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Grated inlet pit and surcharge outlet calculator

Enter the grate, where the pit sits, the depth water may pond over it and the flow. The calculator gives the inlet capacity (a sag pit by Equation 5.4.10.1, an on-grade pit at 80% of a design aid), checks the ponding against the floor, and sizes a surcharge outlet and the pit.

NCC 2025 calls up AS/NZS 3500.3 for stormwater drainage (Volume One F1D3 for Class 2 to 9 buildings, Volume Two H2D2(1)(a) for houses and Class 10 buildings), and this calculator applies its sag and on-grade pit, surcharge outlet and pit size provisions.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 5.4.10.1 and Equation 5.4.10.1 (sag pits); Clause 5.4.10.2 (on-grade pits)
  • Clause 5.4.12 and Equation 5.4.12.2; Table 7.5.2.1

Sag pits

Where water ponds over a grate at a low point, Qi = 1600 bf P dp^1.5 in L/s, with P the open perimeter (sides against a kerb or wall excluded), dp the ponding depth up to 0.12 m and bf a blockage factor, commonly 0.5. The Standard gives no method for deeper ponding, so deeper ponds are worked at 0.12 m. The pond must stay 300 mm below the floor or damp course: enter the floor height and the calculator checks it (Clause 5.4.10.1(b)).

On-grade pits

The Standard gives no equation for a pit on a slope. Take its capacity from a street drainage design manual or the maker's data for that grate, grade and approach flow; Clause 5.4.10.2 takes 80% of it for blockage, and the rest bypasses the pit. Place it so bypass flow in a minor storm causes no nuisance (Clause 5.3.6).

Surcharge outlets

A surcharge outlet lets the drain overflow harmlessly when it backs up. Its grate passes the drain's whole flow at that point with the drain fully blocked: A = Q / 150 m², an exit velocity of 0.15 m/s. The grate sits 300 mm below the lowest floor and 75 mm above unpaved ground or level with paving, wholly within the property, clear of buildings, where a discharge is noticed, with an overflow path.

A worked example

The calculator's example inputs, worked through step by step.

Inlet capacity

45.5 L/s

The grate takes 45.5 L/s with 100 mm of ponding, more than the 15.1 L/s arriving. A surcharge outlet for 15.1 L/s needs 1005 cm² of open grate.

  1. Equivalent impervious areaΣCA = 1.0 Ar + 0.9 Ai + Cp Ap = 0.0 + 0.9 × 300.0 + 0.00 × 0.0 = 270.0 m²AS/NZS 3500.3:2025 Clause 5.4.6
  2. Design flow, 5% AEPQ = ΣCA × I / 3600 = 270.0 × 201 / 3600 = 15.1 L/sAS/NZS 3500.3:2025 Eq 5.4.8
  3. Perimeter clear of kerbs and wallsP = sum of open sides = 1.80 mAS/NZS 3500.3:2025 Eq 5.4.10.1
  4. Sag pit inlet capacityQi = 1600 bf P dp^1.5 = 1600 × 0.5 × 1.80 × 0.100^1.5 = 45.5 L/sAS/NZS 3500.3:2025 Eq 5.4.10.1; Clause 5.4.10.1(c)
  5. Surcharge outlet grate area, full blockageA = Q / 150 = 15.075 / 150 (this inlet's flow) = 1005 cm² (0.1005 m²)AS/NZS 3500.3:2025 Eq 5.4.12.2

Questions

How much water does a grated pit take?

At a sag, AS/NZS 3500.3:2025 Equation 5.4.10.1: 1600 × blockage factor × open perimeter × ponding depth^1.5. A 450 mm square grate clear on all sides takes about 45 L/s with 100 mm of ponding at 50% blockage.

How do I size a grated pit on a slope?

AS/NZS 3500.3:2025 gives no equation for on-grade pits. Take the capacity for your grate, grade and approach flow from a street drainage design manual or the maker's data, and use 80% of it (Clause 5.4.10.2); what the grate does not take runs past it. At a low point, Equation 5.4.10.1 applies instead.

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