Supporting Material for Lewis et al. “Calculating sediment trapping
efficiencies for reservoirs in tropical settings: a case study from the
Burdekin Falls Dam, NE Australia”
Background information on study area
The Great Barrier Reef (GBR), NE Australia, is claimed to be the best managed and
protected coral reef system in the world (Wilkinson, 2004; Brodie and Waterhouse, 2012).
However, coral cover has declined greatly in the past few decades (Bruno and Selig, 2007;
Sweatman et al., 2011) and this has been attributed to increased runoff of terrestrial pollutants
(Fabricius et al., 2005; Brodie et al., 2012) among other causes (Hughes et al., 2011; Brodie
and Waterhouse, 2012). Watershed models suggest that sediment and nutrient loads exported
from the GBR watershed have increased by 3 to 5 fold and 4 to 10 fold, respectively, since
European settlement in the region c. 1850 (Furnas, 2003; McKergow et al., 2005a, b; Kroon
et al., 2012). Much of this increase has been attributed to soil erosion associated with
increased grazing pressure (McCulloch et al., 2003; McKergow et al., 2005a).
One of the largest rivers discharging to the GBR, the Burdekin River, drains an area of
130,000 km2 (~30% of the GBR watershed) and exports ~30% of the suspended sediment
load delivered to the GBR lagoon (Furnas, 2003; Kroon et al., 2012). The Burdekin Falls
Dam (BFD), constructed in 1987 (capacity 1,860 × 106 m3), regulates 88% of the watershed
area (115,000 km2) and is thought to trap a large proportion of the suspended sediment load
that enters the reservoir (Prosser et al., 2002; Post et al., 2006; Kinsey-Henderson et al.,
2007). The spillway depth of the BFD is 37 m and the total embankment is 873 m long with
a reservoir area of 220 km2 and a maximum depth of 40 m. A low level outlet releases water
for irrigation purposes with a minimum release requirement of 10 m3 s-1 to maintain
environmental flows. Importantly, once the BFD overflows, the release gates are shut until
water levels recede below the spillway. Since construction, the BFD reservoir has rarely
been under 60% capacity and rapidly fills (within 1 to 2 days) during intensive inflow periods
triggered by wet season rainfall during the summer months (December to April).
The average sediment flux from the Burdekin River has been estimated using the SedNet
(Sediment River Network) model (Prosser et al., 2002; McKergow et al., 2005a; Fentie et al.,
2006; Post et al., 2006; Kinsey-Henderson et al., 2007). In this regard, Prosser et al. (2002)
used the Heinemann (1981) equation (a modification of the Brune, 1953 curve) to calculate
that 90% of incoming suspended sediments are trapped in the BFD. However, recent model
runs have applied an empirical modification to this equation to account for the highly
episodic seasonal flows in the Burdekin River which lowers the trapping efficiency
calculations to ~80% (see Brodie et al., 2003; Post et al., 2006). However, studies on the
sediment dynamics within Lake Dalrymple suggest that considerable (but not fully
quantified) amounts of suspended sediment pass through the reservoir as intrusions during
large flow events due to thermal stratification within the reservoir (temperature difference of
6 to 8° C from upper to lower water column: Griffiths and Faithful, 1996; Faithful and
Griffiths, 2000). The BFD reservoir, known as Lake Dalrymple, does not receive significant
inflow throughout the year because the rivers above the BFD are ephemeral, with practically
all of the inflow (and thus the sediment load) delivered during the summer months.
While geochemical studies of coral cores have examined historical changes in sediment loads
(e.g. McCulloch et al., 2003; Lewis et al., 2007) and monitoring and modelling activities have
refined the ‘average’ suspended sediment load for the Burdekin River (Kinsey-Henderson et
al., 2007; Kroon et al., 2012; Kuhnert et al., 2012), the actual trapping efficiency of the BFD
is unresolved. If the BFD traps ~90% of incoming suspended sediments as predicted by the
Heinemann (1981) equation, then remedial works to reduce suspended sediment delivery to
the GBR lagoon by reducing erosion in the Burdekin watershed should be targeted within the
considerably smaller area downstream of the BFD. Recent Australian Government
investment for remedial works under the Reef Rescue Initiative (Anon, 2010) has prioritised
this particular area.
In Australia, many reservoirs have an actively (diurnally) mixed surface layer 4 - 10 m deep
(Chudek et al., 1998; Sherman et al., 1998; Jones and Sherman, 1999; Sherman et al., 2000).
