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Dry season suspended sediment concentration and sedimentation in the Richmond River estuary, northern NSW, Australia.

Introduction

Suspended sediments play an important role in fluvial and coastal ecosystems by changing the physical and chemical properties of the bottom sediments and water column and by providing an important transport medium for pollutants (Cowen and Lee 1976; Bostrom and Pettersson 1982; Bostrom 1984; Edmond et al. 1985; Armengol et al. 1986). Changes in land use patterns such as deforestation and agricultural activities in the upper catchments, development of floodplains, and urbanisation in lower coastal catchments has significantly increased the supply of sediment to fluvial and coastal waters. This increased sediment load has resulted in water quality degradation, loss of primary production and benthic habitats, an increase in flooding, and increased cost associated with the removal of sediment from navigation channels (Pringle 1986; Kinsey 1990; Robertson and Lee Long 1990; Arakel et al. 1993; Arakel 1995).

Previous studies have demonstrated that sedimentation in estuaries is complex (Allen et al. 1977, 1980), and suspended sediment transport through the estuary is controlled by many factors including river flow, tidal flows, tidal range, salinity, density, circulation, and wind (Ward et al. 1984; Pejrup 1986; Vale et al. 1993). Under low river flows, sediment transport in an estuary environment during the fortnightly tidal cycle maintains a large volume of sediment in motion and may trap the sediment in the estuary through repeated cycles of deposition and erosion (Salmon and Allen 1983). In contrast, tidal asymmetry and variation of tidal amplitudes during fortnightly tidal cycles may create a turbidity maximum in the estuary which increases sediment escape to the sea during an ebb tide if the longitudinal semidiurnal tidal excursion is large enough (Castaing and Allen 1981). In micro-tidal estuaries, wind-generated waves can increase suspended sediment concentrations by stirring the bottom sediments, and strong turbulent water mixing can also promote flocculation in addition to that occurring at the salt-fresh water interface (Gibbs 1977; Pejrup 1986).

To date, most studies have been carried out in North American and western European estuaries (Swift et al. 1981; Yarbro et al. 1983; Kesel et al. 1992) and few studies have been undertaken for Australian subtropical estuaries (Hossain et al. 2001; Hossain and Eyre 2002). This paper contributes to our understanding of dry season sediment transport dynamics by investigating detailed dry season suspended sediment concentration and short-term sedimentation rates for a typical subtropical Australian micro-tidal estuary using direct field measurement data.

Methods

Richmond River estuary

The Richmond River estuary (Fig. 1) is situated in the subtropical zone of northern New South Wales (NSW), Australia. The surface area of Richmond River estuary is about 15 [km.sup.2]: the estuary is classified as micro-tidal (Davies 1973; Hayes 1975), bar-built type (Fairbridge 1980). The average depth of the estuary varies from 4 to 5 m; its semidiurnal tidal amplitude for neap tide varies from 0.5 to 1.5 m and for spring tide from 0.2 to 2 m. During dry seasons the estuary extends about 60 km from its entrance at Ballina to Coraki and remains well mixed. The tidal wave propagates up to Lismore and Casino in the Wilsons and Richmond River subcatchments and up to 15 km upstream from Coraki in the Bungawalbin Creek subcatchment (Hossain 1998).

The upper catchment area of the Richmond River estuary is ~6900 [km.sup.2], which includes 3 main tributaries, the Wilsons River, the Richmond River, and the Bungawalbin Creek, and 2 small coastal catchments. The upper catchment area has slopes steeper than 15[degrees] in 20% of the area and undulating land with slopes 3[degrees] - 15[degrees] in 40% of the area. The remaining area is flat with extensive floodplains (Anon. 1980). The dominant surface lithologies for the Richmond River catchment are basalt (31%), alluvial deposition (30%), and sandstone (28%) (Anon. 1996). The dominant land uses in the Richmond River catchment are cattle grazing (53%), timber cutting (42%), and cropping (3.4%). Only 22% of the Wilsons River subcatchment is forested compared with 42% of the Richmond River and 75% of the Bungawalbin Creek subcatchments (Anon. 1996). In an average year, Richmond River estuary receives about 2490 x [10.sup.6] [m.sup.3] of runoff from the upper catchments, of which only about 10% of yearly runoff discharges into the estuary during the dry season (Hossain 1998).

