alternative press serving the lower columbia pacific region

Some Realities of Landslides and Other Mass Movements on the Landscape

Duane Slide 2013

Slide on Duane and 5th St in Astoria in 2013.


Mass movements are important geomorphic processes in many parts of the world.  Landslides and other mass movements can occur whenever the particular landscape is unstable.  Land Stability relates to the resistance to movement.  The term best used to describe this resistance is ‘Shear Strength’.  The factors involved in shear strength include: level land; solid material [rock]; compact material such as clay or sand; non-expanding clays such as illite; vegetation cover [trees, grass, etc.]; good internal drainage so that water does not accumulate within the material; and typically conformity of material [for example all sand].

Land Instability relates to slope failure and the movement of materials.  The term best used to describe the susceptibility to slope failure is ‘Shear Stress’.  The factors involved with increasing shear stress include: steep slopes; unconsolidated, loose material; unconformity of different materials [for example sand over rotted rock over solid rock]; cracks or joints in rocks; expanding clays such as vermiculite or montmorillonite; loss of vegetation [especially the binding effect of roots]; saturation with water [for example springs or heavy rainfall events]; added weight such as structures or roads; undercutting of slope such as a roadcut; heating and cooling; wetting and drying; and freezing and thawing.

The basic causes of mass movement relate to the excess of stresses to resistance.  In other words, when shear stress [instability] becomes greater than shear strength [stability] a slope will fail.  Increased stress may come from added materials from upslope that can cause rapid failure.  Heavy rains can increase pore pressures to expand or push material away from the slope face.  Such movements can be related solely to a heavy rainstorm, BUT most often there is a lag time from the rain event to the infiltration of rainwater to the buildup of pressures in the material to the slope movement.  The author has seen such a ‘lag’ on numerous occasions in St. Vincent and Jamaica in the West Indies and even here in Oregon.

The importance of any given mass movement ‘event’ is determined by:  the amount of energy and material involved; how often it happens; what occurs between events to stabilize the slope; and the timing and size of the event.  In other words, how much is the event able to transform the landscape – from a major ‘slide’ to a few rocks tumbling down a hillside, AND what impact does it have on roads, structures, and human safety.

Mass movement of any material on slopes is under the influence of gravity and can be rapid or slow.  Rapid movements generally can be explained by such events as earthquakes; vibrations from road traffic or machinery; weight of water; lubrication by water; removal of material at the bottom of a slope; or the weight of buildings and other structures.  Often the sudden alteration of a landscape is caused by high amounts of precipitation.  Bedrock can give way on a joint or fault or crack.  Such failure often is related to the bedrock layers sloping in the same direction as the general land surface slope.  The failure is compounded if the rock layers are composed of different minerals and textures of materials [such as a combination of mudstone, sandstone, and basalt].

Unconsolidated materials can experience shallow or deep landslides depending on the character of the materials involved.  Deep slides are the result of shear stress increasing with depth and exceeding the shear strength.  The material gives way as the mass becomes unstable.  Other natural factors may be of lesser importance overall, but do contribute to movement in addition to precipitation.  Freezing, heating, and wetting processes all contribute to the expansion of unconsolidated materials and the pushing up of these materials from the surface.  The ‘pushing’ often is uneven because the mixture in the material may freeze, wet, or heat in variable ways depending on the size of the individual particles and the minerals involved.  As these same materials thaw, dry, or cool the materials contract in varying degrees again depending on the particle sizes and minerals.  Thus, materials are more loosened on the sloping landscape and subtly moved down slope by gravity alone.

Different terms are recognized depending on the types of movements that occur.  ‘Falls’ describe breaks at joint planes or cleavages in rocks and result in the breaking off of rock faces on a hillside leaving a steep, exposed cliff with fractured bedrock.  Angular blocks and slabs pile up at the base of the slope.  Trees and other plants are broken and scarred by the rocks.  ‘Slumps’ usually occur where there are differing shear strength layers and failure comes in the layer or layers where shear stress exceeds shear strength.  A steep back wall exposes the rock or unconsolidated material.  The movement often is a back rotation that uproots trees and other plants and tips them back toward the slope.  ‘Flows’ usually need water that can come naturally from rainfall or springs and also from irrigation or watering of lawns.  The flow has a lobate form that buries ground plants and soils and can bend trees.  ‘Slides’ slip over one or more surfaces and are helped by wetness [can be slippery or solution of minerals], earthquakes, and/or steep slopes.  A concave scar exposes rock or other material at the back of the slide.  The displaced mass may move a few feet to more than a mile downslope depending on the size of the event.  Trees and other plants are broken and bent.  ‘Creeps’ usually are quite slow, deforming the slope and tilting objects on the surface.  Tree roots usually are bent to reveal the down slope movement.  Otherwise, creeps are the most subtle mass movements.  However, creeps can expand into something more dramatic if the shear stress increases for any reason.

