|題名||105-107年曾文水庫放淤對下游河道變遷影響分析/總成果報告(光碟版)The Impact Analysis of the 2016 to 2018 Zengwen Reservoir Silt Disposal Operation Strategy on the Downstream Channel Morphology Transition
To resolve the sedimentation problems caused by Typhoon Morakot of 2009, several desilting projects have been planning and executing for Zengwen Reservoir, including the desilting tunnel for hydraulic flushing. This project proceeded various investigation and analysis tasks for future operation of the flushing tunnel on its influences of returning sediment to the downstream channel transition. For this three-year project spanning from 2016 to 2018, research team will focus on several topics, including basic information collection, channel topographical survey, sediment sampling and test at estuary and channel, monitoring and analysis of estuary and channel transition, numerical simulation of sedimentation at estuary and channel, investigation, impact analysis, and evaluation of water quality and ecological monitoring, and proposing protection measures for potential influenced estuary and channel. Besides, result briefing meetings and documentation of project update reports are also the requirements for the project. The outcomes of these research topics are concluded in following sections.
1. Basic data collection
From the review of literature, cases, and regulations, this project found that the Japan cases had almost identical operation strategies to those planned for Taiwan’s reservoirs. Since the geological environment is really similar in both countries, the operation approaches of Japan cases can be useful to this project. The cases in United States and western countries often had the goals of river environmental restoration or ecological habitat creation such that they are considered as examples for ecological restoration in reservoir management.
Beside the collection of topographical, geographical, hydrological, ecological, and tidal datum along with reservoir sediment movement, discharge release records, watershed ecological databases, and satellite images, this year project had completed two field patrols, 26 samples of channel sediment, eight times heavy metals detection ,and six times of water quality and channel physical habitat investigation for the later analysis.
2. River cross-section survey
In 2016 the amount of silt disposal operation Strategy reservoir is 350 thousand cubic meters, according to the channel sand source is not enough.
Reference to the location of large section piles of the 6th River Management Bureau. The plan requires 60 transects to be measured. Measurements were carried out every year after the flood season, for a total of 3 years.
The monitoring cross-section of 142~158 in 2017. The results show that the channel of this cross-section is transferred from the deep groove erosion in 2013 to the siltation trend. It should be related to the siltation of reservoir and the supply of channel sand by silt transport. Interval analysis of river sediment erosion and deposition variation in 2017-2018. Change within the range of 34-36%, 37-41% of the scope of the scour and silt range 29-23%. The range of changes between 2016 and 2017 is limited. As a whole, there is no obvious change of channel caused by reservoir flood discharge and sand discharge. The monitoring data of cross section 1~137 in 2018 showed that the average bed height showed a maximum downward brush trend of 0.5 m, and a slight desilting trend of cross section 137~178, about 0.05 ~ 0.44 m. In 2018, the range of changes increased by 2％, the range of continuous brushing increased by 4％, and the range of siltation decreased by 6％, and the overall range of change was limited.
3. Estuary and channel siltation sampling and testing
During the reservoir release periods, water sediment concentration sampling were applied at four bridges over the main channel and the conjunctions of four tributaries in order to identify the influences of desiltation and branch watersheds on the change of concentration. In 2017, the river was temporarily deposited about 134 thousand cubic meters of silt.
The sediment sampling and habitat survey in 2017-2018 includes four cross sections and two estuaries, covering the upper, middle and lower reaches of the silt temporary zone. Particle size analysis through 2017-2018, the particle size of monitoring section is approximately the same as that in 2016. The upstream section 144 is affected by river channel arrangement, and the size composition of river channel changes a little larger. No abnormal changes in habitat survey.
In May 2017, the reservoir mud pumping operation began to test pumping. In June 2017, during the heavy rain event, the Zengwen Reservoir carried out two regulating releases of water, which was a trial operation stage. By the end of July, the amount of mud pumped was about 136,000 cubic meters. The concentration of water quality sampling in from July 29th to 31st, carried out 1 times of flood operation. The concentration of artificial sampling is low (3,821~5ppm) during the period of typhoon regulation. The overall water quality concentration in the front of Zengwen No. 1 Bridge was lower than that in the past years, indicating that the lower siltation amount was related to the lower scour capacity.
The 2018 dredging operation suspended silt up to 1.3 million cubic metres in the river before August. After August 20 silt prevention tunnel water trial operation and August 23 heavy rain through reservoir regulation and discharge. Full amount of silt has been scoured by temporary silt in the river course. During this period, the maximum water quality concentration measured by manual sampling was 90,793 ppm (Zengwen No. 1 Bridge). The concentration of water quality does not exceed the concentration of desilting in the past.
4.Numerical analysis for sedimentation of channel and estuary
With CCHE2D and HEC-RAS as the tools for numerical simulation on the effects of silt disposal at the reach near the Zengwen Dam, the model constructed for the downstream channel morphology transition in previous Year 2012-2015 Project had demonstrated its reliable performance.
