Assessing Geomorphic Change in Restored Coastal Dune Ecosystems Using a Multi-Platform Aerial Approach

Abstract

Uncrewed aerial systems (UAS) provide an effective method to examine geomorphic and vegetation change in restored coastal dune ecosystems. Coupling structure-from-motion (SfM) photogrammetry with RGB orthomosaic imagery allows researchers to characterize spatial-temporal geomorphic responses associated with differences in vegetation cover. Such approaches provide quantitative data on landscape morphodynamics and sediment erosion and deposition responses that allow scientists and land managers to assess the efficacy of dynamic restoration efforts and, in turn, make informed decisions for future restoration projects. Two different restored coastal foredune sites in Humboldt County, California were monitored between 2016–20 with UAS (quadcopter and fixed-wing), kite aerial photogrammetry (KAP), and terrestrial laser scanning (TLS) platforms. We compared our KAP- and UAS-SfM elevation models to concurrently collected TLS bare earth models for five of our fifteen collections. The goal of this study was to inform on the potential of a multi-platform aerial approach for calculating geomorphic differences (i.e., topographic differencing), in order to quantify sediment erosion and deposition, and vegetation change over a coastal dune ecosystem. While UAS-SfM datasets were relatively well fit to their TLS counterparts (2.1–12.2% area of difference), the KAP-SfM surfaces exhibited higher deviations (23.6–27.6%) and suffered from systematic collection inconsistencies related to methods and susceptibility to external factors (e.g., the influence of wind speed and direction on variable altitude, image overlap, and coverage extent). Finally, we provide commentary on the logistical considerations regarding KAP and UAS data collection and the construction of uncertainty budgets for geomorphic change detection (GCD), while providing suggestions for standardizing methods for uncertainty budgeting. While we propose an approach that incorporates multiple levels of collection- and processing-based uncertainty, we also recognize that uncertainty is often project-specific and outline the development of potential standards for incorporating uncertainty budgets in SfM projects.

Photogrammetry as a tool to improve ecosystem restoration

“…However, unlike the 3D mesh, the DEM will only be 2.5D, having lost any information on overhanging structures. restoration [10]. In the marine realm, applications of 3D technology have been largely restricted to aerial photogrammetry in intertidal coastal settings [11], including vegetation mapping in wetlands and estuaries [12,13] and the monitoring of restored oyster reefs [14] (Box 2).…”

“…For example, 3D models of tree canopies can be used to monitor growth trends of individual trees or of entire forest patches [7]. They can help assess geomorphic (e.g., spatial variation in sediment erosion and deposition) and vegetation change (e.g., cover, shape of patches, and invasive species) in river [8] and coastal dune restoration and link patterns to potential drivers, such as wind and wave regime [10]. Large-area 3D mapping thus provides maps and geospatial information needed to assess ecosystem dynamics (e.g., predator-prey relationships; invasive species) and recovery trajectories of entire ecological communities, key to assessing the success of restoration interventions in any ecosystem.…”

Ammophila Invasion Ecology and Dune Restoration on the West Coast of North America

“…Davidson-Arnott et al [126] modeled dune-lined sandy shorelines and predict that under an equilibrium sediment budget, the upper beach and foredune will migrate in tandem with sea level rise. However, in a sea level rise adaptation demonstration site in northern California, the A. arenaria foredune scarp, which was vertical and tightly bound by rhizomes, resisted ramp building much longer than the native foredune [127], which quickly underwent slumping that contributed to ramp building. This behavior is consistent with Davidson-Arnott's [3] model of migration of the foredune morphology inland.…”

“…The project objectives were to re-establish dune process, remove Ammophila spp., and create habitat for rare plants. An experiment is underway at the Lanphere Dunes in northern California which encompasses manual and burning/herbicide removal of A. arenaria, revegetation with native species, and spatio-temporal eco-geomorphic monitoring [127]. This project is part of a six-year collaboration between a land management agency (U.S.…”

Using High-Spatial Resolution UAV-Derived Data to Evaluate Vegetation and Geomorphological Changes on a Dune Field Involved in a Restoration Endeavour

“…This can be fundamental to dune systems study, because it allows researcher to perform multi-scale analyses, that are crucial to understanding the complex connection between dynamic factors influencing dune fields [32]. Among RS techniques, the most appropriate for usage in dune environment are: (i) Laser Scanning (LS), airborne or terrestrial, especially for high-resolution monitoring of linear-shaped morphologies [33,34]; (ii) Structure from Motion (SfM) photogrammetry, usually from Unmanned Aerial Vehicles (UAV) [35][36][37][38][39][40]. Sediment distribution related to vegetation types on coastal dunes can be investigated combining airborne multispectral and LiDAR dataset [41], but low cost UAVs have several advantages, being usually light-weight devices, that are easily transported and very fast performing.…”