Tidal Junctions Research Articles

Tidal impacts on the discharge division and freshwater transport in the Berau Delta, East Kalimantan, Indonesia

Flow division at bifurcations is important in controlling material carried from the terrestrial to the coastal zone in tidally influenced deltas. The present study aims to identify the impacts of tides on flow division and freshwater transport in the Berau Delta, East Kalimantan. The Princeton Ocean Model (POM) is applied to simulate the hydrodynamics with forcing from river discharges and tides. Tides at open seas and observed river discharge at upstream locations were used to set model boundaries. Model validation was accomplished by comparing measurements of tidal elevation and salinity time series with model results. The model results reproduce the observed in temporal variations of tidal elevation and salinity. Model results highlight that the tidal amplitude has large influence on discharge division at the tidal junction. Tides enhance equal subtidal flow distribution in the river junctions, with the influence of tide is 15% on subtidal flow distribution in the bifurcations. Freshwater discharge at the Berau Delta flows mainly to the north and middle outlet with 90% of the total discharge. Based on an analysis model results, the decomposition of freshwater transport reveals that advection and tidal pumping are major factor, which are fluctuate in fortnight cycle.

Sensitivity of Junction Width to Tidal Nonlinearity on Small and Shallow Tidal Junction

Observations of water elevation in the short and small tidal junctions of the Ota River, Japan, showed an increase in tidal nonlinearity at the apex of the junction. To quantitatively estimate the increase in nonlinearity, the barotropic hydrodynamic model was applied in an idealized junction domain, inspired by the Ota River Estuary junction. Even though the model was simplified, it successfully reproduced the increase in nonlinearity at the junction apex. A sensitivity analysis of tidal nonlinearity to the width of the upstream channel at the junction was performed by varying the upstream channel width from the same width as the branch channel width to three times the branch channel width. The relationship between the upstream channel width at the apex and tidal nonlinearity was not linear. Tidal nonlinearity was maximized when the apex width was twice the branch channel width. The convergence of the tides in the small width junction induced an increase of some positions of quarter-diurnal tidal constituent that raised the tidal nonlinearity. In the case of a wider channel, the flushing from river runoff dampen the tidal constituents, making it decrease tidal nonlinearity

Determining the subtidal flow at a stratified tidal junction by a novel repeated measurements method

The division of freshwater into separate channels at tidal junctions plays an important role in determining the hydrodynamics and material transport in multichannel estuaries. To obtain a better understanding of the net material transport, it is essential to precisely measure the residual flow near the tidal junction. In this study, the phase-averaging method (PAM) was applied to investigate the residual current and freshwater division at the lower reaches of the Modaomen Estuary (ME), China. This is the first PAM-based quantitative evaluation of the freshwater division at the tidal junction, which controls the freshwater division between the mainstem of the ME and the Hongwan Waterway (HW). The accuracy of the PAM results was examined by comparison with the subtidal flow obtained at a moored site near the estuary mouth. The results showed that the HW received freshwater discharge from the ME during both spring and neap tides. The stratification in the HW was determined by water masses advected from the mainstem of the ME. During the spring tide, the ME had a stronger longitudinal salinity gradient. Via interaction with the vertical shear of the along-channel velocity, the tidal straining tended to enhance the vertical salinity gradient, which generated more vigorous stratification in the channel during the spring tide than during the neap tide. In accordance with the stratified and longitudinal salinity gradient, the mainstem of the ME exhibited a stronger exchange flow during the spring tide. Harmonic analysis and error estimation showed that the PAM can effectively remove the tidal signals. The accuracy of our results suggests that the PAM can be applied to survey other tidal junctions.

Mapping tidal current and salinity at a shallow tidal channel junction using the fluvial acoustic tomography system

Applying the fluvial acoustic tomography (FAT) system to shallow tidal junctions for studying the flow division and spatiotemporal difference of velocity and salinity is vital to understand the tidal hydrodynamics in multi-channel networks. To the best of our knowledge, this is the first study that monitors continuous 2D current and salinity distributions at a shallow tidal junction using six FATs for ~34.4 days. The horizontal distribution and spatiotemporal variation of the currents and salinity were efficiently estimated by the inverse method. These results demonstrate that FAT is a potential tool for the continuous mapping of variable 2D currents and salinity at shallow tidal junctions. The reciprocating patterns of the current and salinity during the spring tide at the junction responded well to the tide. High salinity occurred around high water, whereas salinity was negligible at low water. During flood tides, significant landward currents flowed with the maximum speed of ~0.4 m/s, and significant seaward currents with the maximum speed of ~0.55 m/s occurred during ebb tides. During neap tides, the salinity pattern began to develop landward from the low water and reached a mature phase around the high water; salinity at the ebb slack remained high. Inverted FAT results indicated a counter-clockwise circulation around the low water during neap tide; some of the currents continuously flowed landward at the downstream of the junction (S4–S5), whereas others continuously flowed to the Tenma River. The behaviors of the currents varied slightly with the tide. The behaviors of the current and salinity during the neap tide mostly resulted from the density-driven current phenomenon. Furthermore, tidal harmonic analyses of the reconstructed currents were performed to clarify the river-tide interactions at the tidal junction. The results reflected the increased tidal wave deformation that occurred with the gradually increasing tidal range and demonstrated the role of the limited river flow on the tidal asymmetry at this tidal junction. This study advances the understanding of river flow dynamics in shallow tidal junction systems.

