1811005 Flood Report - FINAL 2 · where practical and will not exceed the earthwork fill...

FELTON MATHEW 90-104 Felton Mathew Ave, St Johns

F L O O D R E P O R T

CIVIL ENG INEERING SURV EY ING LAND DEV ELOPM ENT

CLIENT

JOB NUMBER

REVISION

DATE

Fletcher Living

168002

A

05/10/18

PREPARED

Jignesh Patel,

Senior Engineer

REVIEWED

William Moore,

Director

© MAVEN ASSOCIATES LTD 2018 This document is and shall remain the property of Maven Associates Ltd. The document may only be used for the purposes for which it was

commissioned and in accordance with the Terms of Engagement for the commission. Unauthorised use of this document in any form

whatsoever is prohibited.

09 571 0050 Level 2, 12-14 Walls Road, Penrose PO Box 11605, Ellerslie, 1542 www.maven.co.nz

90-104 Felton Mathew Ave, St Johns Flood Report

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2 Maven Associates

CONTENTS

1.0 INTRODUCTION 3

2.0 OVERLAND FLOWPATH 4

3.0 FLOODING 5

4.0 HAZARD RISK ASSESSMENT 5

5.0 CONCLUSIONS 7

APPENDECIES ENGINERING DRAWINGS


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3 Maven Associates

1.0 INTRODUCTION 1.1 PROJECT The purpose of this report is to provide a flood assessment (inclusive of risks) associated with the proposed future residential development at 90-104 Felton Mathew Ave, St Johns. We understand that the residential development is dependent on a private plan change request, with the site currently zoned Business – Light Industry. The proposed residential development is contained to the northernmost portion of the site, which is currently undeveloped. This report is to be read in conjunction with the appended drawings and calculations. The location of the site and the overland flowpath (OLFP’) are indicated in Figure 1, below.

Figure 1: GeoMaps overland flowpath and flood sensitive area

The assessment included in this report is a ‘desktop’ analysis based on information available at time of issue. Final design plans for will be supplied at Engineering approval stage as required. In terms of overland flow and flooding an analysis to determine the extent of issues and identification of OLFP location has been undertaken for any future proposed development. This report provides information in relation to the future residential development of the site and provides comment on matters to be considered pursuant to construction. The following matters are addressed in this report:

Introduction including site and locality Overland flowpath analysis Flood assessment Consent requirements and conclusions


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4 Maven Associates

1.2 SITE DESCRIPTION

The overall site area is 2.283ha. The area subject to the future residential development is contained within a 3975m² area to the north of the site. The site is bound by Felton Mathew Ave to the west and a Council Reserve to the east which is adjacent to the Eastern Railway line.

The site is located within he Omaru Creek catchment of which an Integrated Catchment Management Plan (‘ICMP’) has been prepared by Harrison Grierson (‘HG’) on behalf of Auckland Council.

A tributary of Omaru Creek commences on the northern boundary of the Merton Reserve, flowing within an open drain along the rear of the light industrial properties fronting Farmhouse Lane. The drain is then conveyed via a stormwater line under the subject site, and discharges on the eastern side of the site.

This pipe under the site is 1200mm diameter at the inlet and increases to 1650mm diameter before passing under the subject site. The pipe likely reaches capacity during large storm events with excess stormwater flow overtopping the pipe, with OLF via sheet flow across the site.

2.0 OVERLAND FLOWPATH

Auckland Council GeoMaps identifies a major OLFP which traverses the site from east to west. The OLFP is in the form of sheet flow across the site during large rainfall events.

The OLFP falls to a tributary of the Omaru creek located at the western boundary of the site, which flows towards the Eastern Railway line to the east before it again enters a culvert under the railway. Stormwater builds up and ponds against the raised railway corridor before exiting through the existing culvert (1450mm diameter at entrance).

An assessment of the existing OLFP traversing the site during a 100-year rainfall event has been undertaken to confirm OLFP extents. The extent of the existing OLFP is approximately 15-35m wide with a depth of flow up to 0.48m as shown in the attached Engineering Plans.

Peak flow in the OLFP is 7.54m3/s from the information provided by Auckland Council. For pipes greater than 1050mm in diameter they are assumed to be 10% blocked under the Auckland Council Stormwater Code of Practice (‘AC SWCoP’).

An option has been investigated to reroute the OLFP through the site with equal flow within the adjacent carpark area to the south of the proposed residential development. It is understood that the carpark will be remodelled allowing for the OLFP.

The proposed OLFP has been calculated using the 100-year rainfall event flows and manning’s calculations over sections though the site. The appended calculations and sections determine that we can offset the OLFP from the proposed residential area and maintain similar OLF extents to the existing situation.

The proposed OLFP will provide a consistent flowpath depth of 200mm over a greater area before merging back into existing carpark levels, thus allowing the existing extent of flooding to be maintained.

