Civil Drainage Design in Melbourne: What Your Site Needs

Every new warehouse, factory, office building, townhouse group or car park changes how rain runs off the land. Roofs and pavements shed water far faster than grass, so councils require a civil drainage design that shows where the water goes, how much leaves the site and how quickly. Principal Built Engineering prepares drainage designs alongside the structural design for commercial, industrial and residential projects across Melbourne, so the footings, slabs, retaining walls and drainage are coordinated in one set of documents.

What Civil Drainage Design Covers

Civil drainage design (often called stormwater drainage design) sits between the architectural drawings and the council's infrastructure. It turns the site layout into a working system that can be built, inspected and maintained. A complete design for a commercial or industrial site normally includes the following.

• Catchment plan showing roof, pavement and landscaped areas and where each drains
• Pit and pipe layout with sizes, grades, invert levels and cover levels
• Connection to the legal point of discharge nominated by council
• On-site detention (OSD) tank or underground storage, sized to the council's permitted site discharge
• Overland flow path for storms that exceed the pipe capacity
• Water sensitive urban design (WSUD) treatment where council or Melbourne Water requires it
• Pavement grading, kerb and channel and trench grate details for hardstand and car park areas
• Construction notes and details that a plumber or civil contractor can price and build from

For larger industrial subdivisions the scope can extend to internal roads and external works in the road reserve. For a single warehouse or a group of units, the design is usually a site drainage plan with supporting calculations. The stormwater drainage design service page explains the full scope Principal Built Engineering provides.

Council Requirements in Melbourne: Legal Point of Discharge and Detention

The starting point for any drainage design is the legal point of discharge (LPOD). This is the location where council allows stormwater from the property to connect to its drainage network, usually a pit, a kerb outlet or a drainage easement. Under the Victorian Building Regulations, the relevant council issues a report and consent nominating this point, and the building surveyor relies on it before issuing a building permit.

For commercial, industrial and multi-unit development, the council's LPOD response usually comes with conditions. The most common are:

Requirements differ between councils. The City of Greater Dandenong, Casey, Monash, Kingston and Hume each publish their own detention and drainage guidelines, and some sites also fall under a Melbourne Water development services scheme. Checking these early avoids a redesign after the architectural layout is fixed.

How the Drainage System Is Sized

The design follows the Australian standard for plumbing and drainage, AS/NZS 3500.3 (stormwater drainage), together with Australian Rainfall and Runoff guidance and the council's own engineering requirements. The engineer works through the site in a set order.

Catchments and runoff

The site is divided into catchments: roof areas, sealed pavements, gravel hardstand and landscaping. Each surface has a different runoff coefficient. A concrete warehouse apron sends almost all rain straight to the drains, while garden beds absorb a large share. The total runoff sets the flows the pits and pipes must carry.

Pits, pipes and grades

Pipe sizes and grades are chosen so the system flows without surcharging in the minor storm. Pit spacing, inlet capacity and the depth of the pipe network are checked against the levels available at the legal point of discharge. On flat industrial sites this is often the critical constraint, because the outlet is shallow and the pipes cannot fall far.

Detention storage

Where the council limits the site discharge, the difference between the developed runoff and the permitted outflow is stored on site. The volume is calculated across a range of storm durations, and the largest required storage governs. Storage can be an above ground tank, an underground tank, oversized pipes beneath a car park or a shallow ponding area on hardstand.

Overland flow

No pipe network carries every storm. The design must show where water travels when the pipes are full, and that this path keeps water away from buildings and neighbouring properties. Finished floor levels are set above the overland flow level with a freeboard margin.

The Civil Drainage Design Process, Step by Step

What to Have Ready Before You Engage an Engineer

A drainage design moves fastest when the engineer has the right information from the start. Gather these documents before requesting a fee proposal.

• Feature and level survey of the site, including the street kerb and any existing council pits
• Current architectural site plan and floor plans, showing roof and pavement areas
• Certificate of title and plan of subdivision, showing any drainage easements
• Council legal point of discharge report, or the property address so it can be requested
• Planning permit and endorsed plans, if a planning permit applies
• Geotechnical report, where infiltration, detention depth or groundwater is a concern

For sites that also need retaining walls, the drainage behind the wall should be designed with the site drainage. The retaining wall drainage guide covers how subsoil drainage connects into the stormwater system, and retaining wall engineering explains the structural side.

Civil Drainage on Industrial and Commercial Sites

Industrial and commercial projects bring specific drainage issues that residential jobs rarely face.

Principal Built Engineering designs the structure of these buildings as well as the drainage. See industrial structural design and commercial engineering design for the structural services that sit alongside the drainage work.

How Long Does a Drainage Design Take?

Timeframes depend mostly on the council. The engineering design for a single warehouse or a small group of units is usually prepared within one to two weeks of receiving the survey and the legal point of discharge. Council responses to an LPOD request and to engineering plan lodgement add to this, and vary between municipalities. Starting the drainage design at the same time as the structural design keeps it off the critical path for the building permit.

For a broader picture of fees for engineering work, see structural engineer costs Melbourne, or request a quote with your plans and survey for a fixed fee proposal.

Frequently Asked Questions

What is civil drainage design?

Civil drainage design is the engineered layout and sizing of the pits, pipes, detention storage and overland flow paths that collect stormwater from a site and discharge it to the council's legal point of discharge at a controlled rate.

Do I need a drainage design for a building permit in Victoria?

Most commercial, industrial and multi-unit projects need a stormwater drainage design before the building permit is issued. The building surveyor relies on the council's legal point of discharge report, and many councils require drainage plans and detention calculations as a permit condition.

What is on-site detention?

On-site detention is storage, usually a tank or oversized pipes, that holds stormwater during heavy rain and releases it slowly through a restricted outlet. It stops the development from overloading the council's drains.

What is a legal point of discharge?

The legal point of discharge is the location nominated by the council where stormwater from a property may connect to the public drainage system. It is issued in a council report and consent and often includes conditions such as a maximum discharge rate.

Can a structural engineer prepare the drainage design?

Yes, where the engineer has civil drainage experience. Principal Built Engineering prepares drainage designs together with the structural design, which keeps footings, slabs, retaining walls, tanks and pipes coordinated in one set of documents.