Proceedings of the Twenty-fourth (2014) International Ocean and Polar Engineering Conference www.isope.org Busan, Korea, June 15-20, 2014 Copyright © 2014 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1 880653 91-3 (Set); ISSN 1098-6189 (Set)

Bathymetric Changes Related to Wave and in the Tenryu Delta-front, Japan

Takumi Okabe, Shigeru Kato and Kakeru Takaoka Department of Architecture and Civil Engineering, Toyohashi University of Technology Toyohashi, Aichi, Japan

assessing complex morphology in multichannel estuary. In spite of this, ABSTRACT there are financial constraints on the frequent update of bathymetric charts. This study presents bathymetric changes and its characteristics due to high waves and fluvial are examined for wave-dominated delta- There has recently been an extensive information technology front of Tenryu river, a -rich river on the Pacific coast (Enshu- innovation with which the size and cost of high-performance wireless nada) of central Japan. Frequent continuous bathymetries around the mobile communication devices and GNSS sensors are reduced. In river mouth are obtained from records of coastal fishing activity of coastal area, many researchers and engineers have currently much small vessels. Dominant feature of 4-year bathymetric trend are interest in observations applying the devices for real-time and spatio- exhibited by concentrated (approximately 3 m vertically) within temporal monitoring (e.g. human activities on coastal recreation, the delta-front terminal and the lobe migration. Wave-induced regional water-quality monitoring and shoreline survey using webcam retrogradation and flood-induced of the delta-front during images). In these vast quantities of information, named coastal ‘Big short-term meteorological event are examined by transect analysis of Data’ (Millie et al., 2013), obtained from coastal monitoring by low- the terminal lobe. A tentative estimate, cross-shore variations of the cost sensor devices or data logger, there are possibilities for new lobe are explained in terms of offshore wave energy and the river flood findings concerning various coastal processes. discharge. Flexible terrain analysis of the bathymetry by use of the data and the bathymetric changes of different time periods are For amplifying coastal morphological data based on the Big Data, the shown in this paper. authors have developed a monitoring system of regional bathymetry from 2007 in Enshu-nada coast, Japan (Okabe et al., 2009; Okabe and KEY WORDS: River mouth; delta-front; coastal fishery; Kato, 2013). This new bathymetry method is from collection and morphology monitoring; high wave; fluvial flood. analysis activity data of coastal fishing vessels that operate in the shallow sea. Bathymetric map during some time period (week, month, INTRODUCTION etc.) can be obtained to evaluate and to quantify the topographic changes for sediment management. Similarly, fishing activity mapping Sedimentation and morphological change of river-delta result from the for marine ecosystem management by the Big Data using Vessel interaction of fluvial floods and coastal processes. The morphological Monitoring Systems (VMS) has developed in regions of the UK and systems around river mouth play an important role in not only estuarine European continental shelf (Witt and Godley, 2007; Lee et al., 2010; environment but also a regional sediment management. It is important Lambert et al., 2012). The spatial patterns of activities by fishing gear to develop insight into the morphodynamic processes in an estuary in class were estimated using VMS dataset contained location, timestamp order to estimate impacts for environmental assessment (e.g. coastal and vessel identification number in their studies. In both monitoring erosion, estuarine habitats). methods, the key to collect a large amount of coastal data frequently at low-cost is passive data sampling technique in collaboration with the The observation of such regions is difficult owing to complex flow fisheries. field due to wave-current interaction. In particular, much cost is needed to high-resolution bathymetric surveys around river mouth and inlet In this study, we investigate the large dataset from the coastal fishing because of unsuitable for shallow water with wide swath Multibeam activities in Enshu-nada to describe characteristics morphological sonar (Hughes Clarke et al., 1996). Moreover, river-delta morphology pattern of response to high-wave and fluvial flood conditions at the can occur on all timescales from a few hours to months, so that frequent Tenryu river delta-front. To discuss wave-induced retrogradation and sequential bathymetries are essential to find sediment process flood-induced progradation of the delta-front during short-term containing the vicinity. For example, Pluymaekers et al. (2007) has meteorological event and obtain evaluation of short-term movement of analyzed 17 two-weekly Multibeam Echo Sounding (MBES) data sets for the terminal lobe by their energy balance, we test a time-series analysis

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