Sawfly

On this webpage is information relating to the Statement of Proposal for sawfly remediation works proposed in a Statement of Proposal used by Hawke's Bay Regional Council for public consultation from 14 March – 15 April 2005.

You can also source the maps showing the extent of sawfly damage on the Ngaruroro, Tutaekuri and Lower Tukituki rivers, proposed work programme, previous meeting papers of 17 February and 10 March, and required amendments to the LTCCP 2004-14. Financial statistics and the public notice (14 March) of the Summary of the proposal is also available.

On this page

Statement of Proposal for a change to Hawke's Bay Regional Council's Long Term Council Community Plan 1 July 2004 to 30 June 2014 to incorporate sawfly remediation works for the Heretaunga Plains Flood Control Scheme.

• Summary of the Proposal
• Reason for the Proposal
• Details of the Proposal
• Analysis of the reasonably practicable options
• Other matters
• Consultation process and timeline.

1.0 SUMMARY OF THE PROPOSAL

This proposal is to change the Hawke’s Bay Regional Council’s Long Term Community Plan 2004-2014 (LTCCP) to reflect the cost and funding of sawfly remediation works for the Heretaunga Plains flood control scheme and, in particular, flood control measures between the river edge and stopbanks on various parts of the Ngaruroro, Tutaekuri and Tukituki rivers that cross the Heretaunga Plains.

These works are necessary because of the significant damage caused by the willow sawfly to the live edge protection that is predominantly comprised of various species of willows which help control the active channel of all three rivers and help to safely convey a 1% Annual Exceedence Probability (AEP) flood safely to the sea. The damage caused by sawfly to the live edge protection has substantially increased the risk of flooding to significant parts of the Heretaunga Plains because of the risk from lateral erosion of the river and the risk to the stopbank system from breaches which could result.

Details of the work proposed, costs and funding are as follows:

• Structural works comprised of wire rope and steel rail permeable groynes, hard engineering works in those areas that are now at extreme risk of stopbank failure from lateral erosion from the river and extensive planting of a variety of tree and shrub species to help bind both the river edge and berm areas to reinforce the berms and protect them from lateral erosion.
• Costs for the three year remediation works are estimated at $8.8M.
• $6.16M representing 70% of the cost of the works will be met by the targeted ratepayers contributing to the Heretaunga Plains Flood Control Scheme targeted rates and will be funded by loans for $5.5M, redirected capital works budgets and schemes reserves for the balance. This will require an increase in targeted rates for the scheme of $125,000 (10%) in 2005/06 and a further $138,000 (10%) in 2006/07.
• $2.64M representing 30% of the cost of the works will be met by Council’s general funding sources which will be funded: $940,000 from Council’s general funding operating reserves; $500,000 from Council’s general funding disaster damage reserves; and loans totalling $1.2M. These loans will require a rate increase in the 2007/08 financial year of $120,000 which will be an increase in the general rate which is levied on land value across all rateable properties in the region. Based on the general rate at 1 July 2007 this is estimated to be an additional 3.8% increase.

Council is proposing this work programme be completed within these years so the risk of flooding is reduced as quickly as possible and the current level of service provided by the Heretaunga Plains Flood Control Scheme for the Ngaruroro, Tutaekuri and Tukituki rivers is reinstated at the earliest opportunity.

A cautious estimate of the amount of damage that could be created by stopbank breaches in key parts of the Ngaruroro, Tutaekuri and Tukituki river systems is $426M. This excludes any allowance for damage to publicly or privately owned infrastructure as well as the social and emotional damage that would be caused from such flooding. This is the reason for this Proposal and the associated amendment to Council’s Long Term Council Community Plan 2004-2014.

2.0 REASON FOR THE PROPOSAL

The level of flood protection on the Ngaruroro, Tutaekuri and Tukituki rivers has reduced as a result of the sawfly damage. A cautious estimate undertaken by Council indicates that if stopbank breaches do occur in the most vulnerable areas of the flood protection works, flood damage to property (excluding roads and other infrastructure) is likely to be in excess of $426,000,000. If this sort of damage did occur then there would also be significant disruption to the community and additional indirect costs because of that disruption.

