DRAINS Modelling for Stormwater Design: ARR 2019, OSD & Pipes

MUSIC for quality, DRAINS for quantity

If MUSIC answers “is the water clean enough?”, DRAINS answers “where does the water go, how fast, and does anything flood?”. Developed by Watercom, DRAINS is the dominant software for urban stormwater quantity design in NSW — councils routinely require DRAINS output to justify pipe sizes, on-site detention (OSD) systems and overland flow behaviour for development applications and Construction Certificates.

DRAINS combines two disciplines in one model:

• Hydrology — converting design rainfall into runoff hydrographs from each catchment, using rainfall-runoff models such as ILSAX-type horton infiltration or storage-routing approaches
• Hydraulics — routing those hydrographs through pits, pipes, channels and storages, computing flows, velocities and hydraulic grade lines (HGL), and reporting any surcharge or overflow

ARR 2019: from single design storms to ensembles

The biggest methodological shift in recent memory is Australian Rainfall and Runoff 2019. Under the superseded ARR 1987 approach, you analysed one design storm per duration. ARR 2019 recognises that real storms of the same depth and duration have very different internal shapes, so it provides ensembles of ten temporal patterns for each storm duration, drawn from regional pattern sets.

DRAINS automates this: it downloads design rainfall depths (from the BoM IFD service) and temporal patterns (from the ARR Data Hub), builds the ensemble of storms for each duration and annual exceedance probability (AEP), runs them all, and reports results statistically — typically adopting a representative storm close to the mean or median of the ensemble for each duration rather than the worst single pattern. For NSW, additional state-specific guidance applies, including probability-neutral burst initial losses. The practical consequences for designers:

• A “design check” is now hundreds of model runs, not a handful — durations × ten patterns × AEPs
• The critical duration can differ between locations in the same model (a pit may peak in a short burst while a basin peaks in a long storm)
• Councils differ in how they ask for results to be reported, and some still request comparison checks — always confirm the consent authority’s current modelling requirements

OSD sizing in DRAINS

On-site detention remains a standard requirement across Sydney councils: post-development discharge must not exceed the permitted site discharge (PSD), with the difference stored on site (the site storage requirement, SSR). DRAINS models OSD explicitly:

• The storage (tank, basin or paved storage area) is entered with its stage–storage relationship
• The outlet control — usually an orifice plate and/or weir — is modelled hydraulically
• The ensemble runs confirm the storage and orifice combination keeps discharge below the PSD across all durations and patterns, not just one nominated storm

This is where ARR 2019 genuinely changed outcomes: storages that passed a single 1987-style storm can fail longer ensemble patterns, and vice versa. Council OSD policies vary — some publish deemed-to-comply PSD/SSR figures, others require full DRAINS justification, and several growth-area councils have their own regional detention rules. Checking the local OSD policy before sizing the tank avoids redesigning a basement.

Pipe network analysis and HGL checks

For the piped network, DRAINS performs the analysis NSW councils expect to see on drainage plans:

• Pipe sizing for the minor storm (commonly the 20% or 10% AEP event, per council policy)
• HGL analysis confirming the hydraulic grade line stays below pit surface levels and freeboard requirements at critical locations
• Pit inlet capacity using published inlet capture relationships, with bypass flows tracked downstream
• Major storm overland flow (typically the 1% AEP) checking that surcharge follows safe overland flow paths rather than entering buildings

The output — pipe schedules, HGL long-sections and pit summaries — feeds directly onto the civil drawings submitted for approval.

Common review comments we see

• Ensemble results reported for a single duration only, with no critical-duration check
• OSD orifice sized for one storm rather than verified across the ensemble
• HGL above gutter level at upstream pits, with no freeboard discussion
• Catchment imperviousness inconsistent with the architectural site plan
• Old ARR 1987 parameters reused where the council requires ARR 2019

A tidy DRAINS model, consistent with the drawings and the council’s policy, is one of the fastest ways to shorten the assessment cycle.

Contrive Consultants designs WSUD systems and prepares MUSIC and DRAINS models accepted by NSW councils — from OSD sizing to full pipe network and overland flow analysis. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us to scope your project.