Fast and optimal decision for emergency control of sudden water pollution accidents in long distance water diversion projects

WDPs

Water diversion projects

SNWDP

South-to-North Water Diversion Project

MR-SNWDP

Middle Route of South-to-North Water Transfer Project

LDWDP

Long distance water diversion projects

AHP

Analytical hierarchy process

x_ij

Observation value

X = (x_ij)_3×6

The observation value matrix of each cluster object

A

The gate closing mode evaluation judgment matrix

E

The evaluation matrix

W

The weight of each index

σ_i^k

The comprehensive clustering coefficient

AT

Arrival time of the peak concentration

CP

The peak concentration in monitoring points

MV

Monitored value

CV

Calculated value

RE

Relative error

In recent years, with rapid economic development and increasing anthropogenic activities, sudden water pollution accidents have become the focus of environmental problems (Rui et al. 2015). Unlike other pollution accidents, sudden water pollution accidents have characteristics of varied and complex pollution sources (Zhang et al. 2010; Moss et al. 2015), the pollution pathway and degree are unpredictable (Morscheidt et al. 2013). To address the problem of uneven distribution of water resources on economic development, an increasing number of water diversion projects (WDPs) have recently been implemented, such as the California State Water Project and the Central Arizona Project in the United States and the South-to-North Water Diversion Project (SNWDP) in China (Huang & Niu 2015). Open-channels, complex operation conditions, higher water quality requirements, diverse types and the increasing number of water pollution risk sources are the characteristics of long distance water diversion projects (LDWDP). Once a sudden water pollution accident occurs, if the treatment is unreasonable and not timely, it will not only cause harm to the water channel but also bring great economic and environmental disasters to humans and society (Zhang et al. 2012; Shi et al. 2014; Fan et al. 2015). In addition to property damage and loss of lives, water pollution caused by sudden accidents in WDPs is a significant issue in China (Posthuma et al. 2014; Mi et al. 2015). Emergency control plays a key role in disaster mitigation, it is important to develop a fast and optimal decision to ensure that the emergency control of sudden water pollution accidents is reasonable. Therefore, a fast and optimal decision can help dispatchers to make correct and timely optimal decisions, which is an important scientific and technological support for the safety of a project.

At present, many scholars have done a lot of studies on the early warning systems, emergency control systems, and environmental risk analysis and response systems. These studies have played an important role in accident treatment and evaluation (Tang et al. 2015), water quality and risk assessments (Grifoll et al. 2011; Shi et al. 2017), risk prediction and analysis (Tang et al. 2016) and reducing the hazards of sudden water pollution accidents in recent years. For example, a real-time rapid emergency control model was proposed to cope with sudden water pollution accidents (Xu et al. 2017). An emergency response system was established to properly and proactively deal with safety incidents (Duan & He 2015). A simple conceptual model was developed to support local authorities making rapid technical decisions. (Rebelo et al. 2014). These systems have provided beneficial support for early warning and regulation of pollution accidents. But, further analysis of these systems showed that many of these systems still have several limitations. Specifically, in the study of emergency control, the systems rarely focused on fast optimization; and mainly concentrated on qualitative research, rather than quantitative studies. For example, in 2011, an oil event spill occurred in the Qiantang River, which is the source of drinking water for several cities in Zhejiang Province. The emergency team just determined the pollution scope through sampling analysis and then took measures to deal with it (Hou et al. 2013).

Due to the lack of an effective emergency control system for sudden water pollution accidents, the main purpose of this study is to establish a fast and optimal decision for emergency control of sudden water pollution accidents in LDWDP. This fast and optimal decision applies to sudden soluble water pollution accidents and sudden floating oil pollution accidents. Furthermore, the optimal decision is examined with a sudden water pollution accident that occurred in the Middle Route of the South-to-North Water Diversion Project(MR-SNWDP), China.

In this study, the proposed fast and optimal decision covers four stages, namely qualitative analysis, quantitative calculation, qualitative decision-making, and quantitative decision-making. The frame diagram of the fast and optimal decision is given in Figure 1.