Below this surface layer the water column is essentially quiescent. Condie and Bormans
(1997) provide a thorough analysis of particle sedimentation in such multi-layered systems in
the absence of inflow and outflow. Furthermore, the summer-dominant inflows will
frequently enter the reservoir as intrusions at an intermediate depth (Faithful and Griffiths,
2000). In this case the 'active' volume of the reservoir relevant for residence time
calculations is the volume between the bottom of the intrusion and the full surface level of the
reservoir. This will have the effect of decreasing the trap efficiency by reducing the
residence time available for sedimentation. Finally, if normal operating procedures require
drawdown of the reservoir for flood control prior to the runoff season, the time required to
refill the reservoir can be considered an additional contribution to the residence time beyond
the standard calculation.
Moreover, the trapping efficiency of reservoirs is influenced by upstream dams where the
particle size distribution is altered as sediments pass through these systems. In this regard,
Churchill (1948) produced a different curve to account for the finer sediments that pass
through upstream reservoirs (i.e. the upstream sediment curve). In the case of the BFD, there
are no major upstream reservoirs (on the upper Burdekin tributary the Charters Towers Weir
holds only 5.2 × 106 m3 and further upstream the Paluma Dam drains <0.5% of the tributary
area and holds only 11.4 × 106 m3) and so the ‘local silt’ curve of Churchill is more
appropriate to apply.
Additional Methods
Samples were collected from the surface of the river (top 50 cm of water column) with a
bucket and rope and well mixed with a stirring rod before being sub-sampled into 1 L
containers and transported to the laboratory for analysis. Where possible the samples were
collected from the centre of the stream, although on some occasions (i.e. when the centre was
inaccessible) samples were collected from the edge ensuring that the main flow was captured.
Samples collected laterally across stream transects confirmed that the surface of the rivers
were laterally well-mixed (with respect to TSS) to within laboratory error measurements.
Additional replicate samples were collected for precision estimates and for inter-laboratory
comparisons. In the 2009/10 wet season, ISCO Avalanche autosamplers were installed on
the Cape, Belyando and Suttor tributaries for more thorough collection over the flow
hydrograph with the sample intake approximately 1 to 2 m above the bed.
Analytical methods
Total suspended solids
TSS analysis was performed at the TropWATER Laboratory Services Unit at James Cook
University, Townsville and at the Queensland Department of Science, Information
Technology, Innovation and the Arts Chemistry Centre laboratory, Brisbane. Samples were
filtered through pre-weighed Whatman GF/C filter membranes (nominally 1.2μm pore size)
and oven-dried at 103–105°C for 24 h and reweighed to determine the dry TSS weight as
described in APHA (2005; Method 2540D). Duplicate (and triplicate) samples analysed by
the laboratories produced values typically within ±10%.
Particle size data
Aliquots were taken from 149 of the TSS samples collected from 2005/06 to 2008/09 (four of
the five monitored years) for particle size analysis. The aliquots were selected to capture the
rising, peak and falling stages of the flow hydrograph over the four monitored water years
and include 32 samples from Burdekin River at Sellheim, 24 samples from the Cape River,
21 samples from the Belyando River, 22 samples from the Suttor River and 50 samples from
the BFD overflow.
Particle size distribution analysis of the suspended sediment samples are conducted using a
Malvern Mastersizer 2000. This instrument separates particles into 100 bin size classes
which range from 0.02244 μm to 2000 μm. The particles within each bin size class are given
as a percentage of the total sample volume. Because the size fraction >300 μm was
negligible (our samples were collected from the surface of streams which favours a finer
‘washload’ fraction), we report exclusively on the data between 0.02244 μm and 282.5 μm
(83 separate bin sizes). Each aliquot was analysed at least twice and a mean was taken.
Reference list
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Table 1. Summary of the number of TSS samples collected and the concentration ranges
(mgL-1) from the various sites over the five year programme.
Water year
Burdekin
R@
Sellheim
Cape R @
Gregory
Dev Rd
Belyando
R@
Gregory
Dev Rd
Suttor R
@ Bowen
Dev Rd
Burdekin
Falls Dam
@ overflow
2005/06
12
7
12
4
31
2006/07
14
8
9
7
55
2007/08
15
30
33
35
97
2008/09
26
13
8
8
102
2009/10
8
115
93
63
63
Total samples
75
173
155
117
348
Concentration
range (mg L-1)
3 – 3350
16 - 1550
9 - 1320
47 - 980
2 - 620
Table 2. Water year sediment budgets for 2005/06 with mean annual concentration (MAC).
