Which one of the following methods is not suitable for urban rainwater harvesting?

Urban rainwater harvesting primarily focuses on capturing runoff from rooftops and paved surfaces to recharge groundwater or for direct storage. Common urban techniques include rooftop recharge pits, recharge wells, and recharge trenches, which are designed for high-density areas with limited space. In contrast, a gully plug is a traditional rural water conservation method. It involves building structures using local stone, clay, and bushes across small streams or gullies to check runoff flow and conserve soil moisture. While effective in agricultural watersheds and hilly terrains to prevent erosion and facilitate percolation, gully plugs are not suitable for urban environments due to infrastructural constraints and the risk of blocking drainage systems during heavy rainfall, which could exacerbate urban flooding. Therefore, gully plugs are categorized as rural or watershed management techniques rather than urban harvesting methods.