Suspended Sediment Fluxes in an Indonesian River Draining A

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Sciences 1 Discussions Earth System Hydrology and , and P. Hoekstra 1 7138 7137 , S. M. de Jong 1,2 . Considering the wet conditions during the observation 1 − , A. J. F. Hoitink 1 This discussion paper is/has beenSystem under Sciences review for (HESS). the Please journal refer to Hydrology the and corresponding Earth final paper in HESS if available. Institute for Marine and Atmospheric research Utrecht (IMAU), Department ofHydrology Physical and Quantitative Water Management Group, Department of Environmental of the catchment. Theusing consequences a of plot ongoing scale erosionbasin clearing model. of in rainforest 2007, When were rainforest, isa explored which converted factor still to covered between production 50–60 10 %sediment, land, of and the soil the 100. increase loss in is If suspendedsevere sediment expected sediment this stress to flux soil on in increase the loss thecover with global changes is Berau hotspot will river of transported largely coral would seaward depend reef imposesediment as on diversity. a trap. The the suspended impact degree of in land which the Berau estuary acts as a flux increases non-linearly withvations river covered discharge. by the Averagedflux over benchmark was the survey, estimated 6.5 the at weeks tidally 2period, Mt obser- this y averaged figure suspended may be sediment consideredflux as is an significantly smaller upper than limit what of could the have yearly been averaged expected flux. from the This characteristics in the Berau river (Indonesia),sidered which the debouches preeminent into center ariver coastal of draining ocean coral that a diversity. can mountainous, TheFlow be relatively velocity con- Berau pristine was is measured basin an overinfluence that example a of of receives tides, large a abundant using part small rainfall. arogate of Horizontal the measurements Acoustic river of Doppler width Current suspendedtical at Profiler sediment Backscatter a (HADCP). concentration Sur- station Sensor were under (OBS). takenincreases the Tidally with with averaged river an suspended discharge, Op- sediment implying concentration that the tidally averaged suspended sediment Forest clearing for reasonsment of of oil timber palm plantations production,tropics. generally open results The in pit excessively increasing mining highsuch sediment sediment and as fluxes loads the coral in pose the reefs. establish- a threat This to study coastal presents marine observations ecosystems of suspended sediment fluxes Abstract Hydrol. Earth Syst. Sci.www.hydrol-earth-syst-sci-discuss.net/8/7137/2011/ Discuss., 8, 7137–7175, 2011 doi:10.5194/hessd-8-7137-2011 © Author(s) 2011. CC Attribution 3.0 License. Suspended sediment fluxes in an Indonesian river draining a rainforested basin subject to land cover change F. A. Buschman 1 Geography, Faculty of Geosciences,2 Utrecht University, The Netherlands Sciences, Wageningen University, The Netherlands Received: 8 July 2011 – Accepted: 19Correspondence July to: 2011 F. A. – Buschman Published: ([email protected]) 20 JulyPublished 2011 by Copernicus Publications on behalf of the European Geosciences Union. 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , , ; ). are ) that 2010 2005 ). The 2005 ( ), who ). The , ). , Edinger Milliman 2009 2005 2004 2009 , ( , 2005 , , Tomascik et al. Fabricius McLaughlin et al. Tarya et al. ; Syvitski et al. Hoitink ; Fabricius 1990 , Syvitski et al. 1999 ) and , section the tidally averaged de Voogd et al. de Voogd et al. ) observed a clear increase of ; 2003 Rogers ( ). The adjacent Berau continental 2004 ects on coral reproduction, growth 1997 ( ff ). , 2007 , Milliman et al. 2005 observations , , where details of the geology, climate and ). The Berau catchment can be considered 7139 7140 1 ). Furthermore, the continental shelf is of global Mallela et al. 1 Tomascik et al. Fabricius field site ; ). The disproportionally high sediment load from the Hoitink and Hoekstra Marshall et al. 2001 , 2011 ). At the same time, sediment fluxes from Indonesian islands , ). Especially the lowland and swamp forest support high densi- ) found a 30–60 % reduction of coral species diversity at land- 2001 , 2010 1998 ( , sensitivity of tidally averaged suspended sediment flux to land cover Spalding et al. ). Sediment fluxes in Indonesia are increasing at a higher rate than in ; are described, including the calibrations necessary to obtain suspended sed- 1999 1998 Spalding et al. ), who revealed the spatial structure of the river plume and underlying governing , ). In an embayment on Jamaica, ; , ected reefs over a period of 15 yr in Indonesia. The observed reef degradation The geography of turbidity may result in a zonation of coral reefs ( Edinger et al. Despite the apparent impacts of sediment fluxes on coral reefs, sediment fluxes in The present study serves as a benchmark reference for suspended sediment fluxes This paper continues with the ff Ekadinata et al. found that turbidity levels bear aby complex monsoons, relation and to may the peak tidal above motion reef and slopes to as flows driven a result of cloud formation. coral reef diversity with distancereef from systems the in river general. mouth, Theing which geography is of coral turbidity exemplary reefs of for an was marginal Indonesian studied embayment by host- Both turbidity and sedimentationand have survival, negative which may e result in decreasing species richness2003 ( interest due to the presencethe of endangered several green anchialine sea lakes and turtle for ( its nesting grounds for a is partly due toreduces the the maximal enlarged depth sediment where corals fluxes. can survive Turbidity ( reduces photosynthesis and Indonesian rivers are rarelytinuous being observations monitored of flow continuously. andsituated This suspended in sediment paper concentration Kalimantan, presents in Indonesia con- the (Fig. Berau river section, the ties of the Bornean orangutanshelf ( is host to an extremely diverse coral community ( pristine and anthropogenic disturbance was increasingis rapidly. employed A only simple erosion to model cover investigate changes, which the include sensitivity forest clearingconversion of for into suspended timber oil production, sediment palm open plantations. fluxes pit mining on and land other characteristics ofmethods the Berau basiniment are concentration given. and discharge.suspended In sediment In the concentrations the subsequent are section shown and the discussed. In the subsequent relatively pristine, with 50–60 % of( the land surface being covered by rainforest in 2007 processes. Sediments thatthe are reefs, suspended whereas sediment in that the ischannel transported mouths river as and plume bed forms load may mouth deposits bars. be primarily transported near the to to the Berau coastal shelf for the situation in 2007, when the catchment was relatively shows a marked zonationoceanic with reefs more east turbid of reefs the west chain of (Fig. the Derawan reef chain and coastal oceanography of the Berausubm continental shelf was studied by et al. about 2 % of the landof area the draining global into sediment the transfer global to ocean,Milliman the it oceans and is ( responsible Farnsworth for 13Indonesian to islands 18 % is due to thebasins with high easily topographical eroding relief, rocks the and smallet heavy size rainfall of al. that the charactizes drainage the tropics ( increasing sediment loads pose a serious threat to coastal coral reef ecology ( Indonesia is the countrycenter with of the diversity largest of marine1997 several areas groups of hosting marine coralare organisms reefs, is where disproportionally situated the large. ( Although the Indonesian Archipelago accounts for only other tropical regions, because of large-scale deforestation ( 1 Introduction 5 5 25 20 10 15 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , Man- Voss ˜ na pe- is indica- ) and also 1 MacKinnon Aldrian and ). Since the 1 2004 , ). The logging yields 2002 on the observed sediment , ). This tectonic movement created Moehansyah et al. discussion ; 1996 , ). Time series of rainfall did not show such 1993 , ). The karst formations, caves and limestone ). 