Live trees form the first line of flood protection along the rivers of the Heretaunga Plains and are just about exclusively made up of different cultivars of willows. They have been planted because of their extensive root system which help control the lateral erosion forces of the river. A dense and healthy erosion buffer zone of trees must be maintained through ongoing planting and maintenance of that vegetation. Sawfly has now killed or weakened large sections of willow edge protection making the river edge (and ultimately the stopbanks) vulnerable to attack from the river particularly in a flood. Council wishes to reduce the flood risk to the Heretaunga Plains at the earliest opportunity, to the level that was provided by the Scheme works prior to sawfly becoming established in Hawke’s Bay.

3.0 DETAILS OF THE PROPOSAL

3.1 Willow Sawfly

Willow sawfly (Nematus oligospilus) was first discovered in New Zealand in 1997 and was first found in Hawke’s Bay in April 2000. Since then Hawke's Bay Regional Council (Council) has been monitoring the effect of the willow sawfly on willow trees and trialling alternative river edge protection species.

Willow sawfly is a small insect which, as a grub, feeds exclusively on willow trees. It spreads as a mass infestation, and the willows are periodically defoliated, with up to three or more defoliations over a summer season. Experience and laboratory trial studies have shown that up to 90% of the root mass of a willow is lost due to these successive defoliations, leading ultimately to the death of the tree. The continual defoliation of the willow trees gives rise to root die back, and it is the progressive root loss that has the most serious impact on the erosion protection ability of the willows. There is also a loss in the source trees for repair works.

In the summers of 2000/01 and 2002/03 willow sawfly caused a number of defoliations of willow trees along large sections of the Heretaunga Plains Rivers. The summer of 2003/04 was unusually wet, and there was less damage than in the previous two summers. However, there has been a progressive weakening and die-off of the buffer zone willow trees.

Since the discovery of willow sawfly in Hawke’s Bay and the realisation of the potential severe effects of willow sawfly on Council’s river protection assets, Council has progressed a number of initiatives:

a) A research programme on willow sawfly has been expanded and coordinated at a national level. Financial contributions have been secured from the majority of regional councils and unitary authorities to enable the science provider HortResearch to progress a research programme on willow sawfly at an accelerated rate. The aims of the research programme are to:

• Understand willow sawfly lifecycle, habits, influences and constraints.
• Assess the susceptibility of willows to sawfly damage and to quantify the effects. This included an assessment of numerous willow species to determine palatability and effects on sawfly growth and fecundity.
• Investigate alternative tree species, including alternative willow species, to try and identify one that is resistant to willow sawfly, and other species that may provide similar attributes to willows.
• Investigate whether chemical or biological control of willow sawfly will provide effective control.

b) Council has undertaken a number of on-site trials for various methods including:

• Insecticide has been injected into trees with some positive effects. This process is being further assessed to determine its viability for specific high risk areas and areas of new planting, however it is not viable for each of the many thousands of willow trees used for river control on the Heretaunga Plains.
• Willows have been sprayed with a range of insecticides, in a joint project with New Zealand Biosecure who successfully eradicated the Southern Saltmarsh mosquito from Hawke’s Bay several years ago.
• Extensive planting of Japanese willow, which appears to have a natural resistance to sawfly, has been undertaken. This is a shrub willow variety. Its gravel holding ability has yet to be fully assessed, and it appears to be rather brittle which may mean that it is susceptible to damage during a flood event. Further assessment of these attributes will be made over the next few years and advice on the ability to interbreed willow sawfly resistant attributes with less brittle varieties will need to be progressed.

c) Council has maintained damage assessments each year since willow sawfly first arrived in Hawke’s Bay. These show a band of severe sawfly damage around the mid to upper boundary of the Heretaunga Plains. This band appears to be consistent with the temperature and altitude sensitivity of where sawfly will establish and develop eruptive phases.

d) Alternative tree species including alder, acacia and poplar have been planted to provide flood resistance on the river berms both from the root structure and in maintaining water velocities to design levels on berm land.

Council has also provided $960,000 in the LTCCP for an ongoing programme of planting and maintaining alternative tree species in areas most affected by willow sawfly.

