DISTRIBUTION: Committee Members : His Worship the Mayor B A Carter D E Challis H van Eyk G Laurich B A Keane P Paterson B Pirie P Johnstone R D Young Council Rep : B A Gordon Councillors (alternates) : J H Thorp D H Swales G R Leonard (observer) Staff : Ian McLeod Bruce Stephens Adrian de Laborde Council Secretary A G E N D A WESTERN PLAINS DRAINAGE DISTRICT COMMITTEE DATE: Tuesday, 23 February 2016 TIME: 10.30am VENUE: Council Office Williams Street PAEROA MEMBERS: B A Carter (Chairperson) D E Challis H van Eyk G Laurich B A Keane P Paterson B Pirie P Johnstone R D Young Cr B A Gordon J P Tregidga (Mayor)
Transcript
DISTRIBUTION: Committee Members: His Worship the Mayor B A Carter D E Challis H van Eyk G Laurich B A Keane P Paterson B Pirie P Johnstone R D Young Council Rep: B A Gordon Councillors (alternates): J H Thorp D H Swales G R Leonard (observer) Staff: Ian McLeod Bruce Stephens Adrian de Laborde Council Secretary
A G E N D A WESTERN PLAINS DRAINAGE DISTRICT
COMMITTEE
DATE: Tuesday, 23 February 2016 TIME: 10.30am VENUE: Council Office Williams Street PAEROA
MEMBERS: B A Carter (Chairperson) D E Challis H van Eyk G Laurich B A Keane P Paterson B Pirie P Johnstone R D Young Cr B A Gordon J P Tregidga (Mayor)
1 Document Number: 3858 Document Name: Western Plains Drainage District Agenda -
HAURAKI DISTRICT COUNCIL
WESTERN PLAINS DRAINAGE DISTRICT NOTICE IS HEREBY GIVEN THAT A MEETING OF THE WESTERN PLAINS DRAINAGE DISTRICT COMMITTEE WILL BE HELD IN THE COUNCIL CHAMBERS, WILLIAM STREET, PAEROA ON TUESDAY 23 FEBRUARY 2016 COMMENCING AT 10.30 AM
ORDER OF BUSINESS
1. APOLOGIES Pages 2. DECLARATION OF LATE ITEMS
Pursuant to Section 46A(7) of the Local Government Official Information and Meetings Act 1987, The Chairman is to call for late items to be accepted.
3. CONFIRMATION OF MINUTES
3.1 MEETING HELD ON TUESDAY 24 NOVEMBER 2015 (1459531) 3
RECOMMENDATION
THAT the minutes of the meeting of the Western Plains Drainage District Committee held on Tuesday 24 November 2015 be taken as read and confirmed.
4. MATTERS ARISING FROM THE MINUTES 5. DRAINAGE REPORT AND FINANCIAL REPORT TO JANUARY 2016 6
(1495222) 5.1 The Drainage Manager’s monthly report and financial report on the
operations of the Western Plains District Drainage Committee to January 2016 is attached.
RECOMMENDATION
THAT the financial report on the operations of the Western Plains District Drainage Committee to January 2016 be received.
6. GENERAL BUSINESS
The members are invited to comment on any general drainage issues of interest in their areas.