7141 7142 2004 , ˜ no period of 1997 and 1998 and the La Ni 1982 ). 1996 Casson and Obidzinski , , 1982 ). This basin contains pools of coal, gass and oil ( Voss , MacKinnon et al. Bruijnzeel 2001 complete this paper. Voss , El-Hassanin et al. MacKinnon et al. Mantel ). The region that covers large parts of southern Indonesia experiences conclusions shows a rainfall climatology for the Berau catchment, which shows char- ). Ultisols are usually very deep, poorly fertile and aluminium toxic soils, erences were not so pronounced at the meteorological station in the harbor 2003 2 ( ). Ultisols cover most of the Berau district. In the south of the district a more ff 1996 , ), which are important for the district economically. Gold panning is of local eco- 2001 , The rainforest cover in the Berau district is rapidly decreasing (Table Figure The dominant source rocks for soils in Borneo are sedimentary. Active volcanoes Indonesia, since logging of thecially rainforests occurs from at 1998 large onwards scale,timber, both illegally legally which ( and is espe- anwith important establishing resource oil for palm theincrease Berau plantations. dramatically district, ( and As may a be result associated of the logging, soil erosion rates area of the Berau catchmenttive is for 55 % the of Berau thethe catchment. whole period Berau In district of area, our the Table fieldwork,when period the most between total of 2005 forest the and coverBerau observations 2008, decreased catchment for with which was this about encloses still 40 study %. about were In 50–60 made, %. 2007, the total This forest forest cover cover in is the still relatively high for The mean annual rainfall atperiod Tanjung from Redeb 1987 measured to 2105result 2007. mm in over decreasing In the rainfall this ( observation period, rainforest clearing has occurred, which may sediment fluxes towards the sea may increase substantially. 2.2 Climate These di town of Tanjung Redeb. Since thesea meteorological data level was and collected rainfall 20underpredict m is above the mean generally rainfall correlated in the withminimal Berau the at catchment. Tanjung altitude, Spatial Redeb, the and patternsin are of station the maximally rainfall may Berau show 2.2 slightly catchment to times ( be higher at the highest altitudes acteristics of two of the three climatic regions in Indonesia as defined by a decreasing trend. Duringriod the of El 1999, Ni rainfall over Kalimantan was respectively lower and higher than average. Susanto a strong influence of the wet Northwest Monsoon from November to March and of the for conservation ( 2.1 Geology and resources The Berau district,plate. which encloses The Sunda theand plate Berau folded is in catchment, the an Tertiary (65–3 is extension millionment of years of situated ago), mainland the associated on Australian with southeast plate the the Asia northward ( move- that Sunda was uplifted rainforests in this area are considered as one of the most valuable areas of Indonesia flux and the 2 Field site which limits their use for crop production and makes them vulnerable for erosion ( investigated by means of the erosion model. A nomic importance in the Berau district. that are typical fordonesian part most of other the Indonesian islandable minerals Borneo. islands and Sedimentary are rich rocks in absentred-yellow are silica. podzolic generally in When soils poor such Kalimantan, develop, in sedimentary of weather- theet parent which rocks In- al. Ultisols are are well the drained, dominant type ( tel fertile limestone area occurs ( Bulungan basin ( 1982 the topography of Borneo, including thesurrounding high hilly mountain landscape. range in Erosion centralmillion of Borneo years this and has mountain the resulted range in over a thick period sedimentary of deposits about in 15 basins, such as the Berau- 5 5 10 15 25 25 20 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ). A Q ). The 3 erences be- ff during several Buschman et al. 1 − s 3 erence between the wet ff ). In the Berau catchment ), who applied the methods ), which is associated with the 2003 2009 and the highest altitude is about , ( 2 2009 , -estimates to optical backscatter in Volts. c ), as they coincide with suspended sediment ) was obtained from turbidity after calibration 7143 7144 ), which was an average year in terms of rain- c 2009 Buschman et al. ( 2009 Buschman et al. , Aldrian and Susanto ) to convert HADCP velocity data to discharge ( 2009 ( Buschman et al. ). The drainage basin of the Berau river is situated in between the Buschman et al. 1 Hoitink et al. shows the linear regression of 4 gives an overview of the observations taken at Gunung Tabur. We use the ). In September 2007 about 20 sets of profiles were taken across the river 2 2 ), during which the protocol was setup to collect the velocity profiles continuously The bathymetry around Gunung Tabur, where instruments were mounted on an ex- The Berau river discharge at Gunung Tabur averaged 605 m Suspended sediment concentration ( . Figure 2009 bathymetry was obtained by sailingwith transects an across echosounder the and channel aalong at GPS, the least correcting channel. every for 500 water m level variation, and interpolating less exponentially going seaward ( increasing tidal prism. isting wooden jetty, showsThe several maximum deep depth troughs in in the the cross-section outer of bend the wooden of jetty the was tidal 8 river. m (Fig. fall. At Gunung Tabur, therange tidal is regime about is 1 mixed,the m predominantly coastal during semidiurnal. conditions. neap The The tide tidal cross and sectional 2.5 m area of during the spring Berau tide, river which increases is more or similar to 3.1 Obtaining continuous discharge Table same discharge data as analyzed by 3 Methods 1800 m, which reveals the mountainous character of the Beraumonths catchment. in 2007 ( the residue and dividec this weight by the filtered volume, resulted in the estimate of 2.3 The Berau river The Berau river isrivers formed join (Fig. just upstreamlarger drainage of basins the of village thedirection. Kayan of and These the Gunung Mahakam two Tabur, riversThe where large that size two rivers also of run the drain in Berau the eastward catchment highest is mountains about 12 of 000 km central Kalimantan. described by three-day period of HADCP was added to the data series described in (Table throughout a 12.5 h period, attaining a higher temporalwith resolution than in in situ May water 2007. turbidity samples. measurements. In Half of total the 99other samples water half was taken samples at at were 1 1 mwater taken m was simultaneously from from filtered the with the bottom through water and membranethe the surface. filters filter with For in a each pore an water size oven sample of to about a remove 0.45 known µm. water volume Drying and of organic matter from the residue, weighing the variation in monthly rainfall reflectsa both year these is climatic similar regions. asand The in variation dry within the periods first is climatic limited region, as whereas in the the di second. Vertical profiles of turbidityevery were 50 measured m with across the an approximatelyof Optical 400 turbidity m Backscatter were wide (OBS) Berau measured device river coveringan at tidal Gunung intermediate cycles Tabur. Profiles at tidal neap range tide, in at spring May, tide and and at during neap and spring tide in September 2007 ( at a higher sampling rate ofprocesses 2 of Hz. lateral The momentum high-frequency transfer data inthe were a data collected future series to study, but from investigate serve heredata to complement series. 