Review of existing data on erosion rates and sediment yield for...

2002-03/02

Review of existing data on erosion rates and sediment yield for the Motueka catchment: Progress Report

Prepared for

Stakeholders of the Motueka Integrated Catchment Management Programme

June 2003

Landcare ICM Report No. 2002-03/02

Motueka Integrated Catchment Management Programme Report Series: Review of existing data on erosion rates and sediment yield for the Motueka catchment

June 2003

Review of existing data on erosion rates and sediment yield for the Motueka catchment: Progress Report

Motueka Integrated Catchment Management (Motueka ICM) Programme Report Series

by

L.R. BasherA and D.M. HicksB

A Landcare Research, P.O. Box 69, Lincoln B NIWA, P.O. Box 8602, Christchurch

Disclaimer: As this report contains unpublished data from a variety of sources, information

in it may not be quoted without the permission of the authors

Cover Photo: Motueka River at Woodstock in flood.



June 2003

PREFACE

An ongoing report series, covering components of the Motueka Integrated Catchment Management (ICM) Programme, has been initiated in order to present preliminary research findings directly to key stakeholders. The intention is that the data, with brief interpretation, can be used by managers, environmental groups and users of resources to address specific questions that may require urgent attentin or may fall outside the scope of ICM research objectives.

We anticipate that providing access to environmental data will foster a collaborative problem-solving approach through the sharing of both ICM and privately collected information. Where appropriate, the information will also be presented to stakeholders through follow-up meetings designed to encourage feedback, discussion and coordination of research objectives.



June 2003

Introduction

Defining the sources and fluxes of sediment, and its impacts on aquatic ecosystems, has been

recognised as a key issue in the Motueka catchment since the inception of the Integrated

Catchment Management programme (Dunne and Likens 2000). No research on this topic was

undertaken in the first 2-year contract period of the ICM programme. However extensive

consultation with Tasman District Council, Nelson-Marlborough Fish and Game Council,

forestry companies, anglers and Cawthron Institute continually identified sediment as a key

land management issue in the catchment and clarified the nature of the research needed. This

included determining:

- total sediment delivery to Tasman Bay,

- variation in river and stream bed composition spatially and temporally,

- impacts of sediment on aquatic ecosystems (freshwater and marine),

- appropriate management of gravel extraction rates and sources,

- major influences on sediment supply and dynamics (e.g., geology, rainfall, vegetation,

land use and land management practices) and the extent to which sediment supply can

be altered by land management practices.

Since 1985 there has been a widespread and significant decline in the abundance and biomass

of trout in the Motueka River. This decline has been linked to increased input of fine

sediment into the river (particularly sand from the Separation Point granite) and its direct and

indirect effects on trout habitat and trout populations, primarily the proportion of sand and silt

in the bed of the river which is believed to have increased in many parts of the river system.

While the decline in the trout fishery is fairly well established the causes are not well known.

There is little data on riverbed characteristics throughout the Motueka, spatial and temporal

variation in rates of sediment supply, the time scale for sediment movement through the river

system, the major sources of sediment, the relative role of natural erosion and that related to

land-use (forestry or pasture), and the long-term impact of large storms.

Similarly, coastal productivity is dependent on an adequate supply of light for photosynthesis

and sediments discharged into the coastal environment from the Motueka can affect available

light in the waters of Tasman Bay. When these sediments settle to the sea floor, they can play

an important role in dictating the physical and chemical properties (and consequently the

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June 2003

plant and animal communities) of the benthic environment. However little is known of the

total quantity of sediment discharged to Tasman Bay, the temporal variation in sediment

supply, and the physical and chemical characteristics of the sediment.

This report reviews existing published and unpublished information on erosion rates and

sediment yield for the Motueka catchment and identifies areas where improved information is

required.

Sediment yield

There has been limited research to measure the total amount of sediment delivered to Tasman

Bay, or the variation in sediment yield within the Motueka catchment. Griffiths and Glasby

(1985) calculated the suspended sediment yield of the Motueka River to the coast at 277

t/km2/yr. However, this estimate was based on a relationship between suspended sediment

yield and rainfall derived from dominantly greywacke and schist catchments throughout the

South Island (Griffiths 1981), and probably overestimated the yield (because of lower erosion

rates on the rock types in the Motueka, and annual rainfall was probably overestimated at

1800 mm). Griffiths (1981) lists no regional regression relationship between sediment yield

and precipitation for the complex geology of north-west Nelson. Similarly, the distribution of

sediment yield across the catchment is not well known. Mosley (1980), using the Griffiths

(1981) method, suggests much of the yield is derived from the high-rainfall, steep terrain of

the west bank tributaries and estimates rates of 119 t/km2/yr from the Dart River and 583

t/km2/yr from the entire Wangapeka River.

These published estimates of sediment yield have been re-examined by compiling all the available data currently in NIWA and TDC archives. Suspended sediment gauging data exists for five sites in the Riwaka and Motueka catchments (Table 1):

- Motueka at Gorge (representing the higher rainfall, mountainous headwaters on Maitai Group sedimentary rocks and Dun Mountain ultramafics)

- Motueka at Woodstock (on the main stem and indicative of the whole catchment sediment yield)

- Stanley Brook at Barkers (one of the larger, low rainfall catchments on Moutere gravels)

- Wangapeka at Walter Peak (large catchment representing the high rainfall, mountainous headwaters on a wide variety of basement rocks)

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June 2003

- Riwaka at Moss Bush (smaller, lower rainfall catchment on marble and basic igneous

rocks)

- Motueka at Woodmans Bend (on the main stem c.10.5 km from the sea, providing the best estimate of discharge to Tasman Bay)

Gauging data are those that have been collected by trained hydrological technicians using depth-integrating suspended sediment samplers (typically USGS D-49 and USGS DH-48 types) deployed at multiple verticals over the river cross-section, and are typically done in conjunction with a water discharge gauging. This permits determination of the suspended sediment discharge and the discharge-weighted suspended sediment concentration (which equals the sediment discharge divided by the water discharge) for the gauging cross-section. The data were collected by hydrological staff of NIWA and precursor organisations (Ministry of Works and Development), and are archived on Water Resources Archive, NIWA, Christchurch (except for Woodmans Bend). Historically, suspended sediment gaugings have tended to be done in phases determined by funding availability and interest, with the last phase terminating in 1995.

