Spatial and temporal variations of suspended sediment responses from the subtropical Richmond River catchment, NSW, Australia.
Like any hydrological process, the delivery of sediment from any catchment area is a stochastic process, which includes deterministic and random components (Novotny and Chesters 1989). The quantity of sediment eroded from a catchment depends on the characteristics of the catchment area, average slope and drainage density, rainfall pattern and intensity, the stream flow hydrograph and channel morphology, vegetation cover, and catchment land use (Williams 1989; Neil and Fogarty 1991; Post and Jakeman 1996). The transport of suspended sediment from a catchment varies seasonally with runoff volume and typically attains its maximum export during floods (Huggett et al. 1980; Ruch et al. 1993), but the availability of sediment may be limited when there is a short period between successive flood events (Wood 1977). Flood events with a 50-year return period may move up to 10 times more than the average annual sediment load (Finlayson 1978). As an example, Burdekin River in Queensland, Australia, discharges 6 times more sediment in a flood year than in an average year, and during drought years the load is only 2% of an average year (Pringle 1991).
Quantitative suspended sediment export studies for Australian catchments are limited (Nell and Galloway 1989; Pringle 1991; Moss et al. 1992; Arakel et al. 1993; Neil and Yu 1996) and there is very little information available for subtropical Australian catchments, which experience a long dry season in winter and in which most of the yearly flow occurs during 1 or 2 months in the wet season. In addition, Australian catchments are characterised by more variable flows than those in other parts of the world (Finlayson and McMahon 1988). To fill the gaps in our understanding a detailed integrated catchment and estuarine study has been carried out from the upper Richmond River catchment to the continental shelf (Hossain 1998). As a part of that wider study, extensive field data were collected from all over the Richmond River catchment and estuary for 2 hydrological years (1994-96). This paper, however, uses a part of the catchment sediment data (a) to quantify yearly and seasonal suspended sediment exports from the 3 Richmond River subcatchments and their variations, (b) to determine suspended sediment response patterns from different subcatchments and their possible relationship with catchment attributes, and (c) to develop a suspended sediment load model for the Richmond River subcatchments to determine possible increase in suspended sediment load due to land-use changes.
Richmond River Catchment
The Richmond River (Fig. 1) is a subtropical coastal catchment (6900 [km.sup.2]) in northern New South Wales (NSW) and has 3 main tributaries: the Wilsons River, the Richmond River, and the Bungawalbin Creek. The catchment has slopes in excess of 15 [degrees] in 20% of the catchment and undulating land with slopes 3-15 [degrees] in 40% of the catchment. The remaining area is flat land with extensive floodplains (Anon. 1980). The dominant surface lithologies of the Richmond River catchment are basalt (31%), alluvial deposits (30%), and sandstone (28%) (Anon. 1996a).
[FIGURE 1 OMITTED]
The Richmond River catchment is a non-metropolitan region with a beef, milk, and sugar production economy. Secondary industries such as building material fabrication, food processing, and abattoirs are mainly concentrated in the urban centres of Lismore, Casino, and Ballina. The dominant land uses in the Richmond River catchment are grazing of cattle (53%), followed by timber cutting (42%) and cropping (3.4%) (Anon. 1996a). Only 22% of the Wilsons River subcatchment (16% for main Wilsons River ann and 25% for Leycester Creek) is forested compared with 42% of the Richmond River and 75% of the Bungawalbin Creek subcatchments (Anon. 1996a). Soil types in the Richmond River catchment show that catchment average soil erosivity index for the Wilsons River subcatchment is 0.027, followed by 0.025 for Richmond River and 0.023 for Bungawalbin Creek subcatchments (Hossain 1998).
Rainfall during study period
Like many other Australian catchments, the Richmond River catchment experienced one of its driest years in the 1994-95 hydrological year and received about 880 mm of rainfall, which was about 65% of the average yearly rainfall. In contrast, about 1530 mm of rain fell in the 1995-96 hydrological year, which was about 14% higher than the average yearly rainfall. Rainfall distributions over the 3 major Richmond River subcatchments during 2 hydrological years are given in Table 1. Although the average rainfall over the 3 subcatchments during the 1995-96 hydrological year was about twice that in the 1994-95 hydrological year, a similar distribution of rainfall was observed over the 3 subcatchments (Hossain 1998).
