What minimum shear stress does a sewer need to self-cleanse?

Yarra Valley Water requires boundary shear of at least 1.6 Pa at ultimate peak dry weather flow for sediment transfer in large sewers, with 2.87 Pa preferred to strip slime and 3.35 Pa as a higher target (YVW Large Sewer Standard YVWCD-2-8119 §6.2). Other authorities use minimum velocities or grade tables instead — this checker applies each authority's own published criteria.

What is the maximum depth of flow (d/D) in a gravity sewer?

It depends on the authority and the sewer's classification. Yarra Valley Water limits peak wet weather flow depth to 70% of pipe diameter for reticulation and branch sewers (30% air space), 82% for main sewers larger than DN975, and 70% for main sewers at major road crossings (YVWCD-2-4853 §4.3). Water Corporation DS50 limits depth of flow to one-half of diameter for DN150 sewers and two-thirds for DN225–DN600. Icon Water's criterion is that the design flow runs without surcharge.

Why do sewers have a minimum and a maximum grade?

Too flat and the sewer cannot self-cleanse — solids deposit because boundary shear or velocity is too low. Too steep and depth, velocity or Froude limits are exceeded. The workable range in between is the grade window; this checker shows it as a band with the criterion that governs each end.

Gravity Sewer Design Checker — Australian Authority Criteria

Checks a single gravity sewer reach against the hydraulic criteria Australian water authorities publish: Yarra Valley Water (ultimate PDWF boundary shear ≥ 1.6 Pa; PWWF depth ≤ 82% of diameter for main sewers, ≤ 70% for branch sewers and road crossings; velocity < 3 m/s; Froude < 0.85), Water Corporation DS50 grade, depth and allowable-flow tables (DN150–600), Icon Water's calculated self-cleansing grades, and SA Water's advisory velocity targets. Manning part-full hydraulics with Colebrook–White full-flow. Free, no login; every figure is verified against the cited source document. Related reading: self-cleansing velocity & tractive force · minimum sewer grades by authority · partially full pipe flow (d/D) · DS50 requirements explained.

Preliminary screening tool. Steady, uniform-flow checks on a single circular reach. It does not model hydraulic grade lines, backwater, surcharge propagation, storage, control structures, transitions, junction losses, dynamic rainfall, pressure flow, hydraulic jumps, odour or network interactions, and does not replace network modelling, project-specific authority requirements, professional review or authority approval.

Modelled flow is constant across the grade sweep. Capacity and operating conditions change with grade; the nominated design flows do not.

Grade window i

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Pipe section i

Flow & available capacity i

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Fixed design flows (horizontal) vs grade-dependent capacity (curves). Flags mark the flattest workable grade per flow. Drag to move the selected grade.

mandatory + preferred passmandatory pass onlya mandatory criterion fails

Operating metrics vs grade i

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Shear, depth, velocity and Froude against grade — real units, profile limits as labelled lines, green tint = pass side of the mandatory limit.

Tractive force — ult PDWF shear

Flow depth — ult PWWF d/D

Velocity — ultimate scenario

Froude — ult PWWF

Hydraulic elements (Camp diagram) i

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Authority & profile criteria matrix

Every figure is verified against the cited source (see Formulae → References). Apply criteria loads a profile's checks into the sidebar and keeps your inputs; Load example also fills the inputs with that profile's worked scenario. Editing any criterion marks the profile as modified.

Live calculation — selected grade

Worked substitutions

Equations & conventions

References