The monitoring involved assessment of :
• Soil samples for particle size distribution and chemical composition ;
• Real Time Kinematic ( RTK ) survey of drain cross section ;
• HOBO water level stage recorders and barometric data loggers ( Figure 41 );
• Rising Stage Samplers ( RSS ) to measure suspended sediments ;
• Continuous rainfall measurements ; and
• Drone ortho-imagery and derived Digital Elevation Data ( DEMs ) using ground control points
To estimate the fine sediment export from the monitoring road drainage points , the percentage concentration of < 20μm particles in the water samples were multiplied by the total event discharge . This is a simplified method which assumes that the sediment concentration from the samples remains consistent throughout the event , which is rarely the case .
Figure 41 . Rising Stage Sampler assembly and water level data logger installation .
Twenty-five water samples were collected during the monitoring period from December 2020 through to April 2021 ( Figure 42 ), however , there were very few paired sample events . The results show variation between sites and between sample events . Over the whole dataset , the mean of the fine sediment export from V-bottom ( control ) table drains was higher ( 950kg – 9 records ) than the mean from the flat-graded ( treatment ) drains ( 381kg – 10 records ). Comparing paired sampling sites however showed that there was too much variation in the data to confidently conclude that one drain type was more effective at reducing fine suspended export than the other . A greater number of sites needs to be incorporated into the sampling design to achieve a significant result . Observation over a longer sampling period and more events would also provide more clarity .
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