E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

doi:10.3850/38WC092019-0457

THE EFFECT OF SEDIMENT REDUCTION INTO WETLAND ON KUSHIRO WETLAND RESTORATIONPROJECT

Taroyamamoto(1), norioishida(2)&toruozawa(3)(1)River Center of Hokkaido, Sapporo, Hokkaido, Japan,

ta.yamamoto@ric.or.jp(2)DoconCo., Ltd., Sapporo, Hokkaido, Japan,

ni1480@docon.jp(3)Hokkaido Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, Sapporo,

Hokkaido, Japan,
ozawa-t22ab@mlit.go.jp

ABSTRACTThe Kushiro wetland is the largest wetland in Japan. The wetland area has been decreasing over the last 50years and that was considered to be the vegetation transition from reed or sedge meadow to alder forest. The
major factors of vegetation transition were assumed that sediment intrusion into the wetland had beenincreasing. Then project for the wetland restoration commenced in 2003 and some practical efforts have been
conducted for reduction of sediment intrusion into center of the wetland.One of the most essential efforts ofsediment reduction is the Sediment Retention Basin (SRB) in Kuchoro River, a tributary of Kushiro River. SRB
is a part of marsh beside the river which is surrounded with low wall made of wooden boards named ArtificialKermi. In 2016, a huge flood occurred in northern Japan. Flood water spread in the SRB and large amount ofsediments were deposited. Sediment accumulation observed in sediment traps set on the ground surface
resulted in around 6 cm thickness of fine sand at the maximum point in SRB. The effectiveness of SRB in
sediment reduction was evaluated using numerical model in comparison between pre- and post-setting SRB.
The total amount of sediment passing the point of the SRB was estimated to decrease by 30 - 40% based on
the numerical model results.
Keywords:wetland restoration, sediment retention, reed and sedge, alder forest

INTRODUCTION
The Kushiro wetland is located in northern Japan, eastern Hokkaido and is the largest wetland in Japan.
It spread in the lower reaches of the Kushiro River and its area is around 250km 2(Figure 1). The climate in thisarea is cool due to the location of northern Japan, and the average annual temperature is around 6 degrees
and the annual precipitation is about 1000 mm. The landscape consists of reed and sedge meadow andmeandering river and it provides habitats for abundant wildlife including rare species such as Japanese cranes(special natural monument) or Japanese Huchen (largest fresh water fish in Japan).
Although such a valuable wetland environment, a serious problem has occurred at the wetland
since several decades ago. The wetland vegetation had been changing gradually. Most of the vegetation in
Kushiro wetland had been reed and sedge community originally. However, alder forest had been invading to thewetland and the ratio of reed and sedge meadow area was decreased from 92% in 1947 to 54% in 2004(Figure2).
The increase of sediment inflow into the wetland was recognized as a key factor influencing thechanging wetland vegetation. The land reconstruction was conducted in whole of Japan after World War II.
Development of farm land was promoted to increase the food production output with population increase. The
unused land adjacent to the wetland had been converted to farmland due to some efforts such as drainage orriver channelizing in Kushiro district. These works, combined with deforestation, resulted in increasing sedimentyield from the area around the wetland.
It was assumed that the increase in the amount of sediment flowing into the wetland would increase theamount of sediment deposition in the wetland, especially at the marginal area. The transition of habitat forE-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

vegetation due to sedimentation led to alteration of vegetation from reed and sedge meadow to alder forest. In
this background, the Kushiro Wetland Restoration Project was commenced in 2003 and its main objective wasconsidered reducing sediment flowing into the central wetland area.

Hokkaido
KushiroDictrict
JapanKuchoroRiv.
SRBKushiro
Wetland
Kushiro Riv.

Figure 1. Map of the Kushiro Wetland showing the location in Japan and the position of the SRB at Kuchoro
River, the tributary of Kushiro River.

alder
reed
all

alderreed
all

Figure 2. In the early 1900s, whole area of the wetland was covered by reed communities, however, alder trees
have been invading the vegetation by 30%. Vegetation alteration triggered the commencement of the wetland

E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

restoration project in Kushiro Wetland.

SedimentRetentionBasin(SRB)

Artificial Kermi
Kuchoro Riv.

August 24, 2016

Figure 3. The Sediment Retention Basin (SRB) was constructed on the side of flood plain area at Kuchoro River.
A large amount of flood water flowed into the SRB and lots of sediment were deposited there on 2016 flooding.

