Evaluation of the Flood Mitigation Effect of a Paddy Field Dam Project

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Evaluation of the flood mitigation effect of a Paddy Field Dam project

N. Yoshikawa a,*, N. Nagao b, S. Misawa c

a Research Center for Natural Hazard and Disaster Recovery, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japanb Toyama Prefectural Federation of Land Improvement Association, 17 Kurosaki, Toyama 939-8214, Japanc Faculty of Agriculture, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan

1. Introduction

Recent climatic change has brought about a significant increase

in the frequency of unusually heavy rain events in Asian monsoon

regions. According to the Fourth Assessment Report of the

Intergovernmental Panel on Climate Change (IPCC, 2007), it is

very likely that heavy rainfall events will continue to become more

frequent at middle and high latitudes. The Extreme Weather

Report 2005 written by the Japan Meteorological Agency (2005)

also reported on the basis of an analysis of the rainfall pattern for

104 recent years that the numbers of days with intense

precipitation exceeding 100 and 200 mm have increased 1.19

and 1.46 times, respectively, when comparing the first 30 years of

thelast century (19011930) with themost recent30 years (1975

2004).

There has been a recent increase in the risk of flood disaster in

Japan owing not only to the changes in frequency and intensity of

rainfall, but also to changes in land use and modernization of

agriculture leading to the expansion of impervious areas and

shortening of the arrival time of floods. Although investments have

been made in infrastructure for protection against flood damage

such as dam construction, bank heightening, river widening, and/

or channel dredging, further investment in such large-scale

structural measures is not realistic owing to increasing concern

about the negative impact on the natural environment, in addition

to the huge costs to the government.

To overcome these problems, making use of paddy fields as a

flood control system has been highlighted. The paddy fields

themselves are considered to possess an innate flood mitigation

function (Abler, 2004; Matsuno, 2006; Groenfeldt, 2006; Kimet al.,

2006; Huang et al., 2006). Various studies at national and regional

scales have been carried out to evaluate this flood control function.

Shimura (1982), for the first time, estimated the flood water

storage capacity of all paddy fields at 8.1 billion m3, which by far

exceeds 2.4 billion m3, the total flood detention capacity of flood

control dams in Japan. Regional cases have also been investigated

taking into consideration topographical features; for example, the

cases of terraced paddy fields in sloped areas (Onishi et al., 2004)

and paddy fields in low-lying flat areas (Nakamura et al., 1994;

Hiramatsu and Shikasho, 2001). Most of these regional case studies

concluded that paddy fields play an important role in increasing

the water storage capacity of river basins and lower the peak flows

of rivers to a certain extent, but not to the same degree as Shimura

(1982) estimated. Some studies found a significant rise in peak

Agricultural Water Management 97 (2010) 259270

A R T I C L E I N F O

Article history:

Received 21 January 2009

Received in revised form 24 September 2009Accepted 26 September 2009

Available online 25 October 2009

Keywords:

Paddy field

Runoff control devices

Flood control function

Unsteady flow model

Kinematic wave model

Water balance analysis

A B S T R A C T

To mitigate flood damage due to a recent increase in the frequency and magnitude of heavy rainfall

events, the Kamihayashi district in Niigata prefecture, Japan, has undertaken flood mitigation measures

using paddy fields by installingrunoff control devices in drainage boxes of paddy field plots.The purpose

of this study is to evaluate theflood mitigation performance of thePaddyField Dam project in terms of a

decrease in discharge volume, drop in channel water level and reduction of inundation damage using

combined hydrologic analyses and flood routing. The model constructed for runoff analysis is composed

of three modules: a hilly/residential area module in which the overland flow is estimated using the

kinematic wave method, a paddy field module in which runoff from paddy fields is calculated using

water balance analysis, and a channel network module in which flood routing is performed using a one-

dimensional unsteadyflow model.The outputs of thefirsttwo modules arethe input of thethirdmodule.

The result of the simulation shows the main channel discharge decreased by 26% and the water level

dropped by 0.17 m in the case of the largest observed rainfall event. The simulated effect was larger for

larger rainfall events. In terms of flood water volume, the runoff control devices have the effect of

reducing the flood damage due to the 50-year return period rainfall event to almost that due to the 10-

year return period rainfall event.

2009 Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +81 25 262 6653; fax: +81 25 262 6653.

E-mail address: [email protected](N. Yoshikawa).

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j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a g w a t

0378-3774/$ see front matter 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.agwat.2009.09.017
mailto:[email protected]://www.sciencedirect.com/science/journal/03783774http://dx.doi.org/10.1016/j.agwat.2009.09.017http://dx.doi.org/10.1016/j.agwat.2009.09.017http://www.sciencedirect.com/science/journal/03783774mailto:[email protected]


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runoff with the transformation of paddy fields to other land uses

such as upland dry fields or the abandonment of cultivation (Chiba

et al., 1997; Masumoto et al., 1997; Wu et al., 2001; Masumoto

et al., 2003). To standardize the evaluation of the flood controlfunction, Masumoto et al. (2006) proposed a method and an index

at a macro scale using the relationship between drainage and

storage capacity.

These studies have contributed to the understanding of the

flood mitigating role of paddy fields and suggest that haphazard

urbanization and abandonment of paddy cultivation may increase

the negative impact on therunoff characteristic. However,it seems

clear that mere conservation of the existing paddy fields will at

most maintain the status quo.

To address the recent increase in the flood risk, an unconven-

tional measure for flood mitigation has been recently introduced

using existing paddy fields with enhanced flood control function as

an inexpensive and environmentally sustainable technique. This

measure is termed the Paddy Field Dam, in which rain water isintentionally stored in the paddy fields temporarily at times of

intense rainfall by installing runoff control devices in the drainage

boxes of paddy field plots. Although paddy fields are not deep,

usually surrounded by a levee 1530 cm high, the areal extentover

which they spread is large and they therefore provide ample water

storage potential.

The purpose of this study is to evaluate the flood mitigation

performance of the Paddy Field Dam, paddy fields with runoff

control devices, using combined hydrologic analyses and flood

routing. The effect is presented by the changes in flow volume and

water level of thedrainagechannel andthe inundation volumeand

duration at the city center.

2. The Paddy Field Dam project

2.1. Description of the Paddy Field Dam project

The Paddy Field Dam is an experimental flood control measure

launched in 2002 in Kamihayashi village, Niigata prefecture, Japan,

takingadvantage of thepondingcharacteristics of paddyfields. The

paddy fields within theproject site areinstalled with runoffcontrol

devices in theform of orifice restriction platesto reinforcethe flood

control function that they innately possess. The reason for the

project being termed the Paddy Field Dam is that the paddy fields

with the reinforced function store excessive rainwater and thus

lowers the peak discharge just like ordinary flood control dams do,

even though it does not fall into the definition of a dam, strictly

speaking.

The basic structure of the paddy fields in the study area is

almost uniform in terms of shape and size as a result of the land

consolidation and reallocation work in the 1980s: 5000 m2 plots

equipped with two concrete drainage boxes, which are connectedto a drainage channel with a 150 mm inner diameter pipe. In

general, the water level in the paddy fields is maintained at

approximately 5 cm above the paddy field surface during the

ponding season by placing a rectangular weir at the inlet of

the drainage boxes just as for standard paddy fields in Japan. The

excess water then flows into the drainage channel along the paddy

field plots.

The runoff control devices are structurally simple; 300 mm 300 mm wooden orifice restriction plates with 50 mm orifices are

installed on the bottom of the drainage boxes, approximately

40 cm below the paddy field surface. This shrinks the drain orifice

of the boxes from the original pipe size of 15050 mm, reducing

the orifice area to one-ninth that of the original pipe as illustrated

in Fig. 1.One may worry about possible negative impacts on the rice

production due to the retardation of drainage. However, there may

be little damage to the rice crops unless they are completely

submerged for several days since rice is one of the most tolerant

crops against submergence (Inoue, 1999). Since the maximum

ponding depth inside the paddy fields is the height of the levees,

complete submergence occurs only when the water level of the

drainage channel adjacent to the paddy fields increases above the

height of the levees. It means that inundation would be

unavoidable whether or notthe runoff control devices are installed

because the effect of the Paddy Field Dam is rather to lower the

water level of the drainage channel by controlling the runoff from

paddy fields.

Although temporary complete submergence may occur whenthe top of the rice plant is still below the top of the levees of the

paddy fields, approximately 1 month after the transplantation in

the beginning of May in Japan, rice plants would already have

grown higher than the levees by the rainy season, which begins in

the middle of June.

2.2. Mechanism of surface runoff control for the Paddy Field Dam

When the runoff control devices are not installed during the

ponding season, the volume of surface water runoff from a paddy

field plot is determined only by the height and length of the

rectangular weir set at the inlet of a drainage box and the overflow

depth above the rectangular weir, which in turn determines the

ponding depth of the paddy field. On the other hand, in the case of

Fig. 1. Paddy field drainage system. Two drainage boxes are installed and equipped with a runoff control device for each 0.5 ha standard-size paddy field plot.

N. Yoshikawa et al. / Agricultural Water Management 97 (2010) 259270260


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runoff control devices being installed, the runoff is regulated by

either the rectangular weir or the orifice size of the runoff control

devices depending upon the water depth of thepaddyfield. That is,

the runoff control devices function only when the water inflow

from the paddy field to the drainage box exceeds the outflow

capacity of the orifice; otherwise, the surface runoff is regulated by

theoverflowing runoff above the rectangular weir regardless of the

existence of the devices. Therefore, the surface runoff QS (m3 s1)

can be expressed by

QS

RectangularWeir Equation

QW 2EbH03=2 2EbH D3=2 when QW QOOrifice Equation

QO 2CaOffiffiffiffiffiffiffiffiffiffi

2gh0

q 2CaO

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2gH d

qwhen QW>QO

8>>>>>:

(1)

where QWis the surface overflow runoffabovethe rectangular weir

(m3 s1), Eis the weir coefficient, b is the weir length (m), H is the

head above the top of the weir (m), H is the water depth of the

paddy field (m), D is the height of the weir (m), QO is the discharge

via the orifice (m3 s1), C is the orifice coefficient, aO is the area of

the orifice (m2), gis the gravitational acceleration (m s2), h0 is thevertical distance between the water surface of the paddy field and

the bottom of the drainage box (m), and d is the vertical distancebetween the paddy field surface and the bottom of the drainage

box (m) (Fig. 1). The number 2 in both equations indicates that

each standard-size paddy field is equipped with two drainage

boxes.

The regulating factors may differ during the season of

midsummer drainage, a common practice of paddy cultivation

in Japan in which paddy fields are dried out for about 10 days

between the end of June and the beginning of July to control

surplus tillering and supply plant roots with oxygen. In this period,

the rectangular weirs are removed to release all the ponded water

and several shallow furrows are excavated on the paddy field

surface toward the drainage boxes to promote quick drainage of

the remaining water. Since the soil dries up during this period, the

surface runoff begins as soon as the soil surface is saturated whenheavy rainfall occurs. In this case, the factors determining the

rainfallrunoff relations are more complex and need to be

quantified without using the rectangular weir equation, although

once the water inflow from the paddy field to the drainage box

exceeds the outflow capacity of the orifice, the orifice size of the

runoff control devices becomes the regulating factor. Because of

the variations in the density and depth of the furrows made on the

surfaces of paddy fields, the runoff characteristics may differ with

respect to each paddy field plot. Therefore, the average rainfall

runoff relations before the runoff control devices come into effect

need to be derived somehow. In this study, a tank model

(Sugawara, 1972) in which the runoff from the control devices

is incorporated is adopted for this purpose.

3. Materials and methods

3.1. Study area

The study was carried out in the Ishikawa River watershed,

Kamihayashi village, Niigata prefecture, Japan. The Ishikawa River

drains the entire 60.6 km2 watershed, which comprises hilly areas

(69%), paddy fields (26%) and residential areas (5%) (Fig. 2). The

Ishikawa River then flows into the Sea of Japan. The watershed can

be divided into two areas by drainage systems: an area of

gravitational drainage and an area of pumping drainage. The city

center of Kamihayashi village is situated in the lower part of the

watershed of the Fuefuki River (6.24 km2), one of the tributaries of

the Ishikawa River, which largely coincides with the gravitational

drainage area. Most of the remaining area coincides with the

pumping drainage area. After flowing through the city center, the

Fuefuki River joins the mainstream of the Ishikawa River. The city

center area (approximately0.099 km2) has an altitude only slightly

above sea level and is, therefore, subject to frequent flood water

damage from the upstream water as well as from backwater

caused by the rising of the Ishikawa River when intensive rainfall

events occur.

The project site comprising paddy fields (3.54 km2) and

residential areas (0.88 km2) is located within the Fuefuki River

watershed. Forthe purpose of this study,the inclusion of thewhole

Ishikawa River watershed in the analysis was necessary because

the water level and discharge of the Fuefuki River are significantly

influenced by the water level of the Ishikawa River, which is in part

determined by thetidalcondition of theSea of Japan.Therefore, the

Fuefuki River watershed is treated as an inner drainage basin and

the Ishikawa River watershed as an outer river basin because the

analysis for the latter was performed only to determine the water

level boundary condition of the Fuefuki River watershed.

3.2. Field measurements

The study period was 2 years between April 2006 and March

2008. The channel water level was continuously measured andrecorded at 10 min intervals at six important sites in the study

watershed using water level sensors (Hi-net,HTV-020KP) and data

loggers (HIOKI, 3635-55). Precipitation was also measured with a

tipping bucket rain gauge (Davis Instruments, Rain collector II) at

10 min intervals. The channel flow velocity was intensively

measured where the water level sensors were installed when

heavy rainfall events occurred.

3.3. Field experiment

To investigate the surface runoff characteristics of the paddy

fields with and without the runoff control devices, field experi-

ments using standard-size paddy field plots were implemented.

The paddy fields plots were intentionally ponded with irrigationwater as deep as possible by clogging the drainage outlets, and

then the ponded water was released from the paddy field plots

through the twodrainage boxes of each paddy field plot. The water

level was continuously recorded with the water level sensors and

data loggers.

In general, the output components of the water budget of a

paddy field plot include percolation and evapotranspiration

besides the surface runoff. However, evapotranspiration is not

taken into consideration in this study on the assumption that

evapotranspiration is negligible during heavy rainfall events.

Percolation was measured by filling the paddy field with water

with the drainage outlets clogged and recording the fall in the

water level at night. This experiment was implemented twice for

different water levels: 120 and 50 mm.

