THE IMPACT OF RESTORED FRESHWATER INFLOW ON TIDAL DISTORTION IN A SHALLOW ESTUARY

Abstract

Te Awa o Ngātoroirangi (the Maketū Estuary) in the Bay of Plenty of Aotearoa New Zealand has had multiple major shifts in the salinity regime, associated with a long history of engineering works on river inflows. The Kaituna River was diverted out of the estuary in 1957 to prevent flooding, resulting in degradation of the estuary, including increased sedimentation, loss of tidal channels, decreased flushing, saltwater intrusion and ecological decline. However, in 2020 and 2021, a total of 20 percent of the river flow was restored back into the estuary in two stages through 12 control gates, with a key driver being to restore the mauri (life force) of the estuary and kaimoana for tangata whenua (Māori people of the land). This study focuses on the immediate effects of the partial (13 percent) restoration of the Kaituna River flow on the tidal distortion at 5 sites throughout the estuary. One of the issues before the partial freshwater restoration was infilling of the estuary with sediment. Infilling can be driven by flood-dominant tidal asymmetry and thus changes to asymmetry are a focus of this study.