FUNCTIONS OF NATURAL LEVEESON THE CONTROL OF INUNDATION WATERS IN THE ARA RIVER BASIN
Shigeru SAITO1 and Shoji FUKUOKA2

ABSTRACT
Abstract: Natural levees are a kind of micro-topography produced by past flood inundation.
Although many studies on geological features of natural levees have been conducted, there are few
suggestions for effective utilization of natural levees to reduce flood damages by inundation waters.
So, we have studied functions of natural levees on flood control and presented the ability to reduce
flood damage. The objective of this study is to clarify effects of sizes and arrangement of natural
levees on behaviors of flood inundation water in entire Ara river inundation area. The effects ofnatural levees on flood inundation are discussed by superimposing simulation results of floodinundation by the Ara River designed flood on topographical classification maps. For this purpose,natural levees in Kumagaya city, Kazo city and Koshigaya city in Saitama prefecture areinvestigated. We identified that functions of natural levees on behaviors of inundation water had
differences every inundation areas. They can block spreading of inundation water and inundationwater is advancing between the natural levees. However, inundation water is concentrated by thenatural levees, and the water depth becomes deeper in the vicinity of the junction point of naturallevees.

1.

INTRODUCTION

To discuss reasonable measures for potential flood inundation areas, it is important to understand
topographical characteristics on inundation water behaviors. Natural levees are a kind of microtopography produced by flood inundations in ancient time. Although many geomorphologicalstudies of natural levees have been conducted, there are few studies focused on effective utilizationof natural levees to reduce flood damages. The objective of this study is to clarify functions of sizes
and arrangement of natural levees on behaviors of flood inundation waters in the Ara Riverinundation area.
In this study, characteristics of natural levees in the cities of Kumagaya, Kazo and Koshigaya in
Saitama prefecture are investigated and compared. Kumagaya city is located on the Ara River
alluvial fan. Kazo city is located downstream of the alluvial fan, and Koshigaya City is downstreamof Kazo city. We identify natural levees by using topographical classification maps (GeographicalSurvey Institute, 1974) and ground level data surveyed by aerial laser profiler. The functions ofnatural levees on flood control are discussed by superimposing simulation results of 1-100 year
flood inundation of the Ara River on the topographical classification maps.
1

Department of Civil Engineering, Graduated Student of Science and Engineering, Chuo University, Japan. (shigerus@civil.chuo-u.ac.jp)
2Research and Development Initiative, Chuo University, Japan. (sfuku@tamacc.chuo-u.ac.jp)

2.

NATURAL LEVEES IN THE ARA RIVER BASIN

Figure 1 Natural levees along rivers (Kagose, Y. 1975)

Figure 2 Natural levees in Ara river basin (Geographical Survey Institute of Japan, 1972)
Natural levees are a kind of micro-topography and are formed along the river (Fig. 1). Generally,
they have developed in the alluvial fan and its downstream. Ara river basin includes some areas
where many natural levees exist. Fig. 2 shows the locations of natural levees and major rivers in Ara
river basin. Natural levees have developed mainly at Kumagaya city, Kazo city and Koshigaya city.

Figure 3 The elevation and ground gradient along the green line in Fig.2.
Fig. 3 shows the elevation and ground gradient in the Ara River basin. The gradient of Kumagaya
city indicates larger than that of Kazo City and Koshigaya City. Magnitude of natural levees haveclose relations with the gradient of the land surface. The gradient of land surface in Kumagaya City
has been steeper because this city locates on alluvial fan. That is, since river flowing throughKumagaya city is prone to channel change, this area has less natural levees developed large and long.
On the contrary, natural levees which in Kazo city and Koshigaya city have developed larger asshown in Fig. 2.

Figure 4 Depth of 1-100 flood inundation water.

Fig. 4 shows the depth of inundation water due to 100 year flood of the Ara River. This
simulation was done by two-dimensional unsteady flow analysis. The inundation water spreads
eastward of the Ara river and flows through Kumagaya city, Kazo city and Koshigaya city.

Figure 5 Future population change Kumagaya city, Kazo city and Koshigaya city. (National Instituteof Population and Social Security Research of Japan, 2010)
Fig. 5 shows the future population change of the three cities. The population of three cities ispredicted to decrease gradually. So, it is considered that residential district becomes narrower andkey municipal functions concentrate in the central area of the city. In that case, we must discuss
flood management as a concept of city planning how to treat inundation are.

3THE FUNCTIONS OF NATURAL LEVEES ON THE FLOOD CONTROL IN
KUMAGAYA CITY

Figure 6 Natural levee distributions and traffic network in Kumagaya city.

Fig. 6 shows natural levee distributions and traffic network in Kumagaya city (GeographicalSurvey Institute of Japan, 2012). In this area, the elevation on the natural levees is about 0.5m higher
than surrounding area. We can see that roads and railways have been constructed so as to connect
the natural levees.
Fig. 7 shows the depth of inundation water at an hour after the dike was breached. Roads andrailways block the spreading of inundation flow. Most roads and railways play a role ofembankments. As long as embankments are higher than the depth of inundation water, inundationwater is restricted by them.
Fig. 8 shows the depth of inundation water at nine hours after the dike was breached. Naturallevees work with the bank of the roads and the railways and restrict flood inundation area.

