1. Introduction
As important flood-control facilities, Flood Detention Areas (FDAs) have been widely applied in flood-prone regions within various major river basins [
1,
2,
3]. When floods surpass the storage and discharge capacity of rivers, these FDAs can take on a certain amount of floodwater, thus reducing the risk of dike breaches and potential flood losses within the basin [
4]. In China, given its large population, many FDAs are densely populated. Some of these areas are located near cities and serve as important spaces for urban expansion. Using these FDAs for flood control could result in significant losses [
5]. Taking the example of the Dongxihu FDA in Wuhan, if it were to be activated for flood storage, the estimated economic loss would be approximately CNY 39 billion (calculated based on the economic development level in 2011) [
6]. As the economy continues to grow, the activation losses of FDAs are progressively increasing year by year.
However, in the context of climate change, the risk of flooding continues to escalate. Despite the implementation of a series of flood-control projects, flood disasters are still occurring [
7]. To mitigate the risk of flooding, FDAs remain essential. Consequently, under the current circumstances, striking a balance between flood control benefits and economic benefits within FDAs and seeking a more scientific development strategy for these areas is one of the critical issues in basin development and flood management [
1]. It is important to emphasize that this allows for the rational use of the FDAs to support regional sustainable development [
8,
9].
In large river basins, crucial flood-control facilities such as embankments, reservoirs, and river regulation works are primarily aimed at reducing the impact of floods on human settlements, thus providing a safer and more sustainable environment, effectively offering flood control benefits for the basin. The assessment of these flood control benefits usually relies on flood risk maps, such as those created through hydrological modeling or GIS system simulations of flooding scenarios [
10,
11,
12,
13,
14,
15,
16,
17], which provide data support for the calculation of flood control benefits. However, these methods often only account for the losses incurred during flood inundation, while the costs associated with the construction and maintenance of the flood control structure, as well as the potential economic benefits they might yield, are neglected.
In recent years, the focus of researchers has shifted to the multiple benefits of flood-control structures, which primarily encompass potential values in water resource utilization and sustainable development. For example, a theory and a quantitative methodology for the multi-dimensional benefit evaluation of reservoirs have been developed, drawing upon interdisciplinary theories from energetics, economics, hydrology, and ecosystem service functions [
18]. Moreover, an effectiveness assessment framework based on the Rainfall–Runoff Model (HEC-HMS) for dams and reservoirs has been proposed to balance the flood control, water supply, and irrigation benefits of reservoirs [
19]. Although the existing multi-benefit assessment methods can be used to quantitative assess flood risk, which is essential for evaluating the benefits of flood control structures, the economic gains and losses of flood control structures have been ignored. They are not suitable for assessing areas that need economic development, such as FDAs.
For flood-control structures like FDAs, which have a low activation frequency and are populated, assessing their benefits necessitates a more comprehensive consideration. A framework for optimizing the site-selection and operational strategies of FDAs has been developed to maximize the overall benefits of flood-protection systems [
4]. This framework adeptly combines a hydrologically based flood simulation module with a comprehensive cost–benefit analysis module, aiming to optimize the system’s efficiency. It achieves this by meticulously balancing flood management in the basin with the advantages offered by flood-protection areas. Furthermore, an agent-based modeling approach has been introduced to simulate various stages of FDA lifecycles, including operation, maintenance, repair, and replacement [
20]. These methods can provide optimal maintenance strategies for FDA networks. However, the economic benefits generated by production in FDAs are often overlooked.
Currently, the benefit assessment for FDAs primarily focuses on improving their flood control efficiency, such as the losses reduced or the economic benefits derived from its flood control capacity, and very few studies focus on a comprehensive assessment of the benefits of all FDAs within a large watershed. To assess the flood control and economic benefits of FDAs in the YRB, the AHP–EW–TOPSIS integrated evaluation method is introduced by this study. Compared with previous studies that mainly focused on the assessment of individual FDAs, a statistically comprehensive assessment method for multiply FDAs was proposed in our manuscript; it can be utilized to evaluate the economic development and flood control capacity of multiply FDAs quantitively. The AHP method is one of the most commonly used subjective evaluation methods, then EW is used to increase the objectivity of the assessment, and finally the TOPSIS method is used to synthesize and rank the results. In order to balance the flood control and economic benefits within the FDAs in the YRB, we have referred to several flood scenarios that required partial storage in these basins. Based on our ranking results, the necessary FDAs for activation were selected. Finally, drawing on these outcomes, recommendations for FDAs were provided, prioritizing either economic development or the construction of flood prevention facilities. This strategy aims to enhance the economic benefits of the FDAs while ensuring safety in the basin in the event of floods. According to our review of the existing literature, there are no studies that provide a comprehensive assessment of multiple FDAs considering both economic and flood control benefits in a holistic perspective, and our study fills this gap.
