Author: Christian Feldbauer
This web app simulates the water level inside a lagoon in response to the ocean tides and swell, and in turn whether the current in a pass flows in or out. The simulator is based on a first-order model borrowed from electrical engineering: the lagoon is modeled as a capacitor, the pass(es) and the channels over the barrier reef are represented by resistors, the ocean tides are a sinusoidal input, and swell and waves are represented by a DC input.
Parameters. The blue input field at the very top holds the 2 ocean parameters: tidal range and swell height in meters. The lagoon has 3 parameters: the pass time constant τpass, the reef time constant τreef, and the swell threshold. The pass time constant is a measure for how much water can run through the pass(es): a small lagoon with a wide pass (open lagoon) has a very small time constant, whereas a large lagoon with a tiny pass (closed lagoon) has a very high one. A swell-filled lagoon reaches its calm-weather equilibrium (tidal steady state) after 5 times the time constant when no more swell enters. This pass time constant can be specified in the bright-turquoise input field either as 5τpassin hours, as the tide delay in the lagoon compared to the outside ocean in hours (measured in calm conditions without swell), or as the tidal range inside the lagoon (in meters or in % of the ocean's tidal range). The dark-turquoise input field is for the reef time constant as 5τreef in hours and the swell threshold in meters. The reef time constant is a measure for how much water can make it into the lagoon over the barrier reef once the swell is higher than the threshold and can be set to fit a measured lagoon level offset (once the pass time constant has been determined).
Limitations. The model clearly oversimplifies most real-world lagoons: above all it assumes the same amount of water can enter over the reef regardless the swell or wave direction. So, different τreef parameters may be used for different wave directions for the same lagoon. The tidal cycle models only the principal lunar semi-diurnal constituent. For a location where the solar tidal force plays an important role, the program needs to be modified.
What's on the graph?
The blue line shows one full cycle of the lunar semi-diurnal tides, from one high water to the next one outside the pass. The turquoise line represents the water level inside the pass. Whenever the lagoon level is higher than the ocean level, the current through the pass is outgoing as represented by the turquoise-filled curve. Similarly, the blue-filled curve indicates an incoming current whenever the ocean level is higher than the lagoon level. Those times where lagoon and ocean are equally high are indicated by vertical green lines labeled as 'slack water.' Without a lagoon level offset (i.e, when the swell is below the threshold), slack water is when the lagoon has high or low water. With increasing lagoon level offset, slack water happens increasingly ahead of high water in the lagoon and increasingly delayed after low water, until the lagoon level is higher than the ocean regardless of the tide and the current is outgoing the whole time.
A cursor appears when you move the mouse over the graph or when you touch it.
Download. You can download this HTML file to your computer (in your web browser select 'Save page as' or similar) and use it offline. Feel free to pass the file on to others who may find it useful. In case you have received this file as a downloaded version, check out our blog for a possibly newer version at www.Pitufa.at/lagoonsimulator.
Donate. If you find this web app useful you might want to buy us a drink when you meet us somewhere around in person or make a donation to our PayPal account.