Two engineers in hard hats reviewing plans on site

Stormwater DesignOSD, Drainage & Surface Water Management

Compliant stormwater engineering for Sydney and NSW developments — from On-Site Detention systems to full drainage design and surface water management plans.

STORMWATER.

Infrastructure Engineering

Every development changes how water moves across the land

When a site is developed — whether it is a single residential lot or a large commercial subdivision — impervious surfaces replace vegetation and soil. Rainfall that once infiltrated slowly now runs off rapidly, increasing peak flow rates, eroding downstream channels, and placing pressure on ageing council drainage networks. Managing that change is a legal requirement under NSW planning legislation, and it begins with sound stormwater engineering.

At IA Engineering, our stormwater team designs practical, council-accepted drainage solutions for residential, commercial, and infrastructure projects across Greater Sydney and NSW. We work with your civil engineer, architect, and certifier to integrate stormwater management into your project from the earliest planning stage — reducing delays, avoiding costly redesigns, and maximising the chance of first-submission approval.

The outcome: a site that handles rainfall safely, meets council SMIS requirements, satisfies WSUD objectives, and discharges water at pre-development rates — all documented and certified for your development application or construction certificate.

Cutaway of a home's stormwater system tracing roof runoff through downpipes, pits, and the piped drainage line to the street
Stormwater Engineering

Drainage design that works with your site, not against it

We assess your catchment, soil type, council controls, and downstream drainage capacity before selecting a stormwater management strategy — so the system is sized correctly from day one, not revised after council review.

  • On-Site Detention (OSD) design & sizing
  • Piped drainage networks & overland flow paths
  • Surface water management plans
  • MUSIC modelling & WSUD compliance
Cutaway diagram of an on-site detention tank installed beneath a waffle pod slab, labelled through the base preparation, geotextile, tank modules, membrane, and slab, with plan, isometric, and section views alongside
On-site detention tank set beneath a waffle pod slab — the space-saving OSD arrangement we design and certify to council SMIS requirements for dual occupancy and small-lot developments across Western Sydney.

On-Site Detention in NSW

What OSD is, and when your development needs it

On-Site Detention (OSD) is a stormwater management strategy that temporarily stores runoff on your property and releases it to the council drainage system at a controlled, pre-development rate. In NSW, OSD is a standard development consent condition for most residential and commercial developments — required by the vast majority of metropolitan councils when an application increases impervious area on a site.

The sizing of an OSD system is based on Bureau of Meteorology Intensity-Frequency-Duration (IFD) rainfall data for the specific location, the pre- and post-development catchment characteristics, and the council's permissible site discharge (PSD) limit. Systems are typically sized for the 1-in-100-year storm event, though some councils require additional freeboard or secondary containment for overland flow.

Getting OSD wrong delays certifier approvals and forces expensive redesigns. Our engineers use council-approved software — including DRAINS and the OSD Advisor method — to size tanks, pits, and orifice plates precisely, and provide all documentation in the format your council's SMIS officer requires.

Key OSD design considerations

  • BOM IFD rainfall dataRainfall intensity is derived from the Bureau of Meteorology's IFD tool for the specific site location — critical for correct tank volume sizing.
  • Permissible Site Discharge (PSD)Each council specifies maximum post-development flow rates based on pre-development runoff coefficients and catchment area.
  • Tank location & overflow routingThe tank must be positioned to intercept all impervious runoff, and overflow must route safely to the street or easement without flooding adjacent properties.
  • Orifice plate sizingThe orifice plate controls discharge rate. Incorrect sizing is the most common reason for OSD rejection — we calculate and detail this precisely.
  • Integration with drainage networkOSD connects to the broader piped drainage system. We design the full network — pits, pipes, and overland flow paths — as a cohesive system.
  • SMIS submission packageMany councils require a Stormwater Management Information Sheet (SMIS) lodged with the construction certificate. We prepare this as part of our standard deliverable.

Our Scope

Stormwater systems and services we design

Our stormwater engineering team covers the full range of drainage and surface water management requirements for residential, commercial, and infrastructure projects across Greater Sydney and regional NSW.

On-Site Detention (OSD) Systems

Complete OSD design from tank sizing through to detailed engineering drawings and SMIS documentation. We size tanks using BOM IFD data and council-approved methodologies, design orifice plates and overflow structures, coordinate tank locations with architectural and civil layouts, and prepare the full submission package for construction certificate sign-off. Both above-ground and below-ground tank options are available, including modular plastic cell systems for constrained sites.

