圖書典藏及影音數位平台

計畫範圍為雲林林厝寮海堤至台子村海堤海域，其中第一年度地形水深測量範圍為箔子寮漁港至台子村海堤海域，沿岸長度約7公里，沿岸長度約15公里，第二年度地形水深測量範圍為林厝寮海堤至箔子寮漁港海域，沿岸長度約9.5公里。

1、The project overview (1) Introduction of research This project works on the part of Yunlin coast, 55 km long extended from Chuo-shui River north to Beigang River south. In the recent years, due to deduction of sand support from rivers, it suffers serious coastal problems, such as retreats of coastline and windbreaks. Therefore, we expect the achievement of this research project that can be established and updated the basic information of this coastal region for future application of coastal management and design, by way of fundamental data collection and analysis. (2) Research scope The measurement range of water bathymetry for the first year is 7.8 km long extended from Bozihliao Fishing Port to Taizih Seawall along the Yunlin coast. The second year is 10 km long extended from Lincuoliao Seawall to Bozihliao Fishing Port along the Yunlin coast. 2、Fundamental data collection and analysis (1) Geographical characteristics In this Coastal and near shore morphology was providing protection from waves and surges by offshore sandbar, such as Bozihliao sandbar and Wai-Shan-Din sandbar. But in recent years, the sandbar area was shown substantial loss due to Industrial zone development, river regulation and land subsidence. (2) Meteorology The meteorological data are collected from 2014 observations. The annual average wind speed is about 6.9 m/sec, the months average wind speed is 4.1~10.8m/sec, and the maximum wind speed is 23.4m/sec(SW,July). The average monthly temperature is between 14.3℃ to 28.7℃,and the annual average temperature is between 22.2℃ to 23.2℃. The rainy and typhoon season during every year are in May to September. The most threatening for invasion routes of typhoon to Yunlin coastal are routes 3, 7 and 9. In 412 typhoon events over the past 117 years (1897~2013), there were 110 typhoon events classified routes 3, 7 and 9, accounting for around 26.70% of all the typhoon events. (3) Oceanography The wave directions in summer are mainly west (W) to south (S), with a wave height below 0.5 meters and 4~6 seconds in the period. And the mainly wave directions in winter are northwest (NW) to north (N), with a wave height between 1~2 meters and 5~7 seconds in the period. According to 2014 statistics Mailiao station (2014) climax bit + 2.676m, the lowest point bit -1.923m; Bozihliao station climax bit + 2.226m, the lowest point bits -1.538m. Tide stations two years (2002 - 2014) the monthly average tidal range of approximately between 2.49m ~ 2.94m (Mailiao station), 2.0m ~ 2.38m (Bozihliao station). The main transport direction of offshore current is controlled by wind-driven flow and ocean current. There is considerable seasonal variation of the offshore current. It is from north to south in winter, and conversely in summer, it is from south to north. (4)Coastal morphological changes There are two main factor items of sediment transport changes, as described below. A) Anthropogenic activities: including channel dredging, land reclamation, sea wall or riverbank protection B) Natural forces: transport of sediment around an estuary is governed by many natural factors such as sediment source, tidal flow and waves. (5) River hydrology There are more than 7 percent rivers with annual river runoff concentrated in April to October .The largest sources of river sand transport in the research area are Jhuoshuei River and Beigang River. From the 2000 to 2014 average sand content, measured data and the maximum amount of sand lost an average amount of sand, showed one absolute extreme value in 2001, but in 2006 and 2010 and gradually decline, from 2011 to 2014, the is the ups and downs of the case occurred. Another source of sand south of Beigang River, measuring the average sand content data from 2000 to 2012. Beigang River in 2001 and reached a peak in 2009 the average amount of sand lost, and then since 2009 after is fast diminishing. Jhuoshuei River is much higher than the Beigang River with average sand content, maximum sand content, and the amount of average sand transport. (6) Land subsidence Yunlin is the most serious subsidence area of entire country .The annual land subsidence rate along the coast is more than 3cm. The maximum subsidence rate is about 6.1cm per year during 2013~2014. Subsidence area of 307.6 square kilometers. (7) Coast protection structures Yunlin coast owns 13 regular coastal protection sea dikes (around 36,000 meters), and 16,716 meters of sea dikes to against tidal waves. There are still two areas with no protective equipment including Coastal Reclaimed Land-Mailiao and Kainan Dao in Kouhu Township. The seawall has become emptied and surface deterioration due to the long-term invasion by tidal-currents and waves. The subsidence rate of the seawalls are about 5 ~ 10cm per year. 3. Control measure In this project, there are sevral coastal controlling piles (In the first year, point numbers are between YL01 to YL07, and the second year are between YL08 to YL16,) with average spacing of 1 km, as a control base of monitoring coastal terrain. 4. Topographic Survey of Coastal Zone (1) Land topography The land topography survey is working with aerial photography, and pairing with a LiDAR (laser scanning). (2) Ocean bathymetry The ocean bathymetry measuring times are action after winter and summer wave season (March 2015 and September-November). And use the two measurement results for analysis of coastal morphological changes. 