Hydrodynamics in the Morganza Floodway and Atchafalaya Basin, report 3 : phase 3; a report for the US Army Corps of Engineers, MRG&P

The Morganza Floodway and the Atchafalaya Basin, located in Louisiana west of the Mississippi River, were evaluated using a two-dimensional Adaptive Hydraulics model. Prior to this study, Phase 1 and 2 model studies were performed that indicated that the existing floodway may not be able to pass the Project Design Flood discharge of 600,000 cubic feet per second due to levee overtopping. In this study, all elevations of exterior and interior levees were updated with current crest elevations. In addition, the Phase 3 effort evaluated the sensitivity of the floodway’s flow capacity to variations in tree/vegetation density conditions. These adjustments in roughness will improve the understanding of the role of land cover characteristics in the simulated water surfaces. This study also provides a number of inundation maps corresponding to certain flows through the Morganza Control Structure.

Flood Assessment and Identification of Emergency Evacuation Routes in Seti River Basin, Nepal

Sudden floods frequently occur in the Himalayas under changing climates. Rapid glacial melt has resulted in the formation of glacial lakes and associated hazards. This research aimed to (1) identify flood-prone houses, (2) determine pedestrian emergency evacuation routes, and (3) analyze their relationships to socioeconomic status in the Seti River Basin. Detailed hazard maps were created using field survey results from unmanned aerial vehicle photogrammetry and the Hydrologic Engineering Center River Analysis System. Questionnaire, focus-group, and key-informant surveys helped identify the socioeconomic situation. Inundation maps revealed that most residents are exposed to future flooding hazards without proper evacuation routes. Highly impoverished and immigrant households were at the highest risk in terms of income inequality and migration rate (p < 0.001) and were located on the riverside. The locations of 455 laborers’ houses were significantly correlated with inundation hazards (p < 0.001). Governmental and associated agencies must develop adequate plans to relocate low-income households. Group discussions revealed the need for stronger adaptive capacity-building strategies for future risk management. Pokhara requires better systematic and scientific land-use planning strategies to address this issue efficiently. A similar approach that combines flood modeling, proper evacuation route access, and socioeconomic survey is suggested for this river basin.

Comparison between Topographic and Bathymetric LiDAR Terrain Models in Flood Inundation Estimations

Remotely sensed LiDAR data has allowed for more accurate flood map generation through hydraulic simulations. Topographic and bathymetric LiDARs are the two types of LiDAR used, of which the former cannot penetrate water bodies while the latter can. Usually, the topographic LiDAR is more available than bathymetric LiDAR, and it is, therefore, a very interesting data source for flood mapping. In this study, we made comparisons between flood inundation maps from several flood scenarios generated by the HEC-RAS 2D model for 11 sites in Norway using both bathymetric and topographic terrain models. The main objective is to investigate the accuracy of the flood inundations generated from the plain topographic LiDAR, the links of the inaccuracies with geomorphic features, and the potential of using corrections for missing underwater geometry in the topographic LiDAR data to improve accuracy. The results show that the difference in inundation between topographic and bathymetric LiDAR models decreases with increasing the flood size, and this trend was found to be correlated with the amount of protection embankments in the reach. In reaches where considerable embankments are constructed, the difference between the inundations increases until the embankments are overtopped and then returns to the general trend. In addition, the magnitude of the inundation error was found to correlate positively with the sinuosity and embankment coverage and negatively with the angle of the bank. Corrections were conducted by modifying the flood discharge based on the flight discharge of the topographic LiDAR or by correcting the topographic LiDAR terrain based on the volume of the flight discharge, where the latter method generally gave better improvements.

Agricultural Flood Vulnerability Assessment and Risk Quantification in Iowa

Agricultural lands are often impacted by flooding, which results in economic losses and causes food insecurity across the world. Due to the world’s growing population, land-use alteration is frequently practiced to meet global demand. However, land-use changes combined with climate change have resulted in extreme hydrological changes (i.e., flooding and drought) in many areas. The state of Iowa has experienced several flooding events over the last couple of decades (e.g., 1993, 2008, 2014, 2016, 2019). Also, agribusiness is conducted across 85 percent of the state. In this research, we present a comprehensive assessment for agricultural flood risk in the state of Iowa utilizing most up-to-date flood inundation maps and crop layer raster datasets. The study analyzes the seasonal variation of the statewide agricultural flood risk by focusing on corn, soybean, and alfalfa crops. It also investigates the crop frequency layers and corn suitability rating datasets to reveal regions with lower or higher productivity ratings. Additionally, a terrain-based flood model is used to analyze performance against the FEMA maps. The research discusses the potential mitigation activities for the most vulnerable watersheds in the state. The analysis shows that nearly a half-million acres of cornfields and soybean fields are located in the 2-year flood zone. We also found that terrain-based flood maps are a reliable alternative for agricultural flood risk assessment based on their dynamic structure, rapid update capability, and performance compared to FEMA maps.

