The Mānā Plain is a land apart, buffered from oceanographic influences by ~3–35 m high backshore deposits, and drained by an intricate, >100-y-old ditch system and modern, large-capacity pumps. Quantifying present and prospective inputs and outputs for the hydrologic landscape suggests that, although sea-level rise (SLR) will begin to impact ditch system operations in 2040, transient, event-based flooding caused by rainfall, not SLR induced, multi-mechanism flooding, will continue to pose the most immediate threat. This is because as sea level rises the ability of gravity flows to discharge storm runoff directly into the ocean will diminish, causing floodwater to pond in low-lying depressions. Estimates of the volume of water involved suggests the risk of flooding from surface water is likely to extend to 5.45 km2
of land that is presently ≤ 1 m above sea level. This land will not be permanently inundated, but weeks of pumping may be required to remove the floodwater. Increasing pumping capacity and preserving some operational ability to discharge storm runoff under the influence of gravity will enhance the ditch system’s resilience to SLR and ensure it continues to fulfill its primary functions, of maintaining the water table below the root zone and diverting storm runoff away from farmland, at least until the end of this century.
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