The dangers of secondary undulations are not widely known. Secondary undulations are a phenomenon in which a rise and fall in sea level occurs in cycles of from a few minutes to a few tens of minutes. There are many aspects of the occurrence mechanism that are not understood, making predictions difficult, with sudden increases in amplitude often occurring without any forewarning. 
Thus, when the Marine Observatory records a significant change in undulation it releases a "Tide Level Information Bulletin, Egiving current status data, forecasts, and advice for disaster preparedness.