Urban Inundation Model Research Articles

A Coupled River–Overland (1D-2D) Model for Fluvial Flooding Assessment with Cellular Automata

To provide accurate and efficient forecasting of fluvial flooding assessment in the river basin, the present study links the well-known CA-based urban inundation modeling (2D-OFM-CA) with a one-dimensional river flow model (1D-RFM) as a coupled 1D-2D river–overland modeling. Rules to delineate the geometric linking between the 1D-RFM and 2D-OFM-CA along embankments are developed. The corresponding exchanged water volume across an embankment is then computed by using the free and submerged weir flow formulas. The applicability of the proposed coupled model on fluvial flooding assessment is then assessed and compared with a well-recognized commercial software (HEC-RAS model) through an idealized fluvial case and an extensively studied real-scale fluvial case in the Severn River Basin. Based on the simulated results concerning the numerical accuracy, the coupled model is found to give similar results in the aspects of the river flow and overland flow modeling in both two study cases, which demonstrates the effectiveness of the linking methodology between the 1D-RFM and 2D-OFM-CA. From the viewpoint of numerical efficiency, the coupled model is 47% and 41% faster than the HEC-RAS model in the two cases, respectively. The above results indicate that the coupled model can reach almost the same accuracy as the HEC-RAS model with an obvious reduction in its computational time. Hence, it is concluded that the coupled model has considerable potential to be an effective alternative for fluvial flooding assessment in the river basin.

Application of a subgrid-scale urban inundation model for a storm surge simulation: Case study of typhoon Haiyan

This study evaluated the individual drag force model (iDFM), a subgrid-scale model for simulating tsunami inundation in coastal urban areas. The iDFM was then applied to characterize Typhoon Haiyan-induced storm surge inundations in 2013. Using a post-typhoon survey dataset containing inundation depths at different survey points with inundation limits, the performance of iDFM was validated, and the results confirmed that iDFM accurately estimates the storm surge inundation depths but overestimates the limit of inundation. In addition, the results revealed that the iDFM flow velocity was a dominant factor, and the inclusion of buildings in the simulated affected the arrival of the leading edge of inundation. Furthermore, the reduction in flow velocity was sensitive to the subgrid-scale parameters, and the spatial distribution pattern of buildings located in the inundated area affected the relative difference in the maximum flow velocity with the regional roughness model. The inundation characteristics of the Typhoon Haiyan storm surge models were compared with a historical tsunami event. The inundation characteristics were similar and model differences were observed mainly in the fluid velocity and momentum flux rather than the inundation depth. However, the magnitudes of relative differences observed in Tacloban were larger for fluid velocity and momentum flux, respectively, than in the tsunami event. The effect of building drag on advection was also substantial in Tacloban due to the differences in the inflow mass and momentum fluxes.

Coupled sink and flow accumulation analyses with single flow direction and multiple flow direction algorithms

Abstract A static flood analysis (SFA) toolset is implemented with the purpose of performing simplified, event-based flood inundation modeling. The simulation process is divided into two steps: a topographic sink analysis of the terrain followed by flow accumulation (FA) of runoff volumes. Both procedures are coupled to account for sink storage effects. The sink analysis procedure determines the morphology of present sinks that are utilized during FA to calculate the sink storage by solving a mass balance, whereby inflows are captured in each sink according to their capacity and overflows are routed further downstream. Each sink is enriched with a set of hydrological attributes such as the total inflow, overflow, and flood depth. The flood depth is then utilized to determine the flood extent and flood depths of each sink. Two main options are made available for the FA procedure: either a single flow direction (SFD) or a novel formulation of the multiple flow direction (MFD) algorithm. Both methods were compared in terms of their accuracy with results from the TELEMAC-2D finite-volume solver for an urban inundation model. Both methods showed good agreement when compared with the validation results, with the MFD method performing marginally better than the SFD method.

