water
ArticleMechanism of Riparian Vegetation Growth andSediment Transport Interaction in Floodplain: A
Dynamic Riparian Vegetation Model(DRIPVEM) Approach
Mahendra B. Baniya 1,2, *, Takashi Asaeda 3,4,5 , Takeshi Fujino 1 ,Senavirathna M. D. H. Jayasanka 1 , Guligena Muhetaer 1 and Jinghao Li 1
1

2
3
4
5

*

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku,
Saitama 338-8570, Japan; fujino@mail.saitama-u.ac.jp (T.F.); jayasanka@mail.saitama-u.ac.jp (S.M.D.H.J.);
gulgina1112@gmail.com (G.M.); ri.k.089@ms.saitama-u.ac.jp (J.L.)
Provincial Government, Ministry of Physical Infrastructure Development, Pokhara 33700, Nepal
Institute Hydro Technology Institute, 4-3-1 Shiroyama Trust Tower, Tranomon, Minato, Tokyo 105-0001,
Japan; asaeda@mail.saitama-u.ac.jp
Research and Development Center, Nippon Koei, 2304 Inarihara, Tsukuba, Ibaraki 300-1259, Japan
Institute for studies of the Global Environment, 7-1 Sophia University, Kioicho, Chiyoda,
Tokyo 102-0094, Japan
Correspondence: baniyam57@gmail.com; Tel.: +81-048-858-9186

Received: 20 November 2019; Accepted: 21 December 2019; Published: 24 December 2019


Abstract: The ecological dynamics of riparian areas interact with sediment transport in river systems,
which plays an active role in riparian vegetation growth in the floodplain. The fluvial dynamics,
hydraulics, hydro-meteorological and geomorphological characteristics of rivers are associated withsediment transport in river systems and around the riparian area. The flood disturbance, sedimentwith nutrients and seeds transported by river, sediment deposition, and erosion phenomena in thefloodplain change the bare land area to vegetation area and vice versa. The difference in riparianvegetation area in the river floodplain is dependent on the sediment grain size distribution which
is deposited in the river floodplain. Mathematical models describing vegetation growth in a shortperiod exist in literature, but long-term modelling and validations are still lacking. In order to coverlong-term vegetation growth modelling, a Dynamic Riparian Vegetation Model (DRIPVEM) was
proposed. This paper highlights the existing modelling technique of DRIPVEM coupled with a
Dynamic Herbaceous Model used to establish the interactive relationship of sediment grain sizes andriparian vegetation in the floodplain.
Keywords: ecological dynamics; riparian area; sediment transport; flood disturbance; deposition anderosion; DRIPVEM

1. Introduction
Riparian vegetation plays a significant role in the ecological balance of a riparian area [1].
Riparian area is a transitional semiterrestrial zone extending from the edges of water bodies to theedges of upland communities and regularly interacts with river water, flow alterations, sediment,
and nutrients [2–4]. Flood disturbance is one of the major causes of vegetation mortality in the riparianzone either by inundation or bank erosion [5–7]. The morphological alterations in riparian area caused
by a flood event either supports or suppresses the riparian vegetation, which depends also on thesediment grain sizes deposited on the original sediment surface or exposed of previous underlying

Water 2020, 12, 77; doi:10.3390/w12010077

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2 of 13also on the sediment grain sizes deposited on the original sediment surface or exposed of previous
underlying sediment surface [8]. The flood hydrology affects the hydrochory in the riparian area
sediment surface [8]. The flood hydrology affects the hydrochory in the riparian area during flood time,
during flood time, recruits seeds, and begins colonization of trees [9]. Water is one of the major
recruits seeds, and begins colonization of trees [9]. Water is one of the major principal seed dispersal
principal seed dispersal agents in the floodplain [10]. The number of seeds floating during a flood
agents in the floodplain [10]. The number of seeds floating during a flood event, magnitude of theevent, magnitude of the flood, duration and frequency of flood occurrence determine the seed
flood, duration and frequency of flood occurrence determine the seed recruitment phenomenon in the
recruitment phenomenon in the riparian area [11–13]. The area of vegetation coverage in the riparian
riparian area [11–13]. The area of vegetation coverage in the riparian zone is directly dependent onzone is directly dependent on the availability of water, nutrients available in water and sediment
the availability of water, nutrients available in water and sediment before and after the flood event,
before and after the flood event, and exchange of nutrients from atmosphere to the floodplain [8].
and exchange of nutrients from atmosphere to the floodplain [8].
Phragmites australis, Typha spp., and Phragmites japonica are perennial plants in the riparian zone
Phragmites australis, Typha spp., and Phragmites japonica are perennial plants in the riparian zone inin Japanese River floodplains and have an extremely high ability to expand their territory by
Japanese River floodplains and have an extremely high ability to expand their territory by extending
extending stolons [14]. The floods provide the habitat with nutrient rich water and sediment in thestolons [14]. The floods provide the habitat with nutrient rich water and sediment in the inundated bar.
inundated bar. Sandy sediment sites are preferable for the growth of herbaceous plants, as moisture
Sandy sediment sites are preferable for the growth of herbaceous plants, as moisture and nutrients inand nutrients in the coarse sediment layers are very limited [15–17].
the coarse sediment layers are very limited [15–17].
The flood controlling structures such as dams and weirs control the natural flood dramatically
The flood controlling structures such as dams and weirs control the natural flood dramatically [5,18][5,18] so that the bed shear stress, specific power, and flow velocity of downstream reaches wereso that the bed shear stress, specific power, and flow velocity of downstream reaches were reduced [19].
reduced [19]. The low interacting flood events in the downstream area also curtail the sediment
The low interacting flood events in the downstream area also curtail the sediment transport andtransport and make the river channel more stable [20]. This phenomenon gradually coarsens themake the river channel more stable [20]. This phenomenon gradually coarsens the downstream active
downstream active riparian zone. From the sustainable reservoir management perspective, theriparian zone. From the sustainable reservoir management perspective, the intermittently artificial
intermittently artificial release of sediment with high nutrients deposited in reservoir through dams
release of sediment with high nutrients deposited in reservoir through dams has been practiced
has been practiced in Japanese Rivers [21], and has increased the depth of fine sediment in thein Japanese Rivers [21], and has increased the depth of fine sediment in the floodplain, which also
floodplain, which also alters the inundation pattern and sediment grain sizes in the riparian zone
alters the inundation pattern and sediment grain sizes in the riparian zone [22]. The released fine[22]. The released fine sediments interlocked the coarsened sediment of the bar after sedimentation
sediments interlocked the coarsened sediment of the bar after sedimentation in the bar, supporting thein the bar, supporting the vegetation encroachment.
vegetation encroachment.
The sediment yield from a catchment depends on numerous parameters including topography,
The sediment yield from a catchment depends on numerous parameters including topography,
gradient of river, rainfall in catchment, temperature, and soil type of the catchment area [23,24].
gradient of river, rainfall in catchment, temperature, and soil type of the catchment area [23,24].
Fluvial dynamics, hydraulics, hydro-meteorological and geomorphological conditions of a river
Fluvial dynamics, hydraulics, hydro-meteorological and geomorphological conditions of a river
determine the transport of sediment from its origin [25]. The flood events with its magnitude,
determine the transport of sediment from its origin [25]. The flood events with its magnitude, duration,
duration, and frequency are associated with sediment transport [26]. The morphologicaland frequency are associated with sediment transport [26]. The morphological characteristics of acharacteristics of a riparian zone was altered by flooding, which triggers erosion or deposition in theriparian zone was altered by flooding, which triggers erosion or deposition in the floodplain [27].
floodplain [27]. The floods have an influence on recruitment as well as vegetation colonization.
The floods have an influence on recruitment as well as vegetation colonization.
The interactive parameters for riparian vegetation growth in a floodplain are illustrated Figure
The interactive parameters for riparian vegetation growth in a floodplain are illustrated Figure 1.
1.
Fluvial dynamics

