圖書典藏及影音數位平台

本計畫旨在探討曾文水庫排砂隧道啟用後，經排砂隧道及水庫前庭放淤等增加濁度之行為，可能增加曾文溪河道通洪量及輸砂量，研究下游段至河口濁度與流量之關係，有助於瞭解不同流量下輸砂之量體，及波潮流作用下河口堆積之特性對曾文溪河口海域地形之影響。

AbstractDue to the construction of reservoir and barrier dam on the upstream of ZengWen River, the sediment transport capacity of the river is reduced and the supply of coastal sand is insufficient. In recent years, ZengWen Reservoir has been actively improving its draining sand and silt disposal facilities, which is expected to supply sand source of ZengWen River and improve the balance of sand supply between river and coast. Hence, this plan observed the change of turbidity on the estuary of ZengWen River during the period of reservoir relaese, and observe the velocity, flow direction and flow discharge at the same time. The trend of river sand transport and the distribution of coastal drift sand were analyzed by numerical simulation, which can enable us to understand whether the river sand has the supplementary effect to the shore and provide the help of engineering application to prevent coastal erosion in the furture.This project includes basic data collection, field investigation of drift sand including wave tidal current, field investigation of whole tidal current, turbidity observation of Guosing Bridge including water level and discharge, topographic survey of river channel and sea area, simulation of wave tidal current and sediment transport in estuary, and improvement and strengthening suggestions of protection facilities, etc. The following is the summary of the key achievements of this plan.1. Basic data collection and analysisThe estuary of ZengWen River used to be a large water area, and gradually evolved to its present position after 1926. Due to the development of Zengwei barrier flat, many fish farms appeared nearby. From the satellite images, it can be seen that the sandbank area at the estuary of ZengWen River was obviously less than that in 1985. From the observation and analysis of drift sand, it can be seen that the drift sand in the estuarine section of the base discharge period was influenced by wave tide and diffused to the sea. The constrution of ZengWen Reservior blocked the supply of sand, and reduced the sources of find sand on the nearshore. With the reduction of coarse sand from upstream, the thining of downstream bed material was increasingly serious. The terrain along the ZengWen river has entered a stage of long-term erosion.2. Field investigation of drift sand including wave tidal currentIn order to understand the change of sediment concentration during the operation period and the unoperation period of the discharge of flood and sand from the ZengWen reservoir. The field investigation of drift sand including wave tidal current was performed each time on "River base discharge period" (21-30 November 2018) and "Reservoir discharge and desilting period" (23 August to September 4 2019). In the "River base discharge period", the sediment transport capacity of the main stream is obviously smaller than that of the sea side, while the outflow of the estuary is affected by the sea current. The drift sand diffused outwards, and the sediment transport in the estuary is moved outwards from the main stream under the influence of tides and waves.In the "Reservoir discharge and desilting period", the sediment change of river flow was obviously great than "River base discharge period", but the sea side sediment discharge was great changes than other stations. The estuarine sediment was mainly affected by wave, tidal current, and sediment direction with the waves and tides. Because the sand moved directly from estuary to opne sea, the sand transport in the estuary did not add significantly to the north and south coasts.3. Field investigation of whole tidal currentThe tidal current field investigation completed on November 25 to 26, 2018 with the whole trend monitoring survey from the estuary to the Guosing Bridge. Then, the investigation results were analyzed harmoniously, the diurnal tidal and semi-diurnal tidal currents are separated, and the elliptic parameters of the diurnal tidal and semi-diurnal tide currents were obtained. The variation of the flow field in estuary and channel with rising and falling tide is calculated. The analysis results are as follows:(1) The elliptic shape of the tidal current in the deep channel is mostly flat and parallel to the direction of the channel. The ellipse parameter shows that the direction of the long axis of the ellipse is roughly along the deep channel. The ellipse radius of semi-diurnal tidal current is larger than that of the diurnal tidal current.(2) The results of hourly tidal current analysis show that the tidal current flows to the open sea in the period of ebbing, and the velocity is faster than that in the middle part of ebbing. During the period of high tide, the tidal current flows inland, and the velocity is faster in the middle part of high tide, while the velocity is slower in the period of dry tide and full tide.4. Turbidity observation of Guosing BridgeThe investigation was completed during the "Reservoir discharge and desilting period" from August 23 to September 4, 2019. During the observation period, the reservoir approaches the full water level, and the reservoir carries out flood control discharge, and releases water through power generation and desilting tunnel. The results showed that the water level on Guosing Bridge was significantly influenced by tide. The water depth of deep channel was about 2.65 meters to 4.13 meters. The maximum depth of the I station on both sides of the high beach was about 0.372 meters, and the maximum depth of K station was about 0.627 meters. At high tide, the flow direction was generally from northwest to north, and at low tide, it changed to south to southeast. The maximum velocity was about 53 cm/s, and the average velocity was about 15.4 cm/s.5. Topographic survey of river channel and sea area(1) Coastal area topographic surveyFor this project, first topographic survey of land and sea area was carried out on July 18 and 24~27, 2019. Second topographic survey of land and sea area was carried out on September 10~12, 2019. The survey results were displayed with bathymetric charts and incorporated in analysis on evolutions of coastal erosion and accretion.