Section 5: Stormwater. Tauranga City Council

Transcription

1 Section 5: Tauranga City Council

2 Contents Scope 5.A General 5.A.1 5.A.2 5.A.3 5.A.4 5.A.5 5.B. Design Standard 5.B.1. 5.B.2 Overall System Design 5..B.3 Secondary Flows 5.B.4. Freeboard 5.B.5 5.B.6 5.B.7 5.B.8 5.B.9 5.B.10 High Levels of Groundwater 5.C Primary System 5.C.1 Connection Destination 5.C.2 5.C.3 Manhole Spacing 5.C.4 5.C.5 Subdivision 4 Lots or More 5.C.6 Infill Subdivision 3 Lots and Less (includes existing Lot) 5.C.7 5.C.8 5.D Service Connections 5.D.1 5.D.2 5.D.3 5.D.4 5.D.5 5.D.6 5.E Manholes 5.E.1 5.E.2 5.E.3 5.E.4 5.E.5 2

3 5.F Layout 5.F.1 5.F.2 5.F.3 5.G Pipe Materials 5.G.1 5.G.2 5.G.3 5.H Bedding and Protection 5.H.1 5.H.2 5.H.3 5.H.4 5.H.5 5.H.6 5.I Testing and Acceptance 5.I.1 5.I.2 5.I.3 5.I.4 Pipe End Caps 5.J Close Proximity Rules 5.J.1 5.J.2 Dispensation 5.J.3 Ancillary Buildings May be Exempt 5.J.4 Carports, Decks and Sunrooms 5.K Resource Consents 5.K.1 5.K.2 5.L Open Stream Systems 5.L.1 5.L.2 5.L.3 5.L.4 5.M Retention Ponds & Detention Ponds 5.M.1 5.M.2 5.M.3 Calculation Criteria 3

4 5.N Pump stations 5.O Construction Provisions 4

5 SECTION 5: STORMWATER DRAINAGE Scope The District Plan chapters; 15, 17, 18, 19 and 20 provide the mechanism for which the Council can provide provisions, guidelines and specifications for stormwater infrastructure, management and disposal. 5.A General 5.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 the effects on adjacent land. 5.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 and solutions. Designers are encouraged to use this document where appropriate to minimize affects on the environment. 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. The economic life of any piped system shall not be less than 100 years. 5.A.3 The stormwater drainage system shall be capable of serving the entire natural upper catchment to provide adequate protection against flooding. The potential of the entire natural upper catchment is defined as the full development of the area as zoned under the current District Plan at time of consent. 5.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. Designer shall describe the effect of the proposed development on the upstream, downstream and neighbouring properties The system (5.A.3-4) may not need to be constructed but it must be demonstrated that it can be successfully constructed at the time of building consent and effectively maintained for the life of the building. 5

6 5.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. 5.B Design Standard 5.B.1 Unless otherwise required, the design of the stormwater system shall be in accordance with one of the following techniques: (a) IPENZ Procedure for Hydrological Design of Urban Systems, December 1980; or (b) an approved catchment/stormwater computer modelling system. For (b) above the Council s Engineer can confirm an acceptable modelling system on request 5.B.2 Overall System Design 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) 5.B.3 Secondary Flows Flows generated by events that exceed the capacity of the primary system shall be conveyed by a secondary system. calculations shall demonstrate the performance of the overall system and the primary piped 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. 6

7 5.B.4 Freeboard Freeboard above the design top water flood/stormwater level shall be provided as shown. Floor levels may be imposed by consent notice to achieve minimum freeboard. The minimum building platform level shall be defined by the Consent Holder s Representative prior to the s224 Application. This shall incorporate the component of freeboard from the table below. The figures required below are to be read in conjunction with the minimum free board requirements of the NZ Building Code. For all major City infrastructure the following will take precedence. Type of building Non habitable residential buildings, garage floors. Commercial and Industrial. Dwellings (habitable floors) Major Facilities 50 yr Return Period (2% AEP) 200mm 300 mm 500mm 500mm 5.B.5 Drawing D1 details rainfall intensities to be used for the drainage element being designed. 5.B.6 The designer shall identify areas that will be prone to inundation/flooding. This includes ponding areas and secondary flow paths. 5.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 as per manufacturer s specifications or requirements. Inundation information is used to update GIS records identifying land within the District that is prone to inundation or potentially prone to inundation. Diameters quoted refer to nominal internal diameters in millimetres 7

8 5.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 the Council and/or Consent Notice which prohibits ground reshaping and the erection of any barriers. 5.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. 5.B.10 High Levels of Groundwater In areas of high or potentially high groundwater, designers shall provide a method for allowing excess groundwater into the stormwater system in a controlled manner. Such a solution may incorporate perforated inlet pipes together with drainage rock and geotextile. Temporary means of the same order as the critical storm s time of concentration Removal of silt from flat lines costs a significant amount of money. 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 10 % AEP event. "TCC has experienced early asset failures where groundwater has entered the stormwater system in an uncontrolled manner and transported surrounding material into the system causing substantial voids externally and excessive silt internally. Controlled entry of excess porewater pressure (but not fines) is the preferred method of dealing with this situation." 8

