From time to time every weir installation will need periodic maintenance; whether to inspect the weir crest or to clean debris out of the weir pool.  A simple and handy accessory to help with this is a clean-out opening or coupling.  In weir boxes and manufactured channels, a clean-out allows the weir pool to be completely drained, while in natural channels it allows the water to drop below the crest of the weir.

When sizing a clean-out it is important to make it large enough to pass the anticipated debris or allow the weir pool to be drained in a reasonable amount of time.

When positioning the clean-out simply make sure to position it towards the headwall / discharge bulkhead and low enough to drain most (if not all) of the weir pool.  In industrial applications it may be necessary to direct the clean-out discharge into a drain or back into the process stream.  Make sure to allow sufficient space to make whatever temporary connection is necessary.

For weir boxes in industrial applications, it may be possible to permanently plumb the discharge to a receiving drain.  If that is so, then a valve should be installed inline so that the operator can accomplish the drain procedure without assistance.