(2) Evolutions of coastal erosion and accretionThe long-term (2003/04~2019/07) coastal evolutions from Sigu Seawall to Qingcaolun Seawall showed that the landward movement of Sinfulun Sandbar resulted in notable accretion at sea area outside Sigu seawall. There was erosion on the existing sandbar area(between -5 and -9 meter bathymetric contour). The area west of Sinfulun Sandbar (bathymetric contour: from -3 to -7 meter) showed accretion. Most of ZengWen River estuary had erosion. Most area between -2 and -13 meter bathymetric contour showed accretion. Most sea area outside Qingcaolun Seawall showed accretion.From 2006 to 2010, the deep channel and river bottom on ZengWen River showed erosion caused by the flood of Morakot typhoon. From 2010 to 2019, river flow route affected topographic change of river channel cross-section. The estuary section between Section 001 and 003 had erosion on right river bank and accretion on left river bank, so the deep channel was moving from the left to the middle of river channel; from Section 004 to 006, since the main flow was closer to left river bank, there is erosion on left river bank and accretion on right river bank. Hence, the deep channel was moving toward left river bank; from Section 007 to 008, the deep channel showed accretion; from Section 009 to 015, since the main flow was closer to left river bank, there is accretion on left river bank and erosion on right river bank. Hence, the deep channel was moving toward right river bank; from Section 016 to 021, the river flow route was relatively flat and straight. Hence, the deep channel showed accretion in total.6. Simulation of wave, tide, current and sediment transport in estuaryThe hydrological and sediment transport behavior in estuary of ZengWen River and sea area around the estuary was very complex. The interaction between oceanic wave, tide and current was the driving factor of sediment transport and topographic change in and around the estuary. With open-source numerical model NearCOM-TVD, this plan summarily simulated the infleunce of possible factors on topographic change around the estuary. Also, this plan collected survey data around ZengWen River estuary for determining and analyzing turbidity change and sediment transport of the estuary. Data including tide level, suspended material density, wave and river discharge rate surveyed on November, 2018 and August and September, 2019 was used to established two numerical models representing “River base discharge in northeast monsoon period” and “Typhoon flood discharge in summer period”. The numerical model simulated sediment transport and topographic change in the estuary and its coastal area nearby under the influence of tidal current, wave and interaction between wave and tidal current. For coastal area aroundd the estuary, the coastal erosion and accretion in “Winter monsoon period” and “Summer typhoon period” were investigated.(1) Wave field simulationThe wave-breaking location was affected by water depth (using 0.78, the default wave height-water depth ratio of SWAN) and background current velocity (interaction between wave and tidal current). The simulation found that the wave-breaking area was changed with tidal level. The river estuary wave height was affected by rising and ebbing of tidal level. Longshore current from the breaking wave on Sinfulun Sandbar also affected sediment transport.(2) Current field simulationThe winter current velocity simulation results corresponded with the survey results at station B, C, D and E around the river estuary. Only simulation of the westward sea current at station A showed notable error. We surmised that station A was located near offshore submerged sandbar, which was vulnerble to wave behavior. Offshore submerged sandbar should be continually monitored because its topography changed seasonally. The summer current velocity simulation results largely corresponded with the survey results at Guosing Bridge, but the simulated current velocity was sometimes slightly higher. At station E in the estuary, the simulated current velocity and phase corresponded with the survey results. At station D outside of the estuary, the simulated current velocity was lower than the survey results. In winter and summer, ebbing tide result in higher current velocity in the estuary. Furthermore, the interaction between seaward river current caused by water level difference and longshore current caused by waves created south-north sediment transport on the offshore side of Sinfulun Sandbar.(3) Sediment transport simulationLongshore current caused by breaking wave removed the sediment on the offshore side of Sinfulun Sandbar and lead to south-north sediment transport. Notable southwest-northeast sediment transport appeared on Haipu Seawall at the turn of ZengWen River estuary; The sediment was transported out of the estuary in Winter (River base discharge period) and was transported southward to Qingcaolun seawall area in summer (Reservoir discharge and desilting period). The simulation results were confirmed by the south-north degradation of 0m bathymetric contour on Sinfulun Sandbar in long-term and the southwest-northeast degradation of 0m bathymetric contour on Haipu Seawall in long-term.(4) Topographic change simulationThe simulation results showed notable erosion on the turn of ZengWen River estuary and offshore side of Sinfulun Sandbar. The sediment was removed by the longshore current caused by wave-breaking on the offshore side of Sinfulun Sandbar. In addition, we simulated adding groins at the estuary in “River base discharge winter period”. The simulation results showed that additional groins slightly increased erosion-accretion mixture on Sigu Seawall and the turn of ZengWen River estuary, but accretion on groin area increased. Hence, groin addition was beneficial for Haipu Seawall area around the turn of ZengWen River estuary.7. Improvement and strengthening suggestions of protection facilitiesWe summarized the above-mentioned field investigation, numerical simulation and past improvement plan for ZengWen River estuary for providing the following suggestion:(1) Because Bailu typhoon brought insiginificant rainfall, the sediment investigation of "Reservoir discharge and desilting period" showed no drastic sediment change. The sediment at the estuary could be different from that of high turbidity and discharge. Furthermore, the topography of deep channel could change. The simulation results showed that obvious deep channel would siginificantly affect sediment transport direction at the river estuary. We suggested that more typhoon period investigation should be performed for understanding sediment transport direction at the river estuary in typhoon period.(2) Long-term evolutions of coastal erosion and accretion showed that Sinfulun Sandbar at the north of ZengWen River estuary gradually moved inland and toward the river estuary. The sediment investigation and numerical simulation showed that Sinfulun Sandbar had siginificant infleuence on changes of river estuary wave and longshore current. We suggested that continuing topographic, wave, tide, current and sediment transport investigation should be performed at the area.(3) Concentrated wave attack at seawalls on both banks of ZengWen River estuary eroded the beach to dyke foot. Groins were built at the northern bank which is the connection area of Sigu Riverwall and Seawall, but the sand-fixing performance waited to be investigated. The beach nourishment suggested in “Research of sediment transport at Zengwen estuary on surrounding coast” could improve dyke protection. We suggested continuing beach nourishment at the northern bank and working on dyke protection and beach nourishment at southern bank, connection area of Qingcaolun Riverwall and Seawall.