A novel approach to discriminate sedimentary characteristics of deltaic tidal flats with terrestrial laser scanner: Results from a case study

Sediments in deltaic tidal flats regulate physical and chemical processes. Grain-size distribution plays an important role in determining sediment dynamics and substrate properties. However, it is challenging to quantify large-scale depositional environments in intertidal flats, due to time-consuming grain-size analyses and sparse sedimentary information extracted from scattered sediment samples. In this study, a novel terrestrial laser scanner (TLS) based method was developed to characterize the substrate of an intertidal flat. Surface sediment samples in the Nanhui flats in the Yangtze Delta, China, and the corresponding waveform amplitudes of TLS echoes at fixed sampling sites were collected for a total of 22 months. A negative logarithmic relationship was found between the sediment sand fraction, average grain size, D-50, and corrected waveform amplitude of TLS echo in different hydro-meteorological conditions. The mean of average grain size of five sediment sampling sites along a transect was 58.78 mu m when measured by traditional grain-size analysis, and 49.48 mu m when calculated with the proposed logarithmic equation. The mean error at each site was up to 21.77%. The mean error for the sand and silt fraction at each location was as high as 27.28% and 21.75%, respectively. The spatial distribution pattern of TLS-based average grain size in the entire study area was consistent with the measured pattern with a Root Mean Square Error of 13.83 mu m. These errors could be caused by the accuracy of the TLS waveform amplitude correction and by limits of the method in recognizing different substrates. The effects produced by the presence of microphytobenthos (for example, cyanobacterial mats or diatom biofilms) or bedforms have not been investigated and may have affected the results. The TLS-based grain-size measurements can rapidly and effectively discriminate sediment characteristics, thus avoiding traditional time-consuming measurements. It is expected that the TLS-based method proposed here will have wide applications in shoreline studies, especially in inaccessible tidal flats.