1 ABSTRACT
The interaction between vegetation, water flow and geomorphology has created diverse
fluvial landforms on the Earth. Investigating the interaction between these elements also hassignificant engineering value. The interaction has been studied extensively in last decades. By
using numerical model that developed rapidly in the last decade, vegetation effects, i.e.,vegetation distribution and flexibility, on the transitional and equilibrium river morphology arediscussed in this study. The effects of vegetation transect distribution on braided rivermorphology, and effect of flexibility on reach-scale river morphology has been firstlydemonstrated. The problems are investigated through simplified scenarios aiming to understandthe fundamental interaction between vegetation and river morphology. The results havecompensated the bio-hydro-geomorphology interaction triangle and deepened the understandingof the interaction.
Firstly, a numerical experiment is performed to investigate the effects of floodplain vegetation
combined with various initial low water channel planforms on the meandering development ina gravel-bedded river. Three different low water channels, whose wavelengths are determinedby an empirical criterion of the mobility of alternate bars through river bends, are studied. Twodifferent floodplain covers are considered: a bare floodplain and a vegetated floodplain. The
simulation configurations are based on the Otofuke River at Hokkaido. Results show that withsuitable initial low water channel planform and floodplain vegetation, meandering develops in agravel-bed river without periodic vegetation establishment on bare bars. The sinuosity in thesimulation is close to the river in the field. Meandering develops from two processes in gravelbed rivers, (1) alternate bar growth with an erodible bank and (2) growth from the initial low
water channel bend. The wavelength and amplitude of developed meandering are similar. Theresults suggest that appropriate initial low water channel and floodplain vegetation is sufficientcondition for meandering initiation in a gravel-bed river. Furthermore, vegetation establishmentIon the floodplain is not a necessary condition for meandering development in short-term rivermorphology development. This chapter demonstrated vegetation on the floodplain can bringserious damage to river embankment in a vegetated river, and implies the important role of wateredge vegetation in the formation of channel meandering.
Since the combination effect of floodplain vegetation and channel planform may induce
serious embankment failure, countermeasures are necessary for the management of a vegetatedriver. In the Satsunai River in Hokkaido, reopening of a closed channel on floodplain is used asa measure to reduce embankment failure risks and remove vegetation coverage. To evaluate theperformance of the reopened side channel, the morphological change of river reach (between
31.0 km and 28.8 km from the downstream confluence) during the 2019 flushing flood issimulated by the hydro-morphological model, iRIC Nays2DH. Results show that numericalsimulation can capture the trend of river morphological change around the bifurcation introducedby the reopening, but the erosion in the reopened side channel is overestimated compared tofield-measured data. The morphology of the side channel is quite stable to the flushing flood
based on the field measurement after both 2019 and 2020 flushing floods. The inactive
morphological response of the reopened side channel to the flushing flood is inefficient inreactivating the river bed. By narrowing the inlet of another side channel near the bifurcation,
more discharge has entered the bifurcation and increased the efficiency of the reopened sidechannel according to the simulation. The results suggest that by artificially changing the low
water channel planform and redirecting the flow, short-term morphological responses can bemodified to meet river management requirements. Combined with previous chapter,
appropriately modifying the floodplain, e.g., artificially creating a new watercourse on thefloodplain, can reduce the risks brought by vegetation expansion.
Second, the effect of vegetation distribution on river morphology in a braided river is studied.
Vegetation distribution along river transects is controlled by hydrological conditions and flow
disturbance. It can also be influenced by human activities. As one of the most dynamic riverIIpatterns, braided river can be significantly influenced by vegetation encroachment, while theeffect of vegetation distribution along river transects on braided river characteristics is still
unknown. A depth-averaged hydro-morphodynamic model is employed to study the influenceof vegetation transect distribution as the development of numerical model in the last decade hasproved its efficiency in studying the interaction between vegetation and river morphology.
Rather than discussing specific reason that induces different vegetation distribution and theireffects on river morphology, the problem is generalized and studied by varying the vegetation
habitat extension. Two patterns of transect distribution of vegetation have been investigated: (1)Vegetation establishment near the low water channel; and (2) Vegetation establishment on bar
tops and keeps distance from the low water channel. The model has successfully reproduced thereduction in braiding index of a vegetated braided river. The results show that transect
distribution of vegetation has a significant influence on the statistical properties of braiding river
bed elevation. Bed variance increases with the increase of vegetation habitat area in both
distribution patterns. Skewness and kurtosis decrease and increase with the increase ofvegetation habitat area in case of type (1) distribution, respectively. With a distribution type (2),the relationships between skewness, kurtosis, and vegetation habitat area are opposite. These
results have provided an extra explanation for the discrepancy in the skewness between fieldobservation and laboratory experiment and show implications for managing vegetated braided
rivers with a gravel bed. For restoration project in a vegetated braided river, vegetation nearwater edge should be removed rather than vegetation on bar top. This chapter shows that onlywith a vegetation belt that is close to the water edge, the river morphology can be significantlyinfluenced. The results further prove the importance of water edge vegetation in the fluvialsystem.
Finally, the effect of vegetation flexibility on river morphology is investigated. The flexibilityof riparian vegetation affects the flow around vegetation patches and alters bed shear stressbelow and surrounding the patches. To investigate the effects of vegetation flexibility in a fluvialIIIbio-hydro-morphodynamic simulation, a model to predict flexible vegetation reconfiguration isincorporated to a bio-hydro-morphodynamic model based on Delft3D. The effects of vegetation
flexibility in a gravel bed river with alternate bars have been studied. Phragmites japonica,
which is an extensively existing reedy grass in riparian environment is the target vegetation.
Results show that vegetation flexibility impacts river hydrodynamics and morphology
development processes. With flexibility accounted for, the water depth decreases, averaged
Shields stress in vegetation patch increases, and active channel width widens. Erosion anddeposition in the channel are reduced. The significance of flexibility depends on multiple factors,i.e., stem density growth rate, rigidity, and sediment size. With a fast stem density growth rateor large gravel size, the difference between flexible and rigid cases can be neglected. Compared
to previous chapters, the effect of flexible water edge vegetation is demonstrated in this chapter.
With flexibility considered, the floodplain formation rate can be better estimated.
In this thesis, the effects of riparian vegetation on transitional and equilibrium rivermorphology have been studied. The results demonstrate the significant influence of vegetation,i.e., its distribution and flexibility, on river morphology. The results emphasize the necessity tounderstand the geomorphological influence of water edge vegetation, including its spatial
distribution and mechanical properties.

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