[1] Subtidal water level dynamics in the Berau river, East Kalimantan, Indonesia, feature a pronounced fortnightly variation. The daily mean water levels at a station about 60 km from the sea are 0.2-0.6 m higher during spring tide than during neap tide. To explain the underlying mechanisms, a local subtidal momentum balance is set up from field data, using continuous discharge estimates inferred from measurements taken with a horizontal acoustic Doppler current profiler. It is demonstrated that terms accounting for friction and variation in the water surface gradient are dominant in the subtidal momentum balance. To further investigate the sources of subtidal water level variation, a generic method of analysis is proposed to decompose the subtidal friction term into contributions caused by river flow, by interaction between tidal motions and river flow, and by the tidal motions alone. At the station under study, mainly the river-tide interaction term is responsible for generating fortnightly variation of the subtidal water level. The contribution from interaction between diurnal, semidiurnal, and quarterdiurnal tides to subtidal friction is significantly smaller. Provided that the reduction of tidal velocity amplitudes with increasing discharges can be predicted from a regression model, the results presented herein can be used to predict changes in subtidal water levels as a result of increased river discharges.

[1] Tides influence distribution of river discharge at tidally affected channel junctions. At the apex of a channel network in an Indonesian delta, observations of flow division suggest that tidally averaged flow division depends on the tidal range. To understand the mechanisms governing the subtidal flow division, an idealized hydrodynamic junction model inspired by the observations has been set up. The barotropic model consists of two exponentially converging tidal channels that connect to a tidal river at the junction and solves the nonlinear shallow water equations. By varying the depth, length, e-folding length scale of the channel width, and hydraulic roughness in one of the two tidal channels, the sensitivity of the subtidal flow division to those four parameters was investigated. For depth, length, and e-folding length scale differences between channels the effect of tides is generally to enhance unequal subtidal flow division that occurs in the case of river flow only. In contrast, for hydraulic roughness differences, the tidal effect partly cancels the inequality in river flow division. The tidal effect may even reverse the horizontal flow circulation that would occur in the absence of tides.

[1] Acoustic Doppler current profilers (ADCPs) can be mounted horizontally at a river bank, yielding single-depth horizontal array observations of velocity across the river. This paper presents a semideterministic, semistochastic method to obtain continuous measurements of discharge from horizontal ADCP (HADCP) data in a tidal river. In the deterministic part, single-depth velocity data are converted to specific discharge by applying the law of the wall, which requires knowledge of local values of the bed roughness length (z 0 ). A new filtration technique was developed to infer cross-river profiles of z 0 from moving boat ADCP measurements. Width-averaged values of z 0 were shown to be predominantly constant in time but differed between ebb and flood. In the stochastic part of the method, specific discharge was converted to total discharge on the basis of a model that accounts for the time lag between flow variation in the central part of the river and flow variation near the banks. Model coefficients were derived using moving boat ADCP data. The consistency of mutually independent discharge estimates from HADCP measurements was investigated to validate the method, analyzing river discharge and tidal discharge separately. Inaccuracy of the method is attributed primarily to mechanisms controlling transverse exchange of momentum, which produce temporal variation in the discharge distribution over the cross section. Specifically, development of river dunes may influence the portion of the discharge concentrated within the range of the HADCP.

Plastic pollution in aquatic environments is an increasing global risk. In recent years, marine plastic pollution has been studied to a great extent, and it has been hypothesized that land-based plastics are its main source. Global modeling efforts have suggested that rivers in South East Asia are in fact the main contributors to plastic transport from land to the oceans. However, due to a lack of plastic transport observations, the origin and fate of riverine plastic waste is yet unclear. Here, we present results from a first assessment of riverine macroplastic emission from rivers and canals that run through a densely populated coastal urban city. Using a combination of field measurements, empirical relations and hydraulic modeling, we provide an estimate of total riverine plastic export originating from Jakarta, Indonesia, into the ocean. Furthermore, we provide insights in its composition, and variation in time and space. We found that most macroplastics in Jakarta consists of films and foils. We estimate that 2.1 × 10 3 tonnes of plastic waste, is transported from land to sea annually, equaling 3% of the total annual unsoundly disposed plastic waste in the Jakarta area.