3.2 Obtaining profiles of suspended sediment concentration in western Indonesia, the Monsoonstween are the suppressed, wet resulting and in small dry di seasons ( dry Southeast Monsoon from May to September. In the second region, that is centered 5 5 25 15 20 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | in (1) e from A Jetten c ; 2003 , ) derived from the C ected by air bubbles ff over the cross section, c Merritt et al. is the total water depth. All d ), suggesting that the sediment 4 cients were used to obtain ffi ) with an overturning period of 3 hours. shows the smoothed suspended sediment 1992 5 7146 7145 ( ). The USLE is an empirical erosion regression from the OMS turbidity correspond well to the C ) grid. Finally, a box filter was applied in time and 2010 , n,σ erent between spring and neap tides and between May ff ). 2 Kinnell ; ). ), defined as: 2006 Schlax and Chelton 5 , σ ciently reliable for analysis. The first of those time series lasted 6.5 ffi within the cross section was investigated. The concentration profiles were c , is the height from the water surface level and z z shows that the estimates of + d 6 d The valid points were interpolated and smoothed on a regular 5 min interval, us- For those two turbidity time series, data processing started by removing the spurious For the two days of September 2007, which had a higher temporal resolution, the = equation based primarily on observations. The annual soil loss per unit area ( and Favis-Mortlock concentration is limited, the pointcross data section from (Fig. the OMS are largely representative for3.4 the Erosion model The Universal Soil Loss Equationtics (USLE) typical is for applied the to Berau aorder drainage assessment hillslope basin. of that the has The sensitivity characteris- plot-scalechanges. of model the enables The sediment to flux USLE make in is a the a first Berau river widely to used land cover soil erosion model ( The resulting turbidity timethe series cross of section September averaged 2007OBS suspended was profiles. linearly sediment regressed concentration AlthoughFig. against ( this approach ignorescorresponding variation OBS-derived values. of Since the spatial variation of suspended sediment data points within 4 standardA deviations last from filtering the procedure medianand was obtained 542.5, necessary over a only when 24 for someobservations h a showed period. of a period low the value. between The measured Julianand highest days turbidities these signal was 538.5 points were attributed to were high, biologicaldid removed. fouling not whereas The exceed subsequent 4 number % of of points the total, removed for from each theing of time the the series methods two series. of weeks and the second timedata series were covered collected the (Table two days when the high timepeaks. resolution Turbidity sampleshigh exceeding turbidity, were 300 removed. NTU, which Next, a were moving obviously window not filter related was applied, to retaining the peaks were su larger part of thoseshells peaks growing were on caused the by instrument. biological fouling, Only with two weeds, data algae series and with relatively few spurious 3.3 Obtaining continuous suspended sediment concentration Turbidity was measured continuouslyMonitoring with Station (OMS) a manufactured 5 byjetty YSI. as minutes The the interval OMS HADCP using wasthe at deployed water a about at 1.5 surface, 600 the m for same equipped Optical above several with the months. a bed wiper andcontained and Although between with an the 2.5 a increasing optical and cage number backscatter to 4.5 m protect of sensor below the spurious was sensor, peaks the turbidity in signal each often of the time series. The The resulting regression has a high correlation (Fig. concentrations for three moments onsediment concentration each varies observation little day, over revealing width that and suspended depth. Regarding this calibration, 7 of the 99 points were excluded based on a 4-sigma test. variation of interpolated to a regular verticalwere interval discarded, of since 0.1 m. theseor measurements The by top could interference 0.3 have m with beennormalized and depth the a the ( bottom bed. 0.5 m The vertical coordinateσ was expressed in terms of characteristics were not very di and September 2007. The obtainedthe regression vertical coe profiles of turbidity. where profiles were interpolated onover a depth ( to smooth the data. Figure 5 5 25 15 20 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , , ) ). 1 (2) (3) (4) − 1978 , averaged er little be- 1 in mm y ff − s ects. Around P 3 ff Yu and Rosewell . is the soil erodibility 3 Renard and Freimund K Wischmeier and Smith is the slope-length factor ), l 1 S − ). The maximum observed h ), 1 is the support practices factor − , which is partly caused by the − Wischmeier and Smith ff y P ). Due to temporal water storage, 2009 1 , − ). Because ebb usually has a longer is an exponent between 0.2 and 0.5 2009 7 , whereas it was 605 m ) and , , 1 − m − s , which occurred at Julian day 545. During , which is the ratio of the long-term soil loss 065) 3 1 . m − 0 7148 7147 s C + 3 α Buschman et al. 56sin . induced by the tides, however, is small with respect to according the annual rainfall can be used to assess the Buschman et al. 4 i was estimated from annual precipitation ( e ). Considering the small variation between mean monthly + Q R e h α R is the slope-steepness factor ( 2 s ), obtained by applying a running mean over 24.8 h period to are usually derived jointly as: i 1996 amounted to 703 m is due to river discharge variations and tidal e S l , which corresponds to a cross section averaged flow velocity of , Q i S 1 7 ), h − 1 Q 41sin . h s − 3 and . P, , which represents slope angle here ( (65 s mm m 61 α m . S 1 1 C ), estimates of l − 2 ) is the result of a regression of data with slopes in between 2 and 10 P S 1 . s s 4 is the rainfall erosivity factor (MJ mm ha is the length of the slope (m) and ), averaged over a plot on a constant sloping hill is ( L . e s 1 22 is the cover and management factor ( 1 KS − L R − 0438 e . y m ) found for the United States, which suggests a universal nature of this relation- Yu and Rosewell ): ): 0 R 1 = C l − = = ), ). S The variation of For the cover and management factor Each factor in the USLE model can be derived or estimated based on physical char- This relation is remarkably similar to the empirical relation that The factors e e s − − 1994 the tidally averaged dischargegradients. is The variation smaller of the during variation periods due to with rivermountainous increasing discharge character variation of water from the level runo catchment. spring tide bottom frictionlevel is gradient than elevated, at which neap results tide ( in a higher tidally averaged water tidally averaged discharge was 1412 m that event, the flowpeaked direction at was 2896 m seaward almost1.0 over m s the entire day. The discharge over 6 months in the same year ( 4.1 Discharge The flow at Gunungtween Tabur is flood bidirectional. tide and Peak ebbduration, discharge tide a magnitudes (top di net panel ofaveraged flow discharge Fig. seaward ( occurs, whentime series averaged of over discharge, aweeks is tidal shown always in positive. cycle. Fig. The The discharge averaged tidally over the full 6.5 from a vegetated area to thecover types long-term soil that loss occur from in a the bare Berau fallow catchment area, are values given for land in Table 4 Observations 1996 R t ha 1978 A where factor (t h MJ ( ( acteristics of a hillslope.able Because for no the high Berauaccording temporal basin, resolution to rainfall an data empirical was relation avail- found for southeastern Australia ( where depending on S degrees. rainfall (Fig. relative soil loss. Equation ( ( ship ( 5 5 20 15 10 10 20 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) ) ). ), S S z u ): 5 (7) (5) (6) c and ). 