8/14/2019 Explain the role plants play in creating meandering rivers.pdf

http://slidepdf.com/reader/full/explain-the-role-plants-play-in-creating-meandering-riverspdf 1/5

Explain the role plants play in creating meandering rivers

ABSTRACT

Paola (2011) notes that unconfined rivers tend to move laterally as a broad sheet of flow rather than incontained channels. Vegetation is a principal factor in the transition from lateral and braided flows tomeandering channels. Tal and Paola (2007) in their laboratory experiments concluded that vegetationhas a role in both instigating and maintaining the changes from a braided to meandering channel.Vegetation colonises exposed areas of a braided river and transforms the braided system to a single-thread channel. This paper identifies the key processes of this transformation as increasing bankstability and cohesion; reducing flow resistance; creating preferential flow routes; and forming log

jams. Vegetation then plays a key role in developing meanders and maintaining a single channel.This paper will discuss how vegetation affects these key processes and, in turn, how they lead to thecreation of single-channel meandering rivers.

INTRODUCTION

The transition from a braided river to a meandering river is a multi-faceted one, in which vegetationplays a large part. Firstly it helps to form a stable single-thread channel through increasing bankstability and cohesion; reducing flow resistance; creating preferential flow routes; and forming log

jams, and then promotes the creation of meanders. Higher flow velocity on the outside of the bendcauses erosion which is accompanied by point bar development and deposition on the inside of themeander. Point bar deposits are colonised by plants at the same rate the outer bend is eroded resultingin a steady but dynamic state (Tal and Paola, 2007). This paper utilises previous research on this topic

and identifies the key roles of vegetation in forming meanders.

DISCUSSION

Flow Resistance

Flow resistance is increased by thepresence of vegetation within the riverchannel. The roots and stems create drag

and turbulence which ultimately slowthe flow rate. This flow disruption iscaused by the mean kinetic energy of theflow being converted to turbulent kineticenergy (Nepf, 1999). Nepf created amodel which links vegetative drag andturbulence intensity and ran experimentscomparing the flow of vegetated and un-vegetated channels (Figure 1). Theresults confirm that turbulence intensity

is “principally dependent on thevegetative drag” (p488). The

Figure 1 Comparison of turbulent kinetic energy within vegetatedand un-vegetated (subscript o) regions. Bed drag coefficients of0.01(solid lines) and 0.005(dashed lines) are considered as well asthree ratios of flow depth to stem cylinder diameter (10, 20 and 50).Note that the scales are dimensionless as the volume of flow domainoccupied by plants is expressed as a fraction.

8/14/2019 Explain the role plants play in creating meandering rivers.pdf

http://slidepdf.com/reader/full/explain-the-role-plants-play-in-creating-meandering-riverspdf 2/5

introduction of vegetation creates competing effects; a reduction of flow velocity (due to drag andfriction) and an increase in turbulence production. These effects combine to increase flow resistanceand thus reduce the flow velocity, which in turn causes a decrease in the shear stress exerted on thebanks and bed, and subsequently there is less energy for erosion and transportation (Thorne 1990).

In addition, vegetation causes preferential flow routing as the water tends to flow over un-vegetatedareas where friction is lowest. This results in a “velocity pulse” which increases the flow rate incertain parts of the channel (Temmerman et al., 2005). When vegetation is present, the flow velocityin the centre of the channel is much faster (Figure 2E) compared to un-vegetated areas which tend tohave an even sheet flow (Figure 2F). Similarly the presence of vegetation means that sedimentation isconcentrated around the edges and banks of the channel (Figure 2G) rather than the centre as the flowrate is too fast.

Tal and Paola (2007) also refer to this phenomenon in their research referring to the water being“corralled” by vegetation into a dominant channel through the elimination of weak flow paths. Theymapped the areas with high discharge and confirmed that these are the fastest moving flows bytracking floating particles on the surface. These fast “pulses” in the flow are eventually linked up by“corralling” from vegetation. The fast flowing water prevents the establishment of new vegetationalong its path which helps the river to maintain a ‘dynamic steady state’ (Tal and Paola, 2007).

