Section 2: Community Profile Washington County Natural Hazard Mitigation Action Plan Update 2010 The following section describes the County from a number of perspectives to help define and Environment and Geography understand the County’s sensitivity and resilience to Washington County is located in northwestern natural hazards. Sensitivity factors can be defined as Oregon, just west of the City of Portland. The those community assets and characteristics that may County is primarily rural on the west and heavily be impacted by natural hazards, (e.g., special developed with urban infrastructure on the east. populations, economic factors, and historic and Washington County is approximately 727 square cultural resources). Community resilience factors miles, and is part of the tri-county metro area can be defined as the community’s ability to manage comprised of Multnomah, Clackamas, and risk and adapt to hazard event impacts (e.g., Washington Counties. It is bounded to the north by governmental structure, agency missions and Columbia County, the to east by Multnomah and directives, and plans, policies, and programs). The Clackamas Counties, to the south by Yamhill information in this section represents a snapshot in County and to the west by Tillamook County. The time of the current sensitivity and resilience factors western part of Washington County, where it meets in the County when the updated Natural Hazard the mountains of the Northern Oregon Coast Range, Mitigation Action Plan (NHMAP) was developed. is forty miles from the Pacific Ocean. Dominant The information documented below, along with the features of the County landscape are the Coast hazard assessment, and background information Range Mountains on the west, the Tualatin provided in Appendix B, should be used as the local Mountains on the north, the West Hills of Portland level rationale for the risk reduction actions on the east, and the Chehalem Mountains on the identified in Section 4: Action Items and south. Also prominent are Cooper and Bull Implementation. Identification of actions that reduce Mountains in the southeast and Bald Peak in the the County’s vulnerability and increase its resilience southwest. The agriculturally rich Tualatin Valley assist in reducing overall risk of disaster. This lies between the mountain ranges and hills. relationship is the area of overlap in Exhibit 2.1 below. Exhibit 2.2: Contextual Map of Washington County Exhibit 2.1: Understanding Risk Source: USGS - University of Oregon Community Service Center, 2006 Source: Washington County Office of Emergency Management, 2004 Most of Washington County is in the Tualatin Valley between the northern Oregon Coast Range to the west and the Cascade Range to the east. The valley ranges from 120 to 300 feet above sea level. The topography is rolling and lacks dramatic changes in elevation except in the foothills and mountains surrounding the central valley. The Washington County Natural Hazard Mitigation Action Plan 2-1 Section 2: Community Profile foothills and the uplands range in elevation from from the Scoggins Creek watershed. The dam forms 1,300 to 3,500 feet. The highest point in the county is a reservoir, Henry Hagg Lake, which provides active Saddle Mountain in the Northern Oregon Coast water storage capacity of about 56,000 acre-feet. The Range near the border of Tillamook and Washington dam and reservoir are owned by the Bureau of Counties with an elevation of 3,464 feet. Reclamation and operated by the Tualatin Valley Irrigation District. The project provides flood Washington County has a long growing season control, irrigation water, municipal water supply, and mild temperatures, which lead to a wide range water quality benefits, and recreation. of agricultural activities. Seasonal flooding, high ground water, and soil erosion cause most of the Climate non-urban drainage problems in the county. When maintained in their natural state, Washington Washington County’s climate is moderate year- County’s wetlands control runoff and decrease soil round. The western edge of the county is only forty erosion and water pollution while reducing miles from the Pacific Ocean, which provides a potential damage from flooding, and helping to modified marine climate. Extreme summer and recharge groundwater supplies. winter temperatures are moderated by the airflow moving across the county from the Pacific Ocean. Tualatin River The Cascade Mountains to the east of the county act as a barrier that prevents the colder continental air The Tualatin River, one of the many natural masses originating in the artic areas of Canada from features of the County, starts in the northern Oregon reaching Washington County. Occasionally, extreme Coast Range and flows to the Willamette River near temperatures can occur when the airflow comes in the city of West Linn. The Tualatin River is a major from the east flowing west through the Columbia source of drinking water for the cities of Cornelius, Gorge and across the Cascade Mountains. If the east Hillsboro, and Beaverton. It is also a source of winds occur when rain is falling, the result