1 cients 5 2009 − 1986 ffi , ects are ). Using , ff ). Rouse s Lane et al. w ) can be ob- uent reaches . Dyer 1986 S 1 ffl , − m. During neap is density of the usion coe 6 vary considerably was estimated as ff above the bed ( − that falls within the s Dyer C s s Hoitink et al. z ρ 10 w d + over depth (Fig. × according to the Stokes d c s , or 2 Mt y amount to 730 kg s ), second order e 29 1 w according to ( 5 − is often only a small fraction = S were well-developed (Fig. s ref S c d z profiles at during spring tide, at level + c c d cult to obtain a reliable estimate of ects to be minor. We choose not to ffi ff ) at level 7150 7149 ref c in the vertical are largest, which can be explained ) can be derived from is lowest, and vice versa near the bed. At the tidal s c is gravitational acceleration, d c over depth and width. The obtained more accurate , the Von Karman constant that has a constant value ∗ ) and the observed g . over the cross section (Fig. u ) ) stress that it is di ∗ uc c from the HADCP flow velocity data ( shows the cross section averaged suspended sediment βκu 2009 ∗ ( 7 , / is water density. During spring tide 2008 u s ( is generally highest around spring tide. The peak value of w ρ was smaller, resulting in a representative ) C ) c ref ), confirming second order e ref , in a tidal river, since tidally averaged z 7 5 z . − 0 S coincides with the highest river discharge, at Julian day 545. + ): , shear velocity )( s , a first order estimate of the suspended sediment flux ( 3 , which corresponds to medium silt with ) showed a maximum deviation of 25 % from the corresponding estimate z d − Hoitink et al. ( 1 , which is a constant of proportionality between the di w C s ρ ( + and inferring − z S French et al. β were derived from a best fit procedure. − 1986 u 0 d − , respectively, which occur during the highest river discharge. Values of is dynamic viscosity, µw s ( , z 1 ref m s and usion of sediment. Under these conditions, ρ − c ( = 18 µ 3 ff g − ref Q 26 kg m ) and . ref Dyer c 0 QC. z 10 and = = The instantaneous and tidally averaged peak value of Given the small variation of The exponent governs the shape of the profile and is named the Rouse parameter. The sediment particle diameter ( = ≈ + × s z s 1997 ( tidally averaged was calculated by integrating estimate of derived from Eq. ( account for the small bias causedthe by coarser resuspension sediment near eventually the will bed,the remain since coast, suspended it and is once pose unlikely the that a river threat e to marine ecosystems. 270 kg s averaged over the full time series amount to and 60 kg s expected to be small.is A highest small near bias the can surface, becycle where during expected spring from tide the variations fact of from that flow coarser velocity sediment ( profiles to of be brought in suspension at that time. Using logarithmic 4.4 Suspended sediment flux From tained from: S sediment particle and 1 tide the variation in clay and fine silt fractions.be considered Both relatively the fine, mean explaining the grain small size variation of of spring tide and neap tide may where c relates to the reference concentration ( C 4.3 Estimating suspended sediment particle size Both during spring tide andFor during those neap concentration tide, profiles, profiles observed of profiles during two were days fitted, in yielding Septemberprofiles 2007, a represent Rouse bulk steady estimate state ofward conditions di the when sediment downward settling fall is velocity balanced ( by up- The middle panel ofconcentration, estimated Fig. from the OMSover measurements. a Values of tidal period.frequent The variation highest of value occurs most often in the ebb phase. The low- d 4.2 Suspended sediment concentration It consists of of 0.4, and of suspended sediment andd water, usually assumed tow be unity ( law ( 5 5 20 25 15 10 10 20 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ): = S (8) 0 and C ected 7 2003 ff , and zooms in i 9 Q for only 8 % of −h 3 Q − = 0 is also elevated. This Meybeck et al. , respectively) are lin- Q i ) estimated that soil loss C C may be due to variations h S typically results usually in a 1987 i and ( 0 i C 0 Q h are likely related to high sediment Q , which places this general view in h S C 5. The tidally averaged suspended Besler . ects of land cover changes on a hills- 0 . ff 1 = within a tidal cycle ( within a day are pronounced (Figs. − within a day only has a small contribution 2 S dominates the dynamics. Tidally averaged r Q C C i 7152 7151 was always directed seawards during the 6.5- C and to land cover S ih is limited, exceeding 150 kg m and Q S C S S h Q of 272 kg s i S h shows peaks that are more than two times higher than the C , i 0 , occurring at the highest river discharge. The peak flow velocities . These dramatic increases of C S C 0 Q or due to variations of h i is usually elevated more during the peak ebb flows and the ebb duration + shows that the product C i h C 8 C showed that tidally averaged ih ). On the steep slopes of central Borneo, 7 . 2 Q i and , which is highest when variations in h was observed. At this day the flow velocity amplitude increases, with spring tide i i C = S Q h C h i The left panels indicate a period around Julian day 513, when the largest variation The contribution of variations of Figure −h ). Since S lope that has characteristics typicaldescribes for plot-scale the erosion processes, Berau we drainage assume basin. that Although the the model provides model first order 5.1 Motivation and assumptions This section describes thea application type of of the(Sect. production erosion land model. results Conversionfrom in of hillslopes a forest with significant to barehillslopes. decrease soil of was We over protection use 10 the to 000 USLE times soil to larger loss investigate than e losses from forested discharge events in onesediment may of be the passing by twoby at flow rivers Gunung velocity Tabur that in variation. the drain Berau into river, remaining the una Berau river.5 Clouds of Sensitivity of tidally averaged particles may be resuspended.elevation, As in a combination result, withresult tidally the in averaged the high highest ebb observed flow velocities and thein long ebb periods, occurring three days later. Concurrently,ebb the periods river of discharge increases. thistidally day, During averaged the two To analyze the periodson with two peak the events. highesttidally averaged suspended The right sediment panelsduring flux, ebb show Fig. are results higher from than during the lower period river discharge, with implying the that also highest coarser bed 4.6 Highest observed tidally averaged peak flood currents are similar. 