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«A J Boyle M R Surman June 2013 Report No. R13/64 ISBN 978-1-927257-62-3 (print) 978-1-927257-63-0 (web) 24 Edward Street, Lincoln PO Box 345 ...»

-- [ Page 1 ] --

Middle Ashley

(Rakahuri) (Gorge –

the Okuku confluence)

bed level investigation

Report No. R13/64

ISBN 978-1-927257-62-3 (print)

978-1-927257-63-0 (web)

A J Boyle

M R Surman

June 2013

Report No. R13/64

ISBN 978-1-927257-62-3 (print)

978-1-927257-63-0 (web)

24 Edward Street, Lincoln

PO Box 345

Christchurch 8140

Phone (03) 365 3828

Fax (03) 365 3194

75 Church Street

PO Box 550

Timaru 7940

Phone (03) 687 7800

Fax (03) 687 7808

Website: www.ecan.govt.nz

Customer Services Phone 0800 324 636 Middle Ashley (Rakahuri) (Gorge – the Okuku confluence) bed level investigation Executive summary This report details the physical setting of the Upper and Middle Ashley (Rakahuri) River catchment, the weather patterns, the structural setting, background, hydrology and hydraulics and notes the deteriorating state of the waterways above the Okuku confluence.

It lists the results of channel analyses that have been carried out and various modelling scenarios.

Between two and four million cubic metres of available shingle have been identified that could be extracted and a strategy for its removal outlined.

The report also addresses other matters related to options for managing the river in this area including interrelationships with the existing Ashley (Rakahuri) River Rating Area downstream.

Environment Canterbury Technical Report i Middle Ashley (Rakahuri) (Gorge – the Okuku confluence) bed level investigation ii Environment Canterbury Technical Report Middle Ashley (Rakahuri) (Gorge – the Okuku confluence) bed level investigation Table of contents Executive summary

1 Physical setting of the Upper and Middle Ashley (Rakahuri) River catchment

2 Weather patterns

3 Structural setting of the upper and middle catchments

4 Background, hydrology and hydraulics

4.1 1% AEP design discharges and results of hydraulic modelling (3 scenarios; as is, cleared fairway and berms, cleared fairway and berms and fairway lowered 0.5m, berms lowered 0.25m)

4.2 10% AEP design discharges and results of hydraulic modelling (3 scenarios; as is, cleared fairway and berms, cleared fairway and berms and fairway lowered 0.5m, berms lowered 0.25m)

4.3 Discussion and results of a further hydraulic modelling scenario (cleared fairway and berms and fairway lowered 1m, berms lowered 0.5m)

5 Other matters

6 References

Appendix 1: Channel analyses Middle Ashley (Rakahuri) River

Appendix 2: Control lines Middle Ashley (Rakahuri) River

Appendix 3: Cross section locations Middle Ashley (Rakahuri) River............... 31

–  –  –

List of Photos Photo 4-1: Ashley (Rakahuri) River and Okuku River confluence, 1977

Photo 4-2: Ashley (Rakahuri) River and Okuku River confluence, 2001

Photo 4-3: Ashley (Rakahuri) River and Okuku River confluence, 2012

Photo 4-4: Ashley (Rakahuri) River looking upstream from near Sunken Road

List of Tables

Table 4-1: 1% AEP (100 year return period) discharges

Table 4-2: Computed Flood Levels 1% AEP (100 year return period event)

Table 4-3: Computed Flood Levels 1% AEP (100 year return period event)

Table 4-4: Design discharges 10% AEP (10 year return period)

Table 4-5: Computed flood levels 10% AEP (10 year return period event)

Table 4-6: Computed flood levels 10% AEP (10 year return period event)

Table 4-7: Computed Flood Levels 10% AEP (10 year return period event)

Table 5-1: Clearing of fairway and berms only

Table 5-2: Clearing and lowering of fairway and berms

Appendix 1 Table A1.1: Channel configuration

Table A1.2: Mean bed levels

Table A1 3: Inter survey differences 2012 - 2001

Table A1.4: Mean bed levels (full section)

Table A1.5: Mean bed levels (fairway only)

Table A1.6: Volumes to Mean Sea Level

Table A1.7: Change in volumes between sections

–  –  –

1 Physical setting of the Upper and Middle Ashley (Rakahuri) River catchment The Upper (upstream of the Gorge) and Middle (Gorge to the Okuku confluence) Ashley (Rakahuri) River is a relatively steep, braided river which drains a catchment of approximately 700 km.

