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

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

Datasets Mangatu (1999..) Waihora (1999..) Te Arai (2001..) Waipaoa at 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 sensor Sampler intake

SSC vs Discharge Relationship: Waipaoa at Kanakanaia No gaugings301 Maximum C (mg/l) Max gauged flow (m 3 /s) 1826 Maximum recorded flow (m 3 /s) Mean annual flood (m 3 /s) SS yield (t/km 2 /yr) Average d 50 (microns) Bias-corrected LOWESS curve Qmaf Qmax Qmean

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

Example storm: 2-12 July 2000 Waipaoa SSC (mg/l) 0 Q (m 3 /s) Waihora SSC (mg/l) 0 Q (m 3 /s) Mangatu SSC (mg/l) 0 Q (m 3 /s)

Example storm: 2-12 July 2000 Loop of loops

Event peak SS Concentration R 2 = 0.07

Effect of large events: Cyclone bola x2

Event-yield magnitude-frequency relationships 1-yr1-mo

Shallow landslip (e.g. Waihora, Te Arai) Gullied (e.g. Mangatu) Characteristic erosion terrains

Event-yield magnitude-frequency relationships Gullied Landslip Mixed

Event effectiveness 50% 2 yr 1.1 yr

Waipaoa River Basin

Hyperpycnal discharge potential

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

Waipaoa/Poverty Bay Conditions River temp ( o C) River density (gcm -3 ) Ocean density (gcm -3 ) LocationOuter Poverty Bay River plume* SeasonWinterSummer Winter Summer C r (mgl -1 ) ,000-41,000 mg/l * Stephens et al., 2002

Hyperpycnal threshold exceedences ~40,000 mg/l

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

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