Continuous measurements of discharge from a horizontal acoustic Doppler current profiler in a tidal river

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 (z0). A new filtration technique was developed to infer cross-river profiles of z0 from moving boat ADCP measurements. Width-averaged values of z0 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 HADC

Subtidal water level variation controlled by river flow and tides

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

Discharge estimation in a backwater affected meandering river

Variable effects of backwaters complicate the development of rating curves at hydrometric measurement stations. In areas influenced by backwater, single-parameter rating curve techniques are often inapplicable. To overcome this, several authors have advocated the use of an additional downstream level gauge to estimate the longitudinal surface level gradient, but this is cumbersome in a lowland meandering river with considerable transverse surface level gradients. Recent developments allow river flow to be continuously monitored through velocity measurements with an acoustic Doppler current profiler (H-ADCP), deployed horizontally at a river bank. This approach was adopted to obtain continuous discharge estimates at a cross-section in the River Mahakam at a station located about 300 km upstream of the river mouth in the Mahakam delta. The discharge station represents an area influenced by variable backwater effects from lakes, tributaries and floodplain ponds, and by tides. We applied both the standard index velocity method and a recently developed methodology to obtain a continuous time-series of discharge from the H-ADCP data. Measurements with a boat-mounted ADCP were used for calibration and validation of the model to translate H-ADCP velocity to discharge. As a comparison with conventional discharge estimation techniques, a stage-discharge relation using Jones formula was developed. The discharge rate at the station exceeded 3300 m3 s-1. Discharge series from a traditional stage-discharge relation did not capture the overall discharge dynamics, as inferred from H-ADCP data. For a specific river stage, the discharge range could be as high as 2000 m3 s-1, which is far beyond what could be explained from kinematic wave dynamics. Backwater effects from lakes were shown to be significant, whereas the river-tide interaction may impact discharge variation in the fortnightly frequency band. Fortnightly tides cannot easily be isolated from river discharge variation, which features similar periodicitie

Diversion of flow and sediment towards a side channel separated from a river by a longitudinal training dam

A human‐made entrance to a side channel separated from the river by a longitudinal training dam can be considered a new, emergent type of river bifurcation. To understand the processes controlling the diversion of flow and sediment towards the side channel at such bifurcations, a comprehensive field‐monitoring programme was performed in the Waal River, which is the main branch of the Rhine River in the Netherlands. Local processes govern the flow field in the bifurcation region. The angle between the main river flow and the flow into the side channel increases with decreasing lateral and longitudinal distance to the bifurcation point, which corresponds to the head of the training dam. The general flow pattern can be well reproduced with a uniform depth, potential flow model consisting of a superposition of main channel flow and lateral outflow. For submerged flow conditions over the sill, the side channel hydraulic conditions influence the exchange processes, yet free flow side weir theory describes the flow field at this bifurcation type qualitatively well. The vertical flow structure in the side channel, which governs the sediment exchange between the main channel and the side channel, is steered by the geometrical details of the sill. The presence of the sill structure is key to controlling the morphological stability of this type of bifurcation given its primary influence on bed load sediment import and exerts an indirect impact on suspended sediment exchange

Morfodynamiek van Nederlandse laaglandbeken

In de eerste helft van de 20ste eeuw zijn veel laaglandbeken in Nederland gekanaliseerd. Dit heeft vaak geleid tot grote veranderingen in de hydrologie en morfologie van beken, met als gevolg een sterke achteruitgang van de bijbehorende karakteristieke aquatische en terrestrische ecosystemen. In de afgelopen 25 jaar zijn de Nederlandse waterschappen begonnen met beekherstel om wateroverlast te voorkomen, verdroging terug te dringen en de ecologische (water)kwaliteit van beeksystemen te verbeteren (KRW). Eén van de meest toegepaste maatregelen hierbij is hermeandering

Quantifying shape and multiscale structure of meanders with wavelets

features in the landscape, but the processes shaping them, act on a wide range of spatial and temporal scales. This results in meanders that curve at several spatial scales with smaller scale curves embedded in larger scale curves. Here, we show how to quantify the multi-scale structure of meanders from the valley scale until the sub-meander scale based on continuous wavelet transforms of the planform curvature. The zero crossings and maximum lines of the wavelet transform capture the main characteristics of the meander shape and their structure is quantified in a scale-space tree (Figure 1). The tree is used to identify meander wavelength and how meanders are embedded in larger scale features. The submeander structure determines meander shape, which is quantified with two parameters: skewness and fattening. The method is applied to the Mahakam River planform, which features very sharp, angular bends. Strong negative fattening is found for this river which corresponds to angular non-harmonic meanders which are characterized by strong flow recirculation and deep scouring

Flood occurence mapping of the middle Mahakam lowland area using satelite radar

Floodplain lakes and peatlands in the middle Mahakam lowland area are considered as 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 occurrence 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 occurrence 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 were collected. A fully inundated region can be easily recognized on radar images from a dark signature. Open water flood occurrence was mapped using a threshold value taken from radar backscatter of the permanently inundated river and lakes areas. 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 occurrence mapping in the vegetated area

Het Geldernsch-Nierskanaal: hoe een recht kanaal gaat meanderen als gevolg van kwel

Aan het eind van de 18de eeuw is tussen de rivieren de Niers in Duitsland en de Maas in Nederland het Geldernsch-Nierskanaal gegraven, om piekafvoeren in het benedenstroomse deel van de Niers af te vlakken. Het Duitse deel van het kanaal is in de loop van de tijd gekanaliseerd gebleven door oeverbeschoeiing. Opvallend is dat het Nederlandse deel, dat de vrije loop werd gelaten, actief is gaan meanderen. Onderzoek laat zien dat kwel de aanzet gaf voor het meanderen