Flood Influence Characteristics of Rail Transit Engineering of Tunnel, Viaduct, and Roadbed through Urban Flood Detention Areas

Abstract

Many subways, light rails, and trains travel through urban flood retention regions via tunnels, viaducts, and roadbeds; however, less is known about the flood influence laws of rail transportation by the crossing ways. Rail transit projects were chosen as research objects for the ordinary subway, light rail, and railway passing through urban flood detention areas in Wuhan, and the flood influence characteristics were systematically compared for the three crossing ways. The study revealed that crossing ways primarily affected the flood storage volume occupied per unit length of lines and that the flood influence of rail projects on flood detention areas was proportionate to the flood storage volume occupied per unit length of lines. Specifically, the flood storage volume occupied per unit length of tunnels was about 1/8.9 that of viaducts and 1/19.7 that of roadbeds. Moreover, the tunnel way had the least influence on the main aspects, such as flood control, floods on engineering, and engineering-related aspects; the roadbed-based way had the largest; and the viaduct way was in the middle. These findings may provide technical support for the decision-making, engineering planning, construction, and management of rail transit and other projects in urban flood detention areas.

1. Introduction

Flood detention areas refer to the low-lying areas and lakes that temporarily store floodwater beyond the backwater surfaces of river dikes, including port gates of flood separation, most of which are historical sites of inundation and storage of river floods. Flood detention areas include flood flow areas, flood diversion areas, flood storage areas, and stagnant flood areas [1,2]. Briefly, flood detention areas are an essential part of the flood control systems of rivers and an effective measure to ensure the safety of flood control in critical areas and mitigate disasters. To ensure the flood control safety of important areas, especially cities, opening up conditional areas as flood storage areas and storing floods in a planned manner is a realistic and economically reasonable need for a basin or regional flood control plan and a global consideration for preserving the overall situation while having to sacrifice local interests. From an overall perspective, preserving the flood control safety of important areas, local losses, and planning flood diversion are necessary and reasonable. Therefore, flood detention areas are restricted development areas, and flood impact evaluation is required for engineering construction in their areas to ensure the normal use of flood detention areas as well as the safety of the projects themselves. Moreover, urban infrastructure construction is growing at high speeds with the rapid development of social and economic development, and rail transit has been rapidly developing as an efficient mode of public transportation. The most common rail transits mainly include the subway, light rail, and railroad, and the most common ways of crossing flood detention areas include tunnels, viaducts, and roadbeds. However, many rail transit projects need to cross urban flood detention areas because of the limitations of urban development spaces, which may affect the regular use of flood detention areas and endanger the safety of projects. Therefore, flood influence laws need to be revealed for urban flood detention areas and their engineering for various rail transit projects by different crossing ways. These will benefit urban infrastructure construction by allowing flood detention areas to be used reasonably, and urban flood control and engineering safety can be realized by breaking the bottleneck of urban development spaces. The line of rail transit projects is long through flood detention areas; it needs to set up stations, and there are many types of projects and crossing ways. However, when the influence of flood control on the projects is evaluated through flood detention areas, generally only one or two crossing ways are usually used to evaluate a rail transit project. For different engineering types and crossing ways of rail transit, there is less research on the impact of flood control on urban flood detention areas. There are fewer results of the impact of flooding on the project itself and insufficient understanding of the laws of flood control affecting the project through the urban flood detention areas. Thus, it restricts the implementation of rail transit projects in flood detention areas. Therefore, the main challenge in the research is analyzing the flood influence characteristics of rail transit projects through urban flood detention areas by tunnel, viaduct, and roadbed-based ways. In this paper, we describe and demonstrate the flood influence characteristics and laws of three crossing ways of tunnel, viaduct, and roadbed crossing urban flood detention areas using qualitative, quantitative, and comparative analysis of engineering cases. These will provide a theoretical basis and technical support for relevant project planning, construction, and management.

In recent years, researchers have carried out a large number of calculations and analyses on the flood influence of flood detention areas; explored the flood influence characteristics of flood detention areas; analyzed the flood influence of different engineering crossing ways of flood detention areas; and achieved a series of results such as numerical calculations, influence analysis, risk assessment, optimal design, and engineering management. Flood detention areas divide into urban, rural, and combined types of urban and rural according to the region where they are located, and regional attributes are essential factors affecting engineering planning and use of flood detention areas. Specifically, the main results of the numerical calculation are as follows: the pre-filling probability of detention facilities for flood control in urban areas [3]. Aspects of influence analysis: the hydrological influence of land and detention basins on urban flood hazards [4] and the use of detention basins to reduce flash floods in urbanization [5]. Aspects of optimal design: the optimal design of urban rain detention facilities [6] and the optimization of reservoir systems for flood reduction and detention in urban drainage areas [7]. Aspects of engineering management: the optimal operation modes of urban detention basins [8] and the use of detention basins for flood mitigation and urban transformation [9]. Related results include: a one- and two-dimensional model is used for the calculation methods of flood influence [10]; an intensity-time-frequency curve of storm models is established for the excess runoff from the Abukuma River [11]; contributions of DOC are studied from surface and ground flow into lakes in Lake Võrtsjärv (Estonia) [12]; and constructed wetlands are used for a flood treatment system in Latvia [13]. In conclusion, most results focus on flood calculation, influence analysis, optimal design, and engineering management of urban detention areas. There is no consideration of the influence of project types and crossing ways on floods in flood detention areas, and there are fewer results on flood control in urban detention areas and other influences on floods. The flood influence of engineering through flood detention areas is closely related to engineering types (subway, light rail, railroad, and highway) and crossing ways (tunnel, viaduct, and roadbed), in addition to the regional properties of flood detention areas. Firstly, there are more results on the influence of highways and bridges on floods in flood detention areas. Specifically, a numerical method is used to simulate the process of flood storage before and after the construction of highway projects in a flood detention area [14]. A formula is proposed for calculating the expected scouring depth of bridge piers [15]. A flood model of vulnerability evaluation is proposed for bridges [16]. However, there is less research on the other influences of roads and bridges on flood control in urban detention areas. Secondly, the influence analysis of several projects crossing flood detention areas is also available, such as the light rail of No. 21 in Wuhan, China [17] and the flood evolution process of the underground gas storage in the flood detention area of Huaiyin around the Hongze Lake of China [18]. Specifically, the rail transit project of Line 21 in Wuhan is taken as a case study to analyze the influence of light rail [17]. A two-dimensional mathematical model of water flow evaluates the influence of the light rail construction project on flood storage and flood recession in the flood detention area. However, no other influence is analyzed for flood control in the flood detention area or other influences on floods, and no comparative analysis is carried out with other crossing ways. Finally, there are several results on the influence of urban transportation in Kinshasa, the Democratic Republic of the Congo [19], the subway [20], and the underground infrastructure in Milan, Italy [21] on flood control. However, these do not involve urban flood detention areas. Therefore, the results mainly focus on highways and light rails for the flood influence of engineering types on flood detention areas, and there are fewer results on subways and railways. The results mainly focus on tunnels and bridges for the flood influence of crossing ways on flood detention areas, and there are fewer results on roadbed ways and combined ways of roadbed and viaduct. In conclusion, the results focus on flood evolution and inundation analysis. There is no comparative analysis of the influence of various engineering types and crossing ways on floods in flood detention areas. There is less research on flood control laws in flood detention areas, the influence of floods on engineering, and the relevant influence of engineering. However, the flood influence rules have not been clarified for rail transit projects through urban flood detention areas in different ways, which makes it difficult to provide technical support for government decision-making, engineering planning, construction, and management. Therefore, it is essential to research the influence of various crossing ways for rail transit projects on floods using existing engineering cases and the actual situation of urban flood detention areas.

Taking urban flood detention areas in Wuhan, the goal was to reveal the interrelationship and laws between the construction of different rail transit projects and the influence of floods in urban flood detention areas. Specifically, many comparisons were carried out for the influence of different crossing ways of rail transit projects on flood control in flood detention areas, the influence of floods on engineering, and the relevant influential characteristics of engineering. For this reason, taking the flood detention areas of Lake Dongxi and Lake Wu as the research objects, three rail transit projects (subway, light rail, and railroad) were used as cases crossing urban flood detention areas in Wuhan. Qualitative and quantitative analyses were carried out with codes to explore the influence of flood control in flood detention areas, the influence of flood on engineering, and the relevant influential characteristics of engineering for the tunnel, viaduct, and roadbed crossing ways of rail transit projects. The study showed that crossing ways mainly determined the flood storage volume occupied per unit length of lines. The influence of the rail transit projects on flood detention areas was proportional to the flood storage volume occupied per unit length of lines. The tunnel way had the least influence on flood control in flood detention areas, flood engineering, and the relevant aspects of engineering; the roadbed-based way had the greatest; and the viaduct way was in the middle. The main strength of this study was a combination of qualitative and quantitative methods to compare and analyze the flooding influence characteristics of three types of rail transit projects through urban flood detention areas by tunnel, viaduct, and roadbed-based ways. It revealed the interrelationships between engineering construction and the flooding influence of different rail transits in urban flood detention areas. For different types and different ways of rail transits to cross the urban flood detention areas, the comparison data and influence rules may provide technical support for project review, planning and design, engineering construction, and management. Thus, these may be used as a reference for the construction and management of rail transit and other projects through urban flood detention areas.

2. Cases and Methods

2.1. Specific Objectives

We compare and analyze the influential characteristics of crossing ways on flood control in flood detention areas for three types of rail transit projects, such as the influence on the implementation of relevant plans, the influence on the use of flood detention areas, the influence on flood control projects, the influence on safety construction facilities, and the influence of river channels and canal systems.

