Sediments of the Anthropocene

In my workplace I have recently had some fun improving my knowledge and application of erosion and sediment control methods. It reminds me that sometimes a little knowledge and the best intentions can actually lead to wasted time or even worse outcomes. I’d like to use this post to look at what erosion and sediment control means for sites that are to be disturbed. This is because of a construction site I visited in my town a couple of weeks ago that made me laugh (I had to see the funny side otherwise I’d always be crying!)

Is this working?

The first point to know is that erosion and sediment control is two things (erosion and… sediment control). They are not one and the same thing. In fact the most important part is the erosion control bit. If you have erosion control you don’t need sediment controls. This lack of distinction I think causes the biggest waste of resources.

Have a look at the picture here. This is a classic example of a waste of time. It is something that was never going to be the solution and inevitably failed and wasn’t even looked after anyway. In this example a small slope was disturbed. This small slope had water running on to it from a grass slope. The people responsible thought “hey, treatment: sediment fence!”… But thought nothing about fixing the problem in the first place. A better solution would have been to do one or a combination of erosion control measures. These could have included:

• Diverting clean storm water around the exposed slope with a mulch bund or similar (many trees were chipped and removed from the area).
• Spraying the surface with a synthetic soil stabiliser.
• Spraying the surface with a hydro mulch or similar with grass seed.
• Covering with a synthetic or biodegradable mesh framework which was then seeded

Is this working?

None of these things would cost much more in time or money than installing and re-installing failed silt fences. And they would have actually fixed the problem in the first place. Just to add a little icing to the cake here is another control measure that was located about 20 metres away. The good old sandbag near a stormwater inlet. At the best of times this can only be considered a supplementary technique that should never be used in isolation. In this case the sandbag has ruptured and the sand appears to have actually gone into the storm water system itself! The small amount of sediment retained seems to only be effective because of weeds growing in the gutter. No thought again, and no checking to make sure things work and no fixing of failed problems for an obviously long time.

Erosion control should always be the first focus and even when using sediment controls consideration needs to be given to whether they will even be effective. For example “silt fences” are actually not good at holding back silt. They only hold back sand! They should be called “sand fences”. Clays, silts and any dispersive soils will pass straight through a silt fence. It is important that people in the know to undertake erosion and sediment control works. This is important were ever significant disturbance is to occur or where sediment may easily enter waterways or other sensitive receptors.

A rock of Gibraltar Range National Park - Part 1.

A lookout on the Gwydir Highway

I was going to write a very long post on the Dandahra Creek Leucogranite but I think it lends itself to two posts. This post will focus on the amazing Gibraltar Range National Park and the second will focus on Australian ingenuity and dating of the Dandahra Creek Leucogranite.

A few months ago I travelled from Glen Innes to Grafton via the Gwydir Highway. The landscape in this area is wonderfully diverse and surprisingly contradictory. For example usually Sandy soils on the plateau give rise to swamps with peat. It is a special area because the link between the geology, vegetation and even bush fire patterns is quite obvious. I'd like to focus on one rock unit that makes up the balance of the Gibraltar Range National Park area, the Dandahra Creek Leucogranite.

The Dandahra Creek Leucogranite was often referred to as the Danhahra Granite (and still regularly called this in botanical circles). It is part of the New England Batholith and has recently been dated at at 237.6 Ma (Chisholm et al 2014). It is the youngest member of the Stanthorpe supersuite of granites. Outcrops are very frequent in the Mulligans Hut area and the Gwydir highway transverses the unit.

The spectacular tors which are major features of the landscape of Gibraltar Range National Park arise from weathering from the Dandahra Creek Leucogranite. These tors form through onion peel weathering (technically called exfoliation or spheroidal weathering). This weathering process is where water enters cracks in the rocks and then freezes over night. As water turns to ice it expands and sheets off rock just like an onion skin. This is usually a fairly slow process except with the last sloughing off of the onion peel occurring quite rapidly.

Tall open forest is a major feature of the landscape of the Dandahra Creek Leucogranite. These eucalyptus dominated forests can have an open, grassy understorey featuring grass-trees and/or tree-ferns. These landscapes are quite fire prone. Indeed their structure is dependent on multi-decadal scale fires.

There are also some more unusual vegetation communities on rock outcrops because the tor outcrops lend themselves to protecting some vegetation from fires. They are also very thin soils with low nutrient content so even carnivorous plants can be found.

Heathlands and grasslands occur around the rock outcrops and are particularly important as they contain the greatest concentration of rare, threatened or geographically restricted species, or species found at the limits of their distribution (NPWS 2005). The grass and heath land burns very frequently often with bush fires only every several years.

The shallow wide valleys that are formed on the sandy granitic derived soils result in common large peat swamps. The shape of the valleys slows down water and the underlying massive granite means that the water does not infiltrate. The swamps contain sedges and other water loving plants.

If you are interested in the bush or interested in rock the Gibraltar Range National Park is for you. If you are in to camping, bush walking, amazing views of rugged valleys the Gibraltar Range National Park is for you. If you are in to spectacular flowers, rainforests, exploring a rocky creek the Gibraltar Range National Park is for you. If you are in to staying in a lodge, want to see some snow, or bathe in a rock pool on a summers day the Gibraltar Range National Park is for you.

References/Bibliography:

*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia

*Clarke, Peter J. & Myerscough, Peter J. 2006. Introduction to the Biology and Ecology of Gibraltar Range National Park and Adjacent areas: Patterns, Processes and Prospects. Proceedings of the Linnean Society of New South Wales

*New South Wales National Parks and Wildlife Service 2005. Gibraltar Range Group of National Parks (Incorporating Barool, Capoompeta, Gibraltar Range, Nymboida and Washpool National Parks and Nymboida and Washpool State Conservation Areas) Plan of Management. February 2005. ISBN 0 7313 6861 4