Abstract
Tidal processes regulating sediment accretion rates and vegetated platform erosion in tidal systems strongly affect salt marsh evolution. A balance between erosion and deposition in a restored salt marsh is crucial for analyzing restoration strategies to be adopted within a natural context. Marsh morphology is also coupled with tidal mudflats and channel networks and this makes micro-tidal systems crucial for a detailed assessment of restoration interventions. Here, we present a methodological approach for monitoring channel morphodynamics and vegetation variations over a time frame of six years in a low tidal energy salt marsh of the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (Maryland, USA). The project is a restoration site where sediment dredged from the shipping channels in the upper Chesapeake Bay is used to restore a tidal marsh habitat in mid-Chesapeake Bay. Aerial surveys with an Unmanned Aerial Vehicle (UAV) have been performed for the high-resolution mapping of a small tidal system. Flight missions were planned to obtain a Ground Sample Distance (GSD) of 2 cm. Structure-from-Motion (SfM) and Multi-View-Stereo (MVS) algorithms have been used to reconstruct the 3D geometry of the site. The mapping of channel morphology and an elevation assessment on the mudflat were performed using orthomosaics, Digital Terrain Models (DTMs) and GNSS survey. The results highlight that the workflow adopted in this pilot work is suitable to assess the geomorphological evolution over time in a micro-tidal system. However, issues were encountered for salt marsh due to the presence of dense vegetation. The UAV-based photogrammetry approach with GNSS RTK ground surveys can hence be replicated in similar sites all over the world to evaluate restoration interventions and to develop new strategies for a better management of existing shorelines.
Highlights
We present the use of Unmanned Aerial Vehicle (UAV) photogrammetry for assessing the behavior of a micro-tidal system in terms of evolution, with regard to the restored salt marsh, the mudflat and the channel network
In order to achieve this result, we tested a methodological approach to collect geospatial information about the cell morphology: acquiring aerial imagery from UAV to evaluate the distribution of the vegetation and to generate Digital Terrain Models (DTMs) of the emerged areas, performing a GNSS RTK survey to integrate missing data in submerged channel areas and to collect the coordinates of Ground Control Points (GCPs)
This work highlights how the use of UAV+SfM-MVS and GNSS RTK surveying allows the monitoring over time of a micro-tidal system
Summary
Contributes in mitigating erosion of the most impacted areas. Such erosion is mainly caused by a high shear stress on the bottom of un-vegetated tidal creeks, often associated with extreme events. The friction of dense vegetation reduces erosion, when the tides are elevated, and the drag created by 2 m-tall marsh plants can be critical in slowing currents [9]. The factors mentioned above impact on the capability of vegetated salt marshes to survive climate change [10,11,12]. Data collected by aerial flight missions and GNSS RTK was compared to the initial status of C–1B (in 2012) provided by the MES. The initial profile of a total of six cross-sections (see Figure 5) was reconstructed using the elevation data.
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