Population exposure among the five volcanoes
The lahar-hazard zones for the five active Washington volcanoes contain 274 km2 of developed land, 191,555 residents, 108,719 employees, 433 public venues, and 354 dependent-care facilities. For each variable, Mount Rainier contains the highest percentage of regional (entire study area) totals in the lahar-hazard zones, ranging from 36% of the public venues to 54% of the employees (Figure 2). Mount Baker and Glacier Peak represent 21% to 30% of regional totals, respectively. Mount Adams and Mount St. Helens represent the lowest percentages at approximately 0% to 5% of regional totals, except for developed land at Mount St. Helens which is 12% of regional totals.
Population exposure at each of the five volcanoes
Mount Baker
The lahar-hazard zone of Mount Baker intersects eight incorporated cities and two counties (Figure 1) and contains 49,212 residents, 24,341 employees at 2,412 businesses, 132 public venues, and 93 dependent-care facilities. The majority of these variables are in the unincorporated areas of Skagit and Whatcom counties, followed by the incorporated cities of Burlington, Mount Vernon, and Sedro-Woolley (Figure 3). While these counties and cities have the highest numbers of populations and businesses in hazard zones, other communities have higher percentages of their people and businesses in the zones. For example, Mount Vernon has 5,426 residents in the lahar-hazard zone, but this represents only 17% of the community; whereas La Conner only has 891 residents in the zone, but they represent 100% of the community. Other small communities with high percentages of populations in the lahar-hazard zone include Nooksack, Sumas, and Hamilton. The one exception to this dichotomy of low numbers versus high percentages is the city of Burlington, which has both high numbers and high percentages. Of the 93 dependent-care facilities that are in the lahar-hazard zone, the majority are schools, adult residential care, child day-care centers, and outpatient-care facilities. The 193 public venues in the lahar-hazard zone include 51 religious venues, 30 overnight accommodations, and 26 parks. Certain public venues in the lahar-hazard zone are high-occupancy tourist sites, such as the Mount Baker Ski Area, the North Cascades National Park, and Mount Baker-Snoqualmie National Forest recreation areas.
Glacier Peak
The lahar-hazard zone of Glacier Peak intersects five incorporated cities and two counties (Figure 1) and contains 40,819 residents, 23,576 employees at 2,379 businesses, 121 public venues, and 87 dependent-care facilities. The section of the Glacier Peak lahar-hazard zone that follows the Skagit River significantly overlaps the lahar-hazard zone from Mount Baker, resulting in several communities (Burlington, Concrete, Hamilton, La Conner, Lyman, Mount Vernon, and Sedro-Woolley) exposed to lahars originating from both volcanoes. For Glacier Peak lahar-hazard zones, the highest amounts of the developed land, population, and business variables are in the unincorporated areas of Skagit and Snohomish counties and the incorporated cities of Burlington and Mount Vernon (Figure 3). Similar to Mount Baker, some communities contain high numbers of people and businesses in lahar-hazard zones (e.g., Mount Vernon) that represent low percentages of each variable while other communities contain low numbers but high percentages of their populations in hazard zones (e.g., La Conner, Darrington, and Lyman). As was the case with Mount Baker, Burlington is an exception with high numbers and high percentages in the Glacier Peak lahar-hazard zone. The majority of the 87 dependent-care facilities and 121 public venues in the lahar-hazard zone are in Burlington and Mount Vernon.
Mount Rainier
Among the five Washington volcanoes, the lahar-hazard zone of Mount Rainier (Figure 1) contains the highest number of incorporated cities (18), counties (4), residents (91,435), employees (58,969 at 3,821 businesses), public venues (158), and dependent-care facilities (153). Although there are 18 cities and 4 counties with land in the Mount Rainier lahar-hazard zone, the majority of the residents (86%), employees (75%), dependent-care facilities (90%) and public venues (87%) in the lahar-hazard zone are in cities of Puyallup, Auburn, Sumner, Fife, Orting, and Pacific, and the unincorporated portions of Pierce County (Figure 3). Tacoma is an exception to this list when considering exposed employees given the high number (12,224) and percentage of all employees in the zone (21%), which represent the concentration of employees near the Port of Tacoma. As was the case with Mount Baker and Glacier Peak, there are many smaller communities (e.g., Algona, Carbonado, South Prairie, and Wilkeson) with lower numbers of exposed populations that represent the majority, if not all in many cases, of the community. There are 151 dependent-care facilities (primarily K-12 schools) in the lahar-hazard zone, with most located in Puyallup. Schools (K-12 grade) are the most abundant and widely distributed type of dependent-care facility identified in the lahar-hazard zone. All of the K-12 schools in six communities (Pacific, Carbonado, Fife, Orting, Sumner, and Wilkeson) are in lahar-hazard zones, representing not only a life-safety issue but a long-term community recovery issue. There are 158 public venues in the lahar-hazard zone (primarily religious organizations, overnight accommodations, and parks), including high-occupancy sites such as casinos, the Puyallup Fairgrounds in Puyallup (over 1 million visitors each September; EventCorp Services 2011), and Mount Rainier National Park (1.7 million visitors in 2010; National Park Service 2011).
Mount St. Helens
The lahar-hazard zone of Mount St. Helens intersects four incorporated cities and three counties (Figure 1) and contains 7,645 residents, 1,656 employees at 151 businesses, 12 public venues, and 18 dependent-care facilities. The majority of each of these variables is in unincorporated Cowlitz County (Figure 3). Unlike the other volcanoes, the exposed land, populations, and businesses comprise a small percentage of community and county totals. The one exception is the city of Castle Rock, where in-hazard-zone percentages are high and range from 33% (public venues) to 80% (dependent-care facilities). Although community totals are relatively low for number of dependent-care facilities in the lahar-hazard zone, often these facilities represent each community’s entire facility count, such as schools in Castle Rock and correctional facilities in Kelso. The public venues in the lahar-hazard zone include three parks, four religious venues, four overnight accommodations, and one library. Also in the lahar-hazard zone are access routes and recreational areas of the Mount St. Helens National Volcanic Monument, which draws more than 600,000 visitors to the Johnston Ridge Observatory each year (www.fs.usda.gov; last visited 18 May 2013) and thousands of more visitors on the south and east sides of the monument.
