Tidal inlets dynamics and environment

The procedure developed during the project to analyse LIDAR data aims at providing estimates of low vegetation, namely lower than the smallest observable difference between the first and the last detected return from the same location on ground. The procedure relies on the principle that the waveform of the backscatter signal depends on the vertical distribution of leaves within the observed vegetation canopy. The estimated travel time of the laser pulse will then be slightly different according to vegetation height. Local statistics of a sample of LIDAR measurements provide information on canopy architecture and vegetation height. The procedure as implemented can be applied to samples of 50 to 100 individual laser measurements (raw data) constructed by using a polygon of arbitrary shape or a short segment of a linear sequence of laser measurements. In either case the reference points need to be geo-located. The frequency distribution of laser measurements in each sample provides information in different ways: difference between maximum observed elevation and ground elevation determined with GPS observations;-difference between maximum and minimum observed elevation;-standard deviation of elevation values. The procedure has been validated in two ways: A.For a set of reference polygons, GPS observations provided the elevation of points along the contour line of the polygon. The mean and maximum GPS elevation was in good agreement with minimum LIDAR elevation in the sample. B. A simple simulation model was applied to estimate the expected laser above ground elevation of a vegetation canopy, given the vertical distribution of leaves within the canopy. The mean values of observed elevation for all reference polygons were in good agreement with simulated elevation, assuming a uniform leaf distribution between the top of plants and the ground surface. The procedure can also be applied to refine maps of ground elevation by re-sampling a map of uncorrected elevation data, with the corrected elevation set equal to the minimum elevation in a sample of uncorrected data. Samples are constructed by running a box filter through the uncorrected data.

The model describes the hydrodynamic flow field in an intertidal area by suitably simplifying the shallow-water equations by equating the gradients of the water surface to the (linearized) friction terms. The product is a poisson-like equation, which uses the position of the channel network as a boundary condition to compute a representative water elevation field. This field allows the computation of flow directions and velocities, together with the values of the bottom shear stress. This tool can be used to map the areas of pertinence of relevant sub-network of tidal channels of interest, and to relate different channel morphologies to the properties of the pertinence area identified. Local authorities in Venice have expressed interest in the tool to obtain characterizations of areas within the Venice Lagoon.

The small-scale spatial variations in the erosion threshold of exposed subtidal and intertidal sediments can be measured using the cohesive strength meter (CSM) (Tolhurst 1999). The CSM is comprised of an on board computer, water reservoir, filter assembly, digital and analogue pressure gauges, optical sensor head, and first stage diving cylinder with hose attached. The chamber (diameter 30mm) is placed onto the sediment surface and secured using a clamp stand and filled with water. A jet of water is directed towards the sediment surface within the chamber from a height of 2cm above the sediment surface. The velocity of the water jet is increased systematically with each jet over a period of time (dependent on which test protocol is selected). A drop in transmission of the infrared light, caused by the suspension of sediment across the chamber, determines the erosion threshold. Data is recorded directly to an onboard central processing unit and can then be downloaded to a PC for further analysis. However, a new version of the dive was required for salt-march studies since the erosion resistance of march sediment can be high. A high pressure CSM system was therefore produced (Sediment Services, SME) but this required a new calibration since the old calibration was only relevant to the low-pressure system.

This result is a technique for the semi-automatic extraction of tidal channel networks from airborne scanning laser altimetry (LiDAR) and high-resolution aerial photography acquired at low tide. The main end-user would be a geomorphologist researching tidal networks. The study of tidal network morphology is important because of the role networks play in tidal propagation and in the evolution of salt marshes and tidal flats. The conventional method of measuring tidal channels is cumbersome and subjective, involving digitising an aerial photograph of the channels and doing field measurement of selected channels. However, LiDAR possesses sufficient spatial resolution to detect even the smallest channels. As LiDAR data are usually collected in conjunction with aerial images, and as aerial photos of inter-tidal zones are presently much more common than LiDAR data, the technique works using either LiDAR data or digital aerial photography alone or with both data sources combined. The algorithm was applied to data gathered at the three study sites used in the TIDE project. The best channel extractions were achieved using LiDAR data. Provided that networks were not too ambiguous, it proved easier to correct the automatically extracted networks than to digitise them manually.

The result consists in a set of methods and algorithms for the selection of remote sensors and data attributes and for performing classification of the resulting data to obtain accurate vegetation maps in intertidal environments. The vegetation maps derived should be used to monitor the state and trends of a tidal environment and to evaluate the possible impacts of human activities or of climatic changes. Local authorities responsible for monitoring the Venice Lagoon have been directly involved in the development of the tools and will presumably use them on a systematic basis for policy-support purposes.

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