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Wainui Stream Catchment Study Technical Report – August 2011

Contents 1.0 INTRODUCTION AND OBJECTIVES................................................................................................................... 1

2.0 SCOPE OF STUDY.............................................................................................................................................. 1

3.0 CATCHMENT AND SUB-CATCHMENT BOUNDARIES AND AREAS .................................................................. 2

4.0 PREVIOUS RELEVANT REPORTS......................................................................................................................... 4

5.0 STREAM CAPACITY UPSTREAM OF THE SH 35 CULVERTS ................................................................................ 8

6.0 FLOODPLAIN HAZARD MAPPING BASED ON THE JUNE 1977 FLOODSPREAD ................................................. 11

7.0 WAINUI STREAM BETWEEN SH35 AND THE SEA; REACHES 4, 5 AND 6 ......................................................... 13

8.0 THE EFFECTS OF CLIMATE CHANGE............................................................................................................... 13

9.0 FUTURE CATCHMENT CONDITIONS ............................................................................................................... 14

9.1 PROJECTED POPULATION CHANGES AND CHANGES TO AVERAGE HOUSEHOLD SIZE.........................................................14

9.2 PRESENT AND FUTURE RUNOFF ...................................................................................................................................15

9.3 IMPACT OF CLIMATE CHANGE ON RUNOFF..................................................................................................................15

9.4 IMPACT OF DEVELOPMENT ON RUNOFF.......................................................................................................................15

10.0 IMPACT OF FUTURE CHANGES AT THE WAINUI STREAM MOUTH ................................................................. 16

11.0 WHOLE OF CATCHMENT STRATEGY............................................................................................................... 18

11.1 A STRATEGY FOR FUTURE DEVELOPMENT .....................................................................................................................18

11.2 STRATEGY FOR REDUCING IMPACT OF EXISTING DEVELOPMENT: .....................................................................................19

12.0 FUTURE WORKS AND MAINTENANCE ............................................................................................................ 19

13.0 CONCLUSIONS ............................................................................................................................................... 20

14.0 RECOMMENDATIONS:.................................................................................................................................... 21

APPENDIX 1: MAINTENANCE SCHEDULE PART 1: WAINUI STREAM AND TAMARAU HILL SLOPE DRAINS..... 23

APPENDIX 2: MAINTENANCE SCHEDULE PART 2: URBAN FLOODWAY AREAS ............................................. 25

APPENDIX 3: LEVEL OF SERVICE 2 URBAN STORMWATER CRITERIA ............................................................... 26

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1.0 Introduction and Objectives

This report, commissioned by the Utilities section of the Gisborne District Council, is to undertake a whole of catchment stormwater and flooding study for the Wainui Stream.

The aim is to review the existing catchment and stormwater system and identify any upgrades required to cater for a ‘fully developed’ catchment based on the Gisborne District Council Combined Regional Land and District Plan land zoning and rules. (For the sake of brevity the Gisborne District Council Combined Regional Land and District Plan shall be referred to as the “District Plan” in this report).

The objectives of this work are to develop the stormwater drainage system to:

- Comply with Gisborne District Council’s ‘Level of Service 2’ (LOS 2) stormwater drainage criteria (refer GDC Code of Practice 21 March 2000).

- ensure that each property has an overland flow outlet for surface water drainage from the property.

- ensure the continuing use and enjoyment of their property by property owners/occupiers.

This revision No. 1 updates the draft flood hazard plan in the Lloyd George road area and for the Wainui Stream.

It should be noted that this is a “desk top” study only, and there has not been any discussion or consultation as yet about how the study may affect property owners and the community at large.

2.0 Scope of Study

The scope of this report is as follows:

2.1 Determination of the Wainui stream catchment and sub-catchment boundaries and areas.

2.2 Review of existing GDC reports and records associated with the Wainui Stream Catchment; and GDC, NWASCO and Gisborne Herald reports on the 1977 flood relevant to the stream and catchment.

2.3 Upstream of the SH2 culverts, determine the existing open drain and pipe system capacity of the Wainui Stream under current conditions and maintenance levels.

2.4 Determine as far as practically possible the extent of the June 1977 floodspread over the floodplain and valley areas; from flood level data already available (and from further historic levels and anecdotal evidence to be provided by GDC staff). Assess the extent of the floodspread from a storm of equal magnitude to the 1977 event on the present catchment, taking into account contour changes to the Sponge Bay subdivision and improvements to the upper Wainui Stream, for potential flood hazard zoning purposes.

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2.5 Assess the extent and cost of improvements to the Wainui stream channel and berms downstream of the SH2 culverts (to the sea) to convey the 50 year design flows within the channel, and to limit flood levels from a rainfall event equal in magnitude to the June 1977 event to 300mm below any existing house floor level; for:

(a) the full extent that existing zoning allows, and allowing for the effects of climate change to the year 2030 on design flood flows as per the MfE guidelines; and

(b) what is likely to be fully developed by the year 2060, and allowing for the effects of climate change to the year 2060 on design flood flows as per the MfE guidelines.

2.6 Assess the impacts of flows on the Wainui Beach outlet for a fully developed catchment, and assess the merits of a barrier or barriers at the downstream end of the Murphy Road culverts, to prevent the ingress of logs and other debris from the sea.

2.7 Investigate and develop, together with GDC staff, a viable “whole of catchment” strategy for the management of the Wainui Stream catchment now and in the future.

2.8 Review existing stream maintenance practices and give recommendations for the future.

Assumptions:

For the purpose of this study it has been assumed that Wainui will not be reticulated.

3.0 Catchment and sub-catchment boundaries and areas

Figure 1 shows the Wainui Stream catchment and subcatchment boundaries and areas, and Table 1 lists the subcatchment areas:

Table 1: Subcatchment areas:

SUBCATCHMENT AREA (ha)

Tamarau 166

Wheatstone 125

Rifle Range 67

Taumotu 47

Coastal 74

Lloyd George 84

Catchment Total: 563

The boundaries shown in the figure have been derived from LIDAR contours, and inspections on the ground. Because of the very flat ground in some areas, the mapped boundaries may vary a little in places from the actual boundaries. In the map in figure 1, the Ellmers subdivision presently under construction opposite Matthews Road on the western side of SH35, has been assumed to drain to the Kopuawhakapata stream as it will do on completion. The areas (rounded to the nearest hectare in the above table), have been calculated from the map overlay on the Council GIS.



In addition to the above, the “contributing subcatchment areas” between each tributary of the Wainui Stream, have been identified and estimated, as shown in Table 2:

Table 2: Wainui Stream; contributing subcatchments

Reach No.

