Measuring and modeling event-based environmental flows: An assessment of HEC-RAS 2D rain-on-grid simulations

Is the existing methods sustainable? A hybrid approach to flood risk mapping

The hydraulic and integrated modeling approaches appear to stand out in the sequence of flood risk models that have been presented because of their predictive accuracy. The former has a high probability of under predicting and the latter has a high tendency to over-predict. This study proposed a methodological approach that combines the hydraulic and integrated models using analytical hierarchical raster fusion techniques to strengthen the weaknesses of the individual models. This study seeks to undertake a flood inundation model, a runoff model, and raster fusion models using GIS and HEC-RAS rain-on-grid methods to map flood risk in the Ona river basin of Ibadan city. •A hydraulic model was used to identify flood depth and inundation areas along a major stream channel, which was then extracted, rasterized, resampled, and reclassified to a spatial resolution of 5 m.•Several raster datasets (indicators) were created from land use, elevation, soil, and geological data layers using advanced GIS techniques.•AHP assisted raster data fusion model was used to combine all of the raster indicators into a single consolidated hybrid flood raster layer that revealed flood risk areas by magnitude.

Are open-source hydrodynamic models efficient in quantifying flood risks over mountainous terrains? An exhaustive analysis over the Hindu-Kush-Himalayan region

The Hindu-Kush-Himalaya is abode to numerous severely flood-prone mountainous stretches that distress vulnerable communities and cause massive destruction to physical entities such as hydropower projects. Adopting commercial flood models for replicating the dynamics of flood wave propagation over such regions is a major constraint due to the financial economics threaded to flood management. For the first instance, the present study attempts to investigate whether advanced open-source models are skillful in quantifying flood hazards and population exposure over mountainous terrains. While doing so, the performance of 1D-2D coupled HEC-RAS v6.3 (the most recent version developed by the U.S. Army Corps of Engineers) is reconnoitred for the first time in flood management literature. The most flood-prone region in Bhutan, the Chamkhar Chhu River Basin, housing large groups of communities and airports near its floodplains, is considered. HEC-RAS v6.3 setups are corroborated by comparing them with 2010 flood imagery derived from MODIS through performance metrics. The results indicate a sizable portion of the central part of the basin experiences very-high flood hazards with depth and velocities exceeding 3 m, and 1.6 m/s, respectively, during 50, 100, and 200-year return periods of floods. To affirm HEC-RAS, the flood hazards are compared with TUFLOW at 1D and 1D-2D coupled levels. The hydrological similarity within the channel is reflected at river cross-sections (NSE and KGE > 0.98), while overland inundation and hazard statistics differ, however, very less significant (<10 %). Later, flood hazards extracted from HEC-RAS are fused with the World-Pop population to estimate the degree of population exposure. The study ascertains that HEC-RAS v6.3 is an efficacious option for flood risk mapping over geographically arduous regions and can be preferred in resource-constrained environments ensuring a minimal degree of anomaly.

Evaluating hydrological alterations and recommending minimum flow release from the Ujjani dam to improve the Bhima River ecosystem health

Numerous anthropogenic activities like the construction of large dams, storages, and barrages changed the watershed characteristics impacting ecosystem health. In this study, the hydrological alterations (HAs) that have occurred in the Bhima River due to the construction of the Ujjani dam were analyzed. The hydraulic analysis is also performed to determine the hydraulic parameter and recommend the lowest flow release from the dam for improving ecosystem health. Fifty-eight years of data starting from the year 1960 to 2018 were gathered at Yadgir station, which is located downstream of the Ujjani dam. The data were divided into pre- and post-construction river flow discharge. To check for the change in the river flow regime for the post-dam construction period, HA was calculated using Flow Health Software (FHS). The results demonstrate that the dam impoundment reduces high flows primarily by storing flood flow for water supply, irrigation, etc. The velocity and depth provided by the environmental design flow for a flow health (FH) score of 0.62 give a very good habitat to fishes. A minimum release of 24.8 m3/s from the dam is recommended. This study will help policymakers mitigate the impacts of degrading ecosystem health of the Bhima River.

