Do wave heights and water levels increase ocean lifeguard rescues?

To investigate the association of wave height and tidal water level changes with the frequency of ocean lifeguard rescues.

All ocean lifeguard rescues recorded by Newport Beach Lifeguards in 2015 and 2016 were linked by time and location to weather and ocean variables contained in other historical databases. We performed separate multivariable analyses using mixed effects negative binomial regression to evaluate the total effects of wave height, mean water level (primarily set by tidal elevation), and rising vs. falling water level, on the frequency of ocean rescue in the study location, controlling for confounding variables.

Newport Beach Lifeguards made 8046 rescues during the study period. In all areas of the beach, rescue frequency increased as waves got larger (IRR: 3.25; 95%CI: 2.91-3.79) but then decreased in large surf (IRR: 0.52; 95%CI: 0.37-0.73). In two sections of beach, lifeguards made more rescues during lower water levels, but in the third section of beach, made more rescues during higher water levels. Rescue frequency increased in two sections of beach with rising water levels, but did not in the other section.

Wave height, water level, and water level direction were associated with rescue frequency, but the environmental factors included in the analysis did not fully account for most variation in rescue frequency. Other factors need to be evaluated to identify major determinants of rescue frequency.