TASMAN DISTRICT COUNCIL

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BEFORE INDEPENDENT HEARING COMMISSIONERS APPOINTED BY THE TASMAN DISTRICT COUNCIL

UNDER: the Resource Management Act 1991 IN THE MATTER OF: Resource consent applications

RM190877, RM190876, RM190878, RM190879, RM190881 and RM190880 associated with the Pōhara drainage improvement project, Pōhara Village, Golden Bay–Mohua.

____________________________________________________________________

STATEMENT OF EVIDENCE IN CHIEF OF DAMIAN NATHAN VELLUPPILLAI ON BEHALF OF

TASMAN DISTRICT COUNCIL Dated: 22 April 2021

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___________________________________________________________________

Filename as received - "Stormwater and Flooding - Damian Velluppillai.pdf"

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CONTENTS

1. INTRODUCTION ... 3

Qualifications and experience ... 3

Background... 3

Code of Conduct ... 4

2. SCOPE OF EVIDENCE ... 4

3. THE PROPOSAL ... 5

4. BACKGROUND ... 5

5. THE SITE AND LOCALITY ... 6

6. MODEL DEVELOPMENT ... 6

7. MITIGATION OPTION SET DEVELOPMENT AND DESIGN ... 8

8. COUNCIL S42A REPORT ... 11

9. SUBMISSIONS ... 11

10. PROPOSED CONDITIONS OF CONSENT ... 13

11. CONCLUSION ... 13

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1. INTRODUCTION

Qualifications and experience

1.1 My name is Damian Nathan Velluppillai.

1.2 I am a Civil Engineer with a Bachelor of Engineering (Hons) from the University of Canterbury.

1.3 I have nearly 20 years of experience of hydrological and hydraulic modelling, flood risk assessment and catchment management. I have applied these skills in numerous land development and subdivision projects around New Zealand, mostly in the Nelson and Tasman regions. I have provided stormwater and flood modelling evidence in Nelson City and Tasman District Council hearings related to many of these subdivision consents, including in Golden Bay. I am a member of Engineering New Zealand (EngNZ).

1.4 Since 2002 I have been employed by Tonkin & Taylor Limited (T+T). I am currently employed as a Senior Water Resources Engineer.

Background

1.5 I have been involved in undertaking the hydrological and hydraulic modelling to assess the existing flood hazard affecting property in Pōhara, and to develop and test mitigation options. I attended the public consultation meeting on 29 August 2017 to present flood hazard information on behalf of the Tasman District Council (the applicant).

1.6 I have been to the subject site and I am familiar with the locality. I have prepared the following reports:

(a) Tonkin & Taylor Limited report titled “Ellis Creek Modelling Report”, dated February 2014 (T+T 2014);

(b) Tonkin & Taylor Limited report titled “Pōhara Stormwater Modelling – Drainage Network Improvement Options” report dated November 2016 (T+T 2016).

The T+T 2016 report was appended to the consent application to Tasman District Council.

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1.7 In addition, I have viewed the following documents:

(c) The Resource Consent application and assessment of environmental effects (the AEE), prepared by Tonkin & Taylor Limited;

(d) Pōhara Catchment Stormwater - Issues and Options Assessment, MWH, June 2012;

(e) Pōhara Subdivision Flooding Investigation, MWH, July 2009.

I have read the Council’s S42A reports, which recommend consent be granted.

Code of Conduct

1.8 I confirm that I have read the Code of Conduct for expert witnesses contained in the Environment Court’s Practice Note as updated in 2014. My evidence has been prepared in compliance with that Code. In particular, unless I state otherwise, this evidence is within my area of expertise and I have not omitted to consider material facts known to me that might alter or detract from the opinions I express.

2. SCOPE OF EVIDENCE

2.1 My evidence is structured as follows:

(a) The proposal;

(b) Background;

(c) The Site and Locality;

(d) Model development;

(e) Mitigation option set development and design;

(f) Council S42A report;

(g) Submissions;

(h) Proposed conditions of consent;

(i) Conclusion.

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3. THE PROPOSAL

3.1 The applicant proposes a set of flood protection measures to protect existing private property in and around Pōhara. These measures are detailed in Section 3 of the AEE and in the S42A officer’s report, and include:

(a) Culvert upgrades;

(b) New flood barriers (combination of timber walls and earth stopbanks);

(c) Minor re-contouring and channel widening to improve flood conveyance in selected locations.

3.2 The purpose of these works is to reduce the number of floors that will be flooded in up to the present day 1% annual exceedance probability (AEP) events.

3.3 In recognition of the difficulty and cost of providing a higher level of service in a low-lying, developed coastal floodplain, the works have not all been designed to meet the standards set out in the current Nelson Tasman Land Development Manual (NTLDM).

4. BACKGROUND

4.1 The Pōhara township and surrounding areas have a history of flooding during heavy/extreme rainfall events. Notably, the extreme rainfall event of December 2011 event led to caused significant, widespread flooding, landslips and debris flows, including extensive flooding of the Pōhara township and immediate surrounds environs.

4.2 Investigations and flood mitigation options assessment by Council date back to at least 2009. In 2013, Council commissioned the development of a detailed flood model of the area to understand better flood hazard and risk, and as a tool to identify, investigate and assess mitigation options.

4.3 This model has since been used to develop a set of mitigation measures and options.

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5. THE SITE AND LOCALITY

5.1 Pōhara township and river floodplain is located along the southern-most stretch of coastline in Golden Bay. The floodplain in the vicinity of the township is low- lying and relatively flat. Catchment flows drain from the hills to the south of the town, into Bartlett, Ellis and Clifton Creeks. Natural drainage paths show evidence of having been significantly modified through farming practices to improve land drainage and residential development. Most of the floodplain drains west into Motupipi Estuary, with a smaller proportion draining east to a coastal outfall under Abel Tasman Drive at the eastern end of the Pōhara Holiday Park.

5.2 The floodplain gradient is low (in the order of 1 in 500), and recession of floodwaters can be further constrained by tide levels in the estuary and at the outlet.

5.3 Much of the existing residential property in the area is located on slightly elevated foredunes along the beachfront. More recent development has occurred in the eastern part of the catchment, above the lower floodplain.

5.4 Bartlett Creek flows through an existing flood detention dam before discharging to Ellis Creek downstream in the vicinity of Selwyn Street.

6. MODEL DEVELOPMENT

6.1 Details of the model build are presented in the Tonkin & Taylor Limited report titled “Ellis Creek Modelling Report” (T+T 2014).

6.2 There are two main components to the flood model:

(a) A hydrological (rainfall/runoff) model of the contributing catchments (including flood detention dam), developed using HEC-HMS software;

(b) A coupled 1d/2d hydraulic model of the lower floodplain and area of interest, developed using DHI’s MikeFlood software.

6.3 Rainfall data used in the hydrological model were taken from NIWA’s High Intensity Rainfall Design System v3. These data were used to create hypothetical, conservative design rainfall profiles, where design rainfall bursts

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for a range of durations up to 24 hours were nested within a single 24 hour storm. These nested profiles are generally considered conservative, as real storm profiles tend to be less adverse; i.e. less “peaky”.

6.4 The modelling used Council’s 2008/2009 LiDAR data as the most up to date survey available at the time, supplemented with 2012 ground-based survey of channel inverts and culvert details, also provided by Council.

6.5 There was (and remains) insufficient hydrological information in this catchment against which to provide a detailed calibration and validation of the model.

However, rainfall records and Council’s mapping of the December 2011 flooding event were used as a point of comparison. This was a large, regional event with an exceedance probability well below 1% AEP report by Council’s Hydrology Department, and in which 450 mm of rainfall fell within 48 hours in some areas within Golden Bay. Rainfall data were spatially adjusted and applied to the model area, with resulting flood mapping compared with Council’s observed flood mapping. The results were generally comparable, giving confidence in the model setup.

6.6 The modelling assumed an “existing development” catchment state in terms of runoff estimation, surface roughness and landform. This assumes that any future development within the floodplain will be required to mitigate any effects on downstream flooding within those development sites, as required by the NTLDM.

6.7 The modelling results presented in T+T 2014 considered both present day and projected 2090 tidal conditions (assuming 0.8 m of sea level rise (SLR)). For the design of flood mitigation works, modelling focused on present day 1% AEP rainfall with present day tidal levels, with the applicant accepting that the level of service provided by any mitigation measures may degrade with time as the climate changes.

6.8 Given the uncertainty inherent in some parameters used in the modelling, particularly with respect to the LiDAR-derived ground surface, any flooding of a depth less than 50 mm is considered to be within the confidence limits of the modelling, and has therefore been neglected. That is, modelled flood depths of less than 50 mm is not considered be a hazard.

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7. MITIGATION OPTION SET DEVELOPMENT AND DESIGN

7.1 Based on the modelled flood hazard, a range of flood mitigation options was conceived and tested.

7.2 Some options were discarded early due to lack of performance, high cost, or anticipated difficulty with consenting. One example was the option to capture Bartlett Creek flows either at the existing detention dam or immediately downstream and direct these into a large pipe that discharges directly to the coast. modelling results showed that the removal of these flows from the floodplain would have negligible effect on overall flood depths/extents in the design storm.

7.3 Commentary on the option development and appraisal process is provided in T+T 2016.

7.4 As some of the proposed works are on private land owned by Richmond Pōhara Holdings Ltd (RPHL), Council consulted with RPHL in the development of mitigation options. However, I note that the proposed Council works are not specifically designed to allow for or enable any future catchment development.

Any future development within the catchment by RPH or other developers will be required to include mitigation of any increased runoff such that downstream effects are managed (refer Resource Consent SH180016, Condition 2).

7.5 The option sets were modelled and then ranked using Council’s cost/benefit scoring system, based on reduction in numbers of flooded floors and properties.

Council’s Option Set 4, in combination with the RPHL option, was ranked highest (most preferred in terms of benefit/cost ratio) for all design storm events (present day and 2100 horizons for both 10% and 1% AEP rainfall events).

7.6 The RPHL option included an earth stopbank along the north (right) bank of Bartlett Creek east of Abel Tasman Drive to provide flood protection to properties east of (and including) the Totally Roasted Café at 734 Abel Tasman Drive. An alternative stopbank alignment along the back of the existing Abel Tasman Drive properties was considered. This would have the potential benefit of increased floodplain storage for Bartlett Creek flows, and any overflow contribution from Ellis Creek. Flood modelling showed that the effect of this additional floodplain storage was negligible in terms of flooding east of Kohikiko Place and west of Abel Tasman Drive, and that it potentially increased flooding

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on Abel Tasman Drive properties by reducing floodplain storage available to those properties. The applicant and RPH agreed on a stopbank alignment along the right bank of Bartlett Creek. This option showed the greatest benefit to Abel Tasman Drive properties.

7.7 Subsequent consultation and design work have resulted in refinement of this option set, with a focus on mitigating the flood hazard in the present day 1%

AEP event. The refined suite of proposed measures has been chosen as the preferred design, with the model used to assess the effects of this refined set of proposed measures on flood levels within private property.

7.8 I summarise the final suite of proposed measures as follows (also described in the AEE):

(a) Upgrade the culvert adjacent to 14B Kohikiko Place, including new local flood wall and preferential overland flowpath during extreme events;

(b) Construct a new earth stopbank along the true right bank of Bartlett Creek between 14B Kohikiko Place and Abel Tasman Drive;

(c) Upgrade the Bartlett Creek culvert under Abel Tasman Drive from a 1350 mm diameter pipe to a 4 m by 1.3 m box culvert;

(d) Construct a new earth stopbank along the south-western edge of Numbers 1 to 7 Selwyn Street properties;

(e) Raising of a low area on Lansdowne Street (paper road section) adjacent to 59 Selwyn Street;

(f) Construct new Lansdowne Street swale to provide alternative overland flowpath for flood water presently expected to flow towards 59a Selwyn Street;

(g) Construct a new retained flood wall and earth stopbank to reduce flooding at 59b and 59c Selwyn Street properties;

(h) Widen the confluence of Ellis and Clifton Creek, and existing constriction, to increase the channel capacity, including rock riprap to reduce erosion risk;

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(i) Upgrade Boyle Street culverts by adding two additional 1200 mm diameter concrete culverts, approximately doubling the existing capacity.

7.9 Modelled differences in flood depths as a result of the preferred proposed flood mitigation option set are shown in Appendix F of the AEE, and are annexed to my evidence as Attachment A.

7.10 Overall, I consider that the proposed flood mitigation works will have a positive effect on the Pōhara community given the expected significant reduction in the number of flooded floors during extreme events. This is particularly true for properties to the east of the Totally Roasted Café at 734 Abel Tasman Drive.

7.11 The mapping shown in Attachment A shows:

(a) A reduction in flood depths (in some places by over 0.5 m):

(i) In residential property east of Abel Tasman Drive including Kohikiko place;

(ii) Affecting dwellings at the south-west end of Selwyn Street;

(iii) In total 59 properties are expected to have reduced flooding as a result of the works. The AEE contains further detail as to the modelled effects.

(b) An increase in flood depths:

(i) To the south of Bartlett Creek in RPHL land;

(ii) In farmland to the west of Abel Tasman Drive (up to 140 mm);

(iii) Behind and within Selwyn Street properties (53 to 83 Selwyn Street, up to 120 mm depth increase, though not expected to result in flooding of any additional habitable floors);

(iv) In total, 34 properties are expected to be affected by increased flood depths, though not so as to result in the flooding of any additional habitable floors. The AEE contains further detail as to the modelled effects.

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7.12 The design level of the proposed flood wall at the western end of Selwyn Street is at RL 3.2 m minimum (Nelson Vertical Datum 1955 – NVD55). This is designed to protect against coastal flooding (i.e. storm-surge rather than catchment-related flooding) for the present day the 1% AEP event, and for approximately the MHWS tide level assuming 1 m of SLR above present-day values.

8. COUNCIL S42A REPORT

8.1 I have read the technical memo prepared by Mr. Leif Pigott (Team Leader – Natural Resource Consents at Tasman District Council) for Council.

8.2 I note that Mr. Pigott (in paragraphs 9.9 and 9.10) refers to 73 and 24 properties with lower and higher flood levels respectively, as a result of the proposed works. I am unclear as to how these numbers were determined. Based on my assessment, and as included in the AEE, I have assessed these numbers to be 59 and 34 respectively.

8.3 Other than this point, I am in general agreement with Mr. Pigott’s statements relating to flooding under the section titled “Key Issue 1 – Changes to flood risk”.

9. SUBMISSIONS

9.1 Rosemary Jones and Daniel Te Tau, 59b Selwyn Street:

(a) I have read the above submission. I note that the submission is in support of the proposed works, subject to conditions imposed regarding the maintenance of stream channel capacity;

(b) I agree with the submission that, particularly for less extreme (more frequent) events, heavy vegetation within the main drainage channel is likely to impede flows, resulting in increased sediment deposition in the channel, and causing out-of-channel flooding at lower flow rates and higher frequency than might otherwise be expected;

(c) For more extreme events, like the 10% AEP and 1% AEP events modelled, the catchment flows are far in excess of the capacity of the main drainage channels within the floodplain, and there will be

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significant overland flow outside of the main channels. In these events, the degree of vegetation within the channels is not a significant contributing factor to the overall flooding extents and depths;

(d) I note that Council proposes minor reshaping of the paper road (known as Lansdowne Street) to provide an alternative flow path in extreme events away from Selwyn Street properties and into Clifton Creek. I expect this to provide a reduction in flow rates in these events within Ellis Creek along the reach discussed in the submission;

(e) I also note that the focus of the proposed works is to provide a reduction in the number of flooded floors and property in extreme events, rather than to improve flooding in more frequent events. Modelling shows that the capacity of the existing drains is low compared with the overall floodplain flows, and therefore that the loss of capacity due to vegetation within the Ellis Creek channel will not significantly change flood levels during extreme events.

9.2 Cloud Dance Trust, 59a Selwyn Street:

(a) I have read the above submission. I confirm that the design and consenting process for Council’s proposed flood mitigation works are independent from the consenting process for the proposed Special Housing Area development at 82 Richmond Road, i.e. that the flood mitigation works are based on existing catchment runoff rates, and that the Special Housing Area development will be required to mitigate any increase in flows on-site, so that downstream property is not adversely affected.

9.3 Hans Stoffregen, 23 Landsowne Road:

(a) I have read the above submission. The submitter requests consideration of “alternatives to the hard engineering options proposed”. One alternative suggested is the use of farmland to the west of Abel Tasman Drive for temporary flood storage. This is essentially the approach being taken by Council, i.e. to increase the capacity of the Abel Tasman Drive culvert upstream to more efficiently convey flood flows into the western floodplain area and utilise flood storage there. This will serve to direct floodwaters away from residential development in the east;

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(b) The submitter seeks realignment of the proposed flood control bund along Bartlett Creek (east of Abel Tasman Drive) to the north, against the properties at 762 to 734 Abel Tasman Drive. Modelling to assess the impact of that change for Council showed that the difference in flooding both east and west of Abel Tasman Drive during 10% AEP and 1% AEP was negligible for this scenario. This proposed flood control bund is located on land owned by RPHL and therefore the applicant was required to negotiate with RPHL as to its location. Given the location has a negligible impact on the performance of the proposed mitigations works, the proposed alignment has been agreed between both parties to suit the objectives of each party.

10. PROPOSED CONDITIONS OF CONSENT

10.1 I note that a draft CEMP is included as a volunteered condition in the AEE, and agree that this is important to manage potential construction-related effects.

11. CONCLUSION

11.1 In summary, it is my opinion that:

(a) The modelling provides a technically robust basis for understanding the effects/performance of the proposed flood mitigation works on flooding within the Pōhara and environs floodplain during extreme events;

(b) The proposed mitigation works have been developed using a suitably robust options assessment process, and the preferred set has the highest benefit/cost ratio according to the Council’s scoring system;

(c) The proposed flood mitigation measures will have an overall positive effect on the wider Pōhara community in terms of reduced extent and frequency of flooding causing damages to existing buildings.

DAMIAN NATHAN VELLUPPILLAI 22 April 2021

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Attachments:

• Attachment 1 – Flood Difference Mapping

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Appendix F: Flood Difference Modelling Maps

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BOYLE ST

CLIFTON CK ELLIS CK

ELLISCK

BARTLETT CK

KOHIKIKO PL

EASTERN DRAIN SELWYN ST

ABEL TASMAN DR

Existing Dam

2D STUDY AREA EXENT Proposed Stopbank

Widening of confluence

Upgrade Boyle St 2x 1200 mm dia culvert pipes

TENNYSON ST

WATINO PL

SANDRIDGE TCE

BAY VISTA DR

POHUTUKAWA PL RICHM

OND RD Proposed Stopbank

Upgrade culvert to 4 m by 1.3 m SELWYN ST

Replace existing 900 mm dia culvert with 2x 1350 mm dia culvert pipes

Removal of soil stockpile orginally extracted from Creek after 2011 storm LEGEND

Modelled Watercourses Proposed Stopbank Property Boundary

Difference in Modelled Flood Depth (m)

< -0.25 -0.25 - -0.1 -0.1 - -0.05 -0.05 - -0.02 -0.02 - 0.02 0.02 - 0.05 0.05 - 0.1 0.1 - 0.25

> 0.25

!

^{Figure 1.}

TASMAN DISTRICT COUNCIL

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POHARA STORMWATER OPTIONS ASSESSMENT TDC IMPROVEMENTS

1% AEP Difference in Modelled Flood Depths Location Plan

1:7,000 871018.3000

Figure1.mxd

EMCF Jun.19 1) Aerial photograph sourced from Top of the South Maps and licensed by LINZ

for re-use under the Creative Commons Attribution 3.0 NZ License (CC by 3.0 NZ).

2) Modelling based on 2011 LiDAR data supplied by Tasman District Council.

Lucas House, 51 Halifax Street, Nelson Notes:

www.tonkintaylor.co.nz

SCALE (AT A3 SIZE) PROJECT No.

ARCFILE DRAWN CHECKED APPROVED

FIGURE No. Rev.

0 75 150A3 SCALE 1:7,000225 300 375 Meters

Path: P:\871018\871018.3000\WorkingMaterial\GIS\FIGURES\Figure1.mxd Date: 7/06/2019 Time: 12:22:39 p.m.

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CLIFTON CK

ELLIS CK

SELWYN ST

56 54 60

62

64

77 65 80

89 93

94

65A

50

59B

59A 84

87 85

83

42 48

61 53 63

72 66

73 78

82 82A

56A

46 52

67 68

69 74 70

76 88

90

95 97

81 44

55 71 59

79 75 91

92

59C

LEGEND Modelled Watercourses Proposed Stopbank Proposed Swale Property Boundary

< -0.25 -0.25 - -0.1 -0.1 - -0.05 -0.05 - -0.02 -0.02 - 0.02 0.02 - 0.05 0.05 - 0.1 0.1 - 0.25

> 0.25

! TASMAN DISTRICT COUNCIL

^{Figure 2} ¹

1:1,330 871018.3000

Figure2.mxd

FIGURE No. Rev.

0 A3 SCALE 1:1,33010 20 30 40 50 Meters

Path: P:\871018\871018.3000\WorkingMaterial\GIS\FIGURES\Figure 1.mxd Date: 7/06/2019 Time: 12:25:28 p.m.

Drain points to be installed to remove local flooding.

Widening of confluence

RM190876 Hearing - Applicant evidence - Stormwater & flooding VELLUPPILLAI - 23 Apr 2021 - page 17 of 20

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ELLISCK

56 54

65

65A 51

718 9 15 13

27 21 31

50 35

59A

11

17 39

42 48

49

61 53

63 33B

56A

45

7 29 25

40

41

46 52

33 33A

44

37 43

47

55 59

19 23

< -0.25 -0.25 - -0.1 -0.1 - -0.05 -0.05 - -0.02 -0.02 - 0.02 0.02 - 0.05 0.05 - 0.1 0.1 - 0.25

> 0.25

! TASMAN DISTRICT COUNCIL

^{Figure 3} ¹

1:1,230 871018.3000

Figure2.mxd

FIGURE No. Rev.

0 A3 SCALE 1:1,23010 20 30 40 50 Meters

Proposed Stopbank

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ELLISCK

56 54

65

65A 51

718 9 15 13

27 21 31

50 35

59A

11

17 39

42 48

49

61 53

63 33B

56A

45

7 29 25

40

41

46 52

33 33A

44

37 43

47

55 59

19 23

< -0.25 -0.25 - -0.1 -0.1 - -0.05 -0.05 - -0.02 -0.02 - 0.02 0.02 - 0.05 0.05 - 0.1 0.1 - 0.25

> 0.25

! TASMAN DISTRICT COUNCIL

^{Figure 3} ¹

1:1,230 871018.3000

Figure2.mxd

FIGURE No. Rev.

0 A3 SCALE 1:1,23010 20 30 40 50 Meters

Proposed Stopbank

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BARTLETT CK

KOHIKIKO PL

EASTERN DRAIN TENNYSON ST

Proposed Stopbank Proposed Stopbank

< -0.25 -0.25 - -0.1 -0.1 - -0.05 -0.05 - -0.02 -0.02 - 0.02 0.02 - 0.05 0.05 - 0.1 0.1 - 0.25

> 0.25

!

^{Figure 4}

TASMAN DISTRICT COUNCIL

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1:1,750 871018.3000

Figure 4.mxd

EMCF Oct.18 1) Aerial photograph sourced from Top of the South Maps and licensed by LINZ

FIGURE No. Rev.

0 10 20 30 40 50 Meters

A3 SCALE 1:1,750

Path: T:\Nelson\Projects\871018\871018.3000\WorkingMaterial\GIS\FIGURES\Figure 4.mxd Date: 2/10/2018 Time: 5:02:21 p.m.