Weirs, Sluices and Penstocks
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, called the primary or main weir. As water should always be spilling over this structure, it must be robust and of sufficient size not to block easily.
After a storm, the volume of water entering a lake will increase. More water will flow out over the primary weir, and if the flow into the lake then exceeds the flow out of the lake, so the water level in the lake rises… at which point a much larger weir is needed to accept the flow from out of the lake. This is the secondary or emergency weir.
Normally, the secondary or emergency weir will be set 100mm to 200mm above normal water level and it must be capably of taking a full storm flow without its surface deteriorating. For short floods, a simple grass weir may be adequate, but for long durations or very fast flows, 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 ancient usage 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
Interchangeable names, used to describe a moveable barrier placed in the flow of water to maintain the upstream level. When a gate is closed, water rises up behind it; finally, it may flow over the top or move down an alternative channel.
Sluices, gates and penstocks are often used at mill sites to control the flow of water 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.
Penstocks can be made from various materials, stainless steel, mild steel, timber or plastic, but they all 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 upstream, or to prevent flood water in a main channel flowing back into outlets along its banks.
These are used in a similar way to weiring penstocks, to 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.