Advances in River Sediment Research – Fukuoka et al. (eds)© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00062-9

Topographic changes of tidal flats in the Ota River estuary
by flood flows
T. Gotoh & S. Fukuoka

Research and Development Initiative, Chuo University, Bunkyo, Tokyo, Japan

Y. Miyagawa

Director of River Office, Otagawa River Office, MLIT, Hiroshima, Japan

ABSTRACT: In the Ota River estuary, there are tidal flats along the riverside in the Ota River floodway.
Since emergency transportation road along the riverside is to be constructed on the tidal flats, a part of thetidal flats will be lost by the road constructions. Therefore, understanding bed variations on the tidal flatsby flood flows is important for minimizing loss of the tidal flats. So, we investigated changes over time in
bed profiles of tidal flats by using surveying data. The observed data indicates that the tidal flat elevations
become lower due to alternate bar movements and a series of flood events. We estimated effects of floods
and the road constructions on temporal and spatial changes in bed profiles of the tidal flats by applying numerical model for flood flows and bed variations. Finally, this study gives the concept of riverside
design to preserve tidal flats in the floodway.
1

INTRODUCTION

In the Ota River estuary, the Ota River floodway
and other five branched rivers compose a channel network on the Ota River delta in Hiroshima
City (Fig. 1). The floodway directly connects to theHiroshima bay where the tidal level changes are
relatively large (Maximum difference of tidal levels
is about 4 m). The construction of the Ota Riverfloodway for protecting Hiroshima City fromflood disasters was begun in 1934 and finished in1967 after 2 years suspension by the IInd World
War. Nowadays, the Ota River floodway has tidal
flats along the riverside which provides habitats formany creatures in brackish rivers.
However, since there is a plan to construct anemergency transportation road along the riverside to prepare for an earthquake disaster, a partof the tidal flats will be lost by the road construction. Therefore, a project to restore and preserve
the ecosystem of estuarine tidal flats is now inprogress. In the project, an artificial tidal flat was
already constructed on the riverside of the leftbank at 0.1 km.
For the construction of emergency transportation road along the riverside, it is necessary toevaluate the effects of the road constructions on
temporal and spatial changes in bed profiles of thetidal flats in the Ota River floodway.
The first objective of this study is to investigatechanges in the bed profiles of the tidal flats since

the floodway had constructed. The second objective is to estimate the effects of the emergency
transportation road constructions on temporalchanges in bed profiles of the tidal flats along theriverside by developing the numerical model for

Figure 1. Air photograph of the Ota River delta in 2007
and July 2010’s flood observation system.

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flood flows and bed. This study’s final goal is to
give the concept for riverside design to preserve thetidal flats in the Ota River floodway.
2

2.1

OBSERVED TEMPORAL CHANGESIN BED PROFILES IN THE OTA
RIVER FLOODWAY
Characteristics of the Ota River floodway

Figure 1 shows the air photograph of the Ota Riverdelta in 2007 and observation system in July 2010’s
flood. The Ota River bifurcates to the Ota Riverfloodway and the Kyu Ota River at 6.0 km. And,
the Kyu Ota River divides into other four branchedrivers which are the Kyo-basi River, Temma River,Motoyasu River and Enkou River. These rivers compose the channel network on the Ota River delta.
At the bifurcation section of the Ota Riverfloodway and the Kyu-Ota River, Gion Weir islocated at the side of the Ota River floodway andconsists of a movable weir with three gates. These
gates are fully opened in a flood period. Oshiba
Weir is located at the side of the Kyu-Ota Riverand consists of a fixed weir and a movable weirwith the three gates. Oshiba Weir controls inflow
discharge of flood flows to the Kyu Ota River by
operating the three gates.
The Ota River floodway is composed of compound cross-sectional channel from 5.8 km to
0.2 km and simple cross-sectional channel from
0.2 km to −3.4 km. The channel widths of thefloodway are about 400 m.
Figure 2 shows the air photograph of the Ota
River delta in 1939 when the floodway was underconstruction. Around that time, there were extensive
tidal flats around the river mouth on the delta. Nowadays, since land reclamations had been conductedaround the river mouth until 1979, the tidal flats areremained only along the riverside of the Ota Riverfloodway and the five branched rivers. The Ota
River floodway was constructed by dredging of the
Fukushima River and by excavating of the Yamate
River (Fig. 1 & Fig. 2). Since the floodway were
designed as compound cross-sectional channels in
the first Ota River improvement planning which
was made in 1933, the main channel was dredgedat the sections downstream of 0.0 km (Fig. 3). But,
the bed near the riverside of the floodway was notdredged as shown Figure 3. So, the tidal flats areremained along the riverside in the downstream of
0.0 km in the Ota River floodway (Fig. 4).
In the sections upstream of 0.0 km, flood plains
are located at intertidal zone which are under waterat high tide and above the water at low tide. Therefore, the flood plains have important functions as
tidal flats (Fig. 5). The stability of the tidal flatsin this section against the erosion and scouring is
kept due to ripraps installed in front of the tidal
flats (Onuma et al. 2010).

