Methods
2.1 Modification of the PASS sampler to operate in ephemeral
waterways.
The PASS sampler was originally designed for use in perennial waterways
(e.g., small permanently flowing streams and rivers) (Figure 1) (Doriean
et al., 2019). However, we propose that reconfiguration of the sampler
to operate in ephemerally flowing systems (e.g., gullies) will provide
an affordable alternative or complimentary monitoring method to those
currently used for the measurement of suspended sediment concentration
and particle size in ephemerally flowing systems, such as gullies.
The following modifications have been made to allow the use of the PASS
sampler in gully systems (Figure 1):
- The peristaltic pump has been placed before the settling column,
rather than after it, to allow the sampler to be deployed dry. In its
original design, the sampler needed to be filled with ambient water
before deployment to ensure the peristaltic pump could generate enough
vacuum to collect a sample.
- A small coarse-sediment trap (or initial settling column) has been
added at the intake of the pump to ensure larger particles (e.g., silt
and sand) do not settle within the sampling tubing or damage the pump.
- The main settling column outlet has been re-configured to include a
vertical 4 mm inside diameter polypropylene tube with a 180 degree
down-turn at the outlet to ensure water or debris cannot enter the
sampler through the outlet (Figure 1).
- A float switch has been added and placed at the same height as the
sampler intake to ensure the pump only operates when water is flowing
in the gully. The float switch requires the application of a timer
(e.g., an hour meter or datalogger) linked to the PASS circuitry, or
alternatively in tandem with a water level logger at the location of
the sampler, to determine the sampling period and thereby the volume
of water sampled.
2.2 Laboratory evaluation of gully monitoring
methods
Water quality conditions typical of a gully flow event, based on
previous observations at relevant field sites, were simulated in a
laboratory setting to evaluate the modifications made to the PASS
sampler design and to compare its performance with the other established
methods. A falling suspended sediment concentration trend (high
(~10,500 mg L-1) to low (4,500 mg
L-1)) over a 6-hour period (typical of flow events in
the active gully used for the field evaluation of this study, determined
by preliminary field study data) was simulated using sediment sourced
from a gully at the field study site (median particle size of 29 µm). An
agitation vessel, similar in design to a churn splitter (20 L
cylindrical polypropylene container with four baffles (vertical strips
of aluminium, 0.5 cm thick and 3 cm wide, placed perpendicular to the
side wall, from the bottom to the top, of the vessel) (Ward et al.,
1990)) was used to create a turbulent flow of water during the
experiment (Figure 2).
A triplicate set of PASS sampler intakes were placed approximately 0.15
m above the bottom of the vessel. Sample inlets for discrete sample
collection, identical to the outlet of a churn splitter (Ward et al.,
1990), (6 mm ID polypropylene tube tapped through agitation vessel wall)
and the automatic sampler (Sigma® 900) inlet were placed at the same
level as the PASS inlets to collect discrete samples. The discrete
automatic sampler was elevated (2 m) above its intake point to simulate
the configuration that would typically be used in the field. A turbidity
logger (Observator, NEP495 (measurement range 40-4000 nephelometric
turbidity units (NTU)) was placed at the same level as the sampler
inlets and programmed to record a turbidity measurement every
ten-minutes.
The RS sampler did not fit inside the agitation vessel, thus, a
substitute dataset using the discrete manual sample data (collected from
the isokinetic outlet) was generated to simulate the RS sampler data and
allow comparison with the other techniques. Laboratory test samples
collected using the discrete collection method and an RS sampler were
compared and found to be similar in suspended sediment concentration and
particle size distribution (< 2% ± 1% for both), which
provided confidence to rely on the discrete sample dataset to simulate
RS sample data. Flow event data gathered during a preliminary study,
from the gullies monitored at the field-test site show that there is
little hysteresis between suspended sediment concentration and water
level. Thus, the RS sample data was constructed based on time after
initial flow, estimating the peak stage to occur relatively early during
the simulated event (i.e., the peak water height of the simulated event
occurred 75-minutes into the 6-hour event).
To simulate the flow event, dry gully soil was weighed, suspended in a
small volume of rain water (collected from the laboratory roof) to aid
dispersion, and then diluted in rain water to a predetermined suspended
sediment concentration with a final volume of 15 L. The sediment was
kept in suspension using an overhead stirrer (OS40-S paddle stirrer)
operating at 500 rpm. The concentration was changed by exchanging the
water and sediment solution in the agitation vessel at 30-minute
intervals. Triplicate PASS samplers continuously sampled water from the
agitation vessel during the simulated flow event and repeat discrete
samples (three samples per method) were collected from the same vessel
using flow-proportional discrete sampling, simulated RS sampling and
discrete autosampling methods every 30 minutes (15-minutes after each
change in concentration).
