ABSTRACT

An Unmanned Aerial Vehicle (UAV)-based system was developed to acquire a digital elevation model (DEM) of exposed tidal flats based on the structure from motion (SfM) algorithm. Global Position System-Real Time Kinematic (GPS-RTK) was utilized to measure the underwater portion, consisting of 111 points in 16 cross sections. Then entire 3-dimensional topography of tidal creek was acquired by combining these two approaches. This method can be used for detection and spatial analyses of tidal creeks and can provide accurate insights into the processes related to natural and/or human-related development of tidal creeks.

INTRODUCTION

Characterized by large width and gentle slope, the muddy intertidal zone is widely distributed in the world (Zhang et al., 2016). The landward part of the muddy flats is covered primarily by halophytic vegetation which are regularly submerged during high tide, while the seaward part is subject to bare flats with the elevation between the low and high tidal water levels. As a sensitive area of land-sea dynamic interaction, tidal flats have important ecological function, coastal protection and socio-economic value (Chen et al., 2017). Tidal creeks are an important geomorphic unit in tidal flats systems, and are formed by ocean dynamics, especially by tidal actions (Teal, 1962). Distribution, spatial and temporal variations and geometric characteristics of tidal creeks control the flow, nutrients, deposition rate flux and biological growth in tidal flat areas (Lerberg et al., 2000; Mallin & Lewitus, 2004). It is an important link to study the response of tidal flat system to environmental change to grasp the regularity of the tidal creeks morphological change.

Although field observations are laborious in the intertidal mudflats (Gong et al., 2017), they are a formidable tool for improving our understanding of tidal flat evolution and the validation of mathematical and numerical models. Compared with traditional manual measurement, remote sensing is a relatively easy way to obtain field data. At present, remote sensing is the main method to study the morphological changes of tidal creeks, but it is limited to the planimetric morphology of tidal creeks (Fagherazzi et al., 1999; Huang, 2004). In-situ field observation has high-precision but difficult to realize since the muddy flats are hard to walk. Existing methods applied to investigate geomorphologic features of tidal creeks are commonly based on physical experiments (Kleinhans et al., 2012; Stefanon et al., 2010; Tambroni et al., 2005) and/or numerical models (Horton, 1945; Xu et al., 2017; Zhou & Coco et al., 2014; Zhou & Olabarrieta et al., 2014). However, the entire 3dimensional topography of tidal creeks including both exposed and submerged portion after the ebb, has not yet obtained by previous study. On one hand the underwater topography cannot be obtained by remote sensing, on the other hand the water depth in the tidal creek sometimes is too shallow because of ebb to use ship-borne echo-sounder measurement. Therefore, the overall morphological feature and the ontogeny of tidal creeks remain elusive.

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