We compare and analyze the influence characteristics of floods on crossing ways for three types of rail transit projects, such as the adaptability of the flood control standards of the construction projects and the chances of using flood detention areas, the influence of inundation, and the influence of scouring and siltation.

We compare and analyze the relevant influence characteristics of three crossing ways for rail transit projects, such as the influence of the existing projects and facilities of water conservancy, the influence of engineering management and flood control, the influence of the construction period, and the influence of the legal water rights and interests of the third party.

2.2. Overview of Urban Flood Detention Areas of Lakes Dongxi and Wu in Wuhan

2.2.1. Overview of Urban Flood Detention Areas in Wuhan

There are six flood detention areas near Wuhan, which bear the heavy responsibility of urban flood storage in Wuhan, as shown in Figure 1 [22] (pp. 126–127) and

Table 1. Basic information on urban flood detention areas in Wuhan.

No.	Name	Flood Storage Level (m)	Flood Storage Area (km2)	Water Catchment Area (km2)	Flood Storage Volume (108 m3)	Arable Land Area (km2)	Population (104 Individual)	Type of Flood Detention Area	Water Level and Conditions for Flood Storage Application
1	Dujiatai	30.00	613.98	3782	22.90	235.33	12.75	Important	The design water level of 35.12 m before Dujiatai Gate
2	Lake Xiliang	31.00	1095.00	2480	42.30	247.60	48.64	General	Flood diversion with a water level of 29.5 m at Wuhanguan
3	Lake Dongxi	29.50	444.00	444	20.00	147.60	29.50	Reserved	The same as the Xiliang Lake
4	Lake Wu	29.10	277.90	540	18.10	124.27	14.59	General	The same as the Xiliang Lake
5	Lake Zhangdu	28.30	309.00	524	10.00	263.00	30.37	General	The same as the Xiliang Lake
6	Lake Baitan	27.50	204.00	474	8.80	78.73	39.98	General	The same as the Xiliang Lake
Total			2943.88	8244	122.10	1096.53	175.84

(Ministry of Water Resources of the People’s Republic of China. National flood storage area construction and management plan, 2009) [23]. The elevation system of the water level is the frozen elevation of Wusong, and the frozen base elevation of Wusong minus 2.105 m is equal to the national elevation benchmark of 1985. Specifically, the flood detention area of Lake Dongxi is located in the west of Wuhan City, north of the Fuhuan River, and is a reserved flood detention area. The flood detention area of Lake Wu is located in the Huangpi and Xinzhou districts of Wuhan on the north bank of the Yangtze River and is a general flood detention area.

2.2.2. Overview of the Flood Detention Area of Lake Dongxi

The current situation of the flood detention area is shown in Figure 2 [22] (pp. 108–109). The flood detention area of Lake Dongxi is located northwest of Wuhan City, west of the Yangtze River, and east of the Fuhuan River, and has never been used for flood diversion. Briefly, the flood detention area was reclaimed in 1958 and consists of the main dike of the Han River (the length of 34.65 km, the dike grade of level 2, and the achievement of standards), the dike of Lake Dongxi (the length of 60.0 km, the dike grade of level 3, and not the achievement of standards), and the dike of Zhanggong (the length of 19.65 km, the dike grade of level 1, and the achievement of standards), with a total dike length of 114.30 km. Specifically, the district has eight administrative streets, three offices, 86 community resident committees, and 60 village committees. There are national and provincial transportation trunk lines, such as Handan Railway, 107 National Highway, Airport Road, Jingzhu Expressway, and Wuhan Outer Ring Line, as well as many enterprises and institutions. There are five first-class large- and medium-sized pumping stations in the district, with 43 installed units, a total capacity of 43,805 kW, a total design flux of about 374.36 m³/s, and many watergates. The types of flood inflow and flood discharge are both artificial breaches; the inlet gate location of flood inflow is between Xiongjiatai and Pengjiatai in the north dike of the Han River. The design width of the entrance is 560 m, and the design flux of flood inflow is 5000 m³/s. The outlet gate is located at pile No. 12+000 of the dike of Lake Dongxi; the design gate width is 180 m; and the design flux of flood discharge is 2500 m³/s. Moreover, the construction of communication and early warning facilities has been carried out since the reclamation. However, the construction of safety facilities has not explicitly been carried out, such as safety zones, safety platforms, and water avoidance buildings.

The planning of the flood detention area is shown in Figure 2. The main dike of the Han River and the dike of Zhanggong meet achievement standards at present, but the dike of Lake Dongxi does not meet achievement standards. Its grade is level 3, the design elevation of the dike top is the design water level plus 1.5 m, the top width is 8 m, the slope gradient of the internal and external slopes is 1:3, and the width of the internal and external platforms is 30 m. Then, the sections of substandard dikes will raise and reinforce the sections of substandard dikes, provide anti-seepage treatment to the body and base of the dikes, and reinforce the four diseased culverts and gates on the enclosure dike. Specifically, a gate of flood inflow will be built in the main dike of the Han River near Xingu Street with a width of 560 m and a designed flux of flood inflow of 5000 m³/s. A gate of flood discharge will be built at the dike of Lake Dongxi of the Fuhuan River near Lijiadun Street with a width of 180 m and a designed flux of flood discharge of 2500 m³/s. The total capacity of the pumping station is 47,005 kW after reconstruction, and the total flux is about 430 m³/s.

2.2.3. Overview of the Flood Detention Area of Lake Wu

The current situation of the flood detention area is shown in Figure 3 [22] (pp. 112–113). The flood detention area of Lake Wu is located in the Huangpi and Xinzhou districts of Wuhan City on the north bank of the Yangtze River. It is located east of Cangbu Town in Xinzhou District, north of Lutai Town in Huangpi District, south of the Yangtze River, and west of the Sheshui River, and has never been flooded. Briefly, the flood detention area of Lake Wu was built in 1969. It consists of the dike of Lake Wu (the length of 3.32 km, the dike grade of level 3, and meets the achievement standards), the east dike of Sheshui (the length of 18.48 km, the dike grade of level 3, and does not meet the achievement standards), and the main dike of Lake Wu of the Yangtze River in Huangpi District (the length of 15.45 km, the dike grade of level 3, and meets the achievement standards). The total dike length is 37.24 km. Specifically, there are four towns, two farms in Huangpi District, and many essential units and enterprises in the flood detention area. There are 629 bridges and gates, 103 drainage and irrigation stations, with a total installed capacity of 20,800 kW. The water of Lake Wu is discharged into the Yangtze River by electric drainage stations, the first and second pump stations, and the control gates of Lake Wu. The gate of flood separation and flood secession is located at Zaiyaotou (the pile number of the central dike is 3+508) in the main dike of Lake Wu of the Yangtze River, with a gate width of 500 m and a flux of 5000 m³/s. However, the safety construction is seriously lagging without practical flood control projects in the flood detention area.

The planning of the flood detention area was shown as follows: The east dike grade of Sheshui is level 3, the design elevation of the dike top is the design water level plus 1.0 m, the top width is 6 m, and the slope gradient of the internal and external slopes is 1:3. It will raise and reinforce the east dike of Sheshui, provide anti-seepage treatment to the body and base of the dike, and reinforce the four diseased culverts and gates in the dike. Two safety zones of Wuhu and Sanli will be built; the total area of the safety zones is 10.56 km², with a planning resettlement area of 8.7 km² and a resettlement population of 66,900 individuals. The total length of the enclosure dike in the safety zone is 17.8 km, including 6.0 km of existing dikes, which are the achievement of standards, and the enclosure dike of 11.8 km will be built with the dike grade of level 2. It will build two drainage culverts with a drainage flux of 21.73 m³/s, one drainage pumping station with a drainage flux of 19.2 m³/s, nine new transfer roads with a length of 41.2 km, 23 new transfer bridges, and expand the construction of 8 transfer bridges. The planning gates for flood separation and flood secession are the same as in their present status.

2.3. Engineering Overview of the Rail Transit through the Urban Flood Detention Areas of Wuhan

2.3.1. Engineering Overview of Subway Line 7 through the Flood Detention Area of Lake Dongxi in Wuhan

The Line 7 north extension project of the rail transit (Qianchuan Line) is an essential part of the city express in Wuhan. Line 7 connects Qianchuan new city, Panlong city, and Wuhan central city, which undertakes the transportation of Huangpi and residents along the line, enhances the gathering and dispersal capacity of Tianhe Airport Hub, and supports the recent construction of the northern part of the city. Briefly, the north extension project of Line 7 is from Machi Road to Huangpi Square, with a line length of 36.44 km, a total of 11 stations, 12 intervals, one access line, and one vehicle section, including one reserved station, five elevated stations, six underground stations, two interchange stations, and four new air shafts. Specifically, there are six bid sections of construction (the sixth bid section is through the flood detention area of Lake Wu), and the planning construction period is from 2020 to 2023, with a total estimated investment of 17.552 billion yuan. The Wuhan Metro Group Co., Ltd. (Wuhan, China) constructed this line; the China Railway Fourth Survey and Design Institute Group Co., Ltd. (Wuhan, China) designed it; and the China Railway Tunnel Co., Ltd. (Zhengzhou, China) built the construction unit of the bid section through the flood detention area of Lake Dongxi; the Yangtze River Academy of Sciences assessed the flood influence. Meanwhile, the project through the flood detention area is mainly from Tangyunhai Station to Machi Station, located at the south part of the enclosure dike of Lake Dongxi (2153.39 m), Machi Station (272.00 m), and the interval between Machi Station and Yuanboyuanbei Station (1897.31 m), with a line length of about 4.322 km (Figure 2).