3.4. Observed rainfall event and design hyetographs

The only heavy rainfall event during the study period occurred

on the 29th of July 2007. Fig. 3 is the hyetograph of the observed

rainfall event. The total rainfall depth was 101.8 mm, the return

period of which is determined as being approximately equivalent

to 3.7 years by fitting the Gumbel distribution to the available 85-

year annual maximum rainfall data. Although the total depth does

not seem to be outstandingly large, about 80% of the rainfall was

concentrated within 8 h between 5.00 and 12.00 a.m.

To evaluate the flood mitigation function of the runoff control

devices forlarger rainfall events, the design storms for 10-year and

50-year return periods were generated using the alternating block

N. Yoshikawa et al. / Agricultural Water Management 97 (2010) 259270 261


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method from intensitydurationfrequency (IDF) curves (Chowet al., 1988) (Fig. 4). The IDF curves for the study area were

obtained from the Niigata prefectural government.

4. Model development

4.1. Watershed model and analytical framework

Runoff analyses were conducted to quantify the effect of the

runoff control devices in terms of the fall in water level and

decrease in discharge of channels. The model constructed for the

runoff analysis is composed of three modules: a paddy field

module in which runoff from the paddy fields is calculated by

water balance analyses, a hilly/residential area module in which

the overland flow is estimated using the kinematic wave method,

anda channel network module in which flood routing is performedusing a one-dimensional unsteady flow model. The outputs of the

first two modules are the input of the third module.

4.2. Paddy field module

Using the coefficients of the rectangular weir and orifice of the

runoff control devices determined through the field experiment,

the runoff from a paddy field plot is simulated by

I O dSdt

; (2)

where I is the rainfall inflow to the paddy field, O is the outflow

consisting of the runoff volumevia therectangularweirs or orifices

Fig. 2. Description of the study area.



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and the percolation loss, Sis the ponded volume and tis time. The

ponded volume can be approximated as

S1 S2 DH 12R1 R2Dt

1

2

QS1

QS2AP

12 qPER1 qPER2 Dt (3)where subscripts 1 and 2 denote the time steps of each variable at

t= t1 and t= t2, R is the rainfall depth (m), QS is the surface runoff

volume (m3 s1), qPER is the depth of the loss caused by percolation

(m) and AP is the area of the paddy field plot (m2).

In the case of the ponding season, the value of QS is calculated

using either the rectangular weir equation or the orifice equation

depending upon the ponded water depth whereas it is calculated

using the tank model with the runoff through the runoff control

devices as shown in Fig. 5 in the case of the midsummer drainage

season.

The constructed tank model reproduces the actual paddy field

structure, where aTis therunoff coefficient, qSrepresents the depth

of surface runoff from the study paddy field block (m), qPERrepresents the percolation depth (m), His the ponded water depth

(m) and HI is the initial ponded water depth (m). To offset the

individual specificity of the paddy fields, the model was applied to

the study paddy field block comprising 12 standard-size paddy

field plots, and the results are compared with the runoff observed

at the channel outlet of the study block. The parameter aT was

optimized using the least squares method to reproduce the

observed runoff. qS is always proportional to the ponded water

depth in the case without the runoff control devices. On the other

hand, the regulating factors of qS shift in the case with runoff

control devices depending upon the ponded water depth;that is, qSis regulated to be proportional to the ponded water depth when qT

(the runoff calculated using the tank model) qO (the runoffcalculated using theorifice equation)and determined by theorifice

of the runoff control devices when qT> qO. The value ofqPER is the

percolation depth observed in the field measurement.

4.3. Hilly/residential area module

4.3.1. Basic concept of a kinematic wave model

A kinematic wave model (Chow et al., 1988; Vieux et al., 1990;

Singh, 1996) was used to simulate the flow in the hilly and

residential areas. The kinematic wave equations can be written in

the form of the continuity equation:

@h

@t @QK

@x re (4)

and momentum equation:

QK ahm; (5)

where QKis the runoff volume (m3 s1), h is the flow depth (m), tistime (s), x is the down slope position, and re is effective rainfall

depth (m). a and m are the kinematic wave parameter and

exponent, respectively. Given that Mannings principle is satisfied,

a and m can respectively be written as

a ffiffi

Ip

n; (6)

m 53; (7)

where I is the longitudinal bed slope and n is the equivalent

roughness.

Fig. 3. Observed rainfall on the 29th of June 2007.Fig. 5. Tankmodel reproducing the paddyfield block.qTand qO represent qScalculated

by the tank model and by the orifice equation at an arbitrary ponded water depth,

respectively. The regulating factors switch depending on the ponded water depth.

Fig. 4. Design hyetographs for the 10-year (left) and 50-year (right) return periods generated using the alternating block method.



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4.3.2. Subcatchments of the hilly area

Since the hilly area comprises several subcatchments, the

runoff from each subcatchment eventually flows into different

channel sections as lateral inflow. To determine the destination of

runoff, the hilly area was divided into subcatchments using a

digital elevation model and a GIS (ESRI, ArcGIS 9.2).