Seasonal suspended sediment sampling in the estuary

Simultaneous 25-h sampling at 3 locations (Coraki, Wardell, Ballina) in the estuary (Fig. 1) was carried out for 2 hydrological years (1994 96) during spring (0.2-2 m) and neap (0.5-1.4 m) tides in dry seasons (5 and 12 November 1994; 2 and 9 September, 25 November and 2 December 1995). At each site, water samples were collected from 3 vertical positions (0.2, 0.6, and 0.8 of the total depth) at 3 locations across the river. Samples from the sites at Coraki and Wardell were collected from bridges and a boat was used to collect samples across the estuary mouth at Ballina. The integrated sample was filtered through a pre-weighted 0.45-[micro]m cellulose acetate membrane filter using a hand-held vacuum pump (30 kPa) and the filter was placed in a vial.

[FIGURE 1 OMITTED]

Velocity measurements were carried out at each site using a propeller type current meter. Water samples and velocity measurements were taken at 1.5-h intervals during the 25-h surveys. Depending on the travel time of the tide, surveys from the middle and upper estuary sites were always started 1 h and 3 h later, respectively, than sampling at the estuary mouth at Ballina. The first sample during all surveys was collected just before the change of tide and sampling then continued for the next 2 semidiurnal tidal cycles. To determine error associated with sampling, 6 additional samples from each site were collected during one randomly selected time during the 25-h sampling period.

Longitudinal suspended sediment transect

Suspended sediment transects in the estuary were carried out along the salinity gradient during inflowing and outflowing water for spring and neap tides in dry and wet seasons during the 2-year study period (1994-96). During each transect run, water samples were collected from 3 depths (0.2, 0.5, 0.8 of the total depth) at each sampling station using a hand pump. Sampling stations on each run were selected at salinity intervals of 3-5 on the partial salinity scale along the axial salinity gradient; salinity was recorded at a depth of 1 m. In-situ echo sounder was used to determine water depths at each sampling station and sampling point depths. A total of 6 dry season longitudinal transects (15 November 1994, 31 January 1995, 10 October 1995, 17 October 1995) were conducted during the study. All transect runs commenced 1 h after the change of tide at the mouth and progressed to the fresh-salt water interface.

A similar procedure was used to filter the water samples as described previously. In the laboratory, salinity values of the depth-integrated water samples from each station were measured using a salinity meter, because in the field, salinity was measured from one depth to locate the sampling point.

Sediment concentration measurement

Depending on the suspended sediment concentration, a water sample of 50 250 mL was filtered through a pre-weighted (dry 4-6 h at 60[degrees]C) 0.45-[micro]m 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.

Short-term sedimentation measurement using marker horizon

The use of marker horizons is the simplest, most economical, and most reliable technique for assessing short-term sedimentation ([less than or equal to] 1 year) in a non-static water environments (Baumann et al. 1984; Cahoon and Turner 1989; Eyre 1994). Red brick dust was used as a marker horizon at 15 sites in the estuary (Fig. 1) from its mouth at Ballina to ~40 km upstream at Woodburn. At each site the marker horizon was laid using a wooden frame over a 1-[m.sup.2] area between the lowest (0.2 m) and normal high tide level (1.5 m). Marker horizons were laid in 2 adjacent positions at each site and the average of the 2 positions was used as the net sedimentation at each location. To relocate the plots, 2 wooden sticks were used at the outside corners of each plot and red florescent tape was tied on to the top of each stick to make it visible. Cores were collected using 250-cm-long 10-cm-diameter PVC pipe, and after collection cores were sealed in the field and stored vertically in a box. Cores were placed in a freezer in the laboratory immediately after they were returned from the field. The frozen cores were sliced in half along the vertical axis using an electric cutter. Special care was taken in slicing the core to avoid disturbance. The depth of sediment accumulation on the top of the marker horizon was then measured with callipers at the centre and at I cm to either side of the centre.

The marker horizons were laid in September 1994 and the first core samples were collected after 2 months in November 1994. During the 2-year study period, cores were collected 9 times at ~3-month intervals.

Flushing time of the estuary

Flushing times were calculated using the fraction of freshwater method (Reid and Wood 1976; Day 1981; Officer 1983; Bowden 1983). To calculate flushing time, the Richmond River estuary was divided into 12 boxes, using 13 cross-sections, which were measured using a depth echo sounder. From the salinity transect data, the volume of freshwater in each box was determined and hence the flushing time was calculated by dividing the total volume of freshwater in the estuary by the freshwater input from the catchment. Three salinity transect runs (November 1994, July and October 1995) were used to calculate dry season flushing time for the Richmond River estuary.

