Landsliding is a significant contributor to continuous erosion
of the 150-mile northern Oregon coast. Direct loss of land to the sea
by landslides occurs along 47 percent of the coast. The remaining 53
percent has minor shifting of sand along depositional areas such as
spits and dunes. These minor movements alter coastal topography
but do not erode material directly into the sea.
The type of landsliding is principally controlled by the lithology
of the coastline. Landslides are classified on the basis of two
features: the lithology of the coastal material in the slide and the type
of movement expressed by the overall shape of the slide. Slump occurs
in deeply weathered sedimentary rocks and in marine terrace
sands overlying seaward-dipping rocks. Rock and debris fall are
mainly confined to headlands. Block glides develop along bedding
planes of relatively unweathered sandstone. Debris shift occurs in
thick terrace and dune sand deposits in which no slip surface
is defined.
Factors contributing to the cause of coastal landslides include
high precipitation, easily weathered rock, and high coastal wave energy.
Frequency of reported landslides is related to periods of high
precipitation and high wave energy. Although rock weathering is continuous
throughout the year, the final phase in disruption of slope
equilibrium often occurs during winter storm conditions.
Landslides on headlands and adjacent coastlines disrupt the
most extensive land area. Wave refraction directly influences this
relationship by focusing wave energy on promentories and on the
coastline within one mile of the headlands. Distribution of coarse and
fine beach material is affected by refraction in restricted coves.
Severe coastal erosion has taken place in local areas. The
average rates of retreat vary according to the lithology of the coastline,
and have been determined as follows: unconsolidated sand and
gravel - 23 feet per year; marine terrace sands overlying sandstone
and clay - 20 feet per year; marine terrace sands overlying mudstone
and sandy shales - 6.5 feet per year.
Knowledge of the processes of marine erosion applied to local
cliff protection measures can assist in increasing the effectiveness
of erosion control attempts. / Graduation date: 1964
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29515 |
| Date | 07 May 1964 |
| Creators | North, William Benjamin |
| Contributors | Byrne, John V. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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