July 1997 Volume 3, Issue 3

In our previous two newsletters we described land where concern for water quality, damage to property by water and maintaining wildlife habitat warrant "sensitive/critical area" designations. We also briefly described the permit process an owner and architect undergo to meet the environmental zoning and building codes that apply to sensitive areas. Water in combination with geological conditions describes another group of sensitive areas: Steep Slope, Erosion and Landslide Hazard Areas." Slopes are beginning to dry out and stabilize allowing construction to begin. In a record slide year city & county building/environment department scrutiny and enforcement have been strict.

Land with potentially "geological instability", "steep slopes" or "geologic hazard" is characterized by percent slope, soil type, geologic material and ground water. Landslide (downhill movement of rock, soil, snow or other material) and Erosion Hazard Areas are deemed unstable because they are vulnerable to forces like earthquakes, wind, rain splash, frost, excessive snow and water but may or may not include steep slopes. The hazard exists if: there has been a slide in the past or there are indications of debris or sediment flow; a stream, wave or erosion has undercut a bank; or snow accumulates creating a potential avalanche. Water from springs or seeps may cause silt and clay soils layered with granular soils to separate and slip but some slope (about 15%) is required for sliding to occur. When slope exceeds 40% the Steep Slope Hazard Area designation prevails whatever other conditions exist.

Steep Slope Hazard Areas are defined in most jurisdictions as land which both "inclines 40% or more and rises vertically ten or more feet in a run of 25 horizontal feet." The city of Lynwood allows "lot disturbance" on half of the square footage of a lot that slopes 25 to 40%. If the lot slopes more than 40% only 30% of the area may be disturbed. Mercer Island defines critical slope between 30 and 50% allowing a 30% disturbance. On slopes greater than 50% (measured in any 40' run) only 20% disturbance is allowed. "Land clearing, grading, filling, and foundation work" may only occur between the first day of April and October" on Mercer Island. King County and Bellevue require a 50' buffer between a steep slope and development. King County like most counties allows modifications to existing structures within the buffer if a hazard is not aggravated.

As you can see, the definition, measurement and consequent developable area differ among jurisdictions. A percent slope measurement results by placing rise over run and multiplying by 100 (see drawing below). The key is how much "run" is taken into account to average changes in topography (25' in King County, 40' on Mercer Island). Slope can be expressed in degrees as well as percentage. A rise of 25' over a distance of 25' would result in a 100% slope. The same slope is equivalent to 45 degrees. A 40% slope is roughly equivalent to 22 degrees.

A geotechnical report provided by the owner and an onsite review by the "sensitive or critical area department" in your jurisdiction will either precede or be a part of the process of filing an application for a building permit.

DePape Associates invites owners of land including designated "Sensitive Areas" to call our office. This land is often naturally extraordinary, quite buildable and therefor valuable. We have found that the benefits like views onto open space, wildlife and privacy associated with sensitive areas more than compensates for the challenges presented in utilizing and caring for this precious land. In addition most jurisdictions offer tax exemptions for the portion of this land that is preserved in its existing natural state. A call to your county's tax assessor's office will clarify your land's eligibility. Our region has little unbuilt land remaining that does not include steep slope, wetland, flood plain, landslide and/or erosion hazard areas. Water after all, and lots of it is what gives the Northwest its character. And we can thank slopes for creating those amazing views!

Let us know if you'd like copies of our Sensitive Areas newsletter series for yourself or acquaintances. And call us if you are considering building or renovating.

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