Recommissioning Nature’s flood fertigation infrastructure

When building natural capital (including beef or wool), increased potosynthesis is the goal of any land manager. Available moisture is, of course, a key factor.

At the time Europeans settled in South-Eastern Australia, many broad upland valleys were described as chains of ponds or swampy meadows. There are a few of these well hydrated, very productive systems, effectively drought proof systems still remaining (for example the Hazell’s property), but the majority have been severely eroded and subsequently drained (click on the following for an outline of the degradation process, in diagrams or the scientific literature).

At Tarwyn Park, Peter Andrews demonstrated the potential primary production benefits from reinstating the original floodplain processes and rehydrating the surrounding landscape.

One way of doing so is by raising the alluvial water table through lateral infiltration (as described in the post Floodplain water storage). The speed this occurs depends on the soil type, but if it’s going to happen any time soon the main driving factor is a fairly constant supply of water from the catchment above.

High in the landscape, inflow from the catchment above is generally only available for a short period of time. Where this is the case, the effectiveness of relying solely on a lateral hydration approach is limited, as a severely drained landscape will take a considerable time (maybe several lifetimes) before the water table is raised high enough to enhance plant growth on the floodplain.

Where short sharp bursts of runoff are available, the fastest return can be achieved by reinstating the old flood flows. Water spreads out across the landscape once more, soaking into the floodplain for the extended use by the plants and soil life. Sediments are also deposited, the process which has made floodplains the rich production zones they are worldwide. Basically, it’s recommissioning nature’s flood fertigation system.

In an intact landscape, there are predictable locations where floodwater is more likely to top the banks, just as there are locations where it’s likely to re-enter:

On a macro-scale, floodplain flow patterns are often closely related to the ridges intruding into the floodplain (Tane, 1999)

Where multiple ponds exist between the major landscape features, braided flood flows (red arrows) generally exit the downstream half and enter the upstream half of a pond (P Hazell, personal conversation)

When siting structures, an understanding of these processes is the key to getting the most bang for your buck. A structure in an inappropriate location may get the water up onto the floodplain, but it will soon spill back into the gully, maybe even worsening the existing erosion. In contrast, a well positioned structure results in the flow heading away from the watercourse, spreading into a more passive flow and hydrating the floodplain surface before re-entering the stream sometimes hundreds of metres downstream.

On Gunningrah, Charlie and Anne Maslin have sited their structures as well as anyone I’ve seen with this goal in mind. Having taken inspiration from Peter Andrews on ‘Australian Story’ and attending a Natural Sequence Farming field day, Charlie has since constructed around 40 leaky weirs on Gunningrah (For more information about the Maslin’s farming prowess, see their profile in the Soils for life case studies).

There are a range of positive results which the Maslins have achieved depending on the landscape position of the works, but the following couple of examples are a good demonstration of utilising the original flooding processes mentioned above.

(Note: To avoid hefty fines, it’s important to adhere to local watercourse regulations. In many places there are few restrictions on ‘dam walls’ within first and second order streams other than the harvestable rights of the property)

Poplar site

Flow had become contained within the incised channel, taking shortest path it could towards the ocean. The only moisture available to the surrounding floodplain was what fell from the sky

An earth wall structure intercepts the flow in the channel, reconnecting it with the floodplain. The flow re-enters more than 500m downstream, with the potential to irrigate about 6 ha of pasture.

The poplars indicate the path of the incised channel, the flow now spreads out across the floodplain

Looking upstream at the same structure, the flow spreads significantly across the paddock.

Debris in the middle of the paddock, around 50m from the main channel.

Hayshed site

Flow path before the works….

….. and afterwards, back to how it once was

An aerial view of the flow before the works were completed, contained within the incised channel

An earth wall intercepts the channelised flow, spilling onto the surrounding floodplain. For an idea of the extra water harvesting potential which results, 0.25 Megalitre is stored for every 25mm of water that’s accepted by the landscape per hectare. A healthy topsoil can receive far more than that.

In case you’re still wondering “How can water flow away from the main watercourse? Isn’t that always the lowest point?” It is in a young landscape, but Australia’s pretty geriatric as far as watersheds go.

In Back from the brink, Peter Andrews talks about water flowing on the high ground (of the floodplain). This phenomena was observed by plenty of early explorers and it’s also well accepted in the scientific literature. In short, when a watercourse spills its banks, the water slows down, depositing the heaviest sediment. In time, a natural levee is built as seen below.

If you’re interested in getting these processes happening once again on your land, contact us to find out about our design, consultancy and implementation services.

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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

References

Tane, H. 1999. Catchment Habitats and Landscape Ecosystems. Centre for Catchment Ecology, 1: 1-12

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