Estimation Of Long-Term Sediment Loads In The Fitzroy Catchment, Queensland, Australia

Transcription

1 Estmaton Of Long-Term Sedment Loads In The Ftzroy Catchment, Queensland, Australa Joo, M., 2 B. Yu, B. Fente and 3 C. Caroll Natural Resources and Mnes, 80 Meers Rd Indoorooplly Qld 4068 Emal: Maranna.Joo@nrm.qld.gov.au 2 Faculty of Envronmental Scences, Grffth Unversty, Nathan Qld 4. 3 Natural Resources and Mnes, Bolsover St, Level 2, Rockhampton Qld Keywords: Sedment ratng curves; sedment, Ftzroy catchment EXTENDED ABSTRACT Wth a catchment area of nearly 40,000 km 2, the Ftzroy dscharges a sgnfcant amount of sedment to the Great Barrer Reef lagoon. Sedment dscharge s related to water turbdty whch has a drect mpact on coastal ecosystems. Prevous estmates of the long-term average sedment of the Ftzroy vary greatly Mt ( Mt = 9 kg). These estmates, however, were mostly based on an extrapolaton of sedment s other catchments n Queensland. In addton, sedment s major trbutares of the Ftzroy have not been assessed for nternal consstency n the estmated sedment s. Measured total suspended solds (TSS) concentratons sx stes n the Ftzroy were used to develop sedment ratng curves. Sedment ratng curves were corrected usng smearng estmates to remove an nherent bas as a result of log-transformaton. These ratng curves combned wth streamflow data were used to estmate long-term mean sedment of the Ftzroy and ts man trbutares for a common 30-year perod Sedment ratng curves for the sx stes show consderable smlarty, and varaton n sedment dscharge at gven flow rates s consderably larger than between-ste varatons n the expected sedment dscharge at the flow rate. The mean sedment was estmated to be 3.09 Mt for the Ftzroy at the Gap for the 30-year perod (see Table below). Most of the sedment of the Ftzroy comes the Nogoa and Comet subcatchments. The combned s of the four major trbutares are consstent wth the total estmated for the Ftzroy at the Gap. Load estmated for the MacKenze ste appeared to be low. At gven level of runoff, sedment concentraton on average was hghest for the Nogoa and Comet subcatchments, and the lowest for the Isaac. An ntensfed perod of water qualty montorng snce 993 concded wth a perod of relatvely low streamflow. for the perod was about half, on average, of the 30-year perod nvestgated. Gaugng Staton Ste locaton, mean flow, and sedment s n the Ftzroy for the perod of Rver Locaton Catchment area (km 2 ) dscharge (GL) sedment Upper 95% predcton nterval of Lower 95% predcton nterval of 3029A Nogoa Duck Ponds 27, A Comet 7.2 km 6, A Isaac Yatton 9,79 2, A MacKenze Coolmarnga 76,645 3, A Dawson Beckers 40, A Ftzroy The Gap 35,757 5,

2 . INTRODUCTION The Ftzroy s the largest coastal catchment n Queensland. Wth a catchment area of nearly 40,000 km 2, the Ftzroy dscharges a sgnfcant amount of sedment to the Great Barrer Reef lagoon. Sedment dscharge s related to water turbdty, whch has a drect mpact on coastal ecosystems. It s therefore mportant to accurately determne the amount of sedment the Ftzroy carres to the coastal areas. The Ftzroy basn s a prorty catchment n the Natonal Acton Plan for Salnty and Water Qualty (NAPSWQ), and has been dentfed as a hgh mpact catchment n the Great Barrer Reef Marne Protecton Plan. Prevous estmates of the long-term average sedment of the Ftzroy vary greatly. Belpero (979) estmated a mean of 2.5 Mt ( Mt = 9 kg) for the Fztroy by assumng the sedment ratng curve (an emprcal relatonshp between water dscharge and sedment dscharge) derved for the Burdekn rver.. Moss et al (992) assumed a constant flow-weghted sedment concentraton of 250 mgl - for catchments south of the Burdekn. Ths would lead to a mean of.3 Mt usng the streamflow data for the Ftzroy at the Gap (see Table 2). Nel and Yu (996) modelled sedment ratng curves for sx catchments n Queensland, and developed relatonshps between runoff (mm) and mean sedment per unt area per unt runoff (t km - mm - ). Usng ths method, Nel and Yu (996) estmated a mean sedment of 2.4 Mt for the Ftzroy under natural condtons and up to.3 Mt when the catchment s fully dsturbed as a result of agrcultural producton and urbansaton. Prevous nvestgatons are mostly based