4. STORMWATER DRAINAGE MINIMUM...

SECTION 4Code of Practice for Development

TAURANGA CITY COUNCIL

Stormwater Drainage

TCC Ref: 728407 Print Date: Feb 2006 VERSION 1 R2 Page 4 -1

4. STORMWATER DRAINAGE MINIMUM REQUIREMENTS 4.A General 4.A.1 Most developments will modify the pre-development drainage pattern and a stormwater drainage system is typically designed and constructed to adequately manage this situation. The primary goals of a stormwater system are to minimize nuisance flooding and flows and to minimize affects on the pre-development. 4.A.2 Affects on the pre-development environment are becoming more important as time goes on. Various methods are available to limit such affects. Auckland Regional Council publication TP124 “Low Impact Design Manual” is a useful document that gives consideration to the relevant issues and offers various techniques an solutions. Designers are encouraged to use this document where appropriate to minimize affects on the environment. 4.A.3 The stormwater drainage system shall be capable of serving the entire natural catchment to provide adequate protection against flooding. The potential of the entire natural catchment is defined as the full development of the area as zoned under the current District Plan at time of consent. 4.A.4 The consent holder shall design and construct a stormwater drainage system that will provide an adequate gravity method of disposing of the flow from each allotment or intended building site, including all hardstand areas.

Note that in the case of 4.C.1(d) and (e) the system need not be constructed but it must be demonstrated that it can be successfully constructed. 4.A.5 Where a stormwater drainage system has been or is to be installed by the Council to serve the natural catchment, contributions on an area basis shall be made in terms of the development impact fees or other agreed cost sharing arrangement.

C.1 TP124 is a document for the Auckland Region. While this limits its strict applicability in the Bay of Plenty the majority of the document is relevant to the issues in this Region. C.2 The economic life of any piped system shall not be less than 50 years. C3 Designer shall describe the affect of the proposed development on the upstream, downstream and neighbouring properties.



Stormwater Drainage


MEANS OF COMPLIANCE 4.B Design Standard 4.B.1 Unless otherwise required, the design of the stormwater system shall be in accordance with the IPENZ ‘Procedure for Hydrological Design of Urban Stormwater Systems’, December 1980.

4.B.2 The design storm, Annual Exceedance Probability (AEP) return period shall be as follows: • Overall system (primary and secondary) in all urban areas 50 year storm (2% AEP). • Primary piped system in all urban areas minimum 10 year storm (10% AEP). • Bridge structures 100 year storm (1% AEP). 4.B.3 Secondary Flows Flows generated by events that exceed the capacity of the primary system shall be conveyed by a secondary system. The secondary system will typically consist of ground shaping to manage surface flows along flowpaths. Such secondary flowpaths are typically protected by easement. Piped secondary systems are discouraged due to the risk of blockage. Piped secondary systems may be permitted if a totally separate and adequate entry system is provided. 4.B.4 Freeboard Freeboard above design stormwater levels shall be provided as shown.

Type of building 50 yr Return Period

Non habitable residential buildings, garage floors.

200mm

Commercial and Industrial. 300 mm Dwellings (habitable floors) 500mm Major Facilities 500mm

4.B.5 Drawing D1 details rainfall intensities to be used for the drainage element being designed.

C4 Stormwater calculations shall demonstrate the performance of the overall system and the primary piped system. C4a Temporal rainfall distributions for applications such as computer modelling are available upon application from the Stormwater Asset Manager C5 Floor levels may be imposed by Consent Notice to achieve minimum freeboards C6 Inundation information is used to update GIS records identifying land within the City that is prone to inundation or potentially prone to inundation.



Stormwater Drainage


4.B.6 Designer must identify areas that will be prone to inundation. This includes ponding areas and secondary flow paths. 4.B.7 The piped system shall be designed to cater for the design flow, without surcharge, based on a “Colebrook - White” pipe roughness co-efficient Ks=1.50 for pipes up to and including 450 mm diameter and Ks=0.60 for larger concrete pipes. K values for other materials are to be approved by the Director of City Services. 4.B.8 The secondary stormwater system shall safely convey stormwater without undue nuisance. Acceptable solutions include:

• Temporary ponding on local and collector roads • Temporary flow along local and collector roads. • Flow across private property. Flow must be in a

defined channel or swale, clear of existing or future building sites and protected by an easement in favour of Council and/or Consent Notice which prohibits ground reshaping and the erection of any barriers.