Some examples help to illustrate the complexities of mass movements.  In 1970 ‘rock avalanches’ attributed to large slope failures killed 18,000 people in Peru.  Experts described complex landslides involving the detachment and flow of masses of rock from a cliff face or steep slope.  At first it was a simple transfer of rock on a planar surface.  The rock rapidly disintegrated as it fell away from the cliff and soon became a flowing body of highly fragmented debris that was deposited as an ‘apron’ beyond the foot of the failed slope.

In a 1982 study of mass movements in the Oregon Coast Range, the kinds of movements varied with the materials and their mineralogy.  ‘Debris avalanches’ occurred in non-expanding clays with low water-holding capacity.  ‘Creep’ was attributed to shear stress and deformation of materials.  ‘Slumping’ was attributed to shear stress along failure surfaces.  Both creep and slumping mostly happened with expandable clays [smectite and montmorillonite].  ‘Earthflows’ were slow and deep with moisture involved and with mixed mineralogy.  Poorly crystallized minerals with ‘gels and coats’ on the mineral grains resulted in abundant pores [water present] that accounted for a fluid behavior.

No matter the type of mass movement, the results seem to create three distinct slope segments.  The ‘Detachment Zone’ often shows a sudden failure along a surface leaving a concave shaped form sometimes referred to as a ‘slump scar’.  The ‘Transport Zone’ involves fragments transformed into a debris flow or fall or creep.  Often there is high velocity movement and little debris is deposited here.  The ‘Depositional Zone’ is where the material loses its momentum and comes to rest when it reaches a gentle slope [often the valley bottom or a near level street].

Once the materials come to rest, the upper slope material [detachment zone] and lower slope material [depositional zone] re-adjust to slope angles of stability.  These angles, called ‘threshold angles’ or ‘angles of repose’, vary with the type of materials transferred or remaining.  In general, the steepest threshold angles are in jointed and fractured rocks [around 45 degrees] and the lowest or gentlest are in clays [around 10 degrees].  Sandy slopes [dunes], talus slopes [broken rock debris], and colluvial slopes [finer gravity transported material] have moderate threshold angles [ranging from 20 to 35 degrees].  Thus, even after a major mass movement event, smaller and still significant further movements can be expected until all the affected materials stabilize at their threshold angle[s].

Evidence of instability and the presence or threat of mass movement events can be observed in any unstable or mass movement prone landscape.  Some of the best evidence includes:  a steep upper slope [greater than 45 degrees]; fresh exposure of soil or other loose material; lobe shaped forms, especially on lower slopes; bent or scarred trees; tilted fence posts and telephone/electric poles; seepage of water along the slope [springs, etc. may indicate a convergence of sub-surface water from different sources]; water drains [gutters] and lawn sprinklers; unconformities of materials [loose over compact] on a slope; buried soil layers; cracks in foundations; and slumping ‘arcs’ of pavement on streets.


A good place to start in looking at the types of materials and their distributions throughout Astoria is the Soil Survey of Clatsop County [U.S. Department of Agriculture, Soil Conservation Service, 1988].  The report contains descriptions of all the soil types that are found in and around Astoria, airphoto maps of the distribution of each soil type, and evaluations of the soil types for varied uses based on the soil properties.  One caution about using this soil survey report is that the boundaries between soil types are somewhat general.  From investigative work by the author, more detailed soil sampling and soil type identification are needed for evaluations of specific sites [such as individual property lots or exact areas affected by mass movements].

