Remote sensing and modelling of suspended sediment concentration in shallow tidal areas

The existence and dynamics of coastal lagoons and estuaries are decisively influenced by non-linear interactions between biological and physical processes. Understanding and monitoring such interactions require observations over a wide range of spatial scales (indicatively 1 m-10 km), which cannot be obtained through traditional in situ point observations. Remote sensing techniques can overcome these spatial limitations and potentially allow access to the desired range of scales, and can be applied both for a better understanding of the relevant bio-geomorphological processes and for the calibration and validation of spatially-distributed hydrodynamic and transport models. Suspended Particulate Matter (SPM) concentration is one of the key properties to be observed when looking at the sediment transport shaping the coastal landscape, but it is difficult to determine, particularly in a space-distributed manner. Remote sensing estimates of SPM concentration in lagoon/estuarine waters, however, are not trivial, due to the very low water depth and to the overlapping signal associated with bottom reflectance. We apply here a simplified radiative transfer model to multispectral data from different sensors (LANDSAT, ASTER and ALOS AVNIR) designed to overcome these limitations. The model, calibrated and validated with observations from a network of turbidity sensors in the Venice lagoon, is used to produce SPM concentration maps which are then compared with results from a hydrodynamic and sediment transport model, which is also properly calibrated using a large number of observations within the domain. The differences and similarities emerging from the comparison of the SPM maps produced by satellite images and modelling make it possible to identify the magnitude of the stabilizing effect of benthic vegetation, which could previously be addressed only at the laboratory scale. The analyses allow to quantify such a stabilizing effect and its parameterization within the sediment transport model. The consistency between remotely-sensed and modelled SPM concentration distributions, and their coherence with known features in the Venice lagoon point to the important role played by benthic vegetation in the stabilization of the bottom sediment at the scale of the entire system suggesting that biostabilization be a major control of the overall bio-geomorphic evolution of intertidal areas.