Straightforward measurement of spatially varying
sediment transport in a tidal-inlet
Saeed Shaeri 1,Shane Casselle 1, Rodger Tomlinson 1
1
Griffith Centre for Coastal Management, Griffith University, Gold Coast campus, Qld 4222
Introduction
Understanding sediment transportation caused by currents in micro and macro tidal
conditions has been extensively researched (Cartier & Héquette, 2011; Davidson-Arnott et
al., 2009; Doucette, 2000; Masselink & Short, 1993; Van Wellen, Chadwick, & Mason, 2000).
As a result, implications of computer modelling for the study of coastal processes have
significantly progressed in recent years. Outputs of numerous models and formulas have
been derived to predict sediment transport and the resulting coastal effects. Despite all this
available research, formulas and modelling, accurate predictions of sediment transport
remain different (Bayram, Larson, & Hanson, 2007). However, acceptable accuracy is
reached by using actual field data for calibration of the model. Additionally, in the case of
depth-averaged (2D) models, the value of actual field data is more pronounced, since the
algorithm to perform depth-averaged modelling, by nature, is based on assumptions and
simplifications. Therefore, the collection of field data has to be consistent with the method
used by the computer model to produce the modelled output. Otherwise, direct comparisons
would not be possible. Likewise, conversion of information from each dataset must be
undertaken in the same manner, otherwise results may be inaccurate, therefore leading to
false conclusions. The other aspect relates to the temporal variation of oceanic parameters
which limits the applicability of the field data in performing a thorough calibration of the
modelling software.
This research is a part of a larger project dealing with morphological changes around a small
tidal-inlet. The necessary current, wave and sediment transport modelling were performed
using the Delft3D package (Shaeri, 2015). The hydrodynamic and wave modules were
calibrated using the relevant field data. Meanwhile, this research was designed to collect
some of the necessary data for the calibration and validation of the sediment transport
module. Given that all parameters cannot be analysed concurrently, a simple method was
applied to collect sediment transport information. As opposed to many labour-intensive and
expensive methods, the adopted method here is deemed to be accurate, cheap and
straightforward. Moreover, the report focuses on the derivation of the relationship between
bed load sediment transport and the mean current velocity.
Study Area
Currumbin Creek is a small tidal inlet located southeast of the City of Gold Coast,
Queensland, Australia (28.127º S, 153.484º E; Figure 1). The creek is a highly popular
destination for residents and tourists and has a long history of construction, maintenance
and dredging. Indeed, a comprehensive project has been introduced (Shaeri, 2015), to
provide supportive information for the appropriate design of Currumbin Creek’s annual
dredging. This study aimed at providing field data which are then used for the calibration and
validation of a computer model for the main project.
As can be seen in the right panel of Figure 1, the creek’s entrance is bounded by a concave
Reef, Range and Red Dust 2015, Caloundra
training wall to the south and a straight training wall to the north. A 24 km creek flows out to
the ocean through this inlet and has an annual discharge as low as 0.5-6.0106 cumecs,
with a negligible sediment transport rate. Due to the unequal length of the training walls,
there is no distinct ebb-shoal. Whereas, a fan shaped flood-shoal is located in the middle of
the 20 hectare back-barrier lagoon. For the past 30 years, dredging has consistently been
utilized to keep the entrance stable and open. However, owing to the very dynamic nature of
the creek, the local municipal authority strives to find a more viable maintenance and
dredging strategy. In this paper, the result of a 14 week period (24th of February to the 5th of
June 2015) environmental sediment transport data collection campaign is discussed. The
result will later assist in the calibration and validation of the hydro-sedimentological model,
used for this study.
Figure 1. Map (Shaeri et al., 2013) and aerial photo of the area including zonation map for
data collection (current in April 2015, adopted from Google Earth)
Background Theories and Data Collection Method
There are a number of sediment transport data collection methods which are used for a
variety of cases; from very strong flow in large rivers with large rocks, to highly dynamic
coastal areas with millions of cubic meters per year of sand transport. Literally, the methods
used for such cases are very much specific for each case. In comparison, this study aimed
in the collection of sediment transport data along the shores of the down-estuary part of the
tidal-inlet where neither the riverine currents nor the tidal currents are strong. In fact, the
Currumbin Creek entrance is regarded as a wave-dominated tidal-inlet (Shaeri, 2015);
therefore, the wave induced currents, amplified by the influence of wind generated and tidal
induced currents, play a significant role in the transportation of sediment.
Regardless of the net direction of longshore sediment transport, the creek entrance is
affected by the sand-bypassing. In the meantime, the creek and its back-barrier are
regarded as a good source, store or sink for the longshore transport. Therefore, through the
local effects of tides and waves, sediment flux diverts into the entrance and is transported
towards the inner/flood shoal. The long term net transport of sediment to the back-barrier is
significant to the extent that currently, an annual dredging campaign is needed to keep the
entrance channel open. This is essential in order to: provide safe navigation for waterway
users; to reduce the likelihood of up-estuary heavy rainfall inundation; or to provide
necessary water circulation and keep the water quality at a reasonable level. However, the
short term net transport is not as significant compared to the annual figures, and this is a key
point for the selected data collection method.