Results

Spring and neap tide suspended sediment circulation

The fortnightly tidal cycle in the Richmond River micro-tidal estuary had little influence on suspended sediment concentration under normal tidal circulation. During 25-h sampling in the dry season, the average suspended sediment concentration at the estuary mouth at Ballina ranged from 19.04 [+ or -] 3.14 mg/L during spring tides to 20.51 [+ or -] 1.66 mg/L during neap tides. There was little relationship between the velocity of inflowing and outflowing water, and the suspended sediment concentration at the estuary mouth under normal tidal circulation (Fig. 2).

[FIGURE 2 OMITTED]

Suspended sediment concentrations in the middle estuary at Wardell were always higher than at Ballina during both dry seasons (Fig. 2). The average dry season concentrations during the spring and neap tides at Wardell were 33.07 [+ or -] 1.59 and 27.31 [+ or -] 3.89 mg/L, respectively. The fortnightly tidal cycle at Wardell showed that suspended sediment concentrations during the neap tides were less than during the spring tides, due to decreasing tidal amplitude (1.8 to 1.2 m) and peak current velocity (0.49 to 0.39 m/s).

Suspended sediment concentrations at Coraki (upper estuary) were always less than at the other 2 sites due to a very low sediment loading from the catchment. The average suspended sediment concentrations at Coraki during the spring and neap tide were 13.71 [+ or -] 1.89 and 9.14 [+ or -] 1.26 mg/L, respectively, during the dry season.

Results of 25-h simultaneous sampling at 3 locations in the 60-kin Richmond River estuary showed that there was no tidal asymmetry between the estuary mouth (Ballina) and the upper estuary (Coraki) (Fig. 2). Due to energy loss by friction and the backwater effect, however, tidal amplitude and flow velocity decreased as the tidal wave propagated upstream. Simultaneous flow velocity measurements at 3 locations showed that the flood tidal flow velocity decreased by about half by the middle estuary (Wardell) and by more than two-thirds by the upper estuary (Coraki) during the spring tide; during the neap tide, flood tidal flow velocity had dropped by about one-third by Wardell and by about half by Coraki (Fig. 3). A similar trend is also observed during ebb tide flow for both spring and neap tides.

[FIGURE 3 OMITTED]

Field-measured velocities and suspended sediment concentrations showed that the Richmond River estuary received a net input of suspended sediment from the Pacific Ocean during the neap tides in both dry seasons (Table 1); the net input of suspended sediment during the neap tide was more than the net loss of suspended sediment during the spring tide in both study years. The rate of exchange of suspended sediment through the middle estuary at Wardell during dry season spring and neap tides was almost the same for both flood tide and ebb tide conditions, indicating that there is very little exchange of suspended sediment between the upper estuary and the lower estuary.

Suspended sediment concentration along the estuary

During the dry months the estuary remained well mixed and suspended sediment concentration throughout the estuary was <30 mg/L (Fig. 4). Because of the lower sediment supply from the catchment area there was no abrupt change in suspended sediment concentration at the zone of fresh- and salt-water interface during the dry months. Although the concentrations remained low during the dry months, higher concentrations were found 10-30 km from the mouth at Ballina. No significant turbidity maximum zone was found either within the estuary or at the salt-fresh water interface.

[FIGURE 4 OMITTED]

During all 6 transect runs the Richmond River estuary was found to be well mixed.

Dry season sedimentation

During the 1994 95 hydrological year, marker horizons showed a larger deposition in the lower estuary (~0.84 [+ or -] 0.31 cm) than the upper estuary (-0.23 [+ or -] 0.08 cm) about 20 km upstream from the mouth (Fig. 5). A similar trend was observed during the 1995 96 dry season with a larger deposition in the lower estuary (-0.48 [+ or -] 0.3 cm) than the upper estuary (-0.28 [+ or -] 0.08 cm). In both dry seasons, beyond 22 km the estuary had uniform deposition along its length mainly because of vary small catchment inputs.

[FIGURE 5 OMITTED]

Dry months flushing time

Flushing time in the Richmond River estuary during dry months (July-December) varied from about 70 to 175 days depending on the volume of the freshwater inputs from the upper catchment (Fig. 6). Figure 6 shows that the estuary can flush all suspended point source inputs during one dry season.

[FIGURE 6 OMITTED]

Error estimation

Sediment flux errors for 25-h sampling 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 were considered to have an error of about [+ or -] 5% (Winter 1981). Therefore, the error associated with sediment flux calculation was about [+ or -] 6%.