Figure 3.

Figure 2. Air photograph of the Ota River delta in
1939.

Designed cross-sectional profile at 0.0 km.

Figure 4. Tidal flats along the riverside at the section ofdownstream of 0.0 km in the Ota River floodway.

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Around 2003, emergency transportation roads
were constructed along both riversides at the section of 0.2 km to 5.4 km in the floodway. Moreover, the emergency transportation road of the leftbank is planned to build an extension to −1.5 km

from 0.2 km (Fig. 1). So, a part of the tidal flats on
the riverside of the left bank will be lost by the constructions of the emergency transportation road.
Therefore, it is important to evaluate the effects ofthe emergency transportation road constructions
on the temporal changes in the bed profiles of thetidal flats.
2.2 Changes in bed profiles of the tidal flatsin the Ota River floodway since
the completion of the floodway

Figure 5. Tidal flats along the riverside at the section of
upstream of 0.0 km in the Ota River floodway.

Figure 6.

Figure 6 shows the contours of the observed bed
profiles in 1987, 1994, 2001, 2005 and 2010 in the
Ota River floodway. We define −1.6 (T.P.m) as thelower limit of tidal flats elevations on which clams
can inhabit. From this figure, it is found that thealternate bars with about 1 km wavelength areformed in the main channel of the Ota River floodway from these figures. The alternate bars move
about 50∼100 m to the downstream with keeping
stable bed profiles by flood flows. Since maximum

Contour of the observed bed profiles in the Ota River floodway.

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Figure 7. Cross-sectional profiles of the tidal flats.

elevation of the alternate bars is lower than low
tide level which is about −2.0 (T.P.m), the alternatebars are under water even at low tide. Tidal flats inthe downstream of 0.0 km are remained along theriverside.
Figure 7 shows the observed bed profilesof channel cross sections at −1.2 km, −0.8 km,−0.6 km and −0.2 km where pools of the alternatebars are located. The elevations of the tidal flats
along the riverside in the downstream of 0.0 km
tend to become lower over time by the flood flows.
These surveying data indicate that the bed scouring on the tidal flats is associated with the alternatebars movements. In the following paragraph, we
explain the above phenomena in detail.
From Figure 6(a), a pool of alternate bars wasformed at left bank around −0.4 km in 1987. After
that, the alternate bars moved to the downstream
by July 1993’s flood. In 2001 (Fig. 6(c)), the poolof alternate bars moved to around −0.6 km byJuly and Sep. 1999’s floods. Therefore, the tidal
flats around −0.6 km at the left bank lowered due
to 1999’s floods (Fig. 7(c)). Moreover, since thepool of the alternate bars moved to −0.8 km by
Sep. 2005’s flood as shown Figure 6(d), the bed
scouring of tidal flats developed to −0.8 km from
−0.6 km (Fig. 7(b)). Since the alternate bar elevations in the main channel are 1.0 m lower than the

tidal flats elevations, the most of bed material loadin the main channel can’t be transported to thetidal flats near the riverside. Therefore, once thetidal flats in the downstream of 0.0 km are scoured
by flood flows, the tidal flats will not be able tonaturally recover.
It is important to evaluate the effects of theemergency transportation road constructions on
temporal and spatial changes in bed profiles of
the tidal flats along the riverside. In the following
section, we develop the calculation method of bed
variations in the floodway with tidal flats by flood
flows.
3

BED VARIATIONS ON THE TIDALFLATS ALONG THE RIVERSIDE
IN THE OTA RIVER FLOODWAY

3.1

Calculation method for flood flows
and bed variations with tidal flatson the Ota River floodway