However, the major impact of willow sawfly on the Heretaunga Plains Flood Control Scheme live willow edge protection has now reached the threshold where an urgent and extensive remediation works programme is required to reduce as rapidly as possible the risk of a major failure of the flood control scheme stopbanks.

3.2 Statutory Responsibility

Council has a statutory responsibility under the Soil Conservation and Rivers Control Act 1941 for flood protection from the major rivers crossing the Heretaunga Plains. The Heretaunga Plains Flood Control Scheme is administered by Council and involves ongoing maintenance of and improvement to stopbanks, willow edge protection planting, active river channels and land associated with the Scheme. The estimated value of these assets is $54,000,000. The annual Scheme budget is approximately $1,800,000. The Scheme helps protect the Heretaunga Plains, including the cities of Napier and Hastings, from frequent flooding from the three main rivers and helps ensure that these rivers remain in their current courses. Funding for the Scheme is met 70% through targeted rates, levied on the basis of capital value on Scheme ratepayers and 30% through Council general funding sources including general funding rates.

3.3 Live Edge Protection

Live edge protection is comprised of 3 specific zones (refer sketch below):

a) River Edge Protection Zone – this is the zone in the first 20 to 40 metres of river edge protection immediately adjacent to the river edge. This zone needs to be of sufficient strength to withstand direct attack from the river, accommodate high river velocities and discourage any lateral erosion of the river from occurring. The tree roots protect the bank from scour, and the vegetation helps control water velocity.

b) Buffer Plantings Zone – this is the zone between the immediate river edge protection zone and the stopbank, or adjoining private property in areas where there are no stopbanks. This zone is intended to act as a filter for flood flows to control velocities to an acceptable level and discourage preferential berm flows from developing. These trees also provide an important source of back up material for flood damage reinstatement work and for emergency edge protection repairs.

c) Near Stopbank Zone – this is the zone in the area immediately adjacent to the toe of the stopbank and usually consists of 2 or 3 rows of shrub willow species. The intent of these plantings is to control water velocities to an acceptable level along the river side of the stopbank face to minimise the risk of erosion to the bank surface.

Figure 1 - Cross Section of River Control Design

3.4 Extent of Damage and Risk

The attached maps show the extent of willow sawfly damage divided into 3 categories; severe, heavy and moderate. The estimated loss of effectiveness from the damage caused is estimated to be 25% loss for moderate damage, 70% for heavy and 90% for severe.

As a result of the weakened willow edge protection, the risk of lateral scour of the river during flood flows has substantially increased, with the potential for significant lateral erosion during any flood greater than a 5-year return period, particularly if flood flows remain in the river over an extended period. Repeated floods in any one season could result in substantial lateral erosion and in particular where the stopbanks are within 100 metres of the active river channel the risk of a stopbank breach is substantially increased.

3.5 Proposed Remedial Works

A range of options for the remediation of sawfly damage have already been trialled. Options considered for the remediation works have taken the learnings from these trials into account, and together with the experience of Council river management staff, the preferred option of alternative species planting strengthened by steel rail and wire rope groynes, and with hard engineering protection for the most vulnerable river banks has been developed.

Council have considered the possibility of extending the timeframe over which the works are undertaken, but believe that in order to reduce the increased risk of flooding at the earliest opportunity the works should be undertaken over the shortest time practicable.

Consequently, the objective of the proposed programme of works is to reinstate a level of protection provided by the river edge protection and stopbanks as quickly as possible to a capability and capacity of conveying a 1% AEP flood safely to the sea. A 1% AEP flood is one with a 1% chance of occurrence in any one year, or in other words, one which is likely to occur on average once every 100 years.

The aim is to have the majority of structural works completed over 1 year and the major planting programme undertaken over a 3 year timeframe. An ongoing programme of planting of a range of tree and shrub species will then continue into the future.

Year 1 of the planting programme will be limited by availability of plants although the severe risk areas will be targeted during this period. Commitments will be made for contract growing of materials for Years 2 and 3.

The proposed work programme is attached.