Bottom-up multilevel flood hazard mapping by integrated inundation modelling in data scarce cities

With increasing urban flood risk due to urbanisation and climate change, flood hazard prediction is ever more crucial for flood risk management and emergency response. However, when sufficient high-quality data are lacking, a standard flood inundation modelling approach has significant uncertainties. Therefore, this study develops a bottom-up approach for urban flood hazard mapping at multiple levels (grid-kilometre-district), built upon the integration of grid-based flood modelling with data acquisition from open sources. This reduces the adverse effects of data scarcity and quality on hazard modelling. In the paper, we first set out an integrated approach for gridded inundation mapping in an urban basin using a hydrodynamic model supported by crowd-sourced social media data. Then, applying the model to flooding in Chengdu in August 2020, we articulate how input data bias and parameter uncertainty both affect urban inundation modelling. The results show that the choice of terrain data and the quantification of urban drainage flows significantly influence the modelled urban inundation extent, depth and duration. This indicates the potential for large variation when using urban inundation mapping merely relying on a single-scale flood hydrodynamic modelling even when supported by crowd-sourced data. The multilevel hazard mapping approach developed here presents multi-layered and comprehensive inundation mapping; thus the effects of data bias or availability are reduced and the coarser hazard mapping shows less sensitivity to the data input quality and model uncertainty, indicating relatively higher reliability at this higher spatial scale. The grid-kilometre-district three level approach provides more reliable flood hazard mapping, which can support rainstorm-induced flood management in data scarce cities.

Surrogate Global Optimization for Identifying Cost‐Effective Green Infrastructure for Urban Flood Control With a Computationally Expensive Inundation Model

AbstractOptimization algorithms and urban inundation models are powerful tools to identify cost‐effective designs of urban green infrastructures such as low‐impact developments (LIDs). Most previous LID design optimization studies are based on one‐dimensional (1D) inundation models, which cannot provide spatial information of flooding. The LID design optimization on two‐dimensional (2D) models or coupled 1D‐2D models was rarely explored due to the expensive computing time. This work investigates the effectiveness of surrogate optimization methods for LID design, which have not been used for the LID design problems. We propose a general LID design optimization framework that searches the optimal LID configurations based on both the spatial flood damage under a series of probable flood events and the life cycle costs (LCCs) of LID. We demonstrate the framework using a case study for an urban catchment with 55 sub‐catchments and 103 LID decision variables. We tested two different surrogate optimization methods designed for high‐dimensional problems: DYnamic COordinate search using Response Surface models (DYCORS) and Trust Region Bayesian Optimization (TuRBO), and one popular non‐surrogate method particle swarm (PSO). The result indicates that: (a) DYCORS is a promising method (significantly faster than TuRBO and PSO) for identifying the optimal LID design to minimize the flood damage cost and LID LCC; (b) Optimized LID design could reduce damage cost by as much as $12.14 million for the urban catchment after eliminating its own LCC compared with no LID implementation; (c) LID is effective in reducing the imperviousness of lands in urban areas.

Using the present to estimate the future: A simplified approach for the quantification of climate change effects on urban flooding by scenario analysis

AbstractUnderstanding and modelling pluvial flood patterns is pivotal for the estimation of flood impacts in urban areas, especially in a climate change perspective. However, urban flood modelling under climate change conditions poses several challenges. On one hand, the identification and collection of climate change data suitable for flood‐related evaluations requires consistent computational and scientific effort. On the other hand, large difficulties can arise in the reproduction of the rainfall‐runoff transformation process in cases when only little information about the subsurface processes is known. In this perspective, a simplified approach is proposed to address the challenges regarding the quantitative estimation of climate change effects on urban flooding for real case applications. The approach is defined as “bottom‐up” because climate change information is not included in flood modelling, but it is only invoked for the interpretation of results. In other words, the challenge faced in this work is the development of a modelling strategy that is expeditious, because it does not require flood simulations for future rainfall scenarios, but only under current climate conditions, thus reducing the overall computational effort; and it is flexible, because results can be easily updated once new climate change data, scenarios or methods become available, without the need of additional flood simulations. To simulate real case applications, the approach is tested for a scenario analysis, where different return periods and hyetograph shapes are used as input for urban inundation modelling in Naples, Italy. The approach can support public and private stakeholders, such as land administrators and water systems managers; moreover, it represents a valuable and effective basis for climate change risk communication strategies.