The Ota River floodway and other five branchedrivers compose the channel network on the Ota
River delta (Fig. 1). For understanding the bed
variations in the Ota River floodway with tidal
flats, it is necessary to evaluate the inflow dischargehydrograph which are boundary ­conditions of1340

upstream end of the floodway. Author et al. (2011)
have developed the calculation method for flood
flows and bed variations in the channel networkon the Ota River delta. In the channel network, it
is difficult to determine channel resistances anddischarge through each channel, since flood flows
in each channel have influence on each other. The
effects of channel resistance, flood discharge distributions and so on appear in the time series ofobserved water surface profiles (Fukuoka et al.
2004, Okamura et al. 2012). So, we determinedchannel resistances so as to agree with time seriesof observed water surface profiles. This calculation
method using the time series of observed water
surface profiles was able to evaluate flood flows,
bed variations and flood discharge distributions in
actual river conditions of the Ota River delta. In
order to understand temporal changes in the bed
profiles of the tidal flats along the riverside, thecalculation method which can accurately evaluate
flood flows and bed variations around the tidal
flats along the riverside is needed. Therefore, thecalculation method in this study has two kinds ofgrids which consist of coarse grids and fine grids.
The fine grids are used in order to evaluate theflood flows and bed variations around the tidal
flats along the riverside. The coarse grids are usedin order to evaluate the flood flows and bed variations in the channel network on the Ota Riverdelta, providing boundary conditions for fine gridregion.
To evaluate bed surface velocities is importantfor more accurate calculations of bed variations
around the tidal flats. So, we applied the unsteadyquasi three-dimensional flow analysis (Uchida &Fukuoka. 2011, Okamura et al. 2012) and 2D bed
variation analysis. The vertical velocity distributionsin this method assumed by a cubic curve are determined by using depth averaged velocity and difference between water surface and ­bottom ­velocities.
The shear stresses on the river bed and vegetations
resistance are calculated by ­equation (1) gn2 gh (τ swξ ,τ swη ) =  1 3 + 2  u 2 + v 2 (U ,V )K his calculated by the modified Egiazaroff formula(Egiazaroff, 1965; Ashida & Michiue, 1972). The
suspended load concentration was calculated bydepth averaged 2D convective diffusion equation.
Figure 8 shows the Water level hydrographs in2010’s flood. In this flood, the high tide occurred
about an hour earlier than the flood peak.
­Figure 9 shows the grain size distributions in thesection studied. The bed materials around 13.0 km
consist of many cobbles and gravel. On the other
hand, the bed materials in the Ota river floodway
and the branched rivers (Kyu Ota River, Temma
River, Motoyasu River) mainly consist of sands
while silt and clay deposits along the river bankin the branched rivers. Table 1 shows Manning’s
roughness coefficients and vegetation permeability
coefficients which were determined so as to satisfy
temporal observed water surface profiles (Author
et al. 2011). It is found that Manning’s roughness
coefficients contain the effects of topography suchas sand waves because the coefficients are larger
compared with those caused by bed materials. (e.g.
Manning’s roughness coefficients in the Temma
River is about 0.012 (m−1/3 ⋅ s) by using Strickler’s
empirical formula).

Figure 8.

Water level hydrographs in 2010’s flood.

(1)

where τ swξ ,τ swη : Channel flow resistances by river
bed roughness and vegetation in direction of ξ andη, n: Manning’s roughness coefficients, K : Vegetation permeability coefficients, U ,V : Velocities in
direction of ξ and η, u, v: Velocities in directionof x and y.
­
The bed variation analysis is used conventional
method which consists of bed load formula­(Ashida & Michiue, 1972) and continuity ­equations
for sediment and grain sizes (Hirano, 1971).
The critical tractive force for sediment mixtures

Figure 9. Grain size distributions in the channels of the
Ota river delta.

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Table 1. Manning’s roughness coefficients and vegetation permeability coefficients.
Ota River, Ota river floodway

Kyu Ota river

Manning’s roughness coefficients (m-1/3 ⋅ s)
12.4–5.8 km
Main channel
0.03
Flood channel
0.035
5.8–2.0 km
Main channel
0.028
Flood channel
0.035
Main channel
0.022
2.0–(-3.4 km)
Flood channel
0.035

6.2–2.2 km
2.2–(-1.6 km)
Kyobashi River
5.2–2.4 km
2.4–(-0.8 km)

Ota River

Kyu Ota River

Vegetation permeability coefficients (m/s)
8.0–9.6 km
Right bank40
10.4–10.8 km
Right bank50
10.8–11.4 km
Right bank40
8.0–8.8 km
Left bank50
9.6–10.6 km
Left bank50

6.2–5.0 km

Temma River
0.025
0.022
0.033
0.02

3.6–(-2.4 km)
Motoyasu River
2.6–(-2.4 km)
Enko River
2.6 km–(-3.1 km)

0.03
0.022
0.035

40

Figure 10. Comparison between observed and calculated water surface profiles in July 2010’s flood.