Four specific areas have been identified as severe risk and therefore requiring hard engineering protection and/or enhanced steel rail and wire rope groyne work. These are:

River	Location	Length (m)	Priority
Ngaruroro	Ngatarawa Stopbank	4,700	1
Tutaekuri	Moteo Stopbank	1,850	2
Tukituki	Tennant and Moore Roads Stopbank	3,200	3
Tutaekuri	Dartmoor Stopbank	2,900	4
	Total length	12,650

Hard protection works will be used where the willow edge protection has been extensively damaged by sawfly and the stopbank is within 100 metres of the existing active channel edge.

3.6 Costs of the remedial works

The remedial works set out on the work programme have an estimated cost of $8,800,000.

3.7 Funding for the remedial works

Funding will be secured from the following sources to pay for the $8.8M remedial works.

Loan funding (Scheme and General Funding)	$6,700,000
Council's disaster damage reserves	$500,000
Heretaunga Plains Flood Control and Drainage Scheme	$660,000
Transfer from Council's cash operating balances	$940,000
Total	$8,800,000

It is the Council's intention for the loan(s) to be repaid over a 10 year time frame. A 10 year repayment period is considered appropriate given:

• The ability of the Scheme to service loans with longer terms is more manageable than for short term loans. Ideally therefore loans should be of a long duration; and,
• The ideal situation is to have intergenerational equity which means that the beneficiaries of the assets (i.e. the present and future owners of land protected by the Scheme) should meet a share of the costs through repayment of long term loans; however,
• The risk of a major flood event causing extensive damage to the new works and resulting in significant reinstatement costs means than the Scheme should aim to repay outstanding loans over the shortest possible time because of the ongoing risk of assets being destroyed or severely damaged by a major event before they are fully established.

A 10 year loan period will therefore meet the immediate needs of the Scheme with regard to sawfly mitigation and the capital works programme.

To provide funding for the servicing of the loan the following rate increases are proposed:

• Heretaunga Plains Targeted Rate (affects scheme rate payers only)

2005/06 plus 10% $125,000
2006/07 plus 10% $138,000
2007/08 and subsequent years plus 3%

• General Funding Rates (affects all ratepayers in region)

2007/08 plus 3.8% $120,000

The expenditure of $8.8M on sawfly remedial works as proposed by Council, constitutes an amendment under Section 97 of the Local Government Act 2002 (the Act) to Council's LTCCP. Section 84(2) of the Act states that a Statement of Proposal must include a draft of the parts of the LTCCP that are proposed to be amended, and be accompanied by a draft of any consequential amendments to the LTCCP that will be required if it is amended in the manner proposed.

Accordingly, included as part of the attachments to this Statement of Proposal, is the revenue and funding for the Land Drainage And River Control group of activities as included in the LTCCP 2004-14.

Also attached is Council's proposed amendment to the cost and funding of this group of activities due to the proposed sawfly remediation works. An analysis of the variances is also provided and other required information.

4.0 ANALYSIS OF THE REASONABLY PRACTICABLE OPTIONS

Council is required to undertake, as it considers necessary and appropriate, an analysis of the reasonably practicable options associated with this proposal. The results of this assessment and analysis is as follows:

a) Actual works

There are very few practicable options for Council to consider in relation to restoring the integrity of the river edge protection as part of the Heretaunga Plains Flood Control Scheme. Live edge protection is a widely adopted form of river edge protection throughout the country and the only other practicable option is the sole use of hard engineering structures (i.e. rock or concrete) which can serve the same purpose. Hard engineering structures have been proposed with the use of large concrete blocks in those areas at greatest risk from the lateral erosion of the river and the subsequent risk this will create for stopbank breaches. Concrete has been chosen as the material to be used because Hawke’s Bay has very little readily available rock of the size and features required to fulfil the requirements for effective edge protection. Consequently rock would have to be sourced primarily from out of the region at approximately double the cost of manufactured concrete blocks. Therefore, in terms of the benefits and costs of using rock as opposed to concrete blocks, it would increase costs by approximately $2.5M with very little benefit over concrete. This would impact on the social and economic well being of the region due to the greater demand that will be placed on ratepayers for required funding. Rock may be aesthetically more pleasing than concrete and therefore has some desirable environmental outcomes but even rock protection will look like a man-made structure and with the planned plantings the manufactured concrete blocks will blend with the environment over time.