The Motueka and Riwaka data are summarised in Table 1, plotted for the individual sites in

Appendix 1, and listed in Appendix 2. They were collected over various time periods, and the

number and usefulness of the data for the purpose of estimating mean annual sediment yield

varies from site to site. Generally, better yield estimates are obtained when the sediment

rating relationship is defined by samples collected over a wide range of discharge – in other

words, the more samples and the wider their distribution over the flow range the better. Other

useful indexes are the maximum sampled discharge in relation to the mean annual flood

discharge and the proportion of the estimated load carried by discharges greater than the

maximum discharge sampled. In these terms, the best datasets are for Riwaka at Moss Bush

and Motueka at Woodstock. Motueka at Gorge has only a few samples at relatively low

discharge, and this dataset is inadequate for estimating a suspended sediment yield. Stanley

Brook has c.50% of the estimated load at discharges where the sediment rating curve has

been extrapolated. The Wangapeka has only four gaugings, but they are at high discharges

and they define a good relationship (Appendix 1). Only limited data is available for the

Motueka at Woodmans Bend and a suspended sediment yield has not yet been calculated.

The data indicate clear differences between sites with the Motueka at Woodstock and

Wangapeka sites having similar suspended sediment concentration-flow relationships, while

the Stanley Brook and Riwaka tend to have higher sediment concentrations for the equivalent

flow (Fig. 1). The Motueka at Gorge samples, although only taken at low flows, tend to plot

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June 2003

nearer the Stanley Brook and Riwaka samples. Samples taken from the Motueka at

Woodmans Bend are tending to plot close to the Woodstock samples.

0.1

1

10

100

1000

10000

0.01 0.1 1 10 100 1000

Flow (cumecs)

Susp

ende

d se

dim

ent c

once

ntra

tion

(mg/

l)

Riw aka

Gorge

Woodstock

Stanley Brook

Wangapeka

Woodmans Bend

Fig. 1 Suspended sediment concentration-flow relationships for the Motueka at Woodstock,

Woodmans Bend and Gorge, Wangapeka at Walters Peak, Stanley Brook at Barkers,

and Riwaka at Moss Bush (south branch).

The sediment rating curves shown in Appendix 1 are the basis of the yield estimates in Table

1, and were generally derived using the Locally-Weighted Scatterplot-Smoothing (LOWESS)

procedure. This fits a local regression model to a limited window of data, and so provides a

better fit than simple regression where the relationship is curved and/or the variance changes

with discharge (e.g., Riwaka). Simple regression was adequate for the Wangapeka. The

sampling error of the rating fits was used to estimate the standard error on the mean annual

yield (Table 1). These errors are factorial – for example, a 1.48 factor means that the standard

error band on the yield lies between the yield divided by 1.48 and the yield multiplied by

1.48. This arises because the ratings were fitted to log-transformed data. These uncertainty

estimates should be regarded as indicative only, since they rely on the assumptions of

normally-distributed and homoschedastic residuals (i.e., independent of discharge) and that

the samples are representative of the population of suspended sediment concentration values

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June 2003

over the period of discharge record (i.e., they are not biased in terms of sampling date,

rising/falling stage, etc).

Two sets of suspended sediment samples have been analysed for particle size, both from

Stanley Brook at Barkers (draining Moutere gravel terrain). The results (Tables 1 and 2),

from depth-integrated samples analysed by SEDIGRAPH (silt-clay fraction) and wet-sieving

(sand fraction), show that the median (D50) size ranged from 25 to 67 microns, while the sand

percentage was 34–51%. These may be regarded as indicative of the size of the suspended

load from the Moutere gravel terrain.

In contrast to published estimates of sediment yield, this analysis of available suspended

sediment data suggests the Stanley Brook on Moutere gravel terrain has a much higher

sediment yield (169 t/km2/yr) than the Wangapeka (46 t/km2/yr) and Riwaka (72 t/km2/yr) on

basement rocks (granite, metavolcanics, metasediments). The relative contribution of erosion

under native vegetation compared to erosion from areas converted to pasture or production

forest in these catchments is not known. It also suggests the total sediment yield (180 t/km2/yr

at Woodstock) may be significantly lower than published estimates (277 t/km2/yr).

In addition to these estimates of sediment yield from moderate to large catchments there is

data on yield from small catchments on Moutere gravels and granite (Table 3). Suspended

sediment yields from two of the Moutere experimental station catchments (0.04 and 0.07 ha),

under an annual rainfall of 1050 mm/yr, were 4 t/km2/yr under pine forest and 79 t/km2/yr

under pasture (Hicks, 1990). The storm events which were most effective in transporting

sediment were those that reoccurred every few months. Smith (1992) also lists measured

sediment yields for three of the Moutere catchments: 21 t/km2/yr under pasture, and 32 and67

t/km2/yr from two catchments under pasture with riparian pine. She suggested increased

erosion in the pasture catchments with riparian pine was associated with poor ground cover in

riparian forest causing overland flow and streambank erosion. In small catchments at Big

Bush Forest (0.05–0.20 ha), under a higher rainfall (1700 mm/yr), sediment yield was 5-35

t/km2/yr under undisturbed native forest (O’Loughlin et al. 1978; O’Loughlin 1980). Over a

Table 1 Summary of suspended sediment gauging data and estimated suspended sediment

yields for sites in the Motueka and Riwaka catchments. Data listed in Appendix 2.

