on-site stormwater detention policy

On-Site Stormwater Detention Policy

APRIL 2015

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1.0 General 3

2.0 Objectives 3

3.0 Policy Application 3

Policy Application - continued 4

4.0 Policy Exclusions 4

5.0 OSD Approval Process 5

6.0 Control Standards 6

6.1 General 6-7

6.2 Discharge and Storage Requirement / Catchment Plan 7

7.0 Design and Construction Parameters

7.1 General 8

7.2 Freeboard 9

7.3 Ponding Limits 9

7.4 Access Requirements 10

7.5 Stormwater Connection 10

7.6 Submerged/Drowned Orifice 11

7.7 Council Drainage Easement / Open Channel /

Catchment Low point /Overland Flows. 11-13

7.8 Drainage Easements 13

7.9 Pumpout Systems 14-15

7.10 Rainwater Tanks 15

Figure 1 – Key Elements of domestic aboveground rainwater tank 15

Figure 2 – Configuration for an underground system 15

Figure 3 – Rainwater Directional Model 16

Australian Standard AS/NZS 3500.1.2 : Water Supply 16

Figure 4 – Design Details of a Dual Rainwater Tank 16

NSW Department of Health 17

Water Authorities 17

Figure 5 – Design detail for a dual water supply system using

Rainwater & mains water 17

Table A – Average Percentage (%) of Rainwater Tank Volume

that can be counted as OSD site 18

8.0 Certification of constructed OSD System 18

9.0 Protection and Maintenance of OSD System 18-19

10.0 On-site Stormwater Detention Bond 19

11.0 Information required with Development Application 19

12.0 Appendix A

A-1 Example of a Submerged Orifice Outlet – Calculations 20

A-2 Example of a Submerged Orifice Outlet – Diagram 21

13.0 Appendix B – Typical Section Details B1, B2, B3, B4 22-25

14.0 Appendix C – Standard Pumpout Design Notes 26

POLICY CONTENTS

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On-Site Stormwater Detention (OSD) involves the temporary storage and controlled discharge of stormwater generated within a site. OSD is required to ensure that the change in stormwater runoff from a site due to development does not increase flooding problems downstream, except in very severe events.

Council requires the submission of fully detailed OSD drawings to assist in determining the likely impacts that the development may have on the existing natural and built environments, both public and private. This will include any impacts on existing stormwater systems, overland flow and flooding conditions and those impacts on the assessment of the proposal with regard to Councils’ Development Control Plans.

Council’s Engineers are able to discuss matters relating to a proposed development application with the applicant in relation to Council’s requirements, policies and procedures for drainage and OSD systems. Matters that relate to specific design issues and technical details in relation to the proposed drainage and OSD system should firstly be referred to an engineer who has the appropriate qualifications and experience to interpret Council’s requirements, policies and procedures.

The objectives of this policy are to:

2.1 prevent any increase in downstream peak flows resulting from new developments or redevelopments by temporarily storing on-site the additional and quicker runoff generated;

2.2 prevent increases in downstream flooding and drainage problems that could:

increase flood losses damage public assets reduce property values require additional expenditure on flood mitigation

2.3 encourage integration of OSD systems into architectural and landscape design and layout of the

development so that adequate storage areas are included in the initial stages of the site design.

2.4 require construction supervision of OSD systems by the OSD designer to improve construction standards.

The OSD policy is to apply to all types of development and re-development on both flood liable and flood free site, including the following:

3.1 all commercial, industrial and special-use development and buildings and structures (including awnings, temporary buildings)

3.2 town houses, villas, home units, duplexes;

3.3 dual occupancies;

Note: OSD shall be created for each lot to clarify and ensure the maintenance responsibility for individual owners. Unless the designer can demonstrate that it is not possible or feasible due to the site constraints.

3.4 subdivisions (including residential) approved after 1991

3.5 tennis courts, basketball courts and any other sealed sporting facilities;

2.0 OBJECTIVES

3.0 POLICY APPLICATION

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3.6 roads, car parks, paths and other sealed areas; and

3.7 Major public buildings.

3.8 On an already-developed property, which is being partly re-developed, the OSD requirements shall relate only to the additional new area to be created.

3.9 On a new development (development of a vacant site or complete re-development of an already- developed property), the OSD requirements shall relate to the whole property.

3.10 In the case of a dual occupancy if the existing dwelling is to be retained, the OSD requirements shall relate only to the second dwelling and surrounding associated impervious areas.

3.11 Where an existing residential property is to be subdivided, the OSD requirements shall only relate to the area of the new allotment, and the OSD storage facilities shall be located on the new allotments.

