Suspended sediment properties and visual clarity of the Manawatū River, New Zealand

ABSTRACT

Studies of river suspended sediment typically focus on mass loads with little attention devoted to sediment properties such as particle size distribution (PSD) or light attenuation. However, environmental behaviour and ecological effects of suspended sediment vary greatly with particle properties. Visual clarity is a particularly important attribute of waters that is strongly affected by particle properties. We studied the relationship between visual clarity and suspended sediment in the Manawatū River – by combining event auto-sampling with long-term state-of-environment monitoring. We fractionated stormflow samples into sand and ‘mud’ (silt + clay) by wet-sieving at 63 μm, and the mud fraction was further analysed for organics, PSD and light beam attenuation (which controls visual clarity). Light beam attenuation correlated very closely with high-frequency field turbidity – providing an absolute optical calibration. The ratio of light beam attenuation to mass concentration (known as ‘optical cross-section’) averaged about 0.5 m²/g – somewhat higher than expected for silt particles in the measured size range, probably due to plate-shaped clays. The visual clarity regime, estimated from field turbidity, was consistent with monthly direct observations. Our study illustrates how certain sediment ‘quality’ metrics can usefully extend sediment monitoring applications, and we encourage closer integration of sediment and water quality monitoring.

KEYWORDS:

• Suspended sediment
• water quality
• visual clarity
• turbidity
• light attenuation
• particle size distribution

Acknowledgements

We thank Murry Hicks (NIWA Emeritus Scientist) for input to the design of the Manawatū River sediment research. In-kind support from Horizons Regional Council (HRC) was much appreciated. Brent Watson of HRC provided high-frequency data and organised auto-sampling at three study sites ‘piggy-backed’ on HRC’s regional hydrometric and sediment monitoring network. Maree Pattinson of HRC provided water quality data. HRC field technicians, notably Paul Peters, promptly retrieved auto-samples collected over storm events for courier freight to the NIWA-Hamilton Water Quality Laboratory. Mike Crump (NIWA laboratory manager) and several skilled laboratory technicians are thanked for wet-sieving large numbers of stormflow auto-samples, followed by analyses of suspended particulate matter concentrations and organic content. Ron Ovenden and Stephan Heubeck, both formerly of NIWA, are thanked for measuring light beam attenuation on selected auto-samples, and Ron also performed many particle size analyses. David Wood (NIWA) is thanked for incisive review of a technical report on which this article is based.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was funded through a subcontract to NIWA within the ‘Smarter Targeting of Erosion Control' (STEC) research programme led by Manaaki Whenua Landcare Research (MWLR) (MBIE Contract number C09X1804).