on extrapolaton of results other catchments n Queensland. Estmates of sedment s of the Ftzroy have been derved measured total suspended solds (TSS) concentratons for the catchment. In addton, sedment s major trbutares of the Ftzroy have not been assessed for nternal consstency n the estmated sedment s. Spatal dstrbuton of sedment and sedment budget on a long-term bass are mportant for valdatng spatal sedment yeld models such as SedNet (Fente et al. 2005). Ideally, streamflow and TSS concentraton are contnuously montored so that the total sedment s can be determned by ntegratng nstantaneous s for the requred perod at a partcular ste. In the absence of extensve and contnuous long-term montorng of TSS concentratons, sedment s commonly estmated usng sedment ratng curves. Wth sedment ratng curves, long-term flow records can be used to estmate nstantaneous s so that the total or average s can be estmated for the requred perod. The lmtatons of usng ratng curves are many and generally known. These nclude a lack of sedment samples durng hgh flows, samplng for certan flow ranges, dsparate sedment dscharge durng rsng and fallng stages of the hydrograph, varatons n TSS concentraton as a result of dfferent sedment generaton mechansms and dfferent locatons of the centre of storm events. Large varatons around the ratng curves lead to uncertantes about the functonal form of the ratng curves and parameter values assocated wth the ratng curves. Despte these lmtatons, wth rregularly collected TSS concentraton data, ths method remans the best avalable, and most accepted one to use to estmate long-term sedment s. The objectves of ths paper are to develop dscharge-sedment relatonshps for sx stes n the Ftzroy catchment, to estmate long-term sedment s for a common 30-year perod, and to assess the qualty of the estmated sedment s. 2. DATA AND METHOD The man trbutares n the Ftzroy catchment are the Nogoa, Comet, Isaac, and Dawson rvers. The MacKenze collects flows the Nogoa, Comet, and Isaac. It then jons wth the Dawson the south to become the Ftzroy (Fg. ). The long-term mean streamflow of the Ftzroy s 5227 GL at the Gap (Table 2) or 39 mm. The Isaac generates most water per catchment area wth 27mm yr -, whle the Dawson has the lowest runoff of 2 mm yr - among the four major trbutares. The Queensland Department of Natural Resources and Mnes collects water qualty data, ncludng TSS concentraton, under ts Surface Water Ambent Network program, snce the 970s and more ntensvely snce 990s. Ths program provdes a large amount of TSS concentraton data to allow an estmaton of long-term sedment s throughout the Ftzroy catchment. For ths study, all avalable TSS concentraton and flow data were extracted the Department s HYDSYS surface water database for the perod between 974 and 2003 when the data were most complete. 62

3 Nogoa 2. Comet 3. Isaac 4. MacKenze 5. Dawson 6. Ftzroy began n 993 under the Department s Surface Water Ambent Network program. Both low (<80 percentle) and hgh flows (>80 percentle) were well sampled at all stes, however the upper 2 percentle of flows were well sampled only at the Nogoa and Ftzroy stes, moderately sampled n the MacKenze and poorly sampled n the Comet, Isaac, and Dawson stes. In terms of maxmum dscharge sampled relatve to the maxmum dscharge on record, the rato was 65% for the Ftzroy ste, 45% for the Comet and Dawson stes, 35% for the Nogoa, 8% for the MacKenze, and was only 5% for the Isaac. Fgure. Subcatchments and TSS samplng stes n the Ftzroy catchment. Long term flow data were avalable 974 at all stes except the Nogoa where flow montorng commenced only n 993. The streamflow at ths ste pror to 993 was estmated by takng the dfferences between flows of the Comet Rver at 7.2 km (30504A) and that of the MacKenze Rver at Carnangarra (area = 45,370 km 2 ) whch s just downstream the Nogoa-Comet juncton. The estmated and measured flows were compared for the perod when streamflow was measured at all three stes (Fg. 2). The scatter for hgh flows s small, although at low flows there s a large amount of scatter around the : lne. When the sedment was estmated usng observed and the estmated flow for the Nogoa, the dfference s no more than 