The impacts of piers on oil spill transport in a typical reach of the Middle Yangtze River


 Oil spill, a frequent pollution in the utilization of rivers, is receiving increasing attention in the study of river ecosystems. Taking the Zhuankou–Yangluo Reach (ZYR) of the Middle Yangtze River as an example, the spatial and temporal behaviors of leaked oil in the river under uniformly arranged piers with varying densities were studied based on a MIKE21 hydrodynamic and oil drift model. The results show that the oil spill spread is less affected by the piers when the upstream oil spill point is located on the other side of the shoreline with piers. However, the influence of the piers on the same shoreline of the oil spill point on oil spill transportation cannot be ignored. The piers significantly reduce the oil spill drift speed in the engineering area, resulting in a significant increase in slick retention time and slick area, especially when the density of piers is greater than 1.25 units/km. These results will provide useful reference for river management, for example, in the upstream river of the water conservation area, especially on the same bank as the water intake, where a large number of piers should not be built.


INTRODUCTION
Due to global economic growth, the demand for petroleum products is increasing rapidly, hence many more oil spill accidents can be expected because of extensive oil exploitation and frequent water transportation (Vethamony et al. ). Oil spills pollute the ecological environment and endanger aquatic organisms significantly, often leading to long-term adverse impacts on the environment, ecology and socioeconomic activity of an area (Burk ; Glémarec ; Literathy ). Oil spill simulation in different scenarios can not only provide emergency plans for the prevention and control of secondary disasters after an accident but also help to distinguish high pollution risk areas according to the spill trajectories.
The study of oil transport processes is mostly carried out Various indicators such as spill trajectory, slick area, and residence time were proposed to present pollution characteristics. Although these studies can help to better understand the factors affecting oil transport in those marine environments, it should be noted that there have been relatively few studies on oil spill transport in inland rivers (Yapa & Shen ). Compared with sea areas, inland waterways are mostly narrow and curved, and the movement of the oil slick down the river is more easily affected by shorelines.
Different from coastal waters, hydrodynamic factors such as velocity and flow direction are profoundly affected by the river boundary. The trajectory of a river oil spill is usually dominated by currents rather than winds. Despite some researchers having paid attention to the influence of flow rate, shoreline type and the characteristics of the spill (size, location, continuous or instantaneous spill) on spill trajectory, retention time, and slick area (Shen & Yapa ; Danchuk & Willson ; Liu et al. ), to date there has been no attempt to study the influences of artificial constructions along river banks. We aim to contribute to filling this gap by integrating pier structure in the hydrodynamic model. In particular, we focus on oil spill transport for multi-scenarios under different pier densities.
With the increase in the scale of shoreline utilization, the piers on both sides of the river in some port cities are dense. These piers increase traffic flow and further increase the hidden risks of navigation accidents. They also directly change the hydrodynamics near the shoreline. Changes in hydrodynamic factors such as water level and flow velocity after the construction of a pier are key issues of project demonstration on flood hazards (Wang et al. ). Many studies have confirmed that the cumulative impacts on the flow caused by the pier groups (pile piers or grouped piers) should not be ignored (Zhang & Stive ). The hydrodynamic changes caused by the construction of piers near the shoreline have significant spatial differences in the local river waters of the project area and its upstream and downstream (Ali & Karim ; Zhang et al. ; Ataie-Ashtiani & Aslani-Kordkandi ). Hence, the piers' shoreline is bound to have a certain impact on oil spill transport, but what kind of impact is a question worth exploring.
As a section of 'golden waterway' in China and also a representative urban reach of the Middle Yangtze River, the ZYR flows through the famous port city of Wuhan and there are a great number of piers distributed along both sides of the river. Therefore, in this context, it is essential to understand the impacts of the piers on oil drift. The MIKE21 OS model was used to simulate the oil transport process. The objectives of this study were as follows: (i) to simulate hydrodynamics of the ZYR after arranging varying pier densities on the shoreline, (ii) to predict the spill trajectories for different scenarios under various pier densities, and (iii) to offer advice for shoreline utilization and plan suitable response strategies for port areas when spill episodes occur.

STUDY AREA
The Yangtze River is the longest river in China, with a total length of 6,300 km, and it is usually divided into upper, middle, and lower reaches on the basis of different geographical and hydrological characteristics (Chai et al. ). The study reach in this work, i.e., the ZYR, is located in the Middle Yangtze River, which spans a total length of 50.6 km, as shown in Figure 1. The upper section of the ZYR, which extends from Zhuankou to Guishan (15.2 km), has a relatively straight channel pattern, whereas the lower ZYR is classified as a slightly curved and branched channel extending from Guishan to Yangluo. One considerable tributary is located in the left bank of this stretch: the Han River, which converges into the main river at There are many piers on the ZYR's left and right banks, and their distributions are relatively irregular. The left bank mainly includes Zhuankou port, Hanyang port, Hankou port, and Yangluo port; the right bank port areas are mainly distributed in the middle and lower sections, such as Nianyutao port, Wuchang port, Qingshan port, etc. The Nianyutao-Xujiapeng Shoreline (NXS) is adjacent to the Wuchang deep trough with adequate water depth. The shape of NXS is smooth and it is suitable for port construction. Moreover, the narrow width in this river section makes the flow easily interfered with after pier construction, which will affect material transport. Therefore, this paper selects the NXS as a typical shoreline. Through numerical experiments, we explore the effect of arranging piers with different densities in the NXS on hydrodynamics and oil spill transport within an idealized ZYR to give insight into river oil spill transport with and without piers.