3.12 On a property with multiple existing lots.

Note:

For sites where the existing dwelling is to be demolished and the impervious area of any proposed dwelling and/or structure is less than the impervious area of the existing dwelling

divided by the number of lots, then OSD is not required. (E.g. If a dwelling of 300m2 impervious building area sits on a 3 lot property, the maximum allowable impervious building area of each

lot shall not exceed 100m2 ). If the existing dwelling is to be retained, OSD shall be required for any development on the

remaining lots. If there is no existing dwelling, OSD shall be required for any development on all lots.

The OSD policy shall not apply to:

4.1 single dwellings, extensions, additions and improvements on existing single residential lots except where OSD is required as a restriction on the title of the property and /or clause 3.12;

4.2 subdivisions of existing dual occupancies;

4.3 buildings in Open Space and Rural/Non-Urban areas;

4.4 one-off minor developments, minor additions and repairs where the proposed development area

is less than 150 square metres (subsequent minor developments or additions shall require OSD). It does not apply to any developments where the development area is greater than 150 square metres nor to dual occupancies;

4.5 one-off minor developments, minor additions and repairs to commercial, industrial and special-

use properties where the impervious area is increased by less than 100 square metres (subsequent minor developments or additions shall require OSD)- applicable only to areas outside the UPRCT boundaries;

4.6 change of use, where no physical changes to the outside of the property are proposed;

4.7 areas within large properties (usually commercial or industrial) not covered by the development or

construction certificate application; and

4.8 Any portion of the property used as a defined floodway to convey stormwater Runoff from upstream public land and which shall be protected by an appropriate covenant, restriction or easement.

NOTE: Refer to the Upper Parramatta River Catchment Trust OSD handbook for further clarification of policy application and exclusion.

4.0 POLICY EXCLUSIONS

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OSD is best considered as early as possible in the development process so that the most efficient system can be designed and installed. The following flow chart outlines the OSD approval process.

Determine if OSD is required.

Have a contour survey prepared. Involve the OSD designer in site/lot layout. Note: Survey should include contours of land at max. 0.5m intervals and all existing structures including trees. Survey shall be to AHD for flood affected sites or sites requiring a flood study.

OSD Designer completes design checklist and prepares fully detailed OSD drawings for submission.

Council approves development/subdivision with OSD conditions.

OSD designer addresses consent conditions as required and prepares maintenance schedule. Calculations, maintenance schedule, plans and details submitted with application for Construction Certificate. Plans forwarded to Council or private certifier for issue of approval.

OSD designer supervises construction, complying with inspection points as outlined within the consent conditions and OSD guidelines.

Work as Executed drawings prepared. Positive covenant and Restriction prepared and attached to linen plan. OSD designer certifies hydraulic performance of construction.

Certificate of hydraulic compliance and Work as Executed plan submitted to Council. Positive Covenant and restriction registered.

5.0 OSD APPROVAL PROCESS

Prepare drainage and On- Site- Stormwater Detention

(OSD) drawing

Development Approval

Proposal

Preliminary lot layout

Construction certificate/Engineering Plan

approval

Construction

Finalisation

After approval, release of occupation certificate or

subdivision certificate/request forms.

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6.1 General

6.1.1 The 1% Annual Exceedance Probability (AEP) storm event for developments or re-developments must not increase flooding or stormwater flows in any down stream areas.

6.1.2 Detention storage shall not be located in any natural watercourse or over land flowpath, and shall function independently during all storm events up to and including Council’s adopted design flood standard of the 1% AEP storm event.

6.1.3 OSD basins shall be provided with an overflow spillway directed towards the overland flowpath,

which shall be kept free of obstructions such as fences and minor structures. Overflow spillways shall not be provided on walls adjoining neighbouring properties. Where necessary a 900mm x 900mm overflow pit shall be provided, otherwise the spillway shall be provided on walls adjoining the road reserve or locations where neighbouring properties will not be affected.

6.1.4 Developers and designers are encouraged to use principles of good aesthetics when preparing an OSD design. Long term viability, ease of maintenance and access to the drainage system and storage areas also needs to be considered in the design process. It is recommended that the OSD designer consults with the architect and landscape designer prior to completing an OSD design. This will ensure that all drawings correspond in terms of location of buildings, walls, existing trees being retained and landscaping treatments proposed on the site.

6.1.5 OSD storage’s shall be designed such that run-off in small frequent storms is stored where minimal inconvenience results. In larger storm events, the additional run-off may be stored above ground in landscaped areas, car parks or driveways where it will cause some inconvenience.

6.1.6 Any grated pit in children’s play areas shall be provided with child proof “J” type spring locks.

6.1.7 Any stormwater overflow from the gutters of proposed buildings shall be collected by inlet pits on

the ground and/or drained via overland flows paths into the detention system. Where this is not possible, the gutter and stormwater system shall be designed to convey the 1% AEP storm event to the detention system.