The average longitudinal bed slope (I) of each subcatchment

was also calculated using the GIS. The equivalent roughness

coefficient (n) was determined on the basis of calibration usingsubcatchment A (Fig. 2), where discharge was continuously

measured and applied to all other subcatchments assuming the

surface roughness is similar throughout the hilly area of the study

watershed.

4.3.3. Subcatchments of residential areas

Residential areas including the city center were identified on

the basis of a plan of the drainage channel network obtained from

the local water management office. The average slope of each

district was determined using the digital elevation model. The

equivalent roughness coefficient was assumed to be 0.1, a common

hydraulic recommendation for residential areas (Japanese Ministry

of Construction, 1976) since no direct measurement for calibration

was taken.

4.4. Channel network module

4.4.1. Basic concept of the unsteady flow model

The basic governing equations of an unsteady open channel

flow are the continuity equation:

@aC@t

@QC@xC

QLAT (8)

and momentum equation:

1

g

@vC

@t @hC@xC

12g

@v2C

@xC @z@xC

n2Q2C

a2

C

R4=3 0; (9)

where aC is the cross-sectional area of flow (m2), QC is the flow

volume passing through the cross-section (m3 s1), QLAT is the

lateral inflow per unit length along the channel (m2 s1), t is time

(s), xC is the longitudinal distance along the channel (m), vC is the

average flow velocity (m s1), hC is the water level (m), R is

the hydraulic radius (m),gis gravitational acceleration (m s2),zis

the bottom elevation of the channel section and n is Mannings

roughness coefficient. The numerical scheme adopted to approx-

imate the solution of these partial differential equations was theexplicit finite difference method representing a central difference

approximation for both temporal and spatial derivatives.

4.4.2. Channel network system

The channel network system of the study area was modeled on

the basis of the map of the drainage channel network obtained

from the local water management office (Fig. 6). The total of 59

drainage channels within the study watershed comprised 200 m

longitudinal distance sections (dx), resulting in 299 sections being

created. A 200 m section collects floodwater as lateral inflow from

approximately 10 standard-size (40 m 125 m) paddy field plots.The bed slope of each channel section was determined by the

difference between the altitude of the section and that of one

section upstream, where the altitude information was obtainedfrom longitudinal profile drawings for the main channels and

Global Positioning System (GPS) survey results for small but

important channels. The cross-sectional geometry, approximated

by two trapezoids with a main channel and floodplain for

simplicity, was obtained from cross-sectional profile drawings

and surveyed cross-sections.

The values of Mannings roughness coefficient (n) were

calculated as composite values determined depending upon the

ratio of the wetted perimeter of compound channels and the

roughness coefficient, adopted from the literature as 0.015 for

the concrete flume, 0.03 for the natural main channel, and 0.05 for

the vegetation-covered flood plain (Dingman, 2002).

The lateral inflow (QLAT) was calculated as the sum of the inflow

from paddy fields (QP), hilly areas and residential areas (QK). In

Fig. 6. Description of the modeled stream network.Each channel sectioncontains information on thecross-sectional geometry andaltitude.The lateral inflows to the channel

sections are identified using the map of the drainage channel network and field surveys. The values of lateral inflow are determined by the paddy field module and hilly/

residential area module.



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addition, the rainfall directly falling on the river and channel was

treated as direct lateral inflow and calculated as the product of the

rainfall intensity, the width and the longitudinal distance reaches

of each river and channel section.

4.4.3. Time steps

A time step was set to satisfy the following criterion to avoid

divergence of the numerical calculation:

DtDxC

vmax ffiffiffiffiffiffiffiffiffiffiffiffiffighmax

p ; (10)where vmax (m s

1) and hmax (m) are the assumed maximum flow

velocity and maximum stage, respectively. The maximum values

obtained by the field measurement were vmax 1:6 m s1 andhmax = 1.8 m, and thus Dt= 34.5 s given Dx = 200 m. Although 30 s

would be a sufficiently small time step to satisfy the criteriongiven

above, assuming a larger rainfall event, Dt was set at 10 s.

4.4.4. Inverted siphon

There are two inverted siphons crossing a drainage channel

within the project site (Fig. 6). The discharge volume in the section

of the inverted siphon was calculated by estimating the friction

loss using the DarcyWeisbach equation with Mannings friction

coefficient:

vS2gDhS

2n2Sg=R1=3S l=RS

( )1=2; (11)

where vS is the flow velocity (m s1) in the inverted siphon, DhS is

the head difference between the entrance and exit of the inverted

siphon (m), nS is Mannings roughness coefficient, RS is the

hydraulic radius (m), and l is the length of the inverted siphon (m).

4.4.5. Drainage pumps

There are three pumping stations in the pumping drainage area

outside the project site. The discharge volume of the actual

pumping stations is dependent on thewaterlevel of thechannel. In

the simulation, pump discharge volumes were determined

according to the channel water levels that reproduced the actual

pumping operations as listed in Table 1.

