecology of submerged breakwater and the ecological succession mechanism are not fully grasp. Therefore, it is necessary to go through further discussion about whether the plans achieved their goals over the years. The length of the survey area is from Meilun River to Hualien River (approximately 4 km), width is from about two kilometers away the seawall as the main survey range. The main project is the surrounding environment ecological survey in the program area; and the main study of ecological engineering methods includes Nanbin, Bebin and Huaren coast structures such as seawall, offshore submerged breakwater, jetty, and diversion dike as well. The main work of the project includes the collection and research of basic information and related documents, the status and terrain changes survey of the Hualien coast structures, the status assessment of the Nanbin and the Huaren coast protection, feasibility and efficiency assessment of the new method, information collection and preliminary simulation of currents and terrain changes, information collection and research analysis of ecological species, and consolidate the first year’s ecological submerged breakwater environment outcome as well as preliminary analysis and discussion of coastal protection. Hoping to review the overall outcome of ecological environment submerged breakwater, we realized the impact of the surrounding habitat and propose the improvement strategy at the end. After comparing the results from land measuring of topography, we discovered that the coordinate system referenced from Ministry of Interior geodetic datum TWD97@2010 coordinate system shifted to East side for 0.1m to 0.2m which is slight substance after comparing to the last year record. After the Multi-beam coast survey, the cross sections of Nanbin, Bebin and Huaren offshore submerged breakwater area were compared with original ones, outflows of dike body #2, #8, #9 were found. Underwater survey on marine area around submerged breakwaters of Nanbin and Huaren were conducted within three times in June 6, July 29-31 and after a typhoon on August 18 in 2015. We investigated by scuba diving, underwater photography and sampling analysis. In the survey on polychaetes-built reefs, density and thickness of their tube in Nanbin were higher than in Huaren, indicating a more suitable habitat for the polychaetes in Nanbin, and the data was almost the same to the previous studies. In addition, thickness and density of the tube from the upper layer were less than the lower layer, which might suggest that the wave were more restraining to the polychaetes growth than the bottom sediment was or it could also be the success in stream improvement works on Jian River that reduced the effect of deposited sediment .In all three fish survey, 107 individuals were identified to 19 species belonging to 11families and 17 genera in the first time. And 3036 individuals were identified to 36 species belonging to 16 families and 26 genera in the second time. The population and species diversity were greatly improved from the first to the second time. Species Pelates quadrilineatus had contributed the most to this population increase, suggesting a seasonal shift in fish composition for some species. The third time was conducted after a typhoon had passed by. And only 233 individuals were left belonging to 14 families, 20 genera and 23 species. Fish population and species number decrease a lot, mentioning that the typhoon had tremendous impact on the fish ecology. In environment and adhesive organisms, submerged breakwaters were covered with abundant adhesive organisms, including polychaetes, seaweeds, sponges, polyps, corals, sea squirts and zoanthids, etc. Species number and density were high enough that no large bare surface of breakwater was found. But the wave washed down the adhesive organism and broke the corals, sponges and large seaweed after the typhoon. The bare part reached to 35-45% of the surface area of breakwater, indicating that the typhoon could strongly influence the ecology of submerged breakwaters. The profiling data showed that, the range of sea surface temperature was 25.1?C to 26.1?C, salinity 34.2 psu to 34.6psu, dissolved oxygen 6.4mg/L to 8.6mg/L, Chlorophyll-a 0.2μg/L to 1.38μg/L, turbidity 0.48NTU to 3.98NTU and pH 7.7 to 8.1. Thermohaline distribution of middle and surface layer were different, the outside sea surface temperature was lower than inside, and outside salinity was higher than inside. The outside dissolved oxygen in the middle layer was higher than inside while Chl-a was lower than surface layer. In our results, a total of 30 larvae and 169 fish eggs representing 16 families and 17 species were collected before the typhoon. The abundance of fish larvae was the highest, approximately 1517 ind./1000m3, at station 7 and lowest, approximately 83 ind./1000m3, at station 2. The spatial distribution of fish larvae abundance was higher in nearshore waters than in offshore waters. The diversity index values ranged from 0 to 1.95 and the Evenness index values ranged from 0.99 to 1.The wet weight of zooplankton was between 147 to 488 g/1000m3. The volume of zooplankton was between 126 and 694 mL/1000m3. The abundance of zooplankton was between 116138 and 530439 ind./1000m3. The diversity index values ranged from 4.05 to 4.74 and The Evenness index values ranged from 0.77 to 0.87. The most dominant species were Copepod, Radiolaria and Appendicularia, contributed 63.74%, 11.76% and 4.98%.The results of phytoplankton surveys showed that a total of 91species of Diatoms, 6 species of Dinoflagellates, 2 species of Green algae, 1 specie of Silicoflagellates and 1 specie of Blue-green algae. Total cells of phytoplankton was between 308 cells/L to 11550 cells/L. The diversity index values ranged from 1.01 to 4.74 in surface layer and the middle layer diversity index values ranged from 1.73 to 4.42；the Evenness index values ranged from 0.64 to 0.89 in surface layer and the middle layer Evenness index values ranged from 0.49 to 0.86. After typhoon (Aug. 18),the mean salinity drops to 33.9psu, the mean dissolved oxygen increased to 7.3mg/L, while the mean Chl-a concentrations have increased slightly. The abundance of larvae was decrease and the diversity index values was increase, the abundance of zooplankton was increase and the diversity index values was decrease, the abundance of phytoplankton and the diversity index values were both decrease. From ecological species collection and analysis, we realized that it is hugely beneficial to the environmental surrounding if the setting position and reef body type of ecological submerged breakwater area were well-designed. Also, the enhancing biodiversity and poly fish function may be similar to the natural reef environment or even better. Comparing ecological changes of species in the same year of different areas, it is found that the dominant species like zooplankton and plants would change with the time but not benthic marine organisms. In order to detailed study the change of environmental and ecological species, the survey sampling frequency will go over two seasons, which is conducive to the comparison and research of data analysis. Therefore, it is suggested that increasing of other season’s field sampling survey is necessary for the enhance of background information analysis stringency of ecological site survey after coast submerged breakwater build of Phase II plan. According to historical data of storm flow and terrain measured, the simulation results from MIKE-21 shows that the longshore currents slow down after three jetties at Bebin coast set up; and the back silt appears, which plays a protective effect. Status of the Huaren coast from the land side disposed too close, which caused the coastal current not easy to introduce and lead to the difficulty of river sand entrance form Hualien River. According to numerical simulation, the best solution of the Bebin coast offshore bank building is Option III. This option will change the third jetty into anchor like jetty which will reserve coastal drift sand efficiently. The Huaren coast takes Option II as well, adjust the direction and position of offshore bank #12 to #14, forming an opening with #15 for north coastal current flows into the back embankment, leads to the better opportunity forming deposition at embankment #9 to #11. And it is recommended to use higher Kd-value ones(like Dolos) or increase the weight(40) to strengthen the anti-wave ability in the future clipping block reinforcement.