4.5 Variation of tidally averaged Figure weeks observation period. The variationof in tidally averaged h early correlated, yieldingsediment a flux thus skill increases score non-linearlythe with of river period discharge. of Due to highthe this tidally time. non-linearity, averaged Half of the suspended sediment flux isto concentrated in only 25 % of time. exceed 20 % of theperspective. instantaneous The peak underlying value reason of a is simple that harmonic the and a tidalebb constant, curves currents representing are being discharge, not much whichebb larger a would result than superposition phase in peak of is peak flood longer currents. to Instead, account the for duration the of river the discharge, while peak ebb currents and where the quotes indicate theC deviation from the tidal average: discharge and suspended sediment concentration ( of the instantaneous peak values. In the Berau river, tidally averaged values of 8 is longer than floodpositive duration, contribution to the the correlation seaward term directed flux. 5 5 25 20 15 10 10 20 15 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . ) 1 ). 1 3 − − y y 2 2 . Part − 1999 − 1 , − y 2 − . This rela- ) represents ). Moreover, 1 3 6 ). Annual soil − ). The factors y 5 2 − results from Eq. ( 1978 (Table , e m Milliman et al. R C during the 6.5 weeks when 1 − ) estimated that suspended sediment yields cient additional sediment transport capacity Wischmeier and Smith ( ffi 7154 7153 1 − 2011 ( mm 1 − ), the plot length standard in USLE of 22 m and a slope of 4 observations were derived. In this period, rainfall was about to land cover type appears to be high (Table ). The sediment yield from these Tertiary rocks is about an or- S e A ). 1999 , ). The sediment yield of the Mahakam river, which also drains to the 2005 , Milliman and Farnsworth 1999 result from Eq. ( , l Douglas gives an overview of the default input variables for a hillslope that has char- S 4 for the strongly weathered Ultisols, which are the dominant soil type in the basin, K and The relatively low sediment yield from the Berau river may also be due to the high de- The calculated fluxes of suspended sediment in the Berau river can be compared The sensitivity of Increasing soil loss also implies elevated suspended sediment concentration in s Douglas Storms et al. The tidally averaged suspended sediment flux in the Berau river varied between 0.4 gree in which rainforest coverhigh protected rainforest the cover soil may against also erosion explain the in small 2007. variation The within relatively the observation period. der of magnitude lower than( from tectonically active partseast of coast Indonesia with of volcanoes Borneo,for supports Indonesia. that The sediment yield is yield( similar from as east the Borneo figure for is the relatively Berau low river, being 180 t km Borneo ( tively high yield isrelief, due relatively to young the and generallyAlthough erodible small the drainage rocks, Berau basin river andcatchment, areas, is heavy even an high its rainfall example topographic maximal ( ofof sediment such yield this a is discrepancy small much can river lower be draining than explained a 1000 t by km mountainous the old and deeply weathered rocks in east tion of Tanjung Redeb issediment indicative flux for may the be whole consideredsediment catchment, flux. as the an observed upper suspended limit of the yearly averagedwith suspended what can beyearly averaged expected flux, from the theIn sediment contrast, literature. yield from Based thein on Berau Indonesia, the catchment the maximal is Phillipines figure 170 t and km of Taiwan the are higher than 1000 t km The observed suspended sediment fluxvalid averaged tidally 2 Mt averaged y two times higher thanrainfall was average higher at than the the average station in the of wettest three Tanjung months. Redeb Assuming (Table that the sta- 6 Discussion 10–100 with respect to the situation in 2007. loss from an oil palmwith plantation further is the about same 50 conditions. timescultivation, higher which This than factor is from is carried a even outoccurs primary higher by from rainforest in bare the comparison ground, indigenous with which Dayak represents shifting higher population. the than areas The for with soil primary open loss rainforest pitabout that land mining, 50–60 cover. is % Considering 167 in times that 2007, the conversionresult of rainforest in this cover 10–100 rainforest was times into intensive higherrivers, production soil suspended land loss. may sediment If fluxes this in additional the sediment is Berau transported river to may the also increase with a factor S 10 degrees that characterizes theprimary rolling rainforest, plains. The which valuethe covers for steeper a parts large where share soiltaken, the loss of support is the large. practice factor drainage Since was no basin set erosion to and protection 1. especially measures were 5.2 Erosion model results Table acteristics typical for the Berauusing catchment. the annual The rainfall erosivity averaged over factor tor the period 1987 to 2007.is The soil characterized erodibility by fac- 0.013 t h MJ streams and rivers. In theis Berau available river, to su convey additionalconditions sediment are input far from fromcreasing the sediment supersaturated. catchment stress to on Hence, the the additional coral sea, reefs soil since and loss other marine may ecosystems. result in in- estimates of the sensitivity of soil loss to land cover changes in the Berau catchment. 5 5 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ), where accumulation 1 ) found that in small catch- 7142 1999 ( 7156 7155 Douglas et al. 7143 ), which suggests that increased sediment loads may , 2010 7142 , during a 6.5 weeks observation period in 2007. Since the ob- 1 , 2003. − This study was supported by grant WT 77-203 of WOTRO Science for . The peak value was only about 4 times higher than the average, which 7163 1 , − Buschman et al. 7152 er less protection to soil loss. From a comparison of historical and actual bathymet- of Kalimantan Timur (Indonesian1987. Borneo), Earth Surf. Processes Landforms, 12, 195–204, Agric. Ecosyst. Environ., 104,Focus, Chiang 185–228, Mai, Global THAILAND, Workshop NOV 12–13, on 2001, Bringing 2004. the Landscape into doi:10.1002/joc.950 donesia and their relationship to sea surface temperature, Int. J. Climatol., 23, 1435–1452, A plot scale erosion model was used to explore the first order response of soil erosion Over the past decades, the sediment supply of the Berau river has probably in- ff Bruijnzeel, L. A.: Hydrological functions of tropical forests: not seeing the soil for the trees?, Besler, H.: Slope properties, slope processes and soil erosion risk in the tropical rain forest Aldrian, E. and Susanto, R. D.: Identification of three dominant rainfall regions within In- References the field campaigns. Theses byM. K. van Wetser der and Vegt M. for Leenheer commenting were on used an for earlier this version study. of We thank this article. implies that also tidally averaged suspendedWhether sediment or flux not in this the Berau resultsthe river in coastal increases. a ocean, seaward depends shift on of the the degree zonation in ofAcknowledgements. which coral the reefs estuary observed act in Global as Development, a a sediment division trap. ofWe the acknowledge Netherlands M. Organisation of C. Scientific G.the Research van instruments (NWO). Maarseveen (Utrecht and University) hisnumber for of technical preparing students assistance. and maintaining from We Wageningen thank University and A. Utrecht Tarya (Utrecht University University) who and assisted a during to expected land cover changes.50–60 %, In which 2007, the isconverted rainforest high to cover in production compared the land, to Berauassociated such the basin as fluxes was of oil rest palm suspended of plantation, sediment Indonesia. the may supply increase of 10–100 When sediment times. this and This rainforest increase is still relatively high rainforest cover in 2007, which result in limited soil erosion. and 8.6 Mt y