Statistical analysis showed that the Richmond River catchment was subjected to a single minor flood (1 in 2 year return period) during the 1994-95 hydrological year, whereas, during the 1995-96 hydrological year the catchment produced 2 floods: a 1-in-2 year (minor flood) and a 1-in-5 year (moderate flood) return period flood (Hossain 1998).
Materials and methods
Suspended sediment sampling
Eight sites (Fig. 1) in the catchment were sampled on a monthly basis for 2 years from July 1994 to June 1996. During the dry season (January-June) samples were collected from 2 major tributaries at Casino and Tatham in the Richmond River subcatchment. In the Wilsons River subcatchment samples were collected from 5 major creeks. In the Bungawalbin Creek subcatchment samples were from one site at Coraki.
During the wet season (July-December), in addition to monthly sampling, samples were also collected on a daily basis from all 8 sites for any rain event. During flood events, flow-weighted (i.e. rising and falling stages of flood) samples were collected at certain intervals depending on the duration of floods from major 4 outlets of the 3 subcatchments (Fig. 1). Details of 3 flood events rising and falling stages sampling durations and frequencies are given in Table 2.
Because all flood events were sampled manually the number of samples collected during the flood events varied. For the first 2 flood events, samples were collected at 3-h intervals. For the moderate flood in May 1996 at least 4 samples were collected from each site in a day during the rising stage of flood. During the falling stage of the flood, at least 3 samples were collected in the first 2 days, and thereafter at least 2 samples were collected each day.
For sampling error calculation 6 additional samplings were carried out from one randomly selected site during all monthly and flood events.
Sample collection, sediment concentration and load calculation
During sampling at each location, samples were collected from 9 points (0.2, 0.4, and 0.8 of the total depth and at 3 locations: left, middle, and right sections of the cross-section); the 9 samples (each about 250 mL) were then integrated to prepare a homogenised sample. Suspended sediment concentration was determined by filtering and weighing the non-filterable residues. Depending on the suspended sediment concentration, a water sample 50 to 250 mL was filtered through a pre-weight (dry 4-6 h at 60 [degrees] C) 0.45-mm cellulose acetate filter paper. After filtration of the sample, the filter was rinsed with deionised water to remove salt (if necessary) dried overnight at 60 [degrees] C, and then weighed.
Suspended sediment load for the 3 subcatchments at their sampling point (i.e. at Casino, Lismore, and Coraki) was determined by multiplying monthly, daily, and flow-weighted concentrations with the corresponding subcatchment monthly, daily, and flood event flows. Total load from the Richmond and Wilsons River subcatchments were then calculated by multiplying the ratio of corresponding subcatchment area at Coraki to its sampling area. Gauged flow data from the Bureau of Meteorology, Sydney, were used for determining flow from the individual subcatchment (Hossain 1998).
Suspended sediment load model
A suspended sediment load model for the Richmond River catchments was developed using catchment area, runoff response, and vegetation covers. The purpose of this modelling was mainly to determine suspended sediment load responses from the Richmond River catchment due to land use changes. For the simplicity of this modelling 2 major land covers (i.e. grazing and forest) were used, because these land covers account about 95% of the catchment land use (Anon. 1996a). As such, the general form of the model was used:
L = a.[A.sup.b].[R.sup.c].[(F+[alpha]G).sup.d]
where L is suspended sediment load (t), A is catchment area ([km.sup.2]), R is runoff response (mm), F and G are % of forest and % of grazing areas, [alpha] is load response from equivalent grazing land to forestland, and a, b, c, and d are constants.
The value of [alpha] was determined from catchments in Queensland, Australia (Moss et al. 1992). The average value of [alpha] was found to be 4.01. The model constants (a, b, c, and d) are determined from multiple regression analysis using flood event data from the 3 major subcatchments: the Richmond River above Casino, Wilsons River and Leyester Creek above Lismore, and Bungawalbin Creek above Coraki. For the modelling purpose forest coverage for the individual subcatchment was determined using remote sensing data from the Department of Land and Water Conservation, Sydney (Anon. 1996a).
