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Limitations
HapuaMod has several limitations which should be noted.
Hāpua occasionally have more than one outlet. Mostly this only occurs for a short period when a new outlet opens and the old outlet has yet to close. However, some large hāpua have been known to sustain multiple outlets for extended periods.
HapuaMod can only simulate a single outlet channel at any given time. In the event of a potential breach occurring via overtopping or erosion of the barrier HapuaMod prioritises the channel closest to the point the river enters the lagoon. If the potential breach is closer to the river than the existing outlet a new outlet is created at the site of the breach and the old outlet is closed immediately. If the potential breach is further from the river than the existing outlet then no breach occurs.
HapuaMod includes the sediment flux associated with overwashing but not the water flux. This makes the model more vulnerable to outlet closure during major washover events. In real world hāpua seawater entering the lagoon due to washover helps to maintain an open outlet channel as it flows out to sea through the outlet channel.
HapuaMod does not allow closure of the outlet in the online section of lagoon due to wave washover (as opposed to in the outlet channel, where closure can occur). See notes on schematisation for an explanation of lagoon vs outlet channel in the model. This is not believed to be a major limitation as:
- Closure generally only in the outlet channel prior to the lagoon.
- In reality the barrier is not rectangular in cross-section as assumed in the model but it's backshore edge is lower lying, especially during major washover events. As such it tends not to completely block online sections of the lagoon.
- The volume of seawater overwashing the barrier is not modelled in HapuaMod but in reality this makes closure of the online lagoon even less likely.