The Community Outcome of a ‘strong prosperous and thriving economy’ will be compromised to some extent were rock to be used, due to the greater funding requirements for this option. There is considered to be no other difference in terms of using rock or concrete in terms of the other Community Outcomes. The increased cost option of using rock will compromise the Councils capacity to meet the present and future needs in relation to relevant statutory responsibilities of the Council, as the greater requirement for funding for this option will compromise any further remedial works that may be required to deal with further willow sawfly damage. Furthermore remedial works will be at risk of damage from flood events before the extensive plantings have taken full effect in 5 to 8 years and so there is a distinct probability that flood repairs in future will be significantly more expensive than they have been in the past and therefore funding flexibility will be critical to deal with this issue. The use of a cheaper but equally effective product (i.e. concrete over rock) will allow for this flexibility.
Council has had a peer review of the proposed remedial works undertaken by a suitably qualified River Engineer (Mr Gary Williams) who has confirmed the general approach by Council, which affirms the reasonably practicable options available to Council in terms of the actual remedial works being proposed.

b) The ‘do nothing’ option

Council does not consider the ‘do nothing’ option is reasonably practicable. Council has a statutory responsibility to maintain the specified level of service for the Heretaunga Plains Flood Control Scheme, which is to safely convey a 1% AEP flood safely to the sea. Without the proposed remedial works this will be impossible to achieve and in fact the standard has already dramatically reduced from that previously established and identified, and therefore the initiation of this proposal.

c) Reasonably practicable options for funding

• Council does not believe the following are reasonably practicable options for funding:

i) Increase rates to avoid the need for borrowing

Explanation:
Borrowing requirements are significant for the proposed remedial works and would require a four fold increase in rates in Year 1 to meet the cost. This is obviously a significant increase and so is not considered practicable.

ii) Alter the split between targeted and general funding rates for funding the remedial works.

Explanation:
Council undertook an extensive exercise in the mid 1990’s to determine the appropriate split between targeted and general funding rates for the funding of the Heretaunga Plains Flood Control Scheme. This exercise, and the findings and the decisions related to it are considered as relevant today as they were at the time the assessment was completed. The Heretaunga Plains Flood Control Scheme is a long term initiative and therefore the basis of funding for it should be soundly based and changes should only be initiated if there are strong and compelling reasons.

• Council believes that the alternative reasonably practicable options for funding are:

i) Extend the loan repayments from 10 years to a longer period.

This would reduce capital repayments in the first 10 years but increase interest costs. Council is not proposing this option because of the possibility of increased flood damage until the new works become fully established. Council wish to retain funding flexibility in the future, which can be better assured by a shorter period of borrowing. Extending loan repayments would compromise social and economic well-being as well as the Community Outcome of a strong prosperous and thriving economy, as less flexibility will exist in the future and may even affect the Outcome of a lifetime of good health and well being. Such an option could also impact negatively on the Council’s capacity to meet present and future needs because of the lack of flexibility to deal with further unforeseen circumstances as a consequence of the willow sawfly damage.

ii) Defer all other works related to the Heretaunga Plains Flood Control Scheme to reduce as far as possible the impact of the willow sawfly remedial works programme.

Council has made the decision to defer as many other capital works related to the Heretaunga Plains Flood Control Scheme as is possible. However, those works that are still programmed over the next few years are considered essential, and balance the needs for ongoing improvements to the scheme against the urgent requirement to address the willow sawfly damage. Therefore, on one hand a deferment may benefit the social and economic well being of some residents in the region, but equally compromise and affect the social and economic well being of others. The same principles and arguments apply to the safety and security of communities who live on and benefit from the Heretaunga Plains Flood Control Scheme. Council has a statutory responsibility to consider all aspects related to this scheme.

