Tidal amplitude decreases in response to estuarine shrinkage: Tokyo Bay during the Holocene

Tidal changes in Tokyo Bay during the last 10,000 years were investigated by combining a numerical model with newly compiled model bathymetries for the present day, the early 20th century, and every 1000 years from 4000 to 10,000 years ago. Over this time period, sea level has changed by more than 30 m, and the coastline has shifted 50–60 km. Tides during the early 20th century and 4000–9000 years ago were larger than those at present; from 9000 to 4000 years ago, the bay setting was mesotidal, in contrast to the modern microtidal setting. The maximum tidal level at the head of the bay was 72% higher 7000 years ago than at present, and M₂ tidal currents at the bay mouth from 10,000 to 9000 years ago were three times stronger than those at present. When tides were compared at a fixed location, the greatest tidal enhancement was found to have occurred 9000 years ago. The weakening of tides over the last 9000 years can be explained by introducing the concept of estuarine shrinkage together with quarter-wavelength resonance theory. Tidal weakening occurred in two phases: during the first phase, 9000–7000 years ago, it was due to the rise in sea level (type 2 shrinkage), and during the second phase, a decrease in the length of the bay was responsible (type 1 shrinkage). These findings can be applied to the assessment of both past tidal changes and potential future tidal changes in other elongated estuaries with lengths of several tens of kilometers.