is less than what is found in literature. catchment is substantially smallerdiscrepancy than may be from explained other by similar the tropical old catchments. and deeply This weathered parent rock and the A benchmark study was performed aimingin to the assess Berau the fluxes river, oftershed. which suspended sediment is The Berau an river Indonesianan issues ecological tidal its hotspot river of discharge draining global toflux a importance. the averaged relatively The 2 Berau Mt pristine tidally y continental averaged wa- servation shelf, suspended period which sediment was is relativelythe wet, yearly the figure averaged sediment can flux.rau be catchment considered Comparing area as this the with maximal upper other figure limit studies, divided of shows by that the the Be- sediment yield from the Berau decades ( deposit in the estuarine system, rather than being issued to the coastal7 system. Conclusions the absence of extreme peaks could be the reason for thecreased, small since overall transport areas rates. coveredo with rainforest were subjectedric to maps, land the cover morphologicalsuggests types changes in that that the the Berau Berauas river a appeared river to trap. acts be The asof limited, Berau suspended a which river sediment conduit splits may of intothe occur. a most suspended A channel northeastern sediment comparison network channel rather of (Fig. of than bathymetric the maps network revealed has that silted up significantly over the past ments in Malaysian Borneo, the bulk5 of days. suspended Our sediment observations transport indicate occurredsediment that transport in in only is relatively more equally pristine distributed and over large the catchment year. areas, From another perspective, 5 5 25 20 15 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 7139 7142 7141 , 2010. , 2005. 7139 7146 7154 doi:10.1016/S0025- , 2010. 7149 7139 7139 7161 , , 2003. doi:10.1029/2010WR009266 7146 7140 ect of vegetation cover and land slope on 7150 7148 ff , , 7151 ects of land-based pollution, destructive fish- ff 7142 7149 7144 7139 doi:10.1016/j.marpolbul.2004.11.028 7158 7157 , , on the ecology of corals and coral reefs: review and 7150 7141 ff 7146 , 2003. 7144 7143 , 1993. and the Universal Soil Loss Equation family of models: A doi:10.1016/j.jhydrol.2010.01.024 ff , 2008. 7140 7150 , 2006. , 2009. , 2009. ¨ urr, H. H., and Syvitski, J. P. M.: Global variability of daily 7139 , 2009. 7155 doi:10.1016/S0272-7714(03)00181-1 , 1998. ects of terrestrial runo ff and soil losses from the watersheds of Burundi, Agric. Ecosyst. Environ., 43, 301–308, , and reefs, Global Planet. Change, 39, 191–199, 2003. ff ff doi:10.1016/S0921-8181(03)00018-3 total suspended solids and their fluxes in rivers, Global Planet. Change, 39, 65–93, models, Environ. Modell. Softw., 18, 761–799, 2003. S. A.: Use of limestoneSangkulirang karst Peninsula, forests East by Bornean Kalimantan, orangutans7140 Indonesia, (Pongo Am. pygmaeus morio) J. in Primatol., the 69, 212–219, 2007. runo Borneo, The ecology of Indonesia series: vol. III, Periplusfluvial editions, gradient, Singapore, Rio 1996. Bueno, Jamaica, Caribbean J. Sci., 40, 299–311,consultants 2004. report, DHV, BFMP, 2001. Shelf Sci., 45, 433–451, 1997. estuaries: sensitivity to instrumentation and associated data analyses, Estuarine Coastal review, J. 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Trans. ASAE, 39, 559–561, 1996. doi:10.1016/j.biosystemseng.2004.04.013 7147 models for catchment management in Indonesia, Biosystems Engineering, 88, 491–506, model for soil erosionsystem assessment, analysis, Austria, Tech. 1982. Rep. cp-92-76, International institutethe for revised applied USLE, J. Hydrol., 157, 287–306, 1994. Hamburg: Institut f planning, Agriculture Handbook no. 537, US department of Agriculture, Washington, 1978. large islands in the East Indies, J. Sea Res., 41, 97–107, 1999. 7154 thesis, Cambridge university press, Cambridge, United Kingdom, first edn., 2011. Tomascik, T., Mah, A. J., Nontji, A., and Moosa, M. K.: The ecology of the Indonesian seas; Tarya, A., Hoitink, A. J. F., and van der Vegt, M.: Tidal control over river plume spreading in a Tarya, A., Hoitink, A. J. F., and van der Vegt, M.: Tidal and subtidal flow patterns on a Syvitski, J. P. M., Vorosmarty, C. J., Kettner, A. J., and Green, P.: Impact of humans on the flux Spalding, M. D., Ravilious, C., andStorms, Green, J. E. E. A., P.: Hoogendoorn, R. World M., Atlas Dam, R. of A. Coral C., Hoitink, Reefs, A. University J. F., of and Kroonenberg, S. B.: Schlax, M. G. and Chelton, D. B.: Frequency domain diagnostics for linear smoothers, J. Amer. Rogers, C.: Responses of coral reefs and reef organisms to sedimentation, Mar. Ecol. Prog. Morgan, R.: Soil erosion and conservation, Blackwell publishing, Oxford, England, 2005. Yu, B. and Rosewell, C. J.: A robust estimator of the R-factor for the universal soil loss equation, Moehansyah, H., Maheshwari, B. L., and Armstrong, J.: Field evaluation of selected soil erosion Morgan, R. P. C. and Finney, H. J.: Stability of agricultural ecosystems:Renard, validation K. of G. a and simple Freimund, J. R.: Using monthly precipitation data to estimate the R-factor in Wischmeier, W. H. and Smith, D. D.: Predicting rainfall erosion losses- a guide to conservation Voss, F.: Kalimantan Timur Atlas, East Kalimantan Transmigration Area Development (TAD), Milliman, J. D., Farnsworth, K. L., and Albertin, C. S.: Flux and fate of fluvial sediments leaving Milliman, J. D. and Farnsworth, K. L.: River Discharge to the Coastal Ocean: A Global Syn- 5 5 30 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ). 2010 ( Ekadinata et al. 7162 7161 1995 2000 2005 2008 507,510, 516, 620 and 628 Turbidity profiles ) in time in the Berau district after 2 UndisturbedLogged-over 12 196Total 7121 8407 7991 7551 9497 19 317 4319 16 398 5766 17 048 10 085 ∗ Overview of all measurements used for this study, which were taken at the cross Forest cover (km Instrument Period (Julian daysHADCP from 1 January 2006)OMS Variables measured OBS 503–548 and 612–630 518–575 Turbidity at one point Flow velocity During OBS profile measurements also water samples were taken to obtain suspended sediment concentration. section of Gunung Tabur. ∗ Table 2. Table 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Morgan and that represents primary m C 1 − ) h − 1 ( − 1 y m − 1 C − mm 1 − 1 7164 7163 2.8 ). 9820 MJ mm ha 0.013 t h MJ 0.006 2005 , ee (with cover crops) 0.1–0.3 ff l Morgan S Cacao (with cover crops)RubberOil palm (with cover crops)Groundnut 0.1–0.3 Bare ground 0.1–0.3 0.2–0.8 0.2 1 Land cover type Primary rainforestSecondary rainforestTraditional shifting cultivationCo 0.001 0.002 0.006 ; m e s P K S C USLE factor ValueR Units 1987 , derived from rainfall during an average year and e R Besler ; Cover and management factors for various tropical land cover types ( USLE factors for a hillslope that has characteristics typical for the Berau drainage 1982 , rainforest. Table 4. basin, including Table 3. Finney Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent land cover types. ff ) 1 − y 1 ) − 1 − (t ha e . 4 A (mm y P 7166 7165 1987–20076.5 weeks period2007Nov–Jan 1987–2007 4080 2589 2105 2762 erent periods. ff Primary rainforestOil palm without cover cropsTraditional shifting cultivationBare ground 108.1 0.4 2.2 360.3 Estimated annual soil loss per unit area from USLE for di Rainfall for four di Table 6. Table 5. Other model input parameters are as specified in Table Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | during several 1 − s 3 50 km 118.5º E Sand/mud-bank Coral reef Channel Nov Derawan 0 Sep 118.0º E Jul 8 117.5º E Tanjung Tanjung Redeb 4 Gunung Tabur