Low and high flow suspended sediment concentrations
Average monthly suspended sediment concentrations during the 2 dry seasons remained very low in the Richmond River catchment (Fig. 2). During dry seasons average suspended sediment concentrations at Casino in the main arm of the Richmond River were 3.0 [+ or -] 0.4 (mean [+ or -] s.d.) mg/L in 1994-95 and 6.0 [+ or -] 2.9 mg/L in 1995-96. Within the Wilsons River subcatchment, average suspended sediment concentrations in Leycester Creek (8.1 [+ or -] 3.9 and 6.5 [+ or -] 2.3 mg/L) were slightly higher than in the main arm of Wilsons River (6.4 [+ or -] 0.9 and 6 [+ or -] 0.7 mg/L) in both study years. Only a small volume of water flowed from the lower catchment of Bungawalbin Creek during the 2 dry seasons and corresponding average suspended sediment concentrations were 4.9 [+ or -] 0.6 and 8.0 [+ or -] 4.6 mg/L, respectively.
[FIGURE 2 OMITTED]
Flow-weighted sampling in the 3 Richmond River subcatchments (rising and falling stages) during all 3 flood events showed large variations in suspended sediment concentrations among the 3 subcatchments. The first flood occurred in February 1995 as a result of about 240 mm of rainfall over the 3 subcatchments and the maximum measured instantaneous suspended sediment concentration was 346 mg/L in the main Wilsons River, followed by 324 mg/L in the Richmond River and 313 mg/L in Leycester Creek (Fig. 3). Although the average rainfall over the Richmond River and Bungawalbin Creek subcatchments was the same (201 mm), the maximum instantaneous suspended sediment concentration from Bungawalbin Creek (112 mg/L) was only about one-third of that in the Richmond River, probably due to high forest coverage (about 75%) and flat topography.
[FIGURE 3 OMITTED]
In January 1996 about 290 mm of rain fell over the 3 subcatchments, which produced maximum measured instantaneous suspended sediment concentration of 620 mg/L at Casino compared with 256 mg/L in the Wilsons River and 332 mg/L in Leycester Creek at Lismore (Fig. 3). Similar to the previous flood, the maximum suspended sediment concentration in Bungawalbin Creek during the January flood was low (58 mg/L) despite having the highest rainfall (322 mm) among the 3 subcatchments.
The biggest flood during the study period was recorded in May 1996 when 440 mm rain fell over all 3 Richmond River subcatchments. Two periods of rainfall during the May 1996 flood resulted in 2 distinct peaks in the suspended sediment discharge from all 3 subcatchments (Fig. 3). In the Richmond River, maximum measured peaks in the suspended sediment concentration were observed on the third and seventh day of the flood (628 and 554 mg/L, respectively). Despite being the largest flood (i.e. higher rainfall and runoff) during the study, the maximum suspended sediment concentration in the Wilsons arm at Lismore was much lower than in the previous 2 floods; 200 mg/L was recorded on the third day of the flood, and 101 mg/L on the sixth day. The suspended sediment concentration in Leycester Creek reached 240 and 367 mg/L on the same days as the peaks in the Wilsons River. Suspended sediment concentration peaks in the Bungawalbin Creek subcatchment were 117 and 110 mg/L on the sixth and eighth days of the flood, respectively.
Suspended sediment hysteresis patterns
The relationship between suspended sediment concentration and streamflow generally exhibits a positive correlation (Loughran 1977; Griffiths 1982). Flow-weighted sampling from Wilsons River and Leycester Creek at Lismore and the Richmond River at Casino shows that the suspended sediment concentration during the rising stage of a flood was always higher than on the falling stage of the flood and that the sediment peak preceded the flow peak. In the Richmond River subcatchment during the February 1995 flood, both sediment and flow peaks at Casino occurred at the same time. Because the suspended sediment concentration was higher during the rising stage of the flood and peaks before the flow, hysteresis patterns (Fig. 4) from both the Richmond and Wilsons River subcatchments exhibited a clock-wise response. In contrast, the suspended sediment peak in the Bungawalbin Creek occurred later than the flow peak, so the hysteresis pattern was anti-clockwise (Fig. 4).
