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| Titel |
Method to estimate river discharge from a horizontally deployed Acoustic Doppler Current Profiler in a tidal river subject to sidewall effects and a mobile bed |
| VerfasserIn |
Maximiliano Sassi, Ton Hoitink, Bart Vermeulen, Hidayat Hidayat |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055704
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| Zusammenfassung |
| Acoustic Doppler Current Profilers (H-ADCPs) can be deployed horizontally to yield
water level estimates in combination with flow velocity array data across a river
transect. To maximize the percentage of the river width covered by the H-ADCP
profiling range, while avoiding bottom and surface reflections from the acoustic beams,
H-ADCPs are being deployed at relatively deep and narrow cross sections located at
constrictions and bends, where the flow generally features a three-dimensional pattern
due to the effects of curvature, bed topography and the side-wall. A new method
is presented that accounts for the dip in velocity near the water surface, which is
caused by sidewall effects that decrease with the width to depth ratio of a channel.
A boundary layer model is introduced to convert single depth velocity data from
the H-ADCP to specific discharge. The parameters of the model include the local
roughness length and a dip correction factor, which accounts for the sidewall effects. A
regression model is employed to translate specific discharge to total discharge. The
method was tested in the River Mahakam, representing a large river of complex
bathymetry, where part of the flow is intrinsically three-dimensional and discharge rates
exceed 8000 m3s-1. Results from five moving boat ADCP campaigns covering
separate semi-diurnal tidal cycles are presented, three of which are used for calibration
purposes whereas the remaining two served for validation of the method. The dip
correction factor showed a significant correlation with distance to the wall, and bears
a strong relation to secondary currents. The sidewall effects appeared to remain
relatively constant throughout the tidal cycles under study. Bed roughness length is
estimated at periods of maximum velocity, showing more variation at sub-tidal
than at intratidal time scales. Intratidal variations were particularly obvious during
bi-directional flow conditions, which occurred only during conditions of low river
discharge. The new method was shown to outperform the widely used index velocity
method, by systematically reducing the relative error in the discharge estimates. |
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