The use of small-Unmanned Aerial Systems for high resolution analysis for intertidal wetland restoration schemes

Abstract

Coastal and estuarine wetlands provide a range of important ecosystem services, but are currently being damaged and degraded due to human activities, reduced sediment supply and sea level rise. Managed realignment (MR) is one approach used to compensate for the loss of intertidal habitat, however saltmarshes in MR sites have been recognised to have lower biodiversity than natural environments. This has been associated with differences in the physical functioning including the sediment structure, reduced hydraulic connectivity, and lower topographic variability such as the abundance of intertidal creek networks. Intertidal morphology, including creek networks, play an important role in supporting and regulating saltmarsh environments through the supply of sediment, nutrients and water, and in draining intertidal marshes. However, there is a lack of empirical data on the formation and evolution of topographic features and variability in saltmarsh environments. This is likely to be due to creek networks in natural marshes already being in a state of quasi-equilibrium, making MR sites an ideal environment to investigate creek development. However, traditional remote sensing techniques (such as LiDAR) tend to be relatively expensive, infrequent and at a coarse resolution meaning small, but important (cm-scale), changes are often missed. This study advances the ability to detect these small scale changes by demonstrating the suitability and potential applications of using the emerging photogrammetric method Structure-from-Motion (SfM) on images taken using a small-Unmanned Aerial System (sUAS). Three surveys from a rapidly changing, near-breach site were taken at the Medmerry Managed Realignment Site in July 2016, September 2017 and July 2018. A suitable degree of confidence was found between the modelled surface and independent check points (vertical root-mean-square-errors of 0.0245, 0.0704 and 0.1571 for 2016, 2017 and 2018 respectively). DSMs of Difference (DoD) analysis was performed to evaluate elevation change, with areas experiencing up to 85 cm of accretion between 2016 and 2018. However, when considering the error associated with both surveys, between 2016 and 2017, only 34.39 % of the survey area experienced change above the level of detection (LoD). In contrast, 76.97 % experienced change greater than the LoD between 2017 and 2018. Stream order analysis classified the creek networks into five orders in 2016 and four orders in 2017 and 2018, with 2016 having a higher abundance (291 in 2016 compared to 117 (2017) and 112 (2018)) and density (0.44 m/m 2 in 2016 compared to 0.27 m/m 2 in both 2017 and 2018) of creek networks. These results provide an innovative high resolution insight into the evolution of restored intertidal wetlands, and suggest that SfM analysis of images taken using a sUAS can be a useful tool with the potential to be incorporated into studies of MR and natural saltmarsh sites. sUAS analysis can, therefore, advance the management of these environments to ensure the provision of ecosystem se...

A Review of Unoccupied Aerial Vehicle Use in Wetland Applications: Emerging Opportunities in Approach, Technology, and Data

“…Although restoration monitoring is a vital component of ecological management, it is rarely prioritized due to time and personnel constraints associated with ground surveys, lags between project implementation and data analysis, and the need for high-resolution data [26,120]. UAVs are tools that enable restoration assessment at the whole-site levels and allow for cost-effective change analysis of restored environments [8,9,121,122] due to their flexibility in flight times and heights and lack of disturbance to sensitive restoration areas [56]. Furthermore, repeat aerial surveys facilitate adaptive management techniques [118] of iteratively examining and modifying projects, and this continual decision-making capability helps ensure long-term success of restoration projects.…”

“…UAV-based restoration monitoring involves a variety of objectives, including biomass estimates [120], hydrological mapping [8,9,11,123], morphological evaluations [43,57], and classification of plant communities in peatlands [10,56,119,122] and coastal regions [44,117,118,121]. While many researchers leverage RGB aerial imagery for restoration monitoring [8,9,120,122,123], multispectral imagery, and specifically the near-infrared wavelength, which can detect species and topography in shallow wetland environments [155]. This is particularly useful in invasive species identification [121] and peatland biodiversity assessments [56] at restoration sites.…”

“…Additionally, thermal aerial imagery can be used to locate groundwater discharge in restored peatlands, greatly reducing time spent conducting ground-based seep detection studies [11]. Vertical takeoff and landing in constricted or topographicallycomplex areas generates popularity of quadcopter UAV usage for small restoration study regions [8][9][10][11]120,122], while fixed-wing UAVs are preferred when mapping larger restoration projects [56,118,121,123]. UAVs prove to be efficient tools to track human-induced ecological modifications [9,11,44,118,122,123], and cheaper and lighter multispectral sensors for quadcopter, and fixed-wing UAVs will help advance future restoration assessments and analysis.…”

“…UAVs are beneficial to environmental monitoring because they bridge the constraints in complex, dynamic, limited-access environments that historically have been challenging to survey [3]. Furthermore, they reduce the amount of time and labor expended on surveying and sampling on the ground [4][5][6][7], providing time for targeted managerial activities that may otherwise be overlooked, such as restoration assessments [8][9][10][11]. In recent years, technological advances in UAV instruments such as greater spectral complexity via LiDAR and multispectral sensors and increased volumetric estimations from Structure from Motion (SfM) photogrammetric techniques have transformed applications of UAVs in environmental research [12].…”

The Use of Unmanned Aerial Vehicles to Determine Differences in Vegetation Cover: A Tool for Monitoring Coastal Wetland Restoration Schemes

“…In recent years, unmanned aerial vehicles (UAVs) have become increasingly popular as a technique to measure large (site) scale morphological change in high resolution [15][16][17], facilitated by advancements in UAV technology, compatible sensors, and computer software [18,19]. Within coastal wetland environments, UAV technologies are becoming increasingly relevant for assessments of morphological variability [10,20] and ecological functioning [21,22], replacing lower resolution surveying techniques such as LiDAR [9,23,24]. This study explores the potential to employ UAV technologies as a possible solution in order to assess saltmarsh coverage, and the relationship between vegetation cover and morphology, in MR sites.…”