500 7168 7167 May 117.0º E Mar

500

1000 Jan

0 116.5 º E

1000 1000

50 500

250 200 150 100

1000 (mm) Rainfall

1500

1000

2.2º N 2.2º 1.8º N 1.8º N 1.6º 2.4º N 2.4º 2º N 2º 8035 Map of the Berau region showing the Berau river catchment with contour heights (m), Map of the Berau region showing the Berau river catchment with contour heights (m), Monthly rainfall at Tanjung Redeb based on the years 1987–2007. The yearly average the river and estuarine channels and the coral reef islands inIndonesian the rivers east. are rarelytinuous being observations monitored of ﬂow continuously. andsituated This suspended in sediment paper Kalimantan, concentration Indonesia presents in (Figure con- relatively the 1). Berau pristine, The river Berau with catchment 502007 can – be (Ekadinata considered et 60 al., %densities 2010). of of Especially the the the land Borneantinental lowland surface orangutan and shelf (Marshall being swamp is et covered forest host al., by supportthat to 2007). rainforest shows high an a in extremely The marked diverse adjacentand zonation coral Berau oceanic community with con- (de reefs more Voogd turbid eastof et reefs of al., global west the 2009) interest of chain due thegrounds (Figure Derawan to for 1). reef the the chain Furthermore, endangered presence2009). green the of The sea continental several coastal turtle shelf anchialine oceanography (Tomascik is ofet lakes et the al. and al., Berau (2010, for 1997; continental subm), its shelf de who was nesting Voogd studied et revealed by the al., Tarya spatial structure of the river plume and under- Fig. 1. is 2105 mm, which corresponds to an average monthly rainfall indicated by the thick line. Fig. 2. the river and estuarine channels and the coral reef islands in the east. Fig. 1. Monthly rainfall at Tanjung Redeb based on the years 1987 – 2007. The yearly average The Berau river discharge at Gunung Tabur averaged 605 m months in 2007 (Buschman et al.,At 2009), Gunung which Tabur, the was an tidal average regime yearis is in mixed, terms about predominantly of semidiurnal. rainfall. 1 The mcoastal tidal conditions. range during The neap cross tide sectional area and of 2.5 the Berau m river during increases more spring or tide, less which is similar to the The Berau river isrivers join formed (Figure just 1). upstreamlarger The drainage of drainage basins basin the of of village thedirection. the Kayan of Berau and river These the Gunung is Mahakam two Tabur, situated riversThe where large that in size two rivers also between of run drain the the in thealtitude eastward Berau is highest about catchment mountains 1800 is m, ofment. about which central reveals 12,000 Kalimantan. the square mountainous character kilometers of and the the Berau catch- highest 2.3 The Berau river Fig. 2. is 2105 mm, which corresponds to an average monthly rainfall indicated by the thick line. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | m. Drying µ 17 117.515 0.3 500 m 15 ) from in situ water samples. c 0.25 13 117.51 = 0.72 ) ) from in situ water samples. 2 o c= 0.33 T−0.0018 r c 11 0.2 9 T (V) 117.505 0.15 7 9 11 Longitude ( 7170 7169 ) was obtained from turbidity after calibration c 5 117.5 0.1 3 1 0.05 Gunung Tabur 117.495

0 0 0.1

2.18

0.06 0.04 0.02 0.12 0.08

2.178 2.176 2.174 2.182

) (kg/m c

Latitude ( Latitude )

3 o Bathymetry of the Berau river close to the village Gunung Tabur showing depths with Calibration of turbidity with total suspended matter ( Fig. 4. Fig. 3. respect to mean water levelconcentration (m). were Instruments deployed that at obtain the flow location velocity and marked suspended with sediment an X. Bathymetry of the Berau river close to the village Gunung Tabur showing depths with Calibration of turbidity with total suspended matter ( The bathymetry around Gunung Tabur, where instruments were mounted on an ex- Suspended sediment concentration ( exponentially going seaward (Buschmanincreasing tidal et prism. al., 2009), which isisting associated wooden with jetty, the showsThe several maximum depth deep in troughs thebathymetry in cross-section was of obtained the the by outer sailing woodenwith bend transects jetty an across was of echosounder the 8 and the channel m aalong at (Figure tidal GPS, the least 3). correcting river. channel. every for The 500 water m level variation, and interpolating Fig. 3. respect to mean water levelconcentration (m). were Instruments deployed that at obtain the flow location velocity and marked suspended with sediment an X. Fig. 4. (Table 2). Inthroughout September a 2007 12.5 about2007. 20 hours sets period, of attaining profiles a were higher taken temporal acrosswith resolution the in than river situ water inturbidity samples. May measurements. In Half of total the 99other samples water half was taken samples at at were 1 1water taken m m was simultaneously from from filtered the with the through bottom and membrane water the filters surface. with a For pore each size water of sample about a 0.45 known volume of the filter in anthe oven residue to and remove divide water this and weight organic by matter the from filtered the volume, residue, resulted weighing in the estimate of Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | from c