2.3.2. Engineering Overview of the Light Rail Line 21 through the Flood Detention Area of Lake Wu in Wuhan

Line 21 is a subway line connecting Jiangan District, Huangpi District, and Yangluo of Xinzhou District of Wuhan. It starts from Houhu Avenue Station in Jiangan District, ends at Jintai in Xinzhou District, and is an express line with a maximal running speed of 100 km/h. Briefly, the total length of the line is 35.012 km, including an underground line of 9.330 km, a U-channel and roadbed section of 0.282 km, and a viaduct line of 25.400 km. The line has 20 stations, including seven underground stations in Jiangan District and ten viaduct stations in Huangpi District and Xinzhou District, and the average station distance is 2603 m. Construction started in 2015, and the line was completed and opened to traffic in 2017, with a total construction period of three years and an estimated total investment of 16.288 billion yuan. Specifically, the Wuhan Metro Group Co., Ltd. (Wuhan, China) constructed the line; the China Railway Fourth Survey and Design Institute Group Co., Ltd. (Wuhan, China) designed it. Two companies constructed the section through the flood detention area of Lake Wu, such as the China Railway Eleventh Bureau Group Co., Ltd. (Wuhan, China) and the China Construction Third Bureau Group Co., Ltd. (Tianjin, China); the Yangtze River Academy of Sciences assessed the flood influence of the section through the flood detention area of Lake Wu. Meanwhile, the section from the dike of Lake Wu to Wusheng Station is in the flood detention area of Lake Wu, there are six stations about 17 km in length, and all are viaduct stations (Figure 3).

2.3.3. Engineering Overview of the Xingang Railway through the Flood Detention Area of Lake Wu in Wuhan

The line from Shekou to Huangzhou of the Xingang Railway is located on the north bank of the Yangtze River in the east of Wuhan, an important supporting project of Wuhan New Port. It is a freight railroad of railway special line planning in Chinese “four-vertical and four-horizontal” through the five stations of Wutongkou, Xiangxushan, Yangliao, Linsifang, and Tuanfeng. It is constructed according to the standards of the national first-level railroad, and the whole line realizes electrification (Figure 3). Briefly, the total length of the line is about 80 km, the designed speed is 120 km/h, the estimated total investment is 3.91 billion yuan, the planning construction period is 2.5 years, and the line length in the flood detention area of Lake Wu is about 16.85 km (the length of bridges is about 2.70 km, and the length of roadbeds is about 14.15 km). The project is divided into two phases of construction: the first period is the section from Shekou to Xiangxushan, and the second period is the section from Xiangxushan to Huangzhou. Specifically, the Wuhan Xingang Jiangbei Railway Co., Ltd. (Wuhan, China) constructed the project; the Wuhan Survey and Design Institute of China Railway Second Institute, Ltd. (Wuhan, China) designed it; and the China Railway 21st Bureau Co., Ltd. (Lanzhou, China) constructed the project in the flood storage area of Lake Wu. The Hydrology Bureau of the Yangtze River Water Resources Commission assessed the flood influence of the section through the flood detention area of Lake Wu. The construction period of the project is from 2009 to 2021. Meanwhile, there is an old station in Xiangxuoshan and a new station in Wutongkou in the flood detention area. The parallel section line of the downstream line (K6+000-K17+300) will be built through the flood detention area of Lake Wu. The layout of small bridges and culverts is the same as the existing special line of the power plant of Yangluo, with the main layout of four large- or medium-sized bridges, such as the large-sized bridge of the Sheshui River, the bridge of the Bengzhai River, and the medium-sized bridges of Hanshi Highway and the Chang River.

2.4. Evaluation Methods for Flood Influence

2.4.1. Evaluation Methods of the Influence of Engineering on Flood Control

The influence of the implementation of relevant plans was analyzed based on the relevant codes [1,24]. The proposed projects were compared with the plans involving flood detention areas, such as flood control, construction, and management of the Yangtze River Basin flood detention areas. Thus, it could not only evaluate whether the projects met the requirements of the relevant plans for the development of flood detention areas but also analyze whether the proposed projects had any significant adverse influence on the relevant plans and their implementation.

The application’s influence on flood detention was analyzed. Firstly, relevant parameters were collected, and a generalized model was established based on the relevant codes [1,24], the actual situation of projects, and flood detention areas. Secondly, the influence of the projects was analyzed with or without the projects on the usage of flood detention areas by the finite volume method with a two-dimensional mathematical model of water flow. Finally, it included the effect of engineering on flood diversion (flood storage volume, flood flow diversion, process of flood flow velocity, duration of flood diversion, flood storage level of flood detention areas, etc.) and flood recession (flux of flood recession, water level of flood detention areas, duration of flood recession of flood detention areas, flow velocity of flood recession of flood detention areas, etc.).

The influence of the use of flood detention areas on dikes was analyzed according to the change in flow velocity when flood detention areas were used. The influence of engineering implementation was analyzed on the built and planned facilities for safety construction in flood detention areas according to the requirements of codes. Specifically, dike stability (infiltration and slip resistance) was evaluated by the finite unit method or code methods combined with the evaluation criteria of engineering design [25]. The dike’s safety was evaluated by the tunnel way, which passed under the dike of flood detention areas when flood detention areas were used. The influence of the project crossing was analyzed on the flooding and drainage of the river channels and canal systems for the usage of flood detention areas when the project crossed river channels and canal systems.

Quantitative methods analyzed the influence of flood control in flood detention areas, and the length of lines was affected by the size and proportion of the occupied flood storage volume of projects. In order to compare the influence of different engineering types and crossing ways, the proportion of flood storage volume per unit length of lines was defined as follows: it can facilitate the examination of the influence of the laws of different engineering types and crossing ways on the size and proportion of flood storage volume occupied.

R_PF = V_P/V_F,

(1)

UV_P = V_P/L,

(2)

UR_PF = R_PF/L,

(3)

where V_F is the flood storage volume of a flood storage area; V_P is the total flood storage volume occupied by the project; R_PF is the proportion of the total flood storage volume occupied by the project to the flood storage volume of a flood detention area; UV_P is the flood storage volume occupied per unit length of a line; and UR_PF is the proportion of the flood storage volume occupied per unit length of a line to the total flood storage volume of a flood detention area.

2.4.2. Evaluation Methods of Flood Influence on Engineering

Methods were assessed the adaptability of flood prevention standards and inundation influence. Firstly, the engineering design codes were compared with the utilization probability of flood detention areas according to the relevant codes [1,24]. Thus, adaptability was evaluated for the flood prevention standards and utilization probability of flood detention areas according to the actual utilization and development trend of flood detention areas. Secondly, the inundation influence was evaluated for the safety of construction projects and possible inundation according to the water level of flood storage in the mathematical model of two-dimensional water flow. Finally, these were analyzed to determine whether there was the spread of toxic, harmful, and radioactive substances after the inundation of projects in flood detention areas and whether the flood inundation would have ecological and environmental influences.

Methods were used for scouring influence. Firstly, the scouring calculation of river channels used the formula of Xiejianheng [26] and the recommended formula of the code [25]. Secondly, the scouring calculation was calculated for the parallel slope bank’s water flow according to the formulas of the code [25], and the maximal possible scouring depth was comprehensively analyzed for the river cross-section. Finally, the local scouring calculation used the recommended formula for the local scouring of riverbed buildings of clay soil [27] and the recommended formula of the Ministry of Railways Clay Bridges and the Crossings Scouring Research Group [28]. The influence of floods was comprehensively analyzed on the local scouring of critical parts of projects and the siltation.

2.4.3. Evaluation Methods of Engineering-Related Influence

Methods were used for engineering-related influence. Firstly, the existing projects and facilities of water conservancy were investigated in current flood detention areas according to relevant codes [1,24]. The influence of the engineering implementation was analyzed on the existing projects and facilities of water conservancy according to engineering plans. Secondly, the influence of the engineering implementation was analyzed on the management of the existing projects and flood control and rescue based on the existing projects and flood control facilities in flood detention areas. Moreover, the influence of the construction period was analyzed according to the arrangement of engineering-related facilities, engineering crossing dikes, and river channels and canal systems during the construction period. Finally, the situation of the engineering vicinity was investigated, such as facilities of water conservancy, highways, railroads, and factories, and the influence of projects was analyzed for the legal water rights and interests of the third party.

2.5. Comparison Design of Flood Control Influence of the Three Crossing Ways on Flood Detention Areas

2.5.1. Comparison Design of the Influential Characteristics of Engineering on Flood Control

A characteristic comparison of the influence of engineering on flood control mainly included a comparative analysis of the implementation influence of relevant plans (the relationship between construction projects and relevant plans and the implementation influence of relevant plans); a comparative analysis of the influence of the use of flood detention areas (the influence of flood separation in flood detention areas and the influence of flood recession in flood detention areas); a comparative analysis of the influence of flood control projects (the change in the influence of flow velocity on dikes and bank slopes during the use of flood storage areas and the influence of dike on engineering crossing from below/above dikes); a comparative analysis of the influence of safety construction facilities (the influence of built and planned facilities of safety construction in flood detention areas); and a comparative analysis of the influence of river channels and canal systems.

2.5.2. Comparison Design of the Flooding Influential Characteristics on Engineering

The influential characteristics of flooding on engineering mainly included the adaptability evaluation of flood defense standards, the influence evaluation of inundation (the influence on the safety of construction projects and the ecological environment by flood inundation), and the influence evaluation of scouring and siltation.

