Morphodynamics and hydraulics of vegetated river reaches: a case study on the Müggelspree in...
-
Upload
marshall-banks -
Category
Documents
-
view
212 -
download
0
Transcript of Morphodynamics and hydraulics of vegetated river reaches: a case study on the Müggelspree in...
Morphodynamics and hydraulics of vegetated river reaches: a case study on the Müggelspree in Germany
Alexander Sukhodolov and Tatiana Sukhodolova
Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
1. Introduction
Flow
Vegetation Sediment transport
Channel morphology
2 3
1
457
6
8
River flow in vegetated river reaches is affected by channel morphology (1), aquatic vegetation (2) and it drives sediment transport (3) to cause bed deformation (4), that controls vegetation growth (5). Growing vegetation cover changes the flow and thus sediment transport (6) and channel morphology (7), the bend of the plants is thereby dominated by the flow characteristics (8).
General scheme of interactions in vegetated river reaches (after Tsujimoto 1999)
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
1. Introduction
Fluid mechanics:
the effect of vegetation investigatedin laboratory flumes or by numericalsimulations
vegetation is simulated by plasticplants (steams) uniformly distributedIn space
Hydrobiology:
the effect of vegetation is investigatedin field
research is mainly focused on temporal dynamics of plants biomass and spatial characteristics of plants abundance and composition of macrophytes
Goals of this research: 1) to explore morphological structures and their seasonal dynamics in a river reach of a lowland river with submersible vegetation;2) to estimate quantitatively the effect of vegetation-induced morphodynamic processes
on the balk parameters of the flow;3) to investigate spatio-temporal dynamics of the vegetative cover and its relationships to
the river channel and flow
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
2. Measurement program
1 2 3 4
Object of the study: straight river reach of the lowland river Müggelspree near Berlin (width 20-30m, depth 1-2m, velocities 10-60 cm/s)
during May-October the reach is colonized by freshwater macrophyte Sagittaria Sagittifolia (1)
Field measurement program comprised: riverbed and sedimentary deposits surveys, measurement of hydraulic characteristics (balk velocities, free-surface slope), surveys of vegetative cover (2), measurements of plants morphology and biomass (3), and measurements of turbulence characteristics with an array of acoustic Doppler velocimeters (4).
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
3. Results: riverbed morphology without vegetation
Riverbed elevations (arbitrary datum)
Riverbed material distribution (d50%)
alternate bars
A
A
Profile A-A
Macro-forms: (bars and dunes)
Micro-forms: (2-D waves and ripples)
alternate bars: 30-50 m long, 20-40 cm highsand dunes: 10-12 m long, 15-20 cm high2-D waves: 2-5 m long, 7-10 cm high
oblique sand dunes:
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
3. Results: riverbed morphology induced by vegetation
solitary vegetative patch: bedform induced by the solitary patch:
grain size distribution across the bedform:
Vegetation-induced bedforms:
in large-scale patches: 20-25 m long, 4-6 m wide, 20-25 cm high; in solitary patches: 4-6 m long, 1-3 m wide, 15-20 cm high
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
3 .06 2 1 .0 6 5 .07 1 9 .0 7 1 2 .0 8 3 1 .0 8 2 0 .0 9 11 .1 0D ate , d ay .m o n th
0
2 0
4 0
6 0
8 0
biom
ass,
g/m
24
8
1 2
1 6
2 0
2 4
root
dia
met
er, m
m
0
4
8
1 2
1 6
leav
es n
umbe
r
4
8
1 2
1 6
2 0
leaf
wid
th, m
m
0
4 0
8 0
1 2 0
1 6 0
2 0 0
leaf
leng
th, c
m
3. Results: dynamics of the vegetative cover
03.06.04
21.06.04
19.07.04
12.08.04
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
3. Results: flow patterns and hydraulic characteristics
May
July
downstream mean velocity isovels (cm/s):ADV measurements
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
3. Results: flow patterns and hydraulic characteristics
September
December
March
Seasonal dynamics of vegetation-induced flow resistance
mass canopy break up
break up of highly streamlined plants
wash out of vegetative-induced bedforms
Here: S is free-surface slope; S0 is maximum free-surface slope; is averaged height of alternate bars; h is mean depth on the reach
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
3. Results: vertical structure of turbulence characteristics
Vertical distribution of dimensionless turbulence shear stresses:
hzUwu /1'' 2*
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
4. Discussion
Unlike sand dunes and waves created by longitudinal sequences of erosion-deposition, the vegetation-induced morpological structures are mainly formed by lateral erosion-deposition. Therefore secondary circulations driven by turbulence anisotropy play especially important role for formation of vegetative patches and associated morphological structures.
Aggravation of alternate bars because of deposition of riverbed material beneath the vegetative canopies causes an increase in the height of the roughness sublayer and consequent reduction of the free surface slopes.
Submersible form of Sagittaria Sagittifolia is quite sensitive to the characteristics of riverbed substrate, specifically at the beginning of the vegetative period. Selectivity of plants for the substrate can be explained by the physiology of the plants. In lowland rivers this macrophyte reproduces by vegetative propagation and has rhizomes about 3 mm in diameter with tubers of 10-15 mm (1). Plants can find a stable substrate for rooting in case when a grain size is comparable in diameter with their rhizomes. Substrates with smaller grain size (0.6-0.8mm) are less stable and move in form of ripples (20-30 cm long, 1.5-3 cm high) during the vegetative period. Rhizomes of plants subjected to the flow area with finer substrate, active bedload transport, and relatively stronger current bend downstream and the lateral colonization of macrophytes became limited.
1
2
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
5. Conclusions
spatial patterns of vegetative cover formed by submersible freshwater macrophyte Sagittaria Sagittifolia in the initial stage of development depend on the riverbed morphology, morpho- dynamic processes and associated distributions of riverbed material;
complex plant–flow interactions lead to significant modifications of morphodynamic processes and hydraulic characteristics of flow. In the study reach the interactions resulted in the develop- ment of specific morphological structures of the size comparable with dimensions of alternate bars – the larger-scale structures on the reach;
superposition of alternate bars and vegetation-induced bedforms increases the thickness of the roughness sublayer up to two times and significantly reduces the slopes of the free surface;
morphological structures induced by the plant-flow interactions persist quite a long period after vegetative period and impose considerable effect on flow hydraulics. Therefore, the influence of macrophytes on flow hydraulics in lowland rivers is most probably more than merrily plants- flow interactions during the vegetative period and due to complex morphodynamic processes the effect extends almost over the whole year cycle.
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
Further Research: implanting of plants:
uniform patch of plants:
ADV measurements:
underwater photography:
ADV-video recording :
June-August 2005:
Fluid Dynamics Laboratory in the Field: a study of turbulent flow structure over vegatation in natural yet simplified conditions
Simplifications: straight river reach, monospecific vegeta-tion, uniform distribution of plants (plants were artificially implanted)
Measurements: ADV profiling of streamwise distributions of turbulence characteristics (10 profiles per run, 7 points at each vertical, simultaneously 3 point measurements); underwater photo-video recording of the vegetation
Results:4 runs with different densities of plants (one run without vegetation)
A. Sukhodolov and T. Sukhodolova: Morphodynamics and hydraulics of vegetated river reaches
Thank you for the attention!
The research was supported in parts by Deutsche Forschungsgemeinschaft (BU 1442/1-1)and by Collaborative Programme of NATO (ESP.NR.EV 981608)