630

) in the cross 0.01 ) in the cross

0.01 350 3 0.012 0.008

3 0.012 − − −0.0082

629 0.012 300 OMS ) derived from the

0.008 0.01 C 250 628 =0.0011 T over the cross section,

0.012 200 OMS OBS c n (m) NEAP C C 627

150 0.01

0.012

100 626 0.008 50 t=16 hours t=8 hours t=12 hours 625 0 -estimates to optical backscatter in Volts. c 12 14 624 7172 7171 0.07

from the OMS turbidity correspond well to the 0.06 0.08 350

0.08

0.09 C

0.07 Time (Julian days from 1 jan 2006) 623 300

0.1 0.07

0.08 250

0.09 622

0.08 200 n (m) SPRING 621

150 0.08 0.08

0.1

0.09

0.08 0.06

0.09

100

0.07

0.07 0.06

620 0.07 50 full flood full ebb LW slack

619 within the cross section was investigated. The concentration proﬁles were 0 0 0.1

c 0 0 0

0.06 0.04 0.02 0.08

−2 −4 −6 −8 −2 −4 −6 −8 −2 −4 −6 −8

) (kg/m C

z (m) z (m) z (m) z 3 Suspended sediment concentration from optical measurements (kg m Cross section averaged suspended sediment concentration for 11 days in September Cross section averaged suspended sediment concentration for 11 days in September Suspended sediment concentration from optical measurements (kg m The valid points were interpolated and smoothed on a regular 5 minutes interval, For the two days of September 2007, which had a higher temporal resolution, the . Figure 4 shows the linear regression of 2007. Fig. 6. section close to Gunung Tabur atneap three phases tide in [right]. a The tidal cycle black during triangle spring indicates tide the [left] position and of during the HADCP. Fig. 5. 2007. observations showed a low value. Theand highest these signal was points attributed to were biologicaldid removed. fouling not The exceed 4% number of of the points total, removed for from each of theusing the the time two methods series of series. SchlaxThe and Chelton resulting (1992) turbidity with time anthe overturning series cross period of of section September 3 averaged hours. 2007 suspended was sediment linearly regressed concentration against ( Fig. 6. OBS profiles. AlthoughFigure this 6 shows approach that ignores thecorresponding OBS-derived variation estimates values. of of Since theconcentration spatial is variation limited, of the suspended point sediment cross data section from (Figure the 5). OMS are largely representative for the section close to Gunung Tabur atneap three phases tide in [right]. a The tidal cycle black during triangle spring indicates tide the [left] position and of during thec HADCP. Fig. 5. Regarding this calibration, 7The of resulting the regression 99 has a points highcharacteristics were correlation were excluded (Figure not 4), based very suggesting different on that betweenand a the spring sediment September and 4-sigma neap 2007. test. tides The and obtained between May regression coefficients were used to obtain the vertical profiles of turbidity. variation of interpolated to a regularm vertical were interval discarded, since of these 0.1 measurements m. could have been The affected top by 0.3 air bubbles m and the bottom 0.5 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . 1 − for only 8 % of the

3 − 545 averaged over 6 1 − s 3 540 typically results usually in a 540 i 0 C 0 535 Q h 530 530 525 within a day only has a small contribution to , whereas it was 605 m within a day are pronounced (Figures 7 and 〉 17 21 1 〉 7174 7173 C − C 〈 s 520 C

3 Time (Julian day) 〉 520 〉 Time (Julian day from 1 jan 2006) S Q Q’C’ 〈 〈 〈 and , which occurred at Julian day 545. During that event, 1 − 515 Q s is limited, exceeding 150 kg m 3 S 510 510

0 505 50

100 300 250 200 150

−50

0 0 0

, which corresponds to a cross section averaged ﬂow velocity of 1.0 m s 0.2 0.1

500

) s (kg S

0.25 0.15 0.05

3000 2000 1000

−500

〉 〈

) m (kg C C,

−1000 −2000 −3000 〉 〈

) s (kg S S,

−1 〉 〈

−3 −

−1 ) s (m Q Q,

〉 〈

−1 3 s 3 Time series of discharge [top panel], of suspended sediment concentration averaged Time series of discharge [top panel], of suspended sediment concentration averaged Contribution of tidally averaged discharge and suspended sediment concentration, and months in the same year (Buschmanaged et discharge al., was 2009). 1412 The m maximumthe observed flow tidally aver- direction was2896 seaward m almost over the entire day. The discharge peaked at Fig. 7. over the cross section [middle panel]average and of of suspended these sediment flux variables [bottomspring is panel]. tide. The indicated tidally by thick grey lines and the starsseries indicate of discharge, moments is of alwaysshown positive. in The Figure discharge 7 averaged amounted over to the 703 full m 6.5 weeks is usually elevated more during the peak ebb flows and the ebb duration contribution of varying dischargesuspended sediment and flux. suspended sediment concentration to tidally averaged Fig. 8. Fig. 7. over the cross section [middle panel]average and of of suspended these sediment flux variables [bottomspring panel]. is tide. The indicated tidally by thick grey lines and the stars indicate moments of C Contribution of tidally averaged discharge and suspended sediment concentration, and , which is highest when variations in i The contribution of variations of S time. Half of the suspended sediment flux is concentrated in only 25% of time. 8). Since is longer than floodpositive duration, contribution the to the correlation seaward term directed flux. ment flux thus increases non-linearlyperiod with of river discharge. high Due tidally to averaged this non-linearity, the Fig. 8. contribution of varying dischargesuspended sediment and flux. suspended sediment concentration to tidally averaged h Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 547 546.5 is also elevated. This C 546 545.5 Time (Julian day) 545 S 544.5 22 7175 544 514 513.5 Time (Julian day) 513 , occurring at the highest river discharge. The peak flow velocities S , now zoomed in on two periods with highest observed tidally averaged sus- 7 0 0 0 512.5

0.2 0.1

600 400 200

) m (kg C

3000 2000 1000 C,

−200 〉 〈

−1000 −2000 −3000 −3

) s (kg S S,

〉 〈

−1 ) s (m Q Q,

〉 〈

−1 3 As Figure 7, now zoomed in on two periods with highest observed tidally averaged As Fig. To analyze the periodson with the two highest peak suspended events.tidally sediment averaged The flux, right Figure panels 9during ebb zooms show are in results higher from thanparticles during the lower may period river be discharge, with resuspended. implyingelevation, the that As in also highest a coarser combination bed result, with tidally the averaged high ebb flow velocities and the long ebb periods, Fig. 9. suspended sediment flux. 4.6 Highest observed tidally averaged Fig. 9. pended sediment flux.