Figure 7 Depth of inundation water at an hour after the dike breach.

Figure 8 Depth of inundation water at nine hours after the dike breach.

4.

THE FUNCTIONS OF NATURAL LEVEES IN KAZO CITY

Figure 9 Distributions of natural levees in Kazo city.

Figure 10 Contour diagram of elevation in Kazo city.
Fig. 9 shows distributions of natural levees in Kazo city. Natural levees are longer and wider thanthose in Kumagaya City. Fig. 10 shows contour diagram of elevation in this area. The natural levees
are about 2m higher than surrounding areas.

Figure 11 Depth of inundation water at six hours after the dike breach.
FlowFigure 12 Land use in Kazo city. (Geographical Survey Institute of Japan, 2011)
The maximum depth of inundation occurs at six hours after the dike breach. Fig. 11 shows thedepth of inundation water at that time. Spreading of inundation water stops in front of natural levees,

and inundation water flows with rather large depth between natural levees or diluvial uplands.
Meanwhile, inundation water flows over natural levees at A and B. The elevation of natural leveesat these locations is about 1m, and smaller than the depth of inundation water.
Fig. 12 shows the situation of land use in this area. Most of inundation area has been used forpaddy fields. The damage of flooding is less because paddy fields have endurance to the inundationwater. This is one of ideal examples of land use in inundation area. The key municipal functions andresidences must make be located less damaged area of inundation. Inundation area should beremained as paddy fields. It is considered that natural levees can be used as the second levees toprotect residences from flooding.

5THE FUNCTIONS FONATURAL LEVEES ON THE FLOOD CONTROL IN
KOSHIGAYA CITY

Figure 13 Distributions of natural levees in Koshigaya city.
Fig. 13 shows the distributions of natural levees in Koshigaya city. The Naka river, Furutone riverand Motoara river concentrate in this area, and therefore, natural levees also converge near theconfluent point of rivers. Fig. 14 shows contour diagram in this area. Natural levees are about 2mhigher than surrounding as elevation the case of Kazo city.
Fig. 15 shows the depth of inundation water at 4 days after the dike breach. The inundation waters
are avalanching from upper area concentrates at the confluent point of the rivers. Therefore, thedepth becomes large at this point. Natural levees have functions to bring the inundation water to theconfluent points.

Figure 14 Contour diagram of elevation in Koshigaya city.

Figure 15 Depth of inundation water at four days after the dike breach.

Figure 16 Depth of inundation water at seven days after the dike breach.

Figure 17 Situation of residential development in Koshigaya city.
Fig. 16 shows the depth of inundation water at seven days after the dike breach. There is littlechange in the depth of inundation water from four days after. Since large depth of the inundation is
maintained for long time, the land use such as paddy field is desired in this area. However, thepopulation of Koshigaya city has increased as indicated in Fig.5 and residential districts arecurrently spreading.

Fig. 17 shows the situation of housing area development in this area. At the present, residences
tend to be constructed on the places of predicted large inundation depth because less damage area byinundation water have already used for residences. It is important to discuss the measure to construct
endurable houses against inundation waters and to consider the land use reducing damages offlooding.

6.

CONCLUSIONSFunctions of natural levees on flood control vary with of morphological characteristics of theinundation area. Primary conclusions are shown below.
1. In Kumagaya city, the elevation of the natural levees is about 0.5m higher than surrounding area,and roads and railways were constructed so as to connect the natural levees. Flood inundation
area is restricted by the banks of the roads and railways, so it is considered that natural levees
work together with the bank of the roads and the railways to block the inundation flow in this area.
2. In Kazo city, natural levees are larger than those of Kumagaya city. The natural levees are about
2m higher than surrounding area and they can block inundation flow. Since the inundation waters
advance between the natural levees and diluvial uplands, it is considered that natural levees canbe used as the second levees by reinforcing their depressions with banks of roads and railways.
3. In Koshigaya city, natural levees have height and size similar to those in the Kazo city. Somenatural levees concentrate inundation waters at the confluent point. Therefore, it is needed todiscuss the land use to reduce the damages due to flood inundation waters.

REFERENCES
Geographical Survey Institute of Japan. (2012) Map Browse Service. http://wathchizu.gsi.go.jp/
Geographical Survey Institute of Japan. (2011) Survey on Trends of Land Use for Housing.
http://www1.gsi.go.jp/geowww/LandUse/lum-takudo.html
Geographical Survey Institute of Japan. (1972) Landform Classification Map for Flood Control Browse.
http://www.gsi.go.jp/geowww/thema p/1cmfc/index.html
Kagose, Y. (1975) Natural Levee: Case Study of Flood Plain. Kokon Shoin, Tokyo, Japan.
National Institute of Population and Social Security Research of Japan (2010) Population Projections byPrefecture and Municipality. http://www.ipss.go.jp/pp-shicyoson/j/shicyoson08/t-page.asp