2.5.3. Comparison Design of the Characteristics of Engineering-Related Influence

Engineering-related influential characteristics mainly included the influence on the existing projects and facilities of water conservancy; the influence on engineering management, flood control, and rescue; the influence of the construction period; and the influence on the legitimate water rights and interests of third parties.

2.5.4. Comparison Design of Comprehensive Evaluation, Prevention, and Control Measures

The comprehensive evaluation and comparison of prevention and control measures mainly included the comprehensive evaluation and comparison of prevention and control measures for the influence of engineering on flood control in flood detention areas, flood on engineering, and engineering-relevant influence.

3. Results

3.1. Comparison Results of the Influential Characteristics of Engineering on Flood Control

3.1.1. Comparison Results of the Influence of the Implementation of the Relevant Plans

The results are shown in

Table 2. The influence of the results of the implementation of the relevant plans.

Type	Index/Name	Line 7	Line 21	Xingang Railway
Relationship between construction projects and relevant plans	Type	Subway	Light rail	Railway
	Names of flood detention areas	Lake Dongxi	Lake Wu	Lake Wu
	Crossing ways	Tunnel	Viaduct	Roadbed-based (roadbed and viaduct)
	Project status	The length is about 4.322 km through the flood detention area from the underground by shield way. The tunnel does not expose the ground and does not occupy the water crossing area. No safety facilities are planned for construction within the flood detention area.	The length is about 17 km through the flood detention area by the viaduct way, the surface elevation of the station platform is higher than the designed flood storage level, and the project line crosses the planning safety zone of Lake Wu.	The length is 16.85 km through the flood detention area by the roadbed or viaduct ways (the roadbed length is 14.15 km, and the viaduct length is 2.70 km). Project Line K6+300-K8+550 and Wutongkou Station are located in the planning safety zone of Lake Wu.
	Compliance with plans	Compliance and low influence	Compliance and no significant adverse influence	Compliance and no significant adverse influence
Influence of the implementation of plans	Water Conservancy Planning	The lines are all underground sections and do not affect the planning of projects for water conservation.	When the line crosses the planning safety zone of Lake Wu, bridge spans of 75 m and 60 m are used to cross the west and east dikes, respectively, and the net space between the bridge span and the top of the planning dike is greater than 4.5 m.	The project intersects with the planned safety zone dike of Lake Wu. The former track’s surface elevation is lower than the top elevation of the planned safety zone dike. Adjustment of the traffic gate, and the latter adopts the viaduct type to cross.
	Results of the evaluation	No influence	No significant adverse influence	No significant adverse influence

according to the comparison between the engineering design and relevant plans. These include the relationship between the construction project and relevant plans and the influence of the implementation of relevant plans.

From the results of the relationship between construction projects and related plans, as shown in Table 2, the length of the roadbed is 14.15 km for the Xingang Railway crossing the flood detention area, and the length of the bridges is 2.70 km. The roadbed length accounts for 83.98% of the total length, and the roadbed way has a greater influence on the project. Thus, the crossing ways of the roadbed and viaduct of the Xingang Railway can be represented by the roadbed way. From the comparison of the crossing ways of tunnels, viaducts, and roadbed-based ways in Table 2, the relationship between the construction projects and related plans is mainly impacted by the scale and type of construction projects and relevant plans of the flood detention areas, etc. The flood detention area of Lake Dongxi is relatively similar to that of Lake Wu, and the influence of different types of projects through flood detention areas is mainly affected by the crossing ways. Firstly, the tunnel way does not expose the ground and does not occupy a water crossing area, and the influence of construction projects and related plans is small. Secondly, the roadbed-based way needs to expose the ground and occupy more water crossing areas, and construction projects have a certain influence on relevant plans. Finally, the viaduct way needs to partially expose the ground and occupy less of a water crossing area, and the influence of construction projects on related plans is between the two. Although viaducts and roadbeds influence flood detention areas, effective measures can be adopted to prevent and control them. In conclusion, in terms of the influence of construction projects on relevant plans, the tunnel way has the least influence, the roadbed-based way has the greatest influence, and the viaduct way is between the two. Therefore, the tunnel, viaduct, and roadbed-based ways generally meet the planning requirements, and there is no significant adverse influence as long as there is proper planning.

The results are shown in Table 2 for engineering and construction projects based on the implementation of relevant plans. Firstly, Table 2 shows that the lines are underground sections when crossing flood detention areas through the tunnel way. These do not affect other existing or planned projects and have no influence on the implementation of relevant plans in flood detention areas. Secondly, the dikes, channels, and water systems are crossed by bridges, crossing flood detention areas by the viaduct way, and the appropriate span diameter and net space of bridges are used for the dikes in the planning safety zone. These have no obvious adverse influence on the plan’s implementation of water conservation. Finally, through flood detention areas by the viaduct and roadbed-based ways, the dikes, channels, and water systems are still crossed by the viaduct way. If the elevation of the track surfaces is lower than the top elevation of the dikes of the planning safety zone, the traffic gate is generally used to cross in planning. If the elevation of the track surface is higher than the top elevation of the dikes of the planning safety zone, the appropriate span diameter and net space of bridges are generally used. Thus, the engineering implementation has no significant adverse influence on the planning and implementation of water conservation with the above measures. In conclusion, the tunnel way does not affect other existing or planned projects and does not influence the implementation of relevant plans in flood detention areas. With crossing flood detention areas through the viaduct and roadbed ways, the bridge way is mostly used when crossing dikes, channels, and water systems. It is necessary to plan the scheme of construction of traffic gates or to adopt an appropriate span diameter and net space of bridges to cross planning safety zones. Thus, there is no significant adverse influence from these measures.

3.1.2. Comparison Results of the Application Influence of Flood Detention Areas

The influence of engineering on flood separation and flood recession was derived for flood detention areas in

Table 3. The influence of the results of engineering on flood diversion and recession in flood detention areas.

Type	Influence Index	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence of flood diversion	Length of the project through flood detention areas (km)	4.322	17.000	16.850 (roadbed: 14.150; viaduct: 2.700)
	Flood storage level (m)	29.50	29.10	29.10
	Flood diversion flux (m3/s)	5000	5000	5000
	Flood storage volume of flood detention areas (108 m3)	20.0	18.1	18.1
	Flood detention volume-occupied projects (104 m3)	0.95	33.20	66.40
	Proportion of the project to the volume of flood detention areas (%)	0.00048	0.01800	0.03600
	Flood detention volume occupied per unit length of lines (104 m3/km)	0.2198	1.9529	3.9407 (roadbed: 4.3199; viaduct: 1.9529)
	Ratio of flood detention volume occupied per unit length of lines (%/km)	0.00011	0.00106	0.00214 (roadbed: 0.00234; viaduct: 0.00106)
	Maximal reduction value of flood flux at current gates (m3/s)	1.08	6.08	34.00
	Duration of flood diversion at current gates (h)	140.000	106.330	128.889
	Maximal reduction value of flood flux at planning gates (m3/s)	1.56	6.08	34.00
	Duration of flood diversion at planning gates (h)	126.00	106.33	128.89
	Proportion of the maximal reduction value of flood flux to the design flood flux (%)	0.03	0.12	0.68
	Congestion value of the maximal water level (cm)	1.9	5.6	3.2
	Reduction value of the maximal water level (cm)	1.7	4.9	0
	Peak time of current gates (h)	+0.05	+0.07	+0.34
	Peak velocity change of current gates (m/s)	−0.014	−0.042	−0.012
	Peak time for planning gates (h)	+0.03	+0.07	+0.34
	Peak velocity change of planning gates (m/s)	−0.006	−0.042	−0.012
Influence of flood recession	Maximal reduction value of the volume of flood recession (m3/s)	2.51	5.57	7.32
	Maximal change value of water level (cm)	0.09	0.11	0.15
	Change value of flow velocity during the process of flood recession	Little	Little	Little
	Changes in flow velocity during the process of flood recession	Little	Little	Little

and Figure 4 based on the mathematical model of planar two-dimensional water flow and the finite volume method [17] and Equation (3).

The influence of engineering on flood diversion in flood detention areas is shown in Table 3. Firstly, Table 3 shows that the proportion is 0.00048%, 0.01800%, and 0.03600%, respectively, for the flood detention volume occupied per unit length of Line 7, Line 21, and the Xingang Railway to the total volume of flood detention areas. It has some influence on the volume of flood detention areas, but the influence is small. Secondly, the maximal values are 1.56 m³/s, 6.08 m³/s, and 34.00 m³/s, respectively, for the flux reduction of flood diversion in Line 7, Line 21, and the Xingang Railway, and the duration of flood diversion is from 106.330 h to 128.889 h. The flux change of flood diversion in flood detention areas is small, which has no apparent influence on the process of flood diversion in flood detention areas. There is no significant change in flood diversion’s total duration and flow velocity process before and after the engineering construction of Line 7, Line 21, and the Xingang Railway. There is no significant influence on flood separation and flood storage duration. Finally, the congestion values of the maximal water level are 1.9 cm, 5.6 cm, and 3.2 cm for Line 7, Line 21, and the Xingang Railway, respectively. The reduction values of the maximal water level are 1.7 cm, 4.9 cm, and 0 cm, respectively. Thus, the engineering construction has no significant influence on the change process of water level and the final water level of flood storage in flood detention areas. In conclusion, Line 7, Line 21, and the Xingang Railway account for less than 0.1% of the volume of flood detention areas. It has a minor influence on the volume of flood storage, the flux of flood diversion, the flow velocity process of flood diversion, the duration of flood diversion, and the level of flood storage in flood detention areas.