The process of sediment transport occurs in two distinct ways: suspended or bed load
transport. The velocity of the flow and fineness of sediment particles, determines the volume
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of sediments that can float in the water and so be transported from point to point. Whereas,
in case of coarser particles or where a slower current velocity occurs, the particles tend to
move adjacent to the bed. Hence, the main factors attributing to bed load movement are
current velocity, median grain size and bed slope. Consequently, there are three main
modes of bed load particle motion: rolling, sliding or bouncing (Chanson, 2004).
A variety of sediment transport collection methods has been developed. Some methods,
such as depth integrated container (Sterling & Church, 2002) are best suited for suspended
particles. Alternatively, methods which use magnetic rocks (Haschenburger, 1996) or
coloured rocks (Haschenburger & Church, 1998) are largely applicable for measurements of
bed load. The immediate drawback for the latter methods is the effect of rock size on the
accuracy of data collection; that is, in the case where an area of interest is naturally covered
by fine to medium particles, usage of traceable, coloured rocks is irrelevant. Additionally,
such methods are commonly used for riverine studies where data collection occurs in a
relatively narrow channel width. Such issues translated to the idea of using traceable dyes;
however, environmental or local regulations, as well as the dyes greatest suitability being
more appropriate methods for current pattern studies, makes the usage of this method
unacceptable for this study (Kondolf & Piégay, 2003).
Another way to determine total sediment transport is by monitoring the morphological
changes for a limited area such as a river cross-section or a concave river bank. It can be
done, for instance, by GPS markings of the locations and elevations of the area. Changes to
the land and bed elevations can be recorded at intervals. This can be done in cubic meters
to determine the sedimentation rate (Fuller, Large, Charlton, Heritage, & Milan, 2003; Raven,
Lane, & Ferguson, 2010). For larger or more complicated areas, channel surveying can also
be undertaken by aerial photos (Ham & Church, 2000). However, abnormal weather
conditions during the course of measurement and any type of external coastal works (such
as dredging) in the area and surroundings need to be considered to ensure accurate results.
Another collection method involves creating a sinkhole or pit in the current path. In theory, as
the bed load moves along the surface of the channel bed, the void (or pit) creates a vacuum
that sucks in the bed load. Therefore, this method is also called the ‘vacuum trapping
method’ (Blomqvist, 1981; Håkanson, Floderus, & Wallin, 1989; Kozerski, 1994;
Szmytkiewicz & Zalewska, 2014; Zajaczkowski, 2002). In this method, a pit is dug on the
temporarily dry bed, without disturbing the actual flow. However, due to the nature of bed
load transport, by the time the flux of sediment reaches the pit, it needs to fill the pit first,
rather than by-pass it and this is the key to the measuring the of amount of bed load
transport during a short term period. Knowing the volume of the pit, then the amount of
sediment collected over a given time can be determined.
Visual observation and preliminary calculations showed that the process of sediment
transport in the Currumbin Creek back-barrier is deemed mostly of the bed load type.
Accordingly, the design of the data collection method was adjusted. In order to make less
interference with the current in the relatively shallow water depth as well as the thin layer of
bed load materials, the method of vacuum trapping was adopted. Selection of the necessary
collection period is analogous to the size of the pit. That is, having knowledge of the order of
sediment transport during a particular period, determines the dimension of the pit
(Zajaczkowski, 2002). The volume of the pit needs to be large enough to ensure that all the
bed load materials during the collection period are trapped (Håkanson et al., 1989). On the
other hand, the pit volume is limited to the selected method for measuring the amount of
Reef, Range and Red Dust 2015, Caloundra
trapped sediments. The preliminary trials to establish this method proved that the pit walls
also need to be stabilized in one way or another. For this, it was decided to use plastic
rectangular cubic containers which are strong enough to keep the entire pit stable. When the
container is buried inside the pit, the top surface of the container must be completely level
with the surrounding bed to ensure that the bed flow is not interrupted (Figure 2-left).
Likewise, the usage of containers also simplifies the measurement of the collected sediment
particles; that is the weight of the trapped sediment is relative to the amount of the net
sediment mass flow rate. It should be noted that the amount of collected sediment is limited
by the size of the container to be used. Moreover, this method can only record the time
averaged sediment transport of the desired location.
Due to the limitations imposed by the budget, availability of labour and instruments, it was
decided that all pits were to be dug on the shore face, in the elevation range between LAT
and HAT; that is there was no pit fully inside the water and the pits were intermittently wet
and dry during a tidal cycle. Care should be employed that the sediments excavated to
create the pits for the containers need to be dumped as far away as possible to the location
of the pit to ensure that there is not a possibility for those loose sediments to be included in
the collection process. Since the tide at Currumbin Creek is semidiurnal, between the times
of two consecutive lowest tides, there is roughly a 12-hour gap for data collection. Exceeding
this duration, would result in inundation of the pit by another tidal cycle which eventually
would result in inaccuracy of the collected data; that is, while the pit is underwater, there is
no chance to recognize the exact time when the container is full of sediment and starts to
overflow. Note that the amount of sediment trapped in the container is critical in calculating
the sediment mass flow rate.