7. LATE ITEMS
1 Document Number: 1456801 Document Name: Western Plains Drainage District Minutes – 24 November 2015
HAURAKI DISTRICT COUNCIL
WESTERN PLAINS DRAINAGE DISTRICT MINUTES OF A MEETING OF THE WESTERN PLAINS DRAINAGE DISTRICT COMMITTEE HELD IN THE COUNCIL CHAMBERS, WILLIAM STREET, PAEROA ON TUESDAY 24 NOVEMBER 2015 COMMENCING AT 10.30 AM PRESENT Messrs B A Carter (Chairperson), D E Challis, H van Eyk, P Paterson, B
Keane, G Laurich, P Johnstone, R D Young and Cr B A Gordon IN ATTENDANCE Cr J R H Thorp, Mr I D McLeod (Drainage Manager), Mr B Stephens
(Drainage Overseer), Sarah Holmes (Strategic Policy Planner), Katherine Quinn (Strategic Planner) and Ms C Black (Council Secretary)
APOLOGIES RESOLVED
THAT the apology of B Pirie be received and sustained. WPDD15/27 Keane/Paterson CARRIED LATE ITEMS There were no late items. CONFIRMATION OF MINUTES MEETING HELD ON TUESDAY 27 OCTOBER 2015 (1449563)
RESOLVED
THAT the minutes of the meeting of the Western Plains Drainage District Committee held on Tuesday 27 October 2015 be taken as read and confirmed.
WPDD15/28 Paterson/Young CARRIED MATTERS ARISING FROM THE MINUTES The Drainage Manager advised that no letter had been written to farmers who had allowed silage stack tyres to fall into Council drains. A publicity campaign had been introduced, and that follow up telephone called will be made if the practice continues. Piping of the McMillan Place drain A revised estimate for a reduced option had been prepared and a recommendation will be brought to the next meeting. It was confirmed that the floodgate in the toe drain upstream of the Rawerawe West pump was an asset of the WPDD. Cr Thorp and Gavin Laurich attended the meeting at 10.40am.
WPDD Agenda - 23-02-16 Page 3
2 Document Number: 1456801 Document Name: Western Plains Drainage District Minutes – 24 November 2015
DRAINAGE REPORT AND FINANCIAL REPORT FOR THE 4 MONTHS ENDED 31 OCTOBER 2015 (1455902) The Drainage Manager’s monthly report and financial report on the operations of the Western Plains District Drainage Committee for the 4 months ended 31 October 2015 was presented.
RESOLVED THAT the monthly report for October 2015 be received, and THAT the financial report on the operations of the Western Plains District Drainage Committee for the 4 months ended 31 October 2015 be received.
WPDD15/29 Gordon/Johnstone CARRIED
2016/17 ANNUAL PLAN – DRAFT LAND DRAINAGE & STORMWATER PROPOSALS (1454097, 1452961) The Drainage Manager reported on the draft 2016/17 land drainage and stormwater proposals (including deliverables and expenditure budgets) to the Committee for its consideration. The proposals were attached.
RESOLVED THAT the report be received, and THAT the Committee recommends to the Council that the draft 2016/17 proposals including budgeted expenditure be approved for inclusion in a draft annual plan.
WPDD15/30 Keane/Laurich CARRIED GENERAL BUSINESS WAIKATO REGIONAL COUNCIL – UPDATE ON SCHEMES WORK PROGRAMMES Waikato Regional Council Representatives Robert Hicks (Chair of the Waihou Piako Zone Committee), and staff: Kenny Growden, Ian Sara and Jordan Downs were in attendance. The WRC proposed works programmes for the 2015-16 season as they affected the Western Plains Drainage District were presented. Pump upgrade programmes were proposing to standardise on pump inlet configurations and screens, including being able to stand a 12-20 tonne excavator. The practicality of installing silt traps upstream of pumps was discussed. The weed cleaning of the Maukoro Canal upstream of SH 2 was raised on behalf of farmers in that reach. The canal may be re-sprayed in that reach. The members were invited to comment on any general drainage issues of interest in their areas. B Keane on behalf of B Pirie – Upper Maukoro Canal weed. The subject had been discussed with WRC staff.
WPDD Agenda - 23-02-16 Page 4
3 Document Number: 1456801 Document Name: Western Plains Drainage District Minutes – 24 November 2015
Peter Paterson – Paterson’s outlet needs clearing. Ross Young – was advised that the demolition concrete would be delivered to the Harris drain soon. The meeting closed at 12.00. CONFIRMED B A Carter Chairperson 23 February 2016
File reference: Document: 149 5222 Appendix A 147 4140
Meeting date: 23 February 2016
Subject: WPDD Drainage Manager Report January 2016 Recommendation: THAT the report be received.
