A better way to define the foreshore

A better way to define the foreshore

MHWS heights for the Bay of Plenty from NIWA’s model, relative to mean sea-level.

The Mean High Water Spring (MHWS) level is used as the dividing line between land and sea under both the Resource Management and Foreshore & Seabed Acts. Defining this level is becoming increasingly important for local and regional authorities, for coastal hazard risk assessment and resource management planning.

MHWS can be defined in many ways, but is traditionally computed as the long-term average of the highest high tide (‘spring tide’) that occurs after every new and full moon. Defining the MHWS level in a particular region is complicated by the lack of tide measurements for many parts of our coastline and the considerable variance in tidal range and characteristics around New Zealand. For instance, at Kaikoura and Wellington, about 45% of all high tides exceed the traditional MHWS level.

NIWA has overcome these difficulties by developing a model that can predict high tides for any coastal location in New Zealand over a 100–year timespan and calculate the percentage that exceed particular definitions of MHWS. Authorities can use this information to select a pragmatic MHWS level that is appropriate for their needs and their region. For instance, they may choose the sea level which is only exceeded by 10% of all high tides.