[FIGURE 4 OMITTED]
Suspended sediment export
Suspended sediment exports from the 3 major subcatchments (Table 3) showed that during dry seasons, <1% of the suspended sediment load was transported from the Richmond River and Bungawalbin Creek subcatchments compared with about 2% in the Wilsons River subcatchment. During the 1994-95 hydrological year about 72% of the suspended sediment load was produced from the Wilsons River, although it is the smallest subcatchment; the second biggest subcatchment (Bungawalbin Creek) transported <0.5% of the total load. In contrast, during the 1995-96 hydrological year the biggest subcatchment (Richmond River) produced 66% of the total load and the contribution from Bungawalbin Creek was about 8.5%. Overall, the 3 Richmond River subcatchments exported more than 7 times more suspended sediment during the 1995-96 wet year than during the 1994-95 dry year.
During 3 floods in February 1995, January 1996, and May 1996, the Richmond River subcatchment above Casino exported about 145 500 t of suspended sediment. At the same time Wilsons River subcatchment exported about 61 300 t of suspended sediment. Although the Bungawalbin Creek is the second largest subcatchment, it transported only about 16 300 t of suspended sediment.
Sediment flux errors for flood events were calculated from the sums of the squared component errors (Eyre et al. 1998). Errors associated with suspended flux are suspended sediment concentration error during sampling and flow measurement error. Sampling error was estimated to be about [+ or -] 3% (CV of 6 samples) and water flow measurements by the Department of Land and Water Conservation, Grafton, NSW, were considered to have an error of about [+ or -] 5% (Winter 1981). Therefore, error associated with sediment flux calculation is about [+ or -] 6%.
Runoff response, land uses, and suspended sediment load
During the study period flow weighted samplings were carried out for 6 events in the Wilsons River and Leyester Creek, 5 events in Richmond River, and 3 events in Bungawalbin Creek subcatchments. Details of suspended sediment load responses from 3 subcatchments during flood events are given in Table 4. Suspended sediment load responses from 3 subcatchments were then used to develop a general suspended sediment load model for the Richmond River catchment, and multiple regression analysis of the above suspended sediment load with individual catchment area, runoff, and land uses demonstrated a very strong relationship (Fig. 5). The developed relationship equation was then used for predicting suspended sediment load responses from the Richmond River catchment for different runoff responses and vegetation covers (Fig. 6). At present about 48% of the 3 subcatchments is covered by forest and Fig. 6 demonstrates that suspended sediment load from the Richmond Rive catchment has probably increased about 6-fold from its pristine condition.
[FIGURES 5-6 OMITTED]
At the local scale, topography and forest coverage may play important roles in determining the sediment exports from catchments (Wischmeier et al. 1958; Dunne 1979; Johnson 1985; Pringle 1986). Three Richmond River subcatchments showed similar relationships with catchment topography and land cover--more sediment export from the Richmond and Wilsons River subcatchments than the relatively flat and large forested Bungawalbin Creek subcatchment. Of the 2 highest yielding subcatchments, however, suspended sediment export flood from the Richmond River subcatchment was higher than the Wilsons River subcatchment despite relatively the same value of soil erosivity index for both subcatchments. This is probably because in the Richmond River subcatchment there are 73 dairy farms along both sides of the river (Anon. 1996b), and the land is used for dairy and beef cattle grazing. Although the upper catchment is mostly forest, the middle and lower parts of the catchment have been cleared completely for grazing, and as a result, during flood events the middle and lower catchments supply a large amount of sediment. Further, because of the narrow rectangular shape of the Richmond River subcatchment compared with the fan-shape of Wilsons River subcatchment, which reduces travel distance for runoff and sediment, the eroded sediment and runoff can quickly reach the main channel, which increases the river's ability to export sediment (Dickinson and Pall 1982; Ebisemiju 1990). The combination of land-use and different channel morphology in the Richmond River subcatchment therefore results in a higher sediment export per unit area than from the Wilsons River subcatchment.