Location Contributing

Subcatchments Contributing Areas

(ha)

1 Worsley Street to Wheatstone tributary; (100m downstream of Murdoch Road)

Tamarau 166

2 Wheatstone tributary to tributary from Sponge Bay subdivision.

Ditto plus:

22% of Wheatstone

193

3 Tributary from Sponge Bay to tributary 40m downstream of Wheatstone Road.

Ditto plus:

Rifle Range and Taumotu.

307

4 Tributary 40m downstream of Wheatstone Road to tributary 300m downstream of SH 35.

Ditto plus:

78% Wheatstone

405

5

Tributary 300m downstream of SH 35 to tributary 70m upstream of Murphy Road.

Ditto plus:

68% Coastal and

50% Lloyd George

497

6

Tributary 70m upstream of Murphy Rd to the sea. Ditto plus:

32% Coastal and

50% Lloyd George

563

Note that the “contributing areas” in the above table are as measured at the downstream end of each reach.

4.0 Previous relevant reports

There are a number of relevant reports regarding the Wainui Stream, its tributaries, and reticulated stormwater systems. There are reports on rainfall events which have impacted on the catchment, in particular the flood of 21st June 1977; and also reports on the potential impact of the recent Sponge Bay subdivision development on flood levels both within the subdivision and in the Wainui Stream downstream of the development.

These reports contain valuable information and have been utilised as much as possible, however the data, results and conclusions of these reports have been taken at face value, except where noted elsewhere.

The reports, three of which have been written by the author, have been arranged in chronological order and are summarised below. Full copies of these reports may be seen in the appendices in the CD at the back of this report.

• Storm of 21st June 1977; Rainfall Frequency Analysis; NWASCO report to the Chief Engineer, Poverty Bay Catchment Board; 25th January 1978.

This report analyses the frequency of occurrence of the 21st June 1977 storm. This was an extreme event, centred at Glenroy Station about 8 kms north east of Gisborne, where 444 mms of rain was recorded. The Meteorological Office gauge at Gisborne airport recorded 218mms rain over the (27 hour) storm period, with about 240mms falling in the Sponge Bay area.



Large areas of Gisborne city were inundated and many houses flooded during this storm.

The report compares the rainfall intensities during this event with the theoretical 10 minute to 24 hour rainfall totals for storms of different return periods, based on frequency analysis of 1948 to 1977 data at Gisborne airport.

In the report summary it is described as follows: ……”The 1977 storm was an extreme event with a return period in excess of 100 years for the longer durations. However, as the storm was of the thunderstorm type and not of the usual orographic type, it is quite likely that such a storm could occur again at intervals much less then 100 years, but in some other part of the Gisborne region”.

• Urban Subcatchments Stormwater Study; Contract EW 94/17: Tamarau Catchment; Beca Steven, March 1995.

This report was commissioned by Council in a brief dated 27 April 1994. The report by Beca Stevens was to investigate stormwater conditions in the Tamarau subcatchment (see Figure 1), and report on solutions to mitigate flooding conditions identified. Extensive flooding adjacent to the banks of the Wainui Stream occurred on 20th and 21st June 1977 during a severe storm.

The Tamarau subcatchment is located at the northern limits of residential development in Gisborne city. This study undertook a review of the hydrological and hydraulic conditions applying at that time on this subcatchment. Analysis of these conditions was carried out to predict flood flows and flood levels at critical points and establish the extent of the existing and future flooding. For the pipe system hydraulics the unsteady state “SWAN” rainfall-runoff model was used.

Recommendations were made for physical works at a rough order of cost of $118,750. Works actually carried out are outlined in the following report.

Tamarau Subcatchment - Project Report; D Atapattu, Design Engineer, GDC; 28 July 1997. This report describes briefly the status (in July 1997) of the upgrading works recommended in the Beca Steven report.

The stormwater upgrading standards were designed to meet the GDC “Level of Service 2” as set out in clause 3.2.1 of the Gisborne District Council Engineering Code of Practice.

The “percent completed” of the physical works recommended in the Beca Steven report are given in this report. It is assumed that these works have since been completed, and include:

• Hill slope drainage works above Lyell Road, Alice Street and Paraone Road.

• Secondary flow paths at Lyell Road (2), Alice Street (2), Matthews Road to Worsley Street, Lorraine Street to Matthew Road; (shown on District Plan maps); and

• Minor water table improvements.

This report has a comprehensive “Maintenance Schedule” for the new works.



• Storm Event of 30th July 2008; Effects on the Sponge Bay subdivision and Wainui Stream; D H Peacock, 27th August 2008.

An extensive storm which affected most of New Zealand on the 30th July 2008, raised water levels in the Wainui stream and through the partially completed Sponge Bay subdivision.

This report was prepared to answer questions concerning what were unexpectedly high flood levels within the subdivision, and (particularly) high tailwater levels in the Wainui Stream downstream of the site.

While the rainfall was theoretically only about a 5 year event over a 6 hour period, and a more frequent event over shorter periods, the runoff was likely to have been greater in magnitude than indicated by the rainfall statistics.

The investigations for this report led to a report on the June 1977 flood in the vicinity of the Sponge Bay subdivision, described as follows:

• Storm Event of 21st June 1977; Effects in the Sponge Bay area and Wainui Stream; D H Peacock; 27th August 2008.

While investigating the 30th July 2008 flood levels in Wainui stream, a long term resident offered to show the author the 1977 flood level adjacent to his house. This appeared to be about 1.5m above the 30th July flood level and was obviously of major significance with respect to potential house floor levels in the Sponge Bay subdivision which was under construction at that time.

This report summarises the findings on the extent and depth of flooding in the vicinity of the Sponge Bay subdivision in June 1977, and also updates the storm statistics in the NWASCO report based on the NIWA high intensity rainfall distribution (HIRDS) analyses. These statistics show that for rainfall durations of 6 to 24 hours the rainfall at Gisborne airport was at least a “1 in 100 year” event.

As a result of investigations three long term residents of the area provided accurate June 1977 flood marks on their house foundations, which were surveyed by GDC staff. The locations of these levels form a triangle around the perimeter of the subdivision, and the levels were remarkably consistent with each other. Levels were also taken along the centreline of SH 35 which showed that the (present) road would have been underwater over a length of at least 160 metres at the peak of the 1977 event; which is confirmed by anecdotal accounts at the time in the Poverty Bay Herald.

Finally, comment is made in this report on why the runoff in this storm filled this basin to such high levels.