Hydraulics

Hydro- meteorological

Geomorphological

Anthropogenic
disturbance

Sediment, nutrients and seeds transportmechanisms

Atmospheric
Nitrogen
Water withnutrients

Sediment withnutrients

Groundwater
depth

Organic mattertransport

Riparian vegetation
Decomposition

N fixation fromatmosphere

Trees species

Decomposition

Herb species

Figure 1. Interactive parameters for riparian vegetation growth in a floodplain.
Figure 1. Interactive parameters for riparian vegetation growth in a floodplain.

The intermittent floods encountered in a floodplain alter the morphology of the riparian zone and
The intermittent
encountered
floodplainofalter
the morphologyof the riparianzone
the vegetation
coveragefloods
changesby erosionin
oradeposition
sediment[28]. The nutrients
interchange
andthevegetation
coveragechangesbyerosionordepositionofsediment[28].
The
nutrientsfrom the atmosphere, human disturbance, and flooding play a role for riparian vegetation growth [29].

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3 of 13interchange from the atmosphere, human disturbance, and flooding play a role for riparianvegetationvegetation
growth [29].
Primarily,
vegetation
succession
in aoffloodplain
growthofPrimarily,
succession
in a floodplain
startsfrom growth
herbaceousstarts
plants,from
which
undergoherbaceousafter
plants,which undergo
senescencecompletingthebiomass
lifecycle,
and thedecomposedsenescencecompletingthe lifecycle,
and theafter
decomposed
litterbecomenutrients
to thelitterbiomass
becomenutrients
tothefloodplain.
The
supportiveroleofincreasing
nutrientsbyfloodplain. The supportive role of increasing nutrients by primary succession of herbaceous plants
primary
successionof herbaceous
plantscreate a favorable
environment
for tree recruitmentin thecreatea favorable
environment
for treerecruitmentin the floodplain.
The decompositionof defoliatedfloodplain.
The
decompositionof defoliatedof trees andbiomass
abovegroundbiomass(AGB)
andleavesof trees
andaboveground biomass(AGB)leaves
and belowground(BGB) ofherbaceous
plantsbelowgroundbiomass(BGB)ofherbaceous
plantsalso
increased
thenutrients
levelofriparianzone.
also increased the nutrients level of riparian zone. The absence of large floods due to these human
The absence oflarge floods
due to thesehuman interventions
compelled
vegetationininterventions
compelled
vegetationcolonizationin the floodplain,
which removes
thecolonization
natural stonythefloodplain,whichremovesthenaturalstony
surfaceresultingaforestcommunity.
surface resulting a forest community.
The alterations
alterations ofof riverriver hydrologyhydrology andand morphology
morphology after
after dam
dam intervention
intervention andand changes
changes ofofThe
sediment grain
grainsizes
sizeswithwithnutrientsnutrientstransporttransport
riparianzone
makefloodplain
fertile.
Theresediment
inin
thetheriparianzone
makethethefloodplain
fertile.
There
are
are
numerousinteractive
parameters
responsibleforchanging
bare
land
area
to
vegetation
areas.
numerous interactive parameters responsible for changing bare land area to vegetation areas. Numerical
Numerical describing
simulationripariandescribing
ripariangrowth
vegetation
for aandlong
period andsimulation
vegetation
for a growthlong period
validations
arevalidations
still limitedareinstillliterature
limited in[30–34].
the literature[30–34].
is difficult
to address
all theseinteracting
in a single
theIt is difficult
toItaddress
all theseinteracting
parametersin aparameters
single mathematicalmathematicalmodel. It
importantto conduct
this study
in floodplains
to know
the excessivemodel.
It is important
to isconduct
this study
in floodplains
to know
the excessive
vegetation
drivers
vegetation
drivers
and to predictthe changes
in long-term
vegetation
area.
The
and
to predictthe changes
in long-term
riparianvegetationriparian
coveragearea. Thecoverage
objectivesof thisobjectives
of
this
study
are
to
link
the
interacting
parameters
withvegetation
growthinstudy are to link the interacting parameters with vegetation growth in the mathematical modelthetomathematicalmodel to elucidate
the mechanisms
behindriparianvegetation
growth dynamics
in theelucidatethe mechanisms
behind riparianvegetation
growthdynamics
in the perspectiveof long-term
perspectivegrowthof long-term
vegetation
growthofhighlights
a riparianthe
area.
This paper
highlights
thevegetation
modellingof a riparianarea.modellingThis paper
existing
modellingtechniqueexisting
modellingtechniqueof
DRIPVEM[8,9,35]usedtoestablishtheinteractive
relationshipofof DRIPVEM [8,9,35] used to establish the interactive relationship of sediment grain size and riparian
sediment grain
and riparian vegetation in the floodplain.
vegetationin thesize
floodplain.
2. Materials and Methods
2.1.
Study Site
Site Description
2.1. Study
Description
The
The Kuzuryu
Kuzuryu River
River flows
flows throughthrough Fukui
Fukui Prefecture,
Prefecture, Japan
Japan andand merges
merges inin toto thethe seasea ofof JapanJapan(Figure2a,b).
The
length
of
this
river
is
about
116
km.
Several
dams
alter
the
river
continuity.
(Figure 2a,b). The length of this river is about 116 km. Several dams alter the river continuity. The
The
floodplain
is located
about
from
river
mouthandand5656km
kmdownstream
downstream fromfrom thethe Kuzuryu
floodplain
is located
about
24 24
kmkm
from
thetheriver
mouth
Kuzuryu2
River
The river
river basin
The averageRiver dam.
dam. Thebasin has
has catchment
catchment area
area ofof 29302930 km
km2.. The
average annual
annual discharge
discharge ofof thethe33s−1−1selected
study
reach
is
about
86
m
and
the
averageannualrainfall
is
about
2200
mm.
The
historicalselected study reach is about 86 m s and the average annual rainfall is about 2200 mm. The historical
data
data sourced
sourced fromfrom Ministry
Ministry ofof Land,Land, Infrastructure,
Infrastructure, Transport
Transport andand Tourism
Tourism (MLIT),(MLIT), Japan
Japan recorded
recorded3 −1
thethe maximum
maximum flood
flood heightheight was
was 7.54
7.54m
min
inOctober
October2004
2004andandcorresponding
correspondingdischarge
dischargewas
was32213221mms3s−1
◦
0
00
◦
0
(http://www1.river.go.jp/)
KuzuryuRiver
studyreach.
TheThefloodplain(36 (36°06′23′′
06 23 N, 136
16 0600 E)(http://www1.river.go.jp/)ininthe
theKuzuryuRiver
studyreach.
floodplain
N, 136°16′06′′
is2370
m longandand542542m wide(Figure2a,b).
TheThegradientof thestudy
reach
is 0.3%.
E)about
is about
2370
m longm wide(Figure2a,b).
gradientof thestudy
reach
is 0.3%.