Does flooding get worse with subsiding land? Investigating the impacts of land subsidence on flood inundation from Hurricane Harvey

As one of the most devastating tropical storms, 2017 Hurricane Harvey caused severe flooding and damage in Houston, Texas. Besides enormous rainfall amount, land subsidence might be another contributing factor to the Harvey flood. However, few studies have numerically quantified the evolvement of land subsidence over decades, largely due to the lack of reliable methods to realistically estimate land subsidence both continuously and at high spatial resolution. Therefore, this study aims to investigate retrospective changes of regional topology due to 117 years (1900 to 2017) of land subsidence and the consequent impacts on flood inundation. Based on continuous land subsidence, we conduct a series of simulations on the 2017 Hurricane Harvey in Brays Bayou, Texas using a hydrodynamic/hydraulic model. The results indicate that the overall change of flood depth caused by land subsidence is relatively minor with the flood water deepened by six centimeters per one meter of subsided land at the worst impacted location. The impact from land subsidence on flood depth exhibits strong nonlinearity in time, where effects from previous land subsidence hotspots could be altered by later continuing land subsidence. Spatially, changes in flood depth due to the land subsidence are not only heterogeneous but mixed with coexisting increased and reduced flood depths. The results of this study improve the understanding of the dynamic evolvement of flood inundation due to continuous land subsidence so that better planning can be initiated for sustainable urban development for coastal communities, which is imperative under ongoing climate change and sea level rise.

Habitat alteration assessment for the management of environmental flows in regulated basins

The management of environmental flows is of paramount importance in regulated water resources systems to preserve river ecosystems. This work proposes a methodology to assess habitat alteration in river basins altered by management activities. The methodology is based on the joint application of a basin management model (SIMGES, AQUATOOL) and a model to estimate habitat time series (CAUDECO). CAUDECO is based on the weighted useable areas of the species in their different vital stages that, in turn, depend on the flows in each river stretch and the biological periods of the species. The final output is an indicator of habitat alteration, which is defined ad hoc for this work to relate the habitat suitability under regulated and natural regimes. The methodology was applied to a case study in north-western Spain: the Órbigo River basin. The results in the current management scenario highlight that the ecological flows improve the habitat suitability of several species with respect to natural regime conditions. For instance, the mean values of the habitat time series in the Órbigo River for the brown trout and bermejuela under regulated conditions are 69.6% and 88%; whereas in natural regime they are equal to 55.1% and 72.9%, respectively. Based on these results, eight additional scenarios of ecological flows were tested and their effects on both habitat alteration and water demand reliability were quantified and discussed. It was found that increases in the ecological flows up to 30% do not affect the reliability of water demands and reduce habitat alteration (i.e., lead to values of the habitat alteration indicator closer to 1) for all species present in the river basin. These results highlight that the methodology and indicator of habitat alteration proposed in this paper are useful to support the management of regulated river basins, since they allow assessing the implications of ecological flows on both habitat suitability and reliability of water demands.

Earth Dam Design for Drinking Water Management and Flood Control: A Case Study

Water management for natural channels is a frequent challenge due to the inefficient usage of water resources. The 2030 Agenda of the United Nations (SDG 6 of sustainable development) focuses its attention on water and sanitation. The Sara Guerrero site, located in the Mocache municipality in Los Ríos province (Ecuador), has issues related to access to drinking water, flood control, and crop irrigation that affect 4300 people and 24,000 hectares. The river overflows throughout the rainy season (late December to early May), whereas there is a noticeable water shortage during the dry season. This project aims to design a multiple-use earth dam on the Vinces River, simulating the resulting flow in extreme cases due to its possible failure. Such a study implies the development of a contingency plan for the preservation of life. It considers (i) dam breach analysis and design, and (ii) hydraulic model development using the ArcMap and HEC-RAS software packages. The design includes a waterproofing system that controls possible leaks and a cymbal spillway, mainly for raw water collection. The generated model showed that the shorter the failure time, the higher the maximum output flow. Modelling revealed that four towns would be affected for a maximum of 31 h in extreme cases. This approach offers comprehensive management for this community with regards to the earth dam and flood control.

The 21st August 2020 Flood in Douala (Cameroon): A Major Urban Flood Investigated with 2D HEC-RAS Modeling

A major flood event occurred on 21 August 2020 in the densely populated Makèpè Missokè neighborhood in the city of Douala (Cameroon, Africa). Nearly 2210 buildings and 12,376 victims spread over 82 hectares were affected. A 2D HEC-RAS model is applied to simulate and characterize this event. A cross analysis of flood depth and flow velocity is used to classify the flood risk and identify areas exposed from low to high hazard. The simulations provide detailed information on the flood characteristics (extent, depth, velocity, arrival time, and duration). The simulated maximum water surface profiles are consistent with the floods marks with differences ranging from 0.02 m to 0.44 m, indicating a good agreement between the observed and simulated water levels at the peak flow (NSE = 0.94, Erel = 0.92, RMSE = 0.21 m). The maximum inundation level is 4.48 m and the flow velocity is globally low at less than 1 m/s. The average flood arrival time and duration are 5 h and 26 h, respectively, for a threshold height of 0.5 m. These results indicate a fast mobilization of the major river channel for the evacuation of this flood. The level of accuracy of the developed model of the 21 August 2020 flood event is appropriate for flood hazard assessment in the city of Douala and is designed to find operational application in future events.

