Impact of Sea-Level Rise on the Hydrologic Landscape of the Mānā Plain, Kaua‘i

Basil Gomez

doi:10.3390/w13060766

Abstract

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.

Highlights

The introduction of large-capacity drainage pumps a century ago allowed more land to be brought into production, and drainage ditch system operations continue to be optimized in support of agricultural activities
Quantifying the inputs and outputs of the hydrologic landscape suggests that, as sea level rises, the projected increase in groundwater inflow through the backshore deposits will not present a significant challenge to day-to-day ditch system operations
Water levels in the drainage ditch system rise after large rainstorms and, because ditch flows are unable to act in tandem with basin processes to pass the runoff volume even after the plug in Kinikini Ditch has been removed, excess runoff that accumulates in low-lying areas of the Mānā Plain is removed by increasing the rate of pumping in the days or weeks following a storm

Summary

Introduction

The threat SLR induced, multi-mechanism flooding poses to the urbanized coastal plains on O‘ahu has been well researched [3,4,5,6], and the vulnerability coastal wetlands across the State have to SLR has been assessed [7] These and other studies have emphasized that, to provide insight to potential future conditions and evaluate the utility of different adaptation measures in both urban and rural settings, it is necessary to move beyond the “bathtub” model and account for factors other than relative SLR [2].

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Discussion

Conclusion