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)


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


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).


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:


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


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ass,

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3. Results: dynamics of the vegetative cover

03.06.04

21.06.04

19.07.04

12.08.04


3. Results: flow patterns and hydraulic characteristics

May

July

downstream mean velocity isovels (cm/s):ADV measurements


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


3. Results: vertical structure of turbulence characteristics

Vertical distribution of dimensionless turbulence shear stresses:

hzUwu /1'' 2*


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


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, morphodynamic 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 development 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.


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)


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)

Morphodynamics and hydraulics of vegetated river reaches: a case study on the Müggelspree in...

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