This report, together with the peer review by the author, led to a review of the flood hazard to the Sponge Bay subdivision.

• Peer review of the potential effects of the proposed Sponge Bay subdivision on flood levels within the subdivision, and in the Wainui Stream from the subdivision to the sea; D H Peacock, 11th February 2009.

This peer review, to assess whether the design proposed by MTEC Consultants Ltd of Tauranga complied with specific GDC resource consents, was commenced in June 2008. However the scope of the review was widened to investigate the unexpectedly high water levels within and downstream of the subdivision as a result of the 30th July 2008 storm, and subsequently the scope was widened again to include a review of the June 1977 flood and the potential impact of a flood of the same magnitude in the subdivision and downstream.



Following the discovery of the 1977 flood levels in the Sponge Bay area the applicant was requested by GDC to consider how a storm of the same magnitude as the 1977 event could affect the subdivision and what changes would be necessary to protect the subdivision houses from flooding. Subsequently, a proposal was submitted by MTEC to increase the size of the proposed overflow culvert, but this was not practically possible so eventually this design was replaced with a grass lined open channel overflow.

With the (fortunate) discovery of the exceptionally high flood levels reached by the June 1977 flood in the Sponge Bay area, this subdivision was re-designed while under construction to provide adequate protection to houses, while at the same time not significantly raising downstream flood levels.

• Comment on Resource Consent application for construction of a swale drain: Sponge Bay Development Wainui Road; G Tayler, GDC; 1st April 2009.

This report comments on the consent application for a swale overflow drain from the Sponge Bay development. Note that it is not an application for the Sponge Bay Development itself, which was the subject of a consent application in early 2008.

The report investigates the origin of the requirement for the swale overflow drain, and concludes that the application of Regional Rule 5.22.1 should apply to this area, and para (b) of this rule requires that house floor levels are to be 200 mm above the 1977 and/or 1985 flood levels. The report states that: “The application of this rule has the effect of increasing the compliance level from the 100 year flood to something between a 200 and 1000 year flood and it was the resulting increase in flood levels which made it necessary to have the swale overflow”.

The report concludes that the effects of the development on the downstream stormwater system resulting from the 100 year storm is not a significant issue.; and that additional inundation from the effects of the development from a flood equal in magnitude to the extreme 1977 flood should be acceptable if the table in 3.2.1 of the GDC Engineering Code of Practice is used as the standard.

On the assumption that if other developments in the Wainui Stream catchment are allowed on the same basis as this one, then the GDC cannot assume that the consequences will continue to be insignificant. Hence, in a covering letter to the report, five recommendations have been made to protect existing and future assets from flooding. These recommendations are considered elsewhere in this report.

• Wainui South Stormwater Catchment Study; Opus International Consultants; July 2009. Opus International Consultants have undertaken this conceptual study to review the existing catchment and stormwater system and to propose a drainage system for a fully developed catchment, to comply with the GDC “Level of Service 2” (LOS 2) stormwater drainage criteria.

The Wainui South catchment (of approximately 29 hectares), is located between Wainui stream to the north, Murphy Road to the east, Tuahine hills to the south, and the end of Lloyd George Road to the west.

The existing stormwater system is considered inadequate in terms of present day design standards and does not meet LOS 2 requirements. Shallow flooding, high groundwater levels and poor drainage particularly during the winter months, have apparently been a feature of this area for many years.



The Opus report has investigated four possible drainage upgrading options, the preferred one being “Option 3”. Option 3 is a conceptual design which proposes four main piped drains (beneath swales) in the north-south direction, and open cut-off drains in the east-west direction to intercept stormwater off the hills to the south.

It is noted that the GDC Utilities section are now proposing new piped drainage along the south side of (and between Nos. 38 and 60) Lloyd George Road, and under Lloyd George Road; which is a variation of the Opus option 3 proposal.

• Wainui Stream Catchment Study; Questionnaire form results; J Ruifrok, GDC Utilities Engineer, Stormwater; Jan/Feb 2011.

This is a spreadsheet comprising the results of 27 questionnaire returns from residents of Lloyd George Road, Wainui Beach; regarding answers to questions from a survey undertaken in January and February 2011 about the 1977 flood, and how properties are affected by stormwater flooding.

Note that due to privacy issues, only a summarised version of the spreadsheet is available to the public, and is available on request from the GDC.

5.0 Stream capacity upstream of the SH 35 culverts

REACHES 1, 2 and 3: Reaches 1, 2 and 3 of the Wainui stream have been widened and generally improved by the GDC circa 1995, as shown on GDC drawings SW 337. It is known that the improvement works were designed to meet the level of service 2 (LOS 2) requirements as set out in the GDC Engineering Code of Practice.

Unfortunately no report on these improvement works has been found to date, however it is understood by the author that the channel has been designed to accommodate a “1 in 20 year” rainfall event with freeboard.

At the time of the Tamarua catchment report by Beca-Stevens, (March 1995), these works were either under construction or were imminent, so that the SWAN stormwater modelling carried out by Beca-Stevens was based on the design plans for the Wainui stream upgrade.

The following comments therefore apply to the stream and structures in the stream channel between the SH 35 twin culvert (downstream of Wheatstone Road), to Heath Johnston Park, (reaches 1, 2 & 3), as set out in the design plans SW 337.

Whilst the stream bed and banks can be generally restored to the 1995 design profiles and cross sections, and obstructions can be removed; there are three road culverts which would be expensive to replace or duplicate if required to increase stream capacity. Provided that head losses at these culverts do not exceed reasonable limits for a specific design flood, then the channel can readily be improved if need be to convey this maximum flow limited by the culvert hydraulics.

Since the Wainui stream has been calibrated to the record June 1977 event and modelled for a hypothetical “1 in 100 year” rainfall event by Beca-Stevens, it is useful to assess and compare the discharges and levels for these floods over this reach of the stream.



The June 1977 flood was an extreme event, much more than a “1 in 100 year” flood for some rainfall durations. Flood levels for the record June 1977 flood are available or have been derived at some of the culverts. The “100 year” flood modelling carried out by Beca-Stevens in 1995 was based on a “1 in 100 year” rainfall of one hour duration, with an intensity of 49 mm per hour.

In the reach between Murdoch and Wilson Streets the peak flow of the 1977 flood has been estimated in the Beca-Steven report to be approximately double that of the (1995) “100 year” rainfall event.