Commencing with headwaters in the Puketeraki Range in the north-west of the catchment, a succession of landforms continues downstream to the east through Lees Valley, Mt Thomas Range and Loburn Basin to the easternmost point of this investigation reach (the Okuku confluence).

The highest points in the catchment are along the crest of the Puketeraki Range with many of the peaks in excess of 1600 metres. For example, Black Rock at 1708 m, Mt Pember at 1692 m and Chest Peak – the highest at 1935 m.

The Upper Ashley (Rakahuri) River, along with its tributaries the Lillburn, Whistler and Townshend Rivers drains the Puketeraki Range. A further tributary, Duck Creek, drains the hill country in the Pancake Range around to the Okuku Saddle.

Lees Valley, wedged between these ranges and the frontal range to the south-east (Mt Richardson and Mt Thomas), has been infilled by extensive fans of gravel brought down by the tributary rivers to form a broad valley plain.





The Ashley (Rakahuri) River leaves the upper catchment by a deep narrow gorge through the frontal hill range. The main sector of this range is unnamed on Survey and Land Information Maps but is referred to as the ‘Mt Thomas Range’ in this report. Major peaks in this sector include Okuku Hill, 1143 m, Ashley Hill, 1018 m, Mt Richardson, 1048 m and Mt Thomas, 1023 m.

The sector of hills to the south-west of Ashley (Rakahuri) Gorge is locally known as the ‘Oxford Hills’ with Mt Oxford the highest at 1356 m.

The downland section (the investigation reach) referred to here as the Loburn Basin commences downstream of the Ashley (Rakahuri) Gorge. It forms a partially enclosed basin ringed by a crescent of hill country to the north and east and by the Mairaki Downs to the south.

To the west, the foothills at Mt Thomas abruptly terminate on a north-east trending front of steep slopes 600 to 800 metres in height.

The plains of the Loburn Basin are built of a succession of alluvial fan aggradation gravels and silts of varying ages. The youngest named as the Okuku Surface in this report is extensive and forms a large relatively featureless plain with an average seaward gradient of 6 metres per kilometre. Most of the gentler downland standing above this plain is underlain by older fan gravels. The steeper downland and neighbouring hill country, for example, at Mairaki Downs is underlain by compacted silts and gravels more resistant to erosion. These silts and gravels are known as the ‘Kowai Formation’.

From Ashley (Rakahuri) Gorge to the Okuku River the strongly braided river is joined by the Glentui River (a sub-catchment of 20.6 km ), the Garry River (55.8 km ), Mt Thomas Stream (24.0 km ) and Bullock Creek (24.0 km ); these latter two joining just upstream of their confluence with the Ashley (Rakahuri) River.

All the rivers have now cut below the Okuku surface, each leaving behind a succession of river terraces on their flanks during the course of downcutting.

Immediately downstream of the study reach the unconfined floodplain commences on the south side.

On the north side, a terrace continues for some distance downstream almost to Ashley township.

–  –  –

2 Weather patterns North-easterlies tend to dominate the weather pattern in the lower part of the catchment from the Mt Thomas Range to the coast.

On occasions, cyclonic rainfalls associated with southward-moving tropical weather systems also enter the catchment from this direction. Southerlies, south-easterlies and to a lesser extent ‘norwesters’ also contribute to increased river flows.

3 Structural setting of the upper and middle catchments As with other large Canterbury rivers, the geomorphic history has been one of response to long-term variations in climate.

The changes have also taken place in an active tectonic setting.

The upper and middle catchment extends from the headwaters in the Puketeraki Range eastwards across the Mt Thomas Range in the direction of Mt Grey.

This area is made up of hard greywacke, sandstone and mudstones.

The catchment experiences earthquakes and shallow faulting and is subject to the same levels of differential uplift, seismic faulting and folding that characterise North Canterbury.

Vertical uplifts of several kilometres have occurred along range-bounding faults.

Over the study reach, the river has cut into more recent alluvial material and following that into older surfaces.

4 Background, hydrology and hydraulics Protection works have been constructed from just above the Okuku confluence to the sea on the south side of the river as part of the Ashley (Rakahuri) River Control Scheme.

On the left side (looking downstream), there is a containing terrace extending from the Okuku confluence to just above Ashley Township.