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River Riwaka Motueka Motueka Stanley Brook Wangapeka Site Name Moss Bush Gorge Woodstock Barkers Walter Peak

Site No. 56901 57008 57009 57014 57025

Catchment area (km2)

46.2 163 1750 82.3 479

Flow record period

29/12/61 - present

1/1/65 - present

11/2/69 - present

1/1/70 - present 2/4/81 - present

Qmean (m3/s) 2.51 7.14 59.7 1.17 22.6

Qmaf (m3/s) 52.4 338 1086 59.3 437

Max recorded Q (m3/s)

107.4 1448 2149 122.5 953.2

No. SS gaugings 39 11 32 12 4

No. SS particle size analyses

- - - 2 -

Date first SS gauging

8/9/65 27/2/69 6/6/67 22/12/69 7/11/94

Date last SS gauging

14/1/70 23/3/70 18/7/92 28/5/95 14/11/94

Min Conc (mg/l)



June 2003

Time 15:22 15:40 Discharge (l/s) 38800 23199 %



June 2003

during harvesting and 60 t/km2/yr during the post-harvest phase (Hewitt 2000, 2001b). The

reasons for the low sediment yield during the roading phase include low rainfall at this time,

location of tracks and landing pads along ridges, limited soil exposure at any time and rapid

reversion to scrub.

Ogle (1997) investigated the relationship between roading and harvesting operations and

water clarity in Motueka Forest over a 9-month period. She provides visual clarity and

suspended sediment data for the Herring Stream (unharvested control), and at three sites in

the Little Pokororo and Pokororo Rivers (both being harvested) but does not calculate

suspended sediment yield. She found little conclusive evidence for an impact of forest

harvesting on water clarity and suspended sediment concentrations, but notes the very short

duration of the study which coincided with a period of low storminess and flooding.

As part of Landcare Research’s Erosion-Carbon research programme spatially distributed

estimates of sediment yield have been estimated by Hicks et al. (in prep). These estimates are

based on:

• delineating spatial patterns of “erosion terrains”. An erosion terrain is a landscape with a

specific combination of erosion processes, with characteristic rate of erosion processes

and sediment yield. For the South Island, erosion terrains were determined from the New

Zealand Land Resource Inventory based on rock type (including induration, the presence

or absence of significant loess (>1 m), and the degree of weathering), landform and slope,

soils, and erosion association and severity.

• applying relationships between suspended sediment yield and precipitation for each

erosion terrain. These relationships are derived from measured suspended sediment yields

and the spatial distribution of erosion terrains within gauged catchments. The

relationships have the form: 7.1bPSSY =

where SSY is annual specific sediment yield, P is mean annual rainfall (mm) and b is a

coefficient determined by the Erosion Terrain. The P1.7 term may be viewed as a driving

factor, controlling the intensity of rainfall/runoff related erosion and transport processes,

while the b term probably relates to sediment availability.

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Catchment Area

(km2)

Landuse Rock type Annual

rainfall

(mm)

Suspended

sediment

sampling period

Specific sediment yield (t/km2/yr) Source

Motueka at

coast

2076 Native and exotic

forest, pasture

1800 Estimated yield 277 Griffiths and Glasby (1985)

Motueka at

Woodstock

1750 Native and exotic

forest, pasture

Moutere gravel,

basement rocks,

granite 1600

1967-92 180 Hicks (unpublished)

Wangapeka

River

240 Native forest 310Basement rocks,

granite

0 Estimated yield 583 Mosley (1980)

Wangapeka

River

479 Native forest 1700 1994 46 Hicks (unpublished)

Stanley

Brook

82 Exotic forest, pasture Moutere gravel 1100 1969-95 169 Hicks (unpublished)

Riwaka 46 Native forest, pasture,

horticulture

Marble, basic

igneous rocks

1500 1965-70 72 Hicks (unpublished)

Dart River 81 Native forest, pine

forest

Granite 2000 Estimated yield 119 Mosley (1980)

Dart River 16.9 Pine forest, roaded area Granite 2000 1978-79, field

survey

710

Mosley (1980)

Kaiteriteri 0.76 Pine forest, roaded and

harvested

Granite 1500 1995-2001 175 (average) - 365 (during roading),

40-180 (pre-harvest), 378 (harvesting),

56 (post harvesting)

Hewitt (2001a, 2002)

Apahi 0.71 Pine forest, roaded and Granite 1500 1995-2001 205 (average) - 570 (during roading), Hewitt (2001a, 2002)

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Catchment Area

(km2)


rainfall

(mm)

Suspended

sediment

sampling period


harvested 88-296 (pre-harvest), 68-237

(harvesting), 27-148 (post harvesting)

Greenhill 3.09 Pine forest, roaded and

harvested

Granite 1996-2001 32.9 (pre-harvest), 7.5 (during roading,

start harvesting), 81.5 (harvesting), 60

(post harvesting)

Hewitt (2000, 2001b)

Pokororo 23.6 Pine forest, harvested 1825 1997-2001 57 (average) - 21-111 (harvesting), 21

(post harvesting)

Hewitt (2001a, 2002)

Little

Pokororo

8.6 Pine forest, harvested 1825 1997-2001 90 (average) - 44-151 (harvesting), 45

(post harvesting)

Hewitt (2001a, 2002)

Herring 6.1 Pine forest 1825 1997-2001 30 (18-44) Hewitt (2001a, 2002)

Stanley

Brook

81.6 Pasture, exotic forest Moutere gravel 1969-95 169 Hicks (pers. comm. 2002)

Moutere 5 0.04 Pasture Moutere gravel 1050 1983-86 78.5 Hicks (1990)

Moutere14 0.07 Exotic forest Moutere gravel 1050 1986-87 4.0 Hicks (1990)

Moutere C2 0.07 Pasture Moutere gravel 1050 1986-87 21 Smith (1992)

Moutere C3 0.03 Pasture with riparian

pine

Moutere gravel 1050 1986-87 67 Smith (1992)

Moutere C4 0.03 Pasture with riparian

pine

Moutere gravel 1050 1986-87 32 Smith (1992)

Big Bush 1 0.09 Native forest converted

to pine

Moutere gravel 1700 1976-92 5.4 (native), 200-530 (following

harvesting by clearfelling/skidder

O’Loughlin et al. 1978, Fahey et al.

1993, Fahey and Jackson 1993

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June 2003

Catchment Area

(km2)


rainfall

(mm)

Suspended

sediment

sampling period


logging)

Big Bush 2 0.05 Native forest Moutere gravel 1700 1976-92 5-35 O’Loughlin et al. 1978, Fahey et al.