Brief communication: Western Europe flood in 2021: mapping agriculture flood exposure from SAR

In this communication, we present the exposure of agriculture lands to the flooding caused by extreme precipitation in western Europe from 12th to 15th of July 2021. Overlaying the flood inundation maps derived from the near-real-time RAdar-Produced Inundation Diary (RAPID) system on the CORINE land cover map we estimate a 2470 km2 area affected by the flooding, with 57 % representing agricultural land. Among the inundated agricultural land, 36 % of the area is pastures while 33 % is arable land. Most agricultural flood exposure is found in south-eastern France (~1680 km2) along Rhône River and the coastal area of Marseille and Montpellier.

Mini Special Issue on Tsunami Numerical Modeling Benchmarks – Challenges of Tsunami Modeling Hackathon –

Numerical simulation and modeling became an essential technology in tsunami research and disaster management. Various numerical models were proposed and utilized for the development of tsunami risk assessment, inundation maps, and evacuation plans. The model verification and validation standards would be crucial to ensure sufficient reliability of tsunami risk assessment, inundation maps, as well as a consistency among various efforts. Common approach to ensure sufficient accuracy and reliability of numerical modeling is developing benchmark problems of hydraulic experiments and to use them for numerical model’s verification and validation. To satisfy this requirement, “Tsunami Modeling Hackathon” was held in September 2020 to organize new benchmark problems in numerical modeling of tsunamis and to improve their reliability and accuracy. Hackathon is an intensive-gathering event of computer programmers and others involved in software development to create outcomes by the end of the event. This event was organized by Prof. Tomoyuki Takahashi of Kansai University and his colleagues, who led the tsunami research subcommittee in Japan Society of Civil Engineers (JSCE). Tsunami modeling hackathon, in which about 23 teams and 162 researchers joined, included experiment and modeling teams in seven benchmark problems: urban tsunami inundation, landslide tsunami, tsunami loading on seawalls and coastal structures, sediment transport, drift of floating objects. The modeling groups performed the blind tests to cross-validate and interpret the results of their simulations in seven benchmark problems given by the experiment groups and discussed the improvement. This special issue reports the outcomes of the tsunami modeling hackathon, and includes six papers (five in this issue, one in the regular issue). We hope this issue will provide useful insights for tsunami modelers and contribute to establishing a standardized way to ensure that various tsunami numerical models would be validated through the benchmark problems.

Efficient Hazard Assessment for Pluvial Floods in Urban Environments: A Benchmarking Case Study for the City of Berlin, Germany

The presence of impermeable surfaces in urban areas hinders natural drainage and directs the surface runoff to storm drainage systems with finite capacity, which makes these areas prone to pluvial flooding. The occurrence of pluvial flooding depends on the existence of minimal areas for surface runoff generation and concentration. Detailed hydrologic and hydrodynamic simulations are computationally expensive and require intensive resources. This study compared and evaluated the performance of two simplified methods to identify urban pluvial flood-prone areas, namely the fill–spill–merge (FSM) method and the topographic wetness index (TWI) method and used the TELEMAC-2D hydrodynamic numerical model for benchmarking and validation. The FSM method uses common GIS operations to identify flood-prone depressions from a high-resolution digital elevation model (DEM). The TWI method employs the maximum likelihood method (MLE) to probabilistically calibrate a TWI threshold (τ) based on the inundation maps from a 2D hydrodynamic model for a given spatial window (W) within the urban area. We found that the FSM method clearly outperforms the TWI method both conceptually and effectively in terms of model performance.

2D-HEC-RAS Modeling of Flood Wave Propagation in a Semi-Arid Area Due to Dam Overtopping Failure

Dam overtopping failure and the resulting floods are hazardous events that highly impact the inundated areas and are less predictable. The simulation of the dam breach failure and the flood wave propagation is necessary for assessing flood hazards to provide precautions. In the present study, a two-dimensional HEC-RAS model was used to simulate the flood wave resulting from the hypothetical failure of Al-Udhaim Dam on Al-Udhaim River, Iraq, and the propagation of the resulting dam-break wave along 100 km downstream the dam site for the overtopping scenario. The main objective is to analyze the propagation of the flood wave so that the failure risk on dam downstream areas can be assessed and emergency plans may be provided. The methodology consisted of two sub-models: the first is the dam breach failure model for deriving the breach hydrograph, and the second is the hydrodynamic model for propagating the flood wave downstream of the dam. The breach hydrograph is used as an upstream boundary condition to derive the flood impact in the downstream reach of Al- Udhaim River. The flood inundation maps were visualized in RAS-Mapper in terms of water surface elevation, water depth, flow velocity, and flood arrival time. The maximum recorded values were: 105 m (a.m.s.l.), 18 m, 5.5 m/s, and, respectively. The flow velocity decreased from upstream to downstream of the terrain, which means less risk of erosion in the far reaches downstream of the study area. The inundation maps indicated that the water depth and flow velocity were categorized as Catastrophic limits on the terrain's area. The results offer a way to predict flood extent and showed that the impact of a potential dam break at Al-Udhiam Dam will be serious, therefore, suitable management is needed to overcome this risk. Moreover, the maps produced by this study are useful for developing plans for sustainable flood management. Doi: 10.28991/cej-2021-03091739 Full Text: PDF