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Icon Water Sewer Grade

ACT only · STD-SPE-G-011 Version 5 · not a substitute for the signed design

Catchment

Tick every precinct upstream of the pipe. The design flow is calculated on those precincts together, not by adding their peaks. Yellow is this job. Lilac is a value taken from Icon Water STD-SPE-G-011 Version 5 (8 October 2025), the supplement to WSA 02-2014-3.3. Hover a lilac label for the clause.

Up # Dwellings NSA (ha) dw/ha EP/dw EP PDWF PWWF

Design flow —

Flows for the ticked precincts

Grade for each diameter

This is the forward solve: flow known, grade wanted, for every diameter. Lay the pipe steeper than the minimum and flatter than the maximum. Minimum slime control is equation IW.5.5.3.2, 0.0338 / Rp. Minimum self-cleansing is IW.5.5.3.1, 0.0135 / Rp. Both use the hydraulic radius at Qdmp (IW.5.5.3.3). Above DN150 the absolute floor is IW.5.5.3.4, 80 / internal diameter in millimetres. DN150 uses Table IW.4 instead of those shear equations. The maximum is IW.5.5.4, the grade at which peak dry-weather flow runs at 3.0 m/s.

DN Min % Min Why the minimum Slime % Self-clean % Absolute % Max % Max Why the maximum Full at min (L/s) Check

Reverse: the grade is known

The table above starts from the catchment and tells you the grade each diameter needs. This box starts from the grade you already have, usually the fall of the ground between two pits, and runs Icon’s equations the other way. It answers two questions.

First, which diameters can actually be laid at that grade. A diameter is listed only when your grade is steeper than its minimum, flatter than its maximum, and the pipe can still carry the wet-weather design flow. The list is every pipe that passes. You still choose among them.

Second, how much dry-weather flow that grade is “meant” for on one diameter. Pick the diameter. The note gives the peak dry-weather flow at which your grade is exactly the slime-control grade (0.0338 divided by the hydraulic radius), and the flow at which it is exactly the self-cleansing grade (0.0135 divided by the hydraulic radius). If your catchment’s peak is larger than the slime-control flow, this grade is too flat for slime control on that diameter, even if the pipe still passes the minimum-grade check for a different reason.

What the numbers are

Every coefficient below is from Icon Water STD-SPE-G-011 Version 5, issued 8 October 2025. That document is Icon’s supplement to WSA 02-2014-3.3, the Gravity Sewerage Code of Australia. Where Icon rewrites a chapter, this page follows the supplement, not the base code.

Equivalent population per dwelling is Table IW.1: 3.5 below 25 dwellings per hectare of net site area, 2.5 from 25 to 80, and 2.0 above 80. Average dry-weather flow is equation 3.4.2.2, 180 L per EP per day. Peak dry-weather flow is equation 3.4.2.3, 5.83 times that average, divided by total EP to the power 0.1. Groundwater is equation 3.4.3.1. Rainfall inflow is equation 3.4.4.1, with C = 1.2 and the intensity from equation 3.4.4.2. The 21.4 mm/h starting value is the original spreadsheet’s figure. Replace it with the Bureau of Meteorology one-hour depth at 39.35% annual exceedance probability for the site.

The pipe must carry the wet-weather total without surcharge. Equation 3.4.1.1 adds peak dry weather, groundwater and rainfall inflow. The minimum grade uses a smaller flow, Qdmp, from IW.5.5.3.3: three quarters of peak dry weather, plus two thirds of any pumped inflow. Self-cleansing, IW.5.5.3.1, is 0.0135 divided by the hydraulic radius at that flow. Slime control, IW.5.5.3.2, is 0.0338 divided by the same radius. Above DN150, IW.5.5.3.4 also sets a floor of 80 divided by the internal diameter in millimetres. A DN150 straight sewer does not use those shear equations. Table IW.4 sets its minimum from the number of houses that will ultimately drain to that line: 1 house 1.25%, 2 houses 1.20%, 7 houses 1.10%, 12 houses 1.00%, 18 houses 0.90%, 28 houses 0.80%, and 35 houses or more 0.70%. The house box starts as the dwellings in the ticked precincts. Change it when that DN150 reach serves a different number of houses, including houses not built yet. The maximum grade, IW.5.5.4, is 3 m/s at peak dry weather.

STD-SPE-G-011 Version 5, 8 October 2025, equations IW.5.5.3.1 to IW.5.5.3.4 and IW.5.5.4, flows in Section IW.3. Icon Water standards.