THE STUDY ON FLOOD DISPATCH OF YONGDINGXINHE RIVER^*

Chaohong  Zhou¹,  Shouquan  Chang¹,  Jinyuan  Du¹ and Mingxia  Wang¹

¹ Tianjin Institute of Water Resources Research, Tianjin 300061, China

Abstract: Yongdingxinhe River is an artificial channel, which protects Tianjin municipality from flood risk. Recent years, because of channel deposit and dyke depression, the carring capacity of channel has been reduced seriously. In case of flood, it’s very dangerous for Tianjin. So detention basins should be used to diverge flood. A multiply river network model, which bases on junctions group method is adopted for flood dispatch of Yongdingxinhe River. According to the complicated dispatch condition, the original model has been improved to suit for present dispatch measures’ calculation, optimized dispatch measures’ calculation and real-time dispatch calculation. This study indicates that the model can complete calculation in short time steadily and accurately. In this text, the reduction of river channel’s water stage, discharge, storage of detention basins and their damage are calculated. The calculation result of present dispatch measures indicates it is an effective way to reduce water stage in main stem to divert flood into detention basins, but the unreasonable present diversion gate conditions increase the damage in detention basins. If adjusting the diversion gate condition, the damage can be reduced greatly.

Key words: flood dispatch, multiply river network model

1    INTRODUCTION

Yongdingxinhe River is an artificial channel which was excavated in 1971, the four rivers in north Haihe River basin, including Yongdingxinhe River, Beiyunhe River, Chaobaihe River and Jiyun River, flowing together into the sea through Yongdingxinhe River. Its right dyke is the north defence of municipal flood control in Tianjin. Because of channel deposit and dyke depression, the carrying capacity of channel has been reduced seriously. In case of flood, it’s very dangerous for Tianjin. So in order to protect Tianjin from flood, we must take advantage of Xiqilihai, Dianbei and Sanjiaodian to diverge flood.

It’s demanded by the Flood Control Department to forecast flow characteristic in river channel and detention basins and flood diversion affect in using dispatch measure in short time. It must meet the requirement of the people who make decisions based on known and forecasted discharge process in upstream and tributaries and the level processing in downstream, be suit for the pysiography conditions. The calculation must have high accuracy and quickly, and the model must be flexible and easily to modify. In order to provide references for decision, a best measure must be obtained by comparysion.

 *The study is supported by the national Natural Science Foundation of China and the Minister of Water
 Resource (No.59890200).

Yongdingxinhe River including its tributaries and detention basins come into being a union, the flood movement in it has typical flow characteristic in river network, so the application of complex river network model will be effective. And many experience have been accumulated by using this model in Jinjing and Dongting Lake^[1]. We have improved the model based on the real conditions of Yongdingxinhe River and its detention basins. After improvement, the model can be used on some complicated conditions, and the result is content with the requirement of flood dispatch in Yongdingxinhe River.

1.1    Multiply river network model

Based on many learners’ study, Liyitian suggested a new method for river network equation group , he divided junctions into groups. In this method junctions can be divided into willfully groups based on real condition, and the coefficient matrix dimension can be reduced arbitrarily. Thus not only saves the computational time and storage hut also offers high accuracy.

1.2    Basic form of the river network equations

Junction is the joint of two or more reaches, the boundary point can be also looked as a junction, and the channel between two junctions is looked as a reach. The upstream and down stream boundary points, the joint of tributary and main stem, diversion point and detention basing are also looked as junctions. The reach between diversion point and detention basing is a weir reach.

Elementary equations:

Flow continuity equation.                                             (1)

Conservation of momentum equation.             (2)

In the two equations, “A” is area of wetted cross-section, “Q” is discharge, “q” is side inflow volume, “n” is coefficient of roughness, “R” is hydraulic radius, “Z” is level of water, “a” is dynamic modifying coefficient, “g” is gravitational acceleration.

River network junction equations

Flow continuity equation:

The volume that inflow or outflow a junction must equal the increase or reduction of the water volume of the same junction.

                     (3)

In this equation, “M” is the junction number of the river network, “l(m)” is the reach number which linked with junction “m”, Q_m,lⁿ⁺¹ is the volume that inflow or outflow from reach “k”, Q_mⁿ⁺¹ is the whole inflow volume at junction “m”. (when junction “m” isn’t the joint of tributary), Ωm is the storage at junction “m”.

The prerequisite of conservation of momentum:

It is decided by whether velocity head and the loss of resistance at junction are considered. On general condition, we can regard the water elevations of reaches at the junction as the same, and regard its increase as the same.

                  (4)

By using preissmamm quarter-point eccentric implicit Method, following two equations can be gotten through the dispersion.

                       (5)

                  (6)

In the equations, ai, bi, ci, di, ei are difference coefficients from formula (1) after dispersion, ai’,bi’,ci’,di’,ei’ are difference coefficients from formula (2) after dispersion.

1.3    The calculation of the model

Liyitian had proposed a junction group method for unsteady flow in multiply connected networks^[2]. By using this method, the junctions must be numbered from downstream to upstream, then be divided into groups. In order to get the water stage and discharge at any section, we must make out water stage at the junction and discharge at the bottom of reach must be made out by variate replacement. Because the dimension of equations equal the junction number in a group, the coefficient matrix can be reduced by using this method. The detail of this method can be gotten in reference ^[2].

