Peng Wenqi
The Department of Water Environment, China Institute of Water Resources and Hydroelectric Power Research (IWHR),Beijing, 100044, China
Telephone: 86-10-68415522-6723, E-mail: PWQ@IWHR.COM
Abstract:
A numeric model to simulate Yongding flood detention system is presented in the
paper. The model simulates flood evolution coupling with flow under gates and
sediment transportation. Calculation results for flow patterns, changes of water
level in reservoirs and discharge of gates is useful for designing and operation
of Yongding flood detention system.
Keywords: flood detention system, two-dimension numeric model
Yongding river in the west of Beijing, is the key river of capital flood protection. The upstream floods have been well controlled by Guanting reservoir built in 1953, but there has been no effective control works to defense floods in Guanting gorge. Guanting gorge covers the domain from Guanting reservoir to Sanjiadian, its basin area is 1600 km2. It contributed 90 percent flood volume in major historical floods of Yongding river. The treatment of the river zone downstream Sanjiadian through Beijing city has been made several times, and guarantees well the protection of Beijing city. But the flood protection standard of the right bank and downstream river is so low that the flood protection of Yongding river inundated land and Xiaoqinghe diversion zone will be dangerous.
New flood detention reservoir system is planned to control flood from Guanting gorge in order to enhance flood protection standard, which are arranged over the right beach between Zhaotian and Machang (Fig 1). The flood detention reservoir system is made up by three reservoirs, Daning reservoir, Zhaotian reservoir and Machang reservoir. Daning reservoir connects Xiaoqinghe flood diversion gate. An inlet gate with 6 holes whose net width is 10m is set betreen Daning reservoir and Zhaotian reservoir. A linked gate with 5 holes (the net width of each hole is 12m) is built to connect Zhaotian reservoirs and Machang Reservoir. A outlet gate which comprises of 7 holes (8m net width for per hole) locates at the tail of Machang reservoir. The total capcity of flood detention system is 83.65 million m3. The features of flood detention system are shown in table1.
The maximum discharge of Luguoqiao barrage is 2500m3/s, then, while 1/100 or 1/50 flood happens over Yongding river, the part more than 2500m3/s will be diverged through Xiaoqinghe flood diversion gate, and stay in reservoirs. The maximum rate of flow through Xiaoqinghe flood diversion gate will be lifted from 2760 m3/s to 3730m3/s after flood detention system is built. The diverged water first flows into Daning reservoir. When the water level of Daning reservoir rise to 49.0 m, inlet gate will be opened, and linked gate will be opened at the same time. When the water level of Machang reservoir reaches 50.5 m, linked gate will be closed , then only Daning reservoir and Zhaotian reservoir receives flood water. When the water level of Zhaotian reservoir becomes 53.5m, inlet gate must be closed. Discharge gate of Daning reservoir will start to discharge water no more than 214m3/s into Xiaoqinghe diversion zone if the water level of Daning reservoir grows to 61.21m. After the flood is over, depletion gate of Machang reservoir will discharge water no more than 400 m3/s.
Fig. 1 Layout of flood detention system
Table 1 The features of flood detention system
|
Reservoir name |
Elevation at reservoir bottom（m） |
The highest water level（m） |
Capacity （million m3） |
|
Daning |
48.0 |
61.21 |
36.10 |
|
Zhaotian |
47.0~47.5 |
53.5 |
30.80 |
|
Machang |
45.8~46.1 |
50.5 |
13.81 |
In order to ensure the reliability of engineering design and the safety of operation of flood detention system, it is necessary to study flow pattern in reservoirs and main river and evolution of river bed , and to analyze united operation scheme for flood detention system. A two dimensional numerical model to simulate hydrodynamics and sediment transport was developed. The model was verified using the field topographies observed before and after the flood in 1956. The model testing and the study concerning main river is presented in report[1]. The paper focuses on the simulation of flood detention system.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
where
=water level, H=total water depth,
=elevation of river bed,
,
=velocity components in direction
and
,
=Chezy coefficient,
，
=Maning roughness,
=gravity acceleration, R=hydraulic
radius,
=water depth averaged eddy coefficient,
，
,
=friction velocity,
,
=correction factor for water depth averaging,
=sediment concentration,
=turbulent diffusion coefficient,
,
=proportionality factors,
=settling velocity,
=saturation carry capacity,
=dry density.
ADI is adopted in the paper. The control equations are discreted using FVM (finite volume method). Readers interested in details may consult for reference[2].
Gates or weirs are designed in flood detention
system to input or output flood water. How to treat them in numerical model is
the key to successful simulation. All these gates are divided into two types
according to their positions in the calculation domain. The first type may be
regarded as upper or lower boundary, the second as sink and source. The first
type includes Xiaoqinghe diversion gate and depletion gate, the second covers
the others.