5. Erosion/Accumulation analysis of bathymetry (1) Erosion/Accumulation analysis of ensemble bathymetry A. Bozihliao Fishing Port to Taizih Seawall Comparing with the differences between the winter and summer acted periods displayed that the volumes with -1~-3 water depth are slightly increased and the offshore regions out of -6m water depth are eroded. Based on the changes of volumes and mean eroded depths, the winter waves action induced the topographic changes are much larger than the summer waves action induced that. During 2013/10~2014/09, the ensemble sea regions are erosion. The present volume decreased 10.3% compared with the volume of 2013/10. The bathymetries of offshore are still eroding. For the short-term, the local nature forces and the less of sand source are caused the erosion. Beside of above two reasons, the long-term man-made development and the influences of coastal structures both are main reasons for long-term. B. Bozihliao Fishing Port to Lincuoliao Seawall Comparing with the isobaths and erosion/accumulation on the second half of 2014 and Nov. 2015 shows that the variations of shoreline, -2, -5 and -8 isobaths are quite small. The offshore isobaths with -10 and -12 water depth moved to onshore direction and the erosion area increases. Field investigations display that most regions are still erosion, especially in the offshore region. The eroded depths were about 0~3m. The significant accumulated regions are on the behind of the shoreline and particular areas in the offshore with less 2m accumulated depths. The analysis of long-term numerical data from 1990 indicated that the erosion in the offshore regions are quite serious and violent. Some accumulated areas were appeared in the inner of present sandbar and 12m isobath of offshore region of Bozihliao Fishing Port. Even if the variations of volume show the periodically change, however, the present region is the erosion for long-term. (2) Characteristics analysis of coastal profiles A. Bozihliao Fishing Port to Taizih Seawall The cross-profiles, Sec.38~40, Sec.41~43 and Sec.44~47, were applied to represent the north, centre and south sections of the present region. Based on the field measurement, the south part of this region is more serious area for the erosion. The less of supplied sand and the stronger action in the winter are possible reasons for causing the erosion. The analysis of cross-profiles displayed that the most erosions are occurred in the winter. B. Bozihliao Fishing Port to Lincuoliao Seawall Choosing 13 coastal profiles between Lincuoliao seawall and Bozihliao Fishing Port operates the characteristic analysis of coastal profiles. It finds that the significant changed region is on the south sides of Santiaolun fishing port. The possible reasons are the shortage of sand source and strong wave-action in the winter. The analysis also indicates that most erosions are occurred after winter wave actions. (3) The long-term analysis of sandbar change A. Bozihliao Fishing Port to Taizih Seawall Based on the historical data and shape similarity, the change of Bozihliao sandbar can be divided three periods, 1993~2003(ten years), 2003~2010(seven years) and 2011~2014(four years). Between 1993 and 2003, beside of the sandbar was gradually shifting to south. The small sandbar on the offshore also moved to south and was gone on 2001. After 2003, the shape of sandbar gradually and the shoreline of left/right sides were fixed. Since 2011, the outer shoreline was not the significant change, and the length of sandbar was stretched with time. It can obviously observe that the trend of Bozihliao sandbar shifting to south and land side. On the other hand, it also found that there is the simple linear relationship between the displacement of location of southernmost (N) and time(year) from the long-term sandbar change and the averaged moving speed 321 m/year can be estimated. The long-term change of sandbar moved to the south and land side. The south-displacement was high proportion to the time, however, the relationship between the horizontal displacement and time was not significant. The sandbar moving to the south showed that the alongshore sand shift was quite strong, hence, it also became the one of sand sources of Waisanding Sand Bar. The Bozihliao sandbar is continually shifting to the south and will possible link with the Waisanding Sand Bar. The behavior made a shallow continental sea or a lagoon between the Bozihliao Fishing Port and Taizicun Fishing Port. The decreased volume of sandbar were in the regions of outside of Bozihliao Fishing Port and the south segment of Bozihliao sandbar. The sediment were deposited on the centre and north segment. The erosion-deposition distribution indicated that the most serious erosion areas were on the southwestern regions. The shorelines of left/right sides in the north segment of sandbar were stable from the change of surface area and shape. The shape in the south segment of sandbar was slightly necking, and the erosion was occurring in two sides of sandbar. The significant periods of sand shift were on the winter and summer. Most erosion were occurred on the period of waves action on the winter. Typhoons induced the topographical changes are related to the typhoon travelling path and its magnitude. B. Bozihliao Fishing Port to Lincuoliao Seawall Based on the historical data and shape similarity, the change of Santiaolun sandbar can be divided three periods, 1990~1995(five years), 1996~1999(three years) and 2011~2015(five years). The significant characteristic in the first period is that the sandbar is stretched to south. In the second period, the sandbar shapes were starting to break and decrease. The long-zone distribution of sandbar was clearly defined in the final period. The volumes and areas of sandbar periodically change with time between 1991 and 2015. After 1990, the volumes and areas of sandbar slightly decreased. The volumes, areas and local maximum elevation approached the relative maxima in 1999 and 2013, however, they immediately decreased in next year. The fastest decreasing speed was existed on 1999~2003. Even if sandbar grew up periodically, it still didn’t recover the 1999’s state. The temporal changes of ensemble sandbar is decreased so far. The long-term data of 25 years show that the present volume of Santiaolun sandbar is approximately 34% of that, and the area is only 45% of that on 1990. The short-term analysis displays that the sandbar is temporally-periodically growing up. The significant accumulation regions are on the behind of outer shoreline of sandbar. However, the short-term results do not influence the long-term trend. 