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| Titel |
Salt tracer experiments in wetland ponds: will density stratification spoil the outcome? |
| VerfasserIn |
Bernhard H. Schmid, Michael A. Hengl |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250139136
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| Publikation (Nr.) |
 EGU/EGU2017-2314.pdf |
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| Zusammenfassung |
| Wetland ponds are among the treatment options for peatland flows prior to their discharge
into a receiving ambient water course or water body. The removal efficiency and
effectiveness of wetland ponds (free water surface or FWS wetlands) is considered to be
strongly related to the residence time or travel time distribution in the pond, with a
narrow distribution (close to plug flow) being preferable to a wider one. This travel
time distribution is, in turn, reflected by a breakthrough curve of an ideal tracer
injected instantaneously into the flow (entering the wetland). As the term ’ideal tracer’
suggests, such a substance, in real world cases, does not exist and can, at best, be
approximated by a real tracer. Among the tracer groups in most widespread use, salt has the
advantage of low cost, straightforward detection and analysis as well as low related
environmental risk. In contrast, use of radioactive artificial tracers may meet with
resistance from authorities and public, and fluorescent dyes are not necessarily
devoid of problems, either (as recently discovered, there are two structural isomers of
Rhodamin WT, the mixture of which may compromise the validity of breakthrough data
analyses).
From previous work by the authors it is known that density stratification may result from
the injection of a salt tracer into a low Reynolds number free surface flow, which is a frequent
characteristic of wetland ponds. As the formation of density layers in the course of a tracer
experiment is highly undesirable, it may be useful to judge prior to beginning of the field
work, if stratification is to be expected (and the experimental design should, consequently, be
adapted suitably). The current work reported here employs an energy argument to
extend existing criteria for density stratification in turbulent free surface flows.
Vertical mixing is assumed to be sustained by a fraction of the frictional energy loss
(expressed by Manning’s law, but this can easily be adapted to other friction laws such as
Darcy-Weisbach’s). Experimental data obtained by the authors in the course of the
PRIMROSE project (Contract no. EVK1-CT-2000-00065) were used to calibrate the
criterion with respect to the actual percentage of the friction loss that fuels the vertical
mixing.
The distance x (m) needed for (full) vertical mixing of the salt tracer (NaCl or KBr) is
finally derived as:
C0 ⋅(0.802− 0.002⋅Tw) ⋅h R4h∕3
x =-------0.0694-⋅ρw-------⋅(n-⋅u)2
(1)
with
C = --M0---
0 Q ⋅Δt0
(2)
and M0 the tracer mass (g), Q the flow rate (m3/s), Δt0 the injection pulse duration (s), Rh
(m) the hydraulic radius (= flow cross-sectional area divided by wetted perimeter), Tw the
water temperature (˚ C), ρw water mass density (g/m3), Manning’s n in SI-units
(s/m1∕3) and cross-sectionally averaged flow velocity u (m/s). Tracer concentration
C0, as obtained from Eq.(2), is to be expressed in mg/l or g/m3 for use in Eq.(1). |
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