6% n estmated sedment s for the perod 993 and Streamflow data for the Nogoa were extended to 974 to create a complete data set for the 30-year perod October 973 to September 2003 for all sx stes n the catchment. Total suspended sedment concentratons were measured occasonally the 970s at all stes except for the Nogoa where ntensve samplng m odelled (m 3/s) measured (m3/s) Fgure 2 Modelled versus measured flow, Nogoa Rver at Duck Ponds for the perod between 993 and Table. Comparson of measured and estmated mean flow and sedment s for the Nogoa ste ( ). Mea d Est d Estmaton Error (%) (GL) 2,53 2, Load Table 2. Ste locaton, mean flow, and sedment s for the perod of Gaugng Staton Rver Locaton Catchment area (km 2 ) dscharge (GL) sedment Upper 95% predcton nterval of Lower 95% predcton nterval of 3029A Nogoa Duck Ponds 27, A Comet 7.2 km 6, A Isaac Yatton 9,79 2, A MacKenze Coolmarnga 76,645 3, A Dawson Beckers 40, A Ftzroy The Gap 35,757 5,

4 Data on TSS concentratons were collected at rregular tme ntervals, often three to four tmes a year. All the TSS data were used to develop ratng curves for the ste. Measured nstantaneous sedment dscharge was calculated usng TSS concentratons and correspondng flow measurements n the form of: Q s = x * Q * C () where Q s s the nstantaneous suspended sedment s (t/d), Q the flow rate (m 3 /s), C s the TSS concentraton (mg/l), and x s a unt converson factor (0.0864). Measured nstantaneous suspended sedment dscharge was plotted aganst the flow rate for each ste and ratng curves were developed n the form of: b Q s = a Q C f (2) where a and b are regresson constants usng logtransformed flow and sedment data, and C f s a correcton factor to remove an nherent bas ntroduced as a result of log-transformaton (Ferguson 986). Smearng estmate was used to calculate ths correcton factor (Duan 983): C f = n = ε n (3) where ε s the error term, or the resdual, for each measurement: Q s ( a b log Q ) ε = log + (4) and n the sample sze. Standard error of estmates, SE, was calculated as: 2 (log log ) Q Q SE = t s + + (5 n SQQ where t s the Student s t 2 dstrbuton, s = s where s 2 s the varance, n s the sample sze, and S QQ n = ( ) 2 logq = logq. These sedment ratng curves were then used to convert contnuous dscharge measurements nto nstantaneous sedment dscharge estmates. 0.5 More detaled flow data were used n ths study, whle prevous estmates of sedment s were based on monthly and daly flows (Belpero 983; Nel and Yu 996). The tme nterval over whch the flow rate s assumed to be constant vares as sgnfcant change n water stage occurs. For the Ftzroy at the Gap, for nstance, about 50% of flow was recorded at tme ntervals less than sx hours, and 85% of the flow recorded at tme ntervals < 2 hours. Estmated sedment dscharge was ntegrated to determne long-term sedment for the 30 year perod between 974 and RESULTS AND DISCUSSION Ratng curves developed for the sx stes are presented n Fg. 3. The grey plots under each graph show all data all stes to ndcate the dstrbuton of TSS concentratons relatve to other stes n the Ftzroy catchment. Sample sze, estmated parameter values for ratng curves are presented n Table 3. Table 3 also ncludes a sedment ratng curve developed usng all avalable TSS concentraton data all sx stes. Table 3 Sample sze (n), estmated model parameters for ratngs (a and b), and standard errors (ε), for the sx stes n the Ftzroy catchments where sedment dcharge (Q s ) s n td - and dscharge (Q) s n m 3 s -. Rvers na a b ε r 2 Nogoa Comet Isaac MacKenze Dawson Ftzroy Combned data set Fg. 3 shows that of the four major trbutares of the Ftzroy, the Nogoa and Comet have hgher sedment dscharge at a gven flow rate, especally durng hgh flows. Durng low flows, the sedment dscharge s generally lower for the Nogoa than that for the Comet. The Isaac has the lowest TSS and sedment dscharge values of all stes, although TSS measurements at hgh flows are very lmted at the ste. The MacKenze receves water and sedment the Nogoa, Comet, and Isaac subcatchments and the TSS concentratons here appear to be dluted by streamflow the Isaac subcatchment. In the Dawson Rver, the TSS concentratons le n the md-ranges although measurements of TSS 64