4.B.9 Where the design full pipe velocity is less than 1.3m/s, then measures shall be taken to allow for silt collection. This shall typically take the form of unbenched manhole sumps below lowest invert level. The extra depth required is half the line diameter or as otherwise approved.

4.C Primary System 4.C.1 Stormwater Connection Destination The following destinations are possible for stormwater collected on a lot. Any destinations must be approved by Council: (a) A piped system: The first choice in most situations. (b) An open drain: Generally permitted, but only where there is an existing open public drain within or beside the lot and a piped system is not available, or desirable, subject to Regional Council general authorisation and in accordance with all other requirements.

C7 Diameters quoted refer to nominal internal diameters in millimetres C8 “Temporary” means of the same order as the critical storm’s time of concentration C9 Removal of silt from flat lines costs a significant amount. Removal from sumps reduces costs dramatically. It is acknowledged that this velocity is greater than that in NZS4404 but this is because full pipe velocities are only achieved in a 20% AEP event.



Stormwater Drainage

TCC Ref: 728407 Print Date: Feb 2006 VERSION 1 R2 -4

(c) A road sump: Not encouraged. Prior approval from the Director of City Services required. (d) A public roadside channel: Not encouraged except where the lot is remote from a piped drainage system and Council does not consider that a piped system is likely or desirable [refer Drawing D2 (a)] (e) Soakage: Generally not permitted in any area except Papamoa & Mt Maunganui. In these areas generally permitted if soil and groundwater characteristics are favourable and no adverse effects are caused. Design, construction and maintenance to be in accordance with “Guidelines for use of soakage in Tauranga District. 4.C.2 The primary piped stormwater reticulation system shall consist of 200mm diameter and larger pipelines laid to a true grade and line between access manholes located at each change of direction and grade. Manholes are also required at the upper end of all primary, piped stormwater reticulation. Manhole spacings shall not exceed 100m for pipe lines up to 1000mm diameter, above which spacings may extend to 200m. Each branch line shall join the main line at a manhole junction, except: - 225mm diameter sump leads may be saddled into 600mm

diameter pipes or larger, using a factory made fitting. - Branch lines 300mm diameter or smaller may be saddled

onto pipes 1200mm diameter or larger provided a manhole is supplied on the branch line within 50 m of the main line, using a factory made fitting.

4.C.3 Secondary flowpaths shall generally be along the road network. Where such flowpaths are required to cross private property an appropriate easement in favour of Council shall be provided to ensure its protection.



Stormwater Drainage


4.C.4 Subdivision 4 lots or more – Minimum pipe sizes are 150mm diameter for reticulation and 100mm diameter for laterals. In addition, the following shall apply: (a) Lateral with single connection (out of manhole).

• 100mm diameter up to 50m. • Above 50m normal reticulation standards apply, (ie

150mm minimum diameter pipeline with terminating manhole).

• 150mm diameter pipes must be used under all streets.

(b) Lateral with two or three connections (out of manhole).

• 150mm diameter lengths up to 50m. • Above 50m normal reticulation standards apply.

(c) Lateral out of line servicing one allotment.

• 100mm diameter up to 24m. • 150mm diameter pipes must be used under all

streets. 4.C.5 Infill Subdivision 3 lots and less (includes existing Lot) Reticulation of infill subdivisions shall comply with the following criteria:

• Drainage within the parent lot is to remain private and be protected by an appropriate easement.

• Drainage lines are to be separated at the point of connections to the Council’s reticulation system and remain separate for each lot unless specific approval is gained from the Group Manager of Environmental Services.

• Infill subdivisions are to be reticulated on a site by site basis to the approval of the Group Manager of Environmental Services.

• All private drainage works to be constructed in accordance with Building Code regulations.

• All private drainage works to be constructed by a Registered Drainlayer.

• Should the consent holder upgrade and construct the drainage reticulation in accordance with the Code of Practise requirements for servicing (other than 4.C.5) then the drains will be declared public.

