Engineering Tools

Engineering Tools

A set of technical tools for hydrology, hydraulic and stormwater calculations, based on standard published methods and Australian Standards used in drainage design practice.

How to use these tools: these calculators are provided for preliminary estimation and educational purposes, using standard published hydrologic and hydraulic methods (Australian Rainfall & Runoff, AS/NZS 3500.3, Manning’s equation, the Colebrook–White equation). They are not a substitute for a detailed, site-specific assessment by a qualified engineer. For a project-specific assessment, get in touch with our team.
Disclaimer & limitation of liability: these tools are made available free of charge, for general guidance and preliminary estimation only. They do not constitute engineering advice, and no warranty, express or implied, is given as to the accuracy, completeness or suitability of any result they produce for any particular site, project or purpose. Use of these tools, and all results obtained from them, is entirely at the user’s own risk. To the fullest extent permitted by law, Flood Consulting, its directors, employees and agents accept no liability whatsoever — whether in contract, tort (including negligence) or otherwise — for any loss, damage, cost, expense or injury of any kind arising directly or indirectly from the use of, or reliance on, these tools or their results. Responsibility for independently checking, verifying and validating all inputs, assumptions and outputs, and for engaging a suitably qualified and experienced engineer before relying on any result for design, construction, approval or any other purpose, rests solely with the user.



☁HydrologyTime of ConcentrationKinematic wave overland flow and entry time, with manual, BOM IFD coefficient, or BOM 2016 IFD CSV rainfall input.Open calculator →▾HydrologyRational Method Peak FlowPeak design discharge for a small catchment from runoff coefficient, rainfall intensity and area.Open calculator →≈HydraulicsManning’s EquationPipe and open channel flow velocity and capacity using Manning’s equation.Open calculator →◯StormwaterStormwater Pipe SizingSite stormwater drain capacity using the Colebrook–White equation, per AS/NZS 3500.3 Clause 5.4.11.Open calculator →▭StormwaterDownpipe & Eaves GutterRoof catchment area and downpipe sizing from eaves gutter capacity, per AS/NZS 3500.3 Table 3.5.2.Open calculator →▽StormwaterValley Gutter DesignMinimum sheet width, effective depth and width for a valley gutter, per AS/NZS 3500.3 Table 3.6.2.Open calculator →▤StormwaterBox Gutter & SpreaderCompliance check against the AS/NZS 3500.3 limitations for box gutters, sumps and roof spreaders.Open calculator →

Hydrology

Time of Concentration & Overland Flow (Kinematic Wave Method)

Estimates the time of concentration for a small urban sub-catchment by combining an initial entry time, up to three kinematic-wave overland flow segments, and one defined flow (pipe or channel) segment. Based on the kinematic wave equations of Australian Rainfall & Runoff 1987 (ARR87).

Diagram of flow paths for urban drainage catchments, showing the property drainage segment, overland flow segment, second overland flow segment and gutter flow segment to the pit, with the constant time, kinematic wave time and speed-related time components of the time of concentration.

Flow paths for urban drainage catchments: rainfall is routed through a property drainage segment, one or more overland flow segments and a gutter flow segment to the pit, combining a constant time, kinematic wave time and speed-related time. Enter your own catchment geometry in the fields below.

Initial / entry time

Overland flow segments (kinematic wave)

Typical roughness n: smooth paved/concrete 0.010–0.013, gravel 0.02, lawn/turf 0.02–0.03, dense grass/bush 0.03–0.05 — refine using site-specific published guidance.

Defined flow segment (pipe, gutter or channel)

Design rainfall intensity


Look up the intensity for your chosen ARI at a duration approximately equal to the expected time of concentration, from the Bureau of Meteorology IFD Design Rainfall system, then refine by re-running once you have a computed tc.

Paste the A–G coefficients for your site and chosen ARI from the BOM IFD system. Intensity is solved iteratively against the computed time of concentration (duration in hours).

Upload the Intensity (mm/h) CSV exported from the BOM 2016 IFD Design Rainfall Data system for your site (all standard durations and AEPs). The design intensity is read from the table and interpolated for the computed time of concentration — no manual look-up needed. Processing happens entirely in your browser; the file is never uploaded to our server.



Method: kinematic wave overland flow equations from Australian Rainfall & Runoff 1987 (ARR87). Current practice should be checked against Australian Rainfall & Runoff 2019 and current BOM design rainfall data.

Hydraulics

Rational Method – Peak Design Flow

Estimates peak design discharge for a small catchment using the Rational Method, commonly applied to urban catchments up to a few hundred hectares.

Typical published runoff coefficient ranges: roofs & sealed pavement 0.80–0.95; roads 0.70–0.90; lawns & grassed areas 0.15–0.35; bushland/undeveloped 0.10–0.25. Composite catchments should use an area-weighted average C — confirm values against current ARR / local council guidance.

Formula: Q = C·I·A / 360 (Q in m³/s, C dimensionless, I in mm/h, A in hectares) — the standard Rational Method equation used in Australian drainage practice.