The standard error (SE) of sediment load is provided in brackets.
Sediment load
(million
tonnes) with
SE
Proportion
of
incoming
sediment
load
TSS
MAC
(mg L-1)
87.0%
794
1.4%
360
7.4%
651
Total flow
(m3)
Proportion
of total
flow
2.7 × 109
80.0%
9.3 × 107
2.8%
2.8 × 108
8.3%
Suttor River @ St
Anns* minus
Belyando River
2.6 × 108
7.7%
0.0961
(0.0179)
3.9%
370
Ungauged
estimate
4.0 × 107
1.2%
0.01
0.4%
250
Inflow to Dam
3.4 × 109
100%
2.47 (0.3032)
100%
731
Burdekin Falls
Dam overflow
2.1 × 109
Extra capacity due
to release water
(3/1/06 - 10/4/06)
2.0 × 108
Water to fill dam to
capacity (43.7%)
8.1 × 108
Revised inflow to
dam
3.1 × 109
2005/06 water year
Burdekin River @
Sellheim
Cape River @
Taemas
Belyando River @
Gregory Dev Rd
2.1443
(0.3010)
0.0335
(0.0049)
0.1822
(0.0318)
0.3697
(0.1043)
*Equivalent of Suttor River @ Bowen Dev Rd and Rosetta Creek. MAC = Mean annual concentration
184
Table 3. Water year sediment budgets for 2006/07 with mean annual concentration (MAC).
The standard error (SE) of sediment load is provided in brackets.
2006/07 water
year
Burdekin River @
Sellheim
Cape River @
Taemas
Belyando River
@ Gregory Dev
Rd
Suttor River @ St
Anns* minus
Belyando River
Ungauged
estimate
Inflow to Dam
Burdekin Falls
Dam overflow
Water to fill dam
to capacity
(25.3%)
Revised inflow to
dam
Sediment load
(million
tonnes) with
SE
Proportion
of
incoming
sediment
load
TSS
MAC
(mg L-1)
79.8%
697
4.9%
243
Total flow
(m3)
Proportion
of total
flow
4.4 × 109
67.7%
7.8 × 108
12.0%
2.6 × 108
4.0%
0.1283
(0.0330)
3.3%
493
4.6 × 108
7.1%
0.1004
(0.0103)
2.6%
218
6.0 × 108
9.2%
0.36
9.4%
600
6.5 × 109
100%
3.85 (0.4075)
100%
592
6.5 × 109
3.0687
(0.4054)
0.1892
(0.0225)
1.7054
(0.4346)
4.7 × 108
7.0 × 109
*Equivalent of Suttor River @ Bowen Dev Rd and Rosetta Creek. MAC = Mean annual concentration
262
Table 4. Water year sediment budgets for 2007/08 with mean annual concentration (MAC).
The standard error (SE) of sediment load is provided in brackets.
2007/08 water year
Total flow
(m3)
Proportion
of total flow
Sediment
load
(million
tonnes)
with SE
Burdekin River @
Sellheim
6.2 × 109
32.3%
4.6631
(0.4831)
74.8%
752
Cape River @
Taemas
2.3 × 109
12.0%
0.5027
(0.0590)
8.1%
219
Belyando River @
Gregory Dev Rd
3.9 × 109
20.3%
0.2107
(0.0272)
3.4%
54
Suttor River @ St
Anns* minus
Belyando River
5.9 × 109
30.7%
0.7064
(0.0890)
11.3%
120
Ungauged
estimate
9.1 × 108
4.7%
0.15
2.4%
165
Inflow to Dam
19.2 × 109
100%
100%
324
Burdekin Falls
Dam overflow
18.0 × 109
Water to fill dam to
capacity (13.7%)
2.0 × 108
Revised inflow to
dam
18.3 × 109
6.23
(0.4955)
3.1055
(0.6481)
Proportion
of
incoming
sediment
load
TSS
MAC
(mg L-1)
*Equivalent of Suttor River @ Bowen Dev Rd and Rosetta Creek. MAC = Mean annual concentration
173
Table 5. Water year sediment budgets for 2008/09 with mean annual concentration (MAC).
The standard error (SE) of sediment load is provided in brackets.
