Water Corporation DS50 Gravity Sewers (DN150–600) Explained

Where Melbourne's large-sewer standards hand the designer performance criteria and leave the hydraulics to them, Water Corporation's DS50 does most of the hydraulics in advance: for each standard diameter, Table 4.4 states the flattest and steepest permissible grades, the maximum depth of flow, and the flow the sewer is allowed to carry. Checking a reticulation design becomes a sequence of table lookups — provided you understand what each column means.

What DS50 covers

DS50 — Design & Construction Requirements for Gravity Sewers DN150 to DN600 (V2 R4, July 2023) applies to Water Corporation reticulation and branch gravity sewers in Western Australia. Anything larger than DN600 is outside its scope and subject to project-specific requirements.

Flow terms: Daily Flow and SDF

DS50 uses its own flow vocabulary — not the ADWF/PDWF/PWWF family found in the WSA 02-derived codes. The sewer is sized for the Sewer Design Flow (SDF):

When comparing DS50 designs against east-coast criteria, be careful mapping the terms: SDF is a peaking-factor-scaled dry-weather flow, not a wet-weather flow.

Standard sizes

Six standard diameters: DN150, 225, 300, 375, 450 and 600. There is no DN525 in the DS50 series. If a hydraulic calculation suggests an intermediate size, the design moves to the next standard size up.

Grade limits

Table 4.4 sets both ends of the grade window per diameter. Selected values:

Minimum grades relax as diameter grows (larger wetted sections generate more boundary shear at the same grade), while maximum grades tighten. Consult the full table for all six sizes.

Depth of flow

DS50 limits depth of flow to one-half of diameter for DN150 and two-thirds of diameter for DN225–DN600. The margin above the water surface serves the same purposes as elsewhere — ventilation and headroom — with the tighter DN150 limit reflecting how quickly a small sewer's performance degrades once flow approaches the crown (see partially full pipe flow).

Allowable flows

The distinctive DS50 feature: Table 4.4 caps the flow per diameter and grade band. A DN300 sewer laid between 1:400 and 1:250, for instance, has a lower allowable flow than the same pipe laid steeper than 1:250 — at grades of 1:250 or steeper it may carry up to 50 L/s (selected value). The SDF must sit at or under the allowable flow for the actual grade; if it doesn't, the design moves up a size or steepens the grade. This is a capped-utilisation approach — the allowable flows are below the theoretical part-full capacity, building in margin without asking the designer to compute it.

Roughness and method

DS50's hydraulic basis is Colebrook–White with a roughness of k = 1.0 mm — a deliberately conservative operational roughness accounting for slime and sediment, not the clean-pipe value from a product datasheet. DS50 sets no velocity limit; the grade table plays that role.

Summary

• DS50 governs WA gravity sewers DN150–600 in six standard sizes (no DN525).
• Size for SDF = 1.5 × Daily Flow, kept within the Table 4.4 allowable flow for the DN and grade.
• Grade windows per DN: from 1:200–1:5 (DN150) to 1:800–1:80 (DN600), selected values.
• Depth of flow ≤ ½ D (DN150) or ⅔ D (DN225+).
• Hydraulics: Colebrook–White, k = 1.0 mm; no separate velocity limit.
• These are selected extracts — design against the current DS50 revision.