Mount Adams
Of the five Washington volcanoes, the lahar-hazard zone of Mount Adams (Figure 1) contains the lowest number of residents (2,444), employees (177 at 44 businesses), public venues (10), and dependent-care facilities (3). The lahar-hazard zone crosses four counties (Klickitat, Lewis, Skamania, and Yakima) but no incorporated cities. The majority of the people and businesses are in unincorporated Klickitat County (Figure 3), but percentages are low in each of the four counties. The lahar-hazard zone contains three schools, six overnight accommodations, three religious venues, and one park.
Composite indices of community exposure
Composite indices describing the amount and the percentage of assets in the lahar-hazard zone for each community and unincorporated area were developed using sums of normalized data in 5 categories—developed land, residents, employees, public venues, and dependent-care facilities. The City of Puyallup (Mount Rainier) has the highest composite amount index (4.4 out of 5), indicating that this community has the highest number of people and businesses in the lahar-hazard zone (Figure 4a). The only geographic area with a higher index in a category was unincorporated Skagit County for the amount of developed land it has in the Mount Baker lahar-hazard zone. Other communities and counties with high composite amounts include Skagit County (Glacier Peak); Pierce County, Sumner and Fife downstream of Mount Rainier; and Mount Vernon and Burlington for both Glacier Peak and Mount Baker lahar-hazard zones. Tacoma ranks eleventh behind these other communities and counties, due primarily to the highest number of employees in a lahar-hazard zone.
Several communities have composite percentages of 4.8 to 5.0, indicating that they have the highest percentages of their developed land, populations, and businesses in the lahar-hazard zone for each of the five categories (Figure 4b). In all of these towns, the in-hazard populations and businesses represent close to or exactly 100% of the community. These types of communities include Darrington, La Conner, and Burlington downstream of Glacier Peak; Nooksack, La Conner, Sumas, and Burlington downstream of Mount Baker; and Fife, Orting, Carbonado, Sumner, Algona, and Pacific downstream of Mount Rainier.
A total of 15 communities have high (1.0 or greater) final composite scores, of which seven are downstream of Mount Rainier, four are downstream of Glacier Peak, and four are downstream of Mount Baker (Figure 4c). Certain communities are in this top tier of exposure due to the magnitude of exposed populations and businesses (e.g., Puyallup), whereas others have high percentages (e.g., Orting and Pacific). The two communities with the highest composite scores (Fife and Sumner) have moderately high amounts (and less than Puyallup) but some of the highest percentages of the various categories.
Clusters of communities with similar exposure to lahar hazards emerge on a plot of amount versus percentage indices for the five variables (Figure 5). The one exception is the city of Puyallup (41) downstream of Mount Rainier, which stands alone. Cluster A includes the city of Mount Vernon and unincorporated Skagit County (both for Glacier Peak and Mount Baker) and Pierce County (Mount Rainier) and represents areas with relatively high amounts but low percentages of the five variables. Cluster B communities (Sumner and Fife at Mount Rainier and Burlington at Glacier Peak and Mount Baker) have similar amounts as those in cluster A, but higher percentage index values (close to or equal to 1.0). Cluster C communities have similarly high percentage index values, but very low amount index values. Cluster D communities are similar to cluster C, but with lower percentage index values. Cluster E represents the bulk of the communities in the study area, with relatively low amount and percentage index values. Figure 5 also shows how the lower right of the graph (i.e., areas with amount index values greater than their percentage index values) is dominated by the unincorporated portions of the 11 counties.
Changes in community exposure to lahar hazards between 1990 and 2010
Residential populations within lahar-prone areas of Washington State have increased by 48,080 residents over the 20-year time period between 1990 and 2010. More than half of this total increase is from population expansion within the Mount Rainier lahar-hazard zone (+24,619 residents), followed by smaller increases at Mount Baker (+12,003), Glacier Peak (+9,128), Mount St. Helens (+2,067), and Mount Adams (+263). At the community level (Figure 6a), the greatest increases in population during this time period were in the Mount Rainier communities of Orting (+4,595 residents, representing a 214% increase) and Fife (+4,436 residents, representing a 94% increase), as well as Burlington for both Mount Baker and Glacier Peak (+3,900 residents, representing a 90–92% increase). The number of residents in lahar-hazard zones increased in all communities and counties except for small decreases in Enumclaw, Federal Way, Edgewood, Castle Rock, Kelso, Concrete, and the unincorporated areas of Lewis, Yakima, Snohomish, and King Counties.
Although residential numbers increased through time, the percentage of residents in the lahar-hazard zone across the five Washington volcanoes (Figure 6b) remained the same between 1990 and 2010 (4.4% and 4.3%, respectively). Although Orting more than tripled and Fife, Algona, and Burlington nearly doubled their in-hazard-zone population, the communities saw little to no change in the overall percentage of their communities in the lahar-hazard zone. This is because each town was completely within lahar-hazard zones in 1990 and any new growth still resulted in an overall percentage of 100%. Decreases in the community percentage of exposed populations in most of the remaining communities and counties in the study area were not because the in-hazard-zone population decreased but because of much larger increases in residents elsewhere in the community. For the few towns that did have an increase in the community percentage of exposed populations, the increase was relatively small on the order of 1 to 2%.