The proposed OLFP will also maintain the entry and exit points and the capacity of the existing OLFP within the site. The proposed residential development will not cause an increase in risk or hazard to any upstream or downstream property as per the Auckland Unitary Plan – Operative in Part (‘AUP – OP’) requirements.


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5 Maven Associates

3.0 FLOODING

Auckland Council’s GeoMaps identify a flood sensitive area adjacent to the western boundary of the site, refer to Figure 1 GeoMaps overland flowpath and flood sensitive area. Flooding is caused from the build-up and ponding of stormwater against the raised railway corridor.

Auckland Council GeoMaps also identify that the 1% AEP flood plain extends into the site, also caused by the ponding behind the railway line. However, Maven Associates have completed a topographical survey of the area, which has identified that the extent of the1% AEP flood plain is not accurate.

The extent of the flood plain is more consistent with the extent of the flood sensitive area identified within Geomaps, only a small portion of the proposed residential area is identified within the 1% flood plain.

The existing railway level is lower than the majority of the development area and therefore flood water would overtop the railway line before any significant flooding occurs.

Any future development within the residential area will avoid the identified flood plain area where practical and will not exceed the earthwork fill requirements permitted under E12.6.2.(11) that being 10m³ in volume and not raising ground levels more than 300mm.

4.0 HAZARD RISK ASSESSMENT

Below is a ‘hazard risk assessment’ for the proposed residential development in accordance with the special information requirements set out at section E36.9 of the AUP – OP.

AUP – OP Requirements:

Clause E36.9 of the AUP (OP) states that a ‘hazard risk assessment’ must be undertaken when subdivision, use or development requiring resource consent is proposed on land that is subject to hazards. The matters required to be considered by the assessment are addressed below.

(a) the type, frequency and scale of the natural hazard and whether adverse effectson the development will be temporary or permanent.

The main risk to the proposed building relates to flooding during storm and heavy rainfall events, which are infrequent (1 in 100-year events), and the flooding would be temporary in nature.

Any future residential buildings (sensitive activities) will require minimum freeboard above both the 1% Flood Plain, and the height of the OLFP. Subject to minimum floor levels (+500mm) being set, there will be no adverse effects. The proposed design will not alter the current flood height or level of water within the existing carpark area.

Any future development on the site will avoid the identified flood plain area where possible, with any earthworks limited to the permitted thresholds of Standard E12.6.2.(11). The redevelopment of the site will retain the same volume of water within the surveyed extent of the flood plain. Thus, there will be no effect on the flood plain. The flood plain will retain the same capacity and there will be no effects on the upstream and/or downstream properties.

(b) the type of activity being undertaken and its vulnerability to natural hazardevents;

The proposed activity subject to this application is residential living which is considered vulnerable to the effects of flooding. Habitable Minimum floor levels of the proposed building will maintain a freeboard of +500mm over and above the flood water depth.


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6 Maven Associates

(c) the consequences of a natural hazard event in relation to the proposed activityand the people likely to be involved in that activity;

The consequences of flooding could include damage to buildings and their property, as well as health and safety.

Habitable Minimum floor levels of the proposed building will maintain a freeboard of +500mm over and above the flood water depth, therefore it is not envisaged that there would be consequences related to flooding.

Any future development on the site will avoid the identified flood plain area, with any earthworks limited to the permitted thresholds of Standard E12.6.2.(11). The redevelopment of the site will retain the same volume of water within the surveyed extent of the flood plain. Thus, there will be no effect on the flood plain. The flood plain will retain the same capacity and there will be no effects on the upstream and/or downstream properties.

(d) the potential effects on public safety and other property;

The consequences of flooding could include damage to buildings and their property, as well as health and safety.

The movement of goods or displacement/ exacerbation of flooding hazards to the adjacent properties are the main potential effects in this regard. With respect to the movement of goods and cars, proposed storage of goods should be located outside of the flood plain area.

Any future development on the site will avoid the identified flood plain area, with earthworks limited to the permitted thresholds of Standard E12.6.2.(11). The redevelopment of the site will retain the same volume of water within the surveyed extent of the flood plain. Thus, there will be no effect on the flood plain. The flood plain will retain the same capacity and there will be no effects on the upstream and/or downstream properties.

(e) any exacerbation of an existing natural hazard risks or creation of a new naturalhazard risks;

The proposed development will maintain the entry and exit points and the capacity of the existing OLFP. The proposed development will not cause an increase in risk or hazard to any upstream or downstream property as per the AUP – OP requirements, for the same reasons explained above within (c) and (d) .

(f) whether any building, structure or activity located on land subject to naturalhazards near the coast can be relocated in the event of severe coastal erosion, coastalstorm inundation or shoreline retreat;

Not applicable.

(g) the ability to use of non-structural solutions, such as planting or the retention orenhancement of natural landform buffers to avoid, remedy or mitigate the hazard, ratherthan hard engineering solutions or protection structures;

Not applicable.

(h) the design and construction of buildings and structures to mitigate the effects ofnatural hazards;

See (b) above.

