Hydrologic Engineering Center River Analysis System Model Research Articles

Evaluation of post extreme floods in high mountain region: A case study of the Melamchi flood 2021 ​at the Koshi River Basin in Nepal

Extreme precipitation events and associated hazards are becoming more frequent and intense due to climate change. On June 2021, a devastating flash flood occurred in the Melamchi River in Nepal causing immense destruction to lives and property. The flood event was cascading in nature and associated with multiple driving factors. This study evaluates the post-extreme flood conditions of the Melamchi Flood 2021 (MF21). Rainfall, streamflow (discharge), and gauge height (water level) data of the nearest stations were analyzed. Since there was a lack of reliable instantaneous river discharge during the disastrous MF21, this study developed the flow rating curves at hydrometric stations. Post-extreme flood was evaluated considering upstream catchment hydrology, glacial lake outbursts, landslide damming, and the hydrodynamic propagations using the Hydrologic Engineering Center's River Analysis System (HEC-RAS) model and gauge-to-gauge (G2G) correlation. The results were then compared to field observations. The HEC-RAS model was simulated for several flood frequency periods in different return periods (namely 2, 5, 10, 25, 50, 100, 200, 500, and 1000 years). Results showed that the average 24 ​h gauge height and discharge at Nakote station (a few km downstream of landslide damming) are 15.88 ​m and 169.18 ​m3/s, in which the model-based highest frequency of 1-h extreme flood and return period can reach 716 ​m3/s in 1000 years. During MF21, the discharge of the Nakote region at the 10 ​min intervals was estimated to be with a maximum of 7162.10 ​m3/s, an average of 2207.13 ​m3/s and a minimum of 237.44 ​m3/s.Additionally, the outburst discharge from glacial lake at high elevation was estimated to be 295.14 ​m3/s contributing to downstream flooding. The model-based discharge during the past flood period was also estimated in 9 cross-section points of the river in which discharge at the damming area was 63.81 ​m3/s and downstream Dumredovan was 1206.58 ​m3/s. This research provides valuable insights to post-extreme flood evaluation and the approach can be used for other watersheds in evaluating post flood conditions.

Flood inundation mapping under climate change scenarios in the Boyo watershed of Southern Ethiopia

Abstract This research aims to map flood inundated areas under changing climate in the Boyo watershed of Southern Ethiopia. A semi-distributed physically based Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) and Hydrologic Engineering Center-River Analysis System (HEC-RAS) were used to simulate the flood events and maps, respectively, for climate scenarios. The bias-corrected data of four climate models were used for the baseline (1976–2005), mid-term (2041–2070) and long-term (2071–2100) cycles under RCP4.5 and RCP8.5 scenarios. The 50- and 100-year return period flood events were generated from the baseline and future period streamflow data. The HEC-RAS model was used to simulate the inundation areas and depths from the flood events. The result exhibited that the average annual rainfall and maximum and minimum temperatures of the catchment will increase in the future with an increase in annual runoff. The severity of annual floods would increase in the future under RCP4.5 and RCP8.5 scenarios. Approximately, 193 ha of the study may be flooded with flood events having a return period of 100 years under the RCP8.5 scenario in the long-term period, which is an extreme case. The result is a benchmark to reduce the flood risk and management of floodplains in this watershed.

Assessing flood inundation mapping through estimated discharge using GIS and HEC-RAS model

Water discharge is the main parameter in hydraulic modeling for flood hazard assessment. However, the unavailability of data on discharge and observed river morphologies resulted in erroneous calculations and irregularities in flood inundation mapping. The objectives of this study are (i) to investigate uncertainties of hydraulic parameters (width, cross-sectional depth, and channel slope) used in discharge equation and (ii) to examine the influence of estimate discharge on water extent and flood depth with different boundary conditions on interferometric synthetic aperture radar (IFSAR) and modified IFSAR DEMs. Sensitivity analysis was conducted with the Monte Carlo simulation method to generate random data combinations. Bjerklie’s equation was used to calculate discharge based on the three variables, and Manning’s n was substituted into the Hydrologic Engineering Center River Analysis System (HEC-RAS) model. TerraSAR-X was used to distinguish existing flood water bodies and normal water extent. The uncertainty of the combined variables was assessed with the likelihood measures such as F-statistic, mean absolute error, root mean square error, and Nash–Sutcliffe efficiency which compares observed and predicted inundated area as well as flood water depth simulated using the HEC-RAS model.

