drainages

Drainages refers to the system and processes by which water is removed from land, including surface runoff, streams, rivers, and subsurface flow. The term covers natural hydrological networks as well as engineered systems designed to remove excess water from soil or urban areas.

In natural landscapes, drainage is organized by drainage basins or watersheds, bounded by drainage divides, and outlet through rivers. The pattern of streams—dendritic, trellis, radial, annular—reflects geology and topography. Drainage density, defined as the total length of streams per unit area, summarizes how dissected a landscape is.

Subsurface drainage lowers the water table and improves soil conditions for agriculture and construction. Tile drains and perforated pipes collect and transport groundwater to surface outlets.

Urban drainage networks collect stormwater through gutters, sewers, and outfalls; agricultural drainage uses field ditches and tile drains; canals are used for irrigation and navigation.

Drainage affects flood risk, soil stability, groundwater, and wetland ecosystems. Conventional drainage can degrade water quality and biodiversity. Sustainable drainage, or green infrastructure, uses swales, bioswales, rain gardens, permeable pavements, and retention basins to manage water while enhancing infiltration.

Design and management rely on hydrological data, precipitation, soil properties, and drainage coefficients; calculations include discharge estimates and, for networks, drainage density.