When a tsunami travels over a long and gradual slope, it allows time for the tsunami to grow in wave height. This is called shoaling and typically occurs in shallow water less than 100m. Successive peaks can be anywhere from five to 90 minutes apart. In the open ocean, even the largest tsunami are relatively small with wave heights of less than one metre. The shoaling effect can increase this wave height to a degree such that the tsunami could potentially reach an onshore height of up to 30 metres above sea level. However, depending on the nature of the tsunami and the nearshore surroundings, the tsunami may create only barely noticeable ripples.