C10 Clause 4.C.4 relates to the provision of connection for house lots to be public system. It is not intended to be used for general infill reticulation. However, there will be occasions when infill subdivision may need to adhere to those requirements. C11 It is recognised that infill subdivisions require flexibility in their reticulation layout. However, normal trade practises and standards still apply to ensure that the desired service life is still achieved. Common private drains are not encouraged so as to alleviate the friction that sometimes occurs between neighbours when shared services are involved.



Stormwater Drainage


4.C.6 Where practicable and where connection is to be within 6m of the manhole the connection shall be to the manhole.

4.C.7 Where the main line is within 5m of the property boundary a separate connection shall be provided to each allotment. 4.D Service Connections 4.D.1 To provide a service connection to each allotment, a minimum 100mm diameter line shall be extended from the main reticulation system to terminate at a minimum distance of 1.0m within each site, whether created by cross lease, unit title or fee simple subdivision. In all cases service connections shall serve as much of the site as practically possible, but at least the possible building site. 4.D.2 Service connections shall be brought to within 1.2m of the final ground surface and be constructed in accordance with Drawing D2 or D3 (for pipe depths greater than 2.5m). 4.D.3 All service connections to the site boundary shall form part of the public system. 4.D.4 Each Connection shall be marked by a 50mm x 50mm ground treated timber stake with the top painted blue extending from below invert level to 300mm above ground level. The location shall be shown on the ‘As built’ plan. The ‘As built’ plans shall also show the distance of the “Y” junction from the nearest downstream manhole. 4.D.5 All connections to existing main stormwater shall be made by an approved contractor with the consent of the Group Manager of City Services at the consent holder’s expense. 4.D.6 The following size connections can be connected directly to the line. Any combinations outside this list connect via a manhole.

• 100mm diameter connection can connect to 100mm diameter line or greater.

• Above diameter and 150mm diameter connection can connect to 150mm diameter line or greater.

• Above diameter and 225mm diameter connection can connect to 600mm diameter line or greater.

• 300mm diameter connection can connect to 1200mm line or greater.



Stormwater Drainage


4.D.7 To differentiate between sewer and stormwater, the stormwater end cap is to be wrapped or painted with blue tape/blue paint. This is to be in conjunction with an approved marker peg. 4.E Manholes 4.E.1 Standard Stormwater manholes shall be constructed as detailed on Drawings D4 to D11. For Manholes of greater depth than 5m refer to D12. Shallow manholes (Drawing D5) may be used in certain circumstances with the approval of the Group Manager City Services. 4.E.2 Standard 1050mm manhole risers are not suitable where either pipes are larger that 675mm diameter or where multiple pipes enter a manhole causing loss of manhole wall. Designers shall upsize manholes in these cases to maintain wall integrity. 4.E.3 The fall through a manhole is either: (a) The invert of the outlet pipe from a manhole shall be

20mm plus 0.5mm per degree of horizontal angle change between the two lines lower than the lowest incoming line invert.

(b) The soffit of the outlet pipe shall be level with or below

the lowest incoming line’s soffit. (c) The extension of the grade of the steepest pipe

whichever is the greatest. 4.E.4 All stormwater manhole covers to be painted blue (with road marking paint). 4.F Layout 4.F.1 Reticulation lines shall be sited, where possible on the low side of the proposed sites having more than 1.5m cross fall and in areas that will not reduce the building area available, ie within front, side and rear yard areas. 4.F.2 The minimum desirable cover to all pipes from finished ground level shall generally be 600mm in private land and 1.2m for road crossings, (other than sump leads) provided there is generally a minimum depth to invert, at the service

connection point, of 1.2m from the lowest point in the building site. C12 For large diameter manhole risers the lid may be lowered to 2.0m or more above invert with a 1050mm “throat” above, provided that the lid is of suitable strength.



Stormwater Drainage


4.F.3 Pipelines within private property shall generally run parallel to lot boundaries. Where the centreline of a pipeline is further than 1.5m from a lot boundary or not parallel to a lot boundary an easement in favour of Council and/or Consent notices shall be provided to ensure its protection. 4.G Pipe Materials 4.G.1 Stormwater drainage systems constructed of the following materials will be accepted:

• Polyethylene in accordance with AS4130. • PVC in accordance with AS/NZS 1260:1996. DN

100mm and 150mm to have minimum wall thickness of 4.1mm and 6.3mm respectively, defined as grade SN 16 for up to 5m depth. DN175 mm and larger to be grade SN 8 minimum or equivalent. Pipe type to be either plain wall or sandwich construction (refer Tauranga Distict Council approved products).

