Similar names: diversion embankments, diversion bunds, low lying crescent embankments, spate irrigation (general term).
Water spreading bunds is a technique in the category of floodwater farming (also called spate irrigation) which consists of the use of earthen bunds, usually applied where trapezoidal (regular) bunds are not suitable. Regular bunds may not be suited for area where runoff rates are high and they would be damaged, or where the crops may suffer from flooding.
The goal of the water spreading bunds is, as the name suggests, to spread water and not to confine it, like trapezoidal bunds do. They are intended to spread floodwater that has been either diverted from a watercourse or has reached the floodplain. The technique of water spreading bunds is very site-specific, as it can only be used on lands close to a watercourse, usually on alluvial fans or floodplains, and is characterized by almost flat slopes (up to 1%). Water spreading bunds are usually applied on even lands with deep and fertile soil, with a precipitation rate within 150-300 mm. Therefore, hyper-arid or arid areas where floodwater farming is often the only possibility to achieve fodder or crop production.
- Maintenance: as in all other water harvesting systems that involve earthen structures, maintenance is crucial, especially in the first seasons when the bunds are not yet consolidated. It is therefore recommended to carefully inspect the areas after all runoff events. The risk of breaching diminishes over time because bunds have consolidated, and vegetation has developed, helping to bind the soil together. Nevertheless, heavy floods can happen at any time and ruptures can occur at any stage.
- Community buy-in: As this is quite a large-scale intervention, it is important to attain the approval of the community and the necessary authorizations to build such structures over the determined piece of land.
- The soil type plays a fundamental role in the resistance of the bunds: in areas with loose soils, it is essential to make the size of the bunds larger in order to contain larger quantities of water runoff and, if possible, to reinforce the bunds with stones and vegetation.
- An Abney level or line level (If you have a line level and a measuring tape available, this is also a good way to measure the slope of your land).
- Pegs or stones.
- String (not strictly necessary but useful).
- Measuring tape, a graded stick, or something for measuring.
- Wooden triangular right-angle frame (or something to measure 90 degrees angles).
- Instrument to measure the slope: some examples of tool you can use are an Abney level, a line or a spirit level.
Click here to see one way of measuring the slope with a line level.
Water spreading bunds are used to spread water that reaches the floodplain, either because it has been diverted from a watercourse or because the water has naturally reached the floodplain. The earthen bunds slow down the runoff and let it spread over the cultivated land, thus they increase the infiltration rate.
There are two potential designs of the water spreading bunds depending on the slope of the land on which this technique is applied.
- For slopes of <0.5%: the layout consists of straight open-ended bunds placed across the slope. These are meant to spread and slow down the flow of water. They are usually situated at around 50m apart and it is recommended that they overlap so the flow discharged by the bund above is intercepted by the one below.
- For slopes of >0.5%: the structure consists of a series of graded bunds situated along a ground slope of 0.25%, and each successive bund is graded downslope starting from the opposite end. On the upper end, each bund is provided with a short upslope wing that collects the flow from the bund above and canalizes it towards the bund downslope. The spacing between the bunds depends on the slope.
In both designs, the bund cross section is recommended to be maximum 100 m long and 60cm high with a base width of 4.1m, and a top width of 50cm. When preparing the design of the water spreading bunds intervention, the calculation of the catchment/cultivation area ratio is not necessary, and not easy to achieve. This is because, firstly, the floodwater is not impounded, but instead continuously flows through the bunds, and secondly, because only part of a watercourse flow is canalized towards the cultivated area, so the amount of water actually utilized is difficult to estimate.
For a detailed step-by-step please see the instructions given here.
There are several different designs for water spreading bunds and in order to choose the most effective option, one needs to take into account:
- the quantity of water that will reach the structures.
- the slope of the land
- the type of soil
- the availability of labor.