2008/09 water
year
Burdekin River
@ Sellheim
Cape River @
Taemas
Belyando River
@ Gregory Dev
Rd
Suttor River @
St Anns* minus
Belyando River
Ungauged
estimate
Inflow to Dam
Burdekin Falls
Dam overflow
Water to fill
dam to
capacity (12%)
Revised inflow
to dam
Proportion
of incoming
sediment
load
TSS MAC
(mg L-1)
93.7%
750
3.0%
206
0.1067
(0.0121)
0.7%
296
2.7%
0.1373
(0.0134)
0.9%
196
2.2 × 109
8.6%
0.30
1.9%
136
25.6 × 109
100%
100%
627
Total flow
(ML)
Proportion
of total flow
20.0 × 109
78.2%
2.3 × 109
9.0%
3.6 × 108
1.4%
7.0 × 108
25.0 × 109
Sediment
load (million
tonnes) with
SE
15.0068
(2.1253)
0.4732
(0.0487)
16.02
(2.1259)
4.8818
(1.3041)
2.2 × 108
25.2 × 109
*Equivalent of Suttor River @ Bowen Dev Rd and Rosetta Creek. MAC = Mean annual concentration
195
Table 6. Water year sediment budgets for 2009/10 with mean annual concentration (MAC).
The standard error (SE) of sediment load is provided in brackets.
TSS MAC
(mg L-1)
69.5%
682
6.6%
204
0.1626
(0.0129)
6.6%
181
18.9%
0.2234
(0.0160)
9.1%
203
5.3 × 108
9.0%
0.20
8.2%
381
Inflow to Dam
5.8 × 109
100%
2.45
(0.2118)
100%
422
Burdekin Falls
Dam overflow
5.5 × 109
Water to fill
dam to
capacity (25%)
4.7 × 108
Revised inflow
to dam
6.0 × 109
Burdekin River
@ Sellheim
Cape River @
Taemas
Belyando River
@ Gregory Dev
Rd
Suttor River @
St Anns*
minus
Belyando River
Ungauged
estimate
Total flow
(ML)
2.5 × 109
43.0%
7.9 × 108
13.6%
9.0 × 108
15.5%
1.1 × 109
Sediment
load
(million
tonnes)
with SE
Proportion of
incoming
sediment load
2009/10 water
year
Proportion
of total
flow
1.7061
(0.2106)
0.1614
(0.0096)
0.4476
(0.0723)
*Equivalent of Suttor River @ Bowen Dev Rd and Rosetta Creek. MAC = Mean annual concentration
81
Table 7. Parameters used in the Brune and Churchill equations and extra details for all
reservoirs examined in this study.
Reservoir
Burdekin Falls
Dam
Coralville
Reservoir
Corpus
Christi
Reservoir
Imperial Dam
Hales Bar
Reservoir
Gauge
Burdekin River @
Hydro site
Iowa River @ Iowa
Nueces River @
Texas
Colorado River
@ Arizona
Tennessee River
@ Tennessee
6.05 × 107
2.7 × 109
Dam capacity (C in
m 3)
1.86 ×
109
7.15 ×
107
173,000,000
5.1 × 107
8.7 × 108
Mean Inflow (I)
7.2 × 109
2.2 × 109
9.5 × 108
12.6 × 109
33,900,000,000
Length (L in m)*
50,000
36,000
16,000
12,500
70,000
3,781
216,000
Area (A) = C (in m3) / L
37,200
1,986
2,471
3,188
69,600
Several (highly
regulated)
Negligible (data
used to develop
Churchill's 'local
silt' curve)
Upstream Reservoirs
Paluma Dam
(11,400,000 m3)
and Charters
Towers Weir
(5,200,000 m3)
Lake MacBride
(16,700,000 m3)
Choke Canyon
Reservoir
(857,000,000
m3) but
constructed after
Brune study
Operation
Irrigation water
supply
Flood protection/
recreation
Drinking water
supply
Irrigation water
supply
Hydroelectric
Drainage area (km2)
114,000
8,100
43,500
478,000
56,000
Intra-annual COV
1.3
0.47
0.82
0.06
0.58
*Estimated from information provided by Brune (1953) and Churchill (1948). The capacities and mean
inflows of the Corpus Christi, Imperial Dam and Hales Bar Reservoirs were calculated from the table
provided by Brune (1953). Note that the capacities of the Corpus Christi and Imperial Dam Reservoirs
changed over two time periods examined in this study. The lengths of the reservoirs were back calculated
using the Churchill curve.
Table 8. Summary of previously published trapping efficiency data compared to the
predictions using the standard Brune (1953) and Churchill (1948) equations and the new
equations designed to account for intra-annual stream flow variability. Numbers in bold
represent predicted values within 5% of the measured trapping efficiency.