Big Bush 3

and 4

0.08

and

0.20

Native forest converted

to pine

Moutere gravel 1700 1976-92 10.8 (native), 20-85 (following

harvesting by selection logging or

clearfelling/hauler)

O’Loughlin et al. 1978, Fahey et al.


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Estimated sediment yield in the Motueka (Fig. 2) and Riwaka (Fig. 3) ranges from 10–50 t/km2/yr

to 2000–5000 t/km2/yr, with much of the catchments between 50 and 500 t/km2/yr. The highest

sediment yields are estimated to be in the headwaters of the Wangapeka and the upper Motueka,

with small areas of high yield predicted in the Riwaka also.

Erosion processes and rates

Historically there have been concerns about erosion where land has been cleared for pastoral or

orchard development on the Moutere gravels (e.g., McCaskill, 1973). More recently attention has

focused on land disturbance and forestry activities (e.g., roading, landing construction, forest

harvesting) on the Separation Point granite (e.g., Coker and Fahey 1993, 1994; Fahey and Coker

1989).

Soil conservation reserves and experimental stations on Moutere gravels were operated at Appleby

(to investigate erosion management in orchards at a time (1940s and 1950s) when soils were

cultivated and kept bare between the trees), and at Moutere (to investigate erosion associated with

clearing for pastoral development). McCaskill (1973) reports annual erosion rates from small plots

at Moutere up to 360 t/km2 from bare soil and 160 t/km2 from pasture. Soil losses over a 4-year

period averaged 221 t/km2/yr from cultivated and cropped plots, 21 t/km2/yr from grazed pasture

(both on a 7º slope), and 15 t/km2/yr from grazed pasture on a 17º slope (Scarf 1970, Ministry of

Works 1970). Sediment yields from small experimental catchments at Moutere are reported in

Hicks (1990) and Smith (1992) and discussed in the previous section of this report.

The only comprehensive description of erosion types in the catchment is in the New Zealand Land

Resource Inventory (Hunter 1974, 1975a,b, Lynn 1975a, b, 1977a, b, c, Williams 1975). A wide

variety of erosion types were mapped (sheet, gully, debris avalanche, soil slip, scree), generally at

low severity (1-2). The most severe erosion was mapped (Fig. 4) on the soils of the ultramafic rocks

in the upper Motueka (wind and gully), and on high elevation soils on greywacke, schist and granite

(in the

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June 2003

Fig. 2 Predicted spatial distribution of suspended sediment yield in the Motueka catchment (Hicks

et al. in prep.)

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Fig. 3 Predicted spatial distribution of suspended sediment yield in the Riwaka catchment (Hicks et

al. in prep.)

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June 2003

Fig. 4 Map of erosion severity in the Motueka and Riwaka catchments from the New Zealand Land Resource Inventory

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June 2003

western ranges). Severity as mapped in the NZLRI was largely a function of the extent of bare

ground and has an undefined, but probably poor, relationship with rate of erosion. For example,

currently most concern is about sediment generation from erosion on Separation Point granite under

forestry land use yet this is ranked as low severity in the NZLRI. Similarly, the NZLRI gives a poor

impression of the significance of bank erosion to sediment yield as it usually covers a very small

area but may contribute large quantities of sediment.

Indications of significant sediment sources can be inferred from the soil conservation works that

have complemented the 1982 Motueka Catchment Control Schemes and earlier schemes (Green

1982). Complementing river control measures initiated as part of these schemes were soil

conservation farm plans and erosion control schemes, primarily for gully and streambank

stabilisation on Moutere gravel terrain and on granite. Some erosion control has also been

undertaken for shallow soil slips on Moutere gravel terrain. Green (1982) summarises the erosion

problems of the catchment at the time as:

• surface soil losses in some areas including Moutere gravel and the upper Motueka

headwaters;

• slight to moderate soil slip erosion on Moutere gravel;

• active gully erosion in much of the Moutere gravel and granite terrain;

• active streambank erosion in uncontrolled streams and rivers of all sizes particularly on

Moutere gravel and Separation Point granite;

• erosion associated with forestry activities on Moutere gravel and Separation Point granite.

These schemes have also allowed gravel extraction from the riverbed.

Several studies have been carried out on erosion under production forestry on the Separation Point

granite, which is well known for its erosion problems particularly associated with development of

roads and landings.

- Mosley (1980), based on a field survey of forest roads in the Dart valley, suggested the area

of the Dart valley that was roaded had a sediment yield of 710 t/km2/yr (derived from

surface erosion, gullying, mass movement), compared to a background rate for the Dart of

119 t/km2/yr. However, he indicated that the high sediment yield from the relatively small

roaded area had a minor impact on the Wangapeka River because he estimated this river had

a naturally high sediment yield (583 t/km2/yr), and much of the sediment associated with

roading was stored on slopes and in headwater channels.

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- using a plot-based approach (25x4 m plots) Fahey and Coker (1989) estimated sediment

production rates from surface erosion on existing roads in maturing forests in Golden

Downs and Motueka Forest at 37 t/km2/yr. At the time of peak harvesting this was predicted

to rise to 160 to 320 t/km2/yr, compared to a background erosion rate for the Wangapeka of

c.580 t/km2/yr. Infrequent high-magnitude storms were the major contributors to erosion

rates. They suggest that much of the sediment is initially derived from the cut banks,

drainage ditches and sidecast after roads are formed, but as the cut banks stabilise the road

surface becomes a more important source of sediment. Because the roaded areas cover a

small proportion of the forest, it is likely that they make a small contribution to the naturally

high sediment yield of major rivers in this area.

- Four major storms (up to 20-year recurrence interval) in July and August 1990 increased

erosion rates from forest roads by two orders of magnitude. Erosion rate from roads over the

four storms was estimated at 2,800 t/km2 (of which 50% entered streams), mostly from

failures in the cutbanks and sidecasts of roads (van de Graaf and Wagtendok, 1991; Coker

and Fahey, 1993, 1994).

- Coker and Fahey (1994) provided a comprehensive evaluation of the erosion and

sedimentation risk associated with forestry activities on Separation Point granite terrain.