1.0 OPERATIONS
1.1 Chemical spraying
6.35 km of drains were sprayed in Area 3. Drains sprayed by helicopter are yet to be entered into the database. The annual public notice advising landowners of the spray and machine clean programme were published in November.
1.2 Machine cleaning 15.9 km of drains have been machine cleaned in the current year, including the Mangawhero debris trap. 300m of drain in Ngatea was machine cleaned.
1.4 Pump operation Regular inspection runs were undertaken in this period. 121 pump hours were recorded in November, none in December and 115 in January. The summary for the 2015-16 year is attached. The total to the end of January of 8,676 hours is 84% of the annual average. Two man-hours were spent on weed screen cleaning in the month.
1.5 Floodgates No floodgate maintenance has been undertaken in the period.
1.6 Stopbanks No stopbank maintenance was undertaken in the month. A reach of the recently reconstructed Waitakaruru Stream left bank stopbank has been re-grassed.
2.0 BYLAW APPROVALS
Bylaw approvals were granted in the period as follows:
2.1 Culverts: Nil 2.2 Other: Nil
3.0 SUBDIVISIONS COMMENTED ON
Subdivisions commented on during the period as follows: Nil
4.0 RAINFALL The table and graphs of the Ngatea Monthly Rainfall depths since 1993 is attached. The rainfall total for November was 125 mm, December 19 mm (lowest in HDC’s record), and January 103 mm. The total for the year to date is 700 mm, which is 109% of the annual average.
5.0 CAPITAL WORKS 5.1 Proposed Piping of Paul Leonard Drain
Preliminary earthworks involved in the pre-load for the roads in stage 1 of the Ngatea North development have been completed. A further depth of metal for the pre-load has been placed to increase the rate of compaction of the subsoils. The detailed design of the proposed piping and swale construction has been completed and has been approved. The tender for the supply of pipes closed on Friday 13 November. The majority of the pipes for the stormwater reticulation have been delivered. Laying of the mains for stage 1 has been completed. Work on the Paul Leonard drain piping will start with the construction of the outlet headwall and the twin pipes under the access road.
5.2 Mangawhero Road Concrete Flume Channel (Appendix A) The section of the Mangawhero Roadside Drain from the upper road crossing, to a point approximately 250m downstream is a concrete lined channel with a number of gradient control steps in the invert. It is estimated to be around 80 years old. At the time of the WRC taking over responsibility for the Mangawhero drain. It was agreed that the concrete section would remain an asset of the WPDD, as it had been making provision for its replacement by funding the depreciation as DiSP. There have been three instances of failure of sections of the wall of the channel over lengths of 8-10m. One failure was repaired by Council, and two have been repaired by Council’s roading contractor because the failure promoted lateral
erosion of the material behind the wall which was considered to be a threat to the road structure. Council’s Technical Services staff have been directed to undertake a preliminary assessment of the structure, and to propose some options for its eventual replacement. The report from Technical Services is appended for member’s information and discussion. Any replacement of the asset will need to be signalled in a Council Annual Plan or Long Term Plan. The next LTP estimates will be prepared in June 2017.
5.3 Proposed Piping of open drain McMillan Place Ngatea Council staff provided an estimate to pipe the open drain in McMillan Place on the Hauraki Plains College side, from Hayward Road to the access culvert near the end of the street. The estimate was considered too high to justify the works in that area. Following a meeting on site with the College’s BoT Property Officer and the Property Manager, a modified proposal has been developed. This involves a reduction in the length to be piped from 142 m to 68 m, and the availability of spoil from recontouring the road shoulder to fill to ground level. In addition the parking shoulder can be metalled by the College. These reductions have reduced the estimate to $51,500. It is unlikely, due to staff commitment to the Ngatea North development, that any work could be undertaken this year. It is proposed that a recommendation will be brought to a later meeting of the Committee.