The capacity of the Wainui stream and channel structures upstream of the SH 35 culvert are described in more detail, from the downstream end first progressing upstream; as follows:

REACH 3: • SH 35 culverts: Under SH 35 downstream of Wheatstone Road, are two 1800 mm diameter concrete culverts. It has been estimated in the Sponge Bay subdivision peer review that in the June 1977 flood the discharge through both culverts totalled approximately 22 cumecs. However, a flood of the same magnitude today would discharge about 16 to 17 cumecs, the remainder flowing via the overflow channel from the new subdivision into Wainui stream downstream of the twin culverts. Even at this lesser flow the stream channel between Wheatstone Road would be in high flood, with floodwater depths up to about 3.5 metres deep.

These two culverts, being significantly smaller in diameter than all the culverts in the stream channel upstream as far as Heath Johnston Park, could be in danger of blocking or partially blocking during a major flood event. This possibility needs to be addressed by ensuring that floatable debris on the banks and berms upstream of the twin culverts are periodically removed.

• Wheatstone Road culvert; The Wheatstone Road culvert is a 2.6m diameter corrugated steel culvert. This culvert also conveyed very high flows during the 1977 event, with heading up to a level some 1.4 metres above the upstream soffit. In the event of a flood equal in magnitude to the 1977 event, the heading up would be somewhat less now due to the Sponge Bay subdivision overflow channel.

REACHES 2 and 3:

• The stream between Wheatstone and Murdoch Roads:

Over most of its length in this reach the Wainui stream flows through undeveloped rural land, and is generally in good condition. Where the channel has been confined due to bank slumping or encroaching vegetation, this can be easily remedied and restored to the 1995 design parameters.



REACH 1:

• Culverts at Murdoch Road and Wilson Street, and the reach between; These are 2.1 metre diameter concrete culverts under both roads. Modelling results from the Beca-Stevens Tamarau catchment report show some 5.5 cumecs being conveyed by these culverts at the peak of the one hour “1 in 100 year” design rainfall event. Assuming that the channel and banks between Murdoch and Wilson streets are restored to the profile and sections as in the GDC design drawings SW 337, the channel is capable of conveying this flow with some minor bank overflows and a small amount (up to 300mm) of heading up at the upstream end of the two culverts.

Flood levels between the two culverts could be expected to be between 9.0 and 9.5m RL for the “1 in 100 year” rainfall event modelled. Since the channel is quite incised in this reach, even if the banks overtop the floodwaters should still be confined to the sloping berms.

In terms of the 1995 design thought to be based on a “1 in 20 year” rainfall event (with a freeboard allowance), at least this standard should be achievable by the removal of obstructing bridges and fences, and restoration of the channel and banks to its design parameters.

• Heath Johnston Park to Worsley Street: This reach of the stream is in good condition with few if any obstructions; however the channel width may be less than design in places. This probably does not matter since only parkland is likely to be flooded, but near the two culvert outlets under Worsley Street an undersized channel could raise tailwater levels and hence raise levels upstream of the culverts; however this can be easily remedied.

The hinged grates at the outlets to these two culverts could potentially cause major obstructions if not opened prior to a major rainfall event. It is understood that there is a robust system in place to ensure that the grates are opened in time.

• Summary: In summary, restoration of the Wainui stream channel and banks between the SH 35 culverts and Heath Johnston Park to the 1995 design parameters should convey runoff from a “1in 20 year” rainfall event with freeboard, or runoff from a one hour “1 in 50 year’ rainfall event with only minor bank overtopping, and meet the LOS 2 criteria in the GDC Engineering COP.

Under current stream conditions it would be very hard to say with any certainty what the capacity would be because of the nature of the obstructions, nor would it be realistic to model it. However, in the opinion of the author I would suggest that the capacity would most likely be less than two-thirds of the 1995 design capacity.



6.0 Floodplain hazard mapping based on the June 1977 floodspread

The extreme June 1977 rain storm inundated much of the Wainui stream floodplain, as described by residents and from more recent surveys. From this information it is possible to assemble an approximate floodspread map, however there has been one significant change since the 1977 event which would modify the floodspread if exactly the same storm as on the 21st June 1977 was to happen again today; this being the provision of an overflow channel and the change in land contours in the Sponge Bay subdivision, (2008/2009). Both of these changes will have some local impact on the floodspread and hence any hazard map.

While improvements to the Wainui stream upstream of the SH 35 culverts had been made (circa 1995), subsequent encroachment of vegetation, together with channel obstructions, have reduced the effectiveness of this work.

The June 1977 floodspread within the Gisborne city boundaries has been mapped and used as the basis of flood hazard overlays for the District Plan; however when this mapping was carried out in 1991 the area to the east of the city boundary was predominantly rural with no significant development envisaged at that time. However the situation has changed over the past ten or more years, with a number of small subdivisions in the Lloyd George Road area and the major Sponge Bay subdivision.

The author of this report is no doubt that had the development to the east of the Gisborne urban area been envisaged in 1991, then the hazard planning would have been extended to cover all the low lying floodplain to the east of the city. A draft flood hazard plan has therefore been prepared for this area for the purpose of discussion; see Figure 2.

In this draft flood hazard plan, almost all the low lying areas of floodplain which were either fully or partially flooded in 1977 are included in “Flood Hazard Overlay 7” described in the District Plan as “Urban Stormwater Flood Hazard Area”; with the remaining areas being in either flood hazard overlays 8 or 9. Ground contour information from the Sponge Bay subdivision as-built plans, has been used to modify the flood hazard zonings within the subdivision itself; (Figure 3).

In this report (Revision No. 1), the area of Flood Hazard Overlay 7 in Figure 2 has been reduced substantially, by deletion of this (proposed) overlay north of Lloyd George Road. However, a narrow strip of F7 overlay has been added to both sides of the F9 overlay along the lower Wainui Stream to allow for potential blockages in the Murphy Road culverts.



7.0 Wainui stream between SH35 and the sea; reaches 4, 5 and 6

Downstream of the SH 35 twin culverts, in reach 4, the Wainui stream meanders through pastoral land for about 300 metres until it meets a tributary from the “Coastal’ subcatchment. Downstream of this confluence (reach 5), the stream is shrouded from view by rank vegetation and flood debris in and adjacent to the channel. In places the vegetation is so dense that it is difficult to follow the stream.

During even moderate flood events the stream is badly obstructed and the lower lying area in the vicinity of the confluence with the tributary from the Coastal subcatchment is prone to flooding.

A “LIDAR” survey shows that the stream has ample fall over most of its length, except where it meets the tidal influence of the sea at the downstream end of reach 5, (about 160m upstream of the Murphy Road culverts).

