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So what is OSD? It's Storage of your rainwater with a slow release system to council kerb or street system!

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Many council's in New South Wales are requiring new property owners to provide a system that will capture the rainwater that falls on their property, and release it at a slower rate than it would normally do. 

The reason for this is to improve/reduce the flooding problems which occur in less fortunate areas downstream.

Some councils are now allowing a rebate in storage for Rainwater Tank storage you provide, which is also now required under a new legislation called 'Basix'. Ask us how this may affect you, by emailing us your property address. You can also click on your local council area below to get a general idea of their requirements.

Hills Shire Council Alternatives

Horsnby Shire Alternatives

Canada Bay Council Alternatives

Ku-Ring-Gai Council Alternatives

Ryde Council Alternatives

Pittwater Council Alternatives

Warringah Council Alternatives

Willoughby Council Alternatives

Wollongong Council Alternatives

Common questions asked are:

What do they look like? There are generally two types. Above ground and Below ground. The below ground is generally more costly to construct, and the above ground is generally "land hungry". An engineers advice is recommended to assess what are the best and possibly only alternatives available. Every site is different, and the most economical, aesthetic, and environmentally friendly solution is our aim. Typical photos of some system are below. Click on each photo for a larger image.

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1.An above ground OSD system in the Ku-ring-gai Council area.

Below ground system are generally constructed from concrete. They are basically a concrete/brick box underground with access points from the ground surface for maintenance. These access points are generally 900x900mm in size and are 6m spacings to allow for ventilations and safe egress from the tanks.

A new Alternative is for below ground tanks is a system using drainage cells such as  Atlantis or Versitanks. Basically its a plastic cube system that can be shaped to suit your site conditions. To view some photos. Click here. The photos shown are for an absorption system however the concept is the same for rainwater tanks and detention tanks. Minor detailing differences exist between them. Many councils allow this alternative, but some have not yet approved their use. Ask ISC if you can utilize this new product.

What controls the flow?  Click here for a photo.

A Discharge Control Pit or DCP is constructed to control the flow. Their size is dependant on their depth for maintenance, however, they are generally 600mm or 900mm square.

What controls the flow inside the DCP? Click here for a photo.

A metal plate bolted inside a pit with a small diameter hole, reduces the flow exiting the site and in turn backs up the water which stored either in a below or above ground storage facility.

What is an overland flow path? Rainwater runoff from other properties generally follow the valley lines in a catchment. The extent of flow is referred to as the overland flow path. With new developments the aim is to create a safe and formal passage for these flows around structures. Click here for a photo of a minor overland flow path.

Inter-allotment runoff control. Sounds technical, but basically its a method by which rainfall is held back on your property, rather than allowed to run off your property uncontrolled. Easements are created between private properties that cannot drain to a council stormwater system, to allow their runoff to be 'controlled' rather than uncontrolled.

Drainage Terminology

What is a gutter? Basically this is what captures the rainwater from your roof. Click here for a photo

What is a downpipe? Basically this is what captures the rainwater from your gutter. Click here for a photo

What is a pit, storm water pit, drainage pit, inlet pit? There are lots of types of pits, however, they all have the same common use of capturing water flowing on a surface like a road, garden, terrace or roof deck.   For photos of an online OSD pit required by most councils Click here. For photos of an offline OSD pit required by some councils Click here. Click Here for a typical concrete pit photo.

What is pump pit? Pumps are used to push water against the natural grade of the ground (uphill). Only certain local councils allow their usage for roof water to be directed to a kerb. They can be used to direct rainwater from a rainwater tank to be used for the flushing of toilets and laundry use. Click here for a photo

Specific Council requirements:

(Baulkham) Hills Shire Council

So your building in the Hill Shire Council. If your existing home is being knocked down and re-built then generally OSD will not be required. Low level properties are now being assessed by the engineering department on a case by case basis. Charged systems are currently the preference.

If your in a new subdivision area, then you'll most likely require OSD. Councils preference is for above ground OSD's in order to capture 100% of the site. Below ground OSD concrete tank are also accepted if 85% of the site can be captured by it. Below ground tanks must be concrete tanks. Hills Shire does not approve of the Atlantis style OSD tanks.

Below ground tanks can be quite expensive as the storages are quite large and require fairly significant excavations. The bottom of the tanks need to be able to gravity fall out to the street or easement, so its not always suitable to a property if the land has less than 3m fall across it.

