A trash rack (US) or debris screen is a wooden or metal structure, frequently supported by

masonry Masonry is the craft of building a structure with brick, stone, or similar material, including mortar plastering which are often laid in, bound, and pasted together by mortar (masonry), mortar. The term ''masonry'' can also refer to the buildin ...

, that prevents water-borne debris (such as logs, boats, animals, masses of cut waterweed, etc.) from entering the intake of a

water mill A watermill or water mill is a mill that uses hydropower. It is a structure that uses a water wheel or water turbine to drive a mechanical process such as milling (grinding), rolling, or hammering. Such processes are needed in the production ...

, pumping station or water conveyance. This protects

water wheel A water wheel is a machine for converting the kinetic energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a large wheel (usually constructed from wood or metal), with numerous b ...

penstocks A penstock is a sluice or gate or intake structure that controls water flow, or an enclosed pipe that delivers water to hydro turbines and sewerage systems. The term is of Scots origin, and was inherited from the earlier technology of mill ponds ...

, and

sluice gate A sluice ( ) is a water channel containing a sluice gate, a type of lock to manage the water flow and water level. There are various types of sluice gates, including flap sluice gates and fan gates. Different depths are calculated when design s ...

s from destruction during

flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...

s. They are sometimes fitted to storm drain inflows and outflows to prevent debris from entering a natural watercourse. Other names used as trash screen (US) or debris grille. Trash racks composed of vertical wooden

strake On a vessel's Hull (watercraft), hull, a strake is a longitudinal course of Plank (wood), planking or Plate (metal), plating which runs from the boat's stem (ship), stempost (at the Bow (ship), bows) to the stern, sternpost or transom (nautica ...

s separated by narrow gaps are very common and perform extremely poorly. Metal gratings are sturdier and can have narrower strakes, and angling the trash rack properly can allow some self-cleaning from the action of the water. Modern trash racks as used by

hydroelectric plants Hydroelectricity, or hydroelectric power, is Electricity generation, electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other Renewable energ ...

can incorporate such advanced features as wedge-wire screens, the Coanda effect, and cleaning

robot A robot is a machine—especially one Computer program, programmable by a computer—capable of carrying out a complex series of actions Automation, automatically. A robot can be guided by an external control device, or the robot control, co ...

s. In waterways with large amounts of floating debris, various permanently installed "trash rakes" may be required to reduce the labor required for regular cleaning. Trash racks are designed for water velocity of around 2 feet/second (0.6 metres/second) to prevent excessive energy loss due to the head loss across the trash rack. Close spacing keeps out more small floating debris or fish, but may plug up easily with

frazil ice Frazil ice is a collection of loose, randomly oriented ice crystals a millimeter and sub-millimeter in size, with various shapes, e.g., elliptical disks, dendrites, needles and of an irregular nature. Frazil ice forms during the winter in open-wa ...

in cold climates. U.S. Army Corps of Engineers, ''Ice Engineering - Engineering and Design (EM 1110-2-1612)'' page 20-11

References

Watermills {{Industry-stub