JAMES STREET PLIMMERTON: ENGINEERING SERVICING REPORT · The site is currently vacant and was...

NBC PROJECTS LIMITED PLIMMERTON PORIRUA

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JAMES STREET PLIMMERTON:

ENGINEERING SERVICING REPORT

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> CONTENTS

1 Introduction 3

2 Site location and key site features 4

A. Existing services 4

B. Existing open drain and known flood risk 5

3 Earthworks to support development 6

A. Proposed site shaping and earthworks 6

B. Interface with James Street 6

C. Erosion and Sediment Control 6

D. Earthworks, landform, and access recommendations 7

4 Proposed stormwater concept 8

A. Existing services 8

B. Proposed servicing solution 8

C. Stormwater recommendations 8

5 Proposed sewer concept 9

A. Regional Water Standard 9

B. Sewer recommendation 9

6 Proposed water supply concept 10

A. Proposed water main and water connections 10

B. Firefighting water supply 10

C. Residential demand and residual pressure 10

D. Water Supply recommendations 10

7 Utility services 11

8 An assessment of effects 12

A. Section 106 assessment 12

B. Servicing effects 12

C. Construction effects 12

> Appendix A – Engineering concept design plans 13

> Appendix B – Site investigation scala penetrometer tests 14

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Version control:

Version Date of Issue Purpose Authorised by:

1 14 August 2017 DRAFT C. Rodgers

2 18 August 2017 R. Consent Application D. Brittliff

3 16 August 2018 R. Consent Application D. Brittliff

4 5 February 2019 R. Consent Application D. Brittliff

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1 INTRODUCTION

This report describes the engineering aspects of the proposed subdivision in James Street, Plimmerton. The report outlines the existing site and its parameters that influence land development design on the land before providing recommendations on how the land can be developed and appropriately serviced.

Detailed engineering plans are provided as appendices to this report to provide sufficient justification that the concepts are substantiated and the recommendations made in the report are appropriate.

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2 SITE LOCATION AND KEY SITE FEATURES

The proposed development is located at James St, Plimmerton, Porirua (Lot 1 DP 489799) as shown in Figure 1.

Figure 1 – Application site

It is proposed that the land will be subdivided into 12 new lots as shown on the scheme plan W16039-CN101 Issue 7 in Appendix A.

A. Existing services

The site is currently vacant and was created by a recent subdivision where it was separated from James Street and the rural land to the north. The site previously contained a show home in the northern corner of the site, and there is an existing water connection for the former building however no other services exist for the land.

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B. Existing open drain and known flood risk

The site is located within an area known to be of flood risk due to the large rural upstream catchments and downstream culvert restrictions. There is an open drain that runs along the southern boundary of the site.

Awa Environmental Limited are engaged as specialist stormwater modelers to evaluate this catchment and application site to provide recommendations on flooding matters. The result of their assessment is to recommend restrictions on the site in the form of an easement corridor and setting minimum building floor levels compliant with the requirements of the Building Code.

Therefore, matters of flooding are considered in the design proposed for the new lots as a result of the recommendations by Awa Environmental Limited to mitigation future flood risk.

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3 EARTHWORKS TO SUPPORT DEVELOPMENT

The existing site is proposed to be filled to elevate the site to create building platforms that are above any future flood risk. In addition, the earthworks are required to lower a portion of the land (Lot 12) to provide for compensatory flood water storage that result from the areas that are lifted by fill.

A. Proposed site shaping and earthworks

The proposed earthworks and associated volumes are shown on Plan CN301. The proposal shows one area of fill and one of cut. The fill proposed will lift the building platform levels of Lots 2 – 11 to above the levels of the footpath and provide compliant freeboard floor levels to future flood events for the proposed townhouses as recommended by Awa Environmental Limited in their report.

