Floodplain water storage

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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