High-Resolution Coastal Bluff Monitoring through Crowd-Sourced SfM

Abstract:

Coastal bluffs are highly dynamic in space and time, changing weekly, monthly, or annually. Advances in photogrammetry software now enables researchers and managers to process oblique and near-nadir photos into dense elevation point clouds and elevation models for coastal monitoring; however, accurately measuring change over time often requires photos are geotagged with a differential GPS location. This work presents a time-transgressive approach to monitoring coastal bluff change that is not dependent on high-precision GPS or traditional field surveys. A series of photos were collected from the beach and another series of photos were collected from a boat along a 1.5 km section of bluff adjacent to the Elwha River delta in Washington state. Photos were processed in 4D and used to monitor changes in coastal bluff morphology from 2016 to 2020, with either a beach or boat photo set collected close to every month. Mass wasting events were common before early spring 2017, shortly after the Elwha Dam was removed and sediment began building a beach and delta. The amount of vegetation on the bluff face and talus accumulation at the base is also greater beginning in 2017, suggesting the role of marine erosion has been reduced since the beach has accreted. Results from the bluffs near the Elwha River delta demonstrate the potential to monitor and accurately measure changes in coastal cliff morphodynamics from crowd-sourced data without ground-control points or precision GPS.