Seawater flows

A Basic Understanding Of How Much Sea Water May Flow Through The Murray Mouth And The Lower Lakes And Coorong System

By Mike Young & Ken Jury 22/7/09

It is difficult to accurately calculate the volume of water that flow’s from one area to another in a natural environment! It is however, a different issue to calculate water flow through piping because we know how big the pipe is, (it’s diameter) and length and we know what pressure is pushing it, so a reasonable estimate of the flow rate can be determined from those basic parameters.

But in a natural environment, where the water is tidal and will move from one place to another, it is a different situation again.

Firstly, if flow speed increases, then erosion of channel dimensions will occur where the channel widens and deepens causing flow speeds to slow down. The changing dimensions of channels mean that they may widen and deepen when the demand is there and flow rates are high. Larger channels move more water in a given space of time, lessening the demand if the source is limited. For example, a bottle empties quicker if you cut the neck from it! These variables make it difficult to visualise what to expect in the vicinity of the Murray Mouth and the barrages, if we were to use tidal flow to and from the lakes as a restoration mechanism. It would be pointless to try and express in exact units per hour, the expected volumes of sea water which would flow through the existing entrance channels (such as they are), and in any flow–improved channels after a period of time.

Nevertheless, it is far more satisfactory to try and paint a picture of what would happen in simple terms, which would more easily be visualised.

Lake Alexandrina (click here for a map) is something like 30km one way by 15km the other while Lake Albert is about 15km by 10km. It is about 10km from Tauwitchere Island to Point Sturt to the west and add a further 100 sq km to include the Goolwa channel and the remaining smaller anabranches around the islands. The area of the lakes when full is about 800 sq km. The area encompassing the lakes is roughly round. If water entered it from an unlimited source, it would spread over the surface quite quickly because almost un-interrupted, it can gradually fan out over all expanses. But this is not so for the Coorong.

The Coorong is long and narrow and for our example, we’ll use a part of it in this simple exercise. It is about 50km from Pelican Point to Hells Gate, located part-way down the Coorong. For ease of reckoning the figures, we’ll assume the Coorong is on average about 1km wide. That’s equal to 50 sq km of surface area.

The only unit we need to use with regards to volume of water is a gigalitre, 1,000,000,000 litres or a water body 1 square km x 1 metre deep. You may elect to double the depth of the Coorong from one to two metres deep. If so, then you would have a volume equal to 1 sq km x 2 metres deep, being 2 gigalitres. The sq kilometre x 1 metre deep figure is useful when referring to and measuring from a map to calculate how many square km’s would fit. In the previously mentioned 50 sq km section of the northern lagoon of the Coorong, given the elected depth is one metre, a water body of 50 gigalitres would be required to fill the void. If you think it’s only half that depth, then a half metre over 50 sq km would require 25 gigalitres.

Then we need to understand flow rates, eg volume of flow over a given period of time! An example of flow rate is often found on sprinklers showing the rate in litres per hour. Volume over a given time! Pressure at the sprinkler will influence flow rate, dependent on how much you turn the tap on. Turn the tap almost off and the pressure at the sprinkler head reduces because the flow has been reduced by a restriction at the tap.

In our Murray Mouth scenario, we only have tide height to provide the pressure, the difference in surface levels inside the lakes and at sea. The tide variation from high to low tide every 6 hours is more than 1.5 metres during big tides, down to little variation during dodge tides. Dodge tides are known in very few places around the world, but here in South Australia, these do occur. Our local tides miss about two cycles each fortnight because of them.

So, pressure (tide height-or more precisely, difference in height) is what will dictate how fast water will flow from one point to another and the narrowest restriction along the way will be the limiter on how much water will pass through in a given time. A 1metre tide only has a half metre head of water above its ‘average’ height, to provide the pressure. The other half metre is below average.

It is a common expectation to have tidal flow travel at 5km per hour –about walking pace when it’s flowing into and out of areas with tidal influence.

The flow expectation here should not for example, be confused with that of the NW Kimberly’s or that of Broome where massive tides occur. Our tides are quite modest.

The 5km/hour figure provides an indication of the distance the water body would travel at that speed, eg 5km in one hour. Therefore, in a channel 1km wide and 1 metre deep, 5 gigalitres would have passed a given point in one hour. These figures can be adjusted to suit, for example a fifth of a km (200 metres) by 1 metre deep would result in 1 gigalitre passing by (at 5km/hour).

However, the tide only rises for 6 hours. Therefore from half way up on a rising tide, to halfway down takes in the period of six hours total, at the upper half of the tide.

The other half of the tide is halfway down, through the bottom of the tide to half way up again so there is only about 6 hours of “tide pushing” available in each 12 hours. Six hours at the 5km/hour equals 30km distance. But we have already said in our scenario that the upper or Northern Lagoon of the Coorong is 50km long and therefore the water doesn’t quite reach the end. In truth though, only the top 2 hours either side of high tide would be still high enough to maintain the flows in at 5km per hour. The balance is slowing down. So, it didn’t make it much past the halfway mark down the North Lagoon on a single tide, even if the mouth area had been wide and deep. It’s too long and narrow and a little like tipping a bucket of water out into a gutter where it takes a while to move along.