• Reinforced concrete in accordance with NZS 3107. • Other pipes, arches and box structures on a specific

design basis only.

4.G.2 All joints shall be: • Polyethylene. Butt fusion welding to AS2033 with

flanged end gibault joints allowable at endpoints. • PVC. Factory made spigot and socket, rubber ring

jointed to NZS 3107. Flush joints are permitted on 1050mm diameter and larger pipes, provided no ground water or geotechnical requirements dictate the need for fully sealed joints. Joints are to be RRJ or steel banded, to manufacturers specifications.

4.G.3 All fittings and service connections to pipes shall be factory fabricated of compatible material to the proposed system chosen and comply with the relevant standards. PVC fittings and service connections to be in accordance with AS/NZS 1260:1996. DN 100mm and 150mm injection moulded fitting to be the closest grade currently available to the specified grade. DN 175mm and larger factory fabricated fitting to be made of same grade materials as specified for the pipeline. 4.H Bedding And Protection

4.H.1 All drainage lines shall be thoroughly bedded haunched and surrounded as in the Drawing D13 unless installed by no-dig technology as per Drawing D4. C13 (i) SN 16 is the recommended equivalent of SEH C in DN 100 and 150 (ii) SN 8 is the recommended equivalent of SEH in DN 175 or larger. (iii) “Stiffness”, NOT wall section ,is the controlling feature in AS/NZS 1260:1999. The actual ID (and therefore flow rate) of plain wall pipe is likely to be greater than the same size / SN of sandwich construction pipe.



Stormwater Drainage


4.H.2 All drainage lines shall be designed and constructed to withstand al the likely loads they will be subject to during the life of the system. The load carrying capacity in relation to their installation conditions shall be calculated in accordance with AS/NZS 3725:1989 Loads on Buried Concrete Pipes and AS2566:1982 Plastics Pipe laying Design where appropriate. 4.H.3 For drainage pipelines laid at grades steeper than 20% in grade, anti-scour blocks located at pipe joints, not exceeding 6m spacing, are required, as shown on Drawing D15 or as otherwise approved. 4.H.4 Where the minimum cover as outlined in Section 4.F.2 cannot be achieved, specific design of pipe and cover is required to achieve manufacturers requirements. Concrete capping will be designed on a site by site basis taking into account not just pipe protection but roading integrity requirements. 4.H.5 Where public drains are located within rights of way or private access legs all other utility services such as power, telephone and gas shall be located 300mm horizontally clear of the pipe. 4.I Testing And Acceptance 4.I.1 Prior to acceptance of the completed stormwater drainage system, the subdivider’s representative shall have supplied the Council with ‘As Built’ drawings of the work, including any secondary flowpaths, open channel systems and areas prone to inundation, together with certification and a certificate of subdivision construction (Drawing D16). All testing shall be carried out when backfilling is completed and prior to carriageway surfacing. 4.J Close Proximity Rules 4.J.1 No building shall be built over a stormwater pipeline or closer than the greater of:

• 1.5m from the outside edge of the pipeline. • The depth to the centre line of the pipeline from the

existing/proposed ground surface, plus the diameter of the pipeline.

C14 The manufacturer’s specification for bedding shall take precedence.



Stormwater Drainage


4.J.2 Dispensation Dispensation may be approved to site the building down to 800mm off the pipeline centreline subject to the following conditions:

• The depth of the pipeline is less than 3.0m deep from the existing/proposed ground surface;

• Specifically designed footing/piles must be installed to 200mm below the pipeline invert;

• That all connections be a minimum of 1.0m outside the building;

• That all buildings be at least 0.5m off the outside wall of manhole structures.

• 4.J.3 Ancillary buildings may be exempt: Garages (single and double, maximum 7.2m), provided they are detached and do not form an integral part of the house shall be allowed to be built over pipelines, subject to the following conditions:

• Corner foundations are a minimum of 800m off the pipeline centre line.

• Additional foundation requirements of piles are to be installed 200mm below the pipeline invert, a minimum of 800mm either side of the pipeline where it enters and exits the building.

• The Director of City Services must be satisfied with the location of the building.