Discussion

The Richmond River estuary received a net input of suspended sediment to the estuary from the Pacific Ocean during the dry months under normal tidal circulation, which is also observed in several other estuaries (Biggs 1970; Bokuniewicz et al. 1976). The Richmond River estuary receives net input of suspended sediment from the continental shelf mainly because of successive increase of the ebb tide level during the spring-neap cycle which induces a residual accumulation of water in the estuary and, therefore, accumulates sediment within the estuary. During the neap-spring cycle with the decreasing storage of water in the estuary, the net escape of suspended sediment from the estuary remains less than the net input of sediment during the spring-neap cycle. This mechanism during the fortnightly tidal cycle is probably causing net sedimentation in the Richmond River estuary under low flow conditions.

The net exchange of suspended sediment for both inflowing and outflowing water through the middle estuary at Wardell remains the same during the fortnightly tidal cycle in the dry season. Unlike the estuary mouth at Ballina, there was no clear evidence of net sediment input from the lower estuary to the upper estuary during the neap-spring cycle. This is probably because the velocity of the inflowing tide drops by -30 50% by the time it reaches Wardell. The inflowing velocity from neap to spring tide at the middle estuary varies from 0.38 to 0.49 m/s, which is almost equal to the settling velocity (0.42 m/s) of the average [D.sub.50] size (250 [micro]m) of the marine sediment in the Richmond River estuary (Hossain 1998). The exchange of suspended sediment through the upper estuary at Coraki during the dry season was very small due to the greatly reduced input from the upper catchment.

The transport of suspended sediment through the estuary mouth and middle estuary shows that during the dry season the lower estuary acts as a sediment sink. This assessment is supported by the marker horizons, which also show that during the dry months sedimentation in the Richmond River estuary mostly occurs in the lower estuary and source of this sediment is from the continental shelf.

Turbidity maxima under normal tidal variations are generally developed due to a large tidal range, tidal asymmetry, and stratification of the estuary during the fortnightly tidal cycle (Allen et al. 1980; Castaing and Allen 1981; Vale et al. 1993). In some shallow well-mixed estuaries, tidal asymmetry can cause higher suspended sediment concentrations (Eisma 1993). This phenomenon appears not to occur in the Richmond River estuary, probably because the bottom topography has a very gentle slope and there is no abrupt change in the width of the estuary between Coraki and Ballina. These two physical factors help to propagate the tidal wave with little deformation so that no tidal asymmetry is developed. This maintains low suspended sediment concentrations all along the Richmond River estuary during the dry months.

Flushing time of the Richmond River estuary during the dry months varies from 70 to 175 days, which indicates that the estuary under present conditions can flush pollutants during one dry season. This is also an indicator of healthy estuarine environment from a management point of view and demonstrates that the lower estuary would be able to flush any point-source input from urban development at Ballina during the dry months.

Overall, this work illustrates that the Richmond River estuary is relatively clean and remains well mixed during the low flow months without any visible turbidity zone. This implies that the Richmond River estuary is less polluted than other regional estuaries because the estuary has not been subjected to any significant morphological changes resulting from mining or navigational purpose, controlled flow from upstream catchment as a result of dam construction, and/or extensive urban development, which are common factors for ill health of regional estuaries such as the Brisbane River estuary (Eyre et al. 1998).
Table 1. Dry season suspended sediment exchange (t/tide) at
3 locations in the Richmond River estuary

+, Net input; -, net export

 Year Tide type Flood Ebb Net exchange

 Ballina

1994-94 Spring 256 370 -114
 Neap 355 226 +129
1995-96 Spring 440 457 -17
 Neap 280 246 +34

 Wardell

1994-95 Spring 289 299 -10
 Neap 152 153 -1
1995-96 Spring 339 355 -16
 Neap 191 192 -1

 Coraki

1995-96 Spring 35 36 -1
 Neap 23 25 -2


Acknowledgment

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 fieldwork. Gratitude is also expressed to all volunteers for their support during field data collection.

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Manuscript received 21 May 2003, accepted 7 November 2003

Shahadat Hossain (A), Bradley D. Eyre (B), and David McConchie (B)

(A) Corresponding author; Gold Coast Water, Gold Coast City Council, PO Box 5042, Gold Coast MC, Qld 9729, Australia; email: shossain@goldcoast.qld.gov.au

(B) School of Resource Science and Management, Southern Cross University, Lismore, NSW 2480, Australia.
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Author:Hossain, Shahadat; Eyre, Bradley D.; McConchie, David
Publication:Australian Journal of Soil Research
Date:Mar 1, 2004
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