[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.

Natural resources of the Mekong River are essential to livelihood of tens of millions of people. Previous studies highlighted that upstream hydro-infrastructure developments impact flow regime, sediment and nutrient transport, bed and bank stability, fish productivity, biodiversity and biology of the basin. Here, we show that tidal amplification and saline water intrusion in the Mekong Delta develop with alarming paces. While offshore M2 tidal amplitude increases by 1.2–2 mm yr−1 due to sea level rise, tidal amplitude within the delta is increasing by 2 cm yr−1 and salinity in the channels is increasing by 0.2–0.5 PSU yr−1. We relate these changes to 2–3 m bed level incisions in response to sediment starvation, caused by reduced upstream sediment supply and downstream sand mining, which seems to be four times more than previous estimates. The observed trends cannot be explained by deeper channels due to relative sea level rise; while climate change poses grave natural hazards in the coming decades, anthropogenic forces drive short-term trends that already outstrip climate change effects. Considering the detrimental trends identified, it is imperative that the Mekong basin governments converge to effective transboundary management of the natural resources, before irreversible damage is made to the Mekong and its population.

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[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,