Canada Bay Council

So your building in the Canada Bay council area. Your in one of the fair few councils that has a policy which highly rewards the use of rainwater tanks.

Canada Bay council policy requires OSD storage (refer to the above), based generally on your land area. All new homes will require OSD.

For a 1000sqm block of land your OSD storage can vary from 20,000L to 24,000L approx. The rewarding part of this council's policy is that you can reduce your OSD storage by any amount of storage you provide in a rainwater tank.

For example if your OSD storage is 20,000L. You can provide a 24,000L rainwater tank lieu of OSD subject to your basix certificate stating this volume.

Considering that OSD can cost $500 to $2500 per 1000L depending on site conditions and design, the substitution of OSD for OSR can be very cost effective and environmentally beneficial.

In the event your land falls to the rear and cannot obtain an easement, than your property is subjected to building limitations being site coverage (subject to your property not being located in a rocky area). Call our offices to discuss your specific property as there are as these limitations are complex. Also view our Low level Properties page to get a better understanding.

 

Ku-ring-gai Council

So your building in the Ku-ring-gai council area. Your in one of the fair few councils that has a policy which highly rewards the use of rainwater tanks.

Ku-ring-gai council is broken up in to many areas, that require varying amounts of OSD storage (refer to the above), based generally on your land area. All new homes will require OSD, unless their land falls to the rear and cannot obtain an easement or located downstream adjacent a waterway subject to council assessment.

For a 1000sqm block of land your OSD storage can vary from 14,000L to 25,000L approx. The rewarding part of this council's policy is that you can reduce your OSD storage by any amount of storage you provide in a rainwater tank.

For example if your OSD storage is 20,000L. You can provide a 20,000L rainwater tank lieu of OSD. (Note however, that any rainwater tank exceeding 10,000L must be located below ground).

Considering that OSD can cost $300 to $2000 per 1000L depending on site conditions and design, the substitution of OSD for OSR can be very cost effective and environmentally beneficial.

In the event your land falls to the rear and cannot obtain an easement, than your property is subjected to building limitations of a 30% site coverage (subject to your property not being located in a rocky area). This means that only 30% of your land area can be covered by hard surfaces such as roof, paving, driveway etc. (Excl. Pool waterline). Call our offices to discuss your specific property as there are options to exceed this limitation. Also view our Low level Properties page to get a better understanding.

 

Hornsby Shire Council

So your building in the Hornsby Shire Council. If your existing home is being knocked down and re-built then generally OSD will not be required.

If your in a new subdivision area, then you'll most likely require OSD. Councils preference is for above ground OSD's in order to capture 100% of the site. Below ground OSD concrete tank are also accepted if all hard surfaces of the site can be captured by it. Below ground tanks are generally more economical via a series of precast pits. Hornsby Shire does not generally approve the Atlantis style OSD tanks.

The bottom of Below ground tanks need to be able to gravity fall out to the street or easement, so its not always suitable to a property if the land has less than 2m fall across it.

 

Pittwater Council

So your building in the Pittwater council area. Councils current policy known as Pittwater DCP21 Part 5.7, outlines the several options available for different properties.

If your directly connected to the bay or ocean than you only need to comply with Basix.

If you property falls street or to the rear and have access to an easement, and your proposed building hard surface area is greater than 50sqm than the existing hard surface on your property than you'll need OSD. OSD is based on your new hard surface area, which can be interpreted in two ways, but council currently has conveyed the view that it is the difference between the new and the old as opposed to your entire new area. Council policy also allows you to quadruple the OSD storage and provide it as a raintank for re-use rather than letting the water go to waste. This volume is separate to any Basix requirement. Typical volume for OSD vary from 4000L to 20,000L.

If you property falls to the rear without an easement than please refer to our low level properties link at the top left of the page.

 

Ryde City Council

So your building in the Ryde council area. All new dwelling applications in the Ryde council area require OSD where site coverage exceeds 35% or the existing site coverage. (Unless your close to Parramatta River. Call ISC to confirm if your eligible for exemption). The 35% site coverage rule will be assessed on a case by case basis by council if exemption is sought. Storage requirements vary from 7000L to 16,000L for the typical homes sites. Ryde Council policy is based on the amount of hard surface area you're proposing on your landscape plan, and which part of the council area your building in. The council area your building in has only a small effect on the storage requirement.

