3.3 Rainfall Pattern in 819
According to the measured rainfall obtained from GPM, Our team Yishui Environmental Technology drew the 3-day rainfall contour map (from 8:00 on August 17 to 8:00 on August 21) and the maximum 24-hour rainfall contour map (from 8:00 on 19th to 8:00 on 20th). According to the description of events obtained from many news, the rainfall obtained by GPM is very similar in terms of total amount, time history distribution characteristics and spatial characteristics. Therefore, in the absence of official measured rainfall data, GPM rainfall is of great alternative research value.
The average three-day rainfall on the upper reaches of Yeyuan Reservoir was 240mm, and the maximum 2-hour rainfall occurred at the end of the event, reaching 80mm.
8.18-20 3-day rainfall contour map
24 hour isogram of rainfall
The data of three-day and 24-hour rainfall in Shangkou Town, Shouguang City were analyzed. From 6:00 on the 18th to 2:00 on the 20th, it rained continuously for 45 hours, with a cumulative rainfall of 401.5mm, and the rain peak was at the end of the three days. The maximum 24-hour rainfall occurred from 00:00 on the 19th to 24:00 on the 19th, with a cumulative rainfall of 346.1mm; The maximum 2-hour rainfall occurred at 15:00 to 16:00 on August 19, with a cumulative rainfall of 100.0mm. The peak rainfall occurred in the late period of the rainfall process, the soil reached saturation state under light rainfall in the early period, and the yield flow coefficient of heavy rain in the late period was high, which was a very unfavorable situation.
Distribution map of 3-day rainfall duration in the middle and lower reaches of Mihe River
The "Shouguang Flood Control early warning Plan" defines the red level I warning as: "The rainfall has reached 100mm, the rain may continue, and the rainfall area is forecast to accumulate more than 200mm in the past 15 days..." . Judging from three days and the maximum 24-hour rainfall, it should be a level I red alert. The situation is extremely urgent.
3.4 819 Assessment of flood reservoir dispatching mode
3.4.1 Analysis of regulation and storage of Yeyuan Reservoir
From 06:00 on August 18, 2018 to 21:00 on August 19, 2018, the reservoir area experienced continuous rainfall for 40 hours, with a cumulative rainfall of 241mm. The maximum 2-hour rainfall occurred at the end of the rainfall, with a cumulative rainfall of 82mm and the rain peak after, which aggravated the flood risk. During the whole rainfall process, the accumulated inflow water of Yeyuan Reservoir was 111 million m3, the accumulated outflow water was 99 million m3, and the transferred storage water of the reservoir was 12.4 million m3.
The actual operation flow process of Yeyuan 819 rainstorm reservoir
According to news materials, before the rainstorm arrived, the water level of the reservoir was pre-reduced to 136.2m, with a corresponding storage capacity of 82 million m3. The suitable water environment takes this as the initial water level, and the maximum water level in flood is 138.5m through flood control calculation; , the maximum water storage capacity in flood season is 108 million m3, and the flood storage capacity of flood regulation is 26 million m3 in this peak flood.
Yeyuan 819 rainstorm flood control calculation of reservoir water level, storage capacity change process
3.4.2 Potential estimation of flood control storage capacity of Yeyuan Reservoir
In order to make use of more flood control storage capacity, it is assumed that the drainage of Yeyuan reservoir is controlled to 300m3/s (actual 700m3/s) at the time of operation. The flood control calculation shows that the accumulated inflow of Yeyuan reservoir is 111 million m3, and the outflow of Yeyuan reservoir is reduced from the current 99 million m3 to 71 million m3. The whole flood reservoir reserves 40 million m3, which is 26 million m3 more than the actual amount.
The highest water level during the flood increased from the current 138.5m to 140.23m; The maximum storage capacity in flood season increased from 108 million m3 to 131 million m3, and the actual peak flood diversion volume increased from 26 million m3 to 49 million m3.
The verified flood level of Yeyuan Reservoir is 141.95m, and the upstream migration elevation line is 139.0m (from the feasibility analysis of raising the flood limit water level of Yeyuan Reservoir in 2016.4). This is also a constraint. If the drainage volume is reduced, the water level of the reservoir will rise to 140.23m, and the flood loss will be generated in the reservoir area. It can be seen that the Yeyuan reservoir can theoretically continue to use the flood transfer reservoir capacity of about 20 million m3, but it will aggravate the inundated loss in the upper reaches.
The peak flow of 819 into the reservoir occurs only about 2 hours after the rain peak, and the peak is close to 2000m3/s, with 7 million m3 water per hour into the reservoir. If the rainfall continues, the reservoir will exceed the checked flood level after a few hours, and the risk of dam discharge or dam break will be greatly increased.