(b)(a)

Figure 2. Location map of (a) Kuzuryu River study reach and (b) Magnified view of the study river
Figurefloodplain.
2. Location map of (a) Kuzuryu River study reach and (b) Magnified view of the study riverreach
reach floodplain.

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The main physiographic characteristics of the Kuzuryu River study reach is presented in Table 1.
Table 1. Main physiographic characteristics of study river reach.
Parameters

Descriptions

Floodplain size

2370 × 542 m

Mean gradient of river

0.3%

Extreme flood levelfrom normal water level(0.000 m elevation)

7.54 m in October 2004; 6.24m in October 2002; 6.9 m and6.82m in September 1998 and 1989

Extreme flood discharge

3221 m3 s−1 in October 2004; 2400 m3 s−1 in October 2002;
2517 and 2144 m3 s−1 in September 1998 and 1989

Ground elevation ranges from normal waterlevel (0.000 m elevation)

0.102–7.334 m

Precipitation≈2200 mm year−1

2.2. Model Development
The modelling approach is a technique frequently adopted to predict the vegetation dynamics
in the riparian area which is the best tool for the long-term management perspective of riparianarea. The alterations observed in riparian trees by hydro-morphological alterations and climatechanges provided researchers with evidence to be used for developing numerical models [36,37].
Different mathematical and process-based approaches were used to express vegetative succession in ariparian area [38–40] and for growth and yield simulation, planning, decision making, and efficientforest management [41–43]. However, coupling of all the interacting parameters with riparianvegetation growth once at a time in a single mathematical model is a tedious task.
Most of the parameters associated with the riparian vegetation growth mechanism were clearlycompiled in mathematical models for spatial distribution of herbs and trees in DRIPVEM by Asaeda [8,9].
The growth of herbaceous plants for monospecific stand in a riparian zone was described in the
Dynamic Herbaceous Model [44]. The DRIPVEM coupled with a Dynamic Herbaceous Model wasused to describe vegetation dynamics in the riparian zone.
2.2.1. Dynamic Herbaceous Model for Monospecific StandThe herbaceous plants P. australis, Typha, and P. japonica are some of the dominant herbaceousperennial plants in riverine wetlands. These plants can grow on sandy and stony habitats on thefloodplains. Particle grain size of a floodplain plays an important role in the growth of these plants.
Dynamic Herbaceous Models were developed to simulate the growth dynamics of a monospecificstand of P. australis, Typha spp., and P. japonica in freshwater ecosystems. The mathematical models
were formulated by Asaeda [44] and the results were validated in Czech Republic, Australia andJapan [44], U.S.A. [45], Egypt [46], and Japan [5].
In this Dynamic Herbaceous Model, meteorological data such as daily temperature and dailysolar radiation are the input parameters (Figure 3). In the case of lacking meteorological data,
the latitude of the study site is considered as input for calculating the temperatures and solar radiation.
The model consists of five different equations for shoots, inflorescence, roots, old rhizomes, and new
rhizomes [5,44]. The initial rhizome and root biomass are also given as input variables for initiationof growth. The net growth of the whole plant is the combined effect of photosynthesis, respiration,mortality and transfer of assimilated materials.

