Monitoring Discovery Park Bluff Retreat

Coastal erosion can be a risk to public safety, infrastructure, and coastal homes. Bluff erosion results from a combination of various natural processes. These processes include varying climate, tectonic activity, changes in eustatic sea level, degree of wave erosion, and the properties of the material making up the slopes. Bluff retreat is one of several geologic hazards in the Puget Lowland.The North and South Beach bluffs of Discovery Park are actively retreating. With increased erosion rates expected due to the effects of climate change, the stability of these bluffs is further threatened. The Friends of Discovery Park sought to understand the magnitude and rate of bluff recession and erosion to help manage trails and views, to better serve the public, and to provide support to the Seattle Parks Department.

In collaboration with The Friends of Discovery Park, a study was initiated to monitor the long-term coastal erosion and recession. The monitoring plan consists of 3 elements: measuring the rate of retreat, conducting a photo survey, and installing monitoring benchmarks. The intent of this project is to repeat measurements over the next 50 years to monitor change in the bluff position and document retreat rates. Better understanding the rate and factors that influence bluff retreat helps in assessing the impact of future retreat on Discovery Park.

The most common geologic units in Seattle are exposed on the bluffs of Discovery Park. At the South Beach bluff of Discovery Park, Lawton Clay, Esperance Sand, and Vashon till overlie the Olympia beds. The Lawton Clay, Esperance Sand, and Vashon till sediments are from the last glacial advancement on the Puget Lowland. The Olympia beds are alluvial sediments from the last interglacial period. At the North Beach bluff only Vashon till and Lawton Clay are exposed.

Bluff erosion at Discovery Park is caused by various slope failure mechanisms. These failures are caused by wave action undercutting bluffs, wind direction and strength (which affects the waves), and groundwater at the contact between the impermeable Lawton Clay and the overlying permeable Esperance Sand.

Wave action undercuts the base of the bluff. As this undercutting continues, portions of the bluff become unstable causing it to fail. Wind action, in addition to increasing the strength of the waves, causes slow progressive erosion of the bluff face. Groundwater action at the contact between the Esperance Sand and underlying Lawton Clay causes sliding at that contact.

The underlying stratigraphy plays a large role in the mode of slope failure. The common mode of failure on the South Beach bluff is through soil creep that leads to soil sloughing. On the other hand, the North Beach bluff tends to fail through more deep-seated rotational failures. According to Tubbs and Dunne (1977), the South Beach bluff of Discovery Park experiences 3 times more erosion than the North Beach bluff, which is largely due to the direction and intensity of the wind and its impact on wave action.

The following section provides evidence of slope instability observed along the bluffs of Discovery Park.

Overhanging Material is the result of wave action undercutting bluffs. Wave induced erosion removes sediment from the base of the bluff. This results in undermining and destabilizing of the bluff, causing portions of the bluff to fail. Wave induced erosion will likely increase bluff erosion rates in the future as sea level rises.

Debris cones are present along the North and South beaches. Debris cones are caused by wind and water and they result in material eroding from the upper portions of the bluff and raveling down the slope and gathering at its base.

There are many instances where groundwater seepage is observed at the contact of the permeable Esperance Sand and impermeable Lawton Clay. Groundwater seepage decreases slope stability by causing sliding of the overlying material. In the image on the left, we have water seeping over the exposure of the Lawton clay unit. On the right is water draining over a landslide deposit along the south beach. 

Disturbed vegetation is an indicator of slope movement. Some evidence of disturbed vegetation include pistol butted trees, dying trees, exposed slopes, and stretched plant roots. Disturbed vegetation can be observed throughout the bluffs of Discovery Park. 

Estimating Bluff Retreat Rate

The first element of the study focused on obtaining a rate of erosion through spatial imagery analysis using GIS (Geographic Information Systems). Based on the GIS analysis, the bluff retreat ranges from 0.7 ft/year to 1.6 ft/year in the short term to 0.17 ft/year over the long-term. These values are consistent with Puget Sound bluff retreat findings by others in the area. With sea level rise, this rate could increase by as much as 3 times.

Installing and Measuring Monitoring Benchmarks

Installed benchmarks allow the monitoring of the buff crest position over the next several decades. Benchmarks were installed at two distances from the bluff crest at six actively eroding areas around the park. At each of the six locations, two benchmarks were installed. As shown below, one benchmark was installed 20 feet from the bluff crest and another was installed 50 feet from the bluff crest. Directly after benchmark installation, initial distance measurements from the bluff crest were taken. Benchmark measurements will be repeated twice a year for the next 50 years to monitor bluff crest position.

Repeat Photo Surveying

A photo survey of the North and South Beach bluffs was conducted. In the future, these photos will be used for comparison to similar photos in future years. The purpose of photo surveying is to monitor bluff erosion and vegetation conditions and to be able to see where and how the land is changing over time. 

It is recommended that the bluff be continually monitored in order to constrain the magnitude of retreat and identify highly actively eroding areas.

The information gathered in this study will assist the Friends of Discovery Park and the Seattle Parks Department to better revise trails and park infrastructure.