6. The field investigation of sand shift The investigated area for the first year is Bozihliao Fishing Port to Taizih Seawall. The second year is from Lincuoliao Seawall to Bozihliao Fishing Port. Twice field investigations are carried out after winter and summer wave action, respectively. The classic working terms are following as, (1) Investigations, sampling and analysis of bed sediment A. Bozihliao Fishing Port to Taizih Seawall Based on the results in the first year, there are fine sand on the inner of sandbar and offshore with 12m water depth. Except for the inner of sandbar, the grain size distribution of surveyed region is the consistency with the water depth. There are no significant differences between twice investigations. Twice investigated results showed that the region with fine sand is moving to south. B. Bozihliao Fishing Port to Lincuoliao Seawall In the investigated area, bed sediments are sampled and analyzed in each profile with interval 500m to understand the soil distributions. Overall, the soil diameters are significantly smaller than that of first season for north points. (2) Investigations, sampling and analysis of suspended load A. Bozihliao Fishing Port to Taizih Seawall According to filed survey in first year, the dominant directions of sediment transportation are offshore and south in the winter monsoon period. After the action of summer monsoon, the direction of sand shift in the sea area is the south, and near the sandbar is offshore base on the investigation of total sediment transportation rate. In order to understand the Hydrodynamic characteristics of whole region, it used that measured tide and currents data to simulate the local wave and flow fields. Based on the simulated results, after the action of winter monsoon, the region behind of south of the Bozihliao sandbar is good shaded area in the period of rising tide and ebb. There are continually nearshore currents to act on the offshore zone under the action of monsoon waves. The nearshore currents are related to the wave height (wave energy). The estimated results indicated that there are significant flow of nearshore currents on the nearshore zone of Bozihliao sandbar when wave heights were higher than 2m. The waves larger than 2m travel from north and cause the south nearshore current after waves broken. Therefore, it caused that the dominant direction of sediment transportation is south. Hence, it also concluded that there are small topographical change on this area in the summer monsoon period because the wave heights in the summer are usually small. B. Bozihliao Fishing Port to Lincuoliao Seawall Twice investigated results obtained the catching method show that the dominant directions of coarse sand are south and offshore after winter waves action period. After summer waves action period, the south and onshore are dominant direction. The twice investigations also display that the onshore and north are dominant direction for fine sediment. 7. Numerical estimation of seawall runup heights and overtopping Using the numerical model, digital topographies and scenario simulation numerically estimates the influences of offshore sandbars on seawall runup heights and overtopping in this project. Results show that when the elevation of offshore sandbars is higher than the static sea level, the overtopping is only occurred under typhoon conditions. The present seawall elevation is enough to withstand the monsoon induced waves. The times of occurring overtopping are higher than the mean of that is defined as the high-risk regions. The high-risk region occurred the overtopping is located the zone between Sec.32 and Sec.35-1 on south and north sides of Santiaolun fishing port. The one of possible reasons is that the smaller sandbar in front of the zone doesn’t provide enough protection features under the combined typhoon and high tide level event. All sections are occurred the overtopping in the scenarios of sandbar elevation reducing zero and sinking in sea. This is because the elevation of sandbar is dropping into sea and the protection feature is decreasing so that cause the overtopping. The simulated results indicate that when the design elevation of present seawalls are higher that the height of sandbar, the overtopping will be possibly occurred in the south and north sides of Santiaolun fishing port under combine typhoon and high-tide-level event. On the other hand, when the elevation of sandbars slightly drops under the sea level, the overtopping will be occurred in Lincuoliao, Santiaolun and Bozihliao seawalls and times of overtopping and its risk of all sections will be significantly enhanced. The numerical estimation had verified with Dujuan typhoon induced the seawall failure occurred on 28 Sep. 2015 with combining the storm surge and super riding tide. The present results are good agreement with the real event. It indicates that there are higher overtopping risk for the Santiaolun seawalls in case of combining the typhoon and the rising tide.