Figure 4. The SRB is the area surrounded low wall made of wooden boards. The wall was named Artificial
Kermi, derived from ridge of peat moss in Finland.
Sediment Retention Basinin Kuchoro Site
Sediment reduction efforts have been conducted in some districts of Kushiro Wetland Restoration
Project. The Sediment Retention Basin (SRB) is one of the major efforts and was planned on both side of flood
plain area at Kuchoro River, the branch of the Kushiro River(Figure 3).
The SRB is the area surrounded with low wall and the objective is to reduce sediment intrusion into thecentral wetland by impounding inundated flood water and depositing the dissolved sediment in it. The low wall
was made of wooden boards named Artificial Kermi(Figure 4). The original kermi is ridge of peat moss mostly
found in bogs in Finland.
The first SRB on the left side of the river was completed in 2012. The length of the Kermi is 2.4km andarea of the SRB is 27.4ha. Presently, another SRB is being constructed on the right side of the river. The
monitoring investigation of left side SRB has been conducted to validate the effect of sedimentation design.
A huge flood occurred in 2016 in northern Japan, especially in Hokkaido, and Kushiro district had large
flooding as same as other area. The SRB was inundated for a long period during the flood and lots of sedimentwere deposited in it.

E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

Table 1. Estimates of Sediment Reduction Ratioyear

2013
2014
2016
2017

period

7/22-10/10
8/9-9/2
8/3-11/18
9/17-10/18

total
sediments
in flow
m3
1,900
1,600
26,000750

deposited
sediments

reduction
ratio

m3
240
120
2,400
5

0.13
0.08
0.09
0.01

Figure 5. Sediment Trap: Square plates were set on the ground surface in the area. Sediments remained on theplate after inundation.(size: 0.22m×0.22m)
Sedimentation Monitoring
Sedimentation monitoring has been conducted from a year after the left side SRB was completed. Field
observations were performed on suspended loads in the river channel and deposited sediment on the surface
in the SRB.
Suspended loads have been observed at the upstream station of SRB and discharge has been
observed simultaneously at the same place during flooding. The concentration of suspended load wasconverted to the suspended load influx by multiplying the flow discharge.
In the SRB, sediment traps had been set at 21 points on the ground and the thickness of deposited
sediment was measured after flooding, and the amount of sediment deposition in the SRB was estimated(Figure5). The difference between the influx of suspended load at the upstream of the SRB and the amount of sedimentdeposition in the SRB is the quantity of sediment flowing into the wetland.
Monitoring is important for confirming how much SRB can reduce the amount of sediment flowing into thewetland. The results of sedimentation monitoring on total sediments in flow, deposited sediments and reduction
rate are shown in Table 1 for 2013-2017.
It was confirmed from the result of the sediment reduction ratio that approximately 10% of thesuspended load was deposited in the SRB. This is the effect of the SRB reducing the amount of sediment inflow
to the center of the wetland. Although the reduction rate was significantly lower at 1% in 2017, it is consideredthat the amount of inundated water from the channel was small because the flood discharge was small andwater level didn’t rise much.
Evaluation
A simulation model was established to evaluate the sediment reduction effect by comparing the casewhere the SRB was set or not. The 2D flow was calculated using the shallow water equations, and the sedimenttransportation was calculated by the advection-diffusion equation of suspended loads.
Calculation results of the 2016 flood are shown in Figure 6. The figure above shows the case with noSRB, and the figure below shows the case with SRB. The result of the case with no SRB showed that muchsediment flowed into the wetland area and the case with SRB showed that much sediment deposition occurred.
In addition, the annual amount of sediment reduction was estimated utilizing this calculation model. As a result,it was shown that the amount of sediment reduction effect was about 30% by setting the SRB(Figure 7).

E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

Sediment thickness-0.1

0.2-1.0

1.1-5.0

5.0-

Figure 6. Numerical simulation results of 2016 flood (upper: assuming without SRB, lower: with SRB)

V[m3]
30000

6000
5000

20000

4000
3000
2000

10000

1000002013

2014

2015

2017

2016

no kermi(assumption) after setting kermi
Figure 7. Estimated annual sediment inflow to wetland by numerical simulation
CONCLUSIONSThe effect of Sediment Retention Basin (SRB) set to decrease in sediment intrusion into Kushiro
Wetland was validated by field investigation and numerical simulation. Sediment accumulation observed in SRB
after flooding and numerical results showed that the sediment inflow to the wetland is reduced by about 30%.
We will continue to conduct monitoring investigation and confirm the effects.
REFERENCES
Nakui, T., Shimizu, Y., and Fujita, T. (2003). A study on relationship between sedimentation caused by inundationand drying phenomena in Kushiro Wetland. Proceedings of hydraulic engineering, JSCE, Vol.47, 907-912
Simenstad,C., Reed,D., Ford,M., (2006). When is restoration not? Incorporating landscape-scale processes torestore self-sustaining ecosystems in coastal wetland restoration, Ecological Engineering, 26, 27-39