4.4.6. Boundary conditions

The downstream boundary condition was defined by the

oceanic tide stage recorded by the tide gauge at the mouth of the

Ishikawa River. The upstream boundary conditions were set at the

most upstream channel section of each channel and given by the

lateral inflow calculated as outcomes of the hilly/residential area

module. In addition, the water levels of the channel sectionsadjacent to the pumping stations and the inverted siphon were

treated as boundary conditions.

4.4.7. Inundation depth in the city center

The 99,000 m2 city center area is subject to frequent inundation

damage (Fig. 2). The overflow of the channel water begins when

the water level exceeds 2.0 m, the depth of the shallowest channel

in the city. In the model, the channel depth was tentatively set as

greater than 2.0 m to calculate the overflow depth. The overflow

water volume was simply calculated as the channel stage

exceeding 2.0 m multiplied by the width and the flow velocity

of the channel section calculated in the unsteady flow analysis:

QOFt HCCt 2:0Bv

CCt; (12)where QOF is the overflow water volume (m

3 s1), HCC is the

channel water level with an imaginary channel height (m), B is

the channel width (3.5 m), vCC is the flow velocity (m s1). The

inundation depth was determined by dividing the accumulated

overflow water volume by the city center area, assuming the city

center area is perfectly flat and overflow water spreads over the

area instantaneously:

INUNt X QOFtDt

ACC; (13)

where INUNis the inundation depth (m), Dtis the time interval of

the simulation (10 s), and ACC is the area subject to inundation

damage (99,000 m2).

Table 1

Programs of pump operations in the study watersheds.

Pump 1 Pump 2 Pump 3

Water level Discharge Water level Discharge Water level Discharge

Start H>1.00 m 1.80 m3 s1 Start H>0.85 m 0.67 m3 s1 Start H>1.70 m 2.18 m3 s1

Stop H1.80 m 15.29 m3 s1

Stop H1.90 m 3.87 m3 s1

Stop H


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QOF can take a positive or negative value. As long as INUN is

greater than zero and QOF is negative, INUN decreases as flood

water flows downstream.

5. Results and discussion

5.1. Paddy field module

5.1.1. Percolation rate

As a result of the percolation measurement,the percolation rate

was found to be higher at the higher water level. The relationship

between the depth of the percolation loss (qPER) (m) and the water

level (H0) (m) can be expressed by

qPER 0:00154exp0:0164H0 (14)

5.1.2. Discharge coefficient of the orifice and rectangular weir

Fig. 7 shows the results of the surface runoff experiments.

Fig. 7(a) and (b) presents the ponded water depth changes with

time in the cases with and without the runoff control devices

where the regulating factors of the surface runoff are the orifices

and rectangular weirs, respectively. The best-fitting discharge

coefficient for the orifice was calculated as 0.88. Although this

value seems to be a little larger than the textbook value (in general,

C is approximately 0.6), the value includes leakage from the

inevitable slight gap between the runoff control device and the

floorof the drainage box. This gap is due tothe control device being

merely placed into the drainage box without the use of any

adhesives. The discharge coefficient of the rectangular weir, on the

other hand, was calculated as 1.90.

5.1.3. Runoff from a paddy field plot

The results of the runoff from a paddy field plot are shown in

Fig. 8. The runoff in the case of the ponding season was simulated

by merely applying the rectangular weir and orifice equations with

the identified coefficients (Fig. 8(a)).

For the analysis of the midsummer drainage season, the tank

modelparametersfor thesurface runoffhole (aT) andinitial surface

water depth (HI) were carefully adjusted by trial and error, and

Fig. 8. Calculated runoff per5000 m2 standard-size paddyfield plotwith andwithoutthe runoff control devicesresponding to the observed rainfallfor (a) thepondingseason

and (b) the midsummer drainage season.

Fig. 10. Calculated runoff per 5000 m2 standard-size paddy field with and without the runoff control devices responding to 10-year and 50-year return periods for (a) the

ponding season and (b) the midsummer drainage season.

Fig. 9. Comparison between the observed and calculated discharges from the 12

paddy fields without the runoff control devices.



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determined as 0.081 and 15.9 mm, respectively. The result thatthe initial surface water depth takes a negative value implies an

initial rainfall loss storedin the unsaturated zone of the paddy field

top soil and no contribution to the surface runoff. The calculated

discharge from the 12 paddy fields without the runoff control

devices had good accordance with the observed discharge as

shown in Fig. 9.

The surface runoff from a paddy field plot with the runoff

control devices was calculated. Here, the regulating factor is first

the surface runoff hole of the tank model at a low ponded water

level and then the orifice of the runoff control device as soon as the

water level reaches a certain depth (calculated as 35.5 mm) where

discharge from the tank model exceeds that from the orifice. The

calculated runoffs per 5000 m2 standard-size paddy field plot with

and without the runoff control devices during the midsummer

drainage season are shown in Fig. 8(b).