Overland-gully-sewer (2D-1D-1D) urban inundation modeling based on cellular automata framework

A new overland-gully-sewer (2D-1D-1D) urban inundation modeling is herein proposed. The authors have developed an integrated approach for overland-gully-sewer urban inundation simulations, which comprises 2D overland flow modeling (2D-OFM) by solving the two-dimensional non-inertia shallow water equations (2D-NISWEs), one-dimensional gully flow modeling (1D-GFM), and one-dimensional sewer flow modeling (1D-SFM). This approach can achieve good accuracy on urban inundation simulations. Nevertheless, such physically-based 2D overland flow modeling with the non-inertia SWEs (2D-OFM-NISWEs) requires substantial computational procedures, so that it is not suitable for real-time evaluation of inundation in urban areas. To resolve this limitation, the present study proposes a new integrated approach for overland-gully-sewer urban inundation modeling, in which the cellular automata (CA) framework is adopted for 2D-OFM to improve its suitability, and 1D-GFM and 1D-SFM are modeled by SWMM. A novel linkage among 2D-OFM-CA, 1D-GFM, and 1D-SFM is developed. A theoretical comparison of the governing equations and computational procedures between the two approaches is first conducted. Two historical flood events in Taipei city are next selected to perform the numerical accuracy verification and numerical efficiency evaluation. Analysis of inundation depths/velocities between the two approaches is conducted, and investigation of the drained and surcharged/inversed discharges of the two approaches is also made. The numerical comparisons between the two approaches indicate that the proposed approach can maintain good accuracy with significant reduction on its computing time. This newly proposed approach can finish a 3 hours short-duration urban inundation simulation on a case study with 335,621 cells within 12–17 mins. As a result, this CA-based approach is proved to be a useful tool for real-time urban inundation modeling.

Comparison of inundation patterns of urban inundation model and flood tracking model based on inundation traces

Comparison of inundation patterns of urban inundation model and flood tracking model based on inundation traces

Data complement algorithm of a complex sewerage pipe system for urban inundation modeling

Data complement algorithm of a complex sewerage pipe system for urban inundation modeling

Effects of DEM resolution and resampling technique on building treatment for urban inundation modeling: a case study for the 2016 flooding of the HUST campus in Wuhan

In China, flood inundation modeling is often limited by the lack of high quality topographic dataset, especially in small cities where hydrologic informatization is relatively backward. Coarse-resolution digital elevation models (DEMs) often lead to lower accuracy in simulation results due to the loss of small-scale building features in terrain modeling. Increasing the DEM resolution by resampling techniques may be an available solution to building representation problems in data-sparse areas, which has not yet received much attention and needs to be tested in conjunction with urban inundation models. This paper evaluated the impact of DEM resolution and resampling techniques on the building treatment method (BTM) and the output of urban inundation model to determine whether satisfactory inundation simulation can be achieved based on resampled DEMs with fine resolution. Using the 2016 flood event in Wuhan as the study case, a detailed comparison among 19 modeling scenarios differing in DEM resolutions, resampling techniques and BTMs was subsequently conducted. The outcome of the comparison analysis revealed the sensitivity of the simulated inundation depth to the resampled DEM resolution. Furthermore, the scenario using resampled DEMs with high resolution showed better performance in urban inundation modeling because the building features and the flow paths within building gaps were adequately represented by the BTM. The findings of this study may facilitate the application of BTMs for urban inundation modeling to predict (with useful accuracy) the inundation extent and depth in areas where fine topography data are unavailable.