But, if it’s long and narrow, then the wind will also push it along. In the North Lagoon scenario, it does. When the wind blows from the north-west over a period of about two days, it tends to hold incoming high tide water in the North Lagoon by pushing it down towards Hells Gates, where it holds it while the particular wind prevails (often over several days). The levels at Hells Gates rise and some of the flow manages to squeeze through the much reduced and very narrow channel of here. The NW wind then holds the water against a falling tide and slows its escape through the mouth.

When SE winds blow the other way around, Northern Lagoon water is then blown towards the mouth where some is expelled on outgoing tides.

In real terms, the health of the Southern Lagoon of the Coorong was previously maintained by contributions of freshwater as seasonal flows from the South East through Salt Creek, mixing with the tides from North Lagoon. Sadly, for many years, the S.E. drainage scheme has denied the Coorong of this freshwater injection.

Rehabilitation of the Coorong can therefore be achieved by restoration of the South Lagoon freshwater flows and with manipulation of tidal flows from areas fronting the Tauwitchere barrage to the Murray Mouth region, to clear unwanted silt currently restricting the system.

River Murray freshwater has never contributed significantly to the Coorong’s health, which means that this icon water body could be restored to its previous health, independently of the Murray. These are separate entities, only co-joined to the lakes from Pelican Point on to the mouth and they should be treated as such!

Reverting to the lakes, we can see that the area is relatively round with a likely area of 800 sq km when full. That would be 800 gigalites of water if it were 1 metre deep over the total surface area. Today, much of it has dried out resulting in a surface area reduction to about 500 sq km.

The remaining area of 1 metre depth or more would be smaller again due to the saucer shape of the bottom of the lake. It is therefore shallower around the edges. But, if we add 1 metre of tidal water to raise the level a further metre, the shoreline will then extend out again to a position nearer the original 800 sq km surface area.

Its exact position doesn’t matter a whole lot and it won’t be 1 metre deep at the edges. However, if you remove 100 sq km from the equation, that would be equal to removing 100 gigalitres for a reduced overall capacity of 700 gigalitres.

The existing barrage structure uses gates which are opened and closed individually, as a slow and cumbersome operation. We cannot open and close them quickly enough to take advantage of the tides.

If the existing concrete logs (or gates) in the barrages were replaced with single full-depth polyurethane gates and these opened and closed in unison then we could regulate inflow into the lakes through the barrages to make use of higher tides, and release water out on falling tides through an elected channel to bias scouring of silt out of the system on outflow.

Inflow scouring could be limited by reducing flow speed on rising tides, by allowing water into the lakes through all barrages. For example, by allowing inflow through the Goolwa, Mundoo, Boundary Creek, Ewe Island and Tauwitchere Barrages and then out through Tauwitchere and Ewe Island only, it would provide for clearing the silt from the Coorong entrance.

Then, on outflow, allow most water out through Goolwa and Mundoo barrages to keep those channels clear. As channels gradually enlarge, flow rates will increase!

Engineering solutions are, no doubt available to open and close modified barrage gates (poly-tanks material) using pumped water weight or pressure!

In its most simplistic form, picture a pivoted lever over the top of each modified gate, one end of the lever attached to the top of the gate, and the other end holding a large bucket. When water is pumped into the bucket, the gate opens because of the weight of the water. When the bucket is drained, the gate falls and closes again. The water is available onsite (it’s what we are manipulating). A single pump at each barrage site could load all buckets at once and they could be emptied via a simple valve in the bottom of the bucket.

No doubt, engineers would be able to smooth out the principles here.

Tight fitting gates and seals are not necessary as the gate’s role is only to resist against outflow on falling tides to permit a selected channel to carry most of the outflow. The object is to be able to raise or lower multiple gates easily and quickly.

It is expected that modifications will only be necessary on existing structures, not to them. The barrages themselves are likely to be the greatest flow restriction once channel scouring is achieved, and the limitations of the barrages to pass large volumes of water would be to provide limited flow with regard to channel and mouth scouring.

However, water passing the mouth puts us back to the original equation. That is, should it be 200 metres wide and 1 metre deep it would pass 1 gigalitre per hour at a flow speed of 5km per hour. Extending this, that’s 5 gigalitres per hour if it were 5 metres deep. Old records remind us that the mouth was 6 fathoms deep, in parts.

That’s more than ten metres deep so if it were, say at the upper end 500 metres across and 10 metres deep, that’s 20 gigalitres per hour flow. It’s easy to see how much the volume can be, with thoughtful modification. Our figures to-date are based upon 5km flow rate per hour.

What the exercise does show is that the head of the water available in the lake is large enough to provide continuing flow through the available time of a falling tide – say for three hours at its most effective period.

Even if the mouth settles down at only 200 metres width and 5 metres depth, and it flows at 5km/hour; that’s 5 gigalitres per hour, 20 gigalitres for our available 4 tidal hours and then it does that twice daily. This results in over 30 gigalitres/day, with this figure having been previously quoted as the required flow rate to keep the mouth open.

The expectation is for a larger, deeper mouth that will flow faster.

As silt is exhausted from the lakes entrances by thoughtful use of tidal flows, channels would become wider and deeper, increasing the volume of water accessing and leaving the lakes at each tide.

The need to manipulate the barrage gates would diminish as the channels carrying large volumes of water establish, and flow bias may only need to be used from time to time to maintain clear channels. Larger volumes of tidal water entering and leaving the lakes would re-establish some of the historic channels and sandbanks still visible now, on the floor of the lakes.

By Mike Young & Ken Jury 22/7/09.