Scala penetrometer tests were completed across the site that are included at Appendix B to determine the existing bearing of the land. These tests determined that generally the upper 1.2m of material on site provides a low bearing capacity and therefore it’s in situ properties are unsuitable for filling upon. Therefore, the proposed earthworks allow for the fill area to be undercut by 1.2m and replaced with good quality fill material to provide a compliant earthworks surface to support residential development. As a result of this work the anticipated approximate volumes are; cut = 7350m³ and fill = 7650m³. Once lifted to the design height this area will be stabilised with topsoil and metal to facilitate future dwelling construction.

The proposed cut on Lot 12 provides compensatory flood storage to allow the site to retain is flood storage volume on the land following the development.

The process of earthworks on the site will require topsoil stripping, cleaning out suitable underlying ground in preparation for cutting, fill by placing in layers of quality fill with appropriate compaction to the finished subgrade surface levels. The finished levels will be built up and formed with topsoil across the general site and metal where under proposed building areas and pavements.

B. Interface with James Street

Plan CN441 Issue 2 shows the interface with James Street, Lots 2 – 11, minimum floor levels, and driveways that result from the proposed earthworks. The proposed fill therefore complements the current landform and enables positive housing development with effective driveway access levels to suit the minimum floor levels required on this land.

Between Lots 1 and 2 the access to Lot 12 provides an overland flow route to the stormwater attenuation area. This route is identified and reported in the Awa Environmental report to provide continuity to flow paths in extreme events. Normal access to Lot 12 is appropriately achieved along this route between the levels proposed in the earthworks concept design. A secondary, existing, overland flow route is located adjacent to the sump and existing drain at the far eastern end of the site. This overland flow route will be retained

C. Erosion and Sediment Control

Erosion and sediment control will be required to support the proposed construction. We have prepared a concept plan for site management in accordance with the GWRC guidelines for Erosion and sediment Control that consists of:

• A decanting earth bund located in the low area of the earthworks surface for collection and treatment of any dirty water runoff.

• Silt fencing and bunding to prevent dirty water directly entering the existing clear water channels.

• Runoff channels for dirty water collection.

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• A stabilised construction entrance.

An Erosion and Sediment Control plan and methodology will be required as part of the Contract works and this will be incorporated in a Construction Management Plan for the site.

Earthworks are proposed in four stages to be completed and stabilised before commencing the next stage. Our concept is shown on plans CN303 – CN306 in Appendix A that we recommend providing an appropriate Construction Management Plan for the site.

D. Earthworks, landform, and access recommendations

On the basis of our assessment of the site and the proposed earthworks required to support the development, we provide the following recommendations:

• The proposed earthworks shown on the application drawings be implemented including the

proposed cut and fill to provide an appropriate site to mitigate known flooding risk on the land.

• The proposed Erosion and Sediment Control Plan be adopted as the site management tool for

Earthworks.

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4 PROPOSED STORMWATER CONCEPT

As described earlier the wider catchment characteristics are detailed in the Awa Environmental report however we provide direction on reticulation and servicing of each new lot in the development below.

A. Existing services

There is an open drain that runs along the southern boundary of the site flowing from east to west. In accordance with the Porirua City Council Code of Land Development and Subdivision Engineering this will be provided with a drainage easement in favour of Porirua City Council. The easement is proposed to be 8m wide, consisting of a 5.0m wide strip to cover the bed and banks of the drain plus a 3.0m wide access strip on the northern side of the drain. An existing 450mm diameter culvert collects from the nearby catchment and directs flow into the open channel through the site.

On the western side of James Street road sumps drain the existing open channel within the State Highway road reserve. The middle sump drains into the berm of James Street, with the water from the small catchment (400m²) ending up on the application site. The southern sump drains into the existing open channel also on site.

B. Proposed servicing solution

Plan CN501 Issue 2 illustrates the proposed servicing solution for the development. All elements of the proposed network will be formed in accordance with the Council Code of Practice specifications. Specific elements of the solution are:

• A kerb outlet will be provided to Lot 1 and installed as part of the subdivision.

• A new piped system will be provided to reticulate Lots 2 – 11 in accordance with the Regional

Water Standard and installed as part of the subdivision. This then discharges to the southern open

drain via existing infrastructure in James Street.