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Input data
daily total global radiation &mean temperature

Initial rhizome & root biomass

remobilization for spring
growthrespiration

shootstranslocationin senescencemortality

paniclesAGB
photosynthesis

supply for root growth

old rhizomes

roots

BGB

new rhizomes

respiration

respiration

respirationmortality

mortality

mortality

Figure3.3.Schematic
Schematicstructure
structureof
ofthe
theDynamic
DynamicHerbaceous
HerbaceousModel.
Model.
FigureThenetnetproduction
productionof
ofherbaceous
herbaceousplant
plantisiscalculated
calculatedby
by using
using Equation
Equation (1).
(1).
The
dBT
𝑑𝐵𝑇
production
dt (Mortality) ± P
Net Netproduction== P∑ 𝑃ℎ(𝑖) - P∑ (Respiration)=− P∑∑ (Translocation), (1)(1)
𝑑𝑡 =(Respiration) −(Mortality) ±(Translocation),Ph(i) −
i=layeri=layer

organ

organ

organ

organ

organ

organ

where ∑i =Player 𝑃ℎ(𝑖) is sum of layer by layer photosynthesis performed by leaves of a herbaceous
where∑
Ph(i) is sumisoftotal
layer
by of
layer
photosynthesis
performed∑by leavesof a herbaceous plant,(Respiration)
plant,
loss
biomassdue to respiration,
organ
organ(Mortality) is total loss of
i=
layer
P
P
biomassdue to deadof plants
suchastoshoots,
inflorescence,roots, )old
rhizomes,new(Respiration) is total(Mortality
loss oforgan
biomassdue
respiration,is total
loss ofandbiomassorgan
organ
rhizomesof herbaceous plants. ∑organ(Translocation) is transfer
of assimilated materials. The + sign
due to dead of plants organ such as shoots, inflorescence, roots, old rhizomes, and new rhizomes of
Ptransfer of assimilated materials from BGB to AGB and the – sign represents
represents the upward(Translocation) is transfer of assimilated materials. The + sign represents theherbaceous plants.
the downward transferfrom AGB to BGB.
organ
upward transfer of assimilated materials from BGB to AGB and the − sign represents the downward
2.2.2.
DRIPVEM
Coupledwith a Dynamic Herbaceous Modeltransferfrom AGB
to BGB.
long-term
modelling
incorporationof numerous
2.2.2.ForDRIPVEM
Coupledwithanda Dynamic
Herbaceous
Model interacting parameters with riparianvegetation succession subjected to both herbaceous plants and trees, Asaeda [8,9] developed a
For long-term
modelling
and incorporation
numerousand
interacting
parameterswith riparian
riparian
dynamic
model which
can predictthe vegetationofsuccessioninteraction
with floods,
vegetation succession
bothetc.
herbaceous plants and trees, Asaeda [8,9] developed a dynamic
morphology,
nutrients,subjected
sedimenttosize,
model
which
can
predict
thevegetation
succession
and interaction
withfloods,riparian Herbaceous
morphology,
Figure 4 shows the schematic diagramof DRIPVEM
coupledwitha Dynamic
nutrients,
sedimentsize,
etc.
Model. The model consists of HYDRO, TREE, HERB, and NUTRIENTS modules with their interactive
4 shows
themodule
schematic
diagramof up
DRIPVEM
coupled towitha Dynamic
Herbaceousroles Figure[8,9]. The
HYDRO
is usedto linkflood hydrology
TREE
and HERB
modulesModel.
The
model
consistsof
HYDRO,
TREE,
HERB,
and
NUTRIENTS
moduleswith
theirinteractive
which occurred in the floodplain. The flushing effects of flood is also considered in the model. The
roles [8,9].
The HYDRO
moduleis used tree
to link
up flood hydrology
to TREE andmodules initial
which
TREE
modulecalculates
the spatial
distribution
in the floodplain
byHERB
considering
occurredinthefloodplain.
The
flushing
effects
of
flood
is
also
consideredinthemodel.
The
colonization after flooding. The allometric relations with diameter at breast height (DBH) andTREE
age
were also included in the model to predict the biomass growth of tree species such as Salix gilgiana,
Robinia pseudoacacia, and Albizia julibrissin [8,9,47]. The HERB module calculates the herbaceous plants

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biomass
Water 2020,12, 77 with its spatial distribution in the floodplain in which particle size, shading effects
6 of 13 of tree
canopy, as well as the nutrient interactions and effects of floods are incorporated in the model. The
NUTRIENTS module depicts the soil nutrients budget by interacting atmospheric nitrogen, flood
modulenitrogen,calculatesnitrogenthe spatial
tree distribution
in the floodplain
by considering
initialreleaseafter decompositionof litterbiomass, andthecolonization
N- fixationafter
process offlooding.
The
allometric
relationswithdiameteratbreastheight
(DBH)
and
age
were
also included
rhizobium bacteria with soil in the floodplain. The outputs of DRIPVEM were validated
for Japanesein the model
to predict
steep rivers[35].the biomass growth of tree species such as Salix gilgiana, Robinia pseudoacacia,
and Albizia julibrissin[8,9,47].
TheofHERB
moduletheherbaceous
plantsbiomasswith
its
In the HERB
moduleDRIPVEM,
thecalculates
maximum
herbbiomass is
one ofthe input
parameters.
spatial distribution
inthefloodplaininwhichparticlesize,
shading
effectsoftree
canopy,
as
well
as
the
In the absence of observed data of maximum herb biomass in floodplain, a Dynamic Herbaceous
nutrientModelinteractionsof floods
are incorporated
in the model.
NUTRIENTS
moduleis usedandto effects
calculatethe maximum
herbs biomassin the The
floodplain.
The output
value ofdepictsmaximum
the soil nutrientsbudgetbyinteracting
atmosphericnitrogen,flood
nitrogen,nitrogenrelease whichbiomass simulated by the Dynamic Herbaceous Model transfers to HERB MODULE
after decompositionof litter biomass,and the Nfixation
processof rhizobium
bacteria
with soil modelin the with ais one input parameterof DRIPVEM.
This
is a newtechniqueto couple
the DRIPVEM
floodplain.
The
outputsof
DRIPVEM
werevalidatedforJapanesesteep
rivers[35].
Dynamic Herbaceous Model to describe the vegetation growth in riparian zone (Figure 4).

Figure 4. Schematic structure of Dynamic Riparian Vegetation Model (DRIPVEM) coupled with a
Figure 4. Schematic structure of Dynamic Riparian Vegetation Model (DRIPVEM) coupled with a
Dynamic Herbaceous Model.
Dynamic Herbaceous Model.