There are two main groups of soil types that are of importance in Astoria related to mass movements.  One group consists of ‘terraces’ on the more gently sloping landscapes.  The soils of this group are called Knappa loams and Walluski silt loams.  These soil types occur on level to gentle slopes [up to about 10 degrees].  The location of these soil types in Astoria suggests that these materials include colluvium and thus are a part of the ‘depositional zone’ that was described in part I above.  The general locations of these materials are:  on the northside of Astoria eastward from about 5th Street to east of the Performing Arts Center and southward from about Franklin Avenue to Irving Avenue; from the Millpond area eastward to about 37th Street and southward from Marine Drive/Lief Erikson Drive to Irving Avenue; and on the southside northward of the Old Youngs Bay Bridge, mostly west from 7th Street to about Dresden Street and upslope to about McClure Avenue in the eastern sector and to about Niagara Avenue [Former Gray Elementary School area] and Alameda Avenue in the western sector.

The other group consists of colluvium [“soil material, rocks, or both, moved by creep, slide, or local wash and deposited at the base of steep slopes”].  These soil types occur on slopes ranging from near level to 45 degrees and are the soil materials most likely to undergo mass movement again.  The soils of this group are called Ecola silt loams, Templeton silt loams, and Svensen loams.  The Ecola and Templeton soils are derived from siltstone debris and the Svensen soils are derived from sandstone debris.  The general location of the Svensen materials is:  north of Olney Avenue to about Niagara Avenue and east of 7th Street including the area of Astoria Middle School, Shively Park, Coxcomb Hill [Astoria Column], and the Transfer Station/future John Warren football stadium.  The general location of the Ecola and Templeton materials is the rest of the higher portions of Astoria including: the areas upslope from Alameda Avenue and West Bond Street in the west; upslope from Niagara Avenue [near the old Gray Elementary School] and Astoria High School, McClure Avenue [west of 7th Street], Klaskanine Avenue [east of 7th Street] in the south and extending beyond the water reservoir [at Williamsport Road] and wrapping around the south and east sides of Coxcomb Hill; east of the Astoria-Megler Bridge upslope from about West Duane Street to about 5th Street and then upslope following Irving Avenue eastward to join the sector wrapping around the east side of Coxcomb Hill.

The locations of these soil types in Astoria suggest that these are materials in the ‘detachment zone’, the ‘transport zone’, and parts of the ‘depositional zone’ that was described in part I above.  These soil types also represent the landscape above the detachment zone that is very vulnerable to further mass movement as the material either drops off a cliff face or continues to move seeking to achieve the ‘threshold angle’ or ‘angle of repose’ discussed in part I above.  These soil types in the deposition zone also are subject to further movement because they are loose and may be in the process of re-adjusting to reach a threshold angle.

The areas mapped as these soil types include: portions that are very shallow to ‘soft’ bedrock; significant portions that have rotted siltstone, sandstone, or even basalt over this bedrock; and portions that have more than 5 feet of unconsolidated colluvium material over this bedrock.  Thus, it is hard to predict where mass movements may occur or how much material might move because of the uneven, irregular depths to the underlying bedrock.  However, some general traits of these soil types and their materials can be stated.  All of them have increasing clay content with depth.  This clay may not be an expanding clay, but it does hold larger amounts of water which allows the material to be ‘lubricated’ and to move.  Also, the increased weight of this stored water can contribute to elevated shear stress.  All of these soils are described in the soil survey as being vulnerable to collapse of roadcuts and being “subject to sliding and slumping because it is very plastic and underlain by highly fractured bedrock”.  Also, the soil survey cautions that limits to  ‘development’ for recreation sites, building sites [including dwellings], and ‘local streets and roads’ are ‘severe’ due to the slopes and potential mass movements.

In conclusion, about two-thirds of Astoria is subject to mass movements of varying kinds.  The extent and severity of any one event is hard to predict.  However, the soil survey report and its maps provide a good starting point to show where movements can be expected; the survey was used by the author to generate the discussion presented in part II.

Another source of information to be used for future mass movement considerations in Astoria is the new data and maps of landslides released by the Oregon Department of Geology and Mineral Industries (DOGAMI) [released in October 2013].  The DOGAMI scientists used LIDAR, a laser-mapping technology to locate 120 landslides within the Astoria city limits; 83 of these have moved in the last 150 years [and more or less confirm the information in the soil survey report].  The scientists then used the inventory of mapped landslides to create landslide susceptibility maps.  About 55 percent of Astoria is classified as “highly susceptible to shallow slides” where movement occurs along a plane less than 9.5 feet deep.  About 37 percent of the city is “highly susceptible to deep landslides” where movement occurs along a plane at depths greater than 15 feet.


