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The Suspended Sediment Regime of the Waipaoa River

NIWA Taihoro Nukurangi. The Suspended Sediment Regime of the Waipaoa River. Murray Hicks NIWA, Christchurch. Landcare Research Noel Trustrum Ted Pinkney Basil Gomez Gisborne District Council Dave Peacock Hydro-Technologies Bruce Walpole Ingrid Pak Ross Bannister. Thanks to ….

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The Suspended Sediment Regime of the Waipaoa River

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  1. NIWA Taihoro Nukurangi The Suspended Sediment Regime of the Waipaoa River Murray Hicks NIWA, Christchurch

  2. Landcare Research Noel Trustrum Ted Pinkney Basil Gomez Gisborne District Council Dave Peacock Hydro-Technologies Bruce Walpole Ingrid Pak Ross Bannister Thanks to ….

  3. Context NZ total: 211 Mt/y (~ 1% global yield)

  4. 35 Mt/y 15 Mt/y (~0.1%)

  5. Content • NZ and global context • Datasets • SS rating characteristics • C-Q hysteresis behaviour • Event yields • Hyperpycnal discharge potential • Summary

  6. Waihora (1999..) Mangatu (1999 ..) Waipaoa at Kanakanaia (1997..) Te Arai (2001..) Datasets Datasets

  7. Auto- sampler Data-logger Turbidity sensor(s) 0-50,000 mg/l (0-5,000 mg/l) Stage sensor Sampler intake Mangatu at Omapere Continuous monitoring

  8. Bias-corrected LOWESS curve Qmaf Qmax Qmean SSC vs Discharge Relationship: Waipaoa at Kanakanaia

  9. 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

  10. Hysteresis behaviour

  11. 25000 SSC (mg/l) 0 Waipaoa Q (m3/s) 25000 SSC (mg/l) 0 Mangatu Q (m3/s) 25000 SSC (mg/l) 0 Waihora Q (m3/s) Example storm:2-12 July 2000

  12. Loop of loops Example storm:2-12 July 2000

  13. Event peak SS Concentration R2 = 0.07

  14. x2 Effect of large events: Cyclone bola

  15. 1-yr 1-mo Event-yield magnitude-frequency relationships

  16. Shallow landslip (e.g. Waihora, Te Arai) Gullied (e.g. Mangatu) Characteristicerosion terrains

  17. Gullied Mixed Landslip Event-yield magnitude-frequency relationships

  18. 50% 2 yr 1.1 yr Event effectiveness

  19. Waipaoa River Basin

  20. Hyperpycnal discharge potential

  21. River SS concentration Ocean water Density (clear) Sediment mineral density River water density (clear) Reference water density Hyperpycnal discharge threshold concentration Cr > (o0 - r0) / (1 - 0 / s)

  22. River plume* Location Outer Poverty Bay Winter Season Winter Summer Summer Cr (mgl-1) 39946 40705 36733 37494 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 37,000-41,000 mg/l * Stephens et al., 2002

  23. Hyperpycnal threshold exceedences ~40,000 mg/l

  24. 37,000-41,000 mg/l Hyperpycnal threshold exceedences 1-several exceedences in last 40 years?? Need continuous monitoring

  25. 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

  26. 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

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