In view of the seriousness of the river problem in the tidal reach of the lower reaches of the Minjiang River, the historical role of two ancient bridges in Beigang (Wanshou Bridge and Hongshan Bridge) has triggered an important reflection. It is suggested that the overall layout of bridges in the tidal reach of the lower Minjiang River should be further considered, and the bridge design should be carried out according to the current water environment situation of the tidal reach of the lower Minjiang River. It is expected to further improve the ecological environment of tidal reach in the lower reaches of Minjiang River and ensure the safety of drinking water. It is suggested that sand mining activities should be completely banned from the dam site of Shuikou Power Station reservoir to Mawei Port in the lower reaches of Minjiang River.
Keywords: bridge function river regulation tidal reach
1 preface
The study on river regulation of tidal reach in the lower reaches of Minjiang River began in 1993. In undertaking the special work of water resources evaluation, Chengmen Waterworks in Fuzhou City found that when the flow of Zhuqi Station in the main stream of Minjiang River is less than 600m3/s, it all flows into Beigang, while Nangang River is cut off, accounting for 18.7% of the total annual runoff (1993). The results show that the increase and decrease of water diversion in the north port of Minjiang River presents a system of two periods. In the first stage, the water diversion volume of Beigang increased by 7.3 billion cubic meters every year, which was 37% higher than the original water volume of Beigang. The second phase is 1988- 1993, and the water diversion volume of Beigang increases by1620,000m3 every year, which is 78% higher than the original water volume of Beigang. Under the action of river dynamics and blind sand mining in the channel of Beigang, the riverbed of Beigang was seriously undercut, which led to the pier collapse of jiefang bridge (Wanshouqiao Ancient Bridge) in Fuzhou (1September 1994), and many dangerous dikes were found one after another. Announcement of Fujian Provincial People's Government on Prohibiting Sand Mining in Beigang River in the Lower Reaches of Minjiang River (1May 1997) stopped sand mining activities in this river in time, which played a very good role in stabilizing Beigang River.
1In May, 1995, under the great attention of Fuzhou Municipal Party Committee and Municipal Government, and under the organization of Fuzhou Water and Electricity Bureau, a series of works such as river regulation research in Fuzhou section of the lower reaches of Minjiang River were carried out, and the compilation of river regulation research results in Fuzhou section of Minjiang River was completed in February, 1998, which provided rich basic information for river regulation.
In recent years, relevant professional and technical personnel have continuously studied the dynamic changes of tidal reach in the lower reaches of Minjiang River, and professional papers have provided important information, which is commendable.
This paper focuses on the relationship between bridge function and river regulation, hoping to further improve the ecological environment of tidal reach in the lower reaches of Minjiang River and ensure the safety of drinking water.
2 the seriousness of the river problem
2. 1 The amount of incoming sand is far less than the amount of sand collected.
In the natural state of the basin, the average annual sediment discharge at Zhuqi Station in the main stream of Minjiang River is 7.48 million tons, the maximum sediment discharge at 1962 is 20 million tons, and the minimum sediment discharge at 197 1 year is 2.72 million tons. Suspended sediment transport is concentrated in the flood season, accounting for 89.4% of the suspended sediment transport in the whole year from April to September. With the operation of large reservoirs such as Shaxi Ansha Power Station Reservoir in Minjiang River (1976), Jinxi Chitan Power Station Reservoir in Minjiang River (198 1) and Shuikou Power Station Reservoir in the main stream of Minjiang River (1993), bed load transport and partial suspended load transport in the basin above the dam site have stopped. After the completion of Shuikou Power Station Reservoir, the average annual sediment discharge from 2004 to 2005 was only 2.33 million tons, which was only 365.438+0. 1% of the natural sediment discharge of the basin.
According to relevant statistics, since the completion of Shuikou Power Station Reservoir, the annual sand mining amount of tidal reach in the lower reaches of Minjiang River is as high as 5 million tons to100000 tons, resulting in a serious imbalance of sand in the reach.
2.2 The riverbed is cut down and the water level drops.
After the completion of Shuikou Power Station Reservoir on the main stream of Minjiang River, the original bed load and about half of the suspended load were intercepted in the dam, which was similar to the clear water discharge, which increased the scouring capacity of the water flow under the dam and aggravated the river scouring under the dam. According to the observation data of Songtanpu Station under 1998 dam, the average scouring depth of riverbed is 1.92m, and the water level drops at the same flow rate 1.0 1m, and the present situation will be greater than this value.
According to the relationship curve between the water level of Zhuqi Station and the cross-sectional area of Minjiang River, it is calculated that the average scouring depth of riverbed in 2004 is 8.25m relative to 1992, and its main scouring period is in recent years, which is the peak of blind large-scale sand mining in this reach, which has a direct impact on it. The flood peak discharge of the station in June 2005 was similar to that in July 1992, but the corresponding flood peak water level decreased by 2.46m, and the water level decreased by 3.50m at the same low flow rate. In recent two years, the lowest water level of Zhuqi Station has retreated to be close to that of Beigang Li Wenshan Station.
Since 1977, the cross-sectional area of the river at the same water level has been increasing. 1993 and in 2004, the average riverbed scouring depth was 2.80m and 4.60m respectively, and the water level decreased by 1.70m and 3.40m respectively.
2.3 Salt tide upstream, sewage resurgence.
According to long-term hydrological observation data, the tidal current boundary and intertidal zone boundary before 1976 are near Hongshan Bridge and Houguan respectively. Affected by human activities, the reservoir of Shuikou Power Station in the upstream blocked sand, and a large amount of sand was mined in the downstream river, which changed the balance of water and sediment in the river. Under the action of river dynamics, the main stream of Minjiang River and the riverbed of Beigang are seriously cut down, and the water level drops, causing the tidal current boundary and intertidal zone boundary to extend upstream. Field observation and data analysis show that the tidal level boundary extends from Hongshan Bridge (ancient bridge) to over Houguan in dry season, and the tidal level boundary moves 12km from Houguan to over Zhuqi.