Linking Urban Floods to Citizen Science and Low Impact Development in Poorly Gauged Basins under Climate Changes for Dynamic Resilience Evaluation

Cities must develop actions that reduce flood risk in the face of extreme rainfall events. In this study, the dynamic resilience of the Gregorio catchment (São Carlos, Brazil) was assessed. The catchment lacks environmental monitoring and suffers from recurrent floods. The resilience curves were made considering the water depth in the drainage system as the performance index, obtained by simulations with SWMM and HEC-RAS. The calibration of the flood extension was performed using citizen science data. The contribution to increasing the dynamic resilience by implementing decentralized low impact development (LID) practices was also evaluated. For this purpose, bioretention cells were added to the SWMM simulations. The resilience curves were then calculated for the current and future climate scenario, with and without LID, for return periods of 5, 10, 50, and 100 years and duration of 30, 60, and 120 min. Intensity–duration–frequency curves (IDFs) updated by the regional climate model MIROC5 for 2050 and 2100 were used. The results showed a significant improvement in the system’s resilience for light storms and the current period due to LID practice interventions. Efficiencies were reduced for moderate and heavy storms with no significant drops in floodwater depth and resilience regardless of the scenario.

Automated environmental compliance monitoring of rivers with IoT and open government data

Environmental monitoring of rivers is a cornerstone of the European Union's Water Framework Directive. It requires the estimation and reporting of environmental flows in rivers whose characteristics vary widely across the EU member states. This variability has resulted in a fragmentation of estimation and reporting methods for environmental flows and is exhibited by the myriad of regulatory guidelines and estimation procedures. To standardise and systematically evaluate environmental flows at the pan-European scale, we propose to formalise the estimation procedures through automation by reusing existing river monitoring resources. In this work, we explore how sensor-generated hydrological open government data can be repurposed to automate the estimation and monitoring of river environmental flows. In contrast to existing environmental flows estimation methods, we propose a scalable IoT-based architecture and implement its cloud-layer web service. The major contribution of this work is the demonstration of an automated environmental flows system based on open river monitoring data routinely collected by national authorities. Moreover, the proposed system adds value to existing environmental monitoring data, reduces development and operational costs, facilitates streamlining of environmental compliance and allows for any authority with similar data to reuse or scale it with new data and methods. We critically discuss the opportunities and challenges associated with open government data, including its quality. Finally, we demonstrate the proposed system using the Estonian national river monitoring network and define further research directions.

A Systematic Analysis of the Interaction between Rain-on-Grid-Simulations and Spatial Resolution in 2D Hydrodynamic Modeling

A large number of 2D models were originally developed as 1D models for the calculation of water levels along the main course of a river. Due to their development as 2D distributed models, the majority have added precipitation as a source term. The models can now be used as quasi-2D hydrodynamic rainfall–runoff models (‘HDRRM’). Within the direct rainfall method (‘DRM’), there is an approach, referred to as ‘rain-on-grid’, in which input precipitation is applied to the entire catchment area. The study contains a systematic analysis of the model behavior of HEC-RAS (‘Hydrologic Engineering Center—River Analysis System’) with a special focus on spatial resolution. The rain-on-grid approach is applied in a small, ungauged, low-mountain-range study area (Messbach catchment, 2.13 km2) in Central Germany. Suitable model settings and recommendations on model discretization and parametrization are derived therefrom. The sensitivity analysis focuses on the influence of the mesh resolution’s interaction with the spatial resolution of the underlying terrain model (‘subgrid’). Furthermore, the sensitivity of the parameters interplaying with spatial resolution, like the height of the laminar depth, surface roughness, model specific filter-settings and the precipitation input-data temporal distribution, is analyzed. The results are evaluated against a high-resolution benchmark run, and further criteria, such as 1. Nash–Sutcliffe efficiency, 2. water-surface elevation, 3. flooded area, 4. volume deficit, 5. volume balance and 6. computational time. The investigation showed that, based on the chosen criteria for this size and type of catchment, a mesh resolution between 3 m to 5 m, in combination with a DEM resolution from 0.25 m to 1 m, are recommendable. Furthermore, we show considerable scale effects on flooded areas for coarser meshing, due to low water levels in relation to topographic height.