Efficient Urban Inundation Model for Live Flood Forecasting with Cellular Automata and Motion Cost Fields

The mitigation of societal damage from urban floods requires fast hydraulic models for emergency and planning purposes. The simplified mathematical model Cellular Automata is combined with Motion Cost fields, which score the difficulty to traverse an area, to the urban inundation model CAMC. It is implemented with simple matrix and logic operations to achieve high computational efficiency. The development concentrated on an application in dense urban built-up areas with numerous buildings. CAMC is efficient and flexible enough to be used in a “live” urban flood warning system with current weather conditions. A case study is conducted in the German city of Wuppertal with about 12,000 buildings. The water depth estimation of every time step are visualized in a web-interface on the basis of the virtual globe NASA WorldWind. CAMC is compared with the shallow water equations-based model ANUGA. CAMC is approximatively 5 times faster than ANUGA at high spatial resolution and able to maintain numerical stability. The Nash-Sutcliffe coefficient (0.61), Root Mean Square Error (0.39 m) and Index of Agreement (0.65) indicate acceptable agreement for water depth estimation but identify different areas where important deviations occur. The estimation of velocity performs considerably less well (0.34 for Nash-Sutcliffe coefficient, 0.13 ms − 1 for Root Mean Square Error, and 0.39 for Index of Agreement) because CA ignores momentum conservation.

Influence of Uncertainties in DEM Resolution and Surface Friction to the Building Treatment Methods in Urban Inundation Modelling

This study evaluates the influence of uncertainties in DEM resolution and surface friction to building treatment method by conducting several simulations with various input parameters. A common two-dimensional hydrodynamic model is applied in a study area to produce the inundation results for every simulation. Comparison analysis between the simulation results and observed data suggest that the performance of building treatment method and urban inundation modeling is affected by the uncertainties in DEM resolution and surface friction. Moreover, different building treatment methods show disparity in sensitivity to the changes in model input. In practical terms, we can choose one of these methods based on the practical conditions, including available data, basin conditions and time constraints. The findings in this study are important for improving urban inundation models and for using their predictions in decision-making for various urban environments, which may provide some valuable insights into the flood management and risk assessment for the urban sites.

Evaluating the effect of grid size and type in integrated 1D/2D coupled urban inundation modelling on the interacting discharge between the surface and sewerage system

AbstractTo obtain accurate flood analysis results, research using high‐resolution Digital Elevation Models is becoming important and studies to increase the calculation speed and accuracy are being performed. Numerous researchers have studied parallelisation techniques to improve the accuracy of urban flood analysis by grid resolution and by 1D/2D coupled models. However, there have been few studies on the effects of interacting discharge between the surface and sewerage system on urban flooding analysis using parallelisation techniques. Therefore, in this study, the efficiency of the OpenMP parallelisation technique was investigated according to grid size and type to analyse the influence of the distribution of storm drain boxes on the results of fully integrated 1D/2D urban inundation modelling focused on the interacting discharge between the surface and storm drain boxes.

An urban flood inundation model based on cellular automata

An urban flood inundation model based on cellular automata

An urban flood inundation model based on cellular automata

This study develops a modified cellular automata (CA) model to simulate the flash flood inundation extent on a case study of an urban sub-catchment, in New York City. Based on the soil composition, the Horton equation is modified with threshold infiltration rates and applied to different land cover types. Further, the orifice equation is updated with a time variant parameter to account for partial/full blockage in the inlets. We propose a slope weighted flow transfer function to adjust the CA model and address the problem of depth positivity and flow regime changes, occurring due to the partial submergence. Seven ponding points with different levels of inundation are detected in the survey of the area and accordingly compared with the output of the simulation. The results prove the applicability of the developed CA model to reproduce the evolution of water depth.