While natural erosion rates at the whole catchment scale on Separation Point granite were

higher (estimated at c.500 t/km2/yr) than those induced by disturbance associated with

forestry at the local scale (37 t/km2/yr for surface erosion and 280 t/km2/yr for mass

movement), they made a number of recommendations to limit sediment production. These

included regulation of landing size, regulation of roading and cutbank formation, safe

storage of excess sidecast material, avoidance of stream crossings by use of appropriate

culverts, and promotion of revegetation following disturbance. These methods, including

end hauling of roading spoil, are now used routinely in forestry activities on Separation

Point granite and are included in the Tasman Resource Management Plan (TDC 1998).

It is worth noting that these conclusions might need to be visited in view of the data presented in the

previous section suggesting that the sediment yield of the Wangapeka River (46 t/km2/yr) is an

order of magnitude lower than the background rate suggested above (c.500 t/km2/yr).

17

Graynoth (1979) described the short-term effects of forest harvesting on sediment generation from Moutere gravel terrain at Golden Downs Forest by comparing a control stream (in native forest) with three streams whose catchments had been affected by different logging practices (clearfelling with and without a buffer strip, partial clearance). In the control stream there was little sand and silt on the stream bed and suspended sediment concentrations were low (0.3–22 g/m3). Clearfelling to the stream's edge, together with inappropriate roading and bridging techniques, caused large



June 2003

amounts of waste timber and soil to enter streams (much of which was flushed quickly by floods), with stream bedloads, suspended sediment and dissolved solid concentrations increasing. Suspended sediment concentrations in flood flows reached 862 g/m3. These changes persisted for at least 3 years. However, sixteen years after the original survey, stream beds had largely recovered (Graynoth 1992) although one had narrowed as a result of the amount of wood deposited in the stream and there was evidence of bank erosion caused by log jams deflecting the stream.

Little work has been done on the impact of large storms on sediment generation apart from that described for the storms of July and August 1990 (van de Graaf and Wagtendok, 1991; Coker and Fahey, 1993, 1994), and the observations at Big Bush that in both pre-harvest and post-harvest periods most sediment was delivered in a few high-intensity storms (Fahey and Jackson, 1993; Fahey et al., 1993).

There are anecdotal accounts of the impact of the “Big Flood” of February 1877, in the early days of European settlement and forest clearance. This flood followed a long spell of dry weather with three days of heavy rain which climaxed in an overnight thunderstorm and rain continuing through the next day (Beatson and Whelan 1993). Brereton (1947) reports “deposition of ten feet of timber, mud and sand” at the Herring River, “opposite Alexander bridge….eight foot of sand and timber”, “many flats covered to ten feet with coarse gravel”, “the flood widened the river and…raised the banks ten feet”, “hundreds or thousands of slips slid into swollen torrents”, “landslides…blocking the valleys…..and the dams carried away into the main stream”, “the settler may have contributed….to this calamity by chopping some of the forest”. Beatson and Whelan (1993) detail newspaper accounts at the time that record “ the creeks from Greenhill …. vomiting forth their contents to the solid rock”, “not only had the river been terribly high, but the creeks must have been dammed by slips ….. which suddenly burst”, “it was not the slips and landslides which caused the widespread destruction ….. when lakes became heavy enough to burst through their barriers pandemonium was let loose”, and several reports of severe sedimentation on river flats. Marshall (2002) suggests one landslide dam in the Baton, at least 16 m high, filled for 3 days and when it burst sent a wall of water 6 m high down the Motueka valley. It seems likely that this flood caused many landslides, widespread erosion and sedimentation, and changed the character of the river in many areas. Beatson and Whelan (1993) record “the Motueka riverbed …. when the Flood took place was approximately ten feet below its present level. Since the destruction of the bush once protecting the river’s watershed innumerable slips have raised the level of the riverbed” and also comment on the effects of earthquakes raising the riverbed. Marshall (2002) suggests that after the flood the lower Motueka had doubled in width and was shallower.

The flow at Woodstock in the “Big Flood” is estimated to lie between 2,500 m3/s (M. Doyle, Tasman District Council, pers. comm. 2002) and 3,500 m3/s (Green 1982), and was probably the largest since European settlement. More recent floods include August 1990, July 1983, April 1957, and April 1974 with peak flows of >2000 m3/s. Other large floods occurred in January 1895, 1911, July 1929, June 1954, August 1972, October 1988 (Green 1982; Fenemor 1989). Rainfalls in excess of about 150 mm over the catchment produce large floods on the main stem, while more-localised flooding can occur in any of the smaller tributaries in response to localised high-rainfall events. This is particularly true of the Wangapeka and Baton catchments, which can produce large floods at Woodstock from heavy westerly rainfall in their headwaters alone.

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June 2003

Gravel supply and extraction

Supply of river gravel within the Motueka catchment is very low and extraction is limited by Tasman District Council (TDC 1993), based on an assessment of rates of gravel supply and river bed stability. Peterson (1997) describes the geomorphic evolution of the Motueka spit and delta and calculates the long-term supply of gravel to the coast at c. 9000 m3/yr (of which 7000-7600 m3/yr accumulates in the delta and 1000-1500 m3/yr is transported along the coast). He suggests, from the volume of material trapped in the Motueka delta, that there has never been a large volume of gravel supplied to the coast or transported down the coast by long-shore drift. He also suggests gravel is being deposited in the lower Motueka River channel with only sand and silt reaching the coast (Petersen 1997). However, substantial quantities of gravel are present on the tidal delta zone adjacent to the main channel of the Motueka suggesting that gravel may be transported to the coast.

The calculated rate of gravel supply is far less than historic gravel extraction rates, which exceeded 40,000 m3/yr for much of the period between 1969 and 1991 (E. Verstappen, pers. comm. 2002) – see Fig. 5. As a consequence gravel extraction has been limited by TDC (reduced from 34,000 m3 in 1991/2 to 11,000m3 in 1995/96), although these restrictions have now been eased with 35,000 m3 permitted in 1999/2000 (TDC 1993, 2000). Limits are set for the upper Motueka (above the Wangapeka confluence), lower Motueka (below Alexander Bluff bridge) and Motupiko Rivers (TDC 2000). In the lower Motueka River, gravel extraction has in recent years been limited to around 5–10,000 m3/year, and in the upper Motueka to around 10–20,000 m3/year. There has also been a policy of very limited to no extraction from the middle reaches of the Motueka River (Alexander Bluff bridge to Wangapeka River).