6.0 GENERAL 6.1 Proposed Muggeridge Pump Station
Waikato Regional Council Muggeridge Project team which includes HDC staff, has had discussions with the Department of Conservation (DoC) to attempt to gain their agreement to the proposals to allow a non-notified consent application. There are concerns with the long term sustainability of the Torehape Reserve due to the lowering of water tables in conjunction with the eventual settlement of the peat soils. At this stage WRC has put the consent application process on hold, pending further investigations and consultation with DoC. WRC is now considering widening the scope of the consent to include the eventual full catchment. Consequently the detailed design has also been suspended. It is possible that construction of the pumps may not now be completed until the 2017-18 season. Rates would not then be struck until the 2018-19 financial year. The project team met with DoC staff at Hamilton in late January. The meeting included staff from Engineering Consultants AECOM who have undertaken the initial environmental effects of the pump construction and operation as it potentially affects the Torehape wetland reserve. Further study has been commissioned.
The meeting agreed that a partnership approach between WRC and DoC is the right model to progress the project. Further information from the meeting notes can be presented to the meeting.
6.2 Waikato Regional Council Waihou Piako Zone Committee Meeting The WRC Zone committee meeting is scheduled to be held on 18 February. Agenda items of interest to the WPDD are: Key Scheme Projects, including:
• Structural audits of floodgates and pump stations • Scott’s floodgate replacement • Muggeridge Pump project • Piako stopbank stability works • The K2K cycleway project and the MoU between HDC & WRC • Piako Scheme spoil requirements for stopbank reconstruction • As well as the traditional river and flood protection activities
6.3 Combined Drainage Committees Christmas Dinner The combined Drainage Committees’ Christmas dinner was held at the Kerepehi Bowling Club on Tuesday 16 December from 6pm. Attendance was only half of the members due to other commitments. It has been suggested that an earlier date may enable more members to attend.
Opening Reserve Balance as at 1 July 2015 + Drainage RatesOperating Exp+ Depreciation +/- Interest - Capital = Closing Bal(incl Council 15%) (incl DiSP)
Mac, The Mangawhero Rd Flume structure is a concrete lined drain structure some 278m or so in length. It is observed to be in a poor structural condition for much of its length. There is currently one collapsed section of flume wall some 12m in length requiring reinstatement. There are also two historical repair sections evident in the same vicinity as the currently collapsed section. Refer to Photo 1 below showing the collapsed section and Photo 2 showing a typical timber repair section of the drain.
Photo 1 : Currently Collapsed Section of Flume
The collapse of the walls in this area is likely from a combination of factors. The factors observed on site resulting in the flume walls now being structurally inadequate to resist the mass of retained earth are:-
• The stone wall extensions added to the top of the concrete flume walls [see photo 1]. • The original reinforcing steel elements in the original wall casting joints are observed to be
corroding, with some steel elements completely corroded.
• There appears to be tomo’s formed in the base of all five waterfall ‘drop’ points along the flume, creating localised tomo’s in the bed of the flume and likely erosion and scour of earth material behind the adjacent walls.
• There was no flume wall drainage weep holes observed increasing water pressure behind the walls.
• The observed drainage metal behind the walls is likely to have been contaminated with clays and silts, assisting in pushing over the walls.
• The top member compression bars [from top of wall panel to top of wall panel on other side of drain- see Appendix Photo A1] have generally already corroded their steel reinforcing bars, and most [80 to 90%] have fallen off. With so few compression members across the top of the wall panels to assist with restraining the soil pressures on the flume’s concrete walls, the walls may collapse at any time.
• The drain is advised [by the drain neighbour] to overtop about once per year, and the existing timber repair sections have previously had soft fill material wash out from behind the top of the timber wall – the previously repaired timber retaining wall sections require remedial filling with suitably sized rock rip rap to prevent further erosion [see bare earth in Photo below on top left side of photo].
