The Suspended Sediment Regime of the Waipaoa River

Murray Hicks

NIWA, Christchurch

NIWATaihoro Nukurangi

Thanks to ….• Landcare Research

– Noel Trustrum – Ted Pinkney

• Basil Gomez• Gisborne District Council

– Dave Peacock

• Hydro-Technologies – Bruce Walpole– Ingrid Pak– Ross Bannister

NZ total:

211 Mt/y

(~ 1% global yield)

Context

15 Mt/y(~0.1%)

35 Mt/y

Content

• NZ and global context

• Datasets

• SS rating characteristics

• C-Q hysteresis behaviour

• Event yields

• Hyperpycnal discharge potential

• Summary

DatasetsDatasets

Mangatu(1999 ..)

Waihora(1999..)

Te Arai(2001..)

Waipaoaat

Kanakanaia(1997..)

Continuous monitoring

Data-logger

Auto-sampler

Mangatu at Omapere

Turbidity sensor(s) 0-50,000 mg/l (0-5,000 mg/l)

Stage sensorSampler intake

SSC vs Discharge Relationship: Waipaoa at Kanakanaia

10

100

1000

10000

100000

1 10 100 1000 10000

Discharge (m3/s)

SS

C (

mg

/l)

No gaugings 301

Maximum C (mg/l)

36814

Max gauged flow (m3/s)

1826

Maximum recorded flow (m3/s)

Mean annual flood (m3/s)

SS yield (t/km2/yr)

Average d50

(microns)

5286

1346

6750

13

Bias-correctedLOWESS curve

Qmaf QmaxQmean

Annual yields at Kanakanaia

• Mean: 10.7 Mt/yr

• Maximum: 32 Mt (1988, Cyclone Bola)

• Minimum: 2.5 Mt

• Std deviation: 6.7 Mt/yr

1

10

100

1000

10000

100000

100 1000 10000 100000 1000000 10000000

Water discharge (l/s)

SS

C (

mg

/l) 16-Nov-97

1-Jul-98

3-Dec-98

21-Jan-99

5-Jun-99

21-Dec-99

Gaugings 1962-1996

Hysteresis behaviour

Example

storm: 2-12 July 2000

0

200

400

620

Flow

m3/s

2-Jul-2000 1.6 hrs/mm 6-Jul 8-Jul 10-Jul 12-Julsite 19701 Waipaoa at Kanakanaia Br Flow m3/s 10units/mmsite 99996 Final Kanakanaia SSC Calibration July 2002 conc ppm 403units/mm

Waipaoa25000

SSC(mg/l)

0

Q(m3/s)

0

20

40

67

Flow

m3/s

2-Jul-2000 1.6 hrs/mm 6-Jul 8-Jul 10-Jul 12-Julsite 19706 Waihora at No 3 Bridge Flow m3/s 1units/mmsite 99994 Final Waihora SSC from Calibration July 2002 conc ppm 373units/mm

Waihora25000

SSC(mg/l)

0

Q(m3/s)

0

20

40

60

77

flow m

3/s

2-Jul-2000 1.6 hrs/mm 6-Jul 8-Jul 10-Jul 12-Julsite 19712 Mangatu at Omapere Flow flow m3/s 1.149units/mmsite 99993 Final Mangatu SSC from Calibration July 2002 SSC ppm 373units/mm

Mangatu25000

SSC(mg/l)

0

Q(m3/s)

Example

storm: 2-12 July 2000

0

5000

10000

15000

20000

25000

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Q/Qmean

SS

C (

mg

/l)

Kanakanaia

Mangatu

Loop of loops

Event peak SS Concentration

0

10000

20000

30000

0 200 400 600 800Event peak discharge (m3/s)

Ev

en

t p

ea

k S

SC

(m

g/l) R2 = 0.07

10

100

1000

10000

100000

1 10 100 1000 10000

Discharge (m3/s)

SS

C (

mg

/l)

All data

< 3 yrs after Bola

Effect of large events: Cyclone bola

x2

Event-yield magnitude-frequency relationships

0.1

1

10

100

1000

10000

100000

-2 0 2 4 6

Return Period(Extreme value reduced variate )

Yie

ld (

tkm

-2)

Mangatu

Waipaoa

Waihora

Te Arai

1-yr1-mo

Shallow landslip(e.g. Waihora, Te Arai)

Gullied(e.g. Mangatu)

Characteristic erosion terrains

Event-yield magnitude-frequency relationships

0.1

1

10

100

1000

10000

100000

-2 0 2 4 6

Return Period(Extreme value reduced variate )

Yie

ld (

tkm

-2)

Mangatu

Waipaoa

Waihora

Te Arai

Gullied

LandslipMixed

Event effectiveness

0%

20%

40%

60%

80%

100%

0.01 0.1 1 10 100

Return period (yr)

Percent of long term sediment

yield Waipaoa

Mangatu

Te Arai

50%

2 yr1.1 yr

Waipaoa River Basin

Bankfulldischarge atKanakanaia

Cum

ulat

ive

Perc

ent Su

spen

ded

Sedi

men

t Lo

ad

Discharge (m3 s-1)

100

80

60

40

20

00 1000 2000 3000 4000 5000 6000

Hyperpycnal discharge potential

Hyperpycnal discharge threshold concentration

Cr > (o0 - r0) / (1 - 0 / s)

River SS concentration

Ocean waterDensity (clear)

River waterdensity (clear)

Sedimentmineraldensity

Referencewaterdensity

Waipaoa/Poverty Bay Conditions

River temp (oC) 10 16 10 16

River density (gcm-3) 1.00013 0.99965 1.00013 0.99965

Ocean density (gcm-3) 1.025 1.025 1.023 1.023

Location Outer Poverty Bay River plume*

Season Winter Summer Winter Summer

Cr

(mgl-1) 39946 40705 36733 37494

37,000-41,000 mg/l* Stephens et al., 2002

Hyperpycnal threshold exceedences

0.1

1

10

100

1000

10000

100000

0.01 0.1 1 10 100

Q/Qmean

SS

C (

mg

/l)~40,000 mg/l

37,000-41,000 mg/l

Hyperpycnal threshold exceedences

1-severalexceedencesin last 40 years??

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 20 40 60 80

Q/Qmean

SS

C (

mg

/l)

Waipaoa

RatingNeedcontinuousmonitoring

Summary

• East Cape Region supplies ~ 0.3% sediment flux to global ocean– Waipaoa supplies ~ 15 Mt/yr

• SSC vs Q ratings – top-end curvature– clockwise hysteresis during flood events, over

individual peaks and overall, reflecting sediment supply effects

– event peak SSC poorly correlated with peak discharge– maximum sampled SSC ~ 37,000 mg/l

Summary continued …• Event-yield magnitude-frequency distributions

– Show hillslope erosion-process signatures

– Rarer, larger events more important at delivering long-term sediment yield from landslip terrain, common events more important for gullied terrain

– Waipaoa mainstem shows “mixed” behaviour

• Threshold SSC for hyperpycnal river discharges – ~ 40,000 mg/l

– exceeded 1-several times over last 40 years, but very uncertain

– Recommend maintaining continuous monitoring, ideally at Matawhero, nearer coast