Pierce Transit Flood Analysis
This ArcGIS online map displays the percentage of each transit route potentially impacted during flood events using a green to red color gradient.
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Layers may be turned on and off to assess routes using 2012 data (Old) and 2018 data (New).
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The dissolved layers display all transit routes
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Floodplain cross sections display areas that are likely to be impacted by flooding.
osgov.com
Washington is known for long rainy season and Pierce county is no exception, averaging 43" of rain annually. Urban flooding typically occurs in geographically low spots, or when stormwater systems are overwhelmed during heavy rains. During these flood events, transit lines may be vulnerable and if a portion of the route is affected the entire route may be shutdown. This has the potential to disrupt the lives of many Pierce County residents, as annual transit ridership is 9,104,337. This spatial analysis was conducted to determine the degree of impact by route that severe flooding in Pierce County might have on public transportation infrastructure.
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Pierce County is located in western Washington state. This analysis focused on north western Pierce County and the greater Tacoma area.
Pierce County
Transit route and flood hazard shapefiles were downloaded from Pierce County GIS data and clipped to a Pierce County base map.
Data sets were acquired for 2012 & 2018.
Using the intersect tool in ArcMap, portions of transit lines that intersected flood hazard zones were exported as a new data layer.
Identifying flood hazards and transit route intersections
The attributes of the extracted intersection layer were joined with the original data set.
By rejoining this information, the data set now had the total length of each route, and the newly created total length of each route impacted. Simple division could then determine the percentage of each route impacted and therefor the routes most likely to be impacted during flood events.
2012 data pictured
Percentage of route impacted
From the 2012 data analysis it was clear that route 409 was the most susceptible to flooding. This route runs along the Puyallup River, the largest freshwater input into Commencement Bay, and is prone to seasonal flooding.
Routes with the highest potential impact from flooding:
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Route 496 8.2 %
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Route 409 10.8%
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Route 582 31.0%
Most impacted routes 2012
Sources & Image: piercecountywa.org and piercetransit.org
Some routes have been removed since 2012, which is reflected in the 2018 map. Interestingly, 100 % of route 497 lies in a flood hazard area. Route 409 remains susceptible to flooding at 9.5 % impacted. 15.7 % of route 15 and 22 % of route 400 are potentially impacted during flood events. Neither of these routes were included in the 2012 data set. Overall, those routes running along the Puyallup River are the most likely to be impacted flooding. Future analysis will assess the differences in these two data sets to determine if routes have been renamed, decommissioned or rerouted.
Side by side comparison of 2012 and 2018 data
2012 Percent of Pierce Transit Routes susceptible to flooding
2018 Percent of Pierce Transit Routes susceptible to flooding
Click image to download a pdf map