This is a fixed barrier placed across a stream or channel to provide a raised level to maintain minimum upstream water levels. Weirs are often used as a site to measure flow in a river.
Primary and Secondary Weirs, Emergency Spillway
The water-level in a lake is often set by a fixed weir, this is called the primary or main weir, water should always be spilling over this structure so it must be robust and of sufficient size not to block easily. After a storm the volume of water entering the lake will increase. So more water will flow out over the weir, if the flow into the lake exceeds the flow out of the lake, so the water level rises, at this point a much larger weir should be to accept flow from the lake. This is the secondary or emergency weir. Normally this will be set 100mm to 200mm above normal water level and must be capably of taking the full storm flow without the surface of the secondary weir deteriorating. For short flood events a simple grass weir may be adequate, but for long durations or very fast flows then the weir must be protected to prevent erosion.
Typically used in fish farms to control water level and allow the draining of a stock pond by the removal of timber boards. The name is said to derive from their ancient use in the fish ponds of monasteries. A three sided brick or concrete tower has a double or treble set of timber boards in the fourth face to maintain top water level. The boards can be set at any level and be arranged to take water from below the maintained level of the lake to avoid problems with floating debris.
Sluices, gates, penstocks
These names are interchangeable and are used to describe a moveable barrier placed in the flow of water to maintain the upstream level, when closed the gate will cause the water level to build up and it may then flow over the top of the gate or down an alternative channel. These devices are often seen at mill sites to control the flow through the mill wheel or to pen water into a mill pool. A wide fixed weir will often have a sluice next to it to allow the weir to be bypassed.
The penstock can be made from various materials, stainless steel, mild steel, timber or plastic, but all will have some system of screws or jacks to allow the gate to be lifted fairly easily. Penstocks are made in widths and depths to suit the required application. They can be fitted on the wet (on seating) or dry (off seating) sides of a structure or they may be mounted in the middle of a concrete or masonry channel.
A weiring penstock has an adjustable top level, so rather than opening from the bottom, the gate goes up and down at the top to vary the retained level.
In hydro generation schemes the name penstock or drive pipe is used to describe the inclined pipe or channel that contains the water that drives the turbine.
These are removable boards (logs) placed in the water flow, in guides or a frame to maintain an upstream level. They act in a similar way to a gate or sluice but have no winding gear fitted to them and must be physically lifted out of the channel to vary the penned level.
Flap valves are simple hinged lids that prevent water flowing back through a structure or pipe. Typically they are fitted in tidal situations to prevent salt water flowing back up stream, or to prevent flood water in a main channel flowing back into outlets along its banks.
These are used in a similar way to a weiring penstocks and provide an adjustable weir level, often fitted with electric or mechanical float controls they can maintain either a given flow in a stream or a required level both upstream and downstream of a structure in various flow conditions.
For more information or to discuss your particular requirements call Chris Keech on 01460 279200.