Photo 2 : Previous Flume Repair using Pushed Poles and
Currently Collapsed Section of Flume
The remaining concrete sections of walls or are observed to be towards the end of their design lifetime, but are still operational. Parts of concrete wall panels could collapse at any time due to the listed factors above. Structural decay of the reinforcing steel is shown in Appendix Photos A3 and A4. Some wall panels have rotated at their tops some 100mm, as shown in Appendix Photos A5 and A6. It is noted that the flume will likely continue to operate if parts collapse [even before removal of the collapsed concrete panels]. The local soils above the flume invert levels are typically observed to be quite firm, and are unlikely to collapse as far as the road sealed areas in a short period of time. So there is time to repair collapsed sections after each failure. After a long period the integrity of the nearby road would become compromised due to ongoing longer term erosion of the side earth areas that are exposed
after a collapse. The currently collapsed section has reasonably firm soils showing, but does require repairs to be made before any significant erosion occurs. Weathering of the side walls will fritter them, and rain events will also accelerate erosion of the exposed soils. If nothing was done to the remaining concrete flume sections except spot repairs of any collapsed sections, parts of the walls of this drain could easily last another 10 to 20 years as is. The base is likely to last longer than 20 years, except for the tomo areas at all water ‘drop’ sections noted above. Wall panels are most likely to collapse during or shortly after storm events without warning, with the currently rotated panels most likely to be the first to collapse, as well as panels about the drop tomo areas. During any collapse there are adequate secondary overland flow paths that the water would take until any partial blockages from collapse debris are removed. Other problems observed with the flume drain are:-
1. Some existing flume wall panels are observed to have already rotated forward by up to an estimated 100mm at the top of the panels. It is possible to brace them to prevent further rotation and extend their serviceable life.
2. The right hand flume bank is near to a legal boundary and private land. It may not possible to reinstate this bank into a battered rock rip rap lined channel without the farmer’s permission to enter, and possibly loose a narrow strip of land to an enlarged drain structure.
3. The straight sided flume is a high energy environment, and is being subjected to increased flows as the upper hillsides are being de-forested of pine trees and being returned into grassed rural lands.
4. Whilst the existing timber pole wall repair sections are performing adequately, they have had the open cracks extensively filled with epoxy to prevent scour of the fill material behind the timber walls. There is some ongoing maintenance required to maintain the lower joints filled on the timber sections of walls to prevent tomo’s forming behind the walls. Geotextile cloth could be used behind any future wall repairs to retain the fill materials. In the future the timber boards can be installed on the more traditional inside of the timber poles, instead of being face attached to the poles like the existing walls.
5. The existing repair section timber poles were pushed in with an excavator, and require cross bracing to other poles to prevent their movement. The cross bracing elements are observed to be structurally inadequate and have bends in them in some instances. The 150x50 H3 RS cross bracing timbers are too long to resist the required passive and active earth pressure generated compressive forces. A proper self supporting retaining type structure should be there preferred option for future repairs. The existing poles will require re-bracing after they rotate, and there is evidence they have already rotated some 10mm to 15mm against remaining concrete wall elements.
The Options for renewal of the flume that have been considered are:-
a. Re-construct the entire flume wall sections [concrete or timber]
b. Continue to undertake construction of limited timber retaining wall repair sections of the flume when sections collapse, as per the attached concept sketches shown in attachment 1.
c. As per (b) above, but also undertake installation of timber compression member ‘braces’ along the top of the flume walls to return some structural retaining strength to the existing concrete panels [initially rotated panels, but straight panels could be done if desired]. This compression member ‘brace’ is shown in Attachment 2.
Means of re-lining the flume structure walls [retaining the existing concrete base] that have been considered are:-
• Timber pole and facade sides – previous repairs used this method to provide steep sides. This method should perform well if installed properly. It provides a relatively vertical face against the farm boundary within the road reserve. If poles are rammed then both sides of the wall will require to be done to provide cross bracing. This would look like the photos of the current pole wall repair area in Appendix A. If set in proper concreted and deeper pile holes, the wall would not require cross bracing.