3. Results
In the HERB module of DRIPVEM, the maximum herb biomass is one of the input parameters.
The
historicaldata
monthlymaximumlevels(1969–2014)
in Kuzuryu
River study
In the absenceof observedof maximum
herbfloodbiomassin floodplain,a Dynamic
Herbaceous
Modelreach isin Figure5a (http://www1.river.go.jp/).
riparian area
is divided
into
× 10 m meshes
is usedshownto calculate
the maximum
herbs biomass in theThe
floodplain.
The output
value
of 10
maximum
havingelevations
rangesfrom0(normalwaterlevel)
to
7.33
m.
The
upstream
riparianzone
biomass simulated by the Dynamic Herbaceous Model transfers to HERB MODULE which isoneconsistsof the highest
elevations
andismore
50% riparianarea
low elevations
from 0.1 to 3
input parameterof DRIPVEM.
This
a newthantechniqueto couple
thecomposed
DRIPVEMofmodelwith a Dynamicm(Figure5b).
Herbaceous Model to describe the vegetation growth in riparian zone (Figure 4).
3. Results
The historical monthly maximum flood levels (1969–2014) in Kuzuryu River study reach is shownin Figure 5a (http://www1.river.go.jp/). The riparian area is divided into 10 × 10 m meshes having
elevations ranges from 0 (normal water level) to 7.33 m. The upstream riparian zone consists of thehighest elevations and more than 50% riparian area composed of low elevations from 0.1 to 3 m(Figure 5b).

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(b)
(b)

(a)
(a)

Figure 5. (a) Historical flood level (b) Floodplain elevation.
FigureFigure5.5.(a)(a)Historical
Historicalflood
floodlevel
level(b)(b)Floodplain
Floodplainelevation.
elevation.

3.1. Seasonal Growth of Herbaceous Plants
3.1.
GrowthofofHerbaceous
Plants
3.1.Seasonal
Seasonal
GrowthHerbaceousThemonthlymean
observed Plants
AGB and BGB of P. japonica on the floodplain in sandy habitat isThemonthlymean
observed
AGB
and
BGB
of
floodplain
habitatisis
higherthan
gravelly
habitat(Figure6a,b).
The
morphology
of on
this
plant
such as inplant
densityandThe
monthlymean
observed
AGB
and
BGB
ofP.
P.japonica
japonicaonthe
thefloodplain
insandy
sandy
habitathigherthan
gravelly
habitat(Figure6a,b).
The
this
plant
such
densityandheightplant
are higher
in sandy
soil
compared
to gravellyof
soil.
and
sediment
higherof
thangravelly
habitat(Figure6a,b).
Themorphology
morphology
ofthisFloodplantdisturbancesuchasas plant
plant
densityandheightofplant
arehigher
in
sandy
soil
comparedtogravelly
soil.
Flood
disturbanceandsediment
interactionin theareriparianalter theconditions
the riparianzone suchthat theheight of planthigherzone
in sandy
soilmorphologicalcompared to gravellysoil.ofFlooddisturbanceand sedimentinteraction
in
alter
the
conditions
vegetation
changeszone
in thefloodplain.
The monthly
mean ofobserved
AGBzonewas suchsignificantlyinteractiongrowthinthe
theriparian
riparianzone
alter
themorphologicalmorphologicalconditions
ofthe
theriparian
riparianzone
suchthat
thatthethevegetation
changesin
The
AGB
significantly
higherthangrowthat the gravelly(twosample t-test,< 0.05). mean
Similarly,
the monthly
mean
observed
vegetation
growthchangessite
in thethe floodplain.
floodplain.
Thepmonthly
monthlymean observed
observed
AGB waswassignificantly
higherthesite<<0.05).
Similarly,
the
mean
BGB
at thansandy
was significantly
higherthan
gravellysite (twosample< 0.05).
higherthanatatsitesthegravelly
gravellysite(two(twosample
samplet-test,
t-test,atppthe0.05).
Similarly,
themonthly
monthlyt-test,
meanpobserved
observed−2 at at
BGB
at
sandy
sites
was
significantly
higherthan
thegravellysite(twosamplet-test,p<0.05).
The
observedmaximum
AGB
was
1406
gm
0.13
mm
sediment
grain
size(sandysite)
whereasBGB at sandy sites was significantly higher than at the gravelly site (two sample t-test, p < 0.05).TheThe−2−2 at wasobservedmaximum
mm
grain
size
whereasits
its
value was
400 gmAGB
mm gm
sediment
grain
size(gravellysite).
Similarly,
theobserved
BGB
observedmaximum
AGB 15.2
was14061406
gm−2atat0.130.13
mmsedimentsedimentgrainsize(sandy(sandysite)
site)
whereasitsvalue
value
−2
was
400
15.2
sediment
grainsite).
the1147
wasgm−2−2at
themm
sandy
site and808 sizegm
in the gravellysite respectively.
The BGBAGB
is easily
was1147
400gm
gm
atin
15.2
mm
sedimentgrainsize−2(gravelly(gravellysite).Similarly,
Similarly,
theobserved
observed
BGBwas
was
1147
−2
−2
gm
in
the
sandy
site
and
808
gm
in
the
gravelly
site
respectively.
The
AGB
is
easilyremovedsubject−2−2
removedsubject
floodsandgm
thereafter
the photosynthesis
is curtailed
throughout
the remaining
gm in thesandytosite
and 808in the gravellysite respectively.
The AGBis easily removed
subjecttoandthephotosynthesis
isiscurtailed
growthofgrowthperiodsof the year,
althoughthe secondary
shootsthroughout
grow if the theflood
occurs earlyin theperiods
growing
tofloods
floodsandthereafter
thereafterthephotosynthesis
curtailedthroughout
theremaining
remaining
growthperiods
of
−2
the
year,
althoughthesecondaryshoots
grow
if
the
flood
occursearly
inthegrowingseason[48,49].
season[48,49].
The
Dynamic
Herbaceous
Modelpredicted
thatthemaximum
AGBs
were
1347
gm
the year, although the secondary shoots grow if the flood occurs early in the growing season [48,49].
−2 at the sandy site
−2 at theThe
Herbaceous
Modelpredicted
thatthemaximum1347
attheDynamic
sandy site
and 462 gm
gravelly
site.
Similarly,
theAGBs
modelwerepredicted
the−2maximum
BGBs
The
Dynamic
Herbaceous
Modelpredicted
thatthemaximum
AGBs
were
1347gmgm
at the sandy
site
−2−2
−2
and
at
site.
Similarly,
thepredicted
maximum
BGBs
were
853gm
thegravelly
sandy site
and541 gm−2
atmodelthe gravelly
site.theThe
comparisonof wereobserved
andand462462
gm−2
atatthethegravelly
site.
Similarly,
the
model
predicted
themaximum
BGBs
were853
853gm
gm−2
−2
2 = 0.92)
2 = 0.85)site.
2 = 0.51) and
at
site
at
The
observed
simulated
AGB(Rand
and−2BGB(Rgravelly
at
sandy
soil
and AGBofand
BGB(R2 = AGB
0.66)
atthe
thesandy
sandy
site
and541
541gm
gm
atthe
the
gravelly
site.
Thecomparison
comparisonof(Robserved
andsimulated
simulated
AGB
2 = 0.92) and BGB (R2 = 0.85) at sandy soil and AGB (R2 = 0.51) and BGB (R2 = 0.66) at gravelly soil
(R
2
2
2
2
at(Rgravelly
Japan)
are shownin Figure6a,b.
= 0.92) soiland(Kumagaye,
BGB (R = 0.85)
at sandy
soil and
AGB (R= 0.51) and BGB (R = 0.66) at gravelly soil
(Kumagaye,(Kumagaye,Japan)
Japan)areareshown
shownin
inFigure
Figure6a,b.
6a,b.
(a)
(a)