[1] Tidal junctions play a crucial role in the transport of water, salt, and sediment through a delta distributary network. Water, salt and sediment are exchanged at tidal junctions, thereby influencing the transports in the connecting branches and the overall dynamics of the system. This paper presents observations of water, salt and sediment transports in three channels that connect at a stratified tidal junction. Flow variation in one channel was found to lag behind flow variation in a connected channel by more than 1 h, which is largely attributed to channel length differences from the junction to the sea. The water columns in the three channels were periodically stratified during spring tide, whereas the salinity structure represented a salt wedge during neap tide. Salinity differences between the three channels were substantial. The channels contain water bodies of different salinity and act largely independently. Flow velocities in the upper and lower layers differed substantially. Flow in the lower layer was generally in the direction of acceleration produced by the baroclinic pressure gradient. Interestingly, baroclinic pressure gradients were sometimes directed landward, indicating the presence of saltier water at the land side of the estuary. In sharp channel bends close to the junction, secondary flow was strongest at the highest axial flow velocity during spring tide. In one channel bend, these circulations steered the suspended sediment toward the inner bend, which affected the suspended sediment division.Citation: Buschman, F. A., M. van der Vegt, A. J. F. Hoitink, and P. Hoekstra (2013), Water and suspended sediment division at a stratified tidal junction,

Abstract. Forest clearing for reasons of timber production, open pit mining and the establishment of oil palm plantations generally results in excessively high sediment loads in tropical rivers. The increasing sediment loads pose a threat to coastal marine ecosystems, such as coral reefs. This study presents observations of suspended sediment loads in the Berau River (Kalimantan, Indonesia), which debouches into a coastal ocean that is a preeminent center of coral diversity. The Berau River is relatively small and drains a mountainous, still relatively pristine basin that receives abundant rainfall. In the tidal zone of the Berau River, flow velocity was measured over a large part of the river width using a horizontal acoustic Doppler current profiler (HADCP). Surrogate measurements of suspended sediment concentration were taken with an optical backscatter sensor (OBS). Averaged over the 6.5 weeks covered by the benchmark survey period, the suspended sediment load was estimated at 2 Mt yr −1 . Based on rainfall-runoff modeling though, the river discharge peak during the survey was supposed to be moderate and the yearly averaged suspended sediment load is most likely somewhat higher than 2 Mt yr −1 . The consequences of ongoing clearing of rainforest were explored using a plot-scale erosion model. When rainforest, which still covered 50-60 % of the basin in 2007, is converted to production land, soil loss is expected to increase with a factor between 10 and 100. If this soil loss is transported seaward as suspended sediment, the increase in suspended sediment load in the Berau River would impose a severe stress on this global hotspot of coral reef diversity.

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Plastic litter transported in rivers may enter the marine environment. Due to the durability of plastic, the residence times in the aquatic environment are decades to millennia. Plastic litter items come in a wide variety. The density of the item and the flow velocity largely determine the vertical distribution of the plastic litter, although the litter shape and wind can also play a role in a river. Estimates of plastic transport in a river are usually based on observations of visible litter items in the near-surface layer alone. Only a very limited number of studies have observed the vertical distribution in rivers or estuaries. We have carried out 28 sampling surveys in a river bend located in the lower part of the Rhine-Meuse delta that is under the influence of tides. To observe the vertical distribution, we sampled with three nets simultaneously up to 5 m water depth, at both sides of the river bend. In total, almost 90% of the collected debris was organic matter. Plastic litter represented the large majority of the non-organic litter: about 80% by mass, having an average concentration of 0.5 mg/m3, and more than 90% by number. We observed that by mass and by number the plastic mass concentration in the middle net was generally lower than in the lower and upper nets. In total, more plastic litter was present in the two nets lower in the water column than in the surface net. A higher plastic concentration was found at the downwind side of the channel than at the upwind side of the fairway. It was found that the plastic litter mass concentrations were of the same order of magnitude during ebb than during flood tide. The litter size that contributed most to the mass concentration (around 65%) were items in between 25 and 500 mm, which were mostly soft plastics (i.e. foils). In the lower net, relatively more small hard plastic items were found. It was observed that these plastic items were sometimes entangled in organic matter. The plastic flux lower in the water column may follow the transport of organic matter like aquatic plants.