0

50000

100000

150000

200000

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

2001

Year

Volu

me

of g

rave

l ext

ract

ed (1

000

m3 )

Other

Wangapeka/Sherry

Motupiko

Tadmor

Stanley Brook

Dove

Upper Motueka

Middle Motueka

Low er Motueka

200

100

150

50

0

Fig. 5 Volumes of gravel extracted from the Motueka River (source Tasman District Council).

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June 2003

River cross section surveys have been one of the primary methods for investigating trends in mean

bed levels and changes in gravel storage, and surveys have been carried out on upper and lower

Motueka River1 reaches since at least 1957. Parts of this data have been analysed and summarised

by Tasman District Council (1993) Howes (1994), Nottage (1994, 1995, 1996, 1997, 1998) and

Verstappen (1999, 2000). Sriboonlue and Basher (2003) provide a comprehensive analysis of all the

data, using a consistent methodology, to assess long-term trends in bed levels and gravel storage.

Current understanding (TDC 1993, 2000) suggests that on average current riverbed levels in both

the upper and lower Motueka are lower than at any time since 1957. In the upper Motueka the

average decline (1960–2000) was 0.20 m and in the lower Motueka (1957–2001) was 0.64 m

(Sriboonlue and Basher 2003). However, at individual cross sections bed levels were very dynamic,

with considerable fluctuation between degradation and aggradation from one survey to the next.

These changes imply a loss of gravel from the riverbed of c.728,000 m3 from the upper Motueka

River and c.1,000,000 m3 from the lower Motueka. Gravel extraction accounts for most of this loss

(77% in the upper Motueka and 100% in the lower Motueka) and has resulted in accelerated bed

degradation occurring, particularly in the vicinity of bridge sites where generally there is the

greatest ease of access.

The sources of gravel deposited within the Motueka catchment have been analysed by Waterhouse

(1996). Gravel composition varies systematically down the river as a function of the input of gravel

from major tributaries. In the upper reaches of the river clasts from the headwaters (ultramafics and

Maitai Group) and Moutere gravels dominate, but below the Wangapeka confluence clasts from the

western tributary lithologies and granite are most common. The bulk of the clasts in the lower

Motueka are from the western tributaries, with negligible amounts from the headwaters of the

Motueka and from the Moutere gravel. More than half of the western lithologies are from the Baton

and Wangapeka catchments; the Rocky River is also a substantial contributor to gravel at the

mouth.

The latter is the only study to characterise sediment composition in the Motueka River bed, and its

focus is on coarse sediment. There is no information on patterns of sediment composition within the

riverbed, or on rates of change of sediment composition, particularly for sand which has been

implicated as a major driver affecting trout and invertebrate populations in the river.

20

1 Upper Motueka is the main stem reach from the Wangapeka confluence to Norths bridge; lower Motueka is the reach from the coast to Alexander Bluff bridge.



June 2003

Information gaps

This brief review has identified a number of information gaps relevant to understanding sediment

dynamics and impacts within the Motueka catchment including:

- rates of sediment transport through the fluvial system, and the total sediment delivery to

Tasman Bay. Published estimates of sediment yield appear at variance with the available

data.

- an understanding of variation in sediment generation rate from different rock types and

climatic regimes (Moutere low rainfall catchments c.f. basement rocks, high rainfall,

catchments including the upper Motueka);

- an understanding of the relative importance of major sediment generation processes

(landsliding, stream bank, gullying, surface erosion) and the spatial patterns and rates of

sediment generation by different processes;

- the nature of the sediment storages within stream systems, the source of that sediment and

its impacts on aquatic biology. There is a need for information on gravel and sand supply,

storage and transport through the river system.

Acknowledgements

We thank Roger Young and Tony Hewitt for making unpublished data available to us.

References

Beatson, K.E.; Whelan, H. 1993: The river flows on: Ngatimoti through flood and fortune. K.

Beatson and H. Whelan, Nelson, New Zealand.

Brereton, C.B. 1947: No roll of drums. A.H and A.W. Reed, Wellington.

Coker, R.J.; Fahey, B.D: 1993: Road-related mass movement in weathered granite, Golden Downs

and Motueka Forests, New Zealand: A note. Journal of Hydrology (NZ) 31: 65–69.

Coker, R.J.; Fahey, B.D. 1994: Separation Point granite terrain erosion and sedimentation risk.

Unpublished Landcare Research Contract Report LC9394/70, Landcare Research,

Lincoln, Canterbury.

Dunne, T.; Likens, G.E. 2000: Research for integrated catchment management. Unpublished report

for Landcare Research, Lincoln, New Zealand.

21



June 2003

Fahey, B.D.; Coker, R.J. 1989: Forest road erosion in the granite terrain of Southwest Nelson, New

Zealand. Journal of Hydrology (NZ) 28: 123–141.

Fahey, B.D.; Jackson, R.J. 1993: Hydrological impacts of converting old growth Nothofagus to P.

radiata plantation, Big Bush, New Zealand. Pp. 23–24 in International Symposium on

Forest Hydrology, "Water Issues in Forests Today", 22–26 November 1993, Canberra.

Fahey, B.D.; Jackson, R.J.; Rowe, L.K. 1993: Long-term hydrological changes after beech

conversion to pines, Big Bush, Southwest Nelson. Abstracts New Zealand

Hydrological Society Symposium October 1993, Nelson.

Fenemor, A.D. 1989: Motueka and Riwaka catchments water management plan. Nelson Regional

Water Board, Nelson.

Graynoth, E. 1979: Effects of logging on stream environments and faunas in Nelson. New Zealand

Journal of Marine and Freshwater Research 13: 79–109.

Graynoth, E. 1992: Long term effects of logging practices in streams in Golden Downs State Forest,

Nelson. Pp. 52–69 in New Zealand Freshwater Fisheries Report 136, MAF Fisheries,

Christchurch.

