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Final Addendum – October 18, 2005
CHAPTER 2: CRITICAL AREAS ELEMENT
(Critical Areas Element Final Addendum incorporates changes recommended by the Planning Commission in October 28, 2004 Addendum and City Council in September 14, 2005 Final Addendum) Page 5 Line 48:
Aquifer Recharge Areas
As additional Based on information becomes available from ground water studies such as adopted ground water management plans and wellhead protection plans, the County City will formally designate areas with high ground water recharge capabilities that are critical to protect. Page 6 Line 6:
River and Stream Corridors
The City of North Bend is located upstream from Snoqualmie Falls, which functions as a barrier to the upstream migration of fish. However, the Snoqualmie River above Snoqualmie Falls with its three main branches and its many tributaries provide valuable habitat to resident fish species. Figure 2-2 depicts rivers and streams within the North Bend Planning Area. Page 8 Line 16: The goal and related policies of this plan provide guidance in protecting the public from flood hazard and at the same time protect the environment by discouraging development within flood prone areas, including channel migration or avulsion areas. Figure 2-3 in the Comprehensive Plan shows the North Bend UGA River Channel Migration Hazards. The City has repealed development regulations for channel migration areas on the basis that mandatory FEMA flood way regulations are adequate to protect the public health, safety, and welfare in the Silver Creek “avulsion” hazard area and Tollgate open space land.
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GEOLOGICALLY HAZARDOUS AREAS
Geologically hazardous areas are lands which are susceptible to hazards associated with underlying soils and geology. These include areas characterized by steep slopes, landslides, seismic hazards, and erosion. A number of geologic hazards exist within the vicinity of North Bend. For example, landslide or debris flow and erosion hazards are common in hillside areas with steep and unstable slopes. In addition, these lands are at greater risk in the event of an earthquake.
Erosion Hazards
Erosion is a natural process of the wearing away of land surfaces by water, wind and ice. While erosion and sedimentation are natural processes at work in the landscape, they are frequently accelerated by land use modifications and urban development. The susceptibility of soil to surface erosion depends on its physical and chemical characteristics, slope, vegetative cover, the intensity of rainfall, and runoff velocity. Eroded material is moved by surface flows and deposited elsewhere as sediment. The negative effects of increased sedimentation are most pronounced
Chapter 2 – Critical Areas Element Final Addendum October 18, 2005
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where erosion of soils is connected to the surface drainage network. Through sedimentation, soil erosion can result in degradation of surface water quality and/or aquatic habitats. Figure 2-7, Landslide Hazards Erodible Soils and Debris Flow , depicts areas of potential soil erosion or debris flow and steep slope hazard within North Bend and its surrounding area. The map shows areas where soils are particularly susceptible to increased erosion as a result of development. It is important to note that while the map does not show any areas within the city which are characterized by erosion hazards, these conditions do exist here on a site specific or local scale. Soils mapped include those which may experience severe to very severe erosion (soil particle movement) according to the USDA Soil Conservation Service. This definition is consistent with erosion hazard areas as designated in the 2004 King County Sensitive Areas Ordinance and meets the minimum guidelines for erosion hazard areas outlined in the Growth Management Act.
Debris Flow Landslide and Steep Slopes Hazard Areas
The identification of areas susceptible to debris flow landslides debris flow is necessary for informed land use planning and to support land development regulations which reduce the risk of property damage, personal injury, and environmental degradation. Debris flow landslide hazard areas lie principally outside the existing city limits but are evident in the City’s Potential Impact Areas. Debris flow landslide flow paths however can directly impact the incorporated city. Landslide hazard areas are defined by alternate or coexisting landscape conditions, which are based on well established geo-technical determinations of slope stability and considerable experience and research in the Puget Sound area. The specific debris flow landslide hazard area mapped on Figure 2-7 was identified in the Geotechnical Engineering Report prepared for Phase II of the Forster Woods development in 1992 by Associated Earth Sciences. The stability of slopes in landslide hazard areas is highly dependent on the water content of the underlying soils. Water readily percolates through sand and gravel, but ponds above less permeable silt, clay and till layers, thus saturating the overlying deposits. Where a less permeable layer (silt or clay) intersects a slope, water often seeps from the layers above. This combination of sedimentary deposits, topography, and local groundwater flow results in a high potential for landslides. An event that increases groundwater levels and flow, such as a rain storm or discharge of surface water above a slope, can saturate sediments near the surface and cause failure of a slope that is stable under dryer conditions. Likewise, erosion along a stream channel can steepen a slope or expose deposits which may become water saturated, increasing the potential for landslides on a previously stable slope. Most debris flow landslide hazard areas in the North Bend Potential Impact Areas involve a few feet of relatively loose erodible soils on slopes underlain by denser and typically less permeable till or bedrock. All a Areas with erodible surface soils underlain with relatively impermeable soils on slopes of 15 percent or greater and with drainage from topographically higher areas, and all areas with steep slopes greater than 40 percent (except consolidated rock), were identified as potential debris flow or landslide hazard areas and are depicted in Figure 2-7, Landslide Hazard Areas. These unstable slopes represent a major hazard to people and structures and have limited constrained development potential. The plan’s goal and policies provide guidance in decreasing the hazards of developing within landslide hazard and steep slope areas.
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SA - Goal 2: Policies: Protect public safety by discouraging development within the river floodway and its natural systems and by preserving the flood storage function of floodplains.
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SA - 2.3
Limit construction of permanent structures in the floodway (FEMA) or in the flood water flow channels and require that improvements in floodway or floodwater flow channels not change flood elevation or obstruct or divert the natural flow.
SA - 2.4 Locate roads at grade level and build structures at least two or three feet above the 100 year flood elevation depending on proposed development to maintain existing flood storage capacity where development is allowed in the floodplain. Page 16 Line 4:
SA-2.7 Acknowledge the permanence of the roadways bounding the Silver Creek Neighborhood and remove the regulations for river migratory channels as the floodway or shoreline development regulations
that apply to the same general area can provide adequate protection for the public health and safety.
Figures to be revised for critical areas element:
Figure 2-3 River Channel Migration: 1. Modify to remove Moderate Hazard designation in Silver Creek Area where protected by permanent roads consistent with revised definition of Channel Migration Hazard areas in 2004 King County Critical Area Ordinance. Figure 2-4 Aquifer Recharge Area: 1. Add 2004 King County well-head protection area and designate as “Critical Aquifer Recharge Areas” where it overlays High Recharge Areas; 2.Add standard well head protection area for new City of North Bend well at Riverfront Park. Figure – 2-5 Wetland Area:
1. Revise to remove wetland area from former Hampton Inn site based on site assessment prepared by Wetland Resources, Inc. in 1998 as part of the former Hampton Inn development proposal (note: temporary topsoil stockpile was placed on site in 1992 as part of Mountain Valley Center grading project); 2. Revise to remove wetland area from section of Ribary Creek based on Sensitive Area Checklist and SEPA Checklist prepared for Mountain Valley Center Master Site Plan in 1993 (note Mountain Valley Center site was graded and filled in 1992 based on SEPA work done in 1988). Figure – 2-7 Landslide Hazard Area: 1. Edit area and change title to “Erosion and Debris Flow”; 2. Add King County 2004 erosion layer; 3 Edit area debris flow area to be consistent with Geotechnical Engineering Report prepared for Phase II of the Forster Woods development in 1992 by Associated Earth Sciences.
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