Engineering the Life Back into Maketū Estuary

The Kaituna River, located in the Bay of Plenty, originally entered the sea via the estuary at Maketū. Since 1926 there has been major work undertaken in the catchment to reduce flood risk and improve drainage for surrounding land. This has included the construction of stopbanks along parts of the estuary margin, and the 1956 Te Tumu diversion which directed the river out to sea before it reached the estuary. While this work did improve land drainage, it substantially degraded the ecological and cultural values of Maketū Estuary (the estuary), including:

• Accelerated in-filling of the estuary (largely due to increased water volume and speeds through the estuary entrance on the incoming tide) with almost two-thirds of the tidal prism lost since 1956,
• Loss of wetland habitats in and around the estuary due to changed salinity, hydrology, and encroachment of farmland,
• Reduction in shellfish and finfish populations, and harvestable individuals, resulting in a decline in the mauri (life force) of the estuary and lower river, with associated impacts on tangata whenua (people of the land) relationships with the area.

The Bay of Plenty Regional Council (BOPRC) awarded a $13.5M contract in 2017 for the physical works to re-divert almost a quarter of the Kaituna River’s flow back into the estuary. The extra water is predicted to improve the estuary’s health and will restore some of the ecology of the area by allowing salt marsh and other wetlands to return. This will also create more suitable conditions for a range of shellfish and fish species and will halt the in-filling of the estuary. In fact, the increase from 12:10 to 19:10 in the ebb/flood tidal volumes will increase the rate at which the estuary’s water flushes (reducing from 15 to 2.5 tidal cycles to replace the entire tidal prism volume) and will likely lead to long-term gradual erosion of the flood-tide delta sand. According to modelling predictions from DHI Water and Environment (DHI), the re-diversion will increase Kaituna River inflow to the estuary from 100,000 m3 per tidal cycle (mean river flow and mean tide), to nearly 600,000 m3, including 76% fresh water.

This paper reports on some of the project challenges and successes during the design and construction phases. The challenges included: