Auckland Environmental Office The Westhaven, 100 Beaumont ... · The Meola Flood Hazard Mapping is the stated source of the GIS Viewer floodplain, however a more detailed review of

Opus International Consultants Ltd Auckland Environmental OfficeThe Westhaven, 100 Beaumont StPO Box 5848, Auckland 1141New Zealand

t: +64 9 355 9500f: +64 9 355 9584w: www.opus.co.nz

TO Matt Kerr-Ridge

COPY Tracy Talbot

FROM James Reddish

DATE 18 December 2014

FILE 1-95496.00

SUBJECT Fowlds Park - Stormwater Design

This memo summarises the proposed stormwater design for the Fowlds Park artificial turf

upgrade.

1. The Site and Catchment

Fowlds Park is located near the northern boundary of the Meola Stream catchment.

Immediately north of the site, Western Springs Road approximately forms the boundary with

the Motions Creek catchment. The site is outside the stormwater management area � flow

(SMAF areas) defined in the Unitary Plan overlays.

Geotechnical investigations (Opus, 2014) indicate the site sits on top of the Mt St John lava flow. The underlying geology consists of inorganic fill and quarry rubble basalt on top of the basalt formation. Groundwater levels have been measured approximately 9m below the ground surface at one borehole on site.

There is limited stormwater network in and around the Fowlds Park area. The majority of surface water is discharged to ground via soakholes.

It�s location near the boundary of the Meola catchment means the development site is relatively isolated from off-site stormwater runoff and collection during design storm events (up to and including the 1 in 10 year ARI storm event). This means the stormwater management for the development site can be managed in relative isolation. During more extreme events (e.g. the 1 in 100 year ARI storm event, some minor overland flow may occur through the site (refer Section 2 below).

The proposed development consist of approximately 1 hectare of artificial turf and

approximately 2165m2 of footpath and hardstanding area around the perimeter. The artificial

turf is considered a pervious surface on the basis stormwater infiltrates through the surface

and discharges to ground (refer Section 3 below). Although redevelopment of the site

includes removal of approximately 1570m2 of existing impervious surface (existing netball

courts and a small area of the carpark), there is a net increase in impervious surface (590m2).

This increase in impervious surface is attenuated on site so there is no net increase in surface

water runoff up to and including the 1 in 100 year ARI storm event (refer to Section 4 below).

2. Flood Risk

Auckland Council GIS Viewer indicates the main portion of the development site is located

outside of the 1 in 100 year ARI floodplain. This data set also shows the lower field at Fowlds

Page 2

Park, to the west, is within the floodplain. A portion of the retaining wall and filling proposed

as part of development will occur in this part of the lower field.

The Meola Flood Hazard Mapping is the stated source of the GIS Viewer floodplain, however

a more detailed review of the source data (Appendix 1) shows the development area is not

within the predicted 1 in 100 year ARI floodplain (i.e. an area used for flood storage), but

within an area designated �potential overland flowpath low hazard�. This means development

is expected to have negligible effect on flood extent and level, however the proposed retaining

wall may push the overland flowpath slightly further into the lower field.

A number of minor overland flowpath are also shown on the Auckland Council GIS Viewer

passing through the site. During design storm events these flowpaths are expected to be

intercepted by the network of catchpits in the park access road and are not expected to pass

onto the proposed development site. As the proposed topography and direction of fall of the

development site is similar to the existing field, during extreme rainfall events (e.g. 1 in 100

year ARI storm event), when the drainage network is exceeded, the overland flowpaths are

expected to act in a similar manner and route as at present. This is considered to fall within

the permitted activity criteria, i.e. modifying the alignment of an overland flowpath within a

site, but not diverting the entry or exit point or reducing its capacity.

3. Stormwater Management Approach

Stormwater management has been designed in accordance with the Auckland Council Code

of Practice for Land Development and Subdivision (Chapter 4 � Stormwater), October 2013

and Stormwater Disposal via Soakage in the Auckland Region (Technical Report

2013/040), October 2013. Supporting calculations are provided in Appendix 2 of this memo.

The Proposed Auckland Unitary Plan places increased emphasis on maintaining existing

discharge to ground as far as practical (Auckland Council, 2013) and this has been a key

driver in the management of stormwater on site. Discharge to ground, mimicking the existing

discharge arrangement, reduces the requirement for stormwater attenuation, but does not

remove it entirely. During events more extreme than the design storm the artificial turf is

expected to generate runoff in a similar manner as at present.

Soakage testing of the underlying ground conditions indicates a moderate discharge rate into

the underlying basalt and sufficient for stormwater discharge to ground. The basalt is

predicted to have an average soakage capacity of approximately 3.1 l/m2/minute. As the

artificial turf sits across this basalt, water that infiltrates through the artificial turf surface

and sub base layers can infiltrate into the underlying basalt at this 3.1 l/m2/min � equivalent

to approximately 530 l/s across the full extent of proposed artificial turf area. This is greater

than the 1 in 10 year ARI storm event (including climate change effect) falling on this surface

(refer Appendix 2 Sheet 3). Calculations in Appendix 2 Sheet 2 demonstrate the limiting

factor for discharge to the underlying basalt is the basalt surface, rather than the artificial turf

surface and subbase layers, which have a higher predicted infiltration rate.

Although 3.1 l/m2/min is considered a precautionary infiltration rate, there may be some

variability in infiltration. Over time there may also be some filling of voids within the

artificial turf surface and sub base layers with fine material, reducing the predicted

infiltration rate. For these reasons additional drainage measures have been incorporated into

the design - subsoil drainage at the boundary of the artificial turf sub base and the underlying

basalt; and surface level catchpits to collect additional flow. These additional measures are

considered appropriate contingency to manage any uncertainty in infiltration rate.

Page 3

The overall approach to managing stormwater on the development site is:

· Allow surface water that falls on the artificial turf surface to infiltrate through the surface

and sub base layers and into the underlying basalt;

· The surrounding footpaths and hardstanding areas will discharge to three soakholes and

trenches, where stormwater will be stored and infiltrate into the ground;

· Should localised areas of basalt have lower soakage rates than predicted, subsoil drains at

regular intervals will collect the excess build-up of water and discharge it to the

soakholes;

· Any excess surface water that cannot infiltrate through the artificial turf surface will be

collected via catchpits on the western edge of the field;

· Water from the grassed embankment between the field and access road will also be

collected via the grated channel and discharge to the soakholes to avoid runoff onto the

footpaths;

· All catchpits will be fitted with catchpit filters to capture crumb rubber that may migrate

off the playing surface, avoiding blockage of pipes, manholes and soakholes;

· Parks maintenance contractors will be required to inspect and clear catchpits and

soakholes on a regular basis to avoid build up and blockage.

Importantly the peak rainfall intensity adopted in calculating the size of soakage is 130

mm/hr. This is greater than the 1 in 10 Year ARI rainfall intensity of 90.6mm/hr (NIWA,

2014). It is also greater than the 103mm/hr when the effects of climate change on rainfall

intensity are included. Soakage devices are therefore oversized to account for any reduction

in performance over the lifetime of the asset (e.g. if maintenance is not performed as

expected).

Impervious surfaces expected to generate surface water runoff are not expected to generate

significant volumes of sediment or contaminants in the same way as roading or roofing.

Further stormwater treatment beyond the catchpit filters proposed is not considered

necessary.

4. Stormwater Attenuation

Proposed development results in a net increase of 590m2 of impervious area. This is

equivalent to the area of the proposed hardstanding area between the artificial turf field and

the existing clubrooms. Without this proposed hardstanding area, there would be the same

area of impervious surface pre- and post-development and therefore the same rate and

volume of runoff generated.

The Proposed Auckland Unitary Plan requires that for runoff from a development:

The discharge must not result in, or increase, the flooding of other properties in events up to

the 10 per cent AEP or the inundation of buildings in events up to the 1 per cent AEP.

Some properties immediately downstream of the proposed development site, in Brewster

Avenue, are identified as potentially at risk of flooding in the Meola Flood Hazard Mapping

(refer Appendix 1). It is therefore important that flooding is not exacerbated. Although in

Page 4

isolation the additional runoff generated by the Fowlds Park redevelopment would be very

small in the context of the flooding in Brewster Avenue, cumulative development in a

catchment could potentially result in increased risk.

The proposed drainage design means it is very unlikely that runoff from the development site

would occur in events up to and including the 1 in 10 Year ARI storm, however in larger

events, additional runoff could occur if not appropriately mitigated.

It is therefore proposed to include additional storage and drainage capacity in the

hardstanding area between the artificial turf surface and the clubrooms. This will collect,

store and discharge to ground runoff from events up to the 1 in 100 Year ARI storm event

(including allowance for climate change). This requirement relates only to the 590m2 of

additional hardstanding area, however is appropriate to deliver no net increase in surface

water runoff from the site up to this event.

The additional storage necessary is provided by increasing the size of the soakage trench

beneath the hardstanding (Soakage Trench 2), sufficient for the 1 in 100 Year ARI storm

event. The proposed drainage channels and catchpits in the hardstanding area have been

sized so they are sufficient to collect and convey the runoff to the soakage trench.

Additionally, the precautionary nature of the design means there will be a net benefit to

runoff in the catchment as a result of development:

· Using rainfall intensities greater than currently predicted by NIWA means additional

storage is available within soakage trenches for storing runoff;

· Runoff form the site is managed via storage and infiltration including the effects of

climate change. If development wasn�t occurring, the effects of climate change would

result in a gradual increase in runoff from the undeveloped site into the catchment;

· The existing netball courts do not have any formal drainage to collect and discharge

runoff. At present any rainfall falling on the netball courts will runoff. This effect is

removed as a result of development, creating a net benefit in stormwater management

as a result of development.

5. Compliance with Air Land and Water Plan Stormwater Rules

The total impervious area for the development is 2165m2, although the net increase is only

590m2. Table 1 assesses the proposed stormwater management system against the Air, Land

and Water Plan Rule 5.5.1, setting out the requirements for a controlled activity.

Page 5

Table 1: Auckland Air Land and Water Plan Stormwater Rules Con. Description Design Response

A The combined impervious areas (not already authorised

or consented) of the subdivision or development total less

than or equal to 5000m2;

The proposed impermeable

surfaces that form part of the

development are approximately

2165m2.

B It is not otherwise authorised by a stormwater network

consent granted under the provisions of Rules 5.5.10,

5.5.11, 5.5.12 or 5.5.13

Not applicable

C Inside the Urban Area, Stormwater outfalls that are

likely to cause erosion at the outfall incorporate erosion

control measures that do not impede fish passage;

Discharge is to ground, so there

will be no discharge scour or

erosion.

D It does not cause flood levels in a 100 year ARI storm to

rise within 0.5 metres of a habitable floor level authorised

by a Territorial Local Authority, in any dwelling, unless

the relevant District Plan or �Local Authority

Infrastructure Design Standards� establishes an

alternative freeboard requirement (above the 100 year

ARI storm) in which case the District Plan or �Local

Authority Infrastructure Design Standards� freeboard

requirement shall prevail;

Negligible change in the depth of

the floodplain is expected as a

result of development (refer

Section 2 above). Potentially

some benefit from development

(refer Section 4).

E For any stormwater discharged from more than

1,000m2 of impervious area source control,

contaminant reduction or contaminant removal methods

shall be designed to provide for the removal of at least

75% (or the alternative percentage specified in the

relevant network discharge consent for the area) of the

total suspended solids anticipated from the impervious

area on a long term average basis. These methods

include but are not limited to constructed wetlands,

swales, vegetative filters or infiltration practices;

The proposed footpath land use

is not expected to generate

sediment and/or contaminants

in the same way as a road or roof.

Further stormwater treatment

beyond the catchpit filters

proposed is not considered

necessary.

F Overland flow paths shall be provided and maintained for

flows in excess of the primary drainage network capacity

to allow flows up to and including the 100 year ARI storm

to discharge with the minimum of nuisance and damage;

Existing overland flowpaths will

be maintained within Fowlds

Park.

G Where the ownership of the proposed stormwater works

are to be taken over by the Territorial Local Authority

upon deposit of the survey plan, the written approval of

that Territorial Local Authority regarding the operational

and maintenance aspects of the proposed works has been

provided to the ARC.

The proposals are being brought

forward by Auckland Council

Sports, Parks and Recreation.

The stormwater drainage is

expected to be adopted by the

Auckland Council Stormwater

Unit. Approval will be via the

Engineering Plan Approval

process.

Page 6

Under the Proposed Unitary Plan Section 4.14, the proposed development is classified as new

impervious areas discharging to ground soakage or peat soils that does not meet the

permitted activity controls. Table 2 assesses the stormwater proposals as a restricted

discretionary activity and the rules in 4.14.1.

Table 2: Proposed Unitary Plan Stormwater Discharge Rules � Restricted Discretionary

Activity Description Design Response

a. methods of soakage Refer Section 3. Soakage devices have been designed in

accordance with Auckland Council Technical Report

2013/040: Stormwater Disposal via Soakage in the

Auckland Region.

b. measures to minimise the discharge

of contaminants

to groundwater aquifers

Refer Section 3. Minimal contaminants are expected to be

generated by the concrete hardstanding areas surrounding

the fields. Catchpit filters are proposed to capture crumb

rubber from the field prior to it reaching the soakholes.

c. measures to maintain soil moisture

levels in peat soils

Not applicable.

d. disposal of stormwater in excess of

device capacity and the management of

associated adverse effects

Refer Section 2 and 4. Runoff exceeding the capacity of the

on-site drainage system is predicted to replicate the

existing drainage and runoff patterns on site.

e. the management and mitigation of

flood effects, including

on buildings and property

Refer Section 4. There is not predicted to be an increase in

flood risk to buildings or property as a result of

development.

f. operations and maintenance

requirements

Refer Section 3. Auckland Council maintenance staff will

need to clear catchpit filters regularly and inspect and clear

soakholes as necessary. Flushing of sports field drainage

may be required occasionally.

g. monitoring and reporting, including

monitoring and reporting on a

network-wide basis.

Not applicable.

6. Statement of Effects

An erosion and sediment control plan has been prepared to manage the impact of runoff

during the construction phase. Proper application of the erosion and sediment control

measures mean the construction of the sports field is expected to have no adverse effect on

the receiving environment during construction.

The net increase in impermeable area as a result of development will be mitigated through

additional storage provided within soakage trenches. The precautionary approach in the

design assumptions results in a net positive benefit of development.

Impermeable surfaces consist of footpaths and are expected to have low sediment and

contaminant generation, with minimal adverse effect on the receiving environment.

Page 7

Catchpit filters are proposed to capture any crumb rubber that migrates off the field surface.

The stormwater soakage has been designed in accordance with Auckland Council guidance so

that there is no exacerbation of existing flooding issues downstream. The proposals are not

predicted to exacerbate or cause habitable floor flooding.

7. References

Auckland Council (2013), Auckland Unitary Plan stormwater management provisions: Technical basis of contaminant and volume management requirements, Technical report 2013/035. NIWA, 2014, High Intensity Rainfall System database [http://hirds.niwa.co.nz/]. Accessed

on 18th December 2014.

Opus International Consultants (2014), Fowlds Park Artificial Sports Field: Geotechnical

Investigation Report GS14/079.

Page 8

Appendix 1 � Meola Flood Hazard Mapping for Fowlds Park Area

GL = 28.6m

10yr = 28.8m

50yr = 28.9m

100yr = 28.9m

GL = 29m

10yr = 29.5m

50yr = 29.5m

100yr = 29.5m

GL = 26.4m

10yr = 27.1m

50yr = 27.2m

100yr = 27.2m

GL = 28.1m

10yr = 29.5m

50yr = 29.8m

100yr = 29.9m

G

10

50

10

GL = 20.5m

10yr = 22.2m

50yr = 22.9m

100yr = 23.3m

GL = 19.6m

10yr = 20.5m

50yr = 20.8m

100yr = 20.9m

GL = 22.5m

10yr = 23.7m

50yr = 23.7m

100yr = 23.7m

GL = 23.2m

10yr = 23.7m

50yr = 23.7m

100yr = 23.7m

GL = 22.7m

10yr = 24.1m

50yr = 24.1m

100yr = 24.1m

GL = 18.9m

10yr = 19.7m

50yr = 19.7m

100yr = 19.7m

B08017

B08015

NV8370

NS8747

NY7969

NW5348

NO8740

NO8736

NO8690

NO8701 NO8703

NO8700

NO8752

NO8746

NO8743

NO8725

NO8754

NO8751

NO8741

NO8735

NO8734

NO8675

NO8753

NO8744NO8737

NO8707

NO7132

NO8756

NO8726

NO7142

NO8704

NO8696

NO7124NO7141

NO7140

NO7130

NS6095

NS3407

NS3406

23.25

24.61

20.48

22.16

20.61

20.56

20.44

20.64

19.68

20.8

23.95

21.23

21.19

29.36

29.03

26.92

20.71

20.2420.23

20.41

30

25

20

35

40

45

35

20

20

20

20

30

20

25

30

25

20

30

30

20

35

ST LU

KES R

OAD

MALVERN ROAD

ASQUIT

H A

VENUE

BREWSTER AVENUE

NEW N

ORTH R

OAD

SELKIRK ROAD

JESMOND TERRACE

AMAN

DAL

E AV

ENUE

SAINSBU

RY ROAD

9

9

7

5

3

8

4

1

5

7

9

5

7

4

8

9 7

2

5

9

5

3

1

15

18

20

22

15

13

11

17

21

19

14

12

10

23

71

92

73

96

94

18

16

77

75

98

81

50

57

59

61

63

21

19

17

65

67

69

71

54

15

11

15

17

141618 1012

15 13

22 20

2325 21 1719

23

19

21

13

32 30

37

28 26 24

29 273140

44

49

48

46

41

43

47

45

53

55

51

39

23

21

24

20

22

19

17

15

18

13

11

14

12

16

28

26

15A

707

11A

19A

67A

102

100

104

110

108

118

116

120

680

678

676

674

672

692

1-5

688

686

684

697

695

70770

1

699

693

682

691

23A21A

707B

106A

1/5

4

701A

122-1

24

1-2/ 7

01

B08016

B08014

NO8750

NO8748

NO7131

2419.77

19.87

21.08

20.98

20.81

19.73

19.87

21.15

20.9

20.17

20.85

20.09

21.2

SEYC

HEL

LES

STREE

T

ROCKY NOOK AVENUE

SELCOURT RO

AD

CHALLINOR CRESCENT

WESTERN SPRINGS ROAD

SELK

IRK

ROAD

KEY PLAN

Figure 4.Predicted Maximum Probable Development Flood Hazards and HAT Inundation

Notes:

Scale: (A3 size)

15

0 20 40 60 8010

Meters

1:2,000

AMENDMENTSREV BY APPd DATE NAME DATE

APPROVED

FINALDESIGNEDCHECKEDSTATUSPRINTEDFILE NAME

MJ. Summerhays

O. SolarteO. SolarteS.JonesFinal_v2

29/10/201029/10/2010

29/10/2010

5/12/2012MEO_FHM_AppendixE_20121204.mxd

5/12/2012

A FLOOD HAZARD MAPPING OF MEOLA CATCHMENTS ON AUCKLAND ISTHMUS

Meola Flood Hazard Mapping

© 2010 Metro Water Limited and Auckland City Council. All rights reserved.

Information contained in this drawing/report is confidential and proprietary to

Metro Water Limited and Auckland City Council. No part of this drawing/report

may be copied or used without the prior written consent of Metro Water

Limited and Auckland City Council.

Areas beyond modelled network may have flood hazards and flood prone areas;

These have not been modelled and are therefore not represented.

Flood plains and overland flow paths represent modelled surface profiles.

Freeboard has not been included.

These maps must be read in conjunction with the Model Development Report.

GL means ground level at indicated location or lowest point on cross-section.

Map projection is NZTM.

The Building Footprint dataset has been digitised from 2006 orthoimagery.

The data has not been updated since. The data accuracy is a combination of

spatial accuracy of the orthoimagery and manual digitising.

The information contained or referred to in this drawing/report was developed

for use in the project known by Auckland City Council and Metro Water

Limited as Flood Hazard Mapping of Catchments in Meola DMA. Neither

Auckland City Council nor Metro Water Limited accept any responsibility for the

use of the information by any other parties and state expressly that they do not

warrant the accuracy of the information.

Any use of the information by other parties is at their own risk.

LEGEND:

4.14 4.15 4.16

4.244.23

4.08

4.22

4.09

Hazardous Areas

Significant Hazard Overland Flowpath (50yr)

Significant Hazard Overland Flowpath (10yr)

Significant Hazard Flooding (10yr)



Highest Astronomical Tidal Flood Plain

Potential Overland Flowpath or Low Hazard Flooding (50yr)

Potential Overland Flowpath or Low Hazard Flooding (10yr)

Building Footprints

2D Model Extent

Modelled Stormwater Node

Modelled Combined Node

Modelled Wastewater Node

Modelled Soakage (Representative Only)

Property Boundaries

SW Catchment Boundary

Survey CrossSection##1m LIDAR Contours

Modelled Wastewater Network

Modelled Stormwater Network

Modelled Combined Network

Modelled Open Channel

Modelled Stormwater Culvert,

,

!

!

!

"Ò

Building Footprints (Floor Level)

50yr Site assessed floors within 500mm

50yr Site assessed floors at risk

50yr Surveyed floors within 500mm

50yr Surveyed at risk

Draft For Review SQ MS 09/10

FinalB SQ MS 10/10Final_v2: Added node names.C PM MS 12/12

4/12/2012

Page 9

Appendix 2 � Fowlds Park Soakage and Stormwater Calculations

Page 9

Appendix 2 � Fowlds Park Soakage and Stormwater Calculations

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( ��)��+��)�� Date: 11/09/2014

'�%��-��#��!��).��!�� /��%

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!�� )��5� 3��

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TP108: Worksheet 2: Graphical Peak Flow Rate

- ) �� 2) F �(��-� <

Storm #2 Storm #3 Storm #4

7 �� 6��*��5 � 2 10 100

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/ 6��C��C24 = 1000 x Q24A (m3) 27 87 172

Mannings N Calculation

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TP108: Worksheet 2: Graphical Peak Flow Rate

- ) �� 2) F �(��'�7

Storm #2 Storm #3 Storm #4

7 �� 6��*��5 � 2 10 100

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Mannings N Calculation

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