Waste weir
A waste weir on a navigable canal is a slatted gate on each canal level or pound, to remove excess water and to drain the canal for repairs or for the winter shutdown. This differs for a dam or reservoir, for which a waste weir is another name for a spillway, i.e. not having the boards to adjust the water height nor the paddles to drain all the water as on a canal, only to drain the excess.
Uses
A canal will often need some means to maintain a water level. A canal constantly consumes water due to leakage, evaporation, and the operation of lift locks. Nevertheless, excess water from storms or emptying locks could cause problems eroding the banks of a canal, causing washouts, and flooding buildings or adjacent properties. Waste weirs were one of several items used to remove surplus water. The waste weir also functioned as an opening to drain the entire canal prism of water for repairs, or for winter, or in anticipation of flooding.In some cases when a creek was used to feed the canal, a waste weir was necessary for excess water from the creek. For instance on the Morris Canal, the Lopatcong Creek went into the canal to feed it, but it had a tendency to flood, hence the need for a couple of waste weirs in the area around the bottom of Inclined Plane 9 West near Port Warren, New Jersey.
Reservoirs and tanks also use a waste weir for flood discharges.
Design
The maximum flow output in cubic meters/sec over the top of the waste weir with a length and height of water above the crest can be calculated by the formulas:and
where
g=gravitational constant of
and Cd is the coefficient of discharge over the weir.
Various values of Cd can be given as follows:
| Crest type | |
| Weir with crest up to one meter in width | 0.625 |
| Weir with crest greater than one meter in width | 0.562 |
| Rough stone sloping escape | 0.500 |
| Flush escape | 0.437 |