New and improved! Have another look at DNR’s landslide hazard website

There have been some important changes to our landslide hazard website since we last blogged about it on November 1, 2012. We have improved the resolution of the map by adding the National Weather Service (NWS) forecast zones. A shaded relief of elevation, along with the addition of salt and fresh water features, improves the look of the map, and makes it easier to locate the place(s) you may be interested in. Behind the scenes, an improved algorithm and the use of NWS precipitation forecasts makes the hazard level indicated on the map more closely relate to what is happening on the ground.

As was true before, the webpage is a test project of the DNR and NWS intended to raise awareness of shallow landslide hazards caused by periods of prolonged rainfall. The map is not intended to predict landslides at any particular time or location; it only rates the overall risk that one might occur, based on rainfall measurements over the past week and forecasts for the coming 48 hours.

landslide_hazard_20140113

April Image of the Month: Ledgewood–Bonair Landslide, Whidbey Island, WA

Thanks to our hazards geologists and landslide experts, Isabelle Sarikhan and Stephen Slaughter, we were able to bring you detailed pictures (including this month’s photo) and information on this large Whidbey Island landslide event immediately after its occurrence on March 27, 2013! As we blogged last week, the hazards team released a Quick Report on March 28 detailing their initial on-site investigation of the Ledgewood-Bonair (LB) Landslide. The report suggests that the LB event may be a reactivation of a portion of a larger, prehistoric (~11,000 years old) landslide complex, which extends approximately 1.5 miles along the Whidbey Island coastline. According to the report, “the dimensions of the [LB] landslide are approximately 1100 feet long (measured parallel to the shoreline) and about 300 feet into Puget Sound. Initial estimations place the volume of material mobilized as great as 200,000 cubic yards (~40,000 dump truck loads).”

To see more images of the LB Landslide click here to visit the DNR Flickr site.

More on the March 27 Whidbey Island Landslide Event

DGER geologists who responded to yesterday’s large landslide event, now known as the Ledgewood–Bonair Landslide, have released an unofficial, unedited report detailing their on-site reconnaissance, as well as information pertaining to past observations made of that site. You’ll find this most interesting read here!

And click here to download Michael Polenz’s 2009 Geologic Map of the Camano 7.5-minute quadrangle (10 MB, excerpt seen below) that includes the affected area, which he had identified as landslide deposits and slide-prone terrain.

Camano 7.5-minute quadrangle, Island County WA. Washington Division of Geology and Earth Resources, 2009.

Camano 7.5-minute quadrangle, Island County WA. Geologic Map GM-68, Washington Division of Geology and Earth Resources, 2009.

DGER at the Whidbey Island Landslide Event

Although it is a still-developing story, we thought our readers would be interested to know that DGER hazards geologists are on location and assisting at the site of the March 27th landslides on Whidbey Island. For more information about this event, check out DNR’s Ear to the Ground blog.

Hazards geologist, Isabelle Sarikhan, works at the site of the March 27 Whidbey Island landslide event.

Hazards geologist, Isabelle Sarikhan, works at the site of the March 27 Whidbey Island landslide event.

Hazards geologist, Stephen Slaughter, surveys the Whidbey Island landslide site.

Hazards geologist, Stephen Slaughter, surveys the Whidbey Island landslide site.

Aberdeen Landslides and Liquefaction Susceptibility Report Published

The Washington State Geologic Survey announces the publication of Report of Investigations 36. Earthquake-induced landslide and liquefaction susceptibility and initiation potential maps for tsunami inundation zones in Aberdeen, Hoquiam, and Cosmopolis, Grays Harbor County, Washington, for a M9+ Cascadia subduction zone event, by S. L. Slaughter, T. J. Walsh, Anton Ypma, K. M. D. Stanton, Recep Cakir, and T. A. Contreras. This publication is available free online at http://www.dnr.wa.gov/Publications/ger_ri36_aberdeen_liquefaction.zip. Additionally, a limited number have been plotted and will be sold through the Washington Department of Enterprise Services for $38.61.

RI36 image

This report assesses the earthquake-induced ground failure potential, including soil liquefaction and landslides, for the communities of Aberdeen, Cosmopolis, and Hoquiam in Grays Harbor County, Washington.

The probability of soil liquefaction increases with the duration of shaking, and slopes that are stable under static conditions may fail under large ground accelerations. A Cascadia Subduction Zone (CSZ) earthquake of magnitude (M) 9+ could produce ground accelerations on the Washington coast of as much as 0.40 g (g is the acceleration due to gravity) and shaking durations of as much as several minutes, sufficient to initiate soil liquefaction and shallow landslides. Soil liquefaction can damage transportation networks, such as roads and bridges, and landslides; even very small landslides can render a road impassable to automobiles. These ground failures can complicate or prohibit vehicular evacuation, as well as hamper emergency response and recovery efforts.

The objective of this report is to assist city and emergency management officials in evaluating the suitability of existing evacuation routes and assembly areas for potential susceptibility to ground failure from a M9+ CSZ earthquake. Results of this report could indicate the need to modify current evacuation routes and assembly areas. Understanding which areas are more susceptible to ground failure during a large earthquake can help communities prepare for potentially obstructed transportation networks, toppled buildings, and other secondary seismic hazards.

Rain, Rain, Go Away…

The end of November is typically the wettest two weeks of the year, and today is statistically the wettest of 2012. In Washington State, prolonged or intense rainfall means landslides.Today, amidst the reports of several slides and road closures, the National Weather Service office in Seattle has issued a special weather statement warning for elevated landslide risk in Western Washington over the next few days.

The Washington State Geological Survey (a.k.a DGER) has several tools to help Washington residents prepare for landslide season in the Northwest. Our landslide fact sheet (http://www.dnr.wa.gov/Publications/ger_fs1_landslides.pdf), alerts readers to the warning signs of landslides and to what help is available if you are impacted. Our interactive landslide map on the Geologic Information Portal (https://fortress.wa.gov/dnr/geology/?Theme=landslides) can help you locate landslides that have previously occurred, and our landslide hazard map (https://fortress.wa.gov/dnr/landslidewarning/) can help you determine the current landslide risk on a county-wide basis.

Get Involved!

See a landslide? Want to report it?
Visit www.dnr.wa.gov/ResearchScience/HowTo/GeologyEarthSciences/Pages/report_a_landslide.aspx
We use your information to develop better hazard maps and to further our understanding of landslide behavior in Washington State.

Landslide season is upon us!

As you are no doubt aware, summer sun has now given way to fall clouds and rain in western Washington. Unsurprisingly, this weather transition marks the beginning of landslide season. Landslide events impact many more than just landowners of slide locales; resulting power outages, water supply disruptions, and impassable roads have wide-reaching effects, making it helpful to keep current on the landslide risks in your area.

A test project by DNR and NOAA’s National Weather Service (NWS) is trying a raise awareness of landslide hazards, particularly the ones that occur because of heavy and/or prolonged periods of rain. Our online beta (test) map combines recent rainfall measurements from NWS rain gauges with information about the local soils and slide history. The result is a county-wide risk level rating. The map is not intended to predict landslides at any particular time or location; it only rates the overall risk that one might occur based on the amount of rain that has just fallen.

The map is still in its testing phase, but we have placed it on the DNR website so you can be better informed. The DNR “Ear to the Ground” blog has also recently profiled our landslide hazard map. Take a look and see how your county rates now.