The following discussion is not intended to cover all the history of landslides and other mass movements in Astoria.  Rather it is written to illustrate that many parts of Astoria have experienced land movement events – some very large and some small.  The discussion also will point out that there have been important changes to the infrastructure of parts of Astoria because of these previous mass movements.  The discussion uses newspaper articles and photos from the mid-1950s [some of which refer to earlier events] and newspaper articles and photos from about 2003 to the present [some of which also refer to earlier events].

Two particular areas seem to be the focus of mass movements that occurred from about 1949 through 1954 according to a collection of accounts in Astoria and Portland newspapers that the author was able to look at recently.  One location centered on the north side of Coxcomb Hill and the other on West Commercial Street at 1st Street.  On the north side of Coxcomb Hill there were several different events.  Starting in 1949 and extending into 1950 an earth flow happened on the hill between 18th and 21st Streets and affected 23 homes from Irving to Grand.  By October 1950 area residents were relieved to know that 17 inches of rain in that month “failed to cause any new movement of the Coxcomb hill slide which crawled down the slope for many weeks last winter”.  However, in November 1950 there was a “slight movement of earth” about 50 feet above the top of the Coxcomb hill slide area.  This movement was dismissed by city engineers as “merely an adjustment of the ground” and “not connected” to the Coxcomb slide itself.  “The old slide area remained stationary… and a drainage system installed there last summer [1950] by the city was working well”.  In 1952 there was another slide in the same location on Irving Avenue near 22nd Street where several houses were wrecked and “five were moved by owners to other parts of Astoria”.  In December 1953 a mass movement began in the same area of a slide in 1920; “the slippery clay of Astoria’s north slope began flowing at 27th and Irving… producing the city’s third major slide disaster in five years”.  Apparently it was slow for more than a week after about 4 inches of rain, then the earth “speeded its movement”.  The flow was about a block wide between 27th and 28th Streets from Irving to Grand and also affected the end of 25th Street and Grand.  About 10 houses were affected, water and sewer lines were severed, and about 300 feet of pavement buckled on Irving Avenue alone.  The newspaper photos were dramatic!

The statements issued by city officials in December 1953 regarding the mass movement[s] from Coxcomb hill were very revealing.  First, the earth was falling away from Irving Avenue, not pushing down on it.  However, photos of the roadway show severe buckling.  Second, “the system of creeks flowing several hundred feet above Irving Avenue has absolutely no connection with the slide and has no effect on it”.  More recent understanding of water flow and especially subsurface water movements affecting mass movements would refute that statement.  Third, “the recent logging operation on Coxcomb hill has no bearing upon the present earth movement… experts believe removal of trees tends to stabilize the earth”.  In fact, plants, including trees, and especially the roots of these plants helps to stabilize slopes [contributing to shear strength].  The author has seen many, many mass movements following logging on sloping landscapes, especially with soils similar to those underlying the hills of Astoria.  Fourth, “the center of the slide is the center of a fill which was made in a natural draw at least 40 years ago.  A slide occurred in the same spot in 1920”.  There should have been no surprise that another movement event would occur in this location!

A map was published in the January 10, 1954 Oregonian showing “Astoria’s three slide areas” is very revealing [see map attached].  There must have been mass movements both on the north slope and on the south slope of Coxcomb hill since that date because of several ‘missing streets’.  On the south slope, 8th Street now is discontinuous [rather than paralleling 7th as shown on the map] from Klaskanine to Nehalem; now McClure Avenue does not continue east of 7th Street; and Lewis, Nile, Ohio, and Potomac streets do not exist [they were east-west streets between 12th and 15th streets which also no longer exist] near the present location of Astoria Middle School.  On the north slope, now Madison Avenue does not extend east of 16th Street; Lexington and Jerome Avenues do not extend east of 17th Street; 18th Street does not extend south of Grand; 19th and 21st Streets no longer exist; 20th and 22nd Streets do not extend south of Franklin; there now are only dead end streets of  18th, 21st, 22nd, 23rd, and 24th off the ‘reconstructed’ Irving Avenue; and 19th Street no longer is the route to the Astoria Column!

