Wainono Lagoon is a shallow, freshwater lagoon in South Canterbury that is separated from the sea by a narrow gravel berm, as shown below. Our task was to estimate the seepage flow through the berm.
We installed a water level recorder in the lagoon to see if the lagoon level fluctuated with the tide, but there was no evidence that it did. This implied that the rise and fall of the tide does not penetrate through the berm, and this is backed up by the low levels of salinity in lagoon water. Therefore, we expect that any flow that occurs is from the lagoon to the sea. To calculate what this flow is, we need the transmissivity of the gravel and the depth to the impermeable layer beneath the gravel berm. To obtain these, we dug a very large pit in the berm using a hydraulic excavator. We started small and made the pit bigger and bigger until we struck the impermeable layer. Below is the finished pit, which was 5.5 m diameter at the water line. The impermeable layer was about 1.5 m below the water line. We monitored the level in the pit over a tidal cycle using an automatic recorder (blue box).
By comparing the signal in the pit with the tide at sea, we were able to calculate the attenuation in the amplitude and the shift in phase. This allowed us to infer what the transmissivity must be. This, along with the depth to the impermeable layer allowed us to estimate the flow through the berm. For autumn 2003 we estimated the mean seepage flow over the length of the berm to be 0.73 cumecs.
This work was commissioned and funded by Environment Canterbury.
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Last Updated: 17 November 2003
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