2.3 Field evaluation of gully monitoring
methods
The gullies monitored in this study were located at Crocodile Station in
North Queensland (15°40’08.4”S, 144°35’38.4”E), Australia, and drain
directly into the Laura River, which is connected to the coastal waters
of the northern Great Barrier Reef via the Normanby River (Olley et al.,
2013) (SI-1). The gullies are identified as alluvial gullies because
they are located in alluvial soils of the Laura River floodplain (Brooks
et al., 2013). Two gullies were studied to evaluate the accuracy and
limitations of the monitoring techniques for measuring suspended
sediment dynamics of gullies at different stages of erosion: an actively
eroding gully (gully-1) with high suspended sediment output consisting
of fine sediment (<63 µm) and some suspended sand (63-2000
µm); and a gully remediated in 2016 (gully-2) with relatively low
suspended sediment output dominated by fine sediment (<63 µm)
(SI-2). The suspended sediment particle size data used to describe the
suspended sediment characteristics of the test gullies was gathered in a
pilot study conducted at the study site.
The evaluated monitoring techniques consisted of a Sigma® 900
autosampler, a modified PASS sampler, a RS sampler array (six stages),
and an Observator® NEP495 turbidity logger). Instruments were deployed
in close proximity to each other in a straight section of channel
approximately 50 m and 110 m downstream from the head of gully 1 and 2
respectively. The autosampler was placed on the bank beside the channel
(elevated approximately 2 m above its intake) with the intake positioned
in the middle of the channel cross section, 0.2 m above the channel bed
with the inlet facing downstream (SI-3). A float switch, placed at the
intake, was used to initiate and halt sampling. A PASS sampler was also
placed at the midpoint of the channel affixed to a steel fencing post,
driven into the channel bed; the intake and float switch were placed
approximately 0.2 m above the channel bed. RS samplers were placed in a
line along the channel centre at various heights above the channel bed,
ranging from 0.2 to 0.45 m at 0.05 m intervals. The turbidity logger was
placed alongside the autosampler inlet. A level logger (In-situ® R100)
was placed at the midpoint of the channel directly on top of the bed,
fixed to the steel support for the rising stage samplers. A barometric
pressure logger (In-situ® baroTROLL) was placed nearby above maximum
flood elevation, to allow accurate calibration of the level logger. A
rain gauge (Hydrological Services
tipping bucket rain gauges - 0.2 mm/tip with Hobo data logger) was also
placed within the catchment area of the gullies.
Once activated, the autosampler collected a sample of approximately 800
mL every ten minutes, whilst the PASS sampler continuously sampled until
the ambient water level dropped below the float switch. The turbidity
logger was programmed to record a measurement every 10-minutes whilst
deployed. The RS samplers collected a sample when the water level
covered the intake and caused a pressure difference in the sampler,
resulting in rapid filling of the sampler (Braatz, 1961){Braatz, 1961
#362}. Manual flow-proportional samples were collected using a DH-48
sampler using the equal discharge method when flow velocity and depth
were sufficient (>0.3 m sec-1 and
>0.17 m, respectively), or taken directly from the stream
with a sample bottle when flow velocity and depth was too low for
accurate use of the DH-48 sampler (Edwards et al., 1999).
2.4 Sample analysis and
statistics
Samples collected from the laboratory and field evaluations were
analysed for suspended sediment concentration using ASTM standard method
D 3977-97 and particle size distribution using laser diffraction
spectroscopy (Malvern Mastersizer 3000, Malvern Instruments). Discrete
samples from the autosampler were analysed as received, whilst the PASS
samples (composites of main settling column and intake sediment trap),
were placed in cold storage (4°C) for five days to settle, after which
they were decanted to 1 L and analysed. The supernatant was filtered
through a pre-weighed glass fibre filter (Whatman GF/F (0.7 µm)), to
account for the mass of any sediment that may have remained in
suspension. The time-weighted average (non-continuous) suspended
sediment concentration was determined by averaging the concentration of
multiple discrete samples, weighted by the time span between two
sequential samples. The PASS sampler continuously samples whilst in
operation, thus, the time-weighted average suspended sediment
concentration is calculated by weighting the total mass of suspended
sediment collected by time as a function of volume (Doriean et al.,
2019). Turbidity measurements were calibrated using the discrete samples
from the autosampler. A linear regression between turbidity and
suspended sediment concentration was used to convert NTU measurements
into suspended sediment concentration, when appropriate (Rasmussen et
al., 2009). Statistical analysis
was conducted using GraphPad-Prism®. The sample data did not share
similar standard deviations, thus, the unpaired nonparametric
Mann-Whitney t-test method was used to compare differences between
methods (p = 0.05).
2.5 Data quality and
uncertainty
The uncertainty of each measurement method must be considered when
evaluating their relative performance. The uncertainty assigned to a
particular technique was determined based on laboratory evaluations
conducted during this study and the scientific literature. If the
difference in suspended sediment concentration between two sampling
methods was equal to or less than the cumulative error associated with
those methods, the individual results were not considered significantly
different (Horowitz 2017). For example, manual sampling uncertainty is
typically ~10% of the sample suspended sediment
concentration (Sauer et al., 1992), whereas, the PASS sampler was
previously demonstrated to exhibit ~6 to 17%
uncertainty (Doriean et al., 2019). Cumulatively this suggests a sample
concentration difference range in the order of 16 to 27%. Thus,
suspended sediment concentrations of samples collected by these methods
that differed within this uncertainty range were not likely to be
statistically different (Horowitz, 2017).