Once a sudden water pollution accident happens, the pollutants are detected by monitoring systems; the number and position of monitoring points influence the detection time, and timely detection of accidents is crucial for optimal control. Dispatchers or decision-makers will alert about the accident and analyze the characteristics of the accident channel and the nature of pollutants according to the engineering parameters and the results from water quality monitoring points. The accident channel geometry and its hydraulic characteristics, degree of gate opening and pollutant types can then be determined. According to the accident information, the dispatchers or decision-makers can determine whether the gate should be closed or not. If the accident does not affect the water diversion project and water quality, the normal water transfer mode will be maintained. Otherwise, the degree of gate opening should be adjusted. A hydraulic sketch of the channels and the check gates is given in Figure 2.

Once a sudden water pollution accident happens, if the dispatchers or decision-makers cannot quickly and effectively determine the gate closing mode, which expresses the order of gate closing, the gate closing mode is divided into synchronous gate closing, ‘upstream first closing’ of asynchronous gate closing and ‘downstream first closing’ of asynchronous gate closing (Yang & Zhou 2010), this will lead to errors for the control scheme and delay the rescue time. If the gate is closed too fast, water in the channel may continually oscillate and cause the canal system to be out of control (Cui et al. 2014). Besides, unreasonable selection of the gate closing mode will lead to a sharp decrease in the flow, while plants or algae in channels will be affected if they are in this state for a long time (Mallika & Richardson 2009). Therefore, it is important to study the gate closing mode. Based on this, stage 2 is divided into two steps: determining the gate closing mode and putting forward the formula and condition of the decision parameters.

The selection of the gate closing mode mainly depends on the gate control effect and operation technology. The grey fixed weight clustering can objectively and quantitatively evaluate the advantages and disadvantages of different gate closing modes. The analytic hierarchy process (AHP) (Al-Harbi 2001) will change the multi-state variables that affect the control effect of the gate closing into a single-state variable so that the gate control effect can be easily realized, and the results of the state evaluation can be given quantitatively. Therefore, in this paper, the AHP and grey fixed weight clustering are combined to determine the gate closing mode.

First, the evaluation system of the gate closing mode is determined. The gate closing mode is divided into synchronous gate closing, ‘upstream first closing’ of asynchronous gate closing and ‘downstream first closing’ of asynchronous gate closing (Yang & Zhou 2010), which are cluster objects. The observation value matrix of each cluster object is X = (x_ij)_3×6. Emergency decision-makers need to evaluate and judge the corresponding gate closing modes according to the observation value x_ij, and determine the grey class; thereby the gate closing mode is determined. The selection principle of the index is to directly reflect the gate closing effect. To meet the needs of the evaluation of the gate closing mode, the evaluation system established in this paper is shown in Figure 3.

Once a water pollution accident happens, the check gates of upstream and downstream will be closed immediately. In the process of check gates closing, the pollutants will move to the downstream with flow, and the pollution range will grow at the same time. After the check gate is fully closed, pollutants will fluctuate with water, and end in being stable after a certain time. Hence, decision parameters of emergency control (hereinafter called ECPs), which reflect the process of pollutants advection and diffusion, are put forward. They are transport distance, pollutants' longitudinal length, peak concentration, and diving condition, respectively. Through statistical analysis of the numerical simulation results, the formula and condition of the decision parameters are put forward, as shown in Table 4.

