What Happens When You Dig a Big Hole in an Estuary

Sediment Plume in Omaha Bay

Sediment Plume in Omaha Bay

Since the forest was cleared by the pioneers, huge amounts of sediment and topsoil have washed into our estuaries and into coastal waters.  This causes all sorts of environmental problems, with accumulating silt in estuaries promoting rapid increase in mangrove spread, and silt accumulating on rocky reefs offshore damaging marine life sensitive to silt clogging respiratory and feeding mechanisms.

There are two aspects to sedimentation.  The normal one to consider is how much sediment gets trapped in the estuary and slowly builds up the mud flat levels.  This is usually a slow process, but since the forest was cleared our estuaries have filled fairly rapidly to their present level.  At Mahurangi it has been measured at 8 metres so far. In Whangateau I understand it is up to about 2 metres.

Severe Erosion on Pakari Hill after Cyclone Wilma

Severe Erosion on Pakari Hill after Cyclone Wilma

Steep land turned into pasture tends to slip badly, sending sediment into the streams and rivers and ultimately into the estuary and the sea.  Also stock grazing on stream banks pushes a lot of sediment and soil into the streams.  Harvesting of pine forest, building of roads and creation of housing subdivisions all contribute greatly to sediment getting into the streams.

Silt on Whangateau Harbour After Cyclone Wilma

Silt on Whangateau Harbour After Cyclone Wilma

To reduce sediment loads coming in from the catchment Auckland Council and environmental groups have been encouraging farmers to fence off their streams so stock cannot disturb the banks. Whangateau HarbourCare Group has been involved in a couple of stream bank planting initiatives, and will continue to work in this area.  Fencing out stock and planting up riparian margins of streams has been shown to be an effective way of reducing sediment getting into the streams.

The second aspect of sedimentation that I have been looking at is what happens when you dig a hole in an estuary?  A hole will fill up much faster than the normal sediment accumulation rate on the sand or mud flats.  This is nature trying to correct the disturbance, in this case the hole in the ground.  Nature works towards restoring the previous sediment level, that is to fill the hole up to the old level.

Sediment Traps

Sediment Traps

My sediment trap experiments at Whangateau and at Sandspit are investigating this matter.  Prompted by an interest in what may happen at Sandspit if they build the proposed marina, I installed 2 sediment traps at Sandspit and 2 at Whangateau as a control for the Sandspit experiment.  In the first six months so much sediment accumulated in the traps, particularly at Sandspit, that I was concerned that the estimates used by the Sandspit Marina Society to plan for maintenance dredging (30mm per year) were so far divergent from the results I was getting in my sediment tubes (about 300mm per year) that I did a presentation to Auckland Council on 22nd July to get the information out there in the public domain.

You can see my PowerPoint presentation on the Auckland Council website

What happens when you
dig a hole in an estuary?

Mud near a Whangateau sediment trap

Mud near a Whangateau Harbour sediment trap while taking measurements on October 9th, 2013

Comment on October update of sediment trap records.

I measured the sediment traps on 9th October and the updated spreadsheet is attached.  Both Whangateau and Sandspit had a pretty good slug of sediment as a result of the recent heavy rain.  This time there was a lot more sediment in the Whangateau traps than in the Sandspit ones, and there was about 20mm of fresh mud on the sandy area around the Whangateau traps, which left squishy footprints where normally it was firm sand (see picture attached).  Seems a lot more silt came down into the Whangateau in the recent wet weather than into the Sandspit estuary.  This is probably a result of local rainfall variations, with more rainfall in the Whangateau catchment in the latest rainstorm.

To date we have a mean of 261.5mm of sediment accumulated in the traps at Sandspit in less than 10 months, equating to a daily accumulation rate of 0.891mm/day.  The result for Whangateau is 169.5mm in 11 months, or 0.528mm/day.

Comment on update for October 19th, 2013

Rate of accumulation is still very high, with another 21mm in the Sandspit traps (mean) in only 10 days.  The Whangateau rate has returned to its relatively low level compared to Sandspit, at only 13mm in 10 days, but that is still rather high.  The big slug of sediment that came down last time as a result of the heavy rain, has probably recently been resuspended by the strong winds of the last week or so and contributed to a recent high rate of sediment accumulation in the traps.

Comment on update for 6th December, 2013

Sue and I measured the sediment traps this afternoon.  We now have over a year of data for Whangateau, and almost a year for Sandspit.  The data suggest for Sandspit an annual sedimentation rate for the proposed marina of 352mm.  For Whangateau the traps suggest an equivalent rate of 192mm per year.  If we just look at the southern trap at Sandspit the total for a year would be 432.6mm per year, but we have to average the two sediment traps.  Next sampling I want to clean out the traps and start again.

Intention now is to do a comprehensive analysis of one year data including relationship to rainfall data.  Hopefully we can prepare a report or scientific paper and have it peer reviewed and published.

Spreadsheet data below has been updated to 14th January, 2014.


  1. I think Dr Grace has updated the sediment trap results with January and February results.
    Are they available soon please?
    A letter to the Herald yesterday refers to these results.

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