Long Swamp restoration trial update – May 2014

It has been a couple of weeks now since the Long Swamp Restoration Trial got underway, and it might surprise you to hear that in this short period of time we have already learned a great deal. Enough in fact, to enable us to begin planning the next stages of the trial. Let me explain…

Those people that helped us build the first sandbag weir structure on the 9th of May, will have noticed how quickly water levels rose in the channel upstream. This created the challenge of additional pressure on the weir structure before we had it adequately reinforced, which is why we slightly dropped the level on a temporary basis to enable us to reinforce the central section of the weir, before reinstating the top level of bags and capping the structure with heavier duty white geo-fabric sandbags for stability given the speed of the over-topping flow.

Since the heavy rainfall of a couple of weeks ago, water levels throughout Long Swamp were receding at the time we built the structure, as indicated by the levels leading up to construction commencing:

62.5 cm – 5th May

62 cm – 6th May

58 cm – 9th May – 8:50 am (Prior to construction commencing)

The  change in levels immediately upstream of the structure, on the day it was being built, were recorded as follows:

81 cm – 9th May – 11:38 am

104 cm – 9th May – 1:24 pm

105 cm – 9th May – 1:48 pm

107 cm – 9th May – 3:43 pm

107 cm – 9th May – 4:34 pm

Subsequent visits to the site, have shown that levels began to recede immediately from their peak on the 9th, consistent with the background trend prior to construction commencing:

106 cm – 10th May

101.5 cm – 20th May

Meanwhile, our gauge board back a few hundred metres away in Long Swamp swamp itself only showed a very minor change in elevation response during construction, otherwise displaying the background trend of declining levels:

28 cm – 6th May

24.5 cm – 9th May – 1:29 pm

24.75 cm – 9th May – 4:24 pm

24 cm – 10th May

16 cm – 20th May

So what does all this mean?

The speed that levels rose in the drain during construction indicated that we initially mostly filled only the channel itself. This was a very different response to the restoration trial we installed in wet conditions last year at Gooseneck Swamp, where the channel is situated immediately adjacent to the swamp, and hence (due to storage volume) had a more gradual but direct effect on regulating swamp levels.

Some peripheral low-lying areas upstream (between the structure and Long Swamp proper) have now been inundated, but the weir pool effect created by the structure does not quite extend all the way back to the main swamp itself. Given site conditions (early flows), access considerations under those conditions, the geometry of the channel and the height of the banks we have initially anchored the structure to, we were initially limited to working at the current site this year to start the trial, and can’t go any higher at the existing trial structure location. ┬áHaving said this, the trial structure is, and will continue to have the effect of slightly reducing discharge efficiency from the swamp, which is what caused the slight increase in levels at the gauge board in the swamp during the afternoon of construction.

The beauty of this being a trial is that each step we take will inform the next, and lead us (by providing real observational data) to an eventual restoration solution for the site.

Where to next for the trial…?

Because we had limited detailed elevation data before we started, the weir provides us with an excellent platform to now work from. We’ll be taking some further levels at the site, and investigating options for a second, smaller upstream stage to the trial this year.

Summing up

We realise the trial approach is a little different to what people are used to for engineering projects, in that we can’t yet say exactly what the end product will look like; however, we can say that each step we take will bring us closer to the answer; in a more cost-effective manner than would be possible using more conventional approaches. Another key reason for adopting this approach is because we are dealing with such a dynamic environment, and ecology is not as “absolute” as engineering, but this flexible approach allows us to juggle the needs of both at the same time!

More to come soon…

The structure on the 20th May 2014

 



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