Civil drainage and stormwater design calculators

Civil drainage design calculations

Tenselo provides civil drainage calculators for site engineers assessing rainfall runoff, culvert capacity, open channels, detention storage, infiltration and stormwater pump pits. Use the separate calculations to document a design assumption, compare a component or check an operating condition.

Start with rainfall and the contributing catchment

Record the catchment area, surface types, flow paths and discharge point. For Australian projects, use the Australian Rainfall and Runoff guidance and design rainfall resources, with the probability, duration and inputs required by the applicable method and approving authority. STM-005 is described for Australian Bureau of Meteorology IFD inputs.

For a New Zealand site, check the relevant rainfall data and local design requirements. HIRDS design rainfall information is a starting point for location-specific rainfall estimates. This does not establish that every Australian calculator method, coefficient or default is appropriate for a New Zealand project.

Use STM-007, Rational Method Peak Flow, where the method is appropriate for the catchment and accepted for the project. Rainfall intensity, a justified runoff coefficient and consistent area units all matter. Peak discharge is a flow rate; use STM-006, Rainfall Runoff Volume, when the question concerns water volume. See the verified STM-007 example. The time-of-concentration calculator supports a separate catchment timing check.

Culvert capacity and gravity pipe sizing

STM-001, Culvert Capacity, assesses headwater under inlet and outlet control for circular, box and arch culverts using the documented HDS-5 approach. Check the selected shape, dimensions, length, slope, inlet conditions and tailwater. Review blockage, road overtopping and the downstream discharge arrangement as part of the site design.

For a pipe driven by an available elevation difference, HYD-015, Gravity Flow, offers a pipe check and a size-for-flow mode. Its head-based calculation uses pipe dimensions, length, material and roughness assumptions. This is a different calculation from a partially filled stormwater drain or an interacting pit-and-pipe network; confirm the flow condition before choosing the tool.

Open channel flow calculator

STM-004, Open Channel Calculator, uses Manning's equation to assess flow, velocity, depth and hydraulic regime in rectangular, trapezoidal, triangular and parabolic sections. Typical tasks include channel and swale capacity checks. Document the geometry, roughness, gradient, water depth and freeboard.

Capacity at an assumed depth does not establish that the downstream outlet can receive the flow. For related surface-drainage checks, explore kerb and gutter flow capacity, area drain spacing and riprap and rock-armour sizing.

Detention storage, soakwells and infiltration

The on-site detention calculator provides a screening storage estimate. Check it alongside the detention outlet calculation, the permitted discharge rate and relevant storm durations. A final design may need a routed hydrograph assessment.

The soakwell calculator covers infiltration sizing and emptying time. A useful assessment needs contributing area, design rainfall, storage geometry and a site-supported infiltration rate. Check groundwater, setbacks and overflow provision. Related tools cover French drains, bioretention and raingardens and stormwater pump pits.

What does the drainage software provide?

These tools support component calculations and engineering assessment. This catalogue does not describe a connected drainage-network solver, CAD drawing package or automatic council-approval service. The complete site design, local rainfall assumptions and chosen methods still need engineering review.

New accounts require approval. These public pages explain the tools; running project calculations requires authorised application access. See availability and registration, or explore related pipe hydraulics, storage and pumping calculations.