Stormwater Detention Tanks in Melbourne: How They Work

Most people never think about what happens to rain once it hits a car park, until the day it does not drain away fast enough. On commercial and industrial sites across Melbourne, that problem gets solved before the car park is even built, with an underground system most people will never see or think about again.

On this job the CHS team excavated a large pit beside a new commercial building, lined it with geofabric and an impermeable liner, then installed modular detention crates that interlock into one continuous underground void, in this case holding around 270,000 litres. Once backfilled, the system disappears completely beneath the finished surface.

This article looks at what a stormwater detention tank actually does, how it differs from a water storage tank, and why systems like this matter for commercial developments across Melbourne.

Stormwater Detention vs Water Storage: What Is the Difference

The two get confused constantly, and the confusion is understandable, since both involve a large underground void holding water. The purpose is where they part ways.

A water storage tank holds water so it can be used later, for irrigation, for a toilet system, for general non-drinking use around a property. The water sits in the tank until someone draws on it, and the tank is sized around how much water the site wants to have on hand.

A stormwater detention tank does the opposite. It holds water it does not want, temporarily. When rain falls on a roof, a car park or any other hard surface, that runoff has nowhere to soak in the way it would on open ground. A detention system captures that runoff and releases it slowly, at a controlled rate, rather than sending it straight into the stormwater drain the moment it arrives. The water in a detention tank is not being saved for use, it is being held back so the site does not overload the drainage network the instant it rains.

Both systems can sit underground, both can look similar from above once finished, and a single development can genuinely need both a storage tank and a detention system for different reasons. Knowing which one a project actually needs, and sizing it correctly, is where the value of an experienced civil contractor shows up.

Why Commercial Sites Need Detention in the First Place

Before a site is developed, rain falling on it soaks into soil, grass and garden beds at a natural, gradual rate. Once that same site is covered in roof, hardstand and car park, almost none of that rain soaks in anywhere. It runs straight off the hard surfaces and into the stormwater system, all at once, in a fraction of the time it used to take.

Multiply that across a whole street of commercial development and the local drainage network can be overwhelmed fast, particularly during the kind of short, heavy downpour Melbourne gets through summer and early autumn. Councils manage this by requiring new developments above a certain size to detain a proportion of their runoff on site and release it at a controlled rate, so the increase in hard surface does not translate directly into an increase in flood risk for the street and the properties around it.

A detention tank is one of the more space efficient ways to meet that requirement, particularly on a site where there is limited room for an above ground retention basin. Building it underground means the land on top can still be used as a car park, a loading area or landscaped ground, while the detention volume sits invisibly below.

How a Modular Crate Detention System Goes In

The system in this job follows a fairly standard sequence for a modular crate detention tank, though the exact size and layout changes with each site.

Excavation

The pit is excavated to the depth and footprint the engineering design calls for, with allowance for the crate structure itself plus a bedding layer underneath. Getting the excavation square and level matters here, since the crates are rigid modules that need a consistent base to sit on.

Lining

Once the pit is cut, it is lined with geofabric and, depending on the system, an impermeable liner as well. The geofabric protects the liner and crates from puncture by surrounding soil and separates different material layers so fines do not migrate into the void over time. The liner, where used, keeps the detained water contained within the system rather than infiltrating into surrounding soil, which matters where the design calls for controlled release rather than soakage.

Crate Installation

Modular detention crates are then installed across the base of the pit, locking together into one large, structurally sound void that can carry the load of the ground and any structure built above it once backfilled. The crates create the storage volume itself, while taking up a fraction of the space a poured concrete tank of the same capacity would need to construct.

Backfill and Connection

Once the crates are in and wrapped, the system is connected into the site's stormwater pipework, including the inlet that captures runoff and the outlet that controls how quickly water is released, before the whole structure is backfilled. From this point on, the system is invisible, sitting beneath whatever finish goes down on top of it.

How Big Does a Detention Tank Need to Be

Sizing a detention system properly is an engineering exercise, not a guess, and it is worth understanding roughly what drives the number even if the detailed calculation sits with a civil engineer or hydraulic consultant. The main factors are the total area of hard surface draining into the system, the intensity of rainfall the design needs to cope with, and the maximum release rate the local council or water authority sets for that catchment.

A bigger roof and car park footprint means more runoff to detain. A stricter release rate, often set lower for sites near an already constrained section of the drainage network, means the system needs to hold water for longer before it can let it go, which usually means a larger tank. This is part of why two developments of a similar size can end up with very different detention volumes once the site specific conditions are factored in.

Getting this number right before excavation starts matters, because a detention tank built too small can fail its purpose entirely, while one built oversized wastes both space and construction cost. It is a case where the engineering has to lead the construction, not the other way around.

What This Means for a Development

For developers and builders working on commercial or industrial sites in Melbourne, stormwater detention is not an optional extra, it is frequently a planning condition that has to be satisfied before a development can proceed. Getting the sizing and staging of this work right early avoids costly redesign later, particularly on sites where the detention system needs to be installed before other civil works can follow on top of it.

It also pays to work with a contractor who has handled both sides of this kind of civil work, the below ground detention and storage systems and the broader earthmoving and site preparation that surrounds them. Sequencing matters. A detention tank that goes in at the wrong point in a construction program can hold up everything scheduled to happen above it.

CHS has been carrying out civil earthworks and underground infrastructure across Melbourne for more than 30 years, from residential water tanks through to commercial stormwater detention systems on growth corridor developments. The scale of the job changes. The attention to getting what is buried right, since nobody gets a second look at it once it is covered over, does not.

If you have a commercial or industrial project in Melbourne that needs stormwater detention, water storage, or the civil earthworks around either, get in touch to talk through what the site and the council conditions actually require.

Frequently Asked Questions

What is the difference between a detention tank and a retention tank?

A detention tank temporarily holds stormwater and releases it slowly at a controlled rate, eventually emptying completely between rain events. A retention system, by contrast, is designed to permanently hold a body of water, such as a decorative pond or wetland, with stormwater passing through it rather than simply draining away.

Can a detention tank double as a water storage tank?

Not directly, since the two serve different purposes and are typically sized and plumbed differently. Some sites do install both a detention system for stormwater compliance and a separate storage tank for irrigation or non-drinking use, connected to different parts of the site's plumbing.

Why use modular crates instead of a poured concrete tank?

Modular crates are faster to install, require less specialist formwork and pouring time, and can be configured to fit an irregular pit shape more easily than a poured concrete structure. For a given storage volume, a crate system also generally requires less excavation depth and disturbance than an equivalent concrete tank.

Do all commercial developments in Melbourne need stormwater detention?

Requirements vary by council and by the size and nature of the development, but many local authorities require detention or retention measures once a development increases the impervious surface area on a site beyond a certain threshold. Checking the specific conditions that apply to a site early in the planning process avoids surprises once civil works begin.