Green, H.R. 1982: Motueka Catchment Control Scheme. Nelson Catchment Board, Nelson.

Griffiths, G.A. 1981: Some suspended sediment yields from South Island catchments, New

Zealand. Water Resources Bulletin 17: 662–671.

Griffiths, G.A.; Glasby, G.P. 1985: Input of river-derived sediment to the New Zealand continental

shelf: 1. Mass. Estuarine, Coastal and Shelf Science 21: 773–787.

Hewitt, T. 2000: Greenhill hydrological monitoring revisited. Unpublished Report for Carter Holt

Harvey Forests Ltd, Envirolink, Nelson.

Hewitt, T. 2001a: Motueka Forest hydrological study: progress report for 2000. Unpublished Report

for Rayonier New Zealand Ltd, Envirolink, Nelson.

Hewitt, T. 2001b: Greenhill hydrological monitoring: summary of results to date. Unpublished

Report for Carter Holt Harvey Forests Ltd, Envirolink, Nelson.

Hewitt, T. 2002: A study of the effects of forest management practices on sediment yields in

granitic terrain in Motueka Forest. Dip. Appl. Sci. dissertation, Massey University,

Palmerston North.

Hicks, D.M. 1990: Suspended sediment yield from pasture and exotic forest basins. Abstracts, New

Zealand Hydrological Society Symposium 1990, Taupo.

Howes, P. 1994: Motueka flood hazards: A review of the Lower Motueka stopbank system. Draft

report to Tasman District Council, Richmond.

22



June 2003

Hunter, G.G. 1974: Sheet S33 St Arnaud (1st edition) New Zealand Land Resource Inventory

Worksheet. Water and Soil Division, Ministry of Works and Development for National

Water and Soil Conservation Organisation, Wellington.

Hunter, G.G. 1975a: Sheet S25 Matiri (1st edition) New Zealand Land Resource Inventory



Hunter, G.G. 1975b: Sheet S26 Hope (1st edition) New Zealand Land Resource Inventory



Lynn, I.H. 1975a: Sheet S14 Motueka (1st edition) New Zealand Land Resource Inventory



Lynn, I.H. 1975b: Sheet S20 Nelson (1st edition) New Zealand Land Resource Inventory



Lynn, I.H. 1977a: Sheet S19 Tadmor (1st edition) New Zealand Land Resource Inventory



Lynn, I.H. 1977b: Sheet S17/18 Little Wanganui (1st edition) New Zealand Land Resource

Inventory Worksheet. Water and Soil Division, Ministry of Works and Development

for National Water and Soil Conservation Organisation, Wellington.

Lynn, I. H. 1977c: Sheet S13 Cobb (1st edition) New Zealand Land Resource Inventory Worksheet.

Water and Soil Division, Ministry of Works and Development for National Water and

Soil Conservation Organisation, Wellington.

Marshall, R. 2002: Grooby, notes on the life and times of the Grooby family of Nottinghamshire,

England and Motueka, New Zealand.

McCaskill, L.W. 1973: Hold this land: a history of soil conservation in New Zealand. A.H. and

A.W. Reed, Wellington.

Ministry of Works 1970: Moutere IHD Experimental Basin No.8. Annual Hydrological Research

Report No. 4, Ministry of Works, Wellington.

Mosley, M.P. 1980: The impact of forest road erosion in the Dart Valley, Nelson. New Zealand

Journal of Forestry 25: 184–198.

Nottage, D. 1994: River gravel: Controlled extractions for 1994/95. Environmental and Planning

staff report EP94/05/15, Tasman District Council, Richmond.

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June 2003









Ogle, B. 1997: An investigation into the effects of roading and harvesting on water clarity in the

Motueka Forest, Nelson, New Zealand. B.For.Sci. dissertation, School of Forestry,

University of Canterbury.

O'Loughlin, C. L. 1980: Water quality and sediment yield consequences of forest practices in north

Westland and Nelson. In: Seminar on Land Use in Relation to Water Quality, Nelson,

7–8 November 1979. Nelson Catchment and Regional Water Board, Nelson. Pp. 152–

171.

O'Loughlin, C.L.; Rowe, L.K.; Pearce, A.J. 1978: Sediment yields from small forested catchments

North Westland – Nelson, New Zealand. Journal of Hydrology 17: 1–15.

Peterson, R. 1997: The influence of gravel from the Motueka River on the coastal system.

Unpublished report to Tasman District Council, Richmond.

Scarf, F. 1970: Some run-off results from Moutere soil conservation station. Hydrological Research

Progress Report No. 1, Ministry of Works, Wellington.

Smith, C.M. 1992: Riparian afforestation effects on water yield and water quality in pasture

catchments. Journal of Environmental Quality 21: 237–245.

Sriboonlue, S. and Basher, L.R. 2003: Trends in bed level and gravel storage in the Motueka River

1957–2001: results from analysis of river cross section data from the upper and lower

Motueka River. Unpublished Report to Integrated Catchment Management programme

and Tasman District Council, Landcare Research, Lincoln.

Tasman District Council 1993: River gravel management issues and options: a public discussion

paper. Tasman District Council, Richmond.

Tasman District Council 1998: Regional plan (land). Tasman District Council, Richmond.

Tasman District Council 2000: Environment today! Tasman 2000. Tasman District Council,

Richmond.

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June 2003

van de Graaf, M.; Wagtendok, A. 1991: Road-related mass wasting Golden Downs and Motueka

Forests, south-west Nelson, New Zealand. Unpublished Report to Forest Research

Institute, Christchurch and Institute of Earth Science, Free University, Amsterdam.

Verstappen E. 1999: River gravel: Controlled extractions for 1999/2000. Environmental and

Planning staff report EP99/09/12, Tasman District Council, Richmond.

Verstappen E. 2000: River gravel: Controlled extractions for 2000/2001. Environmental and Planning staff report EP00/09/17, Tasman District Council, Richmond.

Waterhouse, C. 1996: Motueka river gravel study. Unpublished Report to Tasman District Council,

Richmond.