Piped Drainage Network Design

Hydraulic design of stormwater pipe networks, inlet pits, and junction structures for residential lots, medium-density developments, and civil infrastructure. Design covers pipe sizing to AS/NZS 3500.3, pit capacities, hydraulic grade line analysis, and connection to council kerb and gutter or drainage easements. We prepare drainage layout plans and longitudinal sections suitable for construction and council review.

Overland Flow Path Analysis

Identification and engineering of overland flow paths to manage major storm events safely — preventing inundation of structures and ensuring water discharges without causing damage to downstream properties. Where overland flow paths traverse developed land, we design swales, channels, and easements that satisfy council flood planning requirements and integrate with the site's finished surface levels.

Surface Water Management Plans (SWMP)

Preparation of comprehensive Surface Water Management Plans for development applications — documenting pre- and post-development catchment hydrology, proposed drainage strategy, WSUD measures, and erosion and sediment controls. SWMPs are required for most medium-density, commercial, and subdivision DAs and are assessed by council's development engineering team as part of the approval process.

WSUD & MUSIC Modelling

Water Sensitive Urban Design (WSUD) is increasingly required by NSW councils to reduce the water quality impact of urban development. We use the Model for Urban Stormwater Improvement Conceptualisation (MUSIC) to model runoff quality, size biofiltration and bioretention systems, rain gardens, and gross pollutant traps, and demonstrate compliance with the council's adopted water quality targets — typically expressed as percentage reduction in total suspended solids, total nitrogen, and total phosphorus.

“A stormwater system that passes council review on first submission starts with the right methodology, not a bigger tank.”

IA Engineering Design Team
AS/NZS 3500.3
Plumbing & Drainage Standard
MUSIC
WSUD Modelling Software
SMIS
Council Submission Package
BOM IFD
Rainfall Data for Every Site

The Engineering

How stormwater design actually works

Stormwater design begins with a hydrological analysis — quantifying how much rainfall falls on a site, what proportion becomes runoff, how fast it peaks, and what volume needs to be managed. These figures depend on the site area, its soil permeability, the proportion of impervious surface, and the design storm return period selected by council. Getting the hydrology right is the foundation of every element of the stormwater system.

Hydraulic design then determines how that water moves through the system — pipe sizes to carry it, pit depths to collect it, overland flow paths to contain major events, and detention storage to hold it before release. Each element interacts with the others: an undersized pipe backs up into pits, an oversized OSD tank occupies space that the builder needs, and a poorly routed overflow path floods the neighbour. Our engineers design the complete system holistically, not component by component.

All calculations, drawings, and SMIS documentation are reviewed internally and certified by a Registered Professional Engineer before issue — giving your certifier and council the confidence to approve without extensive back-and-forth.

Stormwater drainage engineering drawings and hydraulic calculations
Pebble drainage channel running between concrete surfaces on a construction site

Drainage Infrastructure

Piped networks, detention systems, and WSUD treatment trains

Standards & Compliance

Every stormwater package certified by Registered Professional Engineers

All stormwater designs issued by IA Engineering carry the signature and certification of a Registered Professional Engineer (RPE) in New South Wales. Our documentation is prepared in accordance with council development control plans, the NSW Government's Managing Urban Stormwater guidelines, and the relevant Australian Standards — ensuring your certifier and council's development engineering team can accept the package without requesting additional information.

AS/NZS 3500.3 DrainageBOM IFD Rainfall DataWSUD Guidelines (NSW)Council SMIS RequirementsOSD Advisor / DRAINSMUSIC ModellingARR 2019 HydrologyNCC / BCA Compliant

Our Approach

How we work with you

We start every stormwater engagement by reviewing the site survey, council DCP stormwater provisions, and the existing drainage context — identifying the council's PSD limit, the nearest legal point of discharge, and whether an easement to drain is required. If these have not yet been established, we advise on what is needed before we scope the work.

Throughout design, we coordinate directly with your architect, civil engineer, and hydraulic consultant. If stormwater decisions affect site layout — tank location, overland flow corridor, or finished level — we raise these early so the design team can resolve them before they become construction problems. Good stormwater engineering is integrated with the site design, not bolted on at the end.

Once drawings and calculations are issued, we remain available for certifier queries and council RFIs. Where councils require a SMIS lodgement or a hydraulic report as a condition of consent, we prepare the required document in the council's prescribed format. Our aim is a stormwater package that your certifier accepts without a single request for additional information.

Designed for Your Site, Certified for Council

Need stormwater engineering for your development?

From OSD sizing and SMIS documentation to full drainage design and MUSIC modelling, our team delivers stormwater engineering that council accepts on first submission.

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1300 911 671