The water intakes in the lower reaches of Minjiang River are distributed along the river from top to bottom: Minqing Water Plant and Minhou Water Plant in the main stream of Minjiang River; Minjiang North Port West Water Plant, North Water Plant and Southeast Water Plant; Xu Yi Waterworks, Chengmen Waterworks and Fuqing Xianan Water Transfer Project in Minjiang Nangang; Mawei Port has 12 important water intakes such as Mawei Water Plant, Choudong Power Plant and Mingda Power Plant. The largest water supply is Fuzhou West Water Plant, which supplies most of the domestic water and production water in Fuzhou. The water intake is located near the downstream of Li Wenshan in Beigang. At the boundary of tidal current and intertidal zone, it extends upstream. Except for the intake of Minqing Water Plant, other 1 1 intakes are within its range, and the downstream reaches are seriously affected by the upstream salt water. According to the monitoring data, the salt tide has reached the upper limit of the water intake of Xuyi Waterworks in 2003. The water intake reach of waterworks in the southeast region is affected by the resurgence of sewage, and the water quality exceeds the standard in many aspects. The water intake section of Fuzhou West Water Plant, which has the largest water supply scale, is in a marginal state due to the influence of upstream salt tide and sewage resurgence.
Due to the influence of the upper reaches of the salty tide, the water source in the tidal reach of the lower reaches of the Pearl River has taken measures to adjust the water pressure and make up for seawater desalination in the dry season for two consecutive years, which has paid a huge price. Although the influence of salt tide on the water source of tidal reach in the lower reaches of Minjiang River has not reached a serious level, it is important to take preventive measures before it happens.
3. Historical review of bridge beam function
According to Fujian Highway History, the Longevity Bridge in Beigang (later called jiefang bridge) began in 1303, and the Hongshan Bridge in Beigang began in 1475, indicating that these two ancient bridges have a long history and have become important river-crossing passages in a long historical period.
Fuzhou Nantai Island divides the lower reaches of the Minjiang River into south and north outlets. The water diversion capacity of the Minjiang River at the south and north outlets varies with the discharge, and is restricted by the topography of Toukoumen in Huai 'an, which naturally forms the water diversion law at the north and south outlets.
The functions of the two ancient bridges not only control the effect of the water surface gradient in the upstream of the bridge on the water distribution at the north and south outlets, but also control the effect of the tidal capacity in the downstream of the bridge, so that the river course remains relatively stable, and the tidal boundary and tidal zone boundary near Hongshan Bridge and Houguan are basically stable. The water level and discharge along the river have the function of inhibiting salt in dry season, thus protecting the water quality of the water source reach in the lower reaches of Minjiang River.
4 Bridge planning and design and river regulation
In view of the seriousness of the river problem in the tidal reach of the lower reaches of the Minjiang River, the historical role of two ancient bridges in Beigang has triggered an important thinking. It is proposed that the overall layout of bridge planning in tidal reach of the lower reaches of Minjiang River should be further considered, and the bridge design should be based on the current situation of water environment problems in tidal reach of the lower reaches of Minjiang River, considering both flood control and salt water backflow prevention, so as to find its balance point and give full play to the overall role of bridges in river regulation.
With the rapid development of economy and society, seven bridges have been built in Beigang, including Hongshan Bridge (New Bridge), Jinshan Bridge, Youxizhou Bridge, Sanxianzhou Bridge, jiefang bridge (New Bridge), Minjiang Bridge (Second Minjiang Bridge) and Aofeng Bridge (Third Minjiang Bridge). Beigang plans to build Huai 'an Bridge, Gushan Bridge and Qi Kui Bridge. The total number of bridges in Beigang can reach ten. There are five bridges under construction in Nangang, including Tang Hong Bridge, Juyuanzhou Bridge, Pushang Bridge, Bay Bridge and Wulongjiang Bridge. See attached drawings. It is a very important subject to further study the function of bridge in river regulation, which requires the full cooperation of bridge professionals and river regulation professionals to complete its blueprint.
Five suggestions
According to the seriousness of river problems in the tidal reach of the lower reaches of Minjiang River, it is suggested that sand mining activities should be completely banned from the dam site of Shuikou Power Station reservoir to Mawei Port. Organize forces to track and observe the evolution of the river course and the law of tidal current change, and lift the ban on limited sand mining in the river section where sand mining is allowed under the condition of maintaining the overall balance of river bed erosion and deposition and the relative stability of the river course.
refer to
[1] Study on riverbed evolution and regulation of the north and south ports in the lower reaches of Minjiang River at Yangjiatan River boundary [J] Water conservancy and hydropower technology of Ministry of Water Resources 1996, (2)
[2] Jiang Chuanjie re-discusses the evolution of the lower reaches of the Minjiang River and the changes in its hydraulic conditions. Fujian Water Conservancy Science and Technology 2006, (2)
[3] Cheng Yonglong's influence on water resources utilization due to river bed cutting in the lower reaches of Minjiang River. Fujian Water Conservancy Science and Technology 2005( 1)
[4] Chen Xingwei et al. Dynamic simulation of hydrodynamic water quality in the lower reaches of Minjiang River, 2006
[5] Analysis of hydrodynamic characteristics of Shuikou Chahe section of Minjiang River in Zheng Mingfang, Proceedings of the 6th Annual Meeting of Water Conservancy Branch of Provincial Association for Science and Technology, 2006.