Williams, N.M. 1975: Sheet S27 Wairau (1st edition) New Zealand Land Resource Inventory



25



June 2003

Appendix 1:

The following graphs show suspended sediment rating relationships and yield results for sites with

suspended sediment gauging data in the Motueka and Riwaka catchments, exported from NIWA’s

suspended sediment rating toolbox. The scatterplots show the relationship between gauged

discharge-weighted mean suspended sediment concentration and water discharge. The left plot has

log-log scales, the right plot has linear scales. The histograms show the proportion of the total

suspended sediment yield carried within given discharge intervals. Matching these proportions to

the distribution of samples gives a good indication of how reliable the rating curve may be. For

example, if much of the load is estimated by extrapolating the rating curve to discharges greater

than those sampled, then the accuracy is decreased. Key results are summarised in Table 1 and the

individual gauging data listed in Appendix 2.

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June 2003

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June 2003

28



June 2003

Woodmans Bend

0.1

1

10

100

1000

1 10 100 1000

Flow (cumecs)

Susp

ende

d se

dim

ent c

once

ntra

tion

(mg/

l)

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June 2003

Appendix 2: Suspended sediment gauging data for sites within Motueka and Riwaka catchments. Data summarised in Table 1.

Site Site No. Date Time Water

discharge (l/s)Suspended sediment concentration (mg/l)

Riwaka 56901 9/08/65 10:30 1594 3 at Moss Bush 56901 10/05/65 14:45 1546 16 56901 6/20/66 12:00 1036 4 56901 8/15/66 11:10 2110 2 56901 8/18/66 12:40 1138 3 56901 9/05/66 11:25 742 12 56901 9/08/66 12:10 1022 6 56901 9/20/66 12:00 7674 6 56901 9/27/66 10:45 1727 2 56901 11/02/66 12:35 881 4 56901 11/07/66 11:50 4955 12 56901 11/08/66 15:00 3115 64 56901 11/10/66 11:15 3002 2 56901 11/14/66 11:00 1492 9 56901 11/15/66 18:45 37661 248 56901 11/16/66 11:40 31017 54 56901 11/18/66 10:50 4474 2 56901 11/25/66 10:20 1767 5 56901 4/14/67 11:30 2183 12 56901 4/20/67 11:45 1257 9 56901 5/24/67 13:00 19113 11 56901 5/24/67 15:20 16056 10 56901 9/07/67 14:00 1722 5 56901 10/12/67 13:30 1674 7 56901 11/16/67 14:45 651 7 56901 5/23/68 14:30 2297 38 56901 6/28/68 15:15 2888 30 56901 7/24/68 19:00 21775 188 56901 7/24/68 20:10 30299 261 56901 9/10/68 14:30 3993 1 56901 1/28/69 12:10 1087 13 56901 2/12/69 15:30 1040 3 56901 2/27/69 13:10 1010 1 56901 3/03/69 11:25 1470 4 56901 7/22/69 13:00 780 28 56901 8/21/69 13:45 790 27 56901 9/10/69 17:45 35113 231 56901 11/17/69 14:15 748 3 56901 1/14/70 15:20 1146 17 Motueka 57008 2/27/69 12:03 2192 6 at Gorge 57008 3/03/69 11:15 5295 6 57008 5/27/69 13:00 5239 11 57008 7/17/69 13:45 4729 9 57008 8/18/69 12:15 2772 76

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57008 9/08/69 12:15 27128 23 57008 9/12/69 12:30 33980 65 57008 10/30/69 14:25 3483 9 57008 11/18/69 13:45 1940 49 57008 12/05/69 13:30 7447 12 57008 3/23/70 12:10 2653 1 Motueka 57009 6/06/67 14:00 22682 30 at Woodstock 57009 7/28/67 11:15 13847 7 57009 7/31/67 12:00 15235 1 57009 8/04/67 15:30 223138 147 57009 8/11/67 13:00 424755 352 57009 9/08/67 15:00 41626 6 57009 9/28/67 13:20 28600 2 57009 11/01/67 11:45 20643 52 57009 11/18/67 17:50 841015 561 57009 11/19/67 11:00 631469 519 57009 11/19/68 18:15 413428 243 57009 11/20/67 14:15 205298 72 57009 11/21/67 15:15 136488 29 57009 1/16/68 14:30 13819 8 57009 2/23/68 14:15 10760 27 57009 2/26/68 12:00 9146 20 57009 3/18/68 14:00 23135 27 57009 4/04/68 14:15 16396 25 57009 5/14/68 14:40 23305 16 57009 6/19/68 14:30 51537 26 57009 2/26/69 12:35 15093 7 57009 2/28/69 11:10 13479 15 57009 6/30/84 14:30 438654 710 57009 7/02/84 19:30 311433 391 57009 7/13/84 13:45 253270 140 57009 8/07/84 14:45 84340 11 57009 8/15/84 11:55 87784 11 57009 12/17/84 11:12 308262 407 57009 1/24/85 11:45 73131 19 57009 12/09/85 18:50 379021 627 57009 7/08/92 15:29 606224 615 57009 7/08/92 17:15 519700 452 Stanley Brook 57014 12/22/69 14:00 2441 13 at Barkers 57014 1/20/70 13:45 101 1 57014 3/23/70 14:30 300 2 57014 9/26/83 13:45 26377 460 57014 1/25/86 17:25 33695 898 57014 7/08/92 13:56 35980 910 57014 2/22/94 10:37 1400 5 57014 4/26/95 12:40 29400 617 57014 4/26/95 15:22 38800 1,730.00 57014 4/26/95 17:17 37600 1,419.00 57014 5/28/95 15:40 23199 891

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57014 5/28/95 16:37 25100 892 Wangapeka at 57025 11/07/94 20:05 564750 635 Walters Peak 57025 11/07/94 22:21 563997 550 57025 11/14/94 10:45 335657 242 57025 11/14/94 12:40 279592 204 Motueka at 13/02/01 12:56 13600 0.9 Woodmans 21/05/02 15:30 18236 2 Bend 21/05/02 18:30 27565 28 21/05/02 21:30 50016 57 22/05/02 0:30 73691 78 22/05/02 3:30 84898 54 22/05/02 6:30 135099 88 22/05/02 9:30 104609 120 22/05/02 12:30 85439 66

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