• No buildings will be allowed over the pipeline if there is a connection closer than 1.0m to the buildings, unless the connection is relocated to the satisfaction of the Director of City Services.

4.J.4 Carports/Decks/Sunrooms These structures may be sited over pipelines provided:

• There is no connection point closer than 1.0m to the building;

• The structure is designed such that it can be dismantled easily in sections (bolting is the preferred method of construction);

4.K Resource Consents



Stormwater Drainage


4.K.1 Where resource consents from the Regional Council are required for the diversion and or discharge of stormwater, the consent holder is responsible for making the necessary application and obtaining such resource consents before physical work commences on site. 4.K.2 Developers will typically wish to transfer resource consents (and any associated conditions) to the Tauranga Distict Council when assets are vested. Reserves the right to not accept consents if doing so would be detrimental to the Council’s interests. Generally however the Council is prepared to accept consents provided that:

• The conditions are considered to be fair and reasonable. • The required works have been completed. • Full documentation is supplied including the original

application, drawings and compliance certification from the Regional Council.

4.L Open Stream Systems 4.L.1 Where natural open stream systems or formed channels are to be incorporated in the stormwater drainage system, they shall generally be located within a drainage reserve of sufficient width to contain the full design flood flow together with a freeboard. Such reserves shall generally have a batter of 20% (1 in 5) and where access for maintenance is required shall include a 4m wide access berm for the full length. 4.L.2 The flow characteristics of natural open stream systems shall be based on the likely long term stream conditions. 4.L.3 To encourage the best use of the open stream systems the drainage reserves shall be linked with other reserves and open spaces such as school sites, to accommodate off road pedestrian and cycle access. Access points for public use and maintenance shall provided at regular intervals along the system together with footpath and pedestrian bridges, as may be defined in the resource consent.

C15 In parallel with the EBOP application, subdividers are encouraged to submit copies of stormwater discharge consent applications to Tauranga Distict Council to assist future transfer. C16 Open streams, drains and channels through subdivisions can create maintenance and safety misunderstandings and difficulties. Designers need to carefully consider and describe proposed ownership/maintenance arrangements including easements and consent notices etc.



Stormwater Drainage


4.L.4 Where natural open stream areas form part of the stormwater drainage system they shall be cleared of all unsuitable plant growth and replanted to an appropriately approved landscape design. These works shall include protection of the low flow channel against scour and erosion of the stream bed where necessary. Access shall be provided for maintenance purposes. 4.M Detention Ponds 4.M.1 Stormwater detention ponds are an accepted method of improving stormwater quality and reducing downstream flow rates. 4.M.2 Pond design shall follow the guidance of Auckland Regional Council Technical Publication Number 10. Specific points to focus on are:

• Side slopes with safety considerations. • Ease of maintenance including mowing and silt

cleanout. • Shape and contour for amenity value. • An effective outlet structure. • Overflow.

4.N Pumpstations Stormwater Pumpstations are generally not permitted. Any use of pumpstations will be considered only in exceptional circumstances and then only by specific Council approval.

C 17 Cost of cleaning these ponds is significant hence Council has decided to charge a development contribution bases on 2 cleans in first 5 years

SECTION 4 Code of Practice for Development

TAURANGA DISTRICT COUNCIL

Stormwater Drainage

0129257.01 Print Date: May 2002 VERSION 1 R1 Page 4 -13

4.L.4 Where natural open stream areas form part of the stormwater drainage system they shall be cleared of all unsuitable plant growth and replanted to an appropriately approved landscape design. These works shall include protection of the low flow channel against scour and erosion of the stream bed where necessary. Access shall be provided for maintenance purposes. 4.M Detention Ponds 4.M.1 Stormwater detention ponds are an accepted method of improving stormwater quality and reducing downstream flow rates. 4.M.2 Pond design shall follow the guidance of Auckland Regional Council Technical Publication Number 10. Specific points to focus on are:

• Side slopes with safety considerations. • Ease of maintenance including mowing and silt cleanout. • Shape and contour for amenity value. • An effective outlet structure. • Overflow.

4.N Pumpstations Stormwater Pumpstations are generally not permitted. Any use of pumpstations will be considered only in exceptional circumstances and then only by specific Council approval.

C 17 Cost of cleaning these ponds is significant hence Council has decided to charge a development contribution bases on 2 cleans in first 5 years

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