Bank stability and cohesion

Vegetation promotes a shift to a single-channelled state by increasing bank stability and improvingbank cohesion. Root binding physically stabilises the river banks as it creates tractive forces between

Figure 2 Simulated flow patterns and sedimentation patterns from Temmerman et al. modelThe images show the depth-averaged flow velocity (uv) and sedimentation rate (SD) for:(E) Flow pattern with vegetation(F) Flow pattern without vegetation(G) Sedimentation pattern with vegetation

(H) Sedimentation pattern without vegetation

8/14/2019 Explain the role plants play in creating meandering rivers.pdf

http://slidepdf.com/reader/full/explain-the-role-plants-play-in-creating-meandering-riverspdf 3/5

the soil and the tensile resistance in the roots which give it strength (Abernethy and Rutherford, 2001).A root-reinforced channel is able to resist lateral erosion far better than un-vegetated alluvium and cantherefore maintain its structure which allows a single-thread channel to form. Hickin and Nanson(1984) found that, an un-vegetated bank would erode at almost twice the rate of a vegetated channelwhen other factors such as discharge, bend curvature, bank heights, and size of bank materials werekept constant. Moreover, f ieldwork in the Alexandra River, Canada found that root mats of vegetationon the channel banks were up to 20,000 times more resistant to erosion than similar un-vegetatedbanks (Smith 1976).

Jang and Shimizu (2007)conducted a number oflaboratory experiments toexamine the effects onvegetation of the morphologicalbehaviour of channels. One

conclusion drawn was that bankstability is increased because theroughness caused by vegetationprevents the flow “impingingdirectly on the banks,” (p 767).Figure 3 shows the change inflow direction over time causedby vegetation.

Vegetation also promotes theproduction of fine clay sediment(Paola, 2011) which as Smith(1998) points out is necessary asa binding agent to increase thecohesiveness of banks. Smith’s

experiments were successful in using fine cohesive sediment to produce meandering rivers, howeverdue to the absence of vegetation the meanders were not cut off and regenerated (Tal and Paola, 2007).This shows that bank stability and cohesiveness are important factors in creating meandering riversand that as well as producing these two conditions, vegetation is also needed to maintain the meanders.

Anabranching rivers

Vegetation also plays a role in the formation of anabranching or anastomosing rivers. Nanson andKnighton (1996, p. 218) defined anabranching as a system of multiple channels divided by vegetatedstable islands. The stability of the islands differentiates anabranching from braided channels which aredynamic and unstable. Anabranching rivers are still dynamic but are characterised by slow andinfrequent avulsion rather than the rapid high energy channel-switching of braided systems.Vegetation is crucial as it provides stability which allows the islands to resist erosion. Anabranchingrivers are formed through the processes of avulsion and obtrusion (the gradual formation ofalternative channels) (Knighton and Nanson, 1993), which require a high resistance to erosion(provided by vegetation). As Davies and Gibling (2011) note, the first appearances of anabranching or

anastomosing rivers roughly correspond with the Palaeozoic era when terrestrial plants began to

Figure 3 Photographs from the Jang and Shimizu’s flume experiments.

The progression from (a) to (b) clearly demonstrates the formation ofmeanders due to the increased bank stability and preferential flow routingcaused by the vegetation (alfalfa).Arrows indicate the thalweg of the channel. Flow is top to bottom.

8/14/2019 Explain the role plants play in creating meandering rivers.pdf

http://slidepdf.com/reader/full/explain-the-role-plants-play-in-creating-meandering-riverspdf 4/5

establish on earth. It was the ability of these plants to promote channel stability and avulsion whichallowed anabranching systems to form.

Anabranching rivers can be interpreted as a stable but transitional form between braided and single-thread rivers (Eaton et al., 2010). It can therefore be deduced that vegetation leads to the creation ofmeandering rivers by creating anabranching channels which will then form single thread rivers. Oncethe stable islands in an anabranching river have been established, the presence of vegetation enhancesthe development of bars by trapping further quantities of debris or sediment (Hickin, 1984).