can be irrigation water and provides aquaculture and freezing rain and snow in Washington County. recreation opportunities for many Washington County communities. The Tualatin River also serves Much of Washington County is protected from as a channel for the discharge of treated sewage. severe wind and weather conditions by the surrounding mountain ranges. Snowfall is relatively The Tualatin River has numerous tributaries. rare with only about five days of measurable Wapato Creek drains from the Chehalem snowfall each year. The few times that snow falls Mountains; Scoggins and Gales Creeks drain part of each year, it generally melts off within one to three the Coast Range; McFee and Chicken Creeks drain days. Ice can occur more frequently in higher the northeast slopes of the Chehalem Mountains elevations in the county. and Fanno Creek drains the valley floor and Portland's west hills. Beaverton creek, a tributary to The rainy season in Western Oregon and Rock Creek, drains a large portion of the valley and Washington County happens between October and the west hills of the Portland. Dairy Creek with April, when approximately 81% of the yearly McKay Creek as a tributary drains portions of the rainfall occurs. Most of Washington County has an Coast Range and Tualatin Mountains. average annual precipitation of between 30 and 70 inches, with parts of the Coast Range in the west The river and its tributaries flow through low receiving over 70 inches. Strong storm systems can foothills, terraces, and floodplains that naturally develop at higher altitudes in the upper level flow drain the area under normal circumstances. It is over the Pacific during the rainy season and bring generally a slow moving river draining and rain to the lower elevations and snow to the higher preventing flooding through the collection and flow elevations. of water from rain and snow melt in the Coast Range Mountains. Scoggins Dam, located near the town of Gaston in southwestern Washington County, stores runoff Washington County Natural Hazard Mitigation Action Plan 2-2 Section 2: Community Profile Minerals and Soils Environmental Assets The characteristics of the minerals and soils Washington County residents were surveyed as present in Washington County indicate the potential part of the NHMAP update process and asked to types of hazards that may occur. Rock hardness and identify some of the environmental resources soil characteristics can determine whether or not an throughout the County. Respondents noted the area will be prone to geologic hazards such as Tualatin River Wildlife Refuge as a significant landslides. The four mineral and soil types in environmental resource. Respondents also noted Washington County are valley fill and semi- many of the water features in and near the County consolidated sedimentary rocks, basaltic lavas, as key resources. Sites highlighted include: Jackson marine sedimentary rocks, and Eocene age volcanic Bottom Wetlands; Barney Reservoir; Tualatin River; and sedimentary rocks. Gales Creek; West Fork Dairy Creek; Wapato Lake and wetlands; Mullerliele pond; Bannister Creek; The surface material includes unconsolidated, and the Bethany Lake and Park. fine-grained deposits of Willamette silt, sand, and gravel, and recent floodplain deposits. Torrential flood events can lay down large deposits of sand and gravel. Sandy silt and silt containing clay are moderately dense and firm, and are primarily considered to be prone to liquefaction, an earthquake related hazard. Basaltic lava consists mainly of weathered and non-weathered, dense, fine-grained basalt. Though the characteristic of this lava may offer solid foundation support, landslides are common in many of these areas where weathered residual soil overlies the basalt. Understanding the geologic characteristics of Washington County is an important step in hazard mitigation and avoiding at-risk development. Other Significant Geologic Features Washington County, like most of the Pacific Northwest, lies over the area of Cascadia Subduction Zone, where the Juan de Fuca and North American Plates meet under the earth’s crust. The presence of crustal faults within Washington County also indicates potential seismic activity within the county. There are active volcanoes in the vicinity of the county, including Mt. St. Helen in the south of Washington State, and Mt. Hood, southeast of Portland. Identifying risks posed by natural hazards, and developing strategies to reduce the impact of a hazard event can assist in protecting the life and property of citizens and communities. Local residents and businesses can work together with the county to create a natural hazards mitigation plan that addresses the potential impacts of hazard events. Washington County Natural Hazard Mitigation Action Plan 2-3 Section 2: Community Profile the Cities for effective County-wide mitigation Population efforts. Washington County is part of the tri-county Beaverton and Hillsboro remain the largest cities metro area comprised of Multnomah, Clackamas, in the County, but it was the smaller communities, and Washington Counties.