(i) the effect of structures used to mitigate hazards on landscape values and publicaccess.

Not applicable.


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(j) site layout and management to avoid or mitigate the adverse effects of naturalhazards, including access and exit during a natural hazard event.

The future building footprints will be located away from the redirected OLFP and survey extent of flooding margins.

(k) the duration of consent and how this may limit the exposure for more or lessvulnerable activities to the effects of natural hazards including the effects of climatechange.

The activity is proposed to be permanent.

(l) any measures and/or plans proposed to mitigate the natural hazard or the effectsof the natural hazard.

See (b) above.

5.0 CONCLUSIONS

The proposed residential development will maintain the function of the overland flow path and safely convey the 100 year peak flow through the site without increasing the risk of flooding to any upstream or downstream properties.

The proposed OLFP will maintain the entry and exit points and the capacity of the existing OLFP as per the Auckland Unitary Plan – Operative in Part (‘AUP – OP’) requirements.

Flooding within the adjacent reserve area to the west of the site is caused from the build-up and ponding of stormwater against the raised railway corridor which will overtop before any significant flood waters enter the site.

There is a small portion of the proposed residential area identified within the extent of the 1% AEP flood plain, any development in this area will be in accordance with the Auckland Unitary Plan – Operative in Part (‘AUP – OP’) requirements.

ENGINEERING DRAWINGS

1

6

5

0

3

0

0

1

6

5

0

1

6

5

0

1

2

0

0

3

7

5

3

0

0

CH 0

CH 20

CH 40

CH 45

CH 0

CH 20

CH 35

CH 0

CH 20

CH 30

CH 0

CH 20

CH 30

22.0022.00

23.00

24.00

25.00

22.20

22.2022.40

22.40

22.6022

.80

23.20

23.40

23.60

23.80

24.20

24.40

24.60

24.80

Legend

N

Project

Title

Project no.

Scale

Cad file

Drawing no. Rev

Survey

Design

Drawn

Checked

By Date

Ref Revisions By Date

09 571 0050

Maven Associates

[email protected]

www.maven.co.nz

12-14 Walls Road, Penrose

MAVEN 09/18

JP 10/18

JP 10/18

BV 10/18

90-104 FELTON MATHEWAVENUE, ST JOHNDEVELOPMENTFOR FLETCHER LIVING

EXISTING OVERLAND FLOWPATH & EXTENT OF FLOODING

168002

1:500

OLFP SECTIONS

C450 A

Notes1. All works to be in accordance with Auckland

council standards.2. Co-ordinates in terms of NZ Geodetic Datum Mt

Eden 20003. Levels in terms of the Auckland Vertical Datum

1946.

EX BDYEX EASEMENT

EXISTING 100YR OLFPAS PER A.C GEOMAPS

EXISTING 100YR OLFPFLOW = 7.54m³/sec

FELTON MATHEW AVENUE

1-1C451

100YR FLOOD PLAIN RL = 22.90m

EXTENT OF EXISTING 100YR OLFPAS PER A.C GEOMAPS

2-2C451

3-3C451

4-4C451

DRAFT

1

6

5

0

3

0

0

1

6

5

0

1

6

5

0

1

2

0

0

3

7

5

3

0

0

CH 0

CH 20

CH 40

CH 45

CH 0

CH 20

CH 35

CH 0

CH 20

CH 30

CH 0

CH 20

CH 30

22.0022.00

23.00

24.00

25.00

22.20

22.2022.40

22.40

22.6022

.80

23.20

23.40

23.60

23.80

24.20

24.40

24.60

24.80

Legend

N

Project

Title

Project no.

Scale

Cad file

Drawing no. Rev

Survey

Design

Drawn

Checked

By Date

Ref Revisions By Date

09 571 0050

Maven Associates

[email protected]

www.maven.co.nz

12-14 Walls Road, Penrose

MAVEN 09/18

JP 10/18

JP 10/18

BV 10/18

90-104 FELTON MATHEWAVENUE, ST JOHNDEVELOPMENTFOR FLETCHER LIVING

PROPOSED OVERLAND FLOWPATH & EXTENT OF FLOODING

168002

1:500

OLFP SECTIONS

C451 A

Notes1. All works to be in accordance with Auckland

council standards.2. Co-ordinates in terms of NZ Geodetic Datum Mt

Eden 20003. Levels in terms of the Auckland Vertical Datum

1946.

EX BDYEX EASEMENT

EXISTING 100 YR OLFPAS PER A.C GEOMAPS

PROPOSED 100YR OLFPFLOW = 7.54m³/sec

FELTON MATHEW AVENUE

1-1C451

100YR FLOOD PLAIN RL = 22.90m

EXTENT OF PROPOSED 100YR OLFP

2-2C451

3-3C451

4-4C451

DRAFT

1811005 Flood Report - FINAL 2 · where practical and will not exceed the earthwork fill...

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