Between there and the sea similar protection works to those on the south side exist.

On the right side, the constructed stopbanks tie into the terraces of Mairaki Downs.

The original scheme downstream of the Okuku was designed in the 1930s to provide stopbanks 3 -1 (levees) that would carry 2000 m.s with no freeboard. These banks were overtopped and seriously damaged by floods in 1936, 1951 and 1953. The stopbanks were strengthened in 1968 and 1973 to a level approximately two feet (600 mm) above the levels that occurred in the floods of 1951 and 1953.

3 -1 The present scheme was designed in 1976 to carry 2400 m.s which was estimated at that time to be the 1% Annual Exceedance Probability (AEP) discharge.

The design stopbank heights provided a minimum of 600 mm freeboard above the design flood levels.

The existing bank heights were in places higher than the design bank heights and at these places existing bank heights were retained.

–  –  –

However, more recent estimates (Griffiths 2009) suggest the frequency of this event is likely to be greater than was thought at the time, perhaps with a recurrence interval of around 40 – 50 years.

Because there is only limited site data (18 years of record at the Rangiora Traffic Bridge) there is still considerable uncertainty about the likelihood of occurrence of flood events of a certain size, particularly larger floods.

Griffiths (2009):

–  –  –

Upstream of the Okuku confluence (the reach under study in this investigation), no works have been carried out and the river system has become choked with gravel, trees and vegetation (especially willows).

Photos 4.1, 4.

2 and 4.3 show the same reaches of the Ashley (Rakahuri) River and the Okuku River around their confluence in 1977, 2001 and 2012.

There is a marked progressive increase in the density of vegetation within the active fairway of both rivers upstream of the confluence – the active fairway has effectively been narrowed and more substantial islands have formed. This effect is accelerating.

This is not the case downstream of the confluence, where the Rating District maintains the fairway clear of woody vegetation.

The above is clearly illustrated in the photographs which follow.

–  –  –

Photo 4-4: Ashley (Rakahuri) River looking upstream from near Sunken Road 4.1 1% AEP design discharges and results of hydraulic modelling (3 scenarios; as is, cleared fairway and berms, cleared fairway and berms and fairway lowered 0.5m, berms lowered 0.25m) The Ashley (Rakahuri) model (a Mike11 computer model of the lower Ashley (Rakahuri) River from the Okuku confluence to the sea) has been extended back to the Gorge and re-run with a number of scenarios. See Appendix 3 for location plans of the cross-sections used in this extension.

Initially, the model was run:

• With its current state of vegetation.

• With a 100 m fairway cleared and exposed to open shingle and with 100 m berms cleared of vegetation on either side.

• As above with the addition of lowering the central fairway 0.5 m and the berms 0.25 m. This would yield some 2 million cubic metres of material.

The philosophy here is that gravel extraction would initially be directed to the central 100 m fairway and excavators would be free to clear vegetation 75-100 m on either side of this, vegetation that is not regarded as valuable for erosion protection. Following this the berm could be lowered on average

0.25 m in a strategic manner. Initial areas that could be lowered are shaded blue on the plans attached as Appendix 2.

–  –  –

The 1% AEP design discharges initially run through the middle Ashley (Rakahuri) River system are listed in Table 4-1 below.

The reducing values moving upstream are because of the tributaries that enter the main stream on the left side between the Okuku confluence and the Gorge, namely, from west to east, the Glentui, the Garry, Mt Thomas Stream and Bullock Creek; these last two merging into one not far upstream of their confluence with the Ashley (Rakahuri) River.

Table 4-1: 1% AEP (100 year return period) discharges

–  –  –

A comparison of the initial two modelling runs is tabulated in Table 4-2 below.

Table 4-2: Computed Flood Levels 1% AEP (100 year return period event)

–  –  –

22.100 77900 94.500 94.400 0.100 24.500 75500 107.596 107.530 0.066 26.900 73100 122.522 122.266 0.256 29.300 70700 136.228 136.107 0.121 31.700 68300 151.347 151.080 0.267 33.300 66700 161.375 161.175 0.200 34.900 65100 171.715 171.529 0.186 37.300 62700 187.621 187.291 0.330 38.900 61100 198.232 198.078 0.154 41.300 58700 214.059 213.838 0.221

• Not surprisingly, farmed lands between the river and the terraces on either side would be inundated in the 1% AEP event.



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