(b)
(b)
1400
1400

10000
10000

1200
1200

1000
1000
800
800

-2
BGB,g gmm
-2
BGB,

-2
AGB,g gmm
-2
AGB,

1000
1000
100
100

600
600
400
400

10
10
1
1

200
200

0
0
A M J
A M J

J

J

A S O N D
A MonthS O N D
Month

J

J

F M A
F M A

A M J
A M J

J

J

A S O N D
A SMonth
O N D
Month

J

J

F M A
F M A

Figure6. Observed andsimulated biomass
Phragmites japonica
at different habitats.
(a) AbovegroundFigureFigure6.6.Observed
Observedandandsimulated
simulatedbiomass
biomassPhragmites
Phragmitesjaponica
japonicaatatdifferent
differenthabitats.
habitats.(a)(a)Aboveground
Abovegroundbiomass
(AGB)
and
(b)
belowgroundbiomass(BGB).
biomass
(AGB)
and
(b)
belowgroundbiomass(BGB).
biomass (AGB) and (b) belowground biomass (BGB).

3.2. Historical Evidence of Growth of Herbs and Trees in the Floodplain
3.2.
3.2.Historical
HistoricalEvidence
EvidenceofofGrowth
GrowthofofHerbs
Herbsand
andTrees
Treesininthe
theFloodplain
Floodplain
Figure 7 shows the historical aerial images in chronological sequence of development of vegetation
Figure7shows
thehistoricalaerialimagesinchronologicalsequenceofof
Figure7 shows
thestudy
historicalimagesin chronologicalsequenceof development
developmentofgrowthin Kuzuryu
river
reach aerial
from 1982
to 2018.
From the historicalimageryobservations,vegetation
growthin
Kuzuryu
river
study
reach
from
1982
to
2018.
From
the
historicalimagery
vegetation growth in Kuzuryu river study reach from 1982 to 2018. From the historical imagery
observations,observations,one
onecancannotice
noticethat
thatthethesedimentsedimentdeposition
depositionarea
areadecreased
decreasedandandriparian
riparianvegetation
vegetationarea
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Water 2020, 12, x FOR PEER REVIEW

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one can notice that the sediment deposition area decreased and riparian vegetation area increased
year by year. A major flood of 2144 m3 s−1 witha flood level of 6.82 m occurred in September, 1989.
increased year by year. A major flood of 2144 m3s−1 with a flood level of 6.82 m occurred in September,This major flood changed the morphology of river and increased the riparian zone coverage rapidly.
1989. This major flood changed the morphology of river 3andincreased the riparian zone coverage
After 9 years, another major flood of discharge 2517 m s−13 witha flood level of 6.9 m occurred
rapidly. After 9 years, another major flood of discharge 2517 m s−1 witha flood level of 6.9 m occurredin September, 1998. Similarly, a high flood of discharge 2400 m33 s−1−1 with a 6.24 m flood level wasin September, 1998. Similarly, a high flood of discharge 2400 m s with a 6.24 m flood levelwas
encountered in the study reach in October 2002 and a highest flood of discharge 3221 m3 s−1 withencountered in the study reach in October 2002 and a highest flood of discharge 3221 m3s−1 withinundation level of 7.54 was encountered in October 2004 (Figure 5a). These major floods changed theinundation level of 7.54 was encountered in October 2004 (Figure 5a). These major floods changed
morphology of the floodplain.
the morphology of the floodplain.

Figure7.7.Historical
Historicalaerial
aerialimages
imagesininchronologicalchronologicalsequence
sequenceillustrating
illustratingvegetation
vegetationgrowth
growthininKuzuryu
Kuzuryu
FigureRiver,
Japan:
(a)
1982,
(b)
1994,
(c)
2004,
(d)
2018
(Source:
Geographical
Information
AuthorityofJapanJapan
River, Japan: (a) 1982, (b) 1994, (c) 2004, (d) 2018 (Source: Geographical Information Authority ofhttps://mapps.gsi.go.jp/maplibSearch.do#1 andand Google
Google Earth
Earth 2018).
2018).
https://mapps.gsi.go.jp/maplibSearch.do#1