Verification of hydrodynamic model
Since the hydrodynamic module provides data such as water depth and flow field required to drive the oil spill module's operation, accurate hydrodynamic simulation is essential.
The model verification data adopts the water level and velocity data measured in July 2011. The incoming flow of the river during this period is 23,560 m 3 /s. The water level verification results show (Table 1) Wang () found that when the pier groups' density is greater than 1.33 units/km, the influences of the piers on the hydrodynamics will interfere with each other. Thus five densities of piers (0.31, 0.62, 1.25, 2.5, and 5 units/km) were implemented in this study, respectively (i.e. 3, 6, 12, 24, and 48 piers are evenly arranged along the NXS with the length of 9.6 km). Together with the condition without piers, it is composed of six densities, which have a specific generalization, but to some extent reflect the current sparse pier layout and the possible construction of dense pier groups in the future.
As we focus on exploring the impact of hydrodynamic changes caused by the pier shoreline on the spread of  We calculate the spill spread of six different pier densities for each scenario, a total of 18 simulation conditions (Table 2). Referring to related research, the main input parameters selected in each scenario are shown in Table 3.
The statistics and analysis of the results follow the cri-    With the increase of the piers' density, the changes in water level and flow velocity also increase considerably, and even the change amplitude increase by an order of magnitude, reflecting that the impact of the project group is much greater than that of a single project. However, when the density increases to a certain extent, the water wakes caused by the backwater effects of the piers interfere with       is evident that when the slick moves through the engineering area, its tail is affected by the area with reduced flow velocity, leading to the spill patches being continuously elongated. After 6.8 h, the spilled oil begins to spread to the export boundary, and the slick area rapidly decreases.

DISCUSSION
Advection is the dominant mechanism that governs oil slick transport in rivers. The advection of surface oil is driven by the effects of currents under the no-wind condition (Yapa & Shen ). Since the piers are arranged on the right bank, there are significant spatial differences in the impact on water flow conditions (Figure 3). With the same flow level and oil spill volume, the main difference between scenarios 1, 2, and 3 is that the oil spill location is different, which means that the slick is affected by different hydrodynamic conditions during the downward movement of the slick.
The following compares the differences between the three scenarios in terms of the downward trajectory, slick area, and residence time.  For water source protection areas such as water intakes, two critical issues in the oil spill pollution emergency plan are the arrival time of pollutants and the duration of the impact on water intake. Based on the above calculation results, it can be seen that the increase in slick retention time in the engineering area and downstream waters in scenario 3 is most unfavorable. In this regard, it is not advisable to build wading buildings such as pier groups and port areas near the same bank upstream of the water intake. Water intakes should not be arranged on the same bank side downstream of these areas.
It needs to be pointed out that the understanding of this paper is calculated based on the Wuhan reach. In fact, the boundary shape and flow conditions of different river sections are different. There is a certain randomness in the volume and location of oil spills. The situation of other rivers will be different from the results of this paper. However, the ideas, methods, and qualitative lessons of this paper can also lead to a better understanding for similar rivers.

CONCLUSION
Using a depth-averaged two-dimensional hydrodynamic oilspill-drift model, the changes of hydrodynamic and oil spill transport of ZYR under varying pier densities of the local shoreline were simulated. The response characteristics between pier densities and oil spill transport were analyzed.
The main conclusions are as follows: (1) The movement of slicks mainly depends on the currents where the spill episodes occur under the calm condition without piers. It drifts along the flow directions of the initial spill location at first and gradually shifts to the mainstream flow of the river over time.
(2) The shoreline of the piers has little effect on the spill trajectory. With hydrodynamic changes, when oil spill episodes occur different sides of the piers' shoreline, the slick area and drift speed are not significantly affected by the piers. However, when the oil spill occurs on the same side as the piers' shoreline, its movement is more affected by the piers. The oil slick retention time and slick oil area increase considerably when the piers' density is greater than 1.25 units/km.
(3) Considering the impact of pier groups on oil spill transport, it is not suitable to build wading buildings such as piers and port areas on the same bank upstream of the water intake, and thus it is not advisable to arrange water intakes on the same bank side downstream of dense piers.