Through 3 different traffic scenarios, record values associated with automatic hydrological stations. By regression estimation of push water concentration of sand drainage station subsequent possible changes. The transported rates from disposal areas are from 88% such that the total volume of 1.78 million cubic meters within silt disposal areas is feasible.
In addition, the total amount of sand discharged from the reservoir reaches one million eight hundred and seven thousand cubic meters per year, and the main deep channel of the downstream channel changes in scour and deposition. The result of the operation of the assumptions shall be within a reasonable range of change. Two-dimensional numerical analysis, according to the condition of typhoon regulation and discharge on 2017. The typhoon adjusted the conditions of water discharge, taking the silt discharge amount of river channel about 1.78 million cubic meters per year, and the hydraulic sand discharge capacity of reservoir, the single field scouring and silting simulation was carried out, because of the short time of flood discharge and the small amount of flood discharge. The amount of scour in numerical analysis is about 38%, which is slightly lower than that of 67% scour rate in single field in 2013. It is inferred that the design of the earth embankment is different from that of 2013. It is necessary to monitor and evaluate the earth embankment according to the operation of reservoir flood discharge and sand discharge on 2018.
Proposed model for 2018. Suppose the river is temporarily silt up to 1.78 million cubic meters, washed with a discharge of 5 to 7 times the size of Typhoon Nisha in 2017 The trend of scour and siltation in the upstream section is similar to the result of the 2012-2017 desilting analysis. Because of the large amount of reservoir flood discharge and the obvious trend of channel erosion, the overall scour rate of the temporary area can reach 71-88%. When the discharge of reservoir is higher than the regulation of silt prevention tunnel(995 cms), the time difference between regulating the discharge amount of spillway and the discharge time of silt prevention tunnel will shorten the time of mutual operation with the increase of flow rate. If the regulating capacity of the reservoir is less than 995 cms, the silt scouring rate of the first silt temporary site will be reduced. With the increase of reservoir regulation quantity, adding the regulation of spillway can increase the silt erosion rate of the first silt temporary site, and it is more favorable for the silt to return to the natural river course.
Simulation of water trial operation of silt prevention tunnel on August 20, 2018 and regulation and discharge of heavy rain reservoir on August 23. It is helpful to improve the scour rate of temporary silt in river course to prevent the silt from discharging and discharging sand in the silt tunnel at the right time. On Aug. 20, when there is water in the silt prevention tunnel, the tunnel is in operation (Maximum flow 870cms). With the regulation and discharge of the reservoir on August 23(Maximum flow 2,650 cms). Through numerical analysis, the scour rate can reach 96.4％, which is quite close to the actual scour rate of the river.
5. Influence and evaluation on water quality and ecology
Two bridges and the estuary are the investigation sites for river environment factors and physical habitat in this year. 16 environmental factors including chemical factors, RPI, WQI5, pH, and heavy metal detection along three physical indicators, i.e., flow velocity, depth, and discharge were collected seasonally. Before the rainy season in 2017, the water quality indexes were lower than those before the rainy season in 2016.
Comparison of ecological investigations with previous findings. The index of biological diversity is higher than that of prophase.
The suspended load at the sea outlet slightly exceed the water quality standard of class C. Related to channel sand transport and flood discharge. Results of water quality analysis in 2017. The index of water pollution is between unpolluted and moderate pollution. The pollution level of the river is between medium and good. Results of water quality analysis in 2018. The index of water pollution before flood season is between unpolluted and mild pollution. The pollution level of rivers is good. After flood season, the upstream water quality is better than the downstream. Review of past water quality analysis results. Zengwen No. 1 Bridge is located in Nanxi Mountain area. The water quality of each item is stable. The condition that the amount of suspended load is affected by rainfall in mountainous area, resulting in abnormal increase of data. Stability of Chemical oxygen demand and suspended solid Measurement of Mashan Bridge. However, because of the frequent human activities in the adjacent areas of Jadou and Shanhua, ammonia nitrogen, Escherichia coli group and biochemical oxygen demand are often unusually elevated above the water quality standard of class C. The sea tide affects the estuary, and the measured values are stable. The estuary to the sea is close to the Zengwen river security checkpoint port.The estuary to the sea is close to the Zengwen river security checkpoint port. For oyster boat entry and exit, human activities are frequent. In order to collect oyster boats in and out of the main road, human activities are frequent, and another ZengWen river with sand and bioclastic deposits at the sea mouth, presumably easy to cause the increase in the number of Escherichia coli reasons.
Ecological surveys, the results of S4-S9 survey in 2017-2018 show that the index of divergence between birds and butterflies in Zengwen No. 1 Bridge is higher than that in the previous period, and there is an increasing trend of divergence between birds and butterflies. The index of divergence between amphibians and butterflies of Mashan Bridge is also higher than that of earlier period. The index of biological and heterogeneity in the sea mouth is also higher than that in the previous period. The results of water area survey in 2017-2018 showed that the habitat of Zeng Wen No. 1 Bridge had been stable in 2017 after the river course finishing construction in 2016, but the rainfall was not obvious in August and December, and the area of water area was less, so it was less suitable for the Sinogastromyzon nantaiensis. As a result, the number of significantly reduced, 2018 and 2017 survey results consistent. There has been no significant change in plant life , all of which are seasonal and related to changes in the nature of the machine .
