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A secondary scale of bedforms, superimposed on larger, primary dunes, has been observed in fluvial systems worldwide. This notwithstanding, very little is known about the morphological behaviour and characteristics of this secondary scale. This study aims to better characterize and understand how two dune scales coexist in fluvial systems, and how both scales adapt over time and space, considering their interdependence. The study is based on analysis of a biweekly multibeam echosounding dataset that spans a period of approximately three years and 34 kilometers in the Waal River, a lowland sand-bedded river. Results reveal that the secondary dune scale is ubiquitous across space and time, and not limited to specific flow or transport conditions. Whereas primary dunes lengthen during low flows, secondary dune height, lee slope angle and length correlate with discharge. Secondary dune size and migration strongly depend on the primary dune lee slope angle and height. Secondary dunes can migrate over the lee slope of low-angled primary dunes, and their height is inversely correlated to the upstream primary dune height and lee slope angle. In the Waal River, a lateral variation in bed grain size, attributed to shipping, largely affects dune morphology. Primary dunes are lower and less often present in the southern lane, where grain sizes are smaller. Here, secondary bedforms are more developed. At peak discharge, secondary bedforms even become the dominant scale, whereas primary dunes entirely disappear but reestablish during lower flows.

River deltas, where ocean tides can often freely propagate into the river, are focal points of human settlement. Beneath the delta surface, groundwater stored in the soil strata fluctuates with river tides, resulting in pressure variations within the soil strata. Here, we introduce a novel ultra-weak fiber-optic instrument to test the hypothesis that tide-induced groundwater level variations induce periodic soil stratum deformation. Using borehole deployments ranging over 70 m in the subsurface of the Yangtze Delta, we observe semidiurnal and spring–neap cycles of soil stratum deformation and reveal its dependence on lithology and depth. This allows us to coin the new term “soil stratum tide,” defined as the periodic deformation of soil strata in response to pressure fluctuations induced by tides in open water. High-accuracy monitoring of the breathing of tidal deltas enables calibrating Earth observation systems, analysis of delta subsidence, and safeguarding of infrastructure jeopardized by soil stratum deformation.

The Pearl River Delta represents a tide-influenced lowland delta where sand mining has deepened the distributary channels. This study sets out to investigate the impacts of channel deepening on peak water levels across the delta. Based on collected hydrological data before and after sediment mining and hydrodynamic modeling, three regions are identified where sand mining-induced changes in peak water levels have a different underlying cause. In the river-dominated regions, peak water levels have significantly decreased because of the sand excavation, and thus the associated flood risk was reduced. In a transition region, the sand mining practice has increased peak water levels. In the tide-dominated coastal region, the changes in peak water levels follow the rise in sea level and respond to human activities such as land reclamation. To understand the underlying mechanisms, the changes of subtidal friction governing tidal dynamics before and during sand mining are explored, and a simple linear regression model is used to predict subtidal water levels governing peak flood levels. Results from the hydrodynamic model and the linear regression models quantify exactly how the direct effect of channel bed lowering, river-tide interaction, tidal amplification, and sea level rise have contributed to peak water level change, which offers a new analysis framework that can be applied in other systems.

Hydraulic roughness is a fundamental property in river research, as it directly affects water levels, flow strength and the associated sediment transport rates. However, quantification of roughness is challenging, as it is not directly measurable in the field. In lowland rivers, bedforms are a major source of hydraulic roughness. Decades of research have focused on dunes to allow parameterization of roughness, with relatively little focus on field verification. This study aims to establish the predictive capacity of current roughness predictors, and to identify reasons for the unexplained part of the variance in roughness. We quantified hydraulic roughness based on the Darcy-Weisbach friction factor (f) calculated from hydraulic field data of a 78 km-long trajectory of the river Rhine and river Waal in the Netherlands. This is compared to predicted roughness values based on dune geometry, and to the spatial trends in the local topographic leeside angle, both inferred from bathymetric field data. Results from both approaches show the same general trend and magnitude of roughness values (0.019 < f < 0.069). Roughness inferred from dune geometry explains at best 31% of the variance. Efforts to explain the remaining variance from statistics of the local topographic leeside angles, which supposedly control flow separation, were unsuccessful. Unexpectedly, multi-kilometer depth oscillations explain 34% of the total roughness variations. We suggest that flow divergence associated with depth increase causes energy loss, which is reflected in an elevated hydraulic roughness. Multi-kilometer depth variations occur in many rivers worldwide, which implies a cause of flow resistance that needs further study.

River deltas rank among the most economically and ecologically valuable environments on Earth. Even in the absence of sea-level rise, deltas are increasingly vulnerable to coastal hazards as declining sediment supply and climate change alter their sediment budget, affecting delta morphology and possibly leading to erosion

Floodplain lakes and peatlands in the middle Mahakam lowland area are considered as an ecologically important wetland in East Kalimantan, Indonesia. However, due to a lack of data, the hydrological functioning of the region is still poorly understood. Among remote sensing techniques that can increase data availability, radar is well-suitable for the identification, mapping, and measurement of tropical wetlands, for its cloud unimpeded sensing and night and day operation. Here we aim to extract flood extent and flood frequency information from a series of radar images of the middle Mahakam lowland area. We explore the use of Phased Array L-band Synthetic Aperture Radar (PALSAR) imagery for observing flood inundation dynamics by incorporating field water level measurements. Water level measurements were carried out along the river, in lakes and in peatlands, using pressure transducers. For validation of the open water flood frequency map, bathymetry measurements were carried out in the main lakes. A series of PALSAR images covering the middle and lower Mahakam area in the years 2007 through 2010 was collected. A fully inundated region can be easily recognized on radar images from a dark signature. Open water flood frequency was mapped using a threshold value taken from radar backscatter of the permanently inundated river and lakes area. Radar backscatter intensity analysis of the vegetated floodplain area revealed consistently high backscatter values, indicating flood inundation under forest canopy. We used those values as the threshold for flood frequency mapping in the vegetated area.

Suspended sediment dynamics at two fringing coral reef islands in Teluk Banten, Indonesia, were investigated during wet season. Both islands are surrounded by waters of approximately 8 m depth and have a width of 600 m, whereas one is located close to a former delta platform and fully exposed to waves from the Java Sea, and the other is situated in a more sheltered region. Over a period of three weeks, ten OBS ( O ptical B ack S carter) sensors were placed in vertical arrays transverse to the reef fringes. Simultaneously, the prevailing oceanographic conditions were measured from a wooden fishing platform near the island under investigation, using an ADCP ( A coustic D oppler C urrent P rofiler), an OBS sensor attached to a CTD probe ( C onductivity, T emperature and D ensity) and a pressure sensor. Suspended sediment concentrations (SSCs) at these shoreface sites ranged over 9 and 6 mg/l near the exposed and sheltered island, respectively. Seawater salinity throughout the water column was nearly constant during the field study, proving possible river influences to be negligible. Resuspension by currents and waves controlled the SSC levels at the shoreface site close to the former delta. Depth averaged tidal currents were fairly rectilinear and aligned with the bays depth contours. At the shoreface site near the sheltered island, the main diurnal tidal constituent exhibited a 280 orientation difference between the near-surface and near-bed region. Such a vertical rotation was not present at the shoreface site near the former delta. Therefore, tide-induced dispersion of sediments transverse to the coastline was expected to be limited in that area. At the fringing coral reefs, strong stratification was observed and hourly averaged turbidity levels range over 100 mg/l. Relatively clear waters prevailed near the surface, whereas close to the reef toe averages could be twice as large as the bottom meter average at the surrounding shoreface. Spectrum estimates of the near-reef SSC time series peaked at diurnal and semidiurnal frequencies and cohered well with spectra of the oceanographic time series in the diurnal frequency band. Nevertheless, instantaneous current patterns around the islands were not readily understood due to the irregular shape of reef fringes and the occurrence of eddies, which were likely to form at high-energetic stages of the tide.