The area of West Commercial Street and 1st Street seems to be of concern from the late 1940s.  In October 1950 the newspapers reported no movement in the slope area where the fear of a slide caused the installation of a new drainage system in summer 1950.  However, in January 1951 residents of the west Commercial Street hill told city council “there has been earth movement ther this winter despite the new drainage system installed last summer”.  The city officials responded that the movement might have been much worse without the drainage system!  Starting in December 1953 and continuing into 1954, there was a significant mass movement centered on 1st Street and West Commercial Street.  One report indicated that it was about 400 by 500 feet and another report said 300 by 600 feet.  Regardless, the event damaged at least 26 homes and displaced at least 30 families.  In addition to 1st and West Commercial, homes and pavements were affected on Hume, West Bond, and West Duane streets also.  The land was visibly sliding downhill… it “moved as much as 2 feet an hour, tilting and toppling the houses”.  Reports continued to show that houses were moved an average of 12 feet and some up to 20 or 25 feet.  The movement “occurred in a short period of time and slipped more violently and without sufficient warning to get houses out”.  Photo captions included: “fissures opened in paving, deep into earth, as relentless downhill movements persist.  Soapstone, underlying clay soil, is blamed for slide as water seeps down the rock”; and “paving sunken estimated 35 feet below former level”.  In February 1954 the West Commercial slide continued about 50 feet west of the one reported earlier.  In early 1954 Astoria City Council also “heard strong recommendations by state officials that studies be made to determine if there are other potential slide areas in the city”.

In October 1954 Astoria city council responded to another movement event, this time at 38th Street and Harrison Avenue.  Emergency measures were taken to install new drainage flumes in a slide area described as “an old one which has been moving gradually for many years…”.  The newspaper made reference to 1902 maps on which the area “is marked as a slide area, and there has been earth movement there ever since”.  Now, Harrison is discontinuous in this area.

“Geologic studies show that when water seeps down to the soapstone [ie. siltstone] it serves as a tilted, literally greased, skid down which 10 to 30 feet of overlying clay soil slides of its own weight, carrying buildings with it.  The resulting movement is a mudflow, not a landslide in the strict sense of the term”.  There was “some speculation that the severe earthquake of 1948 may have contributed to the movements since”.

Fast forward to 2004 and the author’s arrival to live on the North Oregon coast.  Almost immediately newspaper articles in the Astorian and Oregonian pointed to continuing mass movement issues for Astoria.  The first discussions concerned the 2002 slide that had been edging down hill near 33rd Street and “finally gave way”, at an estimated 35 feet deep.  Associated with this movement event was the contention that it “was triggered by the botched construction of a retaining wall at Marine Drive and 32nd Street”.  Essentially, the construction work related to “a small commercial development on the site” that cut into the base of the slope in the ‘deposition zone’ and almost immediately pushed the shear stress beyond the shear strength level and gravity flow took over.  Apparently, persons were “familiar with the land movement in the neighborhood by the shifting and settling that plagued the original store that sat on the site from 1910 to the 1970s”.  Some of the damage to upslope homes, roads, and other structures was attributed to the building of the ‘new Safeway store’.  However, continuing cracking of foundations, walls, and pavement and severed water and sewer lines.  Related surveys revealed the earth movement was up to 40 feet deep and was the result of the removal of a large amount of earth at the base of the slope.  Sensors placed in the soil measured as much as 16 inches of movement in the neighborhood above the excavation site after the work began.  Twenty-eight properties suffered damages.

In January 2006 a mass movement event occurred in the South Slope neighborhood on Bridge View Court upslope from Astoria High School.  After weeks of rain, the earth beneath a new home began to move downhill.  The home was declared uninhabitable when the backyard gave way leaving “a cracked and muddy incline with fissures 30 feet deep”.  An “arc shaped fault line” extended behind this house and the one next door.  Both the storm sewer and sanitary sewer lines had to be re-routed above ground.  The land down slope is the location of Astoria High School.  Students were “being warned to stay off the hillside, where the deep cracks and tilting trees pose a hazard”.  The homeowner assumed “the site was safe because the city had approved it”.  In fact, “a geotechnical evaluation of a building site on a slope is required by city code only if it’s within 100 feet of a known slide area”.  [The city code should be changed to reflect the data available from the Soil Survey and the DOGAMI landslide maps].  The area of the subdivision “was not a known slide area”.  However, there was a “known slide area several hundred feet away, below Waldorf Circle, where there was a landslide onto the high school track in the late 1950s”.  [The whole hillside upslope from the high school has ‘prime mass movement potential’]!

