BEFORE THE OLD AACO ROAD: A UNIQUE 1826 ACCOUNT

Robert Dawson, the first Chief Agent of The Australian Agricultural Company was a man of great vision and perception. His wonderful treatise The Present State of Australia; a description of the country, its adavantages and prospects with reference to emigration: and a particular account of the manners, customs and condition of its aboriginal inhabitants contains his description of the first recorded journey by a European along the route of The Old AAco Road. This description is of a journey beside the Karuah River at a time when the first contacts between aboriginal people and AA Company were being made. Robert Dawson is at times patronising and superior to the aboriginal people with whom he made contact, but his respect and affection for many of them is clear as his understanding of their mastery of their environment.

………… we returned to Soldier’s Point, recalled the schooner on the following morning, and sent her again up the river (Karuah), where we appointed to meet her on a certain day. The next duty was to convey our horses across the harbour  to the shore immediately opposite (around North Arm Cove area) which we did with considerable difficulty, by the assistance of the government launch, and we then formed an encampment on that side, from which our whole party departed the next day to join the schooner at the head of the navigable river.

During this journey we passed over about twenty miles of country, some parts of which were of a very inferior description, and others of better quality. The forest every where open and grassy, and free from brushwood; but generally thickly timbered with tall trees, both in the vallies and on the tops of the highest hills.

The natives, Tony and Ben, accompanied us, and also two other natives: the first had his gin, (wife) who carried her little boy, about twelve months old, astride on her shoulders, while the little black urchin fastened his fingers in her hair to prevent himself from falling. They were all three as naked as when they were born, and appeared to suffer no inconvenience from the want of covering – such is the luxurious nature of the climate.

On our journey we fell in with a wild, fierce-looking man, about the middle age, with two slender, interesting looking youths, named Wandoman and Booramee, apparently about twelve years of age. The old man was armed with a long spear; his beard was short and bushy like his hair and his body naked; while he had placed in his girdle of twisted opposssum fur, which he wore around his loins, an iron tomahawk and a large piece of half roasted kangaroo flesh. The trio were wandering in search of the rest of their tribe, who had moved to the beach; and as Tony belonged to the same tribe, I requested him to invite the strangers to join us. This was done in their own language, they being unable to speak a single word of English. The invitation was immediately accepted, and we proceeded together on our journey. I was much pleased to find that every considerable brook and hill had a name; and as the old man was conversant with them all, I made memoranda of their names, shapes and positions, to assist my recollection if I should hereafter examine the country more minutely, or be at any time lost in that quarter of the forest when alone.

After two days’ journey we arrived at the station where we had left the party and found the schooner waiting for us.  Pages 15 & 16

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Tahlee in 1840

This simple pen and ink sketch – artist unknown still captures something of the sense of Tahlee today.

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The 1845 Map by Phillip Parker King

(Click on a map to enlarge it)

One of the great insights into the Old AACo Road from Karuah to Tahlee is provided by Phillip Parker King’s lovely map of Port Stephens. It shows the old road in detail as well as the foreshore, Karuah River, Yalimbah Creek and lots of other detail.

Phillip Parker King was a renowned hydrographer and map maker, having gathered a great deal of experience as a ship’s captain. His maps of the coast of Australia are of very high quality. His most famous voyage was probably when he captained the Adventurer, the ship that accompanied the Beagle on Charles Darwin’s famous voyage.

So, when P. P. King was appointed head of the Australian Agricultural Company, it was hardly surprising that some of his old shipmates should visit Tahlee at his invitation. Charles Darwin and his artist colleague Conrad Martens were both visitors at Tahlee. (One of Marten’s paintings of Tahlee sold at  Christie’s Auction House recently for thirty five thousand pounds!)

This 1845 map reveals the fact that the orginal AACo Road crossed Yalimbah Creek (a bridge was there in 1845) and then wound its way around the back of the hill on the Tahlee side. Nowadays, it takes a short cut beside the creek courtesy some extensive stone work. The rest of the road though, follows the same route that it does today.

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Arriving at Tahlee

Finally we reach Tahlee, the beautiful house and gardens, the wonderful views, the wonderful people at the Bible College and ………………the devonshire teas!!!

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Approaching Tahlee

We walked through fields that had been cleared by convicts one hundred and eighty years ago ……. now it’s a forest again!

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Tide and the Salt Marsh (for the technical)

This study was conducted by Gareth Davies from the University of Wollongong. He has given us permission to publish.

Vignettes > Flow in a microtidal channel during within-bank and over-bank tides: Yalimbah Creek, South Eastern Australia.

Flow in a microtidal channel during within-bank and over-bank tides: Yalimbah Creek, South Eastern Australia.

Gareth Davies
University of Wollongong

Shortcut URL: http://serc.carleton.edu/36187

Location

Continent: Australia
Country: Australia
State/Province:New South Wales
City/Town: Karuah
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Computation

Location map of Yalimbah Creek. a) The location of Yalimbah Creek (black box) in Port Stephens, Australia. b) Map of Yalimbah Creek, with major landform categories and measurement sites. Coordinates are in UTM Zone 56H. Created by the author of the page containing this file. Details
Three intertidal environments of increasing elevation. a) Number One cove, which is usually completely inundated except during very low tides. b) Mangrove forest in the lower elevation vegetated intertidal regions. c) Mixed mangrove and salt marsh in an upstream, higher elevation intertidal area. Created by the author of the page containing this file. Details
Time series of water levels and velocities at the ‘velocity measurement’ site depicted in Figure 1b). The horizontal line in the water level time series approximately divides within-bank and overbank tides at this site. Notice how the velocity peaks are much higher during overbank tides than within-bank tides. Details
A comparison of the measured and predicted water velocities at the ‘velocity measurement’ site (Figure 1b), using Equation 1. The water level data used for prediction was measured near the seaward edge of the channel (Figure 1b). The first day of velocity and stage data was used to estimate F(h), using linear regression. F(h) was estimated as a third order polynomial, because increasing the order of the polynomial did not significantly change the predictions. Created by the author of the page containing this file. Details

Description

The study of tidal hydrodynamics plays an important role in geomorphic studies of tidal channels. While the hydrodynamic properties of the landform are largely controlled by its morphology, these flows also drive sediment transport in the creek and its intertidal flats. They thus have a major influence on the evolution of the landform. This vignette will examine a velocity time series from a tidal channel, and consider the extent to which a simple model, based on the conservation of water mass, can explain the observations.

