Yoo, J.; Shin, S.; Do, K.D.; Shim, J.S.; Ha, T., and Jun, K.C., 2014. Laboratory investigations on effects of water level change on surf-zone processes.Tidal level and current often influence the rip current and the morphological changes in the macro-tidal beaches. Effects of water level change on surf zone processes were investigated through laboratory experiments conducted in a two dimensional wave flume. A movable beach was installed using a find sand in the flume starting with 1/50 slope from the offshore and ending with 1/20 slope near the shore. Irregular waves were generated by a piston type wave maker with active absorption. In the meantime, water level was changed gradually by using a pump and drain system to consider waves and tide simultaneously. Herein, three different tidal phases (i.e. flood tide, full tide and ebb tide) were conditioned with a gradual depth change of about 10 ~ 15 cm in the offshore during a period of about 30 minutes per phase except the full tide, while only one wave condition was kept targeting a significant wave height of 12 cm and significant wave period of 1.7 s. Measurements of wave attenuations in the wave breaking zone influenced by the tidal change were made using in-situ sensors and an image-based analysis method. The rates of wave attenuation (i.e. the ratio of wave height to water depth) measured during the both of flood and ebb tides were found to be larger, ranging from about 0.8 to 1.4, compared to those recorded during the full tide with little depth change. In addition, the attenuation coefficient of the flood tide tends to be larger than that of the ebb tide.