12 th International Conference on Hydroscience & Engineering Hydro-Science & Engineering for Environmental Resilience November 6-10, 2016, Tainan, Taiwan. Development of Low-Cost Drifters Array for Nearshore Current Mapping in Coastal Groin Effect Basins Yao-Zhao Zhong 1 , Hwa Chien 1* , Hao-Yuan Cheng 1 , Yu-Chun Chang 1 , Ramakrishnan Balaji 2 1. Institute of Hydrological and Oceanic Sciences, National Central University Taoyuan, Taiwan 2. Department of Civil Engineering, Indian Institute of Technology Madras, India ABSTRACT A low-cost disposal Global Navigation Satellite System (GNSS)- tracked surface drifter is developed for nearshore current mapping. Cluster consisted of more than 30 drifters were deployed in the vicinity waters around Wu-Shi fishery harbor in the northeastern coast of Taiwan where exhibited severe coastal erosion. This paper describes the design of the drifter array system and the results from two field campaigns in Nov. 2015 and Feb. 2016. KEY WORDS: low-cost drifters array; coastal hydrodynamic; groin effect; dispersion coefficient; turbulent kinetic energy INTRODUCTION The hydrodynamic is the dominating factor for the coastal processes such as material transport and mixing, sedimentation, coastal accretion and erosion. Considering the coastal water in a control volume from a macro perspective, the dynamic characteristics are controlled by the combined effect of a variety of external forcing on the boundaries of the volume. These forces include the wind shear stress on the surface, the seabed friction at the bottom, the tidal and larger scale current or riverine discharge at the lateral boundaries, the wave radiation stress on the offshore lateral boundary and the pressure gradient induced by the difference of surface level. The actual situation can be more complex with the changing bathymetry, various stratification conditions and various scales of eddy-shedding caused by the cragged coastline or coastal structures. The understanding of the whole picture of the coastal hydrodynamic is always a challenge. The hydrodynamic environment in coastal zone features significant spatial heterogeneity and temporal variability. Therefore the temporal and spatial information of hydrodynamic parameters should be obtained simultaneously with fine enough resolution. The oceanic observation could be divided into two categories: Eulerian and Lagrangian approaches. Eulerian method could give high temporal resolution data at some fixed locations. The Eulerian method is not satisfactory for coastal hydrodynamic applications because of the rapid increasing cost when deploying a mass number of instruments spread out the domain to obtain the spatial information of hydrodynamic parameters. On the other hand, the Lagrangian method is a sequent tracking observation method and could obtain temporal and spatial features of parameters simultaneously. For the sediment and material transport studies, the Lagrangian method maybe more suitable. In reality, Lagrangian method had been adopted since 1940. The surface drifters were tracked by compass on boat or shore (Shepard et al., 1941; Shepard and Inman, 1950; Sonu, 1972) or by swimmer (Short and Hogan, 1994; Brander and Short, 2000). Besides, the aerial photography technique was employed to track the dye in water continuously (Bowen and Inman, 1974; Rodriguez et al., 1995; Takewaka et al., 2003). In the last decade, the satellite-tracked surface drifters were widely used for water dispersion characteristics analysis (Schmide et al., 2003; Johnson, 2004; Spydell et al., 2015). However, the above-mentioned drifters are not yet widely used in present coastal engineering applications because of the following reasons: (1) the cost of drifters are rather high, and the fee for data transmitting through satellite or GMS network is high; both restrict the mass deployment over the study area to give a valid observation with high spatial resolution; (2) the size of drifters are not small enough to ensure high current following ability for the small scale processes in the coastal zone. In present paper, we address on the development of small low-cost GNSS-tracked surface drifters. We will also demonstrate the results of field experiments, in which more than 30 drifters were deployed in the vicinity waters around Wu-Shi fishery harbor where severe coastal erosions occurred. We found that the turbulent kinetic energy (TKE) in downstream area of breakwater is relatively stronger and the dispersion coefficient around the coast with severe erosion is also relatively higher. METHODS and DATA Drifters Array The whole system consisted of 3 sub-systems, i.e. 1. the drifter, 2. The coastal relay station sub-system for data transmitting, 3. real-time data display and management sub-system that support the In-situ operation. The spherical drifter (Figure 1), with the diameter of 12 cm, exhibits good surface flow following capacity. The drifters are designed for at least 96 hours deployment as the data transmitted back to the shore station every 10 sec. via digital RF network (Tabel 1). Each drifter on the sea can communicate with any other elements in the cluster; once the link is established, the data of the array can be downlink to the shore station. A novel data display and manage sub-system was developed for easy deployment and retrieval.