For convenience, assuming that the size and proportion of the flood storage volume occupied viaducts in projects of the railroad and light rail are equal, the results are shown in Table 3 for the volume of flood storage and proportion per unit length of lines. Table 3 shows that the unit length of the lines of the tunnel, viaduct, and roadbed occupies the volume of flood storage of 2198 m³/km, 19,529 m³/km, and 43,199 m³/km, respectively. The relative ratio among the three is 1.0:8.9:19.7, and the unit length of lines occupies the flood storage volume ratio of 0.00011%/km, 0.00106%/km, and 0.00234%/km, respectively. Therefore, the unit length of lines occupying the flood storage volume is mainly determined by the crossing way, and the relative ratio among the three is 1.0:8.9:19.7. Specifically, the tunnel way is the lowest in flood storage volume occupied per unit length of lines, the roadbed-based way is the highest, and the viaduct way is in the middle. Moreover, the choice of the tunnel way is conducive to reducing the occupied flood storage volume significantly, but its cost is also higher. The proportion of flood storage volume occupied by viaducts and roadbeds is higher. However, when the line length through flood detention areas is limited, the proportion of flood storage volume occupied is still within the control range and can be selected according to requirements. In conclusion, the flood storage volume occupied by the unit length of lines is mainly determined by the crossing way. The influence of engineering on floods in flood storage areas is proportional to the flood storage volume occupied per unit length of lines. The flood storage volume occupied per unit length of the tunnel way is about 1/9.6 of that of the viaduct way and 1/20.6 of that of the roadbed way. Thus, the way to cross flood detention areas needs to be determined according to the actual situation.

Table 3 shows that the flux reduction values are closely related to the flood storage volume occupied lines for the flood separation of planning gates. Firstly, the tunnel way is the smallest for the maximal values of the flux reduction of flood separation under the same/similar planning conditions for the duration of flood separation. In the ratio of the maximal reduction value of flood separation to the design flux of flood separation of the planning gates, the roadbed-based way is the largest, and the viaduct way is between the two. Moreover, the more flood storage volume is occupied per unit length of lines, the greater the maximal value of the flux reduction of flood separation and the ratio of the maximum reduction value of flood separation to the design flux of flood separation of the planning gates under the same/similar planning duration conditions of flood separation. In conclusion, these values are mainly influenced by the size of the flood storage volume occupied by the unit length of lines, such as the maximal value of the flux reduction of flood separation and the ratio of the maximal reduction value of flood separation to the design flux of flood separation of the planning gates. Therefore, the tunnel way is the smallest under the same/similar planning duration conditions of flood separation, the roadbed-based way is the largest, and the viaduct way is between the two.

The influence of engineering on flood recession is shown in Table 3 for flood storage areas. Table 3 shows that the maximal values are mainly impacted by crossing ways for flux reduction of flood recession. Specifically, the tunnel way has the smallest maximal value of flux reduction of flood recession, the roadbed-based way is the largest, and the viaduct way is between the two. Table 3 shows the proportion is 0.00048%, 0.01800%, and 0.03600%, respectively, for Line 7, Line 21, and the Xingang Railway to the volume of flood detention areas, and the proportion does not exceed 0.1% for the engineering lines occupying the volume of flood detention areas. Therefore, when the volume of flood detention areas occupied by engineering lines is minimal (less than 0.1%), the values are minimal, such as the maximal values of water level change in flood detention areas, the values of the flow velocity change of flood recession, and the change process of the flow velocity of flood recession. The influence of lines on flood detention areas is minimal. Thus, the lines will not affect the flood recession in flood detention areas. In conclusion, the maximal values of flux reduction of flood recession are mainly impacted by crossing ways. The tunnel way has the smallest value of maximal reduction flux of flood recession, the roadbed-based way is the largest, and the viaduct way is between the two. Moreover, the influence of tunnel, viaduct, and roadbed-based ways is minimal for the flood recession of flood detention areas when the proportion of engineering lines occupying the volume of flood detention areas is small (less than 0.1%). It will not affect the flood recession in flood detention areas.

3.1.3. Influence of Flood Control Engineering

The application influence of flood detention areas was derived in

Table 4. The influence of the results of flood control engineering.

Influence Index	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Change in flow velocity with the use of flood detention areas (cm/s)	Less than 0.100	Less than 0.050	Less than 0.021
Dike influence on the change in flow velocity with the use of flood detention areas	No	No	No
Dike influence of the project under/across the dike	No	No	No
Infiltration stability of the dike	The maximal exit gradient of the horizontal and vertical dike feet is 0.130 and 0.223, respectively, and these meet the code’s requirements.	The maximal exit gradient of the horizontal and vertical dike feet is 0.225 and 0.041, respectively, and these meet the code’s requirements.	The vertical infiltration gradient behind the west dike of Sheshui is greater than the allowable vertical infiltration gradient of the overlying soil. The seepage stability of the main dike of Lake Wu meets the code’s requirements.
Dike stability of anti-slippage	The safety factors of 1.282–3.478 meet the code’s requirements.	The safety factors of 1.460–2.840 meet the code’s requirements.	The safety factors of 1.460–2.840 meet the code’s requirements.
Stability evaluation of dike	It meets the code’s requirements.	It meets the code’s requirements.	It meets the code’s requirements (there is a permeability stability problem behind the west dike of Sheshui).

based on the planar two-dimensional water flow mathematical model and the finite unit method.

When flood detention areas are used, Table 4 shows the change in flow velocity is less than 0.100 cm/s, 0.050 cm/s, and 0.021 cm/s, respectively, for Line 7, Line 21, and the Xingang Railway. The change in flow velocity in flood detention areas does not affect dike safety. The project under/across dikes has no effect on dike safety. In conclusion, when flood detention areas are used, the influence of the project crossing dikes on dikes is mainly determined by the characteristic indices of flood detention areas and the soil parameters. However, the influence of the crossing ways is not significant.

The results of dike stability are shown in Table 4 based on the calculation formulae for dike infiltration and slip resistance. Table 4 shows that the stability of infiltration and anti-slippage meets the code requirements for Line 7, Line 21, and the Xingang Railway through the dikes of flood detention areas, except for the problem of infiltration stability behind the west dike of Shehui of the Xingang Railway. The infiltration stability requirements of dikes: the permissible seepage gradient of soil is 0.25–0.35 [29]; the anti-slippage stability requirements of dikes: the safety coefficient is not less than 1.20 under normal operating conditions and not less than 1.10 under extraordinary operation conditions [29]. Moreover, Table 4 shows that dike stability is mainly related to projects involving section structures, soil parameters, and application conditions. However, the influence of the crossing ways is not significant.

3.1.4. Influence of Safe Construction Facilities

The influence on safe construction facilities was evaluated in

Table 5. The influence of the results of engineering on safe construction facilities in flood detention areas.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence on the built safe construction facilities	Only communication and warning facilities are available, and there is no influence.	No influence on the built transfer roads and bridges	No influence on the built transfer roads and bridges
Influence on the planning and construction of safe facilities	There is no planning of safety zones, safety platforms, transfer roads, bridges, etc.	Two new safety zones, Wuhu and Sanli, are planned, and the proposed project crosses the planning dikes of the safety zone and intersects with two planning roads.	Intersecting with the planning safety zone and transfer road, the roadbed is located near the planning gates for flood diversion and flood outflow.
Conclusions of influence analysis	No influence and minimal influence	Low influence and moderate influence	Low influence, slightly greater relative influence, and maximal influence

for flood detention areas based on the results of the mathematical model of planar two-dimensional water flow. Table 5 shows that the influence of engineering on safe construction facilities is mainly affected by the built and planned facilities for safe construction in flood detention areas. The safer the construction facilities that are built and planned in flood detention areas, the greater the influence. Specifically, under the same conditions as the built and planned facilities of safety construction, the tunnel way has the least influence on the safe construction facilities in flood detention areas without occupying the ground or intersecting with ground buildings. The roadbed-based way is the largest influence, occupying the most ground and intersecting with many buildings, and the viaduct way is between the two.

3.1.5. Influence of River Channels and Canal Systems

The influence of river channels and canal systems was evaluated in

Table 6. The influence of the results of engineering on river channels and canal systems.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence on river channels	There is no influence on river channels by the shield way through the Zha River and the southeast lake branch of Lake Dongxi.	There is a significant influence on river channels by the bridge ways through the Bengzhan River and Chang River.	There is no influence on river channels by way of bridges and culvert pipes through rivers.
Influence on lakes	No occupying lake volume	Occupying less lake volume	Occupying less lake volume
Influence on canal systems	Tunnel burial depth of 12.44 m and no influence on channel systems	Across canal systems by the bridge way and negligible influence on channel systems	Across channels by way of bridges and culvert pipes, and negligible influence on channel systems
Conclusions of the analysis	No influence on river channels, lakes, and canal systems, and minimal influence	No significant influence on existing river channels, lakes, and canal systems, and moderate influence	No significant influence on existing river channels, lakes, and canal systems; more significant relative influence; and maximal influence

based on the direction of engineering lines and the type of crossing structures. Firstly, Table 6 shows that the project of Line 7 crosses the flood storage area of Lake Dongxi by the underground shield, which does not occupy rivers, lakes, or canal systems and has no influence on rivers, lakes, or canal systems. Secondly, projects of Line 21 and the Xingang Railway cross the rivers, lakes, and canals in the flood detention area of Lake Wu by bridges/culverts, etc. There is no significant influence on the existing rivers, lakes, and canals. Finally, Table 6 shows that the influence of engineering on river channels and canal systems is mainly determined by the number and layout of rivers, lakes, and canal systems in flood detention areas and the crossing ways. Specifically, the tunnel way does not occupy rivers, lakes, or canal systems and does not influence these under the same conditions as crossing flood detention areas. The roadbed-based way occupies more rivers, lakes, and canal systems and has a slightly greater influence on the existing rivers, lakes, and canal systems. The viaduct method is between the two.

