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