Figure 2. A sample pit and container (left panel) and spatial distribution of data collection
(right panel) (aerial photo of the area adopted from Google Earth)
Consequently, owing to pre-knowledge of the process of the sediment transport in the backbarrier lagoon (with areas located closer to the inlet showing a faster current velocity), the
area was divided into five zones/subdivisions (Figure 1-right) where the current velocity
varies depending on which zone. Zones 3 and 4 which are located in a more dynamic area,
have a higher velocity when compared to the more passive zones, such as 1 and 2. Zone 5,
at the place of flood shoal, was chosen as it lies in the path of the most dominant tidal
current velocities. Figure 2-right shows the spatial distribution of about 100 collected data
points. Accordingly, for each zone, a different container size was chosen to guarantee a
correct data collection. Additionally, based on the slope of the shores, the places of the pits
were finalized to ensure stability during the entire collection period; that is, no pit was dug on
a very steep shore face. During initial testing, uplifting of the containers was noticed. To
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counteract this, additional weight (to make it heavier than a simple plastic container) was
used to ensure that the container would not float away during the experiment. Therefore,
prior to placement of the containers, the container and extra rocks were weighted as a dead
weight and then filled with seawater. Subsequently, the container was placed in the pit for
the required period. The following section presents the results of the data collection.
Results and Discussion
When containers were recovered, all seawater was removed by decanting. Then the
containers were weighed. The difference between the dead weight and sediment weight
gives the sediment mass. By using the assumed density of sediment (2650 kg/m3 based on
Howorth et al. (2014)), the volume of sediment inside the container is found. Then, by having
the planar area of the container, the thickness of the sediment in the container can be
calculated. Considering the period of the data collection (in seconds), the result of the
product of the thickness (in meters) and the width of the container (in meters) perpendicular
to the net flow direction, the sediment flux (in m2/s) is derived.
In summary, containers collected between 0.5 to 55 kg of sediment in different locations.
Dependent on the condition of the tide, elevation of the location of the pit along the shoreline
and the size of the placed container, the duration of data collection varied between 1.5 to 9
hours. The results equate to a sediment flux between 0.02 and 5.610-6 m2/s. The graph of
Figure 3 shows the variation of these outputs.
Figure 3. Variation of mass of trapped sediments in comparison to the calculated sediment
flux
To evaluate the accuracy of the collected sediment flux data, three of the commonly used
relationships (Chanson (2004)) between sediment flux and current velocity are employed
here:
a) Einstein (1942):
𝑞𝑠
√(𝑠−1)𝑔𝑑𝑠3
b) Meyer-Peter (1951):
c) Engelund-Hansen (1967):
𝑞𝑠
𝜌(𝑠−1)𝑔𝑑𝑠
)
𝜏𝑜
1.5
4𝜏
√(𝑠−1)𝑔𝑑𝑠3
𝑞𝑠 =
= 2.15 𝑒𝑥𝑝 (−0.391
𝑜
= ( 𝜌(𝑠−1)𝑔𝑑
− 0.188)
𝑠
0.05𝑢5
𝑔0.5 𝐶 3 (𝑠−1)2 𝑑𝑠
where qs= volumetric sediment discharge per unit width (m2/s); = density of water (kg/m3),
s= relative density of sediment particle; g= gravitational acceleration (m/s2); ds= specific
diameter of sediment particles (d50 and based on Howorth et al. (2014) is roughly equal to
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285 m); u= flow velocity (m/s); C=Chézy coefficient (m0.5/s)and o= average boundary shear
stress (Pa) which is calculated by:
𝑢 2
𝐶
𝜏𝑜 = 𝜌𝑔 ( )
In the case of our measurement by which qs is known for a particular period, the mean of the
depth-averaged flow velocity during that period is derived based on each of the three
introduced equations. The graphs of Figure 4 show these relationships. As can be seen in
Figure 4, there are differences between the flow velocities based on different formulas. It is
not the intention of this paper to evaluate the accuracy of the mentioned formulas or discuss
the differences. The velocities can only be used to distinguish any significantly incorrect
sediment data collection. For instance, the majority of the data which was collected in a
shorter period than 3 hours are doubtful. Nevertheless, Castelle et al. (2007) showed that
the velocity around the inlet channel could possibly reach 0.5 m/s; that is, possibly in a short
period such as 3 hours such a strong current could transport a significant amount of
sediment which is inconsistent with the normal trend.
Figure 4. Variation of velocities in comparison to mass of trapped sediments (top panel) and
duration of data collection (bottom panel). E: Einstein, E-H: Engelund-Hansen and M-P:
Meyer-Peter
Conclusion
The simple method of vacuum trapping is proved to be an accurate approach used to collect
bed load sediment transport data in the back-barrier of a small tidal-inlet. Since the current is
mainly driven by tide, therefore current direction varies periodically. However, the tidallyaveraged residual currents are responsible for the transport of the sediments. Three different
empirical relationships between the sediment flux and average velocity are used to ensure
the accuracy of the collected data. Further discussion of the special variation of the collected
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data is not in the scope of this paper.
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