r-ExposR

Models Topographic Exposure to Hurricane Winds.

EXPOS Model

The EXPOS model uses a digital elevation model (DEM) to estimate exposed and protected areas for a given hurricane wind direction and inflection angle. The resulting topograhic exposure maps can be combined with output from the HURRECON model to estimate hurricane wind damage across a region.

EXPOS contains two main functions:

1. The expos_model function estimates topographic exposure for a specified wind direction and inflection angle. The input file is assumed to be a raster of elevation values in GeoTiff format with missing values represented by zero. Cells may be rectangular. The user can specify if coordinates are lat/long; otherwise lat/long is assumed if X values are between -180 and 180 and Y values are between -90 and 90. If coordinates are lat/long, horizontal and vertical units are assumed to be degrees and meters, respectively; otherwise horizontal and vertical units must be the same. Columns are assumed to be closely aligned with true North (0 degrees); if not, the map orientation (azimuth) must be specified in degrees. The name of the input file is assumed to be "dem.tif".

The output file is a raster file in GeoTiff format with the following values: 0 = missing data, 1 = protected, 2 = exposed. Output files are named "expos-xxx-yy.tif" where xxx is the wind direction and yy is the inflection angle.

In previous studies, spatial resolutions of 30 or 60 meters and an inflection angle of 6 degrees were found to work well (see below). Note that increasing the inflection angle tends to decrease the size and number of protected areas while increasing the odds that these sites are protected.

2. The expos_damage function estimates regional hurricane damage where topographic exposure at each location is determined by peak wind direction. If a location is protected, the enhanced Fujita scale (EF) rating from HURRECON is reduced by a specified number of EF ratings. This function requires a hurricane GeoTiff file created by HURRECON, exposure files created by EXPOS for the eight cardinal wind directions (N, NE, E, etc), and a reprojection file in CSV format (reproject.csv) that contains lat/long coordinates for the lower left and upper right corners of the digital elevation model (variables: name, lat_0, lon_0, lat_1, lon_1).

The output file is a raster file in GeoTiff format with the following values: 0 = missing, 1 = no damage, 2 = EF0 damage, 3 = EF1 damage, 4 = EF2 damage, 5 = EF3 damage, 6 = EF4 damage, 7 = EF5 damage. Output files are named "hhhh-damage-yy-z.tif" where hhhh is the hurricane ID, yy is the inflection angle, and z is the reduction in EF rating for protected areas.

Note: for large areas, best results will be obtained by converting the DEM to latitude/longitude coordinates (if necessary) before running expos_model and expos_damage and then converting the resulting damage maps to the desired map projection.

EXPOS is available in both R (ExposR) and Python (ExposPython) versions. The model is an updated version of the original EXPOS model written in Borland Pascal for use with Idrisi (see below for details).

Note: both versions are under development and subject to change.

Getting Started

Here are the basic steps for using the model. Please see below for more details.

1. Download the R or Python version of the model from GitHub. The R version is also available on CRAN as ExposR.
2. Create a directory for a particular study area with subdirectories as described below.
3. Copy the digital elevation file to the dem subdirectory and rename it "dem.tif".
4. Download or create geographic and political boundary shapefiles for the desired study area. Copy these files to the vector subdirectory and rename so the first name of each file is "boundaries".
5. Use the expos_model function to create exposure maps for different wind directions and inflection angles.
6. Use the expos_damage function to create maps of enhanced Fujita scale wind damage for particular hurricanes.

Details

All user functions begin with "expos". The wind direction and inflection angle must be specified in degrees.

The user specifies a directory (exp_path) for a given study area. Input and output files are stored on the following subdirectories of this directory:

exp_path/dem
exp_path/exposure
exp_path/damage
exp_path/vector

The dem subdirectory contains the input elevation file. The exposure subdirectory contains output files from the expos_model function. The damage subdirectory contains input files from the HURRECON model, the reprojection file, and output files from the expos_damage function. Shapefiles that contain geographic and political boundaries for viewing results are stored on the vector subdirectory.

To run the model, run expos.R (R) or expos.py (Python).

The R version can be installed as an R package directly from CRAN. It can also be installed from Github using the devtools package:

devtools::install_github("expos-model/ExposR")

This has the advantage of providing readily accessible help messages for each function.

Model Functions

expos_set_path
expos_get_path
expos_model
expos_damage
expos_summarize
expos_plot

The expos_set_path function sets the path for the current set of model runs. The expos_get_path function returns the current path. Use expos_set_path before using other functions.

The expos_model function creates a raster file of topographic wind exposure as a function of wind direction and inflection angle.

The expos_damage function uses output from EXPOS and HURRECON to create a raster file of wind damage where topograhic exposure at each location is determined by peak wind direction. If a location is protected, the enhanced Fujita scale rating is reduced by a specfied amount.

The expos_summarize function displays summary information for a specified raster file, including the number of rows and columns, spatial extent, cell height and width, and minimum and maximum value.

The expos_plot function creates a plot of a specified raster file.

Examples

expos_set_path("c:/expos/wach_30m")
expos_get_path()

expos_model(wind_direction=90, inflection_angle=6)
expos_damage(hurricane="AL1938-06", inflection_angle=6, protect=2)

expos_summarize("dem")
expos_summarize("expos-090-06")
expos_summarize("AL1938-06-damage-06-2")

expos_plot("dem")
expos_plot("expos-090-06")
expos_plot("AL1938-06-damage-06-2")

History

The original EXPOS model was written in Borland Pascal and depended on Idrisi for spatial visualization. The model was used in published studies of the ecological impacts of historical hurricanes in New England and Puerto Rico:

• Boose, E. R., Foster, D. R., Fluet, M. 1994. Hurricane impacts to tropical and temperate forest landscapes. Ecological Monographs 64: 369-400. doi:10.2307/2937142.

• Boose, E. R., Chamberlin, K. E., Foster, D. R. 2001. Landscape and regional impacts of hurricanes in New England. Ecological Monographs 71: 27-48. doi:10.1890/0012-9615(2001)071[0027:LARIOH]2.0.CO;2.

• Boose, E. R., Serrano, M. I., Foster, D. R. 2004. Landscape and regional impacts of hurricanes in Puerto Rico. Ecological Monographs 74: 335-352. doi:10.1890/02-4057.