6.1.8 Detention Storage shall be provided above ground wherever possible and permissible. Under ground tanks will only be accepted where above ground storage systems are not possible or feasible due to the site constraints.

6.1.9 Where below ground tanks are proposed, Council will only consider the tanks where the access

provisions and internal depths comply with the following criteria:-

Council generally requires the minimum internal depth of the belowground storage tank to be 1200mm; however a 900mm minimum internal depth can be considered where site constraints will not permit the 1200mm minimum depth.

All access grates to the tank shall be a minimum of 900mm x 900mm.

An access grate shall be provided over the Discharge Control chamber.

An access grate shall be provided over the location of the outlet pipe from the tank storage component. (This grate will generally be located adjacent to the access grate of the Discharge Control chamber.)

An access grate shall be provided at the highest point of the tank.

Where the minimum internal depth is 1200mm, a 900mm x 900mm access grate shall be provided at no more than 6m spacings from another access grate.

6.0 CONTROL STANDARDS

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Where the minimum internal depth is between 1200mm and 900mm, a 900mm x 900mm access grate shall be provided at no more than 3m spacings from another access grate.

In accordance with the Occupational Health and Safety requirements, only persons with Confined Space training shall be permitted to enter below ground storage tanks for any required maintenance. Council requires that a Confined Space Danger sign be placed at all access points to the below ground storage tanks. Details of the required sign may be obtained within the Upper Parramatta River Catchment Trust OSD handbook.

6.2 Discharge and Storage Requirements

Whilst the On-Site Stormwater Detention policy applies to the whole Council area, a different standard applies to the area that drains to the Upper Parramatta River than to the rest of Holroyd which drains to Prospect Creek, A’Becketts Creek and Duck Creek. The Upper Parramatta River catchment includes the suburbs of Girraween, Toongabbie, Pendle Hill, Wentworthville, Mays Hill, South Wentworthville and part of Greystanes and Merrylands West. The following permissible site discharge and site storage requirement apply for each area: -

Catchment Max. Permissible Site Discharge (P.S.D.) - Litres/sec/Hectare

Min. Site Storage (S.S.R.)

- cu.metre/Hectare

Upper Parramatta River

80

470

Prospect Creek, 140 300 A’Becketts Creek & Duck

Creek

Note: It is recommended to contact Council to confirm catchment boundaries

UPRCT AREA

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OSD designs shall be prepared by a suitably qualified and registered engineering consultant, in accordance with the requirements of the Upper Parramatta River Catchment Trust. The methods and principles of the design shall be in accordance

with the latest revision of the 3rd edition of the Upper Parramatta River Catchment Trust OSD Handbook and the latest edition of Australian Rainfall and Runoff. A copy

of the latest revision of the 3rd edition of Upper Parramatta River Catchment Trust handbook may be obtained from the Trusts’ internet web site - www.uprct.nsw.gov.au

All engineering works shall be designed and undertaken in accordance with the relevant aspects of the following documents, except as otherwise authorised by a development consent:-

Holroyd City Council’s Works Specification for Subdivisions and Developments. (Current

Version). Holroyd City Council’s Development Control Plan.

7.1 General

7.1.1 An OSD warning sign shall be provided to each above ground detention area. In this regard, the signage shall be in accordance with the Upper Parramatta River Catchment Trust OSD handbook.

7.1.2 Requirements of the Building Code of Australia shall be considered when determining finish

ground levels adjacent to the proposed buildings. Design levels throughout the site shall be provided showing compliance with such.

7.1.3 Spillways within basin walls adjoining road reserves shall be provided for the full length of the

wall and the spillway level shall be clearly shown.

7.1.4 Top of wall levels of above ground detention systems should generally be set a minimum of 100mm above the top of water level, excluding the spillway.

7.1.5 All walls forming the detention basin shall be constructed wholly within the property boundaries

of the site being developed. A notation shall be shown on the drawings indicating such.

7.1.6 Where below ground tanks are proposed, the location of underground storage tanks shall be beneath proposed hardstand areas. (i.e. driveways, pathways)

7.1.7 Basins proposed adjacent to dwellings/buildings shall ensure that access points/ventilation/air

vents to the under side of the structure are above the top of water level of the basin or protected from the OSD storage.

7.1.8 Existing/Proposed sewer manhole access points within above ground storage areas shall be

provided with a suitable watertight gatic lid in accordance with Sydney Water requirements. Any walls or structures (forming part of the OSD system) that is located close to an existing sewer manhole shall also be in accordance with Sydney Water requirements.

7.1.9 Sewer gully traps within above ground storage areas shall be set above the top of water level of

the basin.