2    IMPROVEMENT OF THE MODEL

The model was first used on the flood dispatch of Dongting Lake, the result indicates that the model can be used on not only multiply connected network but also the flood dispatch of detention basins. The water stage and discharge at any section can be gotten, and the flood division time, the division process, the change of water stage in detention basing after division can be also obtained. In addition, when the flood begins, division effect can be forecasted based on real conditions. In the original model, a detention basins is regarded as only one diversion gate, there’re no reaches among detention basins. But the dispatch in Yongdingxinhe River is complicated, there’re more than one diversion gate of flood division from main stem to the same detention basin. In order to apply the model on Yongdingxinhe River, the original model was improved. The model has several new functions now. (i) Detention basins can be connected with each other.(ii) There can be more than one diversion gate linked to a detention basins.(iii) The diversion gate can be opened step by step. It not only conforms to real condition, but also can avoid vibration caused by the suddenly open.(iv) The calculation is stable when the ratio of division discharge and channel discharge is big.

3    CALCULATION OF PRESENT DISPATCH MEASURES

3.1    Calculation condition

The calculation region is from Dawangcun down to the month of Yongdingxinhe river, the channel is 80 kilometers long, including Sanjiaodian, Dianbei, Xiqilihai detention basins, three tributaries ( Beiyunhe River, Beijing Drainage River, Chaobaixinhe River ) and the hydraulic center of Qujiadian.

3.2    Calculation result

We have calculated many measures in different conditions. Table 1 shows the calculation result of the measure, When Beiyunhe River overflows 200m³/s, Beijing Drainage River inflows 200m³/s and Chaobaixinhe River inflows 2100m³/s. Table 2 shows the application result of detention basins .

Table 1, Table 2

Table  1  The Maximum water stage and discharge at controlled sections before and after flood diversion

 Table  2  The application of detention basins

3.3    Result Analysis

The influence of Chaobaixinhe river and Beijing drainage river

The water stage of Yongdingxinhe River and diversion of detection basins are influnced seriously by the inflow from Chaobaixinhe River and Beijing Drainage River. We proposed that the inflow from two tributaries is limited. During flood once every 5, 10, 20 years, Xiqilihai can be avoided to be use. The storage can be reduced 479, 559, 680 million cubic meters, and the damage can be reduced 187, 213, 216.7 millions yuan, respectively. During the flood once every 50 years, the 7+100 diversion gate of Dianbei can be avoided to be used. During the flood once every 100 years, although Xiqilihai and Dianbei must be used, the overflow can be greatly reduced. Their maximum storage respectively reduced 192 and 210million cubic meters, and the damage can be reduced 2290 millions yuan. When the maximum discharge comes, if the inflow from tributaries can be controlled, the flood damage can be greatly reduced.

The influence of Beiyunhe river

The diversion of Beiyunhe River can effect the water stage and discharge at Qujiadian Gate, and further effect the application, storage and damage of Xiqilihai, Dianbei and Sanjiaodian. The diversion of Beiyunhe River can effect the flood damage, especially during great flood water. For example, during the flood once every 20 years, considering the influence of Chaobaixinhe River and Beijing Drainage River. If Beiyunhe River diverted flood, the maximum water stage at Qujiadian Gate can be reduced 0.3 meters, the maximum discharge can be reduced 152.7 m³/s, the maximum water stage of Xiqilihai can be reduced 0.28m, and the flood damage can be reduced 634 million yuan.

The influence of diversion gate conditions

The channel capacity of Yongdingxinhe River is 800m³/s. But when the 28+082 diversion gate is opened, there is still a discharge 200 m³/s into the detention basin when the upstream discharge is lower than 800 m³/s. That means there’s only less than 600 m³/s discharge in the channel, the channel capacity hasn't been fully used , the reason is the low bottom elevation of the diversion gate. The condition of 22+200 and 7+100 diversion gate is the same.

4    THE CALCULATION OF OPTIMIZED DISPATCH

The calculation result of present dispatch measures shows that the loss of detention basins is very large, so the main purpose of optimization is to reduce the loss. The best measure is discharge alternation, which means when peak discharge occurrence in the main tem, the flow in tributaries must be reduced. The measure can be only used when the flow in tributaries is small or the flow can be released from other channel. The application of Beiyunhe River can reduce the usage opportunity of detention basins.

Because the lower bottom elevation of 7+100, 22+200, 28+082 diversion gate, the channel capacity can not be fully used, so the three diversion gates must be widen and raised. The bottom elevation of the three diversion gates is designed as 0.5meters lower than diversion stage. The 28+082 diversion gate is 220 meters wide and the bottom altitude is 4.28 meters, the 22+200 diversion gate is 200 meters wide and the bottom altitude is 4.58 meters, 7+100 diversion gate is 220 in wide and the bottom altitude is 5.31m. Table 3 shows the increase of water stage and the application result of detention basins based on new diversion gate conditions.

Table 3

Table  3  The effect of optimized dispatch

5    CONCLUDING REMARKS

The multiply connected river network model which based on junctions group method is an effective and economic model for the calculation of flood dispatch in river network. According to the complicated dispatch condition, the original model has been improved, to suit for present dispatch measures’ calculation, optimized dispatch calculation and real time dispatch calculation. The model can complete calculation in short time steadily and accurately. When there’re many measures must be calculated before and during flood period, the model has great advantage. The calculation result of present dispatch measures indicates it is an effective way to reduce water stage in main stem to divert flood into detention basins, but the unreasonable of present diversion gate condition increased the damage in detention basins. The flood dispatch measure, based on reducing flow from tributaries, using Beiyunhe River to diverse and taking the reasonable diversion gate condition, can effectively reduce water stage in main stem and damage in detention basins.

References

[1]  Liyitian, The application of numerical model on detention dispatch in Dongting Lake, Wuhan University of Hydraulic and Electric Engineering, Aug 1997.

[2]  Liyitian, The junctions group method for unsteady flow in multiply connected network, Mar.1997.

Fig.1  Diagram of flood rivers