Drainage System Design, Hydraulics and Falls
Sizing, fall and air
Design has three jobs: carry the load, keep the pipe self-cleansing, and protect trap seals. Loads are counted in discharge (fixture) units and converted to pipe size using the AS/NZS 3500.2 tables; the size chosen is then checked against the minimum grade permitted for that diameter. Typical minimum grades quoted in AS/NZS 3500.2 are of the order of DN65 1:40, DN80 1:50, DN100 1:60 and DN150 1:100 - always confirm against the current edition and the consent documents rather than memory.
Grade does two things: it produces a self-cleansing velocity (roughly 0.6 m/s at design flow) and it keeps the drain above the water table and main sewer invert. Ventilation - individual vents, branch vents, stack vents and relief vents, or air admittance valves where permitted - limits the pressure excursions that suck or blow trap seals.
Stormwater is a separate calculation: catchment area × rainfall intensity (rational method Q = CiA/360) with AS/NZS 3500.3 sizing gutters, downpipes and drains, plus detention, freeboard and outlet capacity.
Traps
Gradient ratios versus percentages: 1:60 is 16.7 mm per metre, about 1.67%. Candidates routinely lay out 1:60 as 60 mm/m. Others forget the grade must be checked over the developed length including bends, ignore the loss of fall through a rest bend, or design a long unvented branch beyond the permitted length. Confusing invert level with obvert or ground level is another standard error.
Worked example: a DN100 drain runs 28 m between two inspection points, finished invert 12.450 at the upstream point. Fall at 1:60 = 28 × 16.7 = 467 mm, so the downstream invert is 11.983. If the council main invert there is 12.000, the design does not work and you need to re-route, resize or seek relaxation.