7.1.10 Hot water units located within above ground storage areas shall be set above the top of water level of the basin

7.0 DESIGN AND CONSTRUCTION PARAMETERS

http://www.uprct.nsw.gov.au/

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7.2 Freeboard

Habitable and/or Office floor levels shall be fixed so that they are a minimum of 300mm above the water storage/flow levels for the 1% AEP storm event. Non habitable floors including garages shall have floor levels set a minimum of 100mm above the maximum water surface level based on the 1% AEP storm event. Consideration shall also be given to the maximum water level that can be achieved within a below ground storage before the tank surcharges and the protection to the proposed finish floor levels.

7.3 Ponding Limits

7.3.1 In the interests of safety and amenity, storage water depths for detention systems should not exceed:-

Maximum

Driveways/Parking/Paved Areas 150mm desirable 200mm maximum

Front Setback Landscaped/Common areas 300mm desirable 600mm maximum

Private Courtyards/Rear Yards See below

Covered/Fenced Storage Consult with Council for Limits

Roof Area As required by structural integrity

7.3.2 The following shall be noted with regard to storage in private courtyards:

Prohibited if the courtyard area is less than 25m2

Acceptable if the courtyard area is greater than 25m2

7.3.3 Ponding is limited to the following for courtyard areas between 25m2 and 60m2:

100 mm (maximum) for the 20% AEP storm event (5 year ARI).

Desirable limit of 400 mm for the 1% AEP storm event (100 year ARI).

Maximum 600mm for the 1% AEP storm event (100 year ARI) - subject to Council approval.

7.3.4 The following shall also be considered:

The finish levels of external porch/verandah areas shall be set a minimum of 100mm above the top of water level of the basin. A suitable porch/verandah area shall be provided in all proposals to function as a permanent dry area.

The limit of ponding within private courtyards greater than 60m2 shall be a minimum of 2m clear of any external porch/verandah areas.

The ponding shall begin to occur as far as possible from all external porch/verandah areas.

At least one dry access/escape route shall be available to individual residences.

An additional 20% storage volume, in excess of the design volume, shall be provided in courtyard areas to allow for future unintentional reduction in storage.

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7.4 Access Requirements

7.4.1 Design of OSD storage areas shall consider access requirements for maintenance and inspection purposes. It shall allow for pedestrian movements, i.e. for mowers and bins, from private courtyards to the street frontage.

7.4.2 Satisfactory access shall be provided to all above ground storage basins by use of stairs or batters not exceeding a grade of 1 in 4. Where storage areas are proposed within the front setback area of the site provision shall be made for an access point within the front setback area. Access into a storage area within the front setback via the side setback areas of the property will not be considered as satisfactory

7.4.3 Where vertical drops into any above ground storage area from adjoining pedestrian/pathway areas exceed 500mm, a pool type fence or equivalent barrier shall be provided.

7.4.4 Figure 1

7.5 Stormwater Connection

7.5.1 Stormwater outlet pipe connections into the kerb and gutter/existing Council drainage system will not be permitted to be laid across the footpath area at an angle of greater than 45 degrees from the front boundary alignment of the property. Where connection into the kerb and gutter/existing Council drainage system cannot be laid at or less than 45 degrees to the property boundary alignment, the following will apply:

The pipe shall be run though the property and/or a drainage easement shall be obtained through the adjoining downstream property to a point where the 45 degree limit can be achieved or,

Council will require the existing nearest drainage system under the control of Council to be extended to the property frontage. A grated gully pit, to the relevant Council standards will be required at the end of the street drainage pipe being extended.

7.5.2 All Council drainage pipes shall be a minimum 375mm diameter reinforced concrete pipes. A cash bond/bank guarantee will be required to be lodged as part of the development consent conditions for the required works including restoration works carried out within any Council reserve.

7.5.3 Fully detailed designs, including hydraulic grade lines, long-section of the pipe and a pipe trench cross-section detail shall be submitted for all proposed drainage works within the road reserve at development application stage.

Protective fencing and/or suitable barrier adjoining pathway, where drop into basin exceeds 500mm.

Stair access into basin area

Pathway to permit movement from courtyard to street frontage and vice versa

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7.6 Submerged/Drowned Orifice outlet

7.6.1 The designer shall ensure that the orifice outlet of the proposed detention system flows freely at all times during all projected storm events up to and including the 1% AEP storm event. Where the outlet pipe of the detention system connects into an existing stormwater drainage pipe/gully pit, Council will consider the grate/ground level at the connection point as the level to determine whether the orifice will function as a submerged/drowned outlet. In cases where overland flows or flood levels exist at the stormwater connection point, the top of water level of the overland flowpath/flood level will be used to determine the extent of submergence of the orifice outlet. For example where the invert level of the orifice outlet is below the grate level of a gully pit (where the outlet pipe from the control unit is being connected), the orifice outlet will be considered to be functioning as a submerged outlet.

