Abstract
Floods are the leading cause of loss of property and loss of life due to natural disasters in the United States. Flood inundation mapping is a critical tool for developing emergency action plans that can help lessen flood related losses. Flood inundation maps are developed using hydraulic models to provide information necessary for predicting the impacts of a given flood event based on inundation extents and maximum water surface elevations at critical locations. With this information, local agencies can develop emergency action plans to make informed decisions based on forecasts and real time stream gage data. Recent advances in hydraulic modeling provide the ability to develop flood inundation maps using one-dimensional (1D) and two-dimensional (2D) models. For this study 1D and 2D hydraulic models were developed using the United States Army Corps of Engineers’ (USACE) Hydraulic Engineering Center River Analysis Software (HEC-RAS) version 5.0.3 to compare and contrast the two types of models and to determine which type of model was better suited for flood inundation mapping. The models were developed for a 4-mile reach of the Napa River, through the City of St. Helena, California. The following data sources were used to develop the 1D and 2D model files: terrain data was created using LiDAR data of the Napa River collected in 2014, land cover values and model boundary conditions were determined from the Napa River Federal Insurance Study published by the Federal Emergency Management Agency (FEMA) and the flow data was developed from the United States Geological Survey’s (USGS) rating curve for gage 11456000 ‘Napa R Nr St Helena CA’. The 1D and 2D models were each calibrated to gage heights within a 5% difference from the measured data to ensure acceptable accuracy of the flood inundation extents. The 1D and 2D models were also calibrated to within a 0.33% difference from each other to provide an accurate comparison. The results of the 1D and 2D models were compared based on the following: calibration, run time, velocity, water surface elevation and inundation extents. Calibration results showed that the 1D model required Manning’s n-values 25-34% higher than the 2D model to achieve the same calibration results. This is because the hydraulic roughness factor in the 1D model is used to account for most hydraulic losses, while the 2D model accounts for some of those losses within the computation. The run time of the 1D model was 96% faster than the 2D model due to the significantly larger number of computation nodes within the 2D model grid. The 1D model calculated lower velocities in the overbank and higher velocities in the channel when compared to the 2D model at maximum velocity. The most compelling reason for this difference is because the velocities are averaged over the entire overbank section of each 1D cross section while the velocities are calculated for each grid cell of the 2D model. The grid cells of the 2D model are significantly smaller than the overbank area of the 1D cross section, allowing for more accurate and detailed calculations in the 2D model. The water surface elevations of the 1D model were lower than those of the 2D model by more than 2 feet in some areas. The differences were mostly attributed to the calculation of expansion and contraction losses within the models. The methods used in the 1D model typically lead to an underestimation in losses, causing an underestimation in the resulting water surface elevations. The final point of comparison was the inundation extents which are determined based on the water surface elevations of the model. The differences varied, but overall the inundation extents of the 1D model was less than the inundation extent the 1D model. Based on all model comparisons it was evident that the 2D model was better suited for flood inundation mapping as it pertained to this study. The results provided by the 2D model were more detailed and therefore more accurate in the overbank areas of the model, which is an indication of floodplain extent potential. The 2D results for water surface elevation and inundation extents were also more conservative than the 1D model. The differences are largely attributed to the greater number of computation points in the 2D model which allow for more detailed results as well as the computation methods that more accurately represent hydraulic losses within the model. The 2D model also accounts for lateral flow as well as longitudinal flow allowing for a more accurate representation of overbank flow paths in the floodplain of the model.