• The attenuation area runoff will drain via a culvert into the southern drain at the western end of the

development. This culvert will incorporate a non-return device to prevent water from the southern

drain entering into the development site under normal conditions.

• Lot 12 dwelling would discharge via a private 100mm PVC drainage system to the Western drain.

This system would be installed as part of the Building Consent drainage for the dwelling on Lot 12.

The increase in runoff from the development has been included in the Awa Environmental Limited assessment for the catchment and concluded that it has a less than minor effect during the design events given that it is attenuated in the proposed attenuation area on Lot 12.

C. Stormwater recommendations

We provide the following recommendations for stormwater servicing to support the proposal:

• The proposed reticulation be implemented to service the proposed new lots 1 – 11 as part of the

subdivision.

• Lot 12’s drainage be installed as part of the Building Consent process for any new dwelling on Lot

12. A Consent Notice can be issued for Lot 12 confirming this requirement as part of the

subdivision.

• The proposed floor levels determined by Awa Environmental Limited be implemented for all new

lots.

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5 PROPOSED SEWER CONCEPT

The proposed solution to provide a sewer service to the new lots in the development is to connect to the existing 150mm sewer main in James Street. The depth of this sewer is generally 1.3m below James Street and due to the proposed filling of the application site, providing a Building Code compliant sewer connection of this main is achievable.

The sewer concept solution for the development is shown on plan CN501 in Appendix A. The concept promotes:

• A 100mm PVC connection as required by the Regional Water Standard to lots 1 - 12;

• For Lot 12 a private sanitary pump may be required (dependent on final dwelling detailed design)

to discharge to the proposed connection in James Street. Lot 12’s drainage is recommended to

be installed as part of the Building Consent process for any new dwelling on Lot 12

• Efficient use of infrastructure by utilising the current Council main in James Street.

A. Regional Water Standard

The anticipated flow from the development considering the Regional Water Standard’s design guideline is shown in Table 1 below.

Table 1: RWS anticipated design flows

Lots Population per lot (T5.1)

ADWF (L/p/day)

ADWF (L/s)

PDWF (L/p/day)

PDWF (L/s)

PWWF (L/p/day)

PWWF (L/s)

12 3.5 270 0.13 540 0.26 1080 0.5

The existing 150mm sewer connects to a 375mm main in State Highway 1. The minimal additional flow from the development will have a negligible effect on these mains as the flows event at Peak Wet Weather are simply very minute.

B. Sewer recommendation

We provide the following recommendation for sewer to support the proposal:

• A simple Council standard 100mm PVC lateral connection be provided off the current James Street

sewer main to each new lot.

• Lot 12’s drainage be installed as part of the Building Consent process for any new dwelling on Lot

12. A Consent Notice can be issued for Lot 12 confirming this requirement as part of the

subdivision.

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6 PROPOSED WATER SUPPLY CONCEPT

The water supply concept design for the development is shown on plans CN601 in Appendix A.

A nominal bore 100mm HDPE water main exists within the southern reach of James Street. The existing main ends with a fire hydrant on the eastern James Street corner. This existing main provides a basis for considering the design of a water service solution for the development that we describe below.

A. Proposed water main and water connections

Each new lot will be provided with a service pipe and valve in accordance with the Council’s design guidelines.

A new water main is proposed to provide fire-fighting and residential water supply to the development. The proposed water main would connect to the existing fire hydrant at the corner of James Street and be laid north in the berm between the road and footpath to a new proposed fire hydrant near Lot 10.

A 63od PE PN12.5 rider main is proposed to run in the inside berm to service the 12 townhouses with individual 25 OD connections.

B. Firefighting water supply

The proposed development can be classified as Fire water classification FW2 for the purpose of our evaluation as per SNZ PAS4509:2003 New Zealand Fire Service Fire Fighting Water Supplies Code of Practice for a non-sprinklered building. Classification FW2 requires supply of a minimum of 12.5 Litres/second from within 135 metres of the building and a second supply of 12.5 Litres/second from with 270 metres of the building.