7.6.2 Where an orifice outlet will be functioning as a submerged/drowned outlet the designer shall carry out the necessary calculations and adjustments to allow for the submerged/drowned outlet conditions. The following shall be submitted with the On-Site-Detention drawings:

Design summary calculation for submerged orifice outlet showing revised volume of storage required.

OSD system shall be designed to take into consideration additional volume required due to submerged orifice conditions.

7.6.3 The calculation method used for submerged/drowned orifice conditions is provided within Appendix A of the policy.

7.7 Council drainage easement/ Open channel/Catchment Low point/ Overland flows

7.7.1 Provisions shall be made by the designer to ensure that overland flows generated from the upstream properties, up to the 1% AEP storm event are not obstructed or permitted to drain into the OSD system provided on the development site. The designer shall ensure that such overland flows are intercepted, within the property being developed and conveyed through the site, bypassing the OSD system. (Note: Minor upstream flows from adjoining neighbouring properties may drain into the proposed OSD system if an allowance is made in the OSD design for those flows).

7.7.2 Where the site is located in/or adjacent to a low point in the catchment area, or a Council drainage easement/open channel is located within or adjoins the site, or the upstream catchment is greater than 0.5 Ha, Council’s Stormwater Engineer shall be contacted about the development site. Any relevant information obtained shall be provided with the proposal and the following shall be submitted on the On-Site-Detention drawings:-

Catchment Plan highlighting full upstream catchment area generating the overland stormwater flows is provided.

A detailed hydraulic analysis for the upstream catchment that flows through the Council drainage system. The detailed analysis shall be prepared by a qualified hydraulic engineer having NPER accreditation. The analysis shall include a pre and post construction analysis, showing on a plan the 1% AEP inundation line for each. It is recommended that the applicant's engineer contact Council’s Stormwater Engineer in order to obtain any information that Council may have on the easement. Minimum details shall be as per the following: -

a). Hydrology – Rational Method: -

i). Calculate 1% AEP flood flow, Q100.

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ii). Calculate pipe flow capacity through the site - Determine the pipe slope (If data is not available assume pipe slope is the same as the ground slope). Capacity = Pipe flowing full, but not under pressure, and multiplied by 0.5 to allow for partial blockages, pipe and pit losses, PCapacity

iii). Stormwater overland flow QO = Q100 – PCapacity or,

iv). If using DRAINS model partial blockages, pipe and pit losses are incorporated in the model of the upstream catchment, therefore 0.5 factor (as per (ii) above) is not required. This will only apply if all the upstream drainage system is analysed.

b). Hydraulics – HEC-RAS for overland flow. Use QO as the stormwater overland flow

through site. For long sites, additional QO may need to be included.

c). The cross-sections used in HEC-RAS shall be at least 10m upstream and downstream from the development site. Cross-sections must extend the full width of the surface flow at a maximum spacing of 5m and all levels shall be to the Australian Height Datum (mAHD). Normal depths shall be used as the upstream and downstream limits, unless these limits have been determined by flood studies previously accepted by Council.

d). Increases in stormwater depth of more than 0.01m for the post development overland flow path will not be acceptable.

Note: All electronic models shall be provided to Council.

The applicant shall submit a survey drawing prepared by a registered surveyor that includes existing site contours and spot levels throughout the site along with the location of all existing structures to the Australian Height Datum (mAHD).

Photos of the existing overland flow path throughout the site shall be taken and submitted to Council.

The hydraulic engineer shall also submit details and drawings of the works that are required to convey the 1% AEP stormwater overland flow through the site ensuring that the neighbouring properties do not suffer any increase in water levels or flooding following construction of the proposed structure. The stormwater system for the proposed structure shall also be included on the drawings.

The finished floor level of the industrial/commercial/habitable floor is set a minimum of 500mm above the post development 1% AEP overland flow path level and that the garage floor level is a minimum of 150mm above the same.

The velocity times depth value through the site does not exceed 0.4 sq. metres/second within any proposed overland flow paths on the site. If this cannot be achieved then low hazard category flows must be attained. (Refer to floodplain development manual, section relating to provisional hazard categories).

All existing and design levels on the OSD drawing shall be to the Australian Height Datum (AHD).

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7.7.3 Overland flow/s generated from catchments of less than 0.5 Ha is/are considered to be localised flow/s. Where this/these flow/s currently drain through the pre developed site the applicant’s engineer shall ensure that the following is shown on the OSD drawings: -

Catchment Plan highlighting the full upstream catchment area generating the overland stormwater flows.

Full hydraulic analysis for projected flows generated from the upstream catchment in the 1% AEP storm event.

Full details of works required ensuring that overland flows generated from upstream catchments are conveyed through the site without draining into the detention system or altering existing conditions on adjoining properties.