Formation of Log jams

Hickin (1984) also notes how vegetation controls avulsion, the key process behind anabranchingrivers, through the formation and breaching of log jams. Riparian vegetation is essential for theconstruction of log jams as it provides the Large Woody Debris (LWD) which is defined as pieceslarger than 10cm diameter and 1m length (Platts et al., 1987). The frequency and duration of log jams

are dependent on the surrounding vegetation; the size and decay rates of species will affect thestability and longevity of a jam (Fetherston et al., 1995). The roots and stems of riparian vegetationalso play a role by providing obstructions which can increase the probability of a log jam. Theincreased bank stability as a result of vegetation also increases the likelihood of log jams as it reducesthe chance of logs eroding the banks and being swept away. Log jams create areas of low shear stresswhich causes deposition of sediment. This allows the log jams to be colonised by vegetation andcreate a stable obstruction in the river (Fetherston et al., 1995).

CONCLUSION

Vegetation has important roles to play in both the key processes leading to change and in themechanisms which maintain meandering single-thread channels. Vegetation increases flow resistanceand reduces shear stress resulting in less energy for erosion and transportation. The subsequentsedimentation can create islands in anabranching rivers which are then colonised by vegetation,helping to promote meanders. Vegetation also increases the shear strength and cohesiveness of riverbanks which means the channel is less susceptible to erosion. In this sense, vegetation can be seen toboth initiate and maintain the creation of meandering rivers.

References

Abernethy, B. and Rutherfurd, I. (2001) The distribution and strength of riparian tree roots in relationto riverbank reinforcement. Hydrological Processes , 15 (1), p.63-79.

Davies, N. and Gibling, M. (2011) Evolution of fixed-channel alluvial plains in response toCarboniferous vegetation. NatureGeoscience , 4 p.629-633.

Eaton, B. et al. (2012) Channel patterns: Braided, anabranching, and single-thread. Geomorphology ,120 (3-4), p.353–364.

Fetherston, K. et al. (1995) Large woody debris, physical process, and riparian forest development inmontane river networks of the Pacific Northwest. Geomorphology , 13 (1-4), p.133-144.

8/14/2019 Explain the role plants play in creating meandering rivers.pdf

http://slidepdf.com/reader/full/explain-the-role-plants-play-in-creating-meandering-riverspdf 5/5

Hickin, E. (2008) Vegetation and River Channel Dynamics. Canadian Geographer , 28 (2), p.111-126.

Hickin, E. and Nanson, G. (1984) Lateral migration of river bends. Journal of Hydraulic Engineering ,110 (11), p.1557-1567.

Jang, C. and Shimizu, Y. (2007) Vegetation effects on the morphological behavior of alluvialchannels. Journal of Hydraulic Research , 45 (6), p.763-772.

Knighton, A. and Nanson, G. (1993) Anastomosis and the continuum of channel pattern. EarthSurface Processes and Landforms , 18 (7), p.613–625.

Nanson, G. and Knighton, A. (1996) Anabranching rivers—their cause, character and classification. Earth Surface Processes and Landforms , 21 (3), p.217-239.

Nepf, H. (1999) Drag, turbulence, and diffusion in flow through emergent vegetation. Water Resources Research , 35 (2), p.479-489.

Paola, C. (2011) Co-evolution of rivers and plants. Geomorphology , 4 (9), p.583-584.

Platts, W. S. (1987). Methods for evaluating riparian habitats with applications to management .Ogden, Utah, U.S. Dept. of Agriculture, Forest Service, Intermountain Forest and Range ExperimentStation

Smith, C. (1998) Modelling high sinuosity meanders in a small flume. Geomorphology , 25 (1-2),p.19-30.

Smith, D. (1976) Effect of vegetation on lateral migration of anastomosed channels of a glacier meltwater river. Geological Society of America Bulletin , 87 (6), p.857-860 .

Tal, M. and Paola, C. (2007) Dynamic single-thread channels maintained by the interaction of flowand vegetation. Geology , 35 (4), p.347-350.

Temmerman, S. et al. (2005) Impact of vegetation on flow routing and sedimentation patterns: Three-dimensional modelling for a tidal marsh. Journal of Geophysical Research , 110 (4), p.1-18.

Thorne, C. (1990) Effects of vegetation on riverbank erosion and stability. In: Thornes, B. eds. (2012)Vegetation and Erosion . 1st ed. New York: John Wiley, p.125–144.