The temperature can be greatly variable along with the areas and the time. Therefore, it is necessary to formulate corresponding emergency control strategies for sudden water pollution accidents in different water diversion periods. The emergency control strategy of sudden water pollution accidents in the conventional water transfer period has been proposed by Long et al. (2016). And the emergency control strategy in an icing period is presented. This strategy is divided into two parts: normal channel emergency control and frozen channel emergency control. Soluble pollutants and floating oil pollutants are considered in each part respectively. In China, there are four seasons: spring, summer, autumn and winter, and the temperature changes greatly along with the time. Generally, spring, summer, and autumn are normal water transfer periods, and winter is the icing period. Surface water may be frozen, it is necessary to formulate corresponding emergency control strategies for sudden water pollution accidents, some channels may form stable ice cover in the icing period, ice cover may be damaged if the emergency control is improper, and then secondary disasters such as ice dams or ice plugs will happen, water conveyance flow may reduce or even be interrupted, the consequences are more severe than in a normal water transfer period. So it is necessary to formulate emergency control strategies in an icing period according to factors such as the location of the sudden water pollution accident, the pollutant types, and decision-makers' need. When a sudden water pollution accident occurs in unfrozen channels, emergency control strategies should be selected based on pollutant types and ensures pollutants do not spread into the ice channels. When a sudden water pollution accident occurs in a frozen channel and the pollutants are only on the ice cover, the ice cover is equivalent to a protective layer, pollutants should be quickly coped with using activated carbon, activated carbon fiber felt, jute, straw, and cotton yarn. Once pollutants enter the water body through the ice cover, the emergency control strategies should ensure the ice cover is not damaged. The detailed emergency control strategy in an icing period is shown in Figure 4.

According to the location of the accident, the channels are divided into three parts: upstream sections of the accident channel, accident channel, and downstream sections of the accident channel. For upstream sections of the accident channel, water supply for upstream branches should stay normal by adjusting the inflow. For upstream sections of the accident channel, water supply for upstream branches should stay normal by adjusting the inflow. For the accident channel, water supply should be interrupted if water does not meet quality requirements; pollutants should be coped with as soon as possible. For downstream sections of the accident channel, water supply for downstream branches should be satisfied as far as possible by the operation of the check gates. The hydraulic sketch of the channels and the check gates is given in Figure 2.

According to the emergency control strategy, the emergency control plan should be determined considering the actual situation of the water diversion project and the demand for decision-makers.

The Middle Route of the South-to-North Water Diversion Project transfers clean water from the Taocha Dam of the Danjiangkou Reservoir in Hubei Province, as shown in Figure 5. The Middle Route crosses the Huaihe River, Haihe River, and Yellow River basins and finally arrives at Beijing's Tuancheng Lake. With a length of 1,277 km, MR-SNWDP transfers clean water to nearly 150 cities, 58 million inhabitants and 151 thousand hectares of land via open-channels, culverts, and pipes under extremely strict water quality requirements.

To improve the actual emergency treatment capacity of the emergency water pollution accident, the demonstrative project was conducted in Fangshui to Puyang channel of MR-SNWDP, which is 12.5 km in length, and the flow of water transfer during the demonstrative project was 5 m³/s (Long et al. 2016). Since MR-SNWDP was supplying water to Beijing and Tianjin at that time, consider the safety of water transfer, sucrose was selected as the pollutant material in the demonstrative project. At 9:00–9:05 on March 22, 2014, 1,000 kg sucrose was released into the channel at the Baiyunzhuangbei Bridge, which is 2.148 km from the upstream of Puyang check gate. Three monitoring points were exclusively established in the upstream of the Puyang check gate and the basic information is shown in Table 5.

In this study, we use sucrose to simulate soluble pollutants to validate our fast and optimal decision. The concrete method steps and results are explained as follows.

In this study, we assume that sucrose is a toxic soluble pollutant, the accident channel is the channel for Fangshui to Puyang, the check gate opening degree is 1 m, and there are three water quality monitoring points. The demonstrative channel is divided into three parts: the upstream sections of the accident channel are the upstream sections of the Fangshui check gate; the downstream sections of the accident channel are the downstream sections of the Puyang check gate.

(1) Establish evaluation judgment matrix A

(2) Determination of evaluation index value

The scoring criteria of the evaluation indexes described in Table 1 are then developed for evaluation systems. The scoring criteria of the evaluation indexes in Table 1 are determined based on expert consultation and literature research.