7.7.4 Where overland flow paths are proposed adjacent to habitable floors, including office floors, a minimum freeboard shall be provided from the finish floor level to the top of water level of the flow path. The minimum freeboard will vary between 200mm and 500mm depending on the catchment area generating the upstream flows. Council shall be consulted for minimum freeboard requirements.

7.7.5 Sites affected by mainstream flooding require minimum 500mm freeboard from the top of flood level.

7.7.6 Where overland flow paths/flood levels are adjacent to non-habitable floors, eg garages, the finish floor levels shall be set a minimum of 150mm above the top of water level of the flow path/flood level.

7.8 Drainage Easements

Inter-allotment drainage

7.8.1 OSD systems shall discharge into a Council drainage system, kerb and gutter or piped/channel, via gravity. Where this will occur through a neighbouring property/properties a drainage easement may be required. Council requires proof that the owner of the property where the easement is to be created agrees to give drainage easement rights to the proposed development site. Proof of acceptance shall be in the form of a standard Statutory Declaration Form. The following shall be shown on any OSD drawing where an easement is proposed:

Full details of Stormwater drainage within the easement. (Proposed and existing)

A long-section of the drainage pipe within the easement.

The drainage easement location shall not disturb any structures or root zone of existing trees within the property/properties.

All trees within, overhanging or within 5m of the proposed easement shall be accurately indicated.

Note: Concrete lined swales/channels or ½ pipes will generally not be approved. The

Design Engineer shall provide turfed swales or utilise landscape strips as channels for

intercepting and conveying overland flows.

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7.9 Pumpout System

7.9.1 Pump-out systems for roof and surface water are not permitted and any plans incorporating such a system for disposal of stormwater will be rejected. However Council will permit basement level pumpout systems for disposal of seepage water and runoff from access ramps to basements, which are generally associated with Residential Flat Buildings. The following shall be provided on the OSD drawing: -

7.9.2 Cross-section detail for pump out tank in accordance with Council Standard Section detail (see Appendix B of the OSD policy for Councils’ Standard Pump Out Tank Detail).

7.9.3 Two (2) submersible type pump units shall be installed. The capacity of each pump being calculated to allow for subsoil drainage and any water falling on access points to the basement car park. Stormwater run-off to the sump and pump system shall be calculated for a 1% AEP storm event.

7.9.4 The two (2) submersible type pump units shall be designed to operate on an alternate basis to ensure that both pumps receive equal usage and neither pump remains continuously idle.

7.9.5 Diameter and class of rising main/s from pump out system

7.9.6 Council requires the provision of basement stormwater storage in case of pumpout failure to accommodate the 1% AEP storm event - 12 hr duration storm event. Included in the storage shall be an above ground component which will act as a visual warning to the occupiers of the building. In this regard the basement storage shall consist of the following:

A Below ground storage tank shall be provided and capable of storing the volume of stormwater run-off generated from the catchment area draining to the pump out system in the 1% AEP (100 year ARI) - 90-minute storm event. The tank shall be clearly delineated on the stormwater drawings.

An Above ground storage area shall be provided and capable of storing the difference in volume of stormwater run-off generated from the catchment area draining to the pump out system between the 1% AEP (100 year ARI) 90 minute and the 1% AEP (100 year ARI) 12 hour storm event. The above ground storage area shall be clearly delineated on the stormwater drawings.

7.9.6 Adequate finished surface levels shall be shown for the above ground storage area to ensure it will be constructed correctly.

7.9.7 The basement car parking area shall be graded to fall to the sump and pump system.

7.9.8 Garage floor levels within basement shall be provided and are a minimum of 100mm above the top of water level of the above ground storage within the basement.

7.9.9 Brand and model numbers of the selected pumps shall be provided on the OSD Drawing and pump performance curve and specifications attached.

7.9.10 Council Standard notes for pump out systems shall be included on the OSD Drawing. (see Appendix C of the policy).

7.9.11 The Location of a “pump failure warning sign” and flashing strobe light shall be indicated on the OSD Drawing, which shall be visible to vehicles entering the basement.

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7.10 Rainwater Tanks

A simple sketch of the domestic rainwater system is as per Figure 1. The rainwater tank system consists of house roofing, roof gutters, first flush device, rainwater tank, pump, overflow pipe system (which discharges into the OSD system if OSD is part of the design) and supply pipe to all toilets, washing machines, and outdoor taps.

Depending on site conditions, user requirements and budget, rainwater tank systems can be installed using a number of systems, including: -

Installing tanks above ground (figure 1) or below ground (figure 2)

Figure 1: Key elements of a domestic above ground rainwater system.

Figure 2 – Configuration for an under ground system

To OSD

Overflow

to OSD

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If the tank is installed below ground, rainfall or surface water runoff from ground surface areas is not allowed into the rainwater tank, but shall drain into the OSD system as required.

