Although I am not familiar with the specific practices in Iran, as a general thought, most engineers are taught structural methods over biological. With ditches, they tend to be linear features and lack sinuosity, facets (riffles, pools, etc.) and floodplains that streams use to help manage energy. Also some plants and trees expand their influence over time, thus constraining the ditch dimensions and perhaps function. Biological methods out of control require maintenance that is sometimes not desirable. With adequate design, ditches and waterways might use just biological methods to help control erosion to a manageable level with careful attention to gradient and biological cover type, drainage area size or discharge controls. Soil bioengineering is sometimes used to combine structural with vegetative controls. And not mentioned, but there are all type of ditches, drainage ditches, road ditches, irrigation ditches, flood control bypass ditches, dam overflow protection ditches that can affect design.
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