3.2. Comparison Results of the Flood Influence Characteristics on Engineering

3.2.1. Adaptability Evaluation of Flood Prevention Standards

The adaptation evaluation of defense flood standards is shown in

Table 7. Results of the adaptation evaluation of flood prevention standards.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Names of flood detention areas	Lake Dongxi	Lake Wu	Lake Wu
Flood control standards and the construction of projects	100-year event, and the project by the tunnel way	100-year event, and the project by viaduct way	100-year event, and the project by roadbed and viaduct ways
Application chance of flood detention areas	50-year event, the chance of flood separation and operation is further reduced after the completion of the reservoir groups with the Three Gorges Project as the core	20–30 year event, the chance of flood separation and operation is further reduced after the completion of the reservoir groups with the Three Gorges Project as the core	20–30 year event, the chance of flood separation and operation is further reduced after the completion of the reservoir groups with the Three Gorges Project as the core
Adaptation evaluation of flood prevention standards	Basic adaptation and measures to mitigate the influence	Basic adaptation and measures to mitigate the influence	Basic adaptation and measures to mitigate the influence

, based on comparing flood prevention standards and the application chance of flood detention areas. Table 7 shows that the flood prevention standards of the engineering construction project are a 100-year event for Line 7, Line 21, and the Xingang Railway, and the flood prevention standards of the flood detention area of Lake Dongxi are a 50-year event. The flood prevention standards of the flood detention area of Lake Wu are a 20–30-year event. Specifically, when flood detention areas are used, all ground is submerged for the tunnel way. The line can still operate normally and meet the requirements of flood control standards, but the station needs to be protected. Part of the project lines are submerged for the viaduct and roadbed-based ways, and the sections of submerged lines are part of the line elevation lower than the design level of flood storage. The project lines and stations should be closed to use in time, and we need to take specific measures to mitigate the application influence of flood detention areas on proposed projects. In conclusion, the standard adaptability of flood defense is mainly affected by the flood control standards of construction projects, the chances of using flood detention areas, and other factors. Thus, the influence of the crossing ways is small.

3.2.2. Evaluation of Inundation Influence

The evaluation of inundation influence was shown in

Table 8. Evaluation results of inundation influence.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Inundation influence on engineering safety	When the flood storage level is reached, the line is submerged in the area; the import and export buildings and two wind shafts of Machi Station are submerged, and it does not affect the overall operation of the line.	When the flood storage level is reached, lots of piers of viaducts, parking lots, and connecting sections are submerged, and they need to stop operation.	When the flood storage level is reached, about 11.8 km of the line is submerged, and it needs to stop operation.
Inundation’s influence on the ecological environment	No toxic, harmful, or radioactive substances spread, and little influence	No toxic, harmful, or radioactive substances spread, and little influence	No toxic, harmful, or radioactive substances spread, and little influence
Conclusions of inundation influence	Minimal influence on engineering safety and little influence on the ecological environment	Medium influence on engineering safety and negligible influence on the ecological environment	Maximal influence on engineering safety and negligible influence on the ecological environment

when the flood storage level was reached based on the results of the mathematical model of plane two-dimensional water flow. When the flood storage level reaches, Table 8 shows that only the entrance and exit buildings are affected on stations, wind shafts are affected by inundation for the tunnel way, and the overall operation of lines is not affected. The inundation has very little influence on engineering safety. Parts of lines and parking lots are inundated by the viaduct and roadbed-based ways, and lines need to stop operation. Meanwhile, Table 8 shows that the influence of inundation on the ecological environment depends mainly on whether there are toxic, harmful, and radioactive substances after engineering inundation and whether these substances spread due to flooding independent of crossing ways. In conclusion, the inundation influence on engineering safety is the least by the tunnel way, the most by the roadbed-based way, and the viaduct way is in the middle. Moreover, the inundation’s influence on the ecological environment is mainly related to toxic, harmful, and radioactive substances in flood detention areas and is not associated with crossing ways.

3.2.3. Influence Evaluation of Scouring and Siltation

The influence evaluation of scouring and siltation was derived in

Table 9. Evaluation results of scouring and siltation influences.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Scouring influence	The maximal possible scouring depth of the riverbed of the Fuhuan River is 4.890 m under 1996-type flood conditions, and the minimal burial depth of the tunnels is about 4.020 m after the maximal scouring of the riverbed occurs.	The maximal scouring depth of the bridge piers is 1.490 m, and the scouring of flood storage will not damage the line.	The maximal scouring depth of the bridge piers is 0.779 m, and the scouring of flood storage will not damage the line.
Siltation influence	Low sediment content in floods and low siltation influence	Low sediment content in floods and low siltation influence	Low sediment content in floods and low siltation influence
Evaluation conclusion	Less influence of scouring and siltation	Less influence of scouring and siltation	Less influence of scouring and siltation

based on the formulas for scouring and siltation. Table 9 shows that the maximum possible scouring depth is 4.890 m for the riverbed of the Fuhuan River of Line 7 under 1996-type flood conditions. The minimal burial depth of the tunnel is about 4.020 m after the scouring. The local scouring depths are 0.520 m, 1.490 m, and 0.779 m, respectively, for the critical projects of Line 7, Line 21, and the Xingang Railway, with less siltation and less influence of scouring and siltation. Meanwhile, Table 9 shows both are considered in the influence evaluation of the scouring of the tunnel way for the local scouring depth of the main ground buildings and the scouring depth of the soil layer above the tunnel. However, the local scouring depth is mainly considered in evaluating the viaduct and roadbed-based ways for the bridge piers and other ground buildings. With the low sediment content of the Yangtze River, the siltation influence on engineering is relatively small when the time of flood storage is not particularly long (not more than 72 h). Thus, the siltation influence on engineering is not significant with crossing ways.

3.3. Comparison Results of the Engineering-related Influence Characteristics

3.3.1. Influence on the Existing Projects and Facilities

The influence results of engineering on existing projects and facilities are shown in

Table 10. The influence of the results of engineering on existing projects and facilities.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence on the existing projects and facilities	Less existing projects and facilities, and no influence	The water resistance ratio of bridge piers crossing the Bengzhai River and Chang River is 5.07% and 5.33%, respectively.	There is an influence on the design scheme of the traffic gate for the special line of Yangluo Power Plant, and the top elevation of dikes in several spanning sections needs to be raised.
Influence of the construction on dikes	The construction of the shield and two wind shafts does not endanger the infiltration stability of dikes.	It does not endanger the safety and function of dikes across dikes by the viaduct way.	It does not endanger the safety and function of dikes across dikes by the viaduct way.
Evaluation conclusions	Minimal influence	Medium influence	Maximal influence

, based on the calculation results of engineering design and dike stability. For the track projects on existing projects and facilities of water conservancy, Table 10 shows that the influence is mainly determined by the scale and quantity of existing and planned projects and facilities in flood detention areas and their relative position to the proposed projects. These are related to the crossing ways. Specifically, the tunnel way passes underground and has the least influence on the existing projects, facilities, and dikes. The roadbed-based way has the greatest influence on existing projects and facilities. The viaduct crosses dikes and rivers, which have a negligible influence on dikes and a moderate influence on river flooding.

3.3.2. Influence on Engineering Management, Flood Control, and Rescue

The projects’ current situation and implementation were analyzed, and the results were shown for project implementation on project management and flood control and rescue in

Table 11. The influence of the results of engineering management, flood control, and rescue.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence on engineering management	Fewer infrastructures in the flood detention area and no influence on engineering management	Less influence on engineering management	Less influence on engineering management
Influence on flood control and rescue	The shield method is constructed underground, which does not affect flood control, rescue traffic, or water rescue.	The net space between the bridge and the dike top meets the code requirements. Six viaduct stations in the flood detention area will not affect flood control, rescue traffic, or water rescue.	The net space between the bridge, culvert pipe, and dike top meets the code requirements. The design elevation of track surfaces on the 5.2 km line is higher than the flood storage level, which affects water rescue.
Evaluation conclusions	Minimal influence	Medium influence	Maximal influence

. Table 11 shows that the influence is mainly determined by the number and layout of infrastructure and the crossing ways of the rail transit projects on project management in flood detention areas. Specifically, the tunnel way does not affect engineering management when there is little infrastructure in flood detention areas, and the tunnel way does not affect engineering management. For the viaduct and roadbed-based ways, the effect on engineering management is smaller when there is a certain amount of infrastructure in flood detention areas.

Table 11 shows that the influence is mainly determined by flood control, rescue traffic, and water rescue for the rail transit projects on flood control and rescue. Specifically, the tunnel way does not influence the traffic of flood control and water rescue and has no influence on flood control. When the net space is greater than 4.5 m between the bottom of the viaduct beam across flood detention areas and the top of dikes, the viaduct way does not influence the traffic of flood control and water rescue, and the influence on flood control is small. When the net space is greater than 4.5 m between the bottom of the viaduct beam across flood detention areas and the top of dikes, the viaduct sections do not influence the traffic of flood control and water rescue. However, the roadbed-based way affects water rescue when part of the design elevation of line track surfaces is higher than the flood storage level. In conclusion, the tunnel way does not influence project management or flood rescue, and the influence is minimal. The influence of the roadbed-based way is the largest for the engineering management and traffic of flood control and rescue. Part of the design elevation of line rail surfaces will be higher than the flood storage level, influencing water rescue. The influence of the viaduct way is in the middle, with little influence on engineering management and flood rescue.