The above analysis shows that the 819 rainstorm reservoir operation, Yeyuan reservoir has an additional 26 million m3 relatively safe available flood capacity. However, the precondition is that there must be accurate weather forecast for at least 6-12 hours in the future and advanced real-time model-based flood early warning and forecasting system in the reservoir basin. Otherwise, the decision is extremely risky.
Contrast diagram of the drainage plan of Yeyuan 819 Rainstorm Reservoir (comparison of 700m3/s and 300m3 /s)
Change of water level of Yeyuan 819 rainstorm flood control calculation Reservoir (comparison between 700m3/s and 300m3 /s discharge)
3.4.3 Analysis of potential utilization effect of flood control storage capacity of Yeyuan Reservoir
According to the above assumptions, that is, the maximum discharge of Yeyuan reservoir is controlled to 300m3/s, and the inundated range and risk level are obtained by using model simulation.
According to the actual discharge and the simulation results of this scheme, the flood risk is compared and analyzed. The results show that the scheme controls the discharge of Yeyuan reservoir no more than 300m3/s, and the high-risk area is reduced by 2.6km2, with a reduction rate of 3.5%. The area of the medium-risk area was reduced by 0.21km2 with a reduction rate of 0.5%, and the area of the low-risk area was reduced by 0.77km2 with a reduction rate of 0.8%. The overall risk area was reduced by 3.6km2, with a reduction rate of 1.7%. The overall reduction effect is not obvious, indicating that it has little effect on reducing the downstream flood disaster.
The specific results are as follows:
The comparative analysis of the above table shows that under the 819 rainstorm condition, when the flood control capacity potential of Yeyuan reservoir is used, that is, the discharge flow is controlled at 300m3/s, the downstream flood risk has not been significantly reduced, and it has little effect on reducing the downstream flood disaster. It shows that the local flood effect in 819 rainstorm area plays a significant role.
3.4.4 Analysis of reservoir group scheduling
Before the rainstorm on the 19th, the water level of the reservoir was estimated to be near the flood limit water level (lack of measured data). According to the actual release of 819 storm reservoir discharge (Figure below), the Yeyuan reservoir began to discharge from 200-400m3/s at noon on the 19th, and the Heihushan Reservoir began to discharge from 80-160m3/s at noon on the 19th, indicating that according to the weather forecast, the dispatch management department took pre-discharge measures.
At the peak of rainfall and reservoir inflow, the Heihushan Reservoir discharged about 900m3/s, and the source reservoir controlled the discharge and realized the wrong peak.
There is no data of water level and discharge of Songshan reservoir.
The actual discharge process of rainstorm reservoir
According to the news data, the drainage area of Heihushan Reservoir is 190km2, the total storage capacity is 56 million m3, the highest water level in history is 165.38 meters, and the warning water level is 168.19 meters. The highest water level of Heihushan Reservoir reached 167.84 meters in 819 rainstorm. At 8 am on August 24, the water level of the Heihushan reservoir was 164.22 meters, still 1.22 meters above the end of the flood. It is estimated that the water level in the early stage was slightly high, which led to the rapid priority discharge when the rain peak arrived on the evening of 19, easing the danger of the dam.
3.4.5 Impact assessment of flood limit water level rise in Yeyuan Reservoir
According to Lin Liting et al. 's Feasibility Analysis on Providing Flood Limit Water Level of Yeyuan Reservoir, published in Shandong Water Resources in April 2016, the original flood limit water level of Yeyuan Reservoir is 136.5m (corresponding storage capacity of 79.35 million m3), and it is planned to be raised to 137.72 m (corresponding storage capacity of 96.78 million m3). Increase Xingli storage capacity by about 17 million m3, or reduce flood control storage capacity by 17 million m3.
According to the above analogy of "analysis of potential utilization effect of Yeyuan Reservoir flood control storage capacity", if the original flood limit water level of 136.5m is used for dispatching, and the flood control storage capacity of 17m m3 is increased, the affected area of downstream flood inundated may be reduced by about 1.7%.
Under the premise of the lack of serious study of the research results of raising the flood limit water level, the simple recognition or denial of this research conclusion is a lack of respect or contempt for complex technical work. We believe that the determination of reservoir characteristic water level requires long-term hydrometeorological data, socio-economic and urban development planning data within the scope of interest influence, and the use of water resources, flood control, economic and other technical analysis, comprehensive determination. Due to the social development of water resources demand, changes in water use structure characteristics, the continuous enrichment of hydrometeorological data and the improvement of calculation technology and management level, the correction of these characteristic water levels is not uncommon.