For the 540 metres long heavily vegetated reach 5, the stream is well incised and the channel gradient is generally between 3 and 4 metres per kilometre. The more tortuous bends in this reach of the stream need to be re-shaped, and a proper channel with a bottom width of about a metre is required. Because of the naturally incised floodplain, a composite channel/floodway would be most suitable and cost-effective for this reach. This would comprise a channel up to about one metre deep with side batters of 1:1 (or steeper); and above this 2 (H) to 1 (V) or flatter grassed berms. On the stream bed gradients available, this should provide ample capacity for floods at least up to the “1 in 50 year” frequency, with a total channel/floodway width of approximately 10 metres.

With only LIDAR survey information available there is insufficient detail to determine the volume of channel material that would require excavation, but a budgeting figure to clear a floodway (which would include a maintenance track) and improve the channel has been estimated at about $30,000.

To accommodate potential extra capacity to allow for future catchment development and climate change, all that would be required is to widen the floodway, however this would probably not be required for many years. Provision could be made for this by designating the floodway including the extra width, in the District Plan.

8.0 The effects of climate change

The most recent guidance manual (May 2010) produced by the Ministry for the Environment (MfE) for use by Local Authorities to estimate the effects of climate change on catchment runoff, has been used for this study. The mid-range “scenario A1B” has been selected from Table 3, Chapter 3.2.2 of the Intergovernmental Panel on Climate Change (IPCC) document; which predicts 0.7 C and 1.4 C increases in annual mean temperature for Gisborne, for the periods 1990 to 2030 and 1990 to 2060, respectively. This has been adjusted to a 0.4 C increase for the twenty year period 2010 to 2030; and adjusted to a 1.1 C increase for the fifty year period 2010 to 2060.



As recommended in the Opus report “Development of Design Rainfalls for Gisborne District”, May 2010; the Gisborne Aero design rainfall tables have been used in this report for the year 2010; modified as follows for the effects of climate change.

The latest NIWA High Intensity Rainfall Distribution model (HIRDS V3) has been used to derive the projected rainfall figures for the “1 in 100 year” storm increased by the annual mean temperature rises of 0.4 C and 1.1 C expected by the years 2030 and 2060 respectively.

The 2030 rainfall depths and intensities average 3.25% more than the current statistics, while the 2060 rainfall depths and intensities average 8.8% more than current statistics; for all storm durations.

The Opus figures for 2010 have been increased by the same amounts for the years 2030 and 2060, as in Table 3:

Table 3:

Gisborne Aero Design Rainfall Table 2010, 2030 and 2060

Year ARI (y) 10m 20m 30m 60m 2h 6h

2010 100 16.0 28.0 34.8 45.6 70.3 111.4

2030 100 16.5 28.9 35.9 47.1 72.6 115.0

2060 100 17.4 30.5 37.9 49.6 76.5 121.2

“100 Year” rainfall intensities (mm/hr) – for durations from 10 minutes to 6 hours

Year ARI (y) 10m 20m 30m 60m 2h 6h

2010 100 96 84 69.6 45.6 35.2 18.6

2030 100 99 86.7 71.8 47.1 36.3 19.2

2060 100 104.4 91.5 75.8 49.6 38.2 20.2

The effects of climate change on sea level rise, and the implications of this at the mouth of the Wainui Stream, are discussed in section 10.

9.0 Future catchment conditions

9.1 Projected population changes and changes to average household size Future catchment conditions will largely be driven by future development, which in turn will be driven by changes in population and average household size.

This section of the report is derived from the GDC draft background report: “Population, Households, Gross Floor Area and HUE Projections”; March 2011. The projections in this report are largely based on Statistics New Zealand population and household estimates and projections.

The population in the Gisborne Urban Area (which includes the Wainui catchment) is projected to remain fairly constant at around 34,000 for the projection period to 2051.



The number of households in the Wainui area is projected to increase at an average rate of 4.3 households per year, from 654 in 2011 to 825 in 2051. In the Tamarau area (which is in the Wainui catchment), the average rate of increase in households is even slower (only one every 3 years).

Growth is fastest at the beginning of the period and slows each year. The increase in the number of households is primarily due to the projected decrease in the average household size from 2.7 persons per household in 2006 to 2.4 persons per household in 2031. This reflects an increasing proportion of one- person households and a decrease in the average size of family households. These trends, in turn, are largely driven by the ageing of the population.

9.2 Present and future runoff Designing an open channel for runoff from a “1 in 100 year” rainfall event does not necessarily result in a channel capable of conveying a “1 in 100 year” flood discharge. This is because of a number of factors which influence how much of the rainfall runs off the land for any given storm.

The only accurate method of determining flood frequency is from analysis of a long series of annual maximum flood discharges, and this type of record is only available for major rivers (eg the Waipaoa). All other methods of determining runoff, such as the rational method, the regional runoff frequency (RRF) method and rainfall/runoff modelling, are all runoff estimates.

However it can be said that runoff is approximately proportional to rainfall intensity and the ratio of impermeable to permeable ground cover, and therefore some subjective observations can be made about trends in future runoff due to climate change and land development.

9.3 Impact of climate change on runoff Rainfall depths and intensities have been estimated in chapter 8 to potentially increase by 3% over the next twenty years, and by 9% over the next fifty years. The runoff will be expected to increase by at least these percentages, because the runoff coefficient is also expected to increase due to the increased rainfall intensities. According to the mid-range IPCC scenario adopted for this study, in fifty years time (2060), what is now considered to be a 1 in 100 year rainfall event will become (about) a 1 in 70 year event.

9.4 Impact of development on runoff There are two main impacts of land development on runoff. The first is the increase in the ratio of impermeable to permeable ground cover, and the second is the inevitable straightening and widening of ditches and streams, or replacement with reticulated stormwater systems.

Dealing with the impact of land development on ground permeability first, land which is now zoned as “Rural Residential” (up to two dwellings per hectare), could in the future become “Rural Lifestyle” (up to two dwellings per 5000m2); or even become “General Residential” (up to 25 dwellings per hectare). These changes bring changes in the ratio of impermeable to permeable ground cover, but the heavier the soil type the less the percent change.

The Wheatstone Road subcatchment has been selected as an example of what impact increased development is likely to have on future runoff. The Wheatstone Road subcatchment has been chosen as it is largely undeveloped at the present time. This subcatchment comprises some 70% moderate to steep hill country on which some very low density development has occurred over recent years on the easier slopes and ridges.



There is also about 8% of the subcatchment in the valley floor already developed (under the Rural Residential rules), with the remaining 22% in pasture.

