Journal of Coastal Research, Special Issue No. 65, 2013 Intertidal Bathymetry Estimation Using Video Images on a Dissipative Beach 1439 Intertidal Bathymetry Estimation Using Video Images on a Dissipative Beach Fernando Sobral†, Pedro Pereira‡, Pablo Cavalcanti∞, Rafael Guedes§, Lauro Calliari† †LOG, Instituto de Oceanografia, Universidade federal do Rio Grande, Av. Itália, s/n, km 08, Bairro Carreiros, Rio Grande, RS, 96203-900, BR. fcalmon.sobral@gmail.com lcalliari@log.furg.br §Department of Earth and Ocean Sciences, University of Waikato, Private Bag 3105, Hamilton 3240, NZ. rmcg1@waikato.ac.nz ‡ Centro de Tecnologia e Geociências, Departamento de Oceanografia, Universidade Federal de Pernambuco, Av. Arquitetura, s/n, Cidade Universitária, Recife, PE, 50740-550, BR. psppraias@gmail.com ∞Instituto de Informática, Universidade Federal do Rio Grande do Sul, Caixa Postal 15064, Porto Alegre, RS, 91501- 970, BR. pgcavalcanti@gmail.com INTRODUCTION The nearshore zone dynamism involves many processes acting at the same time and interacting with each other, making its study complex. Progress in coastal understanding has been achieved with physical measurements under natural conditions. However, the task to deploy instruments at this energetic and hostile environment is not easy, requiring a careful planning and being costly. A solution to that problems is the use of video-based remote sensing techniques, which allows physical measurements with low cost and with high spatial and temporal resolutions (Holland et al., 1997). As time and space scales of the nearshore zone ranges from 1 s or even less for fluid motions to up to years for bathymetric response, and from 10 m for beach cusps to 100-1000 m for large scale bar, the video images seem to be a good alternative for the study of coastal processes. Moreover, the nearshore environment is replete with optical signatures that can be explored (Holman and Stanley, 2007), such as sand bars variability (Lippmann and Holman, 1990), intertidal bathymetry (Plant and Holman, 1997; Aarninkhof et al., 2003; Vousdoukas et al., 2011), surf zone bathymetry (Aarninkhof and Ruessink, 2004) and wave attenuation on muddy bottoms (Pereira et al., 2011) are some examples of nearshore studies using video images. Performing remote intertidal bathymetry is an efficient way to investigate the beach response due to changes in offshore wave height and sea levels, long term beach evolution and also the effects of high energetic events. Such information has helped the scientific community to understand the beach environment. The aim of this work is to investigate the suitability of video images associated with a new image processing technique to estimate intertidal bathymetry on a dissipative beach subjected to a micro- tidal regime but also affected by high water level fluctuations due to storm surges. FIELD SITE Located at southern Brazil (Figure 1), Cassino beach is a multiple bar beach system, composed by very fine sand with a very gentle foreshore slope (β=1:45). Cassino can be defined as a wave dominated microtidal system (Calliari and Klein, 1993) having mixed (dominantly diurnal) tides with mean amplitude of 0.47 m (Möller et al., 2001). According to Motta (1963), mean values of offshore significant wave height (H s ) and period (T 1/3 ) are 1.5 m and 9 s, respectively. Northeasterly (NE) winds dominate throughout the year while southwesterly (SW) winds become more important during fall and winter (Moller et al., 2001). The major sea level oscillations at Cassino beach are associated with storm surges, which can result in 2m water elevations on the ABSTRACT Sobral, F.N.C., Pereira, P.S., Cavalcanti, P.G., Guedes, R.M.C., Calliari, L.J., 2013. Intertidal Bathymetry Estimation Using Video Images on a Dissipative Beach. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 1439-1444, ISSN 0749-0208. Located at southern of Brazil, Cassino beach is a multiple sandbar beach. A video technique to map intertidal bathymetry changes was applied using CMYK color model to find the waterline position, hydrodynamic data and runup 2% (R2) parameterizations. Linear regression between waterline video detected and elevation estimated with in situ measured maximum swash, showed a correlation coefficient of r²=0.82 and r²=0.91, respectively. Rmse values for those comparison was 10.14 m (horizontal detection) and 0.12 m (elevation). The empirical swash coefficient ( Kosc) was 1.08 with a total error δz of rms=0.25 m and represents a waterline underestimated to compensate the overestimated horizontal determination. The principal error was found to be at horizontal position, which means that the R2 parameterization was efficient. The estimated bathymetry temporal evolution changes showed a similar morphologic evolution when compared with beach profile with a total rmse of 0.13 m. ADDITIONAL INDEX WORDS: Waterline, runup parameterization, storm surge. www.JCRonline.or g www.cerf-jcr.org ____________________ DOI: 10.2112/SI65-243.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013