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

Amrei David,Britta Schmalz

doi:10.3390/w13172346

Abstract

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.

Highlights

Motivation and Research GapThe use of 2D hydrodynamic models to determine storm hazards in rural and urban catchments based on the direct rainfall method has recently become state-of-the-art in storm hazard flood modeling
For the hydrograph at the catchment outlet and for the flooded area within the catchment there was no difference between the two software versions
The main study was carried out using the current version of Hydrologic Engineering Center (HEC)-RAS 6.0

Summary

Introduction

Motivation and Research GapThe use of 2D hydrodynamic models to determine storm hazards in rural and urban catchments based on the direct rainfall method (in the following, DRM or ‘rain-on -grid’, compare with Ball et al 2012 [1]) has recently become state-of-the-art in storm hazard flood modeling. With the increased availability of high-resolution data of the terrain (‘digital elevation model’, DEM) and the surface (‘digital surface model’) these models increased their level of detail, with their extension from 1D to 2D [11] With this extension, floodplains and backwater effects are modeled in high resolution. Due to the simultaneous increase of flood events—apart from large rivers (urban, pluvial or flash floods [12])—these 2D models were expanded to include the source terms of precipitation. This enables the models to be used in the context of storm-hazard analysis within an entire catchment area [5]

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Conclusion