Measuring Coastal Bluff Recession Rates Throughout the Puget Sound Region

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Bluff erosion rate.jpg

This project, led by Coastal Geologic Services, aimed to better understand the range and drivers of coastal bluff erosion in the Puget Sound region. Coastal bluff recession supplies the majority of sediment on Puget Sound beaches (Keuler 1988, Finlayson 2006). However, there is considerable variability in bluff form, geology, stratigraphy, wave exposure, landslides, and erosion (recession) rates along the 660+ miles of bluff shore. Very little research has analyzed the variability and range of erosion rates (Shipman 2004, 1995). Most of the bluff recession rates measured in the region are from the most rapidly eroding bluffs, with far fewer documented rates from the more slowly eroding bluffs in central and southern Puget Sound. These slowly eroding bluffs account for considerable lengths of Puget Sound shores. A common disconnect between the perceived and actual need for shore protection (armor) due to coastal erosion has been documented among Puget Sound coastal landowners. Measuring bluff recession from only these most rapidly eroding bluffs simply compounds this erroneous perception. Better understanding of the range of bluff erosion rates can also aid in the development of capital projects that aim to restore beach systems. Regional bluff erosion rates have been used by CGS to identify feasible feeder bluff armor removal sites and to assess sea level rise vulnerability in San Juan County. Land use decisions, including setback distances and the installation of shore armor, rarely reflect regional variability in bluff recession rates. This project will increase understanding of the range and dominant drivers of coastal bluff erosion rates throughout the Puget Sound region to inform management, regulations, restoration, and ultimately will aid in preserving feeder bluff function.

The hypothesis for this proposed project is that building and analyzing a database of historical bluff erosion rates throughout the Sound and identifying the dominant drivers of bluff recession will advance understanding and inform management, restoration, research, and preservation. The results of these analyses can also be used to justify better management decisions (such as setback distances) and, importantly, prevent shore armor where unnecessary by allowing for informed application of risk measures in the Marine Shoreline Design Guidelines (Johannessen et al. 2014). These data can be used to direct capital project policy relating to site selection, feasibility, restoration, and protection. For a recent project, CGS paired setback distance data with extrapolated bluff recession data from 39 bluffs in Island and Jefferson Counties to screen out infeasible armor removal projects. The same erosion rate data was used together with bluff height data (from LIDAR) to quantify the potentially restored sediment volume resulting from restoration. These data could also be used to help determine whether relocating a road or other infrastructure is a preferred solution due to ongoing threat of erosion. This information can be used to make better long-term investments and informed management decisions of Puget Sound bluffs.

Project Tasks[edit]

The proposed project consists of several successive elements, the details of which are described below:

Compile a Geodatabase of (Historical) Bluff Recession Rates in Puget Sound[edit]

A geodatabase of historical bluff recession rates will be compiled. Historical bluff erosion rates are typically measured in GIS using historical aerial photography; however, there are several other techniques applied in a previously published compilation by Keuler (1988). The approaches used by Keuler are more appropriate for slowly eroding bluffs. The project geodatabase will include selected shoreforms from the Puget Sound Feeder Bluff Mapping geodatabase (MacLennan et al. 2013a), with supporting attributes including method, accuracy, period of analysis, and source. Other information will be included, such as fetch, orientation, presence of armor, shoretype, geology, and stratigraphy (where available).

Measure Historical Recession Rates from 60–100 Additional Puget Sound Coastal Bluffs[edit]

Power analysis will be performed to identify where and how many additional erosion rates should be measured to ensure accurate representation from the range of conditions in the region. Additional bluff erosion rates will be required in south and west Puget Sound, as well as in south Hood Canal. CGS will determine the number of required bluffs with various combinations of stratification variables (exposure, shore orientation, bluff height, and geology) to represent the range of conditions displayed in Puget Sound bluffs and produce analytical results with adequate statistical power. The bluffs that are chosen for measurement of erosion rates will adhere to selection criteria to avoid potential interference. For example, bluffs should not include anomalous conditions such as deep-seated landslides, recent deforestation, or other dramatic examples of human-induced erosion. New bluff recession rates will be measured in the field and using historical vertical aerial photography in GIS. Both measurement methods are well-accepted methods used in spatial analysis of coastal geomorphology.

Collect Stratigraphy Data from a Sample of Bluffs[edit]

Bluff stratigraphy varies considerably across Puget Sound bluffs and has been identified as relevant to bluff recession rates. Published bluff stratigraphy data is typically limited to profiles in the original Coastal Zone Atlas of Washington and the Washington Division of Geology and Earth Resources 1:24,000 geology maps. Unpublished data from projects underway as well as existing data from previous assessments will be compiled to augment data produced by the state. Similar bluff toe elevation data will also be compiled and analyzed for a subset of bluffs.

Spatial and Statistical Analysis of Recession Rates[edit]

Bluff recession rates will be analyzed in relation to other variables of influence, including shoreform, geology, orientation, and exposure. Stratigraphy, storm event frequency, and bluff toe elevation will be analyzed further from a subset of bluffs. The specific analytical tools will be determined after consultation with technical advisors and after preliminary data analysis.

Report and Presentation of Results[edit]

The final tasks entail reporting and presenting on the methods, analytical techniques, and results from this project. The draft report will be delivered with a comment matrix compilation of all reviewer comments. These comments will be reviewed and addressed as the draft report is finalized. Similarly, CGS staff will present on the overall project, highlighting the methods, analysis, results, and associated recommendations from the project to ESRP and agency staff.