5.0 OTHER MATTERS

Although the work proposed will result in a rapid reduction in risk level, there will be an ongoing risk of lateral erosion of the river in any single flood event until the live edge protection is fully established. The extent of such erosion is likely to be considerably greater in willow edge protection weakened by sawfly than it would be with healthy willow or alternative species edge protection. The Scheme assets will therefore be exposed to that risk until the alternative species edge protection is fully established (i.e. 5-8 years). In addition the cost of repairs to any damaged edge protection will be greater than it has been in the past when healthy willows have been able to be used for flood repair works in establishing a new river edge. With the reduced number of willows available because of the extensive damage by sawfly, and the fact that there are no other alternative species with the attributes of willow, repairs in the future will have a greater element of structural works (wire rope and steel rail groynes) or hard engineering works.

To recognise this increased risk, it is proposed that the Scheme disaster fund be built up more rapidly than is presently budgeted. The revised estimates therefore include an increased annual contribution (increased from $20,000 to $50,000).

The Council has considered numerous papers related to the impact of willow sawfly. The most relevant papers are attached as Appendices to this Statement of Proposal and specifically cover Agenda items on this matter considered at the Asset Management and Biosecurity Committee Meeting held on 17 February 2005 and the Council Meeting on the 10 March 2005.

6.0 CONSULTATION PROCESS AND TIMELINE

The Special Consultative Procedure being adopted for the proposed change to the Hawke’s Bay Regional Council’s LTCCP for the sawfly remediation works is as follows:

a) The Statement of Proposal and a Summary of the information contained in the Statement of Proposal to be considered and adopted by the Hawke's Bay Regional Council at a properly constituted meeting on 10 March 2005.

b) The Statement of Proposal will then be published and made available for public inspection:

• At Council offices at 102 Vautier St, Napier, and its other offices at Guppy Road Taradale, Takapau Road in Waipukurau and Freyberg Street in Wairoa.
• At public libraries in Waipukurau, Waipawa, Hastings, Flaxmere, Havelock North, Taradale, Napier and Wairoa.
• On Council’s website www.hbrc.govt.nz.
• By phoning and requesting a copy from Toni Little at the Council (06 835 9412 or 0800 108 838 (for callers outside the Hastings / Napier toll free zone)

c) The Summary of the information contained in the Statement of Proposal will be publicly notified in Hawke’s Bay Today on 14 March 2005 and then as soon as possible thereafter in the , Napier Courier, Hastings Leader, Wairoa Star and CHB Mail.

d) Public notices will be placed in Hawke's Bay Today, Napier Courier, Hastings Leader, Wairoa Star and CHB Mail, advising of the public consultation process being undertaken and where persons interested in the proposal may be able to obtain a summary of information about the proposal and inspect the full proposal should they choose to do so.

e) At least three public meetings will be held before the close of submissions. Each of these will be advertised in Hawke’s Bay Today and the relevant community newspapers.

f) The Chief Executives of both Napier City Council and Hastings District Council will be approached and asked if their Councils would like to be informed of the issues.

g) A meeting or meetings will also be held with some of the key stakeholder and industry groups.

h) The period within which submissions on the proposal can be made to the Regional Council will close at 4pm Friday 15 April 2005.

i) Any person who makes a submission on the proposal will be provided with written notice acknowledging receipt of the submission. Submitters specifically requesting in their submission to be heard will be given that opportunity at the meeting of Hawke’s Bay Regional Council during the morning of 28 April 2005.

j) The hearing and consideration of submissions will occur in open session, subject to the Local Government Official Information and Meetings Act 1987 with written submissions on the proposal being made available to the public on request.

ATTACHMENTS:

1. MAPS SHOWING EXTENT OF SAWFLY DAMAGE (B/W)
2. PROPOSED WORK PROGRAMME – [1 PAGE]
3. ASSET MANAGEMENT AND BIOSECURITY COMMITTEE MEETING 17 FEBRUARY 2005 – SAWFLY – [20 PAGES]
4. REGIONAL COUNCIL MEETING 10 MARCH 2005 – [18 PAGES]
5. PARTS OF LONG TERM COUNCIL COMMUNITY PLAN 2004-14 (LTCCP) BEFORE CHANGES AND PARTS OF LTCCP IF AMENDED IN THE MANNER PROPOSED – [4 PAGES]
6. FINANCIAL STATISTICS [6 PAGES]
7. SUMMARY OF PROPOSAL - PUBLIC NOTICE 14 MARCH 2005 - [1 PAGE].