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

Comprehensive sediment management model in Yahagi River basin
based on Yahagi Dam sediment Bypass project
K. Tomita
Civil Engineering and Eco-Technology Consultants Co., Ltd., Tokyo, Japan

T. SumiWater Resources Research Center in Disaster Prevention Research Institute, Kyoto University,
Kyoto, Japan

T. Sugita
Yahagi Dam and Reservoir Management Office, Chubu Regional Bureau, MLIT, Toyota City,
Aichi, Japan

ABSTRACT: This study examines measures for the effective and efficient management of sediment inconnection with the operation of the sediment discharge Bypass (BP) being considered for the Yahagi
Dam, including merits and demerits in the downstream river. Specifically, based on the area where excavation for maintenance will be required for flood control after the sediment discharge facility (BP) starts
operation and the amount of sediment to be excavated, examination was made on the measures for effective use of sediment resources by the development of tidal flat at the river mouth and shallow bottom/tidal
flat in the port area and the recycle of excavated sediment (for material etc.). As a result, development of astockyard as an adjustment base for sediment management in the mid-stream area proved to be effective.
Further examination was made as a case study on potential sites for stockyard leading to optimum costeffectiveness and “sediment management” (draft) for optimizing flood control, water utilization, andenvironment for the overall river system.

1

INTRODUCTION

Concern has been raised about the impact on thedownstream river by the sediment discharge Bypass(BP) being considered for the Yahagi Dam.
Sediment discharge from the Dam, when implemented, is expected to have adverse effects on
flood control and water utilization functions due
to rise in bed height of downstream river, while thecoarse grain of the current river bed material will
be refined again due to the fine sediment to be supplied by the discharge to the river, which leads tothe expectation of restoration of sandy dry riverbed, recovery of biota appropriate for the sandy
river bed, and restoration of tidal flat or shallow
bottom in the river mouth and shore area.
In the sediment management aiming for comprehensive management of sediment in the Yahagi
Dam and Yahagi River (Fig. 1), appropriate excavation of sediment, transportation thereof, sediment
replenishment to rivers and seashores, recycling of
sediment, etc. are discussed as a case study so thatthe management may be optimum over the wholeriver system for the stakeholders in conflict ofinterest concerning the river.

Figure 1.

Yahagi River basin.

Specifically, consideration will be required forthe following items.
1. Forecast and consideration of the section of
sedimentation expected from the discharge.
2. Consideration about the section of excavationof river channel and the amount of sediment2185

required for flood control in connection withthe decrease in discharge capacity expectedfrom sedimentation resulting from sediment
discharge from the Dam.
3. Transportation for dealing with excavatedsediment.
4. Determination of the appropriate amount of
sediment replenishment to rivers, beaches, etc.
using sediment effectively.
5. Consideration about uses and possible amountfor recycling of sediment resources.
6. Consideration about the location and capacity of a transfer station and sediment Stockyard (SY) as time-related regulating capacity
concerning the amount of sediment in order torealize sediment management.
7. Consideration about sediment management for
optimal operation of the aforementioned itemsin the river basins.
8. Verification of feasibility and consideration of
economy concerning excavation, transportation, etc.
9. Identification of issues for adjustment withstakeholders, etc.
This study, as a case study, assumed the amountof sediment discharge from the sediment BP to beabout 250,000 m3 per year in average, and areas tobe excavated for maintenance and the amount of
sediment for the sections from the downstream of
the Dam to river mouth or beaches according to
grain sizes of discharged sediment. Based on thisassumption, cost minimization was discussed withthe condition of replenishment of excavated sediment to rivers and beaches and recycling of sediment resources.
As the result, it was proved to be effective toreplenish 30,000 to 50,000 m3 of sediment per year
to river mouths and beaches for environmental
improvement by providing sediment stockyard
around the Koshido Dam in the midstream andto recycle sediment resources including use fornew materials and machines while watching market demand. Moreover, system and organization
framework to realize the studied case was examined by providing a similar practical case.
2

EXAMINATION OF SEDIMENTBALANCE IN DEVELOPMENT OF
SEDIMENT BYPASS (BP)

2.1 Issues with the present sediment balancein Yahagi River
2.1.1

Inflow area of Yahagi Dam(Sediment production area)
The inflow area of the Yahagi Dam is increasingly
devastated since a large amount of sediment isdischarged due to hillside collapse, etc. The areasof collapsed land and bare hills had been decreasing since the 1940’s, but turned to increase sincethe 2000 Keinan Heavy Rain. However, the inflow
of sediment into the Dam has been moderate inrecent years.
2.1.2 Yahagi Dam reservoir
Sedimentation in the Yahagi Dam reservoir has
been increasing at a pace of about 300,000 m3
in yearly average after the completion of theDam (see Fig. 3). Particularly, the 2000 Keinan
Heavy Rain increased one-year sedimentation to
2.8 million m3, about 9 times the yearly average.
The present amount of sedimentation in the reservoir has already exceeded the planned sedimentation capacity of 15 million m3, so excavation ofdeposited sediment is going on as an urgent measure against sedimentation.
2.1.3 Yahagi Dam downstream river (river area)
As a geological feature of the Yahagi Riverbasin, granite on the surface is likely to weather(“masaka”) and collapse. As a result, a large
amount of sediment has been discharged in theevent of rain, forming an alluvial plain.
In the recent years, however, sediment movement in the sediment transport system has been
prevented due to the development of the Yahagi
Dam or electric dams, and the Headworks of
Meiji Irrigation Water, excavation of gravels on
the river bed has been conducted, and disturbancefrequency has decreased as a result of the stableflow regime. Consequently, establishment of waterroute and increase in land area formed by sandbar are proceeding, sandy dry river bed, which isthe characteristic landscape of the Yahagi River,has decreased, and the grain of river bed material is becoming coarse in the down-stream of theDam.
2.1.4 River mouth/harbor areas
In the mouth of Yahagi River, the total area of
reed field reduced in 1982 to less than a half of thearea in 1973 and thereafter resulted in an increaseor decrease of 10 to 20 ha.
Meanwhile, the tidal flat area at the river mouth
had been decreasing every year since 1965, even to
about 20% of the 1965 level in 1985, while littlechange has been recognized after 1985.
Also, in the Mikawa Bay area, the area of tidal
flat and shallow bottom decreased about 1,200 hain several years since around 1965 due mainly
to landfill and channel dredging. Moreover, thedecrease in the tidal flat and shallow bottom
in the Mikawa Bay is said to have increased thetotal number of days of reported red tide. See
Figure 2.

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Figure 2.
(draft).

Image of sediment management scenario in the Yahagi River comprehensive sediment management plan

Figure 3.

Increase in sediment transport by sediment BP.

2.2

Sediment balance after developmentof sediment BP (forecast)

For the forecast of sediment balance after
development of the sediment BP, treