Nearshore currents and swimmer safety in the Netherlands

Artificial beach and foreshore nourishments, as executed widely along the Dutch coast, change the nearshore morphology and current patterns significantly. Over the last decades the size of the nourishments has increased from 0.7 x 106 m3/km to over 6 x 106 m3/km. Groynes which were present along the southern Holland coast since the 19th century are now (partly) covered with sand. As a result, entirely new dynamic coastlines and beach configurations are found at several locations along the southern Holland coast. This major impact on the beach configuration (disappearance of groynes, alongshore variations in shoreline amongst others) changes the currents swimmers and lifeguards were familiar with. This leads to uncertainty and concerns of lifeguards and (local) governments regarding swimmer safety, especially for the beach season after construction of the nourishment.Two research objectives are formulated: 1) investigate the relevant aspects of nearshore currents and beach configurations with respect to swimmer safety and 2) investigate the regime for which rip currents are likely to form on a groyned and groyneless beach along the wind sea dominated South Holland coast.A monitoring campaign of the beach season of 2010 in The Hague was initiated in cooperation with the lifeguards. Experiences and observations of the lifeguards with respect to swimmer safety on the groyned (unnourished) and groyneless (nourished) beach were discussed. Lifeguard reports of the rescue brigades of The Hague and ‘s Gravenzande (another rescue brigade on the frequently nourished Delfland Coast 13 km south of Scheveningen) were collected for a period of over 6 years to assess critical conditions and locations. Rescue research parameters (wind, waves, tide, temperature, cloud cover, precipitation amongst others) at times of the rescues were compared to seasonal occurrence of the parameters to investigate a causal relation between the parameters and swimmer safety. Detailed lagrangian current measurements were performed on the groyned and groyneless (nourished) beach of Scheveningen, using GPS drifters. The measurements took place for a selected set of conditions, which, according to the analysis of the lifeguard rescue reports, caused many rescues in the past. Bottom topography of the nourished beach was surveyed just before or after the current measurements using PWC (jetski) surveys around high tide and low wave energy conditions. The sub aqueous surveys were complemented with RTK GPS 4wd- and walking surveys on the sub aerial beach around low tide, giving a coverage from -5 to +3 m above MSL. The acquired field data is analysed to identify hazardous currents, locations and to quantify the order of magnitude of the hazardous currents. The regime for the occurrence of rip currents along the open nourished beach and near groynes is investigated. In a synthesis of the analysis results seven aspects of nearshore currents and beach configuration were formulated, which need to be evaluated in order to make a comprehensive assessment of the impact of nearshore currents and beach configuration on swimmer safety at a certain location.The occurrence of rip currents on open beaches along the wind sea dominated South Holland coast was shown to be strongly related to and limited by the longshore current magnitude. Rip currents were observed when long- and cross shore current velocities were of the same order of magnitude, which was observed only for near-normally incident waves. No rip currents were observed when longshore current velocities were approximately three times higher than cross shore velocities, which was observerd for breaking wave angles of incidence larger than 10° to 15°. Wave angle of incidence, wave height, wave period, tidal level and vertical variations in bottom topography within the surf zone were identified as crucial factors in the occurrence of rip currents along the wind sea dominated Dutch coast. Conceptual swimmer safety tools were presented for application by lifeguards on a groyned and open beach. Based on the above mentioned (and readily assessable) parameters the relative risk on rip currents can be estimated.