Water spreading bunds is a technique in the category of floodwater farming and consists of earthen bunds, usually applied where trapezoidal (regular) bunds are not suitable. For instance, where runoff rates are high and the trapezoidal bunds 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.
As the name suggests, 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 designs of the water spreading bunds depending on the slope of the land where this technique is applied.
For land slopes of less than 0.5%, the layout consists of straight opened ended bunds placed across the slope. These are meant to spread and slow down the flow. They are usually sited 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 greater than 0.5%, the structure displays a series of graded bunds sited 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 it continuously flows through the bunds and, secondly, because only part of a watercourse flow is canalized towards the cultivated area, so of the amount of water actually utilized is difficult to estimate.
There are several different designs for water spreading bunds and the choice for the most effective one needs to take into account the quantity of water that will reach the structures, the slope of the land, the type of soil and the availability of labour.
As in all other water harvesting systems that involve earthen structures, maintenance is crucial especially in the first seasons when the bunds are not consolidated yet. 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 that helps bind the soil together. Nevertheless, heavy floods can happen time over and ruptures can occur at any stage.
Image source: Food and Agriculture Organization of the United Nations, 1991, Will Critchley and Klaus Siegert with contributions from C. Chapman and M. Finke, Water harvesting, http://www.fao.org/3/u3160e/u3160e00.htm#Contents. Reproduced with permission.