Figure 3 – Rainwater directional model

Australian Standards

The Standard AS/NZS 3500.1.2: Water Supply - Acceptable Solutions provides guidance for the design of rainwater tanks with dual water supply (rainwater and mains water). It categorises cross connections between mains water supply and premises with a rainwater tank to be ‘low hazard’, thereby requiring a non-testable backflow prevention device. Rainwater tanks with dual water supply must maintain an air gap, and be designed and connected in accordance with Figure 4.

Figure 4 – Design details of dual rainwater tank

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NSW Department of Health

The NSW Department of Health does not prohibit the use of rainwater for drinking or other purposes. The Department recommends proper use and maintenance of rainwater tanks and provides a monograph Guidance on the use of rainwater tanks [Cunliffe, 1998] to assist with this task. The focus of NSW Department of Health guidelines is drinking water quality.

Water Authorities

Water authorities cannot prohibit the use of rainwater from tanks on private land. Their primary concern is to maintain the quality of mains water. Accordingly, water authorities may require the installation of an adequate backflow prevention device to prevent contamination of mains water by tank water if the existing water meter does not already have a backflow prevention device.

All tanks should be installed in accordance with The New South Wales Code of Practice: Plumbing and Drainage [Committee on Uniformity of Plumbing and Drainage Regulations in NSW, 1999].

If a development application is required to install a rainwater tank, details shall include:

Location of the tank and relationship to nearby buildings; Configuration of inlet/outlet pipe and the overflow pipe; Tank capacity, dimensions, structural details and proposed materials; and Purposes for which the tank is intended to be used.

In order to maximise water savings and stormwater management benefits, tank capacity should be between 5 kL and 20 kL for each residential dwelling. The required capacity will depend on the number of persons in the household, water use, rainfall and roof area. Design of the rainwater tank should make provision for:

A minimum storage volume (to ensure that water supply is always available); and A rainwater storage volume

The minimum storage volume is the maximum daily water use that is expected from the tank (about 250-750 litres). If the volume of stored water falls below the minimum storage volume, the shortfall can be overcome by topping up the tank with mains water to the required level. A simple float valve system can be installed to do this automatically (see Figure 5). The rainwater storage volume is the total volume available in the tank to store rainwater below the overflow pipe.

Figure 5. Design detail for a dual water supply system using rainwater and mains water.

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Council will permit the following specified percentages of a rainwater tank’s volume to be counted as part of the site’s OSD storage (the site storage requirement) provided that: -

i) the development is residential, or its water usage can be considered to approximate that of a residence;

ii) the rainwater tank volume is between 5 and 15 m3 (or 20 m3 for apartment blocks).

iii) the rainwater tank is plumbed into the household water supply system so that its water is automatically used for non-potable purposes;

iv) the design is in accordance with the guidelines of section 7.00; and

v) all overflows from the rainwater tanks are directed into the OSD Discharge Control Unit.

The average percentage of rainwater tank volume that can be counted as OSD site storage is in the table below for each allotment scenario:

Scenario (%)Volume of rainwater tank counting as OSD storage

No airspace in tank

Single dwelling 42 * Duplex 50 * Townhouse 40 * Walk up apartments 32 *

Table A - Average percentage of rainwater tank volume that can be counted as OSD site

Note: - ‘*’ Minimum OSD requirement will be the greater of the compensated OSD volume or the

stormwater flow into the OSD system, excluding the area contributing to the rainwater tank.

8.1 Following construction and prior to the occupation of the development, a certificate of compliance, in accordance with the standard Council Certificate, shall be prepared by a qualified and registered Civil Engineering Consultant and shall be submitted to Council.

8.2 In addition, a fully detailed work-as-executed (WAE) plan, prepared and signed by a registered surveyor, shall be submitted along with detailed calculations showing the volume of storage provided in the OSD system. The WAE plans shall indicate an “as Constructed” level adjacent to all design levels, including pit/tank cross sections, in red ink on the approved stormwater and OSD drawing.

8.3 Council will only accept the original Certificate of Compliance and Work-as-Executed drawings.

8.4 An OSD WAE Survey and Certification Submission, including Checklist, shall be submitted with

the above. The checklist is located within the Upper Parramatta River Catchment Trust OSD handbook.

9.1 A Restriction on the Use and Positive Covenant under section 88E of the Conveyancing Act 1919 shall be registered on the title of the subject property requiring that the stormwater detention system within the site, as constructed, shall not be altered and shall be maintained in good working order by the owner to the satisfaction of the Council. Council only, shall be empowered to release, vary or modify such restriction and covenant.

