The Suspended Sediment
Regime of the Waipaoa River
Murray Hicks
NIWA, Christchurch
NIWA
Taihoro Nukurangi
Thanks to ….
• Landcare Research
– Noel Trustrum
– Ted Pinkney
• Basil Gomez
• Gisborne District Council
– Dave Peacock
• Hydro-Technologies
– Bruce Walpole
– Ingrid Pak
– Ross Bannister
Context
NZ total:
211 Mt/y
(~ 1% global
yield)
35
Mt/y
15 Mt/y
(~0.1%)
Content
•
•
•
•
•
•
•
NZ and global context
Datasets
SS rating characteristics
C-Q hysteresis behaviour
Event yields
Hyperpycnal discharge potential
Summary
Datasets
Datasets
Waihora
Mangatu
(1999..)
(1999 ..)
Waipaoa
at
Kanakanaia
(1997..)
Te Arai
(2001..)
Continuous monitoring
Autosampler
Data-logger
Turbidity sensor(s)
0-50,000 mg/l
(0-5,000 mg/l)
Stage sensor
Sampler intake
Mangatu at Omapere
SSC (mg/l)
SSC vs Discharge Relationship: Waipaoa at Kanakanaia
100000
No gaugings
301
36814
10000
Maximum C
(mg/l)
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
1000
Bias-corrected
LOWESS curve
100
Qmean
Qmaf Qmax
10
1
10
100
Discharge (m3/s)
1000
10000
1346
6750
13
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
Hysteresis behaviour
100000
SSC (mg/l)
10000
1000
16-Nov-97
1-Jul-98
100
3-Dec-98
21-Jan-99
5-Jun-99
10
21-Dec-99
Gaugings 1962-1996
1
100
1000
10000
100000
Water discharge (l/s)
1000000
10000000
620
Flow m3/s
Waipaoa
25000
400
SSC
(mg/l)
Q
(m3/s) 200
Example
storm:
0
2-Jul-2000 1.6 hrs/mm
6-Jul
8-Jul
10-Jul
site 19701 Waipaoa at Kanakanaia Br Flow m3/s 10units/mm
site 99996 Final Kanakanaia SSC Calibration July 2002 conc ppm
403units/mm
12-Jul
25000
77
2-12 July 2000
flow m3/s
60
Q
(m3/s)
0
Mangatu
SSC
(mg/l)
40
20
0
67
2-Jul-2000 1.6 hrs/mm
6-Jul
8-Jul
10-Jul
site 19712 Mangatu at Omapere Flow flow m3/s 1.149units/mm
site 99993 Final Mangatu SSC from Calibration July 2002 SSC ppm
373units/mm
12-Jul
0
25000
Waihora
40
Flow m3/ s
Q
(m3/s)
SSC
(mg/l)
20
0
2-Jul-2000 1.6 hrs/mm
6-Jul
8-Jul
10-Jul
site 19706 Waihora at No 3 Bridge Flow m3/s 1units/mm
site 99994 Final Waihora SSC from Calibration July 2002 conc ppm
373units/mm
12-Jul
0
Example
storm:
2-12 July 2000
Loop of loops
25000
Kanakanaia
20000
SSC (mg/l)
Mangatu
15000
10000
5000
0
0.0
0.5
1.0
1.5
Q/Qmean
2.0
2.5
3.0
Event peak SS Concentration
Event peak SSC (mg/l)
30000
R2 = 0.07
20000
10000
0
0
800
600
400
200
3
Event peak discharge (m /s)
Effect of large events: Cyclone bola
100000
x2
SSC (mg/l)
10000
1000
100
All data
< 3 yrs after Bola
10
1
10
100
Discharge (m3/s)
1000
10000
Event-yield magnitude-frequency relationships
100000
Yield (tkm-2)
10000
1000
100
Mangatu
Waipaoa
10
Waihora
Te Arai
1
1-mo
1-yr
0.1
-2
0
2
4
Return Period
(Extreme value reduced variate )
6
Characteristic erosion terrains
Shallow landslip
(e.g. Waihora, Te Arai)
Gullied
(e.g. Mangatu)
Event-yield magnitude-frequency relationships
100000
Gullied
Mixed
Landslip
Yield (tkm-2)
10000
1000
100
Mangatu
Waipaoa
10
Waihora
Te Arai
1
0.1
-2
0
2
4
Return Period
(Extreme value reduced variate )
6
Event effectiveness
100%
80%
Percent of
long term
sediment
yield
60%
50%
Waipaoa
40%
Mangatu
20%
0%
0.01
0.1
Te Arai
2 yr
1.1 yr
1
Return period (yr)
10
100
Waipaoa River Basin
Cumulative Percent Suspended Sediment Load
100
80
60
40
Bankfull
discharge at
Kanakanaia
20
0
0
1000
2000
3000
4000
Discharge ( m3 s-1)
5000
6000
Hyperpycnal
discharge
potential
Hyperpycnal discharge threshold
concentration
Cr
River SS
concentration
> (o0 -  r0) / (1 -  0 /  s)
Ocean water
Density (clear)
River water
density (clear)
Reference
water
density
Sediment
mineral
density
Waipaoa/Poverty Bay Conditions
Location
Outer Poverty Bay
Season
Winter
River temp (oC)
10
River density
(gcm-3)
Summer
16
River plume*
Winter
Summer
10
16
1.00013 0.99965
1.00013
0.99965
Ocean density
(gcm-3)
1.025
1.023
1.023
Cr
(mgl-1)
39946
36733
37494
1.025
40705
37,000-41,000 mg/l
* Stephens et al., 2002
Hyperpycnal threshold exceedences
100000
10000
~40,000 mg/l
SSC (mg/l)
1000
100
10
1
0.1
0.01
0.1
1
Q/Qmean
10
100
Hyperpycnal threshold exceedences
50000
45000
37,000-41,000 mg/l
40000
SSC (mg/l)
35000
1-several
exceedences
in last
40 years??
30000
25000
20000
15000
Waipaoa
Rating
10000
Need
continuous
monitoring
5000
0
0
20
40
Q/Qmean
60
80
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