3.3. Prediction of Vegetation Growth by Using DRIPVEM Coupled with Dynamic Herbaceous Plants Model
3.3. Prediction of Vegetation Growth by Using DRIPVEM Coupled with Dynamic Herbaceous Plants Model
Figure 8a,b shows the simulated spatial distribution of herb biomass on the floodplain after
Figure 8a,b shows the simulated spatial distribution of herb biomass on the floodplain after 49
49 years of simulation in sandy (D50 = 1 mm) and stony (D50 = 50 mm) sites. After large flood, nearlyyears of simulation in sandy (D50 = 1 mm) and stony (D50 = 50 mm) sites. After large flood, nearly all
all areas of the riparian zone were inundated except for a few high elevated areas whereby they retain
areas of the riparian zone were inundated except for a few high elevated areas whereby they retain
only herb biomass as well as the soil total nitrogen (TN). Herbs were easily washed away and soil TN
only herb biomass as well as the soil total nitrogen (TN). Herbs were easily washed away and soil TN
concentration also decreases in flood affected areas [8]. After completing the lifecycle of herbaceousconcentration also decreases in flood affected areas [8]. After completing the lifecycle of herbaceous
plants, the AGB undergo senescence and decompose so that nutrients were released in to the floodplain.
plants, the AGB undergo senescence and decompose so that nutrients were released in to the
The retained nutrients were used for the next growing seasons for herbaceous plants and createsfloodplain. The retained nutrients were used for the next growing seasons for herbaceous plants andfavorable conditions for tree growth. The DRIPVEM results after 49 years of simulation showed thatcreates favorable conditions for tree growth. The DRIPVEM results after 49 years ofsimulation
the spatial distribution of herbs biomass in the sandy site varied from 657 to 1124 g m−2 whereas inshowed that the spatial distribution of−2herbs biomass in the sandy site varied from 657 to 1124 g m−2
stony site varied from 205 to 400 g m . Out of 8639 meshes, 7661 meshes are covered with herbs inwhereas in stony site varied from 205 to 400 g m−2. Out of 8639 meshes, 7661 meshes 2are covered with2018 and the DRIPVEM generated 4224 meshes were covered with herb biomass (R = 0.55).
herbs in 2018 and the DRIPVEM generated 4224 meshes were covered with herb biomass (R2 = 0.55).
Figure 9a shows the simulated density wise spatial distribution of trees on the floodplain after
Figure 9a shows the simulated density wise spatial distribution of trees on the floodplain after
49 years of simulation in the sandy site. The density wise distribution of trees varied from 0.02 to
49 years of simulationin the sandy site. The density wise distribution of trees varied from 0.02 to 0.53
0.53 Nos. m−2 consisting of different ages of trees varying from 1 to 18 years old (Figure 9b). Out of−2
Nos. m consisting of different ages of trees varying from 1 to 18 years old (Figure 9b). Out of 8639
8639 meshes, 7217 meshes are covered with trees in 2018 and the DRIPVEM calculated 4224 meshes
meshes, 7217 meshes are covered
with trees in 2018 and the DRIPVEM calculated 4224 meshes were
were covered with trees (R2 = 0.65). The discrepancies were expected due to the external factors,
covered with trees (R2 = 0.65). The discrepancies were expected due to the external factors, forfor example hydraulic factors, temperature, and environmental factors change after flood events.
example hydraulic factors, temperature, and environmental factors change after flood events.

Water 2020, 12, x FOR PEER REVIEW(a) 12, 77
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2020,
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(b)

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99 of

(b)

Figure 8. Simulated spatial distribution of herbs in floodplain. (a) D50 = 1 mm size (b) D50 = 50 mm
size.
Figure 8. Simulated spatial distribution of herbs in floodplain. (a) D50 = 1 mm size (b) D50 = 50 mm size.
Figure 8. Simulated spatial distribution of herbs in floodplain. (a) D50 = 1 mm size (b) D50 = 50 mm
(a)
(b)
size.

(a)

(b)

Figure 9. Simulated spatial distribution of trees in the sandy floodplain. (a) Density wise distribution
FigureSimulated
spatial distribution of trees in the sandy floodplain. (a) Density wise distributionand(b) 9.
age
wise distribution.
and (b) age wise distribution.

4. Discussion

Figure 9. Simulated spatial distribution of trees in the sandy floodplain. (a) Density wise distribution