Yalimbah Creek is a microtidal channel (tidal range usually < 2 m), situated in the north western corner of Port Stephens, a large natural harbour in South Eastern Australia (Figure 1a). The channel is largely sheltered from wave processes, and its small catchment has no inflowing river channel. For most of its length, the channel meanders through a narrow valley filled with intertidal marsh (Figures 1b, 2b,2c), the elevation of which gradually decreases downstream. At its most downstream end, the channel flows into an unvegetated intertidal cove (Number One cove, Figure 1b, 2a), where it gradually shallows until it is no longer clearly distinct from the rest of the cove. The channel is formed largely in unconsolidated muddy sediments, consisting of organic rich silt and clay, with some fine and very fine sand. In many places it is also partly bound by bedrock.

Vegetated intertidal flats are known to exert a strong influence on tidal channel hydrodynamics (e.g. Wolanski et al., 1980; Lessa and Masselink, 1995). As the tide overtops the channel banks, the flats are flooded, and the volume of water stored in the channel/flats system increases rapidly with tidal stage. The flow velocities on the flats tend to be much lower than in the channel, because the former have smaller depths and a greater hydrodynamic drag (induced by vegetation) (Wolanski et al., 1980; Furukawa, 1997). Hence, most water in the channel/flats system is transported via the channel. During overbank tides, velocities in the channel must increase in order to transport these larger volumes of water.

These ‘mass conservation’ type effects are complicated by ‘momentum conservation’ effects. When the intertidal flats are inundated, the propagating tidal wave is also slowed and deformed (Aucan and Ridd, 2000; Fortunato and Oliveira, 2005). This can produce temporarily high water surface slopes (i.e. spatial differences in water levels), both along the channel and between the channel and the flats. These water surface slopes can induce strong accelerations in flow velocities.

Figure 3 shows a water level and velocity time series from a site in Yalimbah Creek, collected in October 2008 (Figure 1b). Notice how the peak velocities are much higher during overbank tides than within bank tides. We will now develop a crude model of this data set, based only on mass conservation considerations.

To do this, assume that: 1) the water surface elevation in the channel/flat system is constant in space; 2) that the flats receive all their water from nearby parts of the channel, and 3) that there is no inflowing fluvial discharge. Although these assumptions are a strong simplification of reality in many situations, they lead to simple calculations, and are accurate in some situations (see below).

At any cross-section in the creek, the discharge in the channel Q_c must be equal to the rate of change in water volume upstream (dVol/dt). Thus,
Q_c= dVol/dt = (dVol/dh)*(dh/dt)
where the last step follows from the ‘chain rule’ for differentiation. Because the average velocity at this site in the creek is v_c = Q_c/A_c (where A_c is the channel cross-sectional area), it follows that:
v_c = [ (dVol/dh) / A_c ]*(dh/dt) = F(h)*dh/dt
where F(h) = [(dVol/dh) / A_c]. The assumptions mean that F(h) is purely a function of h (and the location of the cross-section). Qualitatively, this equation says that the velocity in the creek can be predicted purely from a water elevation time series, by estimating dh/dt (the rate of change of the water level), and F(h). Although the latter is unknown, it can potentially be estimated with morphological data, or statistically as a high order polynomial of h (using a time series of water levels, and velocity at the site).

Does this work for Yalimbah Creek? Figure 4 shows the predicted and measured water velocities over a few tides at the ‘velocity measurement’ site in Figure 1b, using water level data from a site in Number One cove (Figure 1b). The model does a decent job of predicting the velocities during smaller, within channel tides. It also qualitatively predicts the velocity peaks during overbank tides. However, it incorrectly estimates the magnitude of the flood velocity peaks, and the final ebb velocity peak. It also poorly estimates the timing of the ebb velocity peaks.

These results are consistent with the hydrodynamic theory reviewed above. During within channel tides, water slope effects are relatively slight, and so the ‘flat water surface’ model does a good job at predicting the within channel velocities. During overbank tides, the model is able to qualitatively fit the velocity peaks as the flow goes overbank. However, momentum effects make the simple ‘flat water surface’ approximation rather crude at this time, and the velocity is not simply a function of h and dh/dt at the mouth. Thus, the results are not as good. Another likely source of error is the assumption that the measured velocity time series reflects the mean within-channel velocity. In reality, variations in velocity over the cross-section mean that this will not always be a good approximation.

However, continuity-based models have the advantage of simplicity. They can also be extended to two dimensional flows (Fagherazzi, 2002; Fagherazzi et al., 2003). Results from the latter studies also suggest that they perform well in some situations. However, it is important to realise that some basic features of tidal hydrodynamics cannot be explained while ignoring flow momentum conservation. This includes the asymmetry of tidal currents. While much is known about such processes, to understand them you will need to consider the principles of momentum conservation, as well as mass conservation.

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Estuarine Shore Crabs

EstuarineShoreCrabs-compFinal

Find out about them here! Click above.

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