What Happens When You Dig a Big Hole in an Estuary

The following presentation may be of interest to members. It is an independent report presented by marine biologist, Dr. Roger Grace, to Auckland Environmental and Sustainability Forum in July, and is based on a study of siltation rates in the Whangateau and Sandspit Estuaries.

What Happens When You Dig a Big Hole in an Estuary

Sediment Plume in Omaha Bay
Sediment Plume in Omaha Bay

 

Severe Erosion on Pakari Hill after Cyclone Wilma
Severe Erosion on Pakari Hill after Cyclone Wilma

 

Silt on Whangateau Harbour After Cyclone Wilma
Silt on Whangateau Harbour After Cyclone Wilma

 

July 29, 2013By Roger GraceSince 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.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.

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.

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.

Sediment Traps
Sediment Traps

You can see my PowerPoint presentation on the Auckland Council website

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