Evaluation of coastal erosion hazard zones along dune and bluff backed shorelines in Tillamook County, Oregon: Cascade Head to Cape Falcon

Metadata:

Identification_Information:
Citation:
Citation_Information:
Originator: J.C. Allan
Originator: G.R. Priest
Publication_Date: 20011231
Publication_Time: Unknown
Title:
Evaluation of coastal erosion hazard zones along dune and bluff backed shorelines in Tillamook County, Oregon: Cascade Head to Cape Falcon
Edition: Preliminary
Geospatial_Data_Presentation_Form: vector digital data
Series_Information:
Series_Name: Open File Report
Issue_Identification: O-01-03
Publication_Information:
Publication_Place: Portland, Oregon
Publisher:
Oregon Department of Geology and Mineral Industries
Other_Citation_Details:
A text accompanies the vector digital data.
Online_Linkage: www.oregongeology.com
Description:
Abstract:
This report describes and documents a range of coastal hazards zones distinguished for the Tillamook county coastline. The report focuses on identifying maximum potential erosion distances for bluffs and for dune backed shorelines using two quite different but complementary approaches. In both types of shorelines four zones were defined, an active hazard zone characterized by existing, active erosion processes, and three zones of potential future erosion, high, moderate, and low risk zones that respectively depict decreasing risks of becoming active in the future. Of most interest to planners are the landward boundaries of the high and low risk zones. The landward boundary of the high-risk zone defines a conservative but reasonable limit of expansion of the active hazard zone in the next 60-100 years. The landward boundary of the low-risk zone defines the outermost limit of expansion of the active hazard zone in a worst-case scenario. This scenario could be a catastrophic event such as a great earthquake on the Cascadia subduction zone, coupled with severe storms. For example, a Cascadia earthquake would directly cause widespread landsides on steep slopes, while remobilizing existing landslides. Near instantaneous subsidence of the coast by ~3 feet during a Cascadia event would lead to extensive coastwide retreat of dune and bluff backed beaches. Finally, the report identifies all landslides, slide blocks, and earth flow deposits located within about six miles of the coast, and classifies these features as active, potentially active, or prehistoric. For dune-backed beaches the study defines maximum potential erosion distances (MPED) determined from a geometric model developed for dune-backed beaches. Under this model, property erosion occurs when the total water level produced by the combined effect of extreme wave runup (R) plus the tidal elevation (ET), exceeds some critical elevation of the fronting beach, typically the elevation of the beach-dune junction (EJ). Three scenarios were used to model coastal hazard zones on dune-backed beaches: Scenario 1 (HIGH-risk) is analogous to the 2-3 March 1999 La Niña winter storm. This scenario is based on the storm waves occurring over the cycle of an above average high tide, coincident with a 3.3 ft storm surge. Under this scenario, the designated HIGH-risk hazard zone was found to range from 250 to 280 ft, and varies between the four littoral cells due to subtle differences in the beach morphologies among the littoral cells. Scenario 2 (MODERATE-risk) is one of two "worst case" situations in which a severe storm event is coupled with a sea level rise of 1.3 ft. Maximum potential erosion distances (MPED) estimated for the Tillamook coastline under this particular scenario varies considerably, with an average MPED that ranged from 430 to 490 ft. Scenario 3 (LOW-risk) is the second "worst case" scenario, and is the same as scenario 2, but incorporates a 3.3 ft vertical lowering of the coast as a result of a Cascadia subduction zone earthquake. MPED estimated for scenario 3 ranged from 490 to 540 ft. Three complementary erosion hazard scenarios were mapped for bluffs utilizing bluff erosion rates, potential for block failures, and angles of repose for the bluff materials. These three scenarios with similar risk levels to the dune hazard scenarios are: Scenario 1 (HIGH risk) portrays the zone of bluff retreat that would occur if only gradual erosion at a relatively low mean rate were to occur after the slope reaches and maintains its ideal angle of repose. The time interval of erosion was assumed to be 60 years. Scenario 2 (MODERATE risk) portrays an average amount of bluff retreat that would occur from the combined processes of block failures, retreat to an angle of repose, and maximum erosion for ~60-100 years. The zone boundary is derived by placing it halfway between the high and low risk boundaries. Scenario 3 (LOW risk) illustrates a worst case for bluff retreat in ~60-100 years. The zone portrays the bluff retreat that would occur if a bluff suffered a maximum slope failure, then eroded back to its ideal angle of repose and continued to gradually retreat for ~100 years. For bluffs composed of Holocene to Pleistocene dune sand the additional bluff top retreat from rapid response to short-term wave erosion events and potential subaerial erosion is achieved by adding a 50 percent safety factor to erosion calculations and making sure that resulting bluff top retreat corresponds to at least a 50 percent factor safety for the ideal angle of repose (i.e. a 2:1slope). Landslide hazard areas at and near the coast were also mapped. Landslides were discriminated as active, potentially active, or prehistoric. Landslides were also discriminated by type of slope failure: landslide blocks, complex landslides, or earth flows.