(3) Determination of triangle whiten function

(4) Determination of comprehensive clustering coefficient σ^ik

The weight W = (η₁,η₂,…,η_j)(j = 6) and triangle whiten function f_j^k(x) are incorporated into Equation (4). Finally, the comprehensive clustering coefficient of each index is obtained, as shown in Table 3.

The value of the dispersion coefficient is 3.43 m²/s in the demonstrative project. According to the formula (Table 4), the REs between MV and CV in each monitoring point are shown in Table 7. Compared with the practical monitoring, their REs are less than 18%.

The demonstrative project of MR-SNWDP was carried out in April. There was no ice in the channel at that time, which can be considered as a conventional water transfer period.

For the upstream sections of the Fangshui check gate, water supply for upstream branches should remain normal by adjusting the inflow. For the accident channel (Fangshui to Puyang), water supply will be interrupted if water does not meet quality requirements, the pollutants should be coped with as soon as possible. For the downstream sections of the Puyang check gate, the water supply for downstream branches should be satisfied as far as possible by the operation of running at a normal level.

In this case, v is equal to 0.07 m/s. T^b is 33 min. For the accident channel, according to the formula of the decision parameters (Table 4), the Fangshui check gate and Puyang check gate should be closed simultaneously within 60 min after the accident occurrs. And the values of transport distance, pollutant longitudinal length, and peak concentration are 94 m, 2,326 m and 29.86 mg/L, respectively.

This paper has proposed an entire framework of a fast and optimal decision system for emergency control of sudden water pollution accidents in the Long Distance Water Diversion Project. The proposed fast and optimal decision system has been examined in the demonstrative project conducted in the Middle Route of the South-to-North Water Diversion Project in China. Fast and optimal decision plays a fundamental role during the process of an emergency spill. Through the above research, the paper draws the following conclusions:

• The proposed framework for fast and optimal decision is reasonable and logical; it can be used as a reference for managers and other emergency control systems.

• The analytical hierarchy process (AHP) and grey fixed weight clustering can be combined to solve the quantitative calculation problem of optimal decision. The method can objectively and quantitatively evaluate the advantages and disadvantages of different gate closing modes. The combined method is simple and satisfied with fast and optimal decisions.

• The presented emergency control strategy in an icing period is described in detail. The strategy is divided into two parts: normal channel emergency control and frozen channel emergency control. Soluble pollutants and floating oil pollutants are considered in each part respectively. The emergency control strategy can be applied in other emergency controls of sudden water pollution accidents.

• The presented emergency control strategy in different channels is described in detail. For upstream sections of the accident channel, water supply for upstream branches should remain normal by adjusting the inflow. For the accident channel, water supply should be interrupted if water does not meet quality requirements; pollutants should be coped with as soon as possible. For downstream sections of the accident channel, water supply for downstream branches should be satisfied as far as possible by the operation of the check gates.

• The Middle Route of the South-to-North Water Diversion Project is selected as a demonstrative project to examine the proposed framework for fast and optimal decision, the case study was examined under the scenario of a sucrose spill in the demonstrative project in China; the relative errors of the arrival time of the peak concentration and the peak concentration in monitoring points between the actual monitoring values and the formula calculation values are less than 18%.

• Scholars and managers should combine with the actual situation of the water diversion project and the demand of decision-makers, to establish fast and optimal decision for emergency control of sudden water pollution accidents especially, because hydraulic characteristics, engineering parameters, and pollutant types may be different in each project.

• The case study in this paper did not examine the emergency control strategy in an icing period; we expect that scholars can do in-depth research into the emergency control strategy in an icing period, and apply the presented emergency control system to improve the framework and methods.

This study is supported by the Major National Science and Technology Project (2017ZX07108-001), National Natural Science Foundation of China (51609167) and Open Research Fund of State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin (China Institute of Water Resources and Hydropower Research) (IWHR-SKL-201704). The writers also acknowledge the assistance of anonymous reviewers.