 |
| Titel |
The rising air bubble technique for streamflow measurement |
| VerfasserIn |
Koen Hilgersom, Willem Luxemburg |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047221
|
|
|
|
|
|
| Zusammenfassung |
| In the world of hydrological measurements we know a large variety of streamflow
measurement techniques, each having their own up- and downsides as it comes down to
accuracy under different circumstances. One of the simplest methods to measure
discharge is by using a float to obtain the water velocity and multiplying this with
the cross-section. The major disadvantage of this method is that the velocity at
the surface level of the waterbody is incorrectly taken as an depth-averaged water
velocity or that a correction factor is applied that is highly dependent on unknown
channel and bed characteristics. So wouldn’t it be better to release an air bubble
from the bottom of the stream and let this float average stream velocity over the
depth? Measurements have proven this ’rising air bubble technique’ to give accurate
results and that there is high potential for this method even to be used for fixed
measurements.
The basic concept of this method is that within the time an air bubble needs to surface
with a constant rising velocity, the bubble is displaced over a certain distance by the
depth-averaged velocity. By simple math it can be shown that the specific discharge follows
from multiplying the displacement with the rising velocity of the air bubble. By releasing air
bubbles over the full width of the stream one can sum the specific discharges over the width,
obtaining a discharge both integrated in depth and in width. A single measurement is
performed only by taking a photograph of the bubble pattern at the water surface. This way
one can easily build a time series by taking several consecutive pictures of how the pattern
evolves over time.
The concept and some field applications were earlier described by D.M. Sargent (e.g.
1982), but ever since there have only been few publications by others. This though there
has been a huge progress in digital imagery analysis techniques, enabling us to
easily read out displacements from our photographs. Besides, advances in pattern
recognition techniques cleared the road towards the automated localization of surfaced
air bubbles. This would make it possible to calculate discharge without human
intervention and enable the possibility to apply this method for fixed discharge
measurements.
The major limitation still in this method is the difficulty to obtain constant rising
velocities. We performed lab experiments to find appropriate air pressures and nozzle sizes to
release our air bubbles with. |
|
|
|
|
|
|