4. Discussion
4.1. Growthof Herbaceous
Plants in the Riparian Zone
and (b) age
wise distribution.

4.1. Growthof Herbaceous
PlantsModelin the RiparianZone
The Dynamic
Herbaceous
calculated
the net growth of the plant stand is the integral effect
4. Discussionof photosynthesis,
and translocationof assimilatedmaterials
shootseffectandThe Dynamicrespiration,
Herbaceousmortality,
Model calculated
the net growthof the plant
standbetween
is the integral
belowgroundorgans[5,44,50,51].
The
transfer
betweenthe
abovegroundandbelowgroundbiomassin
4.1.
Growth of Herbaceous
Plants in
the Riparianof photosynthesis,
respiration,mortality,
andZonetranslocation of assimilated materials between shoots
theof perennialorgans
plants [5,44,50,51].
differentiatesThetheirmorphology
dependingon their plasticity[48,52,53].
andcasebelowgroundtransfer
betweenthe abovegroundand belowground
The Dynamic
Herbaceous
Model incalculated
the net growthof the plant
stand
is of
theplants
integraleffectThe
moistureandnutrients
limitationcoarsesediment
sites
decreases
thegrowthwhereasbiomass in the case of perennial plants differentiates their morphology depending on their plasticityofphotosynthesis,
respiration,mortality,andtranslocationof assimilatedmaterials
betweenshootstheavailableThehighermoisture
nutrients
in sandy
sites
increases
the growthof herbaceous
plantsso[48,52,53].
moisture
andandnutrients
limitationincoarsesedimentsites decreases
the growthofandbelowgroundorgans[5,44,50,51].
The
transfer
betweenthe
abovegroundandbelowground
that
the
total
biomassinfine
sediment
was
more
than
coarsesediment.
plants whereas the available higher moisture and nutrients in sandy sites increases the growth ofbiomassthe case of perennial
plants differentiates
theirdependingon their ofplasticity
Thein
abovegroundparttheof herbaceous
plantsbeginsin morphology
thewasmidmorespringwiththe expense
herbaceous
plants so that
total biomass
in finesediment
thancoarsesediment. reserve[48,52,53].
The
and nutrients[54].
limitationin coarse
sediment sites
decreases
growthofrhizomesbythemoisture
upwardparttranslocation
This upward
translocation
continues
upexpenseto theearly
The abovegroundof herbaceous plants
begins in
the mid springwith theofsummerreserveplantswhereastheavailablehighermoistureandnutrients
in
sandy
sites
increases
thegrowthofandrhizomedecreases
so that [54].
abovegroundbiomass
continuously
increases.
rhizomesby biomass
the upward
translocation
This upward
translocation
continues
up toWhen
early upward
summerherbaceous
plantsso thatthetotal biomass
in fine sediment wasmore starts
than coarse
sediment.
translocation
was
near
the
saturation
stage,
photosynthesizedmaterial
downwardtranslocation
and rhizome biomass decreases so that aboveground biomass continuously increases. When
upwardThe abovegroundpartof herbaceous
plantsAdditionally,
begins in thebiomass
mid springwith theexpenseofreservetorhizomes
reservingfornext
growingseason.
ofshootscontinues
to
grow
and
translocation was near the saturation stage, photosynthesized material starts downward
rhizomesbytheupwardtranslocation
[54].
This
upwardtranslocation
continues
up
to
early
summermaximumAGB
occursatautumn[55](Figure7a,b).
The
aboveground
parts
die
or
are
removedbythetranslocation to rhizomes reserving for next growing season. Additionally, biomass of shoots
andrhizomebiomassdecreases production
so that abovegroundbiomass continuously
increases.
When
upward
flood
flow.
The
photosynthesisishighlycurtailedduringtheflooding
periodintheyear,continues to grow and maximum AGB occurs at autumn [55] (Figure 7a,b). The aboveground partstranslocation
was nearthesaturation
stage,
material
startsThe
downward
althoughshootsgrow
if theThefloodoccursphotosynthesized
early inproduction
the growing
seasoncurtailed[49].
sediment
die or aresecondary
removed by
the floodflow.
photosynthesis
is highly
duringthetranslocation
torhizomes
reservingfornext
growingseason.
Additionally,
biomassof shoots
trapping
behaviorofherbaceous
plantsdeposited
sediments
inthe
upstreamofherbaceous
plantsandflooding period in the year, although secondary shoots grow if the flood occurs early in the growing
continues
to grow
andthemaximumAGB
occursat autumn[55] (Figure7a,b).
parts
scour
behindvegetation
withchanging
velocity
profile[56].The
Theabovegroundsediment
trappedseasonformed[49]. The
sediment
trapping
behaviorof flowherbaceous
plantsdeposited
sediments
in thedieorareremoved
by
the
flood
flow.
The
photosynthesis
productionishighlycurtailedduringthezone
aroundherbaceous
plantsin thefloodplain
contains
higher nutrients.
Herbaceous
plants
upstreamof theherbaceous
plantsand[57]scour
formedbehindthe vegetation
with changing
flow velocity
flooding
periodintheyear,
althoughsecondaryshoots
grow
if
the
flood
occursearly
inthegrowingsubjected
to Theannualfloods,trapped
therefore,
graduallyshrinks
year after
year,
particularly
if floods
occur
profile [56].
sedimentzone
around theherbaceous
plants[57]
in the floodplain
containsseason[49].
The
sediment
trapping
behaviorofherbaceous
plantsdeposited
sedimentsinthewhen the plants are in active growth stage.
upstream of herbaceous plants and scour formed behind the vegetation with changing flow velocity
profile [56]. The sediment trapped zone around the herbaceous plants [57] in the floodplain contains

Water 2020, 12, 77

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The flushing of suspended sediments through dams also transported high nutrients into thedownstream area [58]. These interactive relationships between flooded water, erosion or depositionarea, sediment, and nutrients contributed in creating riparian zones more fertile [28], which created thefavorable condition for rapid growth of riparian vegetation.
4.2. Growth of Trees in the Riparian Zone
The substrate texture of floodplain changes after flood flows. Thus, in case of low flow disturbanceand shallow inundation, the substrate texture changes from coarse to fine, as fine sediment deposits on
the top surface of floodplain [59] such that the growth of herbaceous plants are enhanced. Subjected tohigh flow disturbance and deep inundation, existing fine sediments of top surface of floodplain
wash away exposing the underlying coarse textures on the floodplain, and suppresses the growth of
herbaceous plants. Atmospheric nitrogen, nutrients release after decomposition of AGB and BGB partsof herbaceous plants [60] also supports to increase nutrients in the riparian zone, creates favorable
conditions for trees growth. The vegetation was colonized after the large floods, therefore gradually
covered the floodplain with trees year after year. Finally, the whole sediment deposited area wascovered with herbs and mature trees.
DRIPVEM coupled with a Dynamic Herbaceous Model considered most of the physical, chemical,
and biological interactive parameters, for examples flood disturbance, riparian morphology, mediansediment sizes (D50 ), erosion depth, deposition areas, allometric relationships of trees growth, shading
effects of trees canopy, self-thinning phenomenon, availability of nutrients in water, and sediment withriparian zone, etc. The simulated results of spatial distribution of herbs (R2 = 0.55) and trees (R2 = 0.65)
by using the coupled DRIPVEM tool have an acceptable agreement.
5. Conclusions
The interactive parameters associated in fluvial flow, erosion and deposition area of floodplain,and nutrient availability support riparian vegetation growth in the floodplain. The long-term riparianvegetation growth in the floodplain is quantified by using mathematical models. The Dynamic
Herbaceous Model can predict the maximum biomass of herbs in the floodplain, which links to
DRIPVEM as an input part. The DRIPVEM coupled with the Dynamic Herbaceous Model can simulate
the spatial distribution of herbs and trees in respect of biomass as well as age-wise distribution of trees in
the floodplain. The potential use of this study is to predict the long-term riparian vegetation changes infloodplain. The model could be used in mixed forests in different scenarios by incorporating allometricobservations of different species wise growth of trees and availability of water and sediment withnutrients on floodplains with elevation wise sediment grain sizes (D50 ) and morphology of riparianzones. This model could be very useful tool for long-term forest planning, resources management anddecision making.
Author Contributions: Conceptualization and methodology, M.B.B. and T.A.; software, validation, formal analysis
and writing—original draft preparation, M.B.B.; writing—review and editing M.B.B., T.A., T.F., S.M.D.H.J. andG.M.; supervision T.A.; data curation M.B.B., T.A., G.M. and J.L. All authors have read and agreed to the publishedversion of the manuscript.
Funding: This research was supported by Grant-in-Aid for Scientific Research (B) (18KK0116) JSPS funding.
Acknowledgments: The authors would like to give thanks to Saitama University for managing the research
environment. We would also like to thank to Ministry of Land, Infrastructure, Transport and Tourism, Japanfor accessing historical fluvial, flood level, water quality, and climate data and also would like to give thanks to
Geographical Information Authority of Japan for sequential historical images.
Conflicts of Interest: The authors declare no conflict of interest.

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