However, the correction of the characteristic water level needs to consider the corresponding measures such as the supporting flood prevention plan, the construction of flood control projects in the protection scope, the technology of early warning and forecast, risk assessment and dispatching, and the release and application of flood risk maps.
3.4.6 Evaluation of reservoir dispatching
Based on the above analysis, the technical decision of reservoir dispatching is basically reasonable under the conditions of reservoir water condition and urgent rain condition on the 19th.
The improvement space lies in the accuracy of weather forecast, the application of real-time watershed flood model and rainfall uncertainty prediction and decision technology, the optimization of water reservoir group and the research and formulation of emergency joint operation rules. At 8:00 on August 19, 2018, the National Meteorological Observatory released the rainfall forecast for the next 24 hours (see the figure below). The total amount and coverage described by the forecast are very similar to the GPM rainfall characteristics obtained by this research team. The meteorological forecast and water situation assessment based on which the rainstorm flood control and management departments rely are technically worthy of study and review.
National Meteorological Bureau 2018.8.19 Rainstorm forecast in the next 24 hours
3.5 Horizontal trend characteristics of Shouguang River section
Browsing Google image data in 2018, it is generally found that the river trend in the lower reaches of Mihe Shouguang City began to gradually show a wandering pattern, and the curvature increased. This is the result of the dynamic balance between the river and nature for many years. As can be seen in the picture, the Mi River is built with rubber DAMS in sections, and there are forks in the lower reaches, with large beach space. This kind of river regime, once a flood occurs, because of the kinetic energy of the water flow and straightening inertia, whether the flood plain is well protected is very important.
2018 "Shouguang Mihe River Flood Control Emergency Plan" according to the provincial prevention reference "Notice on the preparation and revision of the province's Backbone River flood control plan" (Lu Xunhan character [2004] No. 37) requirements, Shouguang City diversion control section status under the condition of water conservancy projects and Mihe flood capacity, to determine the Mihe main stream flood, so as to divide the flood level, Combined with the measured vertical and horizontal section data results, the water surface line calculation.
The flood grade is divided as follows:
- "General flood" branch flow ≤2722m3/s.
- "Current standard flood" branch flow 2722 m3/s ~ 3943 m3/s.
- "Over standard flood" branch flow ≥3943 m3/s.
This study estimates that the discharge below Mihe Shouguang is less than the above "general flood" level.
The downstream section of Mihe Shouguang City is complicated, and affected by bifurcation control project and bifurcation channel, the flood control capacity of the upper and lower reaches of Mihe River and the corresponding flow estimation technology are very important. This will affect the reasonable layout of the project, the construction investment and the flood safety of the protected area to a large extent.
4. Conclusion
According to the spatial distribution of rainfall, time history characteristics and model results, the 819 Shouguang flood is mainly caused by:
- Local heavy rainfall is the main cause of Shouguang flood. Extremely unfavorable rainstorm rainfall conditions of more than 300mm in the local 24 hours; Due to nearly 100mm of rain in the previous two days, 819 heavy rain almost 100% runoff production; According to the Shouguang flood control plan, the local rainstorm has exceeded the level I red warning, the danger itself is extremely urgent;
- Mi river downstream flood discharge capacity decline or not yet supporting, resulting in Shouguang City to the downstream section of Mi River flooding. Model analysis shows that the flood discharge capacity of the lower reaches of Mihe Shouguang may only be about 1500m3 / s, which is insufficient to meet the orange warning requirement of Grade II flood control plan of Shouguang City.
- The water level of Heihushan reservoir may be slightly high in the early stage, resulting in rapid discharge when the rain peak comes. Jieyuan Reservoir realized the wrong drainage peak and intercepted the inflow flood peak of nearly 1000m3 / s, and the flood control effect was remarkable. Although there is still more than 20 million m3 of available flood control storage capacity, it is considered that the operation is appropriate under the conditions of comprehensive consideration of the safety risks of major facilities of the dam and urgent decision-making.
- The optimal operation of 819 flood in one, two, middle and three reservoirs in the upper reaches has some potential. However, due to the limitation of the current hydrology and flood forecasting technology, the accuracy and self-confidence of the judgment of water situation and risk situation are slightly insufficient. Although this is not the main factor of the Shouguang flood disaster, but in order to play the comprehensive benefits of the reservoir, in the utilization of water resources, flood control risk management decision-making, support downstream social and economic development and urban safety, the improvement of decision-making technology has been very necessary and urgent.
