Video-based nearshore bathymetry estimation in macro-tidal environments

Abstract

Video-based depth inversion through the linear dispersion relation for free surface waves using the cross spectral correlation analysis, cBathy (Holman et al., 2013), is applied for the first time in a highly energetic macro-tidal environment in the South West of England at Porthtowan. This application of cBathy reveals two main issues: 1) inaccurate depth estimations on inter camera boundaries when multiple cameras are used and 2) significantly less accurate depth estimates over the whole domain during spring tide compared to neap tide (inaccuracies of around 35% of the local depths are found during spring tide). These two issues are not only important in macro-tidal environments: the camera boundary issue has been reported in numerous video-camera sites and the deviation in accuracy during tidal levels is a function of the tidal range in combination of the vertical camera position. To overcome the two issues, a camera boundary solution and a floating pixel solution (meaning moving pixels in a horizontal plane as function of the tidal elevation) are proposed here. With the modifications, cBathy is capable of estimating depths in the sub-tidal zone with an accuracy in the order of 10% of the local depth irrespective to the local tidal regime. However, for the very upper part of the beach face less accurate results are found due to the reduced validity of the linear dispersion relation in that region due to the non-linear behaviour of breaking waves and wave-current interactions. The improvements persist across all bathymetry survey campaigns at Porthtowan and when compared to other well known Argus video-system sites the importance of the floating pixels is apparent.