Prediction of sedimentation in reservoirs by combining catchment based model and stream based model with limited data

Estimation of sedimentation in reservoirs helps in the management and design of the reservoir׳s useful capacity. This research was done on the Awash River basin at the Koka Dam Reservoir in Ethiopia. The method applied was the loose integration of the Soil and Water Assessment Tool (SWAT) model and Hydrologic Engineering Center-River Analysis System (HEC-RAS) model for the estimation of the sediment load reaching the reservoir. The SWAT model was used for the estimation of erosion at the catchment level, and the HEC-RAS model was applied to estimate the sediment transport in the river channel. The implemented method allows sedimentation in the floodplains and bed shear stress to be considered in the sediment modeling, which cannot be considered in the SWAT model. In addition, the river cross sectional properties and the hydrodynamic processes in the rivers were considered in the modeling process. The data used in this study are a combination of i) observed data collected by government agencies, ii) data available online in data repositories, and iii) data extracted from remote sensing in the Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model (DEM). The calibration and validation of the SWAT model was done by using Sequential Uncertainty Fitting (SUIF-2) calibration and validation tools. The HEC-RAS model was calibrated by adjusting the roughness factor. The output from the integrated approaches gives better estimates of flow and sediment near the inlet to the reservoir, with coefficients of determination of 0.85 and 0.67, respectively, and Nash Sutcliffe coefficients of model fit efficiency of 0.90 and 0.62, respectively, for daily simulations.

Analysis of impacts of polders on flood processes in Qinhuai River Basin, China, using the HEC-RAS model

Abstract Flood control with polders is prevalent in East China. Their impact on flood processes is critically important for flood control, but has not been well documented. The Qinhuai River Basin was selected as the study area. A Hydrologic Engineering Center – River Analysis System (HEC-RAS) hydraulic model was developed to simulate and predict storm flood processes and the associated impact of polders. The study shows that the HEC-RAS model is capable of simulating the impact of polders on flood processes in the Qinhuai River Basin. The polders increased the water level outside of the polders. The polders in upstream watersheds have a greater impact on the water level than polders close to basin outlets when individually distributed. The maximum water level at Dongshan section shows an increasing trend for different sized flood with the increasing number of polders in the basin, and a linear increasing trend associated with urbanization. The smaller the flood scale is, the greater the maximum water level changes.

Estimation of hydrodynamic pattern change of Ichamati River using HEC RAS model, West Bengal, India

The physically-based hydrodynamic model can simulate the water flow dynamics of a stream network against time varying boundary conditions that can be implemented for the case of Ichamati River. Such river models are often an important component of flood forecasting, tidal fluctuation system that forecasts river levels in flood prone regions of the middle stream of Ichamati river. The measurements of river bed depth/slope, water quality (river cross section), floodplain mapping and boundary condition flow are essential for the set up of a river model. But one can use proxy approaches relying mostly on remote sensing data from space platforms for the purpose. In this study, we set up the one dimensional river analysis system (RAS) model of the hydrologic engineering center (HEC) over the stream network of the Ichamati river basins. Good quality in situ measurements of river hydraulics (cross section, slope, flow) were available only for the upstream and middle stream flood prone region of the basin. The de-siltation of the stretch of Ichamati river under survey i.e. middle part showed impacts which were positive in ways more than one. The river velocity in the top strata is 0.3 m/s from model and the river regained its ecological flow. Also, the increased salinity of the river helped in cultivation of different species of fishes, which are socio economically relevant in the adjoining areas. Due to the increased capacity of the channel in the middle part possibility of flood has also decreased. The total volume of silt accumulated has been estimated 277,589.92 m3. The rate of sedimentation was then calculated and found to be 16,328.82 m3/month silt deposition during monsoon and 11,109 m3 silt deposition during non monsoon period. It may be concluded that out of 6 % annual silt deposition, 5 % silt deposition took place during the month between June and October and 1 % silt deposition during the month between November and May. Silt excavation along 20 km stretch of Ichamati river showed distinct rejuvenation of the river stretch. The program of silt excavation along other stretches of river Ichamati study area would certainly bring positive impact on the river ecology and environment. Ecological flow needs to be maintained in rivers in the upstream due to chocking the silt deposition. Ecological flow in Ichamati river is absent in many stretches of the river due to heavy silt deposition.

Landslide dam and subsequent dam-break flood estimation using HEC-RAS model in Northern Pakistan

On the morning of January 4, 2010, a massive landslide swept the Attabad and Sarat villages into the Hunza River. The debris from the landslide blocked the low-lying river, creating a barrier lake in the area and poses a major threat to the villages located downstream. The aim of the current study was to evaluate the environmental advantages and disadvantages created by the formation of the artificial lake. For this purpose, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were used to create the contours and triangulated irregular network (TIN) of the region. Data from ‘Google Earth’ image on March 19, 2010 were used as the reference and to determine the river bed elevation of the study area. Landsat satellite data of Enhanced Thematic Mapper Plus (ETM+) sensor on January 10, March 15, April 24 and May 2, 2010, were used for constructing the Geographic Information System (GIS) layers of the river banks, land use area, overbank flow area and water area estimation. Our results show that the area covered by the water in the lake has increased from 1.28 km2 on January 10, 2010 to 6.25 km2 on May 2, 2010. The total upstream urban area affected by the river blockage is 13.99 km2. We also applied the Hydrologic Engineering Center River Analysis System (HEC-RAS) model to estimate the potential catastrophes due to dam burst for different peak outflow scenarios with conclusions and recommendations.