Improved Building Treatment Approach for Urban Inundation Modeling: A Case Study in Wuhan, China

This paper describes an improved building treatment approach (IBTA) for use in urban inundation modeling. In this approach, the ground surface elevation was raised by the threshold (h) of the building entrance height to account for both the blockage and storage effect of areas with dense building coverage. A higher roughness coefficient was assigned to the areas where buildings were located to compensate for the resistance effects caused by the inner wall of the structure. The campus of Huazhong University of Science and Technology (HUST) in Wuhan City, China, was used as a case study. Comparison between IBTA and several traditional building treatment approaches suggested that the model results were sensitive to the building treatment method and the threshold used for terrain preprocessing in dense building regions. Furthermore, as the interaction between the surface water flow and dense buildings were adequately represented by using a new terrain preprocessing approach, the proposed IBTA provided better performance in terms of maximum inundation depth and the peak depth time than the traditional approaches in areas with dense building coverage, such as that of the campus.

Study on the influence of sewer network simplification on urban inundation modelling results.

도시유역에서 발생한 유출은 지표면뿐만 아니라 하수관망을 통해 배수되며, 도시침수 모의 수행시 하수관망을 수문학적 배수 시스템의 한 구성요로서 고려하는 것은 매우 중요하다. 그러나 도시 침수 모의를 수행하는 대부분의 연구자들이 적절한 기준에 준하지 않고 직관적으로 하수관망을 단순화시켜 모의를 수행하는 실...

Urban stormwater inundation simulation based on SWMM and diffusive overland-flow model.

With rapid urbanization, inundation-induced property losses have become more and more severe. Urban inundation modeling is an effective way to reduce these losses. This paper introduces a simplified urban stormwater inundation simulation model based on the United States Environmental Protection Agency Storm Water Management Model (SWMM) and a geographic information system (GIS)-based diffusive overland-flow model. SWMM is applied for computation of flows in storm sewer systems and flooding flows at junctions, while the GIS-based diffusive overland-flow model simulates surface runoff and inundation. One observed rainfall scenario on Haidian Island, Hainan Province, China was chosen to calibrate the model and the other two were used for validation. Comparisons of the model results with field-surveyed data and InfoWorks ICM (Integrated Catchment Modeling) modeled results indicated the inundation model in this paper can provide inundation extents and reasonable inundation depths even in a large study area.

Field application studies of the effect of an underground storage chamber using an urban inundation model

Recently, urban inundation disasters resulting from torrential rain have led to serious problems in many countries worldwide. To mitigate the damage of urban inundation, various structural strategies have been carried out, one of which is the creation of underground storage chambers attached to sewerage systems. In this strategy, part of the storm water within a sewer pipe is diverted over the side weir into the storage chamber. The function of this storage chamber is to manage the storm water runoff at a developed site to prevent flooding, and it is essential for providing a temporary storage area for excess storm water. In this study, the differences in the effects of the storage chamber using an urban inundation model due to several conditions such as the rainfall type, changes in the weir shape, installation locations that have different areal features, and changes in the storage capacity volume are verified. Lastly, a suitable installation site is verified via assessments of the inundation damage costs. The results show that the criteria for the degree of mitigation that can be expected in these conditions.

Reducing Computational Runtime of Two-Dimensional Urban Inundation Model by Dynamic Domain Reshaping

AbstractCoupling rainfall-runoff and one-dimensional (1D)/two-dimensional (2D) hydrologic models provides the current state-of-the-art approach for predicting the extent of urban inundation, although the computational cost for running 2D models is still a major concern. This paper presents a procedure to dynamically reshape 2D numeric domain according to flooding extent evolution. The numeric domain is updated at each selected time step for reshaping (TSR) and includes grid elements directly connected to the 1D stormwater network, flooded grid elements, or grid elements within a buffer distance threshold (BDT) identified in both previous criteria. Results for an urban catchment in South Brazil are presented considering two spatial resolutions and three return-period rainfall events. An optimal relation between TSR and BDT was found and resulted in runtime savings greater than 86% in comparison to adopting a fixed boundary as catchment limits, and greater than 69% in comparison to using a fixed boundary be...