5 TSS concentraton vs. dscharge TSS vs. dscharge Nogoa TSS (t/d) Comet 00 0 TSS (t/d) Isaac 00 0 TSS (td/) MacKenze TSS (td/) Dawson TSS (t/d) Ftzroy TSS (t/d) TSS all subcatchment TSS ndvdual subcatchments 80th percentle of flow for ndvdual subcatchments maxmum recorded flow for ndvdual subcatchments Fgure 3. Sedment flow relatonshps n the Ftzroy catchment concentraton at hgh flows are not well represented. The Ftzroy ste receves water MacKenze and the Dawson.. Fg. 3 shows that the Ftzroy and MacKenze stes have smlar sedment ratng curves, and TSS concentratons reman relatvely low at the two stes. It s nterestng to note that the varablty n the sedment-dscharge relatonshp at any one ste s greater than the varaton among dfference stes. Varatons n the expected sedment dscharge among the sx stes are consderably smaller than the varatons n measured TSS concentratons at a gven ste. From Table 3, t can be seen the standard error for the ratng curve usng combned 65

6 dataset s of a smlar magntude when compared to the standard error for ndvdual stes. It s nterestng to note that the varablty n the sedment-dscharge relatonshp at any one ste s greater than the varaton among dfference stes. Varatons n the expected sedment dscharge among the sx stes are consderably smaller than the varatons n measured TSS concentratons at a gven ste. From Table 3, t can be seen the standard error for the ratng curve usng combned data set s of a smlar magntude when compared to the standard error for ndvdual stes. Usng all the avalable concentraton data, the flow-weghted TSS concentraton of the Ftzroy at the Gap s 533 mgl -. The average TSS concentraton based on the long-term sedment and streamflow Table 2 s 59 mgl -. In both cases, the average TSS concentraton was sgnfcantly hgher than 250 mgl -, a value assumed by Moss et al (992) to assess sedment exports Queensland catchments. Of the four major trbutares of the Ftzroy, the Nogoa has the hghest sedment, followed by the Comet, the Isaac and then the Dawson subcatchments. The same pattern holds n terms of sedment yeld (sedment per unt area). The combned catchment area of the four major trbutares s 3,77 km 2, or 76% of the catchment area of the Ftzroy. The combned the four trbutares s 2.44 Mt yr -. If the s adjusted by takng nto account the dfference n the catchment area, the projected of the Ftzroy at the Gap, based on estmated s of the four trbutares would be 3.9 Mt yr -, whch s qute consstent wth estmated at the ste of 3.09 Mt yr -. Estmated sedment of the MacKenze was.73 Mt yr - (Table 2), some 0.47 Mt less than the combned the Nogoa, Comet and Isaac. Ths would suggest a net loss or deposton of sedments along the reaches downstream the three stes on the trbutares and along the MacKenze upstream Coolmarnga. On the other hand, the combned the Mackenze and the Dawson s.98 Mt yr -, whch s consderably less than the estmated of 3. Mt yr - for the Ftzroy at the Gap. The sedment budgets for the Dawson, MacKenze, and Ftzroy ndcate a consderable addton of sedments along the reaches between the three stes. It s hghly probable that the sedment of the MacKenze at Coolmarnga was underestmated, because t s unlkely to have dramatc fluctuatons n deposton and eroson along the lower catchment reaches. Confdence ntervals for estmates at 95% level are presented n Table 2. Predcton uncertantes were determned for ndvdual tme ntervals usng equaton (5) and then ntegrated over the 30 year perod. The average error bar (half the confdence nterval) s 74% on average, rangng 56% for the Isaac to 8% for the Comet. Sedment concentraton (mg/l),000, Runoff (mm) Nogoa Comet Isaac MacKenze Dawson Ftzroy Fgure 4. runoff versus flowweghted sedment concentraton n the Ftzroy catchment. The relatonshp between runoff and flow weghted mean sedment concentratons shows three dstnct clusters for the Ftzroy catchment. The Nogoa and Comet have the hghest sedment concentraton for a gven level of runoff n the Ftzroy catchment. However, n dry years wth low runoff, the sedment concentraton of the Nogoa s consderably lower than that of the Comet, whle durng wet years, Nogoa generates hgher sedment s. The second group conssts of the MacKenze, Dawson