3.3.3. The Influence of the Construction Period

The influence results of the construction period are shown in

Table 12. The influence of the construction period.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence of construction on water conservation projects	The tunnel is tunneled by a shield structure, the interval wind shaft is constructed by the open cut method, and dike safety can be ensured during the construction period.	Dike safety can be ensured during the construction period.	Dike safety can be ensured during the construction period.
Influence of construction sites	The nearest distance between the construction site of the two wind shafts, Machi Station, and the dike foot of the backwater slopes of the dike of Lake Dongxi is 160 m and 1600 m, respectively.	The distance between the construction sites and the backwater slopes of the dikes meets the relevant regulations.	The distance between the construction sites and the backwater slopes of the dikes meets the relevant regulations.
Disposal of abandoned soil	The Beicha Base of Huangpi District is an abandoned soil site	There are many places with abandoned soil	There are many places with abandoned soil
Evaluation conclusions	Low influence during construction	Low influence during construction	Low influence during construction

, based on the analysis of the engineering implementation. Table 12 shows that the influence of the construction period mainly includes the influence of construction on the projects of water conservancy, the influence of the construction sites, and the disposal of abandoned soil. Firstly, the influence of construction on water conservancy projects is mainly determined by the influence of construction methods on crossing buildings. Secondly, the influence of construction sites is mainly determined by their dikes and canal systems. Finally, the influence of disposal soil is mainly determined by the size of disposal sites and transport distance. In conclusion, regardless of crossing ways, as long as the construction is carried out according to codes, the construction sites are reasonably arranged, and the disposal of abandoned soil is appropriate, the influence during the construction period was small.

3.3.4. Influence on the Legal Water Rights and Interests of the Third Party

The influence of engineering on the legal water rights and interests of the third party was shown in

Table 13. The influence of the results of engineering on the legal water rights and interests of the third party.

Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence on other water-related projects	There are Tongjiahu Gate and Tartu Pumping Station upstream, 2.1 km–10.16 km, and the distance is far from this project.	For the special bridge of the Sheshui River for the special line of Yangluo Power Plant, there is no obvious change in the water flow condition near it after flood separation.	For the special bridge of the Sheshui River for the special line of Yangluo Power Plant, there is no obvious change in the water flow condition near it after flood separation.
Influence on other projects and important facilities	There is a bypass highway and a second channel of the airport upstream (7 km), which are far away from the project.	There is no significant change in the water flow conditions after flood diversion near the special lines of Yangluo Power Plant, Hansh Road, and Wuying Expressway.	There is no significant change in the water flow conditions after flood diversion near the special lines of Yangluo Power Plant, Hansh Road, and Wuying Expressway.
Evaluation conclusions	Minimal influence	Medium influence	Maximal influence

based on the analysis of the current situation of other water-related projects, other projects, and essential facilities. For the engineering on the legal water rights and interests of the third party, Table 13 shows that the influence is mainly related to the distance between the projects and other water-related projects, other projects and important facilities, their mutual influence, and crossing ways. Specifically, the influence of the tunnel way is the smallest for the legal water rights and interests of the third party, the roadbed-based way is the largest, and the viaduct way is between the two.

3.4. Comparison of the Results of the Comprehensive Evaluation and Prevention and Control Measures

The systematic summary of the influence of flood detention areas is shown in

Table 14. Comparison of the results of the comprehensive evaluation and the prevention and control measures.

Type	Content	Line 7 (Tunnel)	Line 21 (Viaduct)	Xingang Railway (Roadbed-Based)
Influence of flood detention areas on flood control	Influence of implementation of relevant plans	Minimal influence and no significant adverse influence	Medium influence and no significant adverse influence	Maximal influence and no significant adverse influence
	Application influence on flood detention areas	Minimal influence, a proportion of flood storage volume occupied per unit length line of 0.00011%/km, and a small influence on flood separation and flood recession	Medium influence, the proportion of unit length lines occupying flood storage volume of 0.00106%/km, and no obvious influence on flood separation and flood recession	Maximal influence, the proportion of unit length lines occupying flood storage volume of 0.00214%/km, and no obvious influence on flood separation and flood recession
	Influence of flood control projects	Determined by engineering and soil properties	Determined by engineering and soil properties	Determined by engineering and soil properties
	Influence of facilities on safety construction	No ground occupation and a minimal influence	More intersecting with buildings and a medium influence	Ground-occupying, more intersecting with buildings, and a maximal influence
	Influence of river channels and canal systems	No occupation of river channels, lakes, and canal systems; no influence; and a minimal influence	A small amount of occupation of rivers, lakes, and canal systems and a medium influence	More occupation of river channels, lakes, and canal systems and a maximal influence
	Influence the conclusions of flood detention areas on flood control	Minimal influence and no significant adverse influence	Medium influence and no significant adverse influence	Maximal influence and no significant adverse influence
	Prevention and control measures	No	The bridge spans of 60–75 m are used to cross the planning dikes of the safety zone of the Wu River, with a net space of more than 4.5 m between the bridge and the top of the planning dike	Adjustment of the design scheme of the original planning and construction of the traffic gate and the use of a viaduct type to cross the dikes of the planned safety zone of Lake Wu
Influence of floods on engineering	Flood prevention standards	Basic adaptation and defensive measures are required	Basic adaptation and defensive measures are required	Basic adaptation and defensive measures are required
	Influence of inundation	Minimal influence, the lines, station, and wind shafts will be flooded when the flood storage is used, and the influence on the ecological environment is small	Medium influence; part of the lines will be flooded; the use is affected when the flood storage is used; and the influence on the ecological environment is small	Maximal influence; part of the lines will be flooded; the use is affected when the flood storage is used; and the influence on the ecological environment is relatively small
	Influence of scouring and siltation	Local scouring and overburden soil scouring, minimal scouring influence, and little siltation influence	Local scouring, medium scouring influence, and little siltation influence	Local scouring, maximal scouring influence, and little siltation influence
	Influence the conclusions of floods on engineering	Minimal influence and ensuring engineering safety with the use of flood storage	Medium influence and ensuring engineering safety with the use of flood storage	Maximal influence and ensuring engineering safety with the use of flood storage
	Prevention and control measures	Strengthening exploration, hole blocking, and seepage prevention treatment; strengthening scouring observation; and ensuring engineering safety with the use of flood storage	Supplementary ground survey; foundation pit support and precipitation; scouring, wind, and wave prevention; local scouring observation; and ensuring engineering safety with the use of flood storage	Scouring prevention and wind and wave prevention; filling the pond of the west dike of Sheshui; observation of bridge piers; and ensuring engineering safety with the use of flood storage
Relevant influence on engineering	Influence on the existing projects and facilities of water conservation	Minimal influence of the crossing way of the tunnel	Medium influence of the crossing way of the viaduct	Maximal influence of the crossing types of the viaduct and roadbed
	Influence on engineering management, flood control, and rescue	Minimal influence and essentially no influence	Medium influence and essentially no influence	Maximal influence and influence on water rescue
	Influence on the construction period	Low influence during the construction period	Low influence during the construction period	Low influence during the construction period
	Influence on the legal water rights of the third party	Minimal influence and no significant adverse influence	Medium influence and no significant adverse influence	Maximal influence and no significant adverse influence
	Relevant influences on the conclusions of floods on engineering	Minimal influence and no significant adverse influence	Medium influence and no significant adverse influence	Maximal influence and influence on water rescue
	Prevention and control measures	Dike monitoring during construction; and no usage of lines, stations, or wind shafts in advance with the use of flood storage	Dike monitoring; increasing water rescue facilities; and no usage of the project with the use of flood storage	Design adjustment of the traffic gate; dike monitoring; increasing rescue facilities; and no usage of the project with the use of flood storage

on flood control, the influence of floods on engineering, the relevant influence of engineering, and the prevention and control measures.

The influence analysis of flood detention areas on flood control is shown in Table 14. Table 14 shows that the tunnel way has the least influence on implementing relevant plans, the application of flood detention areas, facility safety construction, and river channels and canal systems. The roadbed-based way has the greatest influence, and the viaduct way has the middle influence. Moreover, the influence on the engineering of flood control is mainly determined by the project and soil characteristics. Therefore, all three ways will have no adverse effect on the application of flood detention areas through proper planning and scheduling.

The influence analysis of flooding on engineering is shown in Table 14. Table 14 shows that the influence is the smallest for the tunnel way on inundation, scouring, and siltation; the influence of the roadbed-based way is the largest; and the influence of the viaduct way is in the middle. Moreover, the influence is mainly determined by the engineering design standards and application of flood detention areas for flood prevention and dike stability standards. Therefore, when the flood storage of flood detention areas is applied, the influence of floods on engineering is small for the three ways by appropriate measures. These include additional ground investigation, attention to seepage prevention, bridge piers and roadbed scouring prevention, wind and wave prevention, strengthening scouring observation, and ensuring the flood safety of engineering lines and stations.

The analysis of the relevant influence of projects is shown in Table 14. Table 14 shows that the influence of the tunnel way is the smallest on the existing projects and facilities of water conservancy, engineering management, flood control and rescue, and the legal water rights and interests of the third party. The influence of the roadbed-based way is the greatest, and the influence of the viaduct way is between the two. Moreover, the influence on the construction period is not related to crossing ways. Therefore, the three ways will not produce significant adverse engineering-related influence by strengthening the dike monitoring during the construction period and increasing the facilities for water rescue, scheduling, and other measures.