9 5.C Primary System 5.C.1 Connection Destination The following destinations are possible for stormwater collected on a lot. Any destinations must be approved by Council: (a) (b) (c) (d) (e) A piped system: The first choice in most situations, applies to green and brown field, and infill subdivisions. 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. A road sump or bubble-up sump: These options are not encouraged and are not permitted in greenfield situations. A public roadside kerb and channel: Permitted provided the kerb connection can be shown not to cause or increase flooding in the downstream catchment for the street. See drawing D2a. Soakage: Disposal of stormwater by ground soakage or ground water recharge is suitable in some areas of Tauranga and most areas of Papamoa & Mount Maunganui. In these areas ground soakage is generally permitted where soil and groundwater characteristics are favourable and no adverse run off effects are caused. Design, construction and maintenance of soakage systems shall be in accordance with the Guidelines for Use of Soakage in Tauranga City shown in Appendix D of this Code. 9

10 Where ground soakage is permitted then this usually provides for; Soak Holes/Soakage Rings/Soak Pits Soak holes (soakage pits) are usually utilised in Tauranga and soakage rings in Mount Maunganui and Papamoa. Each method must be designed to cater for the on site disposal of stormwater in a 10% AEP 10 minute storm. Each system must be tested using a soakage test to confirm that the rates of soakage for the system will meet the required disposal rates. Where the rates of soakage do not meet the required disposal rate then the disposal system will need to incorporate storage to cater for the design storm above or a reticulated system is to be provided. Ground soakage systems shall be designed with a factor of safety built into them to ensure that the system is able to cater for the design life of the building. They shall be provided with silt trap mechanisms and inspection lids to ensure they are able to be maintained. Secondary overflow must not enter a building during a 2% AEP storm. All specific designed ground soakage systems will be Consent Noticed prior to any building consent, requiring the ground soakage system to be complied with on an ongoing basis. Alternative Soakage Other methods of disposal that can be used in "soakage" areas are soakage fields or storage modules with soakage or slow release to the ground. These must be specifically designed for the design storm above and take into account access, silt and maintenance issues. Low Impact Disposal Alternative and innovative stormwater disposal use, where appropriate, shall be considered by Council, especially where low impact disposal systems are proposed. Such disposal methods could include swales, rain gardens, or wetland treatment of areas instead of the conventional retention ponds or in conjunction with ponds. (f) Pumps pumps are not permitted in Tauranga City. 10

11 5.C.2 The primary piped stormwater reticulation system shall consist of 225mm 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. 5.C.3 Manhole Spacing Manhole spacings shall not exceed 120m for pipelines up to 1000mm diameter, above which spacings may extend to 200m. Each branch line shall join the main line at a manhole junction, except: 225/300mm 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 50m of the main line, using a factory made fitting. 5.C.4 Secondary flowpaths shall be in accordance with s5.b.8 above. 5.C.5 Subdivision 4 lots or more Minimum pipe sizes are 225mm diameter for reticulation and 100mm diameter for laterals. In addition, the following shall apply: (a) (b) (c) Lateral with single connection (out of manhole). 100mm dia. up to 50m. Above 50m normal reticulation standards apply, (i.e. 150mm min. dia. pipeline with terminating manhole). 150mm dia. pipes must be used under all road reserves. Lateral with two or three connections (out of manhole). 150mm dia. lengths up to 50m. Above 50m normal reticulation standards apply. Lateral out of line servicing one allotment. 100mm dia. up to 24m. 150mm dia. pipes must be used under all road reserves. 11

12 5.C.6 Infill Subdivision 3 lots and less (includes existing Lot) Reticulation of infill subdivisions shall comply with the following criteria: within the parent lot is to remain private and be protected by appropriate easements. lines are to be separated at the point of connections to the Council s reticulation system and remain separate for each lot unless approved by Council. Infill subdivisions are to be reticulated on a site by site basis to the approval of Council. All private drainage works to be constructed in accordance with NZ 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 this Code for servicing (other than 5.C.6), then the drains will become public assets. 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. 5.C.7 Where practicable and where connection is to be within 6m of the manhole the connection shall be to the manhole. 5.C.8 Where the main line is within 5m of the property boundary a separate connection shall be provided to each allotment. 5.D Service Connections 5.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. 5.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). 5.D.3 All service connections to the site boundary shall form part of Councils public assets. 12

13 5.D.4 Each Connection shall be marked by a 50 x 50mm timber stake which shall extend from the invert to a minimum of 300mm above finished ground level. A blue PVC marker tape shall be attached to the connection pipe, brought up and tied to the top of the stake. The end cap shall be painted blue to distinguish it from a wastewater connection. 5.D.5 All connections to an existing stormwater network shall be undertaken in accordance with S2.8 of this Code. 5.D.6 The following connections can be connected directly to the stormwater main. These connections are all to be factory manufactured. Any combinations outside the following shall 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. 5.E Manholes 5.E.1 Standard 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 subject to the approval of Council. 5.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. 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. 13