In January 2007 another landslide happened in the same area as the 1953-54 ‘West Commercial Street’ slide described above.  “Persistent heavy rain over the last several years, including a record 22 inches in November, gradually raised the water table and eventually reactivated an old slide”…  The new slide area was bounded by Bond, West Duane, Hume, and First streets and the land was owned by the city which “has prohibited its redevelopment”.  The city closed Commercial Street between 1st Street and Hume Avenue and 1st Street between Duane and Commercial Streets.  Eventually, Bond Street was opened as a one-way street westbound.  By March, 3 inches of rain in one weekend caused “significant movement”.  The slide continued to move “in the same boundaries as the old slide [1954] and is almost a mirror image of it”.  The recent heavy rains “allowed loose clay on top of 30 foot deep siltstone formations to slide downhill”.  Geotechnical engineers said the earth was moving “as far down as 20 to 30 feet below the surface”.  There were 10 to 12 foot vertical drops from the edge of the slide to the material moved below.  Eventually, this slide expanded beyond the bounds of the 1954 slide, especially down slope onto Bond Street and upslope into Duane Street.

In December 2012 a landslide brought down trees and mud behind the Clatsop County Jail on Duane Street which dead ends behind the jail.  Strong winds swung the trees around in the soil that had been saturated by rain.  The trees then toppled down a steep hillside.  [In looking at this location, the steep hillside represents the ‘slump scar’ of a former mass movement or the ‘detachment zone’ described in part I.  The debris from the landslide covered Duane Street [the ‘transport zone’ as described in part I].


Two important questions can be raised related to the preponderance of mass movements in Astoria.  1] What has been known about the ‘potential’ for mass movement events to occur?  2] What can or should the City Council do ‘going forward’ from the present to protect people and property in Astoria?

1] The potential for mass movement events first can be reflected in the history of known events over the past century.  Newspaper articles, photos, personal accounts and other evidence alone are enough to convince one that mass movements in Astoria are a real threat.  The city of Astoria is “so well known for its unstable geology that the ‘Astoria Formation’ is a term found in geology texts”.  “The city has all the ingredients needed for a ‘high-hazard classification’ for landslides”.

Resolution No. 08-23 ‘A Resolution of the City of Astoria Adopting the City of Astoria Multi-jurisdictional Natural Hazards Mitigation Plan Addendum’ adopted by the City Council on October 20th, 2008 and approved by the Mayor on the same date in part recognizes the threat of landslides:
*“Where it appears a landslide, or other earth movement hazards may be present… may require a site investigation and report by a city approved engineering geologist or soils engineer”;
*“Land divisions in areas of steep slopes, unstable soils, weak foundation soils, or landslide potential will be permitted only after a favorable site investigation report has been completed”;
*“The City has drafted a Geologic Hazard and Hillside Development Ordinance which will guide development related to earthquakes and landslides”;
*“A map showing past slides can be found within city records”;
*“The majority of the city is located in areas of high landslide hazards” [map explanation];
*“Astoria is at risk of landslides because of its location on the hillside above the Columbia River and Young’s Bay.  The extent of the landslide hazard includes most of the residential portion of the city”; *“The city of Astoria ‘Areas of High Water and Past Slides’ map originally developed in 1974 and updated as recently as 2008 identifies previous occurrences, location and extent of earth movement in the City of Astoria”;
*“The city of Astoria’s vulnerability to landslides… is high due to location of critical facilities and residential development within landslide prone areas… the probability of landslides is high”.