and Ftzroy. In ths group there s lttle dfference between the stes durng drer years. For wetter years, however, the Ftzroy has a hgher sedment concentraton, followed by the MacKenze and then the Dawson. The Isaac has by far the lowest flow-weghted sedment concentraton n the catchment, and the sedment concentraton at the Yatton does not change as much drer to wetter years as at other stes n the Ftzroy catchment. To assess the qualty of the estmated sedment, especally the effect of havng TSS concentraton data collected dfferent perods on the long-term estmates, we recalculated the mean for the perod between 994 and 2003 when TSS concentraton data were collected at all sx stes. In addton, we re-developed the ratng curves usng the TSS concentraton data snce 993, and re-calculated the estmates usng the new TSS data set a common perod of montorng, but wth a reduced sample sze. 66

7 Table 4 Streamflow and estmated sedment s for varous perods endng n 2003 n the Ftzroy catchment. Rvers Annual (GL) 994 Annual Sedment Load TSS TSS Nogoa Comet Isaac MacKenze, Dawson Ftzroy 2, Table 4 shows that the perod of ncreased water qualty montorng snce 993 has also been a perod of relatve low flows. flow for s only about 50% of the 30-year average ( ) for the Ftzroy at the Gap. The decrease of flow of the three major trbutares, namely the Nogoa, Comet, and Isaac, s much greater, rangng 42% to 62%. Ths decrease n flow would lead to a dramatc decrease n estmated sedment for the perod 994 to Table 4 shows that the decrease n sedment would be 65% for the Ftzroy, and vary 5% to 7% for the three trbutares mentoned above. If TSS concentratons for the common perod snce 993 for all stes are used, the amount of decrease n sedment s as a result of the reduced flow snce 994 s qute smlar. If the ratng curves for the common samplng perod snce 993 were appled to the 30 year perod, all stes except McKenze and Nogoa show a notceable ncrease n the estmated sedment. The ncrease n the estmated sedment for the Ftzroy at the Gap s qute dramatc. 4. CONCLUSION TSS concentratons and flow data for sx stes n the Ftzroy catchment were used to develop sedment ratng curves to estmate sedment dscharge for a common 30 year perod Sedment ratng curves are broadly smlar for all the sx stes nvestgated. Varatons n the expected sedment dscharge among the sx stes are consderably smaller than the varatons n measured TSS concentratons at a gven ste. The long-term sedment of the Ftzroy was estmated at 3.09 Mt yr - wth a 95% confdence about 74%. The estmated was hgher than prevous estmates based on extrapolatons other catchments n Queensland. The perod snce 994 when water qualty montorng ntensfed concded wth a perod of low flows. Sedment for the perod snce 994 was consderable lower than the longterm average. Most of the sedment came the Nogoa subcatchment the west. The sedment of the Ftzroy at the Gap s consstent wth the combned ts four major trbutares, namely the Nogoa, Comet, Isaac, and Dawson. 5. REFRENCES Belpero, A.P. (979), The combned use of wash and the bed materal ratng curves for the calculaton of total : an example the Burdekn Rver, Australa. Catena 6: Belpero, A.P. (983), Terrgenous sedmentaton n the central Great Barrer Reef lagoon: a model the Burdekn regon. BMR Journal of Australan Geology and Geophyscs 8(3): Duan, N. (983), Smearng estmate: A nonparametrc retransformaton method. J Amer. Stat. Assoc. 78: Fente, B., M. Joo, B. Yu, H. Hunter, N. March and C. Carroll (2005), Comparson of mean suspended s estmated by the Sednet model and ratng curves n the Ftzroy catchment, Australa. Ferguson, R. I. (986), Rver s underestmated by ratng curves. Water Resour. Res. 22: Moss, A.J., G. E. Rayment, N. Relly and E. K. Best (992), A prelmnary assessment of sedment and nutrent exports Queensland Coastal Catchments. Queensland Department of Envronment and Hertage Techncal Report No. 5. (Queensland Government, Brsbane, Australa) Nel, D, and Yu, B. (996), Fluval sedment yeld to the great Barrer Reef Lagoon: Spatal patterns and the effect of land use. In Downstream effects of Land Use. (Eds. H.M. Hunter, A. G. Eyles and G.E. Rayment ) pp Department of Natural Resources, Brsbane, Australa 67