14 5.E.3 The fall through a manhole is either: (a) (b) (c) 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. The soffit of the outlet pipe shall be level with or below the lowest incoming line s soffit. The extension of the grade of the steepest pipe whichever is the greatest. 5.E.4 All stormwater manhole covers to be painted blue (with road marking paint). 5.E.5 Flexible joints shall be provided on concrete pipes within 0.5m of the manhole for pipes of 450mm diameter or less. 5.F Layout 5.F.1 Reticulation lines shall be sited, where possible on the low side of the proposed sites having more than 1.5m crossfall and in areas that will not reduce the building area available, ie within front, side and rear yard areas. 5.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. 5.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. 5.G Pipe Materials 14

15 5.G.1 Council permits stormwater drainage systems to be constructed of the following materials: Polyethylene in accordance with AS4130. Polypropylene in accordance with AS/NZS5065. PVC in accordance with AS/NZS 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. Reinforced concrete in accordance with NZS Other pipes, arches and box structures on a specific design basis only. (i) (ii) SN 16 is the recommended equivalent of SEC in DN 100 and 150 SN 8 is the recommended equivalent of SEC in DN 175 or larger. (iii) Stiffness, NOT wall thickness, is the controlling feature in AS/NZS 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. 5.G.2 All joints shall be: Polyethylene: Butt fusion welding to AS2033 with flanged end gibault joints allowable at endpoints. Polypropylene: All jointing and branching to the manufacturers specifications. Structural design and installation and testing requirements for flexible buried pipelines are given in AS/NZS PVC. Factory made spigot and socket, rubber ring jointed to NZS 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. 5.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 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. 5.H Bedding And Protection 5.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. 15

16 5.H.2 All drainage lines shall be designed and constructed to withstand all 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 Loads on Buried Concrete Pipes and AS/NZS 2566 Plastics Pipe laying Design where appropriate. The manufacturer s specification for bedding shall take precedence. 5.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. 5.H.4 Where pipe servicing in the carriageway is required, the trench re-instatement works shall follow the minimum requirements shown in Section 4 drawing R32. Pipe trenches within a carriageway shall be backfilled using an approved hardfill placed immediately above the pipe surround and compacted in layers not exceeding 200mm in depth. In existing sealed streets, the top section of the trench shall be backfilled as specified by TNZ M/4. The depth of base course and type of finishing coat seal shall conform at least to the standard of the existing road construction, but may require being greater. This drawing specification is the minimum standard for all services under the carriageway. 5.H.5 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 shall be designed on a site by site basis taking into account both pipe protection and roading integrity requirements. 5.H.6 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. 5.I Testing And Acceptance 5.I.1 Prior to acceptance of the completed stormwater drainage system, the subdivider s representative shall supply 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 D19). All testing shall be carried out when backfilling is completed and prior to carriageway surfacing. 16

17 5.I.2 5.I.3 pipes shall not be pressure tested unless specifically required by Council. The Consent Holder s Representative s test will be undertaken in the presence of a Council Engineer. Council shall request the Consent Holder s Representative to undertake a CCTV inspection of pipelines as and when required prior to acceptance. Specifications are given in appendix C. Council may request either a water or air pressure test. 5.I.4 Pipe End Caps See s5.d.4 for requirement. To enable the distinction between stormwater (blue) and wastewater (white) end cap piping when in close proximity. 5.J Close Proximity Rules 5.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. 5.J.2 Dispensation Dispensation may be approved to site the building to within 800mm off the side of the pipe subject to the following conditions: The depth of the pipeline invert 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. 5.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 800mm 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. Council must be satisfied with the location of the building. Connections closer than 1.0m to the buildings shall be relocated. 17

18 5.J.4 Carports, Decks and 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); 5.K Resource Consents 5.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. 5.K.2 Developers will typically wish to transfer resource consents (and any associated conditions) to the Tauranga City Council when assets are vested. Council reserves the right to decline to accept consents if doing so would be detrimental to the Council s interests. Generally however the Council is prepared to accept consents provided that: In parallel with the EBOP application, subdividers are encouraged to submit copies of stormwater discharge consent applications to Tauranga City Council to assist future transfer. 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. 5.L Open Stream Systems 5.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. 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. 5.L.2 The flow characteristics of natural open stream systems shall be based on the likely long term stream conditions. 18

19 5.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 be provided at regular intervals along the system together with footpath and pedestrian bridges, as may be defined in the resource consent. 5.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 a landscape design approved by Council. 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. 5.M Retention & Detention Ponds 5.M.1 detention ponds are an accepted method of improving stormwater quality and reducing downstream flow rates. 5.M.2 Pond design shall follow the guidance of Auckland Regional Council Technical Publication Number 10. Cost of cleaning these ponds is significant hence Council has decided to charge a development contribution bases on 2 cleans in first 5 years 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. 5.M.3 Calculation criteria The calculation criteria for retention and detention ponds shall be in accordance with the above ARC Technical Publication #10. 5.N Pumpstations pump stations for use in the stormwater system are generally not permitted. Their use will only be considered in exceptional circumstances. Approval of Council is required. 19

20 5.O Construction Specification The stormwater network shall be constructed in accordance with the construction specifications provided in Appendix C of this Code. 20