Based on our proposed configuration, the existing fire hydrant is within 225m of all proposed buildings and the proposed new fire hydrant is within 115m of all proposed townhouses.

Therefore, the existing and proposed hydrant spacing provide sufficient fire protection for the proposed application in accordance with the New Zealand Standard.

C. Residential demand and residual pressure

In accordance with the Regional Water Standard the 12 new lots would generate a residential demand of 0.19L/s when considering the Council’s demand of 450 Litres/person/day. This flow can be supplied via the proposed 100mm and 50mm reticulation proposed for the development.

During firefighting the FW2 design flow can be supplied via the 100mm reticulation with an anticipated network pressure drop of less than 5m of head.

D. Water Supply recommendations

Based upon our assessment of the site, the existing water main can provide a compliant water supply to the Regional Water Standard and New Zealand Standard for Fire Fighting Water Supplies as shown on Plan CN-601. Each new townhouse will be provided with a separate connection that can be installed in accordance with the Building Code.

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7 UTILITY SERVICES

Utility services to support the development exist within James Street. These utilities would be extended to each lot in the development to provide the necessary power, telecommunications, and street lighting services.

In terms of utility services, no extraordinary requirement or effect on these networks is expected with the proposed scheme in this application.

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8 AN ASSESSMENT OF EFFECTS

A. Section 106 assessment

The concept design for the site provides a sound and modern integrated engineering solution for development of the land that proposes filling to remove the risk of inundation and stipulating Building Act minimum floor levels as recommended by the flood modelling consultants on the land.

In terms of Section 106 of the Act we confirm that all sites are provided with legal and physical access and there is no risk of inundation from stormwater or erosion due to stormwater runoff in the development or the catchment if the recommendations of the flood modelling consultants are followed for Lots 1 - 11.

For Lot 12 an engineered overland flow path between Lots 1 and 2 is proposed as part of the subdivision that can act as a vehicle access route. This access route and Lot 12 is designed to inundate during storm events. Development on the land is accepted where minimum floor levels as recommended by Awa Environmental Limited are followed. The velocities over this Lot as result of the flood modelling are low as reported by Awa Environmental Limited and therefore not expected to create erosion risk to adjacent fill batters, drains, or downstream recipient catchments.

B. Servicing effects

Sewer, stormwater, water, and utilities can be provided to support the proposal that can be provided to an appropriate level of site servicing set by Council’s Code of Practice.

In summary, the current landform can be developed in a manner that meets technical guidelines and expectation of construction management that will have no adverse effect from an engineering servicing perspective.

C. Construction effects

A detailed Construction Management Plan can be established that supports the phasing of the site works, and particularly earthworks, which demonstrates how the development can be implemented in accordance with Industry Best Practice and also with the Greater Wellington Regional Council’s Guidelines for Erosion and Sediment Control. By adopting this Construction Management Plan the effects of Construction can be considered short term and appropriately managed to current authority expectation.

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> APPENDIX A – ENGINEERING CONCEPT DESIGN PLANS

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> APPENDIX B – SITE INVESTIGATION SCALA PENETROMETER TESTS

Client: Jenian Homes

Location: James Street, Plimmerton

Job Number: W16039

Date: 01-03-17

Depth in Metres Blows per 100mm

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1.3 11

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1.6 15

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Scala Penetrometer Results

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A: 1A/10 Surrey Street, Tawa, Wellington P: PO Box 56051, Tawa, Wellington 5249 T: 04 232 0973 W: www.orogen.nz



Job Number: W16039

Date: 01-03-17


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Job Number: W16039

Date: 01-03-17


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Job Number: W16039

Date: 01-03-17


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Job Number: W16039

Date: 01-03-17


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Job Number: W16039

Date: 01-03-17


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JAMES STREET PLIMMERTON: ENGINEERING SERVICING REPORT · The site is currently vacant and was...

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