Purpose:
To assess existing and potential coastal erosion hazards along the Tillamook County shoreline, extending from Cascade Head in the south to Cape Falcon in the north. Further, to assist County planners in effective decisionmaking adjacent to the shoreline.
Supplemental_Information:
Digital map data files include: Historical shorelines - 1927, 1953, 1955, 1986, 1994, 1997, 1998; Dune hazard data - Neskowin cell beach data, Netarts cell beach data, Rockaway cell beach data, Sand Lake cell beach data; Erosion data - Spot erosion rates; Hazard zones - Active hazard zone, Bluff hazard zones, high dune hazard zone, low dune hazard zone, moderate dune hazard zone; Landslides - Tillamook coastal landslides; Misc. geology -Basalt, Slide block widths Tillamook
Time_Period_of_Content:
Time_Period_Information:
Single_Date/Time:
Calendar_Date: 2001
Currentness_Reference: 20010625
Status:
Progress: In work
Maintenance_and_Update_Frequency: Final report in late 2002
Spatial_Domain:
Bounding_Coordinates:
West_Bounding_Coordinate: -124.03
East_Bounding_Coordinate: -123.919
North_Bounding_Coordinate: 45.8028
South_Bounding_Coordinate: 45.0473
Keywords:
Theme:
Theme_Keyword_Thesaurus: None
Theme_Keyword: Geology
Theme_Keyword: Coastal erosion hazard
Place:
Place_Keyword_Thesaurus: None
Place_Keyword: Oregon
Place_Keyword: Tillamook County shoreline
Stratum:
Stratum_Keyword_Thesaurus: None
Temporal:
Temporal_Keyword_Thesaurus: None
Access_Constraints: None
Use_Constraints:
This regional investigation is not intended for use as a site-specific analysis tool. It can be used to identify areas in need of more detailed site-specific geotechnical studies.
Point_of_Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: Jonathan C. Allan
Contact_Organization:
Oregon Department of Geology and Mineral Industries
Contact_Position: Coastal geomorphologist
Contact_Address:
Address_Type: mailing and physical address
Address: 313 SW 2nd, Suite D
City: Newport
State_or_Province: Oregon
Postal_Code: 97365
Country: USA
Contact_Voice_Telephone: 541 574-6642
Contact_Facsimile_Telephone: 541 265-5241
Browse_Graphic:
Browse_Graphic_File_Name: OFR01-03pages 1-87 and OFR01-03 pages 88-126
Browse_Graphic_File_Description:
PDF representation of explanatory text accompanying digital map data.
Browse_Graphic_File_Type: PDF
Security_Information:
Security_Classification_System: None
Security_Classification: Unclassified
Security_Handling_Description: None
Native_Data_Set_Environment: MapInfo GIS v 6.0
Data_Quality_Information:
Completeness_Report: Preliminary report for Tillamook County shoreline
Positional_Accuracy:
Horizontal_Positional_Accuracy:
Horizontal_Positional_Accuracy_Report:
Horizontal_Positional_Accuracy:
Quantitative_Horizontal_Positional_Accuracy_Assessment:
Horizontal_Positional_Accuracy_Value:
±33 feet for the eastern side of the polygons; ±40 feet for the western side
Horizontal_Positional_Accuracy_Explanation:
Eastern side of polygons: standard error for 1:12,000-scale USGS digital orthophoto quadrangles used for plotting. Western side of polygons: standard error for 1:24,000-scale USGS digital raster quadrangles used for shoreline position. 2.4.2.2.1 Vertical Positional Accuracy Value: Varies depending on contour interval for the respective USGS topographic quadrangle. 2.4.2.2.2 Vertical Positional Accuracy Explanation: This maximum vertical positional error is one contour on the respective USGS quadrangle.
Lineage:
Source_Information:
Source_Citation:
Source_Scale_Denominator: 12000
Type_of_Source_Media: digital orthophoto quadrangle
Source_Time_Period_of_Content:
Process_Step:
Process_Description:
1. Work with local advisory groups to guide the investigation. 2. Obtain appropriate historical aerial photography. 3. Search the literature for hazard data. 4. Map existing open coastal shoreline landslides, classifying them as prehistoric, potentially active, or active. 6. Collect shoreline topographic profiles and other data necessary to estimate the width of a zone of shoreline variability on dune-backed open coastal shorelines. 7. In collaboration with County and technical advisers, establish a methodology for mapping erosion hazard areas. 8. Map the erosion hazard areas. 9. Digitize all line data. 10. Transmit all digital data to County. 11. Provide follow-up advice on use of the maps in policy development.