3.2

Bed variations in the Ota River floodway
with the tidal flats during July 2010’s flood

July 2010’s flood was middle scale flood for the
Ota River floodway. The flood peak discharge wasabout 4500 (m3/s) at Ygauchi-daiichi observation
station (the Ota River: 11.6 km). Author et al.
(2011) indicated that the discharge to the floodway was about 60% of the total discharge in thisflood.
Figure 10 shows the comparison between
observed and calculated water surface profiles. The
calculated water surface profiles are good agreements with observed water surface profiles. In thisflood, the water surface profiles around the rivermouth in flood receding period are steeper thanthem in flood rising period due to ebb tide.
Figure 11(a) shows the contour map of the initial bed profiles in the downstream of 0.0 km in the

Ota River floodway. The initial bed profiles weresurveyed in 2005. Figure 11(b) and Figure 11(c)show the observed and calculated contour mapsof the bed profiles after the flood. And, the blacklines in these figures show the edges of the tidal
flats after July 2010’s flood. The red lines show theedges of the tidal flats before this flood. The calculation results explain the observed movements ofalternate bars. From these figures, it is found thatthe bed scouring on the tidal flats along the riversides are induced by movements of the alternatebars. When the pools of alternate bars approachto the tidal flats on the riverside, the bed scouring on the tidal flats is induced by bed variations
around the pools. The above calculation results
demonstrate the characteristics of the observed
bed variation in the floodway with the tidal flats asshown in Figure 11(c).

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Figure 11. Comparison between observed and calculated contour of bed profiles in the Ota River floodway after
July 2010’s flood.

3.3 Estimation of effects of the emergency
transportation road constructions
on changes in bed profiles of the tidal flats
In this section, we estimate effects of the road constructions on the temporal and spatial changes in
bed profiles of the tidal flats when the major flood
events for past 30 years (Fig. 12) are repeated. The
estimations are conducted by using the previous
calculation method under various riverside conditions (Fig. 13). Each condition is as follows.
Case 0 is the present river conditions. Condition of Case 1 is that the emergency transportation road is constructed along the riverside of leftbank from 0.0 km to −1.5 km. In Case 2, ripraps
are installed in front of the tidal flats for protecting
the tidal flats against erosion and bed scouring as
shown Figure 13.
Figure 14 and Figure 15 show the calculated contour map of the bed profiles in Case 0 and Case 2,
respectively. The travel distance, wave length andwave height of alternate bar are similar in eachcase. At the downstream of the artificial tidal flat
(0.1 km), the sandbar which is above the water
surface at low tide is formed and moves to downstream. The leading edge of the sandbar in eachcase reaches to about −0.7 km by the flood flows(Fig. 14 & Fig. 15). Figure 15 shows the calculated
contour maps around the tidal flats after 8th flood
event of Case 2. The orange line shows the edge oftidal flats of Case 1. The black line shows that ofCase 2 which installs ripraps in front of the tidal
flats. It is found that the tidal flats area decreases
due to the bed scouring at around −0.8 km in

Figure 12. Boundary conditions of upstream end anddownstream end when major flood events for past 30
years are repeated.

Figure 13. Calculation conditions of riverside at theleft bank where the emergency transportation road isplanned to be constructed.

1343

Figure 14. Calculated contours of bed profiles of Case 0.

Figure 15. Calculated contours of bed profiles of Case 2.

Figure 16. Calculated contours of bed profiles around the tidal flats after 8th flood events of Case 2.

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flats in the actual river conditions of the Ota
River floodway. Moreover, we estimated effectsof the road constructions on changes in bed
profiles of the tidal flats by using the numericalmodel. As a result, it is found that the emergency
transportation road induces the bed scouring of
the tidal flats in front of the road. Finally, it isindicated that ripraps installed in front of thetidal flats along the river side is effective for preserving tidal flats against the erosion and bed
scouring by flood flows.
Figure 17. Calculated cross-sectional bed profiles
around the tidal flats at −0.8 km.

case that ripraps were not installed in front of
the tidal flats (Case 1). Figure 17 shows the calculated cross-sectional bed profiles near the riversideof left bank at −0.8 km. Height of the tidal flats
decreases over time in case that ripraps were notinstalled in front of the tidal flats along the riverside (Case 0 & Case 1). The emergency transportation road induces the bed scouring on the tidal
flats in front of the road (Case 1). On the other
hand, the tidal flats are remained by installing theripraps in front of the tidal flats along the riverside
as Case 2. Therefore, to install ripraps in front of
the tidal flats is effective for preservation of tidal
flats in the Ota River floodway.
4

Conclusions

The following conclusions were derived from inthis study.
1. The alternate bars with about 1 km wave length
are formed in main channel of the Ota Riverfloodway. Although the alternate bars areunder water even at low tide, the tidal flats areremained along the riverside in the floodway.
The surveying data indicates that height of thetidal flats along the riverside decreases due toalternate bars movements by a series of flood
events.
2. The numerical model for flood flows and bed
variations was developed to evaluate temporaland spatial changes in bed profiles of the tidal

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