• Gabion rock baskets – feasible and expensive, steep sided possible that may fit within road reserve.
• Rock rip rap sides – feasible to do the whole drain in this manner at once, but not practical as a repair method for portions of the flume. It is difficult to marry into existing vertical concrete and timber sections as, steepest rock batter possibly achievable is approaching 1.5V : 1H. This requires farmer’s permission to possibly loose a strip of land to an enlarged drain. This method would require boundary redefinition to confirm actual space available for the drain to be enlarged within existing road reserve, but there is the advantage of an enlarged drain that is provided. This method suits replacing all of the drain at once.
• Cast in Situ or precast counterfort concrete wall sides - an expensive option, but can provide vertical walls. Can be done in sections as repairs are required for the flume, can allow for some enlargement of the flume towards the road. Found previously to cost nearly twice as much as a precast U-channel when investigating options for preliminary Paul Leonard Drain piping/lining options evaluation in 2007.
• Precast ‘U’ channels or similar units could be designed to sit upon the existing drain base, and replace the existing walls with a new structurally adequate wall system. Any short sections of matching U-channel will require a short make up section cast between the old and new sections, in order to tie them into the existing walls. Ordered in a piecemeal manner these units could cost a significant amount more than purchasing hundreds of metres at a time. This system could allow an enlargement of the drain width, and also the height of the walls. The Shuler Precast yard in Thames is capable of producing these units if supplied with a structural design for them by Council.
• Natural lined sides – ongoing maintenance and likely armouring repairs of erosion areas would be required to keep the drain in a linear alignment. There is currently not enough surface space for the required width this option with the current road location. The gradient of the waterway exceeds 2%, and the channel will try and erode back to a meandering shape over time. A meandering drain will affect the road and adjacent farm, and lining of the walls would be required ultimately.
• Liner over natural battered sides above say 1m of rock rip rap above the concrete channel area. This option retains the existing concrete base, and would intend to use rock rip rap [but could use gabion baskets] about the lower flume levels where there is a high energy environment constantly. It is still suggested to use the drop structures to mitigate some of the erosive energy. The stream side wall above the rip rap could be cut back to possibly as steep as 1v:2h. Using a geotechnical liner to create a steeper and stable batter wall for much of the flume has a lower installation costs. This system requires farmers permission to possibly loose a small strip of land to an enlarged drain [requires boundary redefinition to confirm actual space available for the drain to be enlarged]. The product shown below is heavier Enkamat A20. A lighter version would be used which is similar to that used at the Tetley’s Quarry water intake for river bank erosion, and is performing well. Liners are best above the typical water level where plants and grasses grow through it, it is not a long term solution lower down in active and high energy channels such as the Mangawhero Flume.
Treatment Costs of Wall Repair Types A rough order cost estimate of each sort of treatment has been prepared for this report, but is subject to detailed design and supplier quotations for the proposed amount of product, as the more that is purchased, the lower the costs. The cost estimates are based upon HDC Construction and Maintenance Department remediating the entire drain length of 278m [excluding drop structure repairs and any special armouring].
