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When erosion control work is carried out with floodplain rehydration in mind, a more sustained creek flow at the base of the property is one of the most common outcomes. Yet, of all the water harvesting and hydration concepts I’ve discussed with people, this remains the one which draws the most skepticism.

The following diagrams illustrate the way this process works:

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This is a fairly typical erosion gully in the Southern Tablelands. The major erosion happened decades ago, therefore the floor of the gully has mostly revegetated and stabilised. However, the alluvial aquifer remains drained down to approximately the base of the gully.

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With the gully incised, in many locations even the largest runoff events remain contained within the channel. For the short period the creek is running high, there is some lateral infiltration into the alluvial aquifer, but it’s often a fairly insignificant amount.

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Depending on the order of the stream, porous structures (or leaky weirs) of varying heights and types can be constructed (ie vegetated earth-banks, rock gabions, log sills, fascines, brush mattresses etc).

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Whenever there is sufficient flow from above, the structure causes a pool to form. As well as enabling riparian and wetland vegetation to establish with the associated bed stability and habitat benefits, the raised water level in the pool encourages water to laterally rehydrate the surrounding floodplain.

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When flood flows occur, depending on the height of the structure, access to the floodplain is now available once again. With the water spread in a thin sheet across the land, it not only reduces the energy and erosive potential within the channel, but also gives more opportunity for the alluvial aquifer to recharge, with infiltration from above.

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In time, depending largely on how porous the floodplain sediment is, the alluvial aquifer will be raised. (The closer to the surface the water table, the more important the flooding process becomes, due to the freshwater lens it creates over the heavier saline groundwater.)

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Due to porous nature of the structures and floodplain sediments, during extended periods without flow coming into the system, the pools can begin to drop. At such time, water stored in the floodplain begins to feed back into the creek, providing an extended base flow, potentially creating a perennial flow.

At the same time, depending on how high the water table has been raised, floodplain vegetation will benefit from moisture available through capillary action. Deep rooted perennial grasses and riparian trees such as Casuarina and Populus will benefit sooner of course, and provide construction material for further channel repair.

Disclaimer: Where water flow is concerned there are substantial risks involved. While the information and images we publish are formulated in good faith, with the intention of raising awareness of landscape rehydration processes, the contents do not take into account all the social, environmental and regulatory factors which need to be considered before putting that information into practice.  Accordingly, no person should rely on anything contained within as a substitute for specific professional advice.

Article and Images © Cam Wilson, Earth Integral, 2012

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This is a pictorial tour of the degradation and dehydration process that the Australian landscape went through post European settlement, along with one of the major aims of Peter Andrews’ Natural Sequence Farming approach, namely the rehydration of the Australian landscape.

If you were one of the early explorers, walking into a wide floodplain system in the early 1800s, more than likely you would have found some form of discontinuous watercourse. One example is known as a ‘chain of ponds’, in which you’d find small bodies of open water, about a metre below the level of the floodplain, held in place and separated from the next pond by a marshy plug of reeds such as Phragmites.

These ponds weren’t the whole story though. They were just the tip of the iceberg and indicated the level of the water table under the rest of the floodplain-step. That is, moisture within a metre or so of the toes of all of the plants on the floodplain.

When a decent flow occurred, rather than it rushing downstream, the reed beds would slow the water causing it to gently rise and flow over the banks onto the floodplain. This gave the water plenty of chance to infiltrate and recharge the aquifer below (a wise move for a landscape to make when the next generous rain might be a few months away). You might also notice something strange; the banks of the creek are higher than the rest of the floodplain. This is because when the water spills over the banks, the largest sediment settles out first, building up a levee over time.

With the landscape scouted, settlers soon arrived with their animals, ring barking as they went. There weren’t many stock troughs in those days, so of course the animals had to drink from the creeks.

The hard hooves soon cut tracks into the reeds and were one of the ways the marshy plugs were killed off.

With the plugs gone, coupled with the cleared, burnt and overgrazed hillside up above the floodplain, water could now build some momentum, and soon scoured out the deep erosion gullies we still see today.

With the ponds no longer in place, the gully turned into a really efficient drain….

… lowering the alluvial aquifer….

… down to the base of the incised channel. Once this occurred, rather than plants having moisture 1m below, they’re high and dry and at the mercy of the infrequent rainfall patterns experienced in much of the Australian landscape.

Once a channel is deeply incised, in many places even a large rainfall event is confined to the channel. This deprives the floodplain of the soaking sheets of water and fertile sediment of yesteryear.

Peter’s approach is about replicating the job that wetlands used to do. He creates ‘leaky weirs’ using locally available materials. Vegetation is an important component of the leaky weirs, with the fibrous root system of bioengineering plants such as willow used to tie the boulders together. This approach results in structures that are a fraction of the cost of the highly engineered structures commonly built by authorities. Peter also believes that the exclusion of livestock is important, except for periodic crash grazing.

The weirs enable chains of ponds to re-form, which begin to raise the alluvial aquifer (particularly through buried old creek lines which act as gravelly intake areas along the banks).

The rehydration will obviously happen faster in sandy soils than it will in heavy clay, but slowly the aim is for the alluvial aquifer to be raised. This is water harvesting in the form of reinstating a natural landscape process.

Eventually, the goal is to reinstate a drought-proof landscape.

At such time, flood processes become important once again, by creating a freshwater lens on top of the heavier, saline groundwater

If you’re interested in implementing strategies similar to these on your property, please contact us

Disclaimer: Where water flow is concerned there are substantial risks involved. While the information and images we publish are formulated in good faith, with the intention of raising awareness of landscape rehydration processes, the contents do not take into account all the social, environmental and regulatory factors which need to be considered before putting that information into practice.  Accordingly, no person should rely on anything contained within as a substitute for specific professional advice.

Article and Diagrams © Cam Wilson, Earth Integral 2012