Flood Inundation Mapping Using Synthetic Aperture Radar Data Single Polarization: A Case Study of Flood in Lake Tempe, South Sulawesi-Indonesia

Remote sensing data can be used to help disaster management and environmental management because this data has advantages in terms of speed, is more efficient, can reach large and remote areas. Aside from that, it has consistency in measurement, can make repeated measurements, and has measurable accuracy. Floods in Lake Tempe occur almost every year due to overflowing of Lake Tempe. This research will detect flood inundation from Sentinel-1 data with single-polarization of flooding in Lake Tempe, South Sulawesi. The data used were Sentinel-1 data before flooding (2 May 2018) and after flooding (26 May 2018). The Single polarization of the Sentinel-1 can be used very well to identify floods event. The difference of input band uses the threshold of -6, and enables smoothing of 10. Utilization of tidal junction between Lake Tempe and Lake Sidenreng for agriculture and settlement has caused flooding due to lake water overflowing in the rainy season. This single polarization method can already be used in flooded areas, although in some locations it is still overestimates.

Numerical Modeling for Hydrodynamics and Near-Surface Flow Patterns of a Tidal Confluence

Dai, W.; Bilal, A.; Xie, Q.; Ahmad, I., and Joshi, I., 2020. Numerical modeling for hydrodynamics and near-surface flow patterns of a tidal confluence. Journal of Coastal Research, 36(2), 295–312. Coconut Creek (Florida), ISSN 0749-0208.Because of the flow influx of tributaries, a confluence forms a unique environment carrying interesting hydrodynamic features and other attributes. The understanding of flow behavior here is important, particularly if it is on a tidally influenced channel in a harbor metropolitan. Because of communal requirements, there is a possibility of building wading structures, which may interplay with the flow in this zone. The knowledge of unidirectional river or flume confluences so far is not readily applicable for similar features in channels near coastal areas that have tidal flow in addition to river runoff. In this study, a tidal confluence that has a highly dynamic bidirectional flow is investigated. Near-surface flow patterns in a tidal cycle are simulated by using a numerical model. A field survey provides the bathymetry, time-series boundary conditions, and corresponding verification data. Good agreement is reached between calculated and measured results. Based on the condition of tidal current, four scenarios are selected for which confluence flow patterns are observed, both spatially and temporally. The results indicate that at least one recirculation is always in the tidal confluence for all flow conditions, which rotates counterclockwise for the ebb flow and clockwise for the flood flow. In addition, there is no absolute slack water condition at the tidal junction in the study area. The study also finds that the flows of all three connected channels at the confluence change in a looped pattern with respect to one another. Furthermore, the study reports unique relationships among the ratios of different flows.

Tidal influence on the discharge distribution over the Pearl river Delta, China

The discharge distribution is influenced by the tidal behaviour at tidally affected junctions. The Pearl River Delta (PRD) is one of the most complicated deltas and highly affected by human interventions. The tides are reinforced by the riverbed downcutting caused by intensive sand dredging in the PRD. While some efforts have been made to interpret flow division at the apex of the delta, little attention has been paid to the role of tides on discharge distribution. A 2D hydrodynamic model covering the entire PRD has been established and applied to quantify the tidal influence on subtidal flow division at the tidal junctions. The discharge asymmetry index is introduced to quantify the subtidal discharge division. The method of factor separation is used to decompose the subtidal discharge into the contributions of tides, river discharge and river-tide interactions. Results indicate that the tides alter the allocation of river discharge over distributary channels in the PRD, especially in the dry season. Generally, the effect of tides is to reduce unequal subtidal flow division that occurs without tides. The tidal influence increases seaward with a maximum value of the order of 110% in the dry season, which indicates that the tidal influence on the flow distribution can overwhelm the impact induced by river discharge only. Sensitivity analyses also demonstrate that the tidal constituent M2 makes the largest contribution on flow division at junctions, followed by K1, O1 and S2. In addition, the tidal influence is more apparent due to the increased water depths caused by long-term and extensive sand excavations.

Intra-tidal division of flow and suspended sediment at the first order junction of the Pearl River Network

At tidal junctions, flow, sediment, and, indirectly, flooding risk are distributed over the downstream river branches. To characterize the hydrodynamic pattern related to the transport and distribution of suspended sediment, an intra-tidal field measurement was conducted at the first order tidal junction in the Pearl River Network, Makou-Sanshui (MK-SS), which was the first order junction and the apex of the delta. Two 13h boat-mounted ADCP surveys were implemented and 468 water samples were taken synchronously across three cross sections at wet and dry seasons during the spring tide. Observed spatiotemporal velocities were applied to elucidate the intra-tidal flow pattern in the tidal junction, and the acoustical backscatter intensity collected by ADCP was the inverse of the cross-sectional suspended sediment concentration (SSC). Additionally, vertical profiles of velocity and SSC allowed us to fit the lateral distribution of shear velocity and Rouse number, respectively. From these, we obtained the spatiotemporal variations of settling velocity, which then made it possible to interpret the time lag between flow and suspended sediment. Intra-tidal substance exchange, including flow flux produced by tidal wave and net suspended sediment, was inferred by Stokes transport, integrating axial velocities and the SSC. As a consequence of tidal impact, the flow processes of MK and SS were out of phase by 1–1.5 h. As the connecting channel of MK and SS, Ganggen (GG) experienced more frequent substance exchange, displaying a significant difference in flow pattern. A substantial clockwise secondary flow field was detected during the intra-tidal survey at GG, which was affected by the curved channel. This phenomenon was related to the cross-sectional distribution of SSC. Furthermore, the intra-tidal transport of suspended sediment exhibited an inverse direction during the wet and dry seasons. Sediment during the dry season favoured delivery from MK to SS, while it displayed an opposite trend during the wet season. The results reveal the pattern of intra-tidal partitioning of flow and sediment at tidal junctions.

Water and suspended sediment division at a stratified tidal junction

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.

Subtidal flow division at a shallow tidal junction

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.