Hydraulic gradient and wave height control the success of estuary artificial entrance openings

Abstract

Intermittently Open/Closed Estuaries (IOCEs) are dynamic, sensitive estuaries that periodically close as a function of competing marine and fluvial energies. Once closed, IOCEs are often artificially opened to relieve localised flooding. Artificial openings are generally effective as a short-term flood mitigation strategy, however, sometimes IOCEs can close immediately after excavation without draining the lagoon. In this situation, the opening does not achieve its management purpose and reimplementation is required – costing many thousands of dollars at a time and leading to adverse impacts on estuary biota. Using a dataset of 137 openings at 37 IOCEs globally, we show that the hydraulic gradient (or grade) at opening and offshore significant wave height determine whether an estuary will remain open and drain its lagoon. The hydraulic gradient represents the energy slope between the estuary and the ocean, and wave height determines the capacity of waves to infill the channel with sediment to counter offshore erosion. Artificial openings are only successful when IOCEs are opened with a hydraulic gradient steeper than 0.017 m/m (equivalent to a grade of 1:60) and when significant wave height is below 4.30 m. These findings highlight that the estuary water level alone is not a good predictor of opening success, despite it being widely used to decide when to implement artificial openings. Using near-continuous monitoring of geomorphic change at 28 artificial openings, we show that the hydraulic gradient (and grade) at the time of opening controls the rate of channel expansion, outflow velocity at the mouth, lagoon drainage rate, and the time to reach peak channel width and outflow velocities. The tidal stage at opening can be used to either speed up or slow down the rates of change, potentially reducing the occurrence of fish kills associated with rapid drainage. Although the decision to open an estuary is a complex balance between environmental, cultural, socio-economic, and practical factors, we present a simple and cost-effective way to predict if artificial openings will be successful. The thresholds of our study can be used to complement existing decision support tools, but as a standalone method, are best suited for emergency openings where draining the lagoon to reduce flooding is the main management priority.