In the Richmond River catchment, the Richmond and Wilsons River subcatchments showed a clockwise hysteresis pattern and Bungawalbin Creek subcatchment showed an anti-clockwise response even during 2 subsequent flood peaks. Sidle and Campbell (1985) noted that a clockwise loop is more common during the early part of a wet season than during the later part, which was not apparent in the 3 Richmond River subcatchments. Small streams normally exhibit clockwise response (Heidel 1956), which was not the case for 3 Richmond River subcatchments, because streams are prominent and bigger in the Richmond and Wilsons River subcatchments than the Bungawalbin Creek subcatchment. By contrast, an anti-clockwise response was observed when there is an irregular stream path that delays sediment movement (Axelsson 1967). This cannot be justified for 3 Richmond River subcatchments either because the Wilsons River subcatchment exhibited clockwise sediment discharge relationship despite having a fan shape of stream pattern. Considering all catchment attributes, it is apparent that slope and forest coverage are probably 2 major factors influencing suspended sediment response patterns in the Richmond River subcatchments.
On a yearly basis, about 75-91% of the suspended sediment load from the Richmond River catchment was exported in <5% of the time. A similar response was observed in the River Creedy in Devon, UK (Webb and Walling 1982) where 90% of sediment was also transported in 5% of the time. Sediment export from the Richmond River catchment varied from about 5 t/[km.sup.2] during the dry year to about 40 t/[km.sup.2] during the wet year, which is similar to other Australian catchments, but less than other parts of the world (Table 5). Temporal variation of sediment exports from Australian catchments, however, is relatively high (Belperio 1979) and is also evident in the Richmond River catchment (i.e. more than 7-fold increase in suspended sediment export from dry year to wet year). Similar intra and inter annual variations were observed for nitrogen and phosphorus exports during the same study period from the Richmond River subcatchments (McKee et al. 2000).
Wilson (1972, 1973) concluded that on a worldwide basis the most important single control that increases sediment export from catchments is land use and several more recent studies have reached a similar conclusion (Pringle 1991; Brasington and Richards 2000; Longfield and Macklin 2000). In the Richmond River catchment, land use changes modelling also indicates significant increase in suspended sediment load (about 6-fold) from its pristine condition, which is similar to the 3-5-fold increase estimated for Queensland catchments (Moss et al. 1992). Suspended sediment load modelling also indicates a possibility of more than 10-fold increase in sediment load from the Richmond River catchment if 100% forest is cleared. A similar response (about 10-fold) was observed in Wyvuri catchments, Queensland (Gilmour 1977).
In the sediment flux calculation, error associated with the frequency of sampling can increase the overall sediment flux error more than 6%. In this study it was beyond the scope to do more frequent sampling during flood events, mainly because of 5-6 h travel time involved for collecting data manually from all over the catchment. Despite having a possibility of higher error in sediment flux calculation, suspended sediment exports from the Richmond River subcatchments are similar to other Australian catchments. Any increase in error, however, would not have any impact on overall catchment sediment response patterns and their relationships with catchment attributes.
The findings from this study would enable local catchment management organisations to develop sustainable land use management policies for 3 Richmond River subcatchments, which will ultimately maintain and improve total catchment and estuarine ecosystem by reducing sediment export from the Richmond River catchment.