Fig. 10 shows the runoff from a paddy field plot in the case of

the ponding season (Fig. 10(a)) and midsummer drainage season

(Fig. 10(b)) when the design hyetographs of the 10-year and 50-

year return periods are applied. It is important to note the peak

runoff volumes with the runoff control devices being installed

(approximately 0.011 m3 s1) do not differ significantly regard-

less of the rainfall intensity or season. This is because (1) the

runoff control devices are installed on the bottom of the 400 mmdeep drainage boxes, and the change in the paddy field surface

water depth is small in comparison with the total water depth

(the depth of the drainage boxes plus the paddy field surface

water depth) that determines the runoff from the orifice and (2)

the runoff from the runoff control devices is proportional to the

1/2 power of the total water depth according to Eq. (1). On the

other hand, the runoff reacts sensitively when the runoff control

devices are not installed since the runoff depends directly on the

paddy field surface water level, being proportional to the 3/2

power of the paddy field surface water depth in the case of the

ponding season and the first power in the case of the midsummer

drainage season.

5.2. Hilly/residential area module

The peak discharge and time series discharge behavior of the

representative hilly subcatchment that was simulated by the

kinematic wave model had fairly good agreement with those of

the observed surface runoff (Fig. 11) applying the calibrated

equivalent roughness (0.62). Likewise, the runoff from residential

areas was simulated using the kinematic wave model. These

results added to the base discharge were used as the input of the

lateral inflow to the corresponding channel reach sections of the

channel network module.

5.3. Results of integrated runoff analysis by the channel network

module

5.3.1. Model validation

To validate the performance of the developed algorithms and

calculation procedures, the model was tested using the simulated

flood waterlevel anddischargefor therainfall event of the29th of

June 2007 under the actual installed runoff control devices of 80%.

The simulation results were compared with the observed

hydrograph where the water level sensors are located; that is,

Fig. 11. Comparison between the observed and calculated discharges fromthe hilly

subwatershed A.

Fig. 12. Results of the model validation. The observed and calculated water levels

and discharges are in good accordance. (a) Observed and calculated water levels at

observation site 1. The cable of the water level sensor was cut by the flood water

around 19:00. (b) Observed and calculated water levels at observation site 2. (c)

Observed and calculated discharges at observation site 2.



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atobservationsite1 onthe Fuefuki Riverand observationsite2 on

the main drainage channel from the paddy field area (Fig. 12(a)

and (b)).

The simulated time series of the water level reproduce well the

observed fluctuation. Although there were only two discharge

measurements taken at observation site 2 (Fig. 12(c)) owing to

time constraints,thesevalues arein accordance with thesimulated

values. Since the time series water level and discharge are

simulated successively on the basis of the continuity and

momentum equations (Eqs. (8) and (9)), continuous observation

of the water level and two discharge measurements are sufficient

to validate the model, given that the boundary conditions and theparameters are correctly defined.

5.3.2. Evaluation of the flood mitigation function of the runoff

control devices

To evaluate the effect of the Paddy Field Dam project, a runoff

simulation using the constructed model was conducted without

(0%) and with (100%) runoff control devices. Since the difference in

the peak runoff from paddy fields between the ponding season and

midsummer drainage season is not significant, the evaluation is

made for the midsummer drainage season. The results for two

representative sites (observation site 1 and the city center

simulation site in Fig. 2) are presented and compared below.

Observation site 1 is located on theFuefuki River approximately

400 m upstream of the confluence with the Ishikawa River, andthere is no inflow between the confluence and this site. Therefore,

all flood water from the 528 ha Fuefuki River watershed collects at

this point. The cross-sectional geometry of the river is two

trapezoids with a 17 m width at the top and 3.3 m depth at the

center.

The city center simulation site is the channel section near the

outlet at the city center where the flood water most frequently

overflows the channel. Overflow occurs in this section because of

its small cross-sectional geometry, a rectangle of 3.5 m width and

2.0 m height. Since this section is a dugout channel without a bank

along the channel, as soon as the water level exceeds the height of

the channel, overflowing water spreads over the city center,

leading to inundation.

5.3.3. Channel stage, flow volume and inundation scale in

the case of observed rainfall

The simulation results for observation site 1 show that the

maximum water level and runoff volumereduced from 2.24 m and

16.06 m3 s1 with 0% installedrunoff control devices to 2.07 m and

11.84 m3 s1 with 100% installed runoff control devices applying

the observed rainfall pattern (Figs. 13(a) and (b)). The differences

between cases with and without the runoff control devices are

0.17 m in water level and 4.22 m3 s1 in runoff volume (Table 2).

With respect to the discharge volume, a peak runoff reduction of

more than one-quarter can be expected by installing the runoff

control devices to all paddy field plots.

At the city center simulation site, the maximum water levelwith 100% installed runoff control devices is calculated as 1.88 m

Fig. 13. Simulation results for the actual rainfall. (a) Water level at observation site 1. (b) Discharge at observation site 1. (c) Water level at the city center simulation site. (d)

Discharge at the city center simulation site.