8.0 CERTIFICATION OF CONSTRUCTED OSD SYSTEM

9.0 PROTECTION AND MAINTENANCE OF OSD SYSTEM

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9.2 A maintenance schedule for the constructed on site detention system along with a drawing showing all major components of the stormwater drainage and on site detention system on the site shall be prepared by a qualified hydraulic engineer and included in the Positive Covenant of the 88B instrument as a cross reference or appendix. The maintenance schedule will assist the future property owners in identifying the key components of the system and their locations on the site and ensuring that regular maintenance is carried out in order to keep the OSD system functioning as required. An example of a maintenance schedule and drawing is contained within the Upper Parramatta River Catchment Trust OSD handbook.

The applicant/developer shall lodge with Council a cash bond or bank guarantee to cover the registration of the Restriction on Use of Land and Positive Covenant on the property title. The amount of the bond required by Council shall be in accordance with Councils current fees and charges schedule and shall be lodged with Council prior to the issue of the Construction Certificate.

To enable checking of the adequacy of the stormwater drainage and OSD design, the following information shall be supplied with the Development Application: -

Completed Upper Parramatta River Catchment Trust “On-Site-Detention Concept Plan and

Detailed Design submission checklist” (Available within the Trusts’ OSD handbook).

Completed Holroyd Council “Stormwater and On Site Detention drawing submission checklist” (Available from Council’s Customer Services).

All relevant documentation required by the OSD policy and drawing submission checklist.

Copies of the fully detailed stormwater and OSD drawings as required by the DA submission Checklist.

A copy of the OSD design summary calculation sheet (Available within the Trusts’ OSD handbook).

10.0 ON-SITE STORMWATER DETENTION BOND

11.0 INFORMATION REQUIRED WITH DEVELOPMENT APPLICATION

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ie. TWL basin -

1.Determine

the required orifice size based on free discharge.

5. Final required

storage volume of basin.

HOLROYD CITY COUNCIL

On-Site Detention Calculation Sheet

Project: Example 1

OSD Area:

UPRCT

UPRCT

Drowned

3.Determine the

effective maximum head.

SL 1% AEP

SiteArea 0.030 0.030 downstream water level.

Basic StorageVolume 14.10 14.10

Basic Discharge 2.40 2.40

Areaof Site to Storage 0.030 100% 0.030 100%

Percentageof Site 100.00 100.00

Storage per haof contributingare 470.00 470.00 2.Perform drowned

Volume/PSD Adjustment 80.15 80.15 calculation for

PSDfor site 2.40 2.40 submerged outlet with orifice

MaximumHead to Orifice Centre 0.670 0.400 determined in1.

CalculatedOrificeDiameter 0.037 0.037

Maximumdischarge 2.402 1.857 4. Determine

Head for highearlydischarge 0.570 0.300 effective head for

HighEarlyDischarge 2.216 92% 1.609 high early discharge. ie SL

MeanDischarge 2.309 1.733 HED-SL 1% AEP

Average Dischargeper Hectare 76.970 57.765 downstream water level.

Final Site Storage Ratio 487

600

SiteStorage Volume 14.60 18.01

Volume Provided 20.00 137% 20.000 111%

12.0 APPENDIX A-1

Example of a Submerged Orifice Outlet Calculation

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The example of a typical submerged orifice outlet diagram shall be read in conjunction with the above OSD Design Summary Calculation Sheet

12.0 APPENDIX A-2

Diagram of a Typical Submerged Orifice Outlet

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13.0 APPENDIX B-1

Typical Section Detail - Configuration of a Discharge Control Unit

and Return Pit

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13.0 APPENDIX B -2

Typical Section Detail - Configuration of a Discharge Control Unit

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13.0 APPENDIX B-3

Typical Section Detail – Belowground Tank and Discharge Control Unit

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13.0 APPENDIX B-4

Typical Section Detail – Belowground Pump Storage Tank

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STANDARD PUMP OUT DESIGN NOTES

The pump out system shall be designed to be operated in the following manner: -

i). The pumps shall be programmed to work alternately to allow both pumps to

have an equal operation load and pump life.

ii). A float shall be provided to ensure that the minimum required water level is

maintained within the sump area of the below ground tank. In this regard this float will function as an off switch for the pumps at the minimum water level. The same float shall be set to turn one of the pumps on upon the water level in the tank rising to approximately 300mm above the minimum water level. The pump shall operate until the tank is drained to the minimum water level.

iii). A second float shall be provided at a high level, which is approximately the roof level of the below ground tank. This float shall start the other pump that is not operating and activate the alarm.

iv). An alarm system shall be provided with a flashing strobe light and a pump failure warning sign

which are to be located at the driveway entrance to the basement level. The alarm system shall be provided with a battery back-up in case of power failure.

v). A confined space danger sign shall be provided at all access points to the pump out storage

tank in accordance with the Upper Parramatta River Catchment Trust OSD handbook.

14.0 APPENDIX C

on-site stormwater detention policy

Documents