Process_Date: 1999 - 2000
Spatial_Data_Organization_Information:
Direct_Spatial_Reference_Method: Vector
Point_and_Vector_Object_Information:
SDTS_Terms_Description:
SDTS_Point_and_Vector_Object_Type: G-Polygon
SDTS_Point_and_Vector_Object_Type: Entity point
SDTS_Point_and_Vector_Object_Type: Point
SDTS_Point_and_Vector_Object_Type: Network chain, planar graph
Spatial_Reference_Information:
Horizontal_Coordinate_System_Definition:
Planar:
Grid_Coordinate_System:
Grid_Coordinate_System_Name:
Universal_Transverse_Mercator:
Universal_Transverse_Mercator:
UTM_Zone_Number: 10
Transverse_Mercator:
Longitude_of_Central_Meridian: -123
Latitude_of_Projection_Origin: 0
False_Easting: 500000
False_Northing: 0
Scale_Factor_at_Central_Meridian: .9996
Planar_Coordinate_Information:
Planar_Coordinate_Encoding_Method: coordinate pair
Coordinate_Representation:
Abscissa_Resolution: 1
Ordinate_Resolution: 1
Planar_Distance_Units: meters
Geodetic_Model:
Horizontal_Datum_Name: North American Datum of 1927
Ellipsoid_Name: Clarke 1866
Semi-major_Axis: 6378206.4
Denominator_of_Flattening_Ratio: 294.98
Entity_and_Attribute_Information:
Overview_Description:
Entity_and_Attribute_Overview:
See accompanying text for a discussion and explanation of digital map data attributes.
Entity_and_Attribute_Detail_Citation:
Oregon Department of Geology and Mineral Industries Open File Report O-01-03
Distribution_Information:
Distributor:
Contact_Information:
Contact_Organization_Primary:
Contact_Organization: Nature of the Northwest Info Center (NNWIC)
Contact_Person: Donald Haines
Contact_Position: manager
Contact_Address:
Address_Type: mailing and physical address
Address: 800 NE Oregon Street
City: Portland
State_or_Province: OR
Postal_Code: 97232
Country: USA
Contact_Voice_Telephone: 503/872-2750
Contact_TDD/TTY_Telephone: unavailable
Contact_Facsimile_Telephone: 503/731-4066
Contact_Electronic_Mail_Address: http://www.naturenw.org
Hours_of_Service: 9:00 a.m. to 5:00 p.m. Monday Through Friday
Resource_Description:
Oregon Department of Geology and Mineral Industries Open File Report 01-03
Distribution_Liability:
Although these data have been processed successfully on a computer system at the Oregon Department of Geology and Mineral Industries (DOGAMI), no warranty expressed or implied is made regarding the accuracy or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. This disclaimer applies both to individual use of the data and aggregate use with other data. It is strongly recommended that these data are directly acquired from DOGAMI, and not indirectly through other sources which may have changed the data in some way. It is also strongly recommended that careful attention be paid to the contents of the metadata file associated with these data. The Oregon Department of Geology and Mineral Industries shall not be held liable for improper or incorrect use of the data described and/or contained herein.
Standard_Order_Process:
Digital_Form:
Digital_Transfer_Information:
Format_Name: MapInfo .tab and .mif/.mid
Format_Version_Number: ver. 6.0
Format_Specification: None
Format_Information_Content: MapInfo native and interchange files
File_Decompression_Technique: No compression applied
Digital_Transfer_Option:
Offline_Option:
Offline_Media: CD-ROM
Custom_Order_Process: None
Technical_Prerequisites: None
Metadata_Reference_Information:
Metadata_Date: 20011231
Metadata_Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: Paul Staub
Contact_Organization:
Oregon Department of Geology and Mineral Industries
Contact_Position: Spatial Data Specialist
Contact_Address:
Address_Type: mailing address
Address: Suite 965
Address: 800 NE Oregon Street
City: Portland
State_or_Province: Oregon
Postal_Code: 97232
Country: USA
Contact_Voice_Telephone: 503/731-4100 x224
Contact_Facsimile_Telephone: 503/731-4066
Contact_Electronic_Mail_Address: Paul.Staub@state.or.us
Metadata_Time_Convention:
Metadata_Access_Constraints: None
Metadata_Use_Constraints: None
Metadata_Security_Information:
Metadata_Security_Classification_System: None
Metadata_Security_Classification: Unclassified
Metadata_Security_Handling_Description: None
Generated by mp on Mon Dec 10 10:59:39 2001