Site
Coralville
Reservoir*
on Iowa
River, Iowa
Water Year
Measured
trapping
Churchill
Std
Technique
Churchill
modified
daily
Brune Std
technique
Brune
modified
daily
Intraannual
COV
1973
68.2
70.8
66.1
59.4
53.6
0.54
1974
86.0
73.3
68.0
63.0
56.5
0.65
1975
57.5
80.0
72.3
72.9
62.3
0.90
1976
46.3
86.8
78.5
82.3
70.0
1.07
1977
90.4
94.6
83.3
91.1
75.7
1.49
1978
92.4
81.0
76.3
74.3
67.4
0.48
1979
83.6
75.9
69.6
66.9
58.6
0.80
1980
88.0
86.0
82.9
81.4
76.6
0.52
1981
95.8
90.9
86.7
87.4
81.3
0.54
1982
84.2
76.3
71.0
67.4
60.0
0.63
1983
5.6
71.5
67.2
60.4
54.9
0.46
1984
88.0
73.2
68.8
62.8
57.0
0.50
1985
80.0
86.0
78.2
81.3
70.1
0.92
1986
88.2
76.3
72.1
67.4
61.6
0.54
1987
84.8
79.9
76.3
72.7
67.4
0.55
1988
75.1
90.0
86.3
86.3
81.1
0.71
1989
70.8
97.9
93.8
93.9
88.9
0.78
1990
66.3
79.1
67.3
71.5
54.9
1.07
1991
75.7
75.0
64.1
65.5
51.6
1.04
1992
87.3
77.9
74.0
69.8
64.3
0.46
1993
59.7
59.9
49.8
45.2
37.4
0.83
1994
89.8
81.7
76.7
75.4
68.1
0.54
1995
81.0
80.1
73.9
73.0
64.0
0.81
1996
81.4
85.0
76.1
79.9
66.7
1.07
1997
73.2
81.7
75.6
75.4
66.4
0.69
1998
62.4
76.9
69.3
68.3
57.7
0.86
1999
68.8
77.9
72.7
69.8
62.2
0.67
2000
82.2
90.5
80.8
86.9
72.8
1.33
2001
49.5
79.5
68.6
72.1
56.7
1.02
2002
74.0
87.8
84.7
83.7
79.0
0.61
2003
75.6
85.4
78.9
80.5
70.7
0.97
2004
79.9
81.2
74.5
74.6
64.8
0.82
2005
74.6
83.5
78.5
77.9
70.4
0.71
Corpus
Christi
Reservoir**
on Nueces
River,
Texas
Imperial
Dam** on
Colorado
River,
Arizona
Hales Bar
Reservoir
on
Tennessee
River,
Tennessee
Overall
(1973 2005)
80.3
79.1
70.2
71.9
59.9
0.47
1934 - 1942
76.7
88.9
59.5
77.1
43.4
0.95
1942 - 1948
73.7
92.7
66.4
85.5
49.1
1.07
1938 - 1942
90.2
80.8
78.4
63.3
59.6
0.10
1943 - 1947
72.3
78.7
78.4
59.5
59.1
0.07
1935 1936**
1937**
30.5
34.4
15.8
30.0
25.7
0.77
25.7
34.8
20.6
30.3
27.1
0.83
1938**
29.7
35.7
30.7
30.9
29.5
0.29
1935 (1)***
30
16.1
8.7
20.4
19.3
N/A
1935 (2)***
20
25.8
16.6
24.9
23.3
N/A
1935 (3)***
55
57.4
56.2
50.2
49.5
N/A
1935 (4)***
40
56.1
49.4
48.9
44.6
N/A
1936 (1)***
0
7.2
0.0
16.9
15.8
N/A
1936 (2)***
60
30.9
5.3
27.8
23.0
N/A
1936 (3)***
60
58.8
57.9
51.9
51.4
N/A
1936 (4)***
56
48.6
44.5
41.2
39.4
N/A
1937 (1)***
2
6.9
0.0
17.0
16.1
N/A
1937 (2)***
35
42.8
40.5
36.3
35.3
N/A
1937 (3)***
50
54.2
53.2
46.8
46.4
N/A
1937 (4)***
40
43.8
41.1
35.8
36.9
N/A
1938 (1)***
25
31.3
28.5
28.3
27.3
N/A
1938 (2)***
25
31.1
26.9
28.0
27.0
N/A
Measured data reported in: *Espinosa-Villegas and Schnoor (2009); **Brune (1953); ***Churchill (1948)