In our evaluation methodology, the Analytic Hierarchy Process (AHP), one of the most frequently utilized Multi-Criteria Decision Making (MCDM) methods, was introduced by Saaty in the 1970s [
21]. The AHP stands out from other MCDM methods due to it being recognized for its ease of use, intuitiveness, and robust capability for consistency checks. The multi-benefit assessment of FDAs encompasses a variety of factors. In dealing with complex decision-making issues, such as in flood vulnerability and risk assessments, many studies have adopted diverse methods, including Frequency Ratio (FR), Cluster Analysis (CA), Principal Component Analysis (PCA), and Multi-Criteria Decision Making (MCDM) [
22]. MCDM is a prevalent decision-making approach used to select the most critical or best alternative among several potential criteria. For example, the flood risk of the Shenzhen metro system was effectively evaluated by researchers using AHP and TFN-AHP, further validating their findings with actual inundation data and showcasing the strengths of the AHP method in risk assessment [
23]. Other researchers have used AHP (the preference ranking organization method for enrichment evaluation) to choose the optimal Low Impact Development (LID) design for urban flood prevention, eventually deriving the most practical solution [
24]. However, due to the reliance of the AHP on expert scoring, which introduces a degree of subjectivity, we integrated the Entropy Weight Method (EW) to minimize this subjective bias [
25,
26].
Lately, the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS), based on Multi-Criteria Decision-Making (MCDM), has been widely used in assessing flood risk and flood control benefits. It has demonstrated its capability for rapid prediction and high interpretability, making it suitable for addressing various interrelated flood-related issues [
27,
28]. TOPSIS evaluates the best or worst solutions based on their distance to the Positive Ideal Solution (PIS) and the Negative Ideal Solution (NIS). While TOPSIS does not inherently assign weights, it can be used effectively in combination with other MCDM methods, such as the AHP, to create a robust evaluation model.
In this study, to comprehensively understand the differences among various FDAs, land use changes, GDP fluctuations, and population shifts in various FDAs were analyzed, comparing them with nearby cities from 1995 to 2020. Subsequently, the AHP–EW–TOPSIS comprehensive evaluation system was employed to assess changes in flood control benefits and economic benefits in these FDAs. The assessment results will provide priority construction decision plans for developing FDAs in the YRB and serve SDG10.1 and 11.5. The primary objectives of this study are as follows:
- (1)
To analyze the changes in economic factors within the Yangtze River Midstream Urban Agglomeration (YRMUA) and FDAs in the YRB to explore the relationships of economic development between FDAs and nearby cities.
- (2)
To construct a framework that encompasses the assessment of economic value and flood control value for FDAs in the YRB and identify their trends along with the underlying reasons.
- (3)
To provide constructive recommendations for the economic development and flood management construction of the FDAs in the YRB based on the results of this study.
4. Discussion
4.1. Changes in Value of FDAs in Yangtze River Basin
Most of the FDAs in the YRB, established during the 1950s and 1960s, are located in low-lying areas with sparse populations. However, as regional economics developed, there has been a certain degree of population growth within these FDAs.
Table 4 compares regional economic developments between the FDAs in the YRB and YRMUA from 1995 to 2020. The values in the table represent the growth rates of various indicators over these 25 years. During this period, the economic development of the FDAs in the YRB was particularly rapid, with the impervious area increasing by 1.8 times and GDP growing by 2.8 times, showing faster development compared to the YRMUA [
49].
Through several comparative experiments, it has been observed that, as cities expand, the economic development of the surrounding FDAs also accelerates rapidly. The closer an FDA is to a city, the greater the impact it experiences. Large cities have a significantly greater impact on the economic development of FDAs compared to medium and small cities. For instance, in the YRMUA, the most developed city, Wuhan (classified as a new first-tier city according to the “2022 City Commercial Attractiveness Ranking”), experienced an almost 3.3-fold increase in impervious area and an almost 3.1-fold increase in GDP over 25 years in its nearby FDAs. The growth rate of these FDAs even surpassed that of Wuhan itself. The main reason for this discrepancy in development is the relocation of parts of Wuhan’s industrial chain to its adjacent FDAs. In contrast, for slower-developing cities like Jingzhou (a third-tier city), the economic growth rate of its surrounding FDAs was roughly consistent with that of Jingzhou, and lower than the FDAs around Wuhan. As for Nanchang (a second-tier city), which differs from the aforementioned cities, the FDAs nearby developed more slowly than Nanchang over the same period. The primary reason for the developmental disparity between Nanchang and its neighboring FDAs is their distance from the city, resulting in less influence from urban development.