A “composite” runoff coefficient “C” of 0.44 for the subcatchment has been calculated assuming a heavy clay soil type and allowing for the present development.

The very gradual population changes and changes in average household size described in 9.1 are essentially an extrapolation of past population and average household size trends into the future. However in this report a much more rapid rate of development has been assumed, as follows:

At the year 2030 it has been assumed that development has proceeded to the full extent allowable under the present District Plan. It is assumed that some more low density development on the moderate to steep land will proceed but this will have an insignificant impact on runoff except for localised erosion issues. The remaining 22% now in pasture is assumed to be fully developed as far as Rural R rules allow. The recalculated composite “C” value is now 0.45. This represents an increase in runoff of only about 2%.

At the year 2060 it has been assumed that rezoning to “General Residential” has been allowed on the valley floor, and 50% of the valley floor has up to 35% site coverage, and the other 50% has 35 to 50% site coverage. The composite C value for the whole subcatchment is now 0.465; an increase of about 6%.

The increase in peak runoff due to the straightening and widening of ditches and streams potentially reduces the time of concentration for a catchment, and an exercise in the Wheatstone Road subcatchment assumes that this straightening and widening happens over most of the length of the flat valley floor. It has been estimated that the time of concentration could be reduced by about 20 minutes and therefore peak flood discharges from the subcatchment will be expected to increase by about 14%.

In summary it can be said that “improvements” to existing streams and drains in this subcatchment at least, will have potentially a greater impact on subcatchment peak runoff than increased urban densities, and at least a similar impact as climate change as far as the year 2060.

10.0 Impact of future changes at the Wainui Stream mouth

The effects of climate change are expected to increase peak flows in the Wainui Stream by at least 9% over the next fifty years, assuming the IPCC mid-range scenario.

In addition to this there is expected to be a small increase in discharge (of up to about two cumecs) at the Wainui stream mouth, but only in major flood events, due to the operation of the new overflow channel at the northeast corner of the Sponge Bay subdivision.

Because future development within the Wainui stream catchment is expected to be “stormwater neutral”, as required by Chapter 8 of the District Plan, there should be little if any significant increase in peak stormwater runoff in the stream up to the 10% AEP event, and only a small increase for storms greater than the 10% AEP.



As a result of future increased discharges at the Wainui stream mouth there could be expected to be a small increase in scour at the mouth (between Murphy Road and the sea); however this would be short lived as the peak discharge would be transitory and wave action from the sea would start to infill any channel as the flood subsides.

The above can be put into perspective by comparing with the normal seasonal movement of sand at any beach; onshore in the summer and offshore in the winter. At Wainui Beach on average some 250,000 cubic metres of sand moves onshore and offshore each year. Over the 4kms of beach this represents some 60 cubic metres of sand per metre length of beach, and at the mouth (between revetments) over 1000 cubic metres of sand is transported onshore and offshore quite naturally by wave action each year.

In the vicinity of the stream mouth, potential sea level rise could well have a more noticeable effect than any increase in peak flows due to climate change or future development.

Mean sea level at Port Gisborne has been accurately measured for the period between 1926 and 1991 by A Radcliffe (former Director of Lands & Survey, Gisborne). For this period the average rate of relative sea level rise is 2.3mm per annum, (which is somewhat faster than the national average of approx 1.6 mm/a).

In Table 1 of the guide for Local Government in New Zealand; “Preparing for Coastal Change”, MfE; March 2009; figures of 150mms and 280 mms for sea level rise to the years 2035 and 2060 (above the 1980 to 1999 average) respectively, are recommended for planning purposes. This equates to average rates of sea level rise of 3.3 and 4 mm/a respectively; which is 2 to 2.5 times the rate for most of the 20th century.

In the opinion of the author the effect of future development at current rates is unlikely to have significant impact on stream flows, while the effects of climate change will be minor but significant with increase in peak discharges in the stream in the order of about 10 to 15% over the next fifty years. Sea level rise at projected rates could also be expected to have a significant impact in the estuarine part of Wainui stream especially by the year 2060. The river mouth area will become more susceptible to flooding from the sea as well as becoming more exposed to coastal erosion.

At present there are sometimes logs and other floatable debris in the estuarine reach of the stream, some of which has come from the sea and some of which from further upstream. Potentially these logs could block or partially block one or more of the culverts under Murphy Road. It would be relatively inexpensive to erect a floating barrier downstream of the Murphy Road culverts to prevent floating debris and logs from being driven upstream from the sea, however the source of logs and branches from upstream of Murphy Road needs to be removed first.



11.0 Whole of Catchment Strategy

The Wainui Stream catchment comprises both moderate to steep hill country and alluvial floodplains, with the largest subcatchment (Tamarau) with at least 60% fully developed residential area. This diversity, both in landforms and level of development, makes it particularly challenging to develop a viable whole of catchment strategy.

There are two main aspects to any proposed strategy, these being a strategy for future development and a strategy for reducing the adverse effects of the existing development. In many ways the strategy for future development is both much less limited and easier to implement than dealing with existing developments; and therefore this is discussed first.

11.1 A strategy for future development In terms of stormwater runoff, viable catchment strategies can be developed for the largely undeveloped subcatchments to mitigate any downstream adverse effects from future development. Chapter 8.13.4 of the District Plan already requires “stormwater neutral” runoff from proposed developments at least to the 10% AEP level; while policies in Chapter 8.8 of the District Plan prefer low impact design approaches to stormwater management where there is a need to:

• Protect or enhance natural heritage and amenity values associated with receiving environments such as waterways and the coastal environment;

• Improve the quality of stormwater discharges;

• Reduce peak flows associated with additional runoff.

More adequate control of subdivision, by the early imposition of stormwater management plans as recommended in the District Plan, will have an important part to play in the future, and should largely prevent the adverse effects of ad-hoc subdivision.

A stormwater management plan for any part of a subcatchment can be designed to negate the adverse effects of speeding up stormwater runoff by ensuring that existing watercourses are maintained in defined meandering courses, with retention ponds or wetlands included if so required to moderate peak flows. Potential watercourses, retention ponds or wetlands can be protected as easements or as reserves before development physically proceeds.

Reforestation is another means of reducing peak stormwater runoff, at the same time as providing protection against erosion. Some of the steeper hill country which is potentially at risk from gullying or landsliding may best be retired into native or exotic forests, or soil conservation plantings established.