Benthos, fish survey less in 2018(September and November). There is no significant difference in the species of Zengwen No.1 Bridge. However, the number of Sinogastromyzon nantaiensis is obviously less, and there is no obvious rainfall in the 2018 survey time. The water volume in the river is obviously less, and the water environment changes to the environment dominated by the lake area, which is not conducive to some species. There were no significant differences in fish species and quantity in the estuary.
The quantity of ecological survey in land area increased a little compared with the previous period, and was mainly affected by cluster phenomenon. Water survey shows the trend of survey of black-faced spoonbills in Tainan, as shown in tables 5 and figure 5.
Overall, the number of species surveyed in 2017 is slightly higher than that of 2016, but the number of Sinogastromyzon nantaiensis is still not high. The number of land-based ecosystems increased in 2018 compared to 2016-2017. There has been no significant change in plant ecology, which is due to changes in seasonality and investigation.
6. The analysis of the effectiveness of silt disposal operation
Before and after the construction of reservoir, the Tsengwen River annual average sediment transport capacities are 11.2 million and 9.3 million tons, respectively. With reducing sediment from the watershed, the estuarine and coastal line backward retreat gradually. In previous four-year project, the silt with total amount of 1.895 million tons was disposed and 99% of them were eroded away. Even with high concentrations of suspended load during the reservoir release periods, the silt can be quickly migrated to the estuary within about 13 hours such that the concentration return to the natural state within about one day. Silt disposal operation with slight impact on aquatic and terrestrial ecosystems and no safety concern both in channel conveyance and bank stability had already proven its feasibility.
To complete the sludge area embankment layout in 2017. The remaining silt in the river after the typhoon is 593 thousand cubic meters. The first sludge temporary field erosion rate of about 95% or more. Statistics show that 517 thousand cubic meters of silt and 503 thousand cubic meters of silt per year were suspended in 2016-2017, scour rate up to 97%. Before June 2018, the cumulative amount of mud pumping was 1.008 million cubic meters per year. Because the reservoir had no regulation and discharge, there was no scour rate for the time being. For the past year 2012-2018, the volume of siltation and the effect of desilting are shown in Table 6, 7 and figure 6. Before August 2018, the cumulative dredging capacity was 1.3 million cubic meters per year. After the trial operation of the silt prevention tunnel on August 20 and the regulation and discharge of heavy rain through the reservoir on August 23, the silt of the temporary site has been completely moved downstream, with a scour rate of 100% per year.
The water quality before and after the deposition is maintained in Class B ~ C. In 2017, the overall water quality analysis showed that the water quality pollution index ranged from unpolluted to moderately polluted, while the river pollution level ranged from moderate to good. The results of ecological investigation were not significantly different after reservoir flood and sand discharge. Only the adhesion of algae, preliminary estimates may be due to water more sediment caused by planktonic algae species and the lower number, time for the November survey showed that the Effect of non reservoir desilting. The water quality remained in Class B ~ C water after desilting in 2018. The investigation of heavy metals in river sediment conforms to the standard of water body. After flood season, the upstream water quality is better than the downstream.
7. The protection strategies and channel morphology transition after silt disposal operation
Comprehensive related project results. The actual situation in 2016-2017 and the influence of hydraulic desilting tunnel construction. Establishment of hydraulic desilting tunnel, large section in No. 36-38 showed very low risk. The actual situation in 2016-2017 and the influence of hydraulic desilting tunnel construction. Establishment of hydraulic desilting tunnel, large section in No. 69-75 showed very low risk.
This project simulates two scenarios in 2018. Simulation of 7 times Nisha Typhoon discharge and 7 times Nisha Typhoon discharge combined with silt Prevention Tunnel Operation. Simulation of measured Section after 106 Flood season. Section 25 above all presented a "very low" risk, and a "low" risk was found in the estuary. Under the circumstance of cooperating with anti-silt tunnel operation. Section 25 shows "very low" risk over section 25, "low" risk over estuary, and "low" risk at only section 142 in upper reaches. The reason is mainly caused by the deep scour of the section. Risk Distribution of Flood Control facilities in Zengwen River Basin under 0823 heavy Rain after Flood season. Section 78 above all showed a "very low" risk, and a "low" risk over the estuary.
According to the analysis of typhoon situation in 2016-2018, reservoir desilting will not do any harm to river flood control facilities.No matter considering current and silt prevention tunnel after construction, the Zengwun River is very low risk of flood control facilities, the preliminary assessment of reservoir desilting no doubts about the safety of the Zengwun River.