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What are CD 526 hydraulic capacity classifications?

In essence, the ‘P, Q, R, S & T,’ ratings are classifications listed in Highways England DMRB document CD526 and are calculated to indicate a gully grating’s hydraulic capacity (or effectiveness at evacuating water from a hard standing area. e.g. carriageway).

It is the classification used by scheme designers to identify how many gully gratings will be required to evacuate rainwater from a project’s hard-surfacing (e.g. roads) and their spacing requirements. Basically, the more hydraulically-efficient the gully grating, the less number are needed on a scheme and hence, savings can be made by limiting the number of gully installations.

Generally, the larger the waterway area, the higher the hydraulic capacity (and the lower the CD 526 classification ‘letter’ with ‘P’ type gratings being the most efficient). Conventionally, for square grating assemblies with compliant BS EN124 grating apertures, ‘P’ classification types normally exceed 600x600mm Clear Area Frames (or are large rectangles, commonly around 450x1000mm Clear Area).