Hydraulics

Manning’s Equation – Pipe & Channel Capacity

Estimates flow velocity and discharge capacity for a circular pipe or a trapezoidal/rectangular open channel using Manning’s equation.

Circular Pipe
Open Channel


use 0 for rectangular

Typical roughness n (AS/NZS 3500.3 Table 5.4.9): PE/PVC 0.009–0.010, smooth concrete 0.011–0.012, trowelled concrete 0.012–0.015, asphalt paving 0.013–0.015, brickwork 0.014–0.016, corrugated metal 0.012–0.015, earth-lined channels 0.018–0.025, rock lining/rip-rap 0.025–0.030, grassed/vegetated channels 0.025–0.075 (depending on growth).

Formula: V = (1/n)·R2/3·S1/2 (SI units, R in m, S as fraction); Q = V·A. Standard open-channel hydraulics (Manning, 1889; Chow, 1959); typical n values from AS/NZS 3500.3:2015 Table 5.4.9.

Stormwater

Stormwater Pipe Sizing (Colebrook–White)

Estimates the hydraulic capacity of a single circular site stormwater drain using the Colebrook–White equation, in accordance with AS/NZS 3500.3:2015 Clause 5.4.11 and the roughness values of Table 5.4.11.2. For a pipe network, check each pipe individually.



Method: Colebrook–White equation, general form using hydraulic radius, consistent with AS/NZS 3500.3:2015 Clause 5.4.11 and Table 5.4.11.2 (water at 20°C). Full-pipe velocity should not exceed 2.0 m/s (Clause 5.4.11.2); velocities of 1.5 m/s or less are recommended at pipe outlets to stormwater pits.

Stormwater

Downpipe & Eaves Gutter Sizing

Step 1 calculates the roof catchment area per downpipe (AS/NZS 3500.3 Clause 3.4). Step 2 looks up the minimum vertical downpipe size for a given eaves gutter effective cross-sectional area (Table 3.5.2).

Step 1 — Roof catchment area per downpipe

Uses Ac = Ah × F (Equation 3.4.3.2(2), Table 3.4.3.2), applicable to domestic-type roofs of constant slope with no flat roof or wall contributing to the catchment. Where a flat roof or vertical wall also drains to the gutter, use Equations 3.4.3.2(1)/(3) or 3.4.4 of AS/NZS 3500.3 instead.

Step 2 — Downpipe size from eaves gutter capacity


Obtain Ae from the gutter manufacturer’s published data (see AS/NZS 2179.1), or from AS/NZS 3500.3 Figures 3.5.2(B)/(C) using your catchment area per downpipe (Step 1) and the site design rainfall intensity — those design graphs are not reproduced here.

Source: AS/NZS 3500.3:2015 Clause 3.4 (catchment area) and Table 3.5.2 (downpipe sizing). Table range: 3500–20 000 mm² for 1:500 and steeper; 4700–26 800 mm² for flatter than 1:500. Outside this range, individual engineering design is required.

Stormwater

Valley Gutter Design

Determines the minimum sheet width and effective dimensions of a valley gutter from the design rainfall intensity, per AS/NZS 3500.3 Table 3.6.2. Applies within the limitations of Clause 3.6.1: roof slopes not less than 1:4.5 (12.5°), nominal valley side angle 1:3.4 (16.5°), and catchment area not exceeding 20 m².

Source: AS/NZS 3500.3:2015 Table 3.6.2, derived from Martin and Tilley (1968). Freeboard (hf) 15 mm; sheet width allows for side rolls or bends of 25 mm. Table range: design rainfall intensity up to 400 mm/h — above this, individual engineering design is required.

Stormwater

Box Gutter & Spreader – Design Requirements

Box gutter, rainhead, sump and overflow device sizing in AS/NZS 3500.3 is read from a set of interrelated design graphs (Appendix I, Figures I1–I8), which cannot be reliably digitized as a calculator. This tool instead checks a proposed box gutter design against the Standard’s stated limitations (Clause 3.7). The sole width, depth, rainhead and overflow device dimensions themselves must be taken from AS/NZS 3500.3 Appendix I or determined by a suitably qualified hydraulics engineer.



Source: AS/NZS 3500.3:2015 Clause 3.7 (limitations, layout and overflow devices). This is a compliance check only, not a hydraulic capacity calculation.

Roof Spreaders

AS/NZS 3500.3 Clause 3.4.5 gives no capacity formula for spreaders (a device at the foot of a downpipe that distributes rainwater onto a roof at a lower level); a spreader is sized by the roof drainage system of the lower catchment taking the combined flow. The Standard requires, where a spreader is used:

  • for a tiled roof, sarking at least 1800 mm wide on each side of the point of discharge, extended down to the eaves gutter in accordance with AS 2050; or
  • for a corrugated metal roof, side laps sealed for the full length, for at least 1800 mm on each side of the point of discharge.

The downpipe and gutter system of the lower catchment must be sized (using the tools above) to take the total flow from both the upper and lower roof areas, and an increased overflow allowance should be considered for the lower gutter.