Multi decadal morphodynamics of
alternate bars in the Alpine Rhine river
Luca Adami
1
, Walter Bertoldi
1
, Guido Zolezzi
1
1
Department of Civil and Environmental Engineering, University of Trento (
[email protected]
)
1 Abstract
2 Study site: Alpine Rhine
3 Remote sensing analysis
4 Results
CONTEXT: The formation and dynamics of alternate bars in straight channels has
been studied since the late ’60s through analytical theories, mobile-bed flume
experiments with fixed banks, and, more recently, with morphodynamical numerical models. Field observations at the time and spatial scales and resolution required to investigate alternate bars long term dynamics are only recently becoming feasible, and they remain still rare. Many studies report on short term analysis focusing on
relatively short reaches, where only a few bars are present. In this work we describe the multi decadal (30 years) morphodynamics of alternate bars in a 40-km reach (400 times the channel width) of the Alpine Rhine river, between Switzerland,
Liechtenstein and Austria. Alternate bars - presently more than 40 bar wavelengths - are present here since the 19th century, providing an almost unique opportunity to
extract a consistent set of bar properties, e.g. bar length, migration, from freely
available Landsat images since 1984. Information on bar amplitude have also been obtained from regularly surveyed cross sections.
AIMS:
1. Describe the long term evolution of alternate bars in a real river :
Time scale: multi-decadal;
Space scale: tens of wavelengths. in terms of:
Bar wavelength (L); Bar migration;
Bar height.
2. Assess the potential of analytical bar
theories (free - forced bars in straight
reaches, e.g. Colombini et al., 1987;
Zolezzi and Seminara, 2001) to replicate observed bar dynamics and to support interpretation of field observations.
Alternate bar
reach STREAM UP- CENTRAL STREAM DOWN-Length [km] 10.7 20.0 11.1 Width [m] 85 95 105 Slope [%] 2.9 2.0 1.3 d50 [mm] 50 ÷ 60 30 ÷ 50 20 ÷ 30 QFW [m3/s] 380 270 230 QFT [m3/s] 830 630 510
FW, FT: Fully Wet, Fully Transporting cross sections
Where: between Switzerland, Austria, Liechtenstein, upstream Lake of Constance, from km 23.3 to km 65.5. When: 1984 ÷ 2013.
What: straigth reaches
linked with bends (b) and ramps (r)
A) Bar wavelength (period 1984-2013)
Longer bars (L ~ 1150 ÷ 1600) up to bend4, then bars tend to be shorter (L ~ 700 ÷ 1000);
role of bends and ramps in the change of bar wavelength.
B) Bar migration (period 2001-2010)
No migration up to bend 3 (b3), then
migration increases up to the maximum in the straight regions bend 5 (b5) ÷ ramp 2 (r2) and bend 10 (b10) ÷ bend 11 (b11)
Examples of alternate bars along the study area:
Rhine
basin
C) Wavelength-migration (averaged)
84% of the short bars migrated > 200 m 81% of the long bars migrated < 200 m
November 26,
2006
December 20,
2009
WAVELENGTH
MIGRATION
1.
a
) Original LANDSAT image
2.
a
) Data conversion to shp
2.
b
) Data conversion to shp
1.
b
) Original LANDSAT image
3.
) Data elaboration
LANDSAT images:
1. Pixel resolution : 30x30 m 15x15 m
2. Covered period: 1984-to date
3. Available images: 78 ( > 2.5 per year)
4. Discharge range: 64 ÷ 540 [m3/s] FLOW Q = 68 m3/s Q = 256 m3/s FLOW
FIELD DATA
ANALITICAL THEORIES
SHORT BARS
LONG BARS
MIGRATING
BARS
STEADY
BARS
FORCED BARS
B
FREE BARS
A
Wavelength / Width
Migration / Width
CHeight /Depth (Q=Q
2)
7 ÷ 12
1.0 ÷ 1.2
1.0 ÷ 1.5
12 ÷ 17
0.0 ÷ 0.2
2.9 ÷ 4.7
A e.g. Colombini et al. 1987 B e.g. Zolezzi and Seminara, 2001 C Values in [year-1]
REFERENCES
Colombini, M., Seminara, G. and Tubino, M. 1987. Finite-amplitude
alternate bars. Journal of Fluid Mechanics 181: 213–232;
Zolezzi, G. and Seminara, G. 2001. Downstream and upstream
influence in river meandering. Part 1. General theory and application of overdeepening. Journal of Fluid Mechanics 438: 183–211.
KEY
FINDINGS
both steady and migrating alternate bars occur in straight reaches of the
Alpine Rhine river study site;
migrating bars are consistently shorter than steady bars;
comparison with analytical theories suggests a dominantly "free" origin of
the migrating bars and a dominantly "forced" nature of the steady bars.
A fairly good agreement between theory and observations is shown in
terms of bar wavelength and
amplitude.
The marked difference between predicted and observed bar
migration is due to the known overprediction by linear theories.