TREATMENT
COST PER SIDE OF FLUME [PER METER]
COST TO DO BOTH SIDES [PER METER]
COST TO DO BOTH SIDES [Overall 278m]
Timber pole and facade sides [2m high pole wall]
$528 $1,056 $293,568
Gabion rock baskets $468 $936 $260,208 Rock rip rap sides $282 $564 $156,792 Cast in Situ or precast counterfort concrete wall sides
$660 $1320 $366,960
Precast ‘U’ channels 1.8m high n/a $1500 $417,000 Liner over natural battered sides above rock rip rap for 1m above concrete channel area
$216 $432 $120,096
Liner over natural battered sides above precast u drain for 1m above concrete channel area
$410 $820 $227,960
Table 1 : Indication of wall Treatment Costs
It would be prudent to allow $2000 per drop area repair of the flume bed [5 in total = $10,000], as these are the area’s most likely to collapse next. Each cross brace [at 3m spacing’s] should cost in the order of $60/brace to install. Up to 30 initial braces to inhibit further wall rotation should be allowed for to prolong the remaining walls lifespan, an allowance of $1800 in total. Further cross braces are likely to be required in the future. The above treatment costs indicate the cost of the works, but do not allow for other site construction factors such as traffic management, and contractor availability in the currently limited market of available civil contractors due to the booming property markets in all three major nearby cities to Hauraki District. Therefore an allowance of up to 25% more must be allowed for on top of these estimates to allow for these uncertainties with the preliminary pricing shown above. Discussion and Considerations It is not considered feasible to return this flume back to an unlined stream structure, as any straight ‘stream’ will attempt to erode into a meandering [snaking] stream, with consequences for the stability of the adjacent farmland and public road. It is possible to retain the current concrete base and line the lower areas of stream wall with gabions, rock rip rap, or similar treatments.
The drain does open out to what appears to be a straight section of ‘man-made’ stream, just after the flume crosses under Mangawhero Rd. The man-made section has much flatter gradient, and takes up considerably more space than the current concrete flume. That space is not available about the current flume location unless the road is moved over to the north to accommodate an enlarged stream and any required intermittent side wall erosion remediation areas. Rock rip rap low in the channel and grass batter with Enkamat type reinforcement up higher is seen as the most cost effective drain re-lining option, but this methodology does not work when repairing short sections of drain. This method effectively opens out the drain and makes it somewhat wider, and is therefore difficult to re-narrow to marry back into any remaining thinner concrete flume sections. A neighbouring farmer [also on the local drainage board] is aware the hills above this drain are being slowly de-forested and returned into dairy and pasture land. This is increasing the runoff to the drain. With a reported drain overtopping being a typically observed annual event, there is likelihood that the drain and the associated road culverts may also need enlarging, in due course, if the level of service of the flume drain and culverts is not acceptable to Council. It is noted that the road culverts on the upstream and downstream of the drain would also have to be upgraded in size, or additional culverts installed, in this case. It is noted that the overtopping of the culverts and flume drain is not indicated to cause significant damage to surrounding areas, as there is safe overland flow paths via the road reserve and fields for the short overtopping periods. Some of the options would open up the drain do provide greater capacity and channel storage, which should be a considered by Council when considering options to repair or replace part or all of the 278m section of flume drain. Most of the drain is still serviceable, but has the potential to collapse. As the risk of damage from collapse is low, and typically only one side of the drain is likely to collapse at any time, the ongoing repair by pole retaining walls [to be deeply drilled, not shallow driven and cross braced] should continue to perform adequately when built to a good design. Above the repaired timber walls needs to be better protected against scour. Recommendations:-
i) If there is limited budget, undertake limited timber retaining wall repairs of the currently
collapsed 12m section of wall. [Refer to Attachment 1 for an indicative design]. ii) Install timber compression braces along all rotated drain wall areas to return some structural
strength to the existing failing concrete panels to elongate their remaining life. [Refer to Attachment 2 for an indicative design].
iii) Repair the Flume bed at all cascade flume drop areas and consider installation of water catching rocks to restore some fish passage availability to the flume. [Refer to Attachment 3 for an indicative design].
iv) Place rock rip rap above existing timber wall repair sections to prevent further scour. [Refer to Attachment 1 for an indicative design].
v) Make allowances in the annual and ten year plan for renewal of the concrete wall sections as they collapse, with an ultimate replacement goal of replacing the flume walls within the next 10 to 15 years.
vi) Install further timber compression cross members every 3 metres along the remaining entire flume wall if a longer renewal period for the observed ‘good vertical’ un-failed panels is required, if there is available budget. As a minimum undertake annual inspections of the remaining vertical concrete wall, and add props to any sections that rotate in the future.