The alternatives to OSD are generally all the above, but councils DCP tends to prefer below ground alternatives due to landscaping. Council has adopted a new DCP in July 2010 which now allows for upto 5000L of OSD to be swamped for extra raintank storage. Naturally some sites are more suitable to specific alternatives. Call our offices to discuss your options further.

In the event your land falls to the rear and cannot obtain an easement, than your property is subjected to building limitations of a 35% site coverage, and your backyard being equal or greater than 35% of your land area. (subject to your property not being located in a rocky area). This means that only 35% of your land area can be covered by hard surfaces such as roof, paving, driveway etc. (Incl. Pool watersurface from the site coverage and backyard area. Any existing or proposed pool must not obstruct the absorption trenches in their direction of flow) . In all instances ISC recommends that you conduct a pre-lodgment meeting with council to obtain advice in writing on which method council will allow or consider. Call our offices to discuss your specific property. Also view our Low level Properties page to get a better understanding.

Warringah Council

So your building in the Warringah council area. Your councils adopted a policy which rewards the use of rainwater tanks. Up to 50% of your OSD storage requirement can be deducted by replacing with a rainwater tank. This includes any storage required under basix. For example, you may need 10,000L for OSD, than council will allow you to reduce your OSD storage to 5000L by providing a 5000L or greater raintank as long as its stated as required on your basix certificate.

All new homes will require OSD, unless their site coverage including eaves, pathways, driveway does not exceed 40% and the site area is greater than 450sqm.

For a 1000sqm block of land your OSD storage is approx. 40,000L using the councils simple calculation method. (700sqm block requires 28000L). This method is now required from the 1-12-12. Council no longer allow an extensive computer modelling approach to simulate the stormwater runoff from your particular site, and provide only the storage required to reduce your new stormwater runoff to the same as the existing, however as no block of land is really standard we still recommend the use of the computer modelling to acheive the intent of council's new policy to avoid dictating which side a driveway should be located. Sounds complicated. The analysis is, but the concept is simple and rewarding in most cases. There are other several other factors which are too technical to explain here.

Considering that OSD can cost $500 to $2500 per 1000L depending on site conditions and design, the substitution of OSD for OSR can be a cost effective and environmentally beneficial.

In the event your land falls to the rear and cannot obtain an easement council has adopted a new policy in 2010 allowing alternative systems subject to the outcomes of a geotech permeability report. Where a rear neighbour has refused in writing to provide an easement council is considering absorption or dispersal as a possible alternative subject to a geotech engineer assessing your soils and finding them favourable. Also view our Low level Properties page to get a better understanding.

 

Willoughby Council

So your building in the Willoughby council area. All new dwellings require On Site Detention. Council provides a concession only for single dwellings that an effective 10,000L raintank can be provided in lieu of On Site Detention.

In the event your land falls to the rear and cannot obtain an easement, than your property is subjected to the limitations of your soil characteristic, in order to provide an absorption system (subject to your property not being located in a rocky area). If your neighbour refuses to provide an easement in writing than council consider an effective 10,000L raintank plus an absorption system (subject to soils). Call our offices to discuss your specific property. Also view our Low level Properties page to get a better understanding.

Click here for a Lists of Manufacturers of Proprietary and Other OSD Products .

 

Click Here for OSD Maintenance Contractors

(The lists above are provided to assist owners to maintain OSD facilities on their properties. No attempt has been made to check the credentials of the listed firms, nor are any specifically recommended by ISC. The selection of a suitable maintenance contractor and agreement on the works to be undertaken are matters for individual owners who will be liable for any losses incurred by either the owner or the contractor.)

 

Wollongong City Council

So your building in the Wollongong council area. All new dwelling applications in the Wollongong council area that exceed an additional 100sqm of hard surface area require OSD. Storage requriements vary from 3000L to 10,000L for the typical homes sites. Wollongong Council policy is based on the amount of hard surface area you're proposing on your landscape plan, and the relative increase from what was currently on the site. The analysis is complex, so accurate volumes can only be obtained once design is completed.

In the event your land falls to the rear and cannot obtain an easement, than your property is subjected to the limitations of your soil characteristic, in order to provide an absorption system (subject to your property not being located in a rocky area). Call our offices to discuss your specific property. Also view our Low level Properties page to get a better understanding.


 

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Last modified: 03-Jun-2013