In conclusion, the influence of the tunnel way is the smallest for flood detention areas on flood control, floods on engineering, and engineering-related aspects; the influence of the roadbed-based way is the greatest; and the influence of the viaduct way is in the middle. Specifically, for flood control engineering, flood prevention standards, dike stability, and construction period, the influence is mainly related to the standards of engineering design and the application of flood detention areas, soil characteristics of dikes, construction methods, the layout of construction sites, the disposal of abandoned soil, etc. Moreover, the index is mainly determined by crossing ways for the flood storage volume occupied per unit length of lines. The index of the tunnel way is about 1/8.9 of that of the viaduct way and 1/19.7 of that of the roadbed way. Therefore, the flood influence is small and controllable for the rail transit projects crossing flood detention areas under the three ways. Regular operation and engineering safety can be ensured in flood detention areas by appropriate engineering and non-engineering measures.

4. Discussion

In this paper, the results showed that the flood storage volume occupied per unit length of lines was mainly determined by the crossing ways. The flood influence of rail transit projects on flood detention areas was proportional to the flood storage volume occupied per unit length of lines. The index of the tunnel way was about 1/8.9 of that of the viaduct way and 1/19.7 of that of the roadbed way. Moreover, the flood influence of the tunnel way was the smallest through flood detention areas, the influence of the roadbed-based way was the largest, and the influence of the viaduct way was in the middle. Specifically, the influence was mainly related to the engineering design standards and application of flood detention areas, soil characteristics of dikes for flood control engineering, flood prevention standards, embankment stability, and construction period. When the flood storage volume occupied by projects was no more than 0.1%, the comprehensive flood influence was negligible in all three ways for the rail transit projects through flood detention areas. Thus, appropriate engineering and non-engineering measures could ensure the regular operation and engineering safety of flood detention areas.

The main cases and results of this paper are as follows: Specifically, the basic situation and main data are shown in Table 1 for the flood detention areas of Lake Dongxi and Lake Wu. The results of flood storage and flood recession are shown in Table 3 and Figure 4. The influence of crossing ways on flooding and engineering was proportional to the flood storage volume occupied per unit length of lines in Table 3 for flood detention areas. The prevention and control measures were shown in Table 14 for different crossing ways on the influence of flood control in flood detention areas, flood on engineering, and engineering relevance.

Relevant papers were compared with the results of the papers in

Table 15. Comparison results of relevant references.

Authors/ References	Main Contents	Available Results	Evaluation
Zhang et al., 2016, [17]	Application influence of light rail on a flood detention area	Viaduct influence on flood storage and flood recession in the flood detention area (the light rail of No. 21 in Wuhan, China)	No analysis of the influence of floods on engineering, engineering relevance, etc.
Peng and Feng, 2012, [14]	Numerical simulation of the flood detention process before and after the construction of highway projects in a flood detention area	Highway influence on flood storage and flood recession in the flood detention area (the Xingtai section of the Xingheng Expressway, China)	No analysis of the influence of floods on engineering, engineering relevance, etc.
Wenrjie et al., 2016, [18]	Numerical simulation of the construction influence of underground gas storage in a flood detention area	Flood evolution processes in the flood detention area of Huaiyin around the Hongze Lake of China	No analysis of the influence of floods on engineering, engineering relevance, etc.
Zope et al., 2017, [4]	Hydrological effects of land use and detention basins on urban flood hazards	Influence of detention basins on urban flood hazards in the Poisar River Basin, Mumbai, India	Preference for the influence of urban detention basins on urban flood hazards
Jacob et al., 2019, [9]	Use of detention basins for flood mitigation and urban requalification	The cell model of the MODCEL of urban flow to evaluate the configuration and optimization of layouts of detention basins in Mesquita, Brazil	Preference for the design and optimization of urban detention areas
Yazdi, 2019, [8]	Optimal operation of urban storm detention ponds for flood management	Optimal operation model of urban detention basins in Tehran	Preference for the operation and management model of urban flood detention areas
Link et al., 2020, [30]	Local scouring and sediment deposition on bridge abutments during flooding	Local scouring and sediment deposition of bridge piers of the Rapel Bridge of Chile during flooding	No specific analysis for flood detention areas
He et al., 2021, [19]	A case study of Kinshasa about the flood’s influence on urban transit and accessibility	Flood influence on urban transit and accessibility in Kinshasa, the Democratic Republic of the Congo	No flood detention areas were involved
Colombo et al., 2018, [21]	Assessment methods for hydrogeological hazards for underground infrastructure	Hydrogeological hazards of underground infrastructure due to rising groundwater levels in Milan, Italy	No flood detention areas were involved
Our paper	Flood influence characteristics of different ways of crossing urban flood detention areas for rail transit projects	Comparison characteristics of flood influence of tunnel, elevated, and roadbed crossing urban flood detention areas in Wuhan, China	Comparing the influence of crossing ways on flood control in urban flood detention areas, floods on engineering, and engineering relevance

. Firstly, the comparison of the numerical results is carried out for the flood influence of light rail [17], highway [14], and underground gas storage [18] in Table 15. The same is true for the numerical methods used for the flood influence analysis of engineering on flood control in flood detention areas. The difference is that these results only analyze the influence of engineering on flood storage and flood recession in flood detention areas. However, we have analyzed the influence of flood on engineering and engineering relevance and compared the flood influence on flood detention areas by different crossing ways (subway, viaduct, and roadbed-based). Secondly, the comparison of urban flood detention areas is carried out for the influence of urban flood detention areas on urban flooding [4], the design optimization of flood detention areas [9], and the operational management of flood detention areas [8] (Table 15). The same is true of the relationship studied between urban flood detention areas and flood control. The difference is that these results focus on the influence of urban flood detention areas on urban flooding, design optimization, and operation management and do not compare the influence of crossing ways on flood control in urban flood detention areas. Finally, the comparison is carried out with the influence of flood control on bridges [30], urban transport [19], and underground infrastructure [21]. The same is true for all of them that carry out the influence on the relevant projects on flood control. However, the difference is that these results focus on the influence of floods on some aspects of engineering facilities without specific analysis for flood detention areas or comparing the influence of different crossing ways on flood control in flood detention areas.

The paper adopted a combination of qualitative and quantitative analysis. It took three rail transit projects crossing urban flood detention areas in Wuhan as examples and compared and evaluated the relevant effects of the crossing ways of the tunnel, viaduct, and roadbed-based ways on flood control in flood detention areas, floods on engineering, and engineering relevance. The major strength of this study was to discover the characteristics of the flood’s influence on the three types of rail transit projects: the subway, light rail, and railway crossing urban flood detention areas, and to reveal the interrelationship between the construction of different rail transit projects and flooding influence in urban flood detention areas.

The flood influence laws for the crossing ways may be used as references in project planning, design, and construction. These are the flood influence laws of rail transit projects of the tunnel, viaduct, and roadbed crossing ways on flood detention areas, the related indices of the flood storage volume occupied per unit length of lines, and the ratio of flood storage volume occupied. Moreover, the results may be helpful for the technical support of government decisions, project planning and construction of rail transit projects, and other projects crossing urban flood detention areas.

However, only an engineering case was chosen for each crossing way, and only two urban flood detention areas were chosen among the selected crossing ways: tunnel, viaduct, and roadbed-based. The number selected was small in crossing ways and urban flood detention areas, thus not very strong in representativeness. Moreover, the combination of the roadbed and viaduct ways was used to cross flood detention areas on the Xingang railway, with the length of the roadbed accounting for more than 83.98% of the total length of the line, but not all of the line was a roadbed way. Thus, its representation of roadbed might have adverse effects on the results. Lastly, since there were relatively few quantitative indicators of flood impact and engineering-related impact, it was difficult to establish a semi-empirical model to quantitatively analyze the flood impact of different rail transit projects crossing urban flood storage areas.

Rainwater problems, pollution caused by flooding, and nutrient input to waters caused by flooding also have some influence on the flood influence characteristics of rail transit through urban flood detention areas and need to be studied. However, it is difficult to carry out relevant studies because the relevant data are short on rainwater problems, pollution caused by flooding, and nutrient input to the waters.

5. Conclusions

The flood influence laws were proposed and demonstrated for the crossing ways of tunnels, viaducts, and roadbed-based rail transit projects in flood detention areas using a combination of qualitative and quantitative comparative methods, relevant codes, and engineering cases. These revealed the mutual relationship between the construction of different rail transit projects and flood influence in urban flood detention areas. Specifically, the crossing ways determined the flood storage volume occupied by unit length lines. The influence of the tunnel way was the smallest through flood detention areas; the influence of the roadbed-based way was the largest; and the influence of the viaduct way was in the middle.

For the flood influence of three crossing ways of rail transit projects on flood detention areas, the results of the quantitative and qualitative analysis may not only be helpful in the construction of rail transit projects through flood detention areas and rivers but also serve as references for other projects such as highways, high-speed railways, and channels through flood detention areas and rivers.

The next step is to collect more cases of tunnel, viaduct, and roadbed crossing ways and urban flood detention areas; more data related to stormwater problems, pollution caused by floods, and nutrient input to waters caused by floods; and engineering cases of a single roadbed way and other crossing ways. Moreover, semi-empirical models should be used for the risk analysis of floods. Last, the influence laws should be studied for multiple crossing ways on flood control in urban flood detention areas. Their influence characteristics should be analyzed for stormwater problems, pollution caused by floods, and nutrient input to waters caused by floods on rail transit through urban flood detention areas.