RECOGNITION OF NON-RULE IDENTIFIED HIGH HAZARD LANDFORMS IDENTIFIED DURING LANDSLIDE HAZARD ZONATION PROJECT MAPPING IN WASHINGTON STATE

Mass wasting assessments currently underway by the Landslide Hazard Zonation Mapping Project (LHZ) seek to identify unstable slopes based on rules adopted by the Washington Forest Practices Board. Landforms in a watershed are assigned Overall Hazard Ratings (low, moderate, high, or very high) based on LHZ protocol. Qualitative ratings based on a semi-quantitative assessment takes into account the number and area of landslides in each landform, normalized to the years of aerial photo coverage studied. Six statewide landforms are rule identified as high hazard due to their instability, likelihood to deliver sediment or debris to a public resource, or threat to public safety. These are: (1) inner gorges, (2) convergent headwalls, (3) bedrock hollows, (4) toes of deep-seated landslides with slopes >65%, (5) ground water recharge areas for glacial deep-seated landslides, and (6) outer edges of meander bends along valley walls or high terraces of unconfined meandering streams. Based on hazard ratings, these landforms require additional evaluation prior to forest management and other landuse activities. The rules also provide the ability to identify instability based on a preponderance of evidence.

Here we describe additional landforms that have high or very high Overall Hazard Ratings and occur in multiple watersheds, but are not currently rule identified by name. These include: (1) Coastal Bluffs where wave action undercuts the toe of the slope, creating steep faces adjacent to the shoreline. Bluff failures commonly occur where glacial outwash-deposits overlie impermeable till, clay, or bedrock. This stratigraphy promotes development of perched water, which facilitates saturation and sliding of the top units. (2) Steep Terrace Faces along reservoirs, where repeated fluctuations in water level occur every year. Repeated wetting and de-watering causes changes in pore water pressure, leading to unstable conditions along the terrace faces. (3) Steep Hillside Slopes (>70%) mapped between bedrock hollows and inner gorges. These slopes often have shallow soils or steep bedrock surfaces. (4) Earthflow bodies and toes with multiple secondary landslides. These occur with slopes <30% and exhibit water piping and sapping out of road cuts. Glacial deposits as well as highly weathered volcanic material show instability.