Table 2

Comparison of the simulated maximum water level and discharge at observation site 1 and the city center simulation site.

Observation site 1 City center simulation site

Actual rainfall 50-year return period Actual rainfall 50-year return period

Without

devices

With

devices

Without

devices

With

devices

Without

devices

With

devices

Without

devices

With

devices

Max water level (m) 2.24 2.07 3.07 2.70 2.07 1.88 2.77 2.13

Difference (m) 0.171 0.364 0.190 0.637

Max discharge (m3 s1) 16.06 11.84 35.64 18.98 2.15 1.55 6.18 3.40

Difference (m3 s1) 4.22 16.66 0.60 2.78

Difference (%) 26.3% 46.7% 27.9% 45.0%



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(Fig. 13(c)). On the other hand, the maximum water level would

have exceeded the channel depth (2.0 m) in the case of no

installation according to the simulation.

The effect of the flood control measure on inundation is

summarized in Table 3. The maximum total inundation volume,

depth and inundation duration for the 0% installation case are

calculated as 205 m3, 0.002 m and 2.07 h (Table 3). Because of the

irregular topography of the ground surface and existence of

buildings in reality, the actual inundation depth in some areas

would have been greater than the simulated value.

Although inundation is not great, what is important is that the

installation of the runoff control devices could have preventedinundation. In fact, the water level did not reach the overflow level

on the day considered with the actual installed runoff control

devices of 80%. According to the officials of the local water

management office, who monitor the water level in this area,

overflow was usually observed with a similar pattern and scale of

rainfall event before the flood control measure was implemented.

5.3.4. Channel stage, flow volume and inundation depth in the case of

the design hyetographs

Applying the design rainfall of the 50-year return period, the

maximum water level at observation site 1 was simulated as being

reduced by 0.36 m from 3.07 to 2.70 m. Since the depth of the

channel is 3.30 m, there is only 23 cmextra bank height in the case

of no flood control measure. With regard to the discharge, the peakdischarge is expected to be reduced by as much as 16.7 m3 s1,

accounting for approximately 47% of the peak discharge without

the flood control measure (Table 2).

At the city center, the maximum total inundation volumes and

depths are 6539 m3 and 0.066 m with the installation of runoff

control devices and 18,223 m3 and 0.184 m without the installa-

tion. The inundation durations are 14.9 and 37.3 h, respectively

(Table 3). Since the maximum total inundation volume for the 10-

year return period rainfall event is simulated to be 6281 m3

without the installation of the devices, the runoff control devices

have the effect of reducing the flood damage due to the 50-year

return period rainfall event to almost that due to the 10-year

return period rainfall event.

6. Conclusion

This study quantifiedand evaluatedthe effectof the Paddy Field

Dam project, a flood control measure taking advantage of the

ponding function of paddy fields, at the watershed scale using

combined hydrologic analyses and flood routing. The simulation

results show that installing runoff control devices would reduce

the discharge of the river responsible for the drainage of the

watershed by 26% and lower the water level by 0.17 m in the case

ofan observed rainfall event,and by47% and 0.36 m in the case ofa

50-year return period design rainfall event. As a result, inundation

damage to the city center would be significantly mitigated. These

results confirmed that the flood control measure is functioning

effectively, as far as the study area is concerned.

The effect of the Paddy Field Damproject would, as a matter-of-

course, depend on the characteristics of the watershed under

study,such as topography, land useand drainage systems, of which

the fraction of paddy field area in a watershed is of the highest

importance. It is crucial, therefore, to select a suitable site for the

Paddy Field Dam project before its application. Further study will

be required fordeveloping a simpler methodto select suitable sites

for the prevalence of the flood control measure in the Paddy Field

Dam project.

Since large-scale investments such as constructing floodcontrol

dams aredifficult owing to budget constraints of national and local

governments, this project is presently attractive to policy makersnationwide as an inexpensive countermeasure to the recent

increase in the frequency and magnitude of heavy rainfall events.

Indeed, some local governments are now exploring the possibility

of introducing the Paddy Field Dam in their paddy field areas. The

project is applicable not only in Japan, but in many Asian monsoon

regions with regular flood damage and where paddy fields are the

primary means of producing food.

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Table 3

Comparisons of the inundation durations, maximum volumes and depths with and without runoff control devices at the city center simulation site.

Actual rainfall 10-year return period 50-year return period

Without

devices

With

devices

Without

devices

With

devices

Without

devices

With

devices

Max volume (m3) 205 0 6281 501 18223 6539

Difference (m3) 205 5779 11684

Max depth (m) 0.002 0.000 0.063 0.005 0.184 0.066

Difference (m) 0.0021 0.0584 0.1180

Duration (h) 2.07 0.00 10.98 3.78 37.27 14.87Difference (h) 2.07 7.20 22.40

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Evaluation of the Flood Mitigation Effect of a Paddy Field Dam Project

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