Lahars started off the
course’s study of natural hazards. Unfamiliarity with the term lead to some
research prior to starting the module requirements. The Javanese word “lahar” used
for a volcanic mudflow originated in the 1920s. Its use increased dramatically
in the 1980s and 1990s, probably in reaction to the frequent volcanic activity
during that time period. In some areas this fast-running mixture of mud caused
by rapidly melted snow and ice poses the greatest risk of any volcanic
activity. Speeds of 45-50 miles per hour and flow depths of 100 feet where the
lahars were confined in valleys have been reported at Mount Rainier. Because of
their concrete-like properties, lahars are capable of demolishing most structures. They
can also occur during periods when no eruptions are occurring in which case
there may be no advance warning of the impending event. For these reasons, the
importance of risk assessment and hazard planning is critical. This exercise
was created to demonstrate that process.
Acting as a private
consultant hired to identify potential inundation zones in the vicinity of
Mount Hood, Oregon, the process involved mosaicking digital elevation models
and then using the Hydrology Tools to determine drainage flow. Coupling that
output with 2010 Census data and Oregon school data, a population analysis and
schools-at-risk identification was performed. State and local officials could
use this information for hazard planning and response time to help keep area
residents and visitors safe from lahars.
After establishing a
geodatabase to keep the project organized and efficient, XY tool was used to
show the location of Mount Hood. Drainage areas were created from the mosaicked
DEMs using some of the Hydrology Tools (Fill to eliminate sinks, Flow Direction
to assign direction of flow to individual cells, and Flow Accumulation to
calculate how many other cells flow into each cell) to create a stream network.
The need for an attribute table for further work called for changing the fill
output from a floating point raster to an integer raster. Using the Con tool (conditional
statement) reduced the stream network to only those areas with sufficient flow
accumulation to qualify as streams. That output was then converted to polyline
vector features – areas most likely to become inundated during a lahar. This whole
process - creating a stream network from DEMs - was rather fascinating to
complete.
Applying buffers to the
streams provided areas of potential inundation by lahars which were then used
in the selection process with Census population block groups and schools data.
This resulted in the areas on which teams should focus for large lahar event
hazard planning. From this information the following map was created:
|
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| Mount Hood Lahar Hazard Assessment |
Populations at risk for being impacted were calculated for the possible inundation zones around drainage
basins within the study area only rather than for the entire dataset. The potentially
impacted population number outside the study area is much greater than the number
calculated and displayed on the map. The values for this map are calculated for
the study area only which has a lower population density than the areas to the
west, so the block group population ranges may differ from those presented by
others. Overall, lahars from Mount Hood have the potential to impact 42,689
people within the study area based on 2010 Bureau of Census figures. Including the
area outside the study area 212,791 people could be impacted.
Although it does result in a
more cluttered-looking map, especially in areas with small census block groups,
the census block group outlines were retained. This was decided primarily to
minimize misinterpretation of the map resulting in an underestimate of the
actual population with the potential to be impacted by a large lahar.
This particular lab demonstrated
the variability in information that could be produced for a client. For
example, the lab requirements were focused on lahars, but in reality a
hazard-planning team would also want to know what populations would be at risk
for other volcanic hazards. In this particular case, one requirement was to
provide the number of schools at risk from lahar inundation. However, just
outside one of the mile-wide lahar inundation zones was an additional school
located closer to Mount Hood. Naturally, one would expect the hazard-planning
team to be made aware of this school, but including that information was outside the scope of the lab
requirements. This also illustrates the importance of communicating similar situations to a client for further clarification or revision of the data to
be provided.
The ability to go back and
review previous geoprocessing steps came in quite handy during this lab!
