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mesh_generation.md

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Mesh generation

Overview

Our method is mesh independent, so theoratically you can use any meshes. However, we require the mesh data must meet the following requirements:

Requirements Note
Format GeoJSON
Spatial reference Geographic coordinate system (GCS) WGS84
Geometry Polygon
Field ID

Structured meshes

PyFlowline provides built-in algorithms for several structured meshes:

Mesh Description Note
Hexagon A projection based hexagon mesh
Latlon A longitude/latitude mesh
Square A projection based rectangle mesh

Because PyFlowline requires mesh data must be GCS system, the generated meshes are automatically converted to the GCS spatial reference.

Unstructured meshes

The MPAS mesh is the only unstructured mesh that is tested in this study.

JIGSAW MPAS workflow

Overview

The MPAS mesh generation process is supported by the JIGSAW tool.

In order to burn the river networks into the mesh so mesh cells aligh with the actual river flowlines, a vector-based flowline GIS dataset is required. This dataset was produced from the NHD dataset, and it was simplified using the PyFlowline to remove small rivers and braided rivers. The coastal line is also used in the mesh refinement.

The final MPAS mesh resolution varies between 3 and 10 km with high resolution near the coastal line and river networks.

USGS National Hydrography Dataset river flowline processing

The raw NHD flowline dataset contains more than 10, 000 flowlines, and cannot be directly used by JIGSAW for mesh generation, which requires the distance between flowline to be less than the desired mesh resolution.

First, we use the stream order to remove small rivers. Second, even with the reduced flowlines, there are still many small rivers and braided rivers.

Currently, the MOSART model and our numerical model design do not support multiple flow directions caused by braided rivers. Therefore, a list of algorithms from the Pyflowline are applied to remove both small flowlines and braided rivers.

The filtered flowlines are then used by the JIGSAW to generate the mesh.

JIGSAW MPAS mesh generation

JIGSAW is used to generate the MPAS mesh. For details on how to use this tool, see JIGSAW.