City of Astoria Comprehensive Plan [of 2009?], Section ‘Geologic and Flood Hazards’ CP.390 ‘Background Summary’ discusses earth slides:
*”The area on which the City of Astoria is located has experienced many earth slides throughout its history”;
*”The sharp escarpment on the north side near the top of the main ridge indicates that a major movement of land took place many years ago”; [most likely this was in 1700 when a significant earthquake triggered the last great tsunami – recognized by local geologists and soil scientists and other experts]
*”Most of these slide areas are in a siltstone and claystone sedimentary rock unit”;
*”There are two types of slides common to Astoria: …shallow earth slippage generally not more than two feet in depth… the deep (and much more serious) landslide caused by rotation or movement along a slippage plane caused by water pressure build up within the earth… preventing construction in landslide areas is the best deterrent”.

CP.395 ‘Conclusions and Problems’ includes comments on landslides:
*”Since 1950, it is estimated that sixty to seventy homes have been seriously damaged by earth movement… cost of street and utility repairs is estimated to be over $2 million”
*”The Engineering Department has detailed information on recent landslides (during the last 50 years)… the City has acquired … much of the active landslide areas on the north slope”;
*”The City Engineer, land agent and Building official all have access to geologic data”;
*”The City and other public agencies own most of the lands on the south slope”;
*”The City has an opportunity… to control how new subdivisions are designed, thereby reducing landslide hazards”;
*”Geological information indicates that the bedding planes under Astoria generally dip toward the south, and that the landslide potential on the south slope… could be considerable as development increases”;
*”Great care should be taken to insure this area does not experience the same problems encountered on the north slope of the City”.

CP.400 ‘Geologic and Flood Hazard Policies’ includes comments on landslides:
*”Where it appears a landslide, or other earth movement hazard is present, the approval of the City Engineer will be obtained before a building or development permit is issued”:
*”The City Engineer and/or Planning Commission may require a site investigation and report… in such cases”;
*”The City Engineer will file copies of all geologic and soils reports… furnish copies of them to interested persons”;
*”Land divisions in areas of steep slopes, unstable soils, weak foundation soils, or landslide potential will be permitted only after a favorable site investigation report has been completed”;
*”The Planning Commission will submit site investigation reports to the City Engineer for evaluation”;
*”The City Engineer and/or Planning commission may require the submission of detailed topographic maps in steep slope areas, indicating the location of drainages, springs or other natural features”;
*”Site investigation reports… will generally indicate where construction may take place without enhancing earth movement hazard… the location of evidence of potential or past earth movement”.

The Oregon Department of Geology and Mineral Industries [DOGAMI] landslide maps at 1:8000 scale have been available to the City of Astoria since about 2008. “The city has long been aware of the jeopardy – it had the maps for five years before they were released publicly last week”.  These maps are in the public domain as of October 2013.  There are 3 different ‘layers’ depicted on 3 map sets: Map set 1 are landslide inventory maps for the City of Astoria – colors on the maps show existing mapped landslides’ Map set 2 are corresponding shallow landslide susceptibility maps – colors show areas at risk for landslides with depths of failure less than 15 feet; and Map set 3 are corresponding deep landslide susceptibility maps – colors show areas at risk for landslides with depths of failure more than 15 feet.  “These maps and results are valuable… they aren’t for site specific evaluations.  However, they give planners an idea of what is out there and where to focus their efforts”.  These maps greatly decrease the risks of making mistakes in development and building decisions.

2] What the City Council can do going forward to protect people and properties from mass movement damage mostly focuses on the strict use of the documents and maps outlined above.
*The DOGAMI landslide maps should function as the basis for all land use planning decisions.
*Site specific work must be done as outlined in the City of Astoria Comprehensive Plan.
*All statements within the Plan that include the phrase “may require” need to be changed to “WILL require”.
*The City Geologic Hazard and Hillside Development Ordinance must be adopted [if it has not been done already].
*The requirements for comprehensive geotechnical reports prior to construction must be enforced and a reputable geotechnical firm hired.
*The subsequent construction must be closely monitored by the City Engineer to insure the report guidelines are followed precisely.
*The City Planner and City Manager must recognize the hazards that go beyond the selling of lots or ‘buildable lands’ in potential mass movement areas – do not allow the philosophy of “let the buyer beware” to sway decisions.  The potential damages to the landscape go far beyond these ‘lands’ to the infrastructure damages – streets, utility poles, sewer lines, water lines, and other things for which the city would be responsible to ‘fix’.  Learning from the past, the costs could be enormous and FEMA or other agencies probably would not be there with funds to help.

Comments are closed.