Lahars and Mt. Hood

This week in Applications in GIS we focused on applying our GIS skills to determining how a lahar would affect a population base using different tools within ArcGIS.  For those who don't know what a lahar is, it is a destructive mudflow that is triggered during a volcanic eruption.  When a volcano erupts, the intense heat melts snow packs and glaciers on the volcano slopes and the resulting flows pick up mud and debris that are then carried down slope destroying everything in its path.  Typically lahars will follow existing stream and river beds as they are the paths of least resistance.  Two somewhat recent lahar flows were those resulting from the Mt. St. Helens eruption in 1980 and the 1985 eruption of Nevado del Ruiz in Columbia whose lahar flow killed over 20,000 people in the town of Armero. 

Our lab this week had us replicate a study of the Mt. Hood stratovolcano and how/where the potential lahar flows would affect the surrounding areas.  The first step to this lab was to obtain the geodatabase that we would be working from.  The data provided within this geodatabase would serve as the foundation for all the processes we would complete in this lab.  A main point to using this geodatabase was keeping a naming convention for newly created elements that made sense and ensuring that we didnt keep useless files within the geodatabase.  The picture below illustrates how my geodatabase ended up at the end of the lab. 

The following steps were used within ArcGIS to create the basis for creating the map that you will see below.  First, a study area was created around the Mt. Hood area that encompassed the Multnohmah, Wasco, Clackamas, and Hood River counties.  This study area would serve as a clip feature to remove unnecessary features later on in the map creation.  

The next step was to create a mosaic raster out of the provided rasters files in the geodatabase.  This was done to make analysis easier by analyzing only one raster vice having to complete the analysis on multiple rasters.  Once the raster mosaic was created the following tools from the Spatial Analysis toolset in ArcGIS were used in the following order: the Fill tool, the Flow Direction Tool, and the Flow Accumulation Tool.  Appropriate file names were used for the resulting outputs.  So what did these do?  They basically identified the likely areas where liquid materials will flow to on Mt. Hood.  In essence, this amounted to a stream network flowing from the peak to the base/surround areas of the mountain.  

Next I used the math Int tool to convert our values from the previous steps to integer.  Originally the pixels had floating point values.  We would then determine what 1% of the value of the total number of pixels were in our stream network.  That would then be used in the Con tool to create what was more likely to be the true stream network.  This output was then converted to a geodatabase feature using the Stream to Feature tool.  

The remaining steps involved conducting the actual hazard analysis by creating a 1/2 mile buffer around our stream feature and determining which population blocks and schools would be affected by the 1/2 mile lahar buffer.  The results were then mapped and the output map is below.  Please let me know if this map works for you.  As a colorblind mapper, I always welcome comments and suggestions to make things better.  Cheers!