1. Introduction
In recent years, global warming-induced temperature rise has increased the occurrence of extreme precipitation events, coupled with urban development leading to heightened runoff volumes, resulting in more frequent and severe urban flooding globally [
1]. With ongoing urbanization and economic growth, the damage from stormwater waterlogging is expected to rise [
2,
3], prompting a need for effective strategies [
4,
5]. Addressing waterlogging involves a comprehensive approach, including pre-flooding forecasts, emergency measures, and post-flooding resilience solutions. Central to this is the development of urban rainstorm waterlogging models and numerical simulations to identify high-risk areas and implement timely measures [
6,
7]. These models provide vital theoretical support for decision-making in urban stormwater flood prevention and mitigation. Generally, coupled models for simulating rainfall waterlogging can be categorized into those simulating flow production processes [
8] and those focusing on water distribution based on flow production [
9], with the latter requiring indispensable pipe network data.
When doing researches on the community-scale areas, it is easy to obtain the pipe network data and input it into the model, and a number of researchers have made studies. For instance, Li et al. [
10] investigated the Yangmei River basin in Tianhe District, Guangzhou City. Similarly, Niu's study [
11] encompassed the flight area of an airport, and Qiu et al. [
12] concentrated on an old district in Shanghai. Szeląg et al. [
13], on the other hand, examined the vicinity of the Si9 canal in Kielce, Poland. However, when expanding the research area into city-scale, there comes to some severe problems [
14], for example, credible pipeline network data are neither readily available nor easy to input into the model in urban scale studies.
When dealing with the pipe network data in city-scale area, the problem is actually twofold. On one side, the network data is hard to obtain. In a large number of urban areas, there is no complete data on the pipeline network [
15,
16]. There are a number of reasons for this, including the fact that the relevant information has been missing from the outset or has been lost over a long period of time for one reason or another, especially given that in some places the pipeline network was constructed decades ago [
17]. Or the actual layout of the pipe network not matching the design drawings for various reasons [
18], or that there are unrecorded blockages and damages in the network [
19,
20]. What’s more, it is quite expensive and difficult to onsite measure the drainage network system [
18]. On the other side, in city-scale area, the complexity of network data increases, significantly reducing the computational speed of the model. Therefore, in attempts to simulate urban-scale study sites, consideration needs to be given to trade-offs in the pipe network, such as generalising the main pipe network [
21,
22], by using this approach, the computational costs can be significantly reduced [
23]. For example, Wang et al. [
24] selected major drainage pipes and simplified minor ones in their simulation of Zhengzhou City, China. Similarly, Yang et al. [
25] employed a similar approach in their simulation of rainwater waterlogging in Fengxi New City near Xi'an City in Shaanxi Province, China, simplifying minor pipes while retaining the major drainage pipes. Shrestha et al. [
26] followed a similar approach in their study of Phoenix City. This approach is justified by the fact that the data requirements of the coupled model for the simulation of the pipe network are limited to the overflow of the pipe network and not to the entire convergence process [
27].
In response to the many problems faced when using pipe network data in the field of urban-scale research, some researchers have attempted to use alternative methods rather than recovering pipe network data, such as the development of synthetic sewerage networks to complement the representation of sewerage systems in urban flood models [
28], or using the empirical calibration method of the rainfall comprehensive runoff coefficient [
29] , or using community mapping [
30], some other researchers have used crowd-sourced data to build models [31]. However, these methods are lacking in hydrology and hydrodynamics principles, therefore, many researchers have proposed corresponding methods based on coupled hydrology-hydrodynamic models, coincidentally, many researchers have replaced the specific drainage network with a parameter called drainage capacity. However, researchers have proposed different approaches on how to substitute this parameter into the model. Some researchers opt to subtract drainage capacity from rainfall [32], while others argue for treating it as an infiltration process, which lead to three methods that are widely used: rainfall reduction method, constant infiltration method, and equivalent infiltration method. In practice, Chen et al. [33] utilized rainfall discounting in their simulations. Chang et al. [34], Leandro et al. [35] used a method of adding drainage capacity into the infiltration process. Wang et al. [36], on the other hand, compared the rainfall discounting method with the constant infiltration method and conclude that the latter produces superior results.
As a matter of fact, all the three methods mentioned above are problematic to a certain extent as far as the principle is concerned. The rainfall reduction method, for instance, produces a rainfall curve that can’t reach a minimum value below zero [36]. This oversimplification disregards the gradual recession of water on the site after the rainfall peak, leading to significant simulation errors. On the other hand, the constant infiltration method converts drainage capacity into a fixed infiltration capacity. While this method can simulate the water recession process following the rainfall peak, it overlooks the temporal changes in the natural infiltration capacity of the site substrate during the actual infiltration process. Consequently, this approach may yield inaccurate results in scenarios where the site substrate exhibits high natural infiltration capacity (e.g., well-landscaped urban areas) or the drainage capacity of the site network is low. As to equivalent infiltration (EI) method, this approach allows the drainage capacity of the pipeline to be influenced by the infiltration curve of the site, an influence that leads to errors that will be explained later.
After analysing the principles, it can be found that among the three methods mentioned above, the equivalent infiltration method is superior to the remaining two. Therefore, this paper chooses to improve the equivalent infiltration method on the basis of the existing research [37] in order to solve the interference brought to the model by the influence of the site infiltration process on the drainage capacity of the pipe network, and proposed a modified equivalent infiltration method (MEI). By using virtual drainage pipes and virtual drainage wells, the MEI method has the same data requirements as the equivalent infiltration method, but with better performance. The method proposed in this study helps to simulate storm waterlogging at urban scale sites where pipe network data are difficult to obtain, and provides a theoretical basis for storm waterlogging prevention and mitigation.
The structure of this article is as follows: section 2 is the principle of MEI method and the evaluation indicators, including the difference between EI and MEI ; section 3 is about the research site and data preprocessing content. In section 4, the error of the model, the comparison between EI and MEI methods, and the influence of well spacing on the simulation results were discussed. And section 5 is conclusion.
5. Conclusions
In this paper, the MEI method is proposed to address the issue of missing data on urban drainage networks, as an improvement of the equivalent infiltration method. This method involves extrapolating the drainage ability of a specific pipe network to the drainage capacity of the entire area and replacing specific wells with virtual wells distributed along the roads. The paper includes both qualitative and quantitative tests based on the measured data of Typhoon "In-Fa." Comparing with the equivalent infiltration method, the improved method can better simulate water-prone areas and the severity of water accumulation under rainstorm conditions with same data conditions, both qualitatively and quantitatively. The MEI’s average relative error of water depth is approximately 31%, with an NSE (Nash-Sutcliffe Efficiency) of 0.818, R2 (coefficient of determination) of 0.945, and RMSE (Root Mean Squared Error) of 2.741.
Additionally, by comparing different virtual well distances, it was found that the simulation results of the waterlogging simulation model constructed using the MEI method remained consistent when varied the parameter of virtual wells’ deployment distance in a suitable range, demonstrating the model's stability.
By employing the model using the MEI method, it becomes possible to simulate the spatial distribution and temporal evolution of flooding more accurately. This capability provides theoretical support for various subsequent studies, including accessibility analysis and other related research areas. Addressing these issues and further refining the model will contribute to its overall effectiveness and utility in practical flood control and urban planning applications.