overflow (relief) gully

see gully

a self-cleansing trap, provided with a suitable loosely fitted grating located above ground surface level, provided for the relief of any possible surcharge and to provide a measure of protection against overflow from a drain

gully with a loosely fitted grating, normally installed slightly above external ground level, so that any overflow condition or surcharge in a drain or sewer will result in external overflow rather than backing up into internal pipework or fixtures