It is of interest to note that on the coastal ridge at Okitu, development there has greatly increased the tree cover from what it was when it was pastoral land. Native forest in particular would provide more diversity to the present landscape as well provide habitat for native birds, and if a continuity of native plantings could be encouraged along the coastal ridge line between the Wheatstone subcatchment and

Wainui Beach, a “native bird corridor” could be established.



Water quality issues in the catchment are outside the ambit of this report. The environmental health section of the Gisborne District Council monitors six sites on the Wainui stream and tributaries downstream of the Sponge Bay subdivision, and report on water quality matters.

However it can be said that maintaining (or improving) water quality in areas of new development would be more difficult then maintaining stormwater neutral runoff in the future.

Although present Rural Residential or Rural Lifestyle zoning allows for only relatively low density development, watercourses can potentially be more at risk from point source contamination than in a purely rural environment. The corollary to this is that present pastoral use is more likely to lead to groundwater contamination via fertilizer application and from livestock.

The policies in Chapter 8.8 of the District Plan which prefer “low impact design”, (see above), should reduce the potential for watercourse pollution from new developments, but it will be much more difficult to reduce watercourse pollution from existing urban development and the road network.

The difficulty in instigating any of the above measures be it by stormwater management plans, stormwater retention ponds or reforestation, is that the land is privately owned and each landowner will have his/her own aspirations as to how they wish the land to be used or developed. The key issue is therefore for the Gisborne District Council to start looking at each subcatchment in detail to identify areas which are most likely to be developed in the near future, and start dialogues with landowners in these areas.

Fortunately the rate of development is not expected to be rapid given the static population and the surplus of available sections to the east of Gisborne at present, so that in most cases Council should have ample lead in time to start consultation prior to the development of stormwater management plans.

11.2 Strategy for reducing impact of existing development: With the roading and housing infrastructure already in place in the Tamarau subcatchment there is little that can be done to reduce runoff; however water quality could be improved at little cost through advocacy and education. There are programmes such as those at the Rethink Environmental Education Centre which educate school groups and others in the safe disposal of waste water, and the Wainui stream could be made the subject of a school environmental project.

12.0 Future works and maintenance

Capital works (or deferred maintenance works) which are recommended to be carried out in the near future, subject to GDC budget constraints, are:

• Lloyd George Road south area: The area between Lloyd George Road and the hills to the south has an unsatisfactory ad-hoc drainage system, which is in urgent need of a comprehensive upgrade. Capital drainage improvement works such as “Option 3” in the Opus report “Wainui South Stormwater Catchment Study” are urgently needed to improve drainage and reduce the flood risk.



• Wainui Stream Reaches 1 to 3; Worsley Street to SH 35 culverts: Major channel and berm works are required here to restore these mainly urban reaches to meet GDC level of service 2 (LOS 2) criteria. (Cost of these works subject to survey).

• Wainui Stream Reach 5: Stream upgrading works and associated berm clearance required over this 540 metre reach of the Wainui Stream; cost subject to survey.

Maintenance work schedules: Two maintenance work schedules are attached as Appendices 1 & 2 to this report, as follows:

• Appendix 1: Maintenance Schedule Part 1: Wainui Stream and Tamarau hill slope drains. This is a schedule of inspections and maintenance works for the various reaches of the Wainui Stream, and for hill slope cut-off drains at the back of properties in Lyell Road, Alice Street and Paraone Road.

• Appendix 2: Maintenance Schedule Part 2: Urban Floodway Areas. This is a schedule of inspections and maintenance works for other urban floodway areas

(FH 9 overlays) not covered in schedule 1. All but one of these FH 9 areas are shown on the maps in the District Plan.

13.0 Conclusions

• The 1995 Wainui Stream upgrade in reaches 1 to 3 was designed to meet level of service 2 (LOS 2) criteria, (see Appendix 3); however due to the encroachment of vegetation and obstructions to the channel and berms, the capacity in reach 1 is now considered to be about two-thirds of the 1995 design capacity.

• Had the development to the east of the Gisborne urban area been envisaged in 1991, then the urban flood hazard overlays would have been extended to cover much of the low lying floodplain to the east of the city, as shown on the draft flood hazard plans in Figs 2 & 3.

• The area of Flood Hazard Overlay 7 in this (revision No. 1) report has been reduced substantially by deletion of the (proposed) overlay north of Lloyd George Road .

• Reach 5 is badly obstructed and during even minor flood events the lower lying area in the vicinity of the confluence with the tributary from the Coastal subcatchment is prone to flooding. However there is plenty of fall available over this 540 metre reach so that even allowing for the effects of climate change and development, channel and berm improvements can be designed to meet the requirements in item 2.5 of the Scope of Works.

• A mid-range IPCC climate change scenario predicts rainfall depths and intensities some 3% to 9% more than current statistics for the years 2030 and 2060 respectively, and stormwater runoff is expected to increase by at least these percentages.



• Because future development within the Wainui stream catchment is required by the District Plan to be “stormwater neutral”, there should be little if any significant increase in peak stormwater runoff in the stream up to the 10% AEP event, and only a small increase for storms greater than the 10% AEP.

• With respect to future development, at least in the Wheatstone subcatchment, straightening and deepening existing watercourses is expected to have potentially a greater impact on downstream subcatchment peak runoff than increased urban densities, and a similar impact as the effects of climate change (as far as the year 2060).

• A stormwater management plan for any part of a subcatchment can be designed to negate the adverse effects of speeding up stormwater runoff by ensuring that existing watercourses are maintained in defined meandering courses, with retention ponds or wetlands included if so required to reduce peak flows.

• Reforestation is another means of reducing peak stormwater runoff, at the same time as providing protection against erosion. Owners of the steeper hill country which is potentially at risk from gullying or landsliding should be encouraged to retire these areas into native or exotic forests, or stabilise these areas with soil conservation works.

• The revegetation of the coastal ridge at Okitu shows that this could also be possible along the coastal ridge further south, and possibly a ‘native bird corridor” could be established with the support of the various land owners.

• More adequate control of subdivision, by the early adoption of stormwater management plans will have an important part to play in the future, and should largely prevent the adverse effects of ad-hoc subdivision.

• Given that the rate of future development in the Wainui catchment is not expected to be rapid, in most cases Council should have ample lead in time to start consultation with landowners prior to the development of stormwater management plans.

14.0 Recommendations:

The following list of recommendations is in order from the highest to the lowest priority:

1. Reaches 1 to 3 of the Wainui Stream be restored to their original (LOS 2) design capacity.

1. Flood hazard overlays for the Wainui Stream floodplain (as shown in draft in figs. 2 & 3) be prepared for consultation and inclusion in the District Plan.