Xiaoqinghe diversion gate is taken as upper boundary, whose
discharge curve (shown in Fig. 2) determinates upper boundary condition. While
flood 1/50 or 1/100 happens to Yongding river, flow rate more than
2500m3/s which is the maximum discharge of Lugouqiao barrage,
will be diverged through Xiaoqinghe diversion gate.
Depletion gate is regarded as lower boundary. It’s flow rate is calculated according to hydraulic formula for outflow under gates. Because it is opened until flood already falls, in the simulation the effect of depletion gate is not counted in.
Inlet gate and linked gate both connecting two reservoirs, may be treated as sink and source couple. Formulas for their flow rate are complex. According to water levels before and behind gate, and structure sizes of gates, flow rate should be calculated for overflow weir (free or submerged), outflow under gate (free or submerged). Of course, the calculation of flow rate of gates should be coupled with the simulation of flow.
Before flood diverging there is no water in all reservoirs.
The growth of water level in three reservoirs during flood 1/50 and 1/100 is
shown
in Fig.4 . The changes of discharge of gates with time are displayed in
Fig.5
During flood 1/100 water level in Machang reservoir reaches limited water level 50.5m in 7.7 hours after diverging, linked gate is closed. Water level in Zhaotian reservoir rises to limited water level (53.5m) nearly at the same time, then inlet gate is also shut About 8.3 hours after diverging water level in Daning reservoir becomes limited level (61.21m), discharge gate should be opened to release water in 214 m3/s. The maximum flow rates through gates into Zhaotian reservoir and Machang reservoir are 2417 m3/s and 1098 m3/s respectively. These peaks arrive late 3.5 hours and 4.5 hours after the peak through diversion gate.
During flood 1/50 no reservoirs receives so much water that water level reaches limited water level, so inlet gate and linked gate are always open, discharge gate is not necessary to operate. The peaks into Zhaotian reservoir and Machang reservoir are 1596 m3/s and 536m3/s, appearing late 4.3 hours and 7.5 hours than the peak of
Fig.
6 Vector of flood 1/100 at 25 Hr.
Flood diverged through diversion goes through bottleneck with velocity more than 12m/s, so some protection engineerings should be arrayed here to keep river banks from being brushed. After flood crowds into Daning reservoir, flood current forms a fan-shaped diffusion zone near right bank. As water level in Daning reservoir grows, main stream become bending with two eddies of unequal intensity formed at two sides
Main stream in Zhaotian reservoir flows straightly at initial time, then began to sway as water level grows, moving near left bank with a eddy’s forming at right side near linked gate. Main stream doesn’t disappear until x=8000.
There is no water flows into Machang reservoir in 2 or 3 hours after water being diverged. Main stream stays in middle when water level is low, and turn to sway with two eddies being generated at two side.
After flood diversion is over, or inlet gate and linked gate are closed according to operation scheme of flood detention system during flood diversion, no water flows into reservoirs, current in reservoirs turns to be stagnant slowly, main stream and eddies disappearing.
There is no silting in bottleneck with width
200m behind diversion gate because of great carry capacity of rapid current.
After current rushes into Daning reservoirs, it’s velocity reduces swiftly at
diffusion zone, a large number sediment settles. Total deposit of three
reservoirs during flood 1/50 is about 2.11 million m3.
Deposit in Daning reservoir is about 1.43 million m3,
68% of sum. So major sediment falls in Daning
reservoir. No silt happens before and behind inlet gate because of high
velocity. Deposit in Zhaotian reservoir is about 0.67million m3,
31% of sum. Sediment almost deposits in foreword two reservoirs, only little
fine sediment (1% of sum) is carried into Machang reservoir.
Fig. 7 Contour of depth of silted layer after flood 1/100
Total deposit of three reservoirs during flood 1/100 is about 6.87 million m3. Sediment of 4.52 million m3, 66% of sum falls in Daning reservoir. Silt layer in Daning reservoir extends in fan-shape, thick in middle and thin at two sides, thick upstream and thin downstream. The largest depth is up to 1.8m. Deposit in Zhaotian reservoir is about 2.22million m3, 32% of sum. The largest depth in Zhaotian reservoir is 0.6m. Sediment about 2% of sum is received by Machang reservoir.
Flood evolution, sediment transportation and flow over weirs and under gates make the simulation of Yongding flood detention system very complex. Numeric model presented in the article found a way to successful simulation.
Results of model show that the operation scheme of detention system can control flood well. About 2/3 of total sediment into detention system deposits in Daning reservoir, about 1/3 in Zhaotian reservoir, litter than 2% in Machang reservoir.
The peaks for various reservoirs come in different time because of detention of reservoir.
The flow patterns in reservoirs are like jets in stagnant water in a way.
References
[1] The report of study on Yongding flood detention reservoirs, Beijing institute of hydraulic research, 2000.
[2] Peng wenqi et al., Theory and practice of numeric simulation of river engineering, 1999, Yellow river hydraulic press.