A New Start for the Sediment Traps at Sandspit and Whangateau

Mud near a Whangateau sediment trap

Mud near a Whangateau sediment trap

A full year of results from the sediment traps at Whangateau and in the footprint of the proposed marina at Sandspit indicated a likely filling rate for the marina of 329.3mm in one year. This is far in excess of the estimate of only 30mm from the marina proposers. There was no experimental verification or ground truthing of that theoretical estimate, despite the SYCMS having ample time to do so.

Analysis of the full year of results from the sediment traps at Whangateau and Sandspit will form the basis of a separate report. This will include relationship to rainfall figures for the catchments, and other matters.

Launching into a new year of sediment trapping experiments, it was time to make a few changes to the sediment trapping programme to improve the precision of the estimates and to make a better job of excluding crabs and fish. Examination of the first year of data suggests that animals getting into the traps were probably responsible for some of the lower sediment accumulation results from time to time. I suspect they tend to stir up the sediment and actually kick some out of the traps. In fact sometimes the sediment level actually went down between readings because of sediment being kicked out.

In January I cleaned out all the traps to start afresh. I also added a third trap to each site, so with three replicate traps at each site this would improve the precision of the estimated sediment accumulation. This also gave the opportunity to correct the one-month difference in start times for each site, and the new start times are only one day apart.

The chicken mesh covers I put on the traps in the early part of last year did a reasonable job of excluding large fishes and crabs, such as the small snapper and the Asian paddle crab which got into the traps. But there were still sometimes a few triplefin fish and quite large numbers of Hemigrapsus crabs which got through the chicken wire mesh. Not only did they kick out sediment, thus causing an apparent lower sediment accumulation rate than reality, but some also died in the tube through lack of good water circulation and were particularly unpleasant to pull out of the traps by hand. Leaving them there of course would have artificially increased the apparent sedimentation rate because of their bulk.

So I made a new improved version of a mesh baffle on top of the traps, using stainless steel wire mesh with 5mm openings, welded on to a short length of PVC pipe to fit over the top of the 100mm-diameter trap tubes. I also painted the mesh with antifouling paint to minimize algal and other biological growth on the mesh. Attaching a mesh baffle to the top of sediment tubes to prevent incursion of nuisance animals is a common practice in use of sediment traps elsewhere, including in the deep sea.

Sediment Data for 02/03/2014:

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