2. A comprehensive land drainage scheme (along the lines of “Option 3” in the 2009 Opus report) be constructed in the low lying area between Lloyd George Road and the hills to the south.

4. Reach 5 of the Wainui Stream be cleared of obstructing vegetation and improvements made to the channel and berms, to the convey the design flows and limit flood levels as set out in clause 2.5 of the Scope of Works.



5. After consultation with the community, stormwater management plans should be prepared for the Wheatstone subcatchment and the undeveloped area between Sponge Bay Road and Lloyd George Road, before any further development proceeds.

Prepared by: D H Peacock BE (Civil); MIPENZ. Director Peacock D H Ltd

10th May 2012.



APPENDIX 1: Maintenance Schedule Part 1: Wainui Stream and Tamarau Hill Slope Drains

The following is a schedule of inspections and maintenance works for the Wainui Stream, and hill slope drains within the Tamarau subcatchment. Note that almost all the inspections and maintenance works will be on private property, and notice should be given before entering properties.

The various reaches of the Wainui Stream are shown in Figure 1 in the report.

The Tamarau subcatchment hill slope drains are as follows; • Lyell Road hill slope drain; GDC Dwg. No. SW 362/1 & 2. • Alice Street hill slope drain; GDC Dwg. No. SW 355/1 to 3. • Paraone Road hill slope drains; GDC Dwg. No. SW 356.

NB: This is not a specification, and all works and procedures should conform to GDC operational requirements, and all sprays applied to be approved for the purpose intended.

Note that in reach 5 maintenance work will not be possible until at least a track is cleared for access.

• Inspections: This will include regular property by property inspections of the stream from the mouth to Worsley Street, and hill slope drains within the Tamarau subcatchment. Frequency of inspections to be:

i) Reaches 1 to 5: Within 5 working days of any significant fresh or flood, and a minimum frequency of 6 monthly inspections thereafter.

ii) Reach 6: Within 5 working days of any significant fresh or flood, or significant storm at Wainui Beach; and a minimum frequency of 6 monthly inspections thereafter.

iii) Tamarau hill slope drains: Visual inspection of all hill slope drains and grates within 5 working days of any significant rainfall event, and a minimum frequency of 3 monthly inspections thereafter.

• Removable of floatable and other material: Remove and dispose of any loose (or potentially loose) or floatable flood debris, rubbish, hedge trimmings, branches or timber from the channel and berms up to at least the 50 year flood level.

Remove and dispose of any overhanging dead or damaged branches which appear likely to fall into the channel or onto the berm within the next 6 months.

Frequency:

i) Reaches 1 to 5: Within 10 working days of any significant fresh or flood, and a minimum frequency of 6 monthly thereafter.

ii) Reach 6: Within 10 working days of any significant fresh or flood, or significant storm at Wainui Beach; and a minimum frequency of 6 monthly thereafter.

iii) Tamarau hill slope drains: Within 10 working days of any significant rainfall event and every 3 months thereafter.



• Spraying and trimming: Spray vegetation on invert of stream only during low flow conditions;

Frequency:

Reach 1: Minimum 3 times per year

Reaches 2 to 5; and Tamarau hill slope drains: As and when required to control excessive weed and shrub growth.

Trim grassed banks regularly with weedeater/s

Frequency:

Reach 1: As required so as grass does not exceed 100 mm in length;

Reaches 1 to 5; and Tamarau hill slope drains: As required (depends on access of stock to the banks)

Spot spray weeds/shrubs on banks and berms within defined floodway width. (Note that the defined floodway width in Reach 1 is 4m wide including the channel).

Frequency: Reaches 1 to 5; as required to control vegetation.



APPENDIX 2: Maintenance Schedule Part 2: Urban Floodway Areas

The following is a schedule of inspections and maintenance works for other urban floodway areas (FH 9 overlays) not covered in schedule 1. All but one of these FH 9 areas are shown on the maps in the Gisborne Regional Land and District Plan (GRLDP), and all but one are on private property. Note that notice should be given before entering private properties.

NB: This is not a specification, and all works and procedures should conform to GDC operational requirements, and all sprays applied to be approved for the purpose intended.

The following list shows the location and GRLDP map number of each of these urban floodway (FH 9) areas:

(i) Worseley Street to Matthews Road: From Worseley Street (opposite Ralph Street) through 18 Matthews Road. Secondary flow path above Donnelly’s piped drain. Location shown on GRLDP map No. u69c.

(ii) Lorraine Street to Matthews Road: From the end of Lorraine Street along the boundary of 22/24 Matthews Road. Secondary flow path; see GDC dwg. No. SW 361. Location shown on GRLDP map No. u68c.

(iii) Alice Street to Matthews Road: From the end of Alice Street to the junction of Matthews Road/Paraone Street. On private property, 50 Matthews Road. GRLDP map No. U69c.

(iv) 7 Lyell Road: Secondary flow path (over top of pipe) along northern boundary of 7 Lyell Road. GRLDP map No. U69c.

(v) 23 Lyell Road: Secondary flow path along gravelled driveway. GRLDP map No. U69c.

(vi) Sponge Bay Subdivision: This is a proposed secondary flow path from the north east corner of the Sponge Bay Subdivision to the Wainui Stream. The location may be seen in Figure 3 of the report.

Inspections and Maintenance works: This will include regular property by property inspections of the above secondary flow paths after any significant rainfall event, and at a minimum of 6 monthly intervals afterwards. Maintenance work should include the removal of any obstructions to flow, and the removal and disposal of any loose (or potentially loose) or floatable flood debris, rubbish, hedge trimmings, branches or timber from the secondary flow path and any inlet gratings to the stormwater system.



APPENDIX 3: Level of Service 2 Urban Stormwater Criteria

The following level of service 2 urban stormwater criteria has been reproduced from chapter 3 of the Gisborne District Council: Engineering Code of Practice:

• Existing houses to have a minimum 50 mm freeboard above the “1 in 50 year” flood level;

• House gully traps to be a minimum of 50 mm above the “1 in 50 year” flood level and a minimum 100 mm above ground level;

• Property flooding (lawns/gardens/sheds etc; but excluding minor ponding) to be no more frequent than once in every 2 years on average;

• Street flooding (over extensive lengths; up to 200mm deep in gutters) to be no more frequent than once in every 2 years on average;

• Where streets become flow paths, the product of the floodwater velocity in m/sec and the depth in metres should not exceed 0.4 m2/sec in a “1 in 50 year” flood event.


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