Table 1. Rainfall distribution (mm) over the three Richmond River subcatchments during two hydrological study year (1994-96) and an average year Subcatchment 1994-95 1995-96 Average year Wilsons River 1067 1762 1564 Richmond River 735 1362 1285 Bungawalbin Creek 793 1467 1200 Table 2. Flood events flow-weighted sampling in the Richmond River catchment Flood event Flood type Sampling site Date start February 1995 Minor Wilsons River, Lismore 15.ii.95 Leyester Creek, Lismore 15.ii.95 Richmond River, Casino 16.ii.95 Bungawalbin Creek, Coraki 17.ii.95 January 1996 Minor Wilsons River, Lismore 9.i.96 Leyester Creek, Lismore 9.i.96 Richmond River, Casino 9.i.96 Bungawalbin Creek, Coraki 10.i.96 May 1996 Moderate Wilsons River, Lismore 1.v.96 Leyester Creek, Lismore 1.v.96 Richmond River, Casino 1.v.96 Bungawalbin Creek, Coraki 1.v.96 Flood event Flood type Sampling period February 1995 Minor 3 days 3 days 2 days 1 day January 1996 Minor 4 days 4 days 4 days 4 days May 1996 Moderate 10 days 10 days 12 days 12 days Table 3. Seasonal and flood events suspended sediment exports (t) from the Richmond River subcatchments (wet season includes flood load) 1994-95 1995-96 Dry Wet Flood Dry Wet Flood Richmond River 60 7860 5700 850 146000 139840 Wilsons River 590 20210 15310 960 54650 45980 Bungawalbin Creek 1 55 49 1 18730 16290 Total 591 28125 21059 1811 219380 202110 Table 4. Runoff and Sediment load responses from three subcatchments during flood and minor events Richmond River Wilsons River Runoff Load Runoff Load (mm) (t) (mm) (t) Flood events Feb. 1995 20 5090 180 7120 Jan. 1996 29 15390 112 4450 May 1996 184 124450 194 6940 Minor events Mar. 1995 10 610 60 1190 Nov. 1995 20 90 Leyester Creek Bungawalbin Creek Runoff Load Runoff Load (mm) (t) (mm) (t) Flood events Feb. 1995 59 4650 0.5 47 Jan. 1996 46 5530 30 1730 May 1996 188 28860 150 14560 Minor events Mar. 1995 25 2370 Nov. 1995 7 110 Table 5. Variation of sediment transport from different parts of the world Note: climate classification is based on Moran and Morgan (1994) Region Climate Sediment yield (t/[km.sup.2]*year) Asia Tropical (India) 23-1029 Tropical-humid (Thailand) 200 (Indonesia) 3400-20000 Subtropical (China) 250-1450 Africa Tropical (Kenya) 20-200 USA Subtropical 64-1000 Europe Temperate (UK) 50-100 New Zealand Temperate-glacial 2-17070 Australia Tropical (Qld) 13-70 Subtropical (Qld & NSW) 21-243 Subtropical Richmond River 5-40 Temperate (ACT) 9-43 Region Climate Source Asia Tropical (India) Subramanian 1987; Vaithiyanathan et al. 1988 Tropical-humid (Thailand) Milliman and Meade 1983 (Indonesia) Suwardjo and Sofijah 1985; Sukartiko 1988 Subtropical (China) Milliman and Meade 1983 Africa Tropical (Kenya) Dunne 1979 USA Subtropical Milliman and Made 1983; Walling and Webb 1992 Europe Temperate (UK) Walling 1990 New Zealand Temperate-glacial Griffiths 1981 Australia Tropical (Qld) Moss et al. 1992 Subtropical (Qld & NSW) Loughran et al. 1992; Moss et al. 1992 Subtropical Richmond River This Study Temperate (ACT) Neil and Galloway 1989
Acknowledgment is extended to the Centre for Coastal Management, Southern Cross University, for providing research grants and other logistic support to carry out the fieldwork. Special thanks to the Department of Land and Water Conservation, Lismore, for providing logistic support during field work, and to the Australian Bureau of Meteorology, the Department of Land and Water Conservation, Grafton, and the Manly Hydraulics Laboratory, Sydney, for supplying necessary data.
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Manuscript received 22 May 2001, accepted 1 October 2001
Shahadat Hossain (AC) , Bradley Eyre (B) , and David McConchie (B)
(A) Gold Coast City Council, PO Box 5042, Gold Coast MC, QLD 9729, Australia.
(B) School of Resource Science and Management, Southern Cross University, Lismore, NSW 2480, Australia.
(C) Corresponding author; email: email@example.com.
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|Title Annotation:||New South Wales|
|Author:||Hossain, Shahadat; Eyre, Bradley; McConchie, David|
|Publication:||Australian Journal of Soil Research|
|Article Type:||Statistical Data Included|
|Date:||May 1, 2002|
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