Significance of Bay Superelevation in Measurement of Sea Level Change


  • Ashish J. Mehta


Bay water head, dredging, inlet hydraulics, river runoff, tide measurement


The issue of whether tide gages located inside bays are suitable for determination of secular trend in the mean sea level has been examined from the perspective of mean water level setup, or superelevation, in the bay relative to mean sea level. As a consequence of the confining dimensions of bays connected to the sea by relatively narrow entrances, the amplitude and frequency responses of bay water level are not coherent with those of sea level. A number of hydrographic factors, chief among them being river runoff, entrance and bay morphology, salinity gradient coupled with runoff, wind and waves contribute towards decoupling mean bay level response from that of mean sea level. Additional constraints to analysis of data from bays arise from the fact that anthropogenic effects including channel dredging and diversion of runoff have introduced biases in tidal signatures which can not be easily removed from available records. As part of a future strategy for mean sea level measurement, therefore, it is recommended that emphasis be placed on the deployment of more open coast primary stations for tidal data collection.