Despite the influence of flood-control policies in the FDAs of the YRB (“Several Suggestions on Strengthening the Construction and Management of Flood Storage and Detention Areas” (Issued on 13 June 2006, by the State Council)), the economic development within these areas has still surpassed that of most surrounding regions. This indicates that the implementation of flood control policies in most FDAs may not be entirely stringent, and the inhibitory effect of flood control policies on economic development is not very pronounced, which is contradictory to the original intention of setting up FDAs. In the event of a major flood, FDAs that do not have completed the flood-control structures will suffer significant losses.
Figure 10 provided an overview of the distribution of flood control values in the FDAs of the Yangtze River in 2020. FDAs with higher flood control values are mainly distributed on the left of Jingjiang River, in the central part of Dongting Lake, and in the western region of Wuhan. These FDAs have comparatively well-established flood-control structures, and most have been used in historical major floods. Although the gradual construction of upstream Yangtze River reservoirs [
50] has significantly reduced flood control pressure in the middle and lower reaches, these FDAs with higher flood control values remain integral to the current flood control system.
Figure 11 depicts the changes in flood control values across the FDAs of the YRB from 1995 to 2020. Regions experiencing a rapid decrease in flood control values are primarily situated around Hong Lake and in the northeastern part of Wuhan. These areas, influenced by urban expansion, have witnessed swift economic development, leading to more pronounced increases in losses when flood-control measures are activated.
Table 5 compares the economic development change of the Jingjiang FDA and the Dongxihu FDA. The Jingjiang FDA, deemed of utmost importance, commenced construction in 1952. It has since witnessed the establishment of floodgates and safety engineering, operating consecutively three times during the extensive Yangtze River flood in 1954. This consistent operation has guaranteed the safety of the Jingjiang embankment and obviously reduced potential losses for major cities in the middle and lower reaches. FDAs of this type should be continuously maintained and continue to regulate their industrial structure. On the other hand, some FDAs have a low probability of activation; the Dongxihu FDA in Wuhan, which has an activation standard exceeding a 200-year return period, has not been used since reclamation efforts began in 1957 [
51]. However, spurred on by a re-evaluation of the economic role of Dongxihu District in the “Wuhan City General Plan (2020)”, the economic development of this area has accelerated. Between 1995 and 2020 its GDP increased by almost three times, impervious area increased approximately 5-fold, and the population grew by 50%, surpassing the average of other FDAs. Despite there being a designated safety region within the area, it remains unconstructed and lacks the necessary conditions for activation. Flood vulnerability in FDAs has been significantly increased by socio-economic development, which poses new challenges for balancing flood-risk management and regional development [
52].
Therefore, accurately and effectively assessing the flood control and economic values of FDAs can provide precise data sources for basin construction and economic development, particularly in discussions about the retention or elimination of FDAs under current conditions.
4.2. Analysis of Watershed Development and Flood Management
Through comparative analysis of nine comparative areas, which include the YRMUA and its surrounding FDAs, a growing contradiction between economic development and flood mitigation policies in the YRB was identified. Unlike many developed countries where populations can be readily relocated from FDAs [
1], a large population resides in the FDAs of the YRB. Residents in these areas are impacted by flood-mitigation policies, whether or not flooding occurs, and therefore should receive some form of compensation.
Our research further reveals that the economic development potential of the FDAs was significantly underestimated at the time of their establishment, particularly the areas around urban areas. The contribution of these areas to urban development far exceeds the average level of the YRMUA. Therefore, in the current circumstances it might be appropriate to reconsider the selection of FDA sites. The FDAs in the YRB were established with the goal of defending against 1954-type floods. With the gradual construction of a large number of comprehensive hydraulic facilities with flood control functions in the upper reaches of the Yangtze River, as well as the completion of the Three Gorges Project, the flood-control capacity in the YRB has been significantly enhanced [
53,
54].
To provide planning recommendations for the existing FDAs, the simulation results in three flood dispatch scenarios facing 1954-type floods were selected by our experiment. These conditions include using flood-control structures such as reservoirs and dams. Only the activation status of the FDAs was concerned, and the reservoirs and dams were omitted. Two of these simulations included scenarios for the utilization of FDAs (Plan 1 and Plan 2) [
55,
56], while the remaining simulation designed two flood control schemes (Plan 3 and Plan 4) [
53]. Based on Plans 3 and 4, we designed the activation conditions for the FDAs. In our design, we strived to consider both their flood mitigation value and economic value as much as possible. The design principles are as follows:
- 1.
Determine the total volume of water that needs to be diverted by the FDAs during flood dispatch. Due to geographical reasons, the FDAs in the YRB are divided into four regions. Each region needs to divert a certain amount of floodwater during flood dispatch. It is necessary to ascertain the volume of water that needs to be diverted in each region.
- 2.
In each region, the FDAs to be activated are selected based on a ranking of their flood control value and the volume of floodwater they can store. The selection of FDAs is performed in descending order of their flood mitigation value.
- 3.
Among the FDAs selected in the previous step, consider their economic value ranking. If a basin has a lower economic value, it is chosen; otherwise, the process reverts to step 2. Finally, verify whether the total volume of floodwater stored meets the requirements. If it does, these FDAs are selected.
Our operation of the FDAs minimizes economic loss while ensuring flood protection, striving to balance economic values and flood control values. However, current flood dispatch in FDAs rarely considers these factors. The final activation results are presented in
Table 6:
In
Figure 12, the total number of activations required for FDAs in four simulated dispatches under existing conditions during a flood similar to the 1954 event is shown. Based on the aforementioned results, we have observed that under current flood control conditions, in the event of a flood similar to that of 1954, effective flood control can be achieved through the combined utilization of embankments, reservoir groups, islands, and FDAs. Some of these FDAs are no longer required. However, due to the impact of human activities and the development along the riverbanks, the flood-carrying capacity of the Yangtze River, from its upper reaches to the middle and lower reaches, as well as in its various tributaries and lakes, has somewhat decreased [
30]. The above-mentioned simulations largely did not consider this aspect. In the event of an extraordinary flood, the actual need to utilize FDAs may increase. Consequently, we have chosen to recommend the removal of the flood detention status for three of the FDAs that exhibit the highest economic development potential but relatively limited flood control effectiveness among those that were not utilized in the simulations. Based on our assessments of flood control value and economic value, the prioritization of the removal of FDA labels for the 33—Dongxihu FDA, 36—Baitanhu FDA, and the 2—Woshi FDA has ultimately been chosen by our results. This recommendation is aimed at fostering local economic development.
During the four simulation scenarios, several FDAs were consistently utilized, including the 29—Honghu West FDA, 30—Honghu Middle FDA, 31—Honghu East FDA, 27—Datonghu East FDA, 18—Chengxi FDA, 23—Qianlianghu FDA, and 17—Gongshuangcha FDA. We recommend strengthening flood-control infrastructure and relocation projects for these areas in preparation for defending against extraordinary floods similar to the one in 1954.
4.3. Limitations and Future Prospects
Despite our evaluation of flood control and economic factors in the FDAs of the YRB and the constructive suggestions we have put forward, there is room for improvement in our experimental results because of data limitations. For instance, we have found slight disparities between the population distribution data we utilized in small regions and the data from national statistical yearbooks during these experiments. This could lead to some degree of deviation in our results. In the future, with more accurate data, the results of our experiment will be modified. Additionally, while we selected parameters with reference to existing research, there still remains a degree of subjectivity. Using a broader set of relevant parameters may enhance the validity of the results. Indeed, the economic development prospects in FDAs, particularly in those near urban areas, present a compelling aspect for examination, and this will be the focus of our forthcoming research [
57,
58].
Although there are some limitations, a comprehensive analysis of flood control and economic development in the FDAs of the YRB has been provided by our results, offering effective data reference for their construction and development. These findings not only supply effective data sources for policy-making in the FDAs of the YRB, but also offer valuable insights for comprehensive evaluation of economic and flood control within other FDAs or a particular region. Further studies can be conducted based on our research findings, such as selecting suitable FDAs and analyzing their future development scenarios.
Furthermore, the continuous improvement in flood-control measures and the introduction of the concept of sustainable development have promoted the gradual acceptance of these ideas [
7]. People have gradually realized the limitations and drawbacks of past management measures. Flood-risk management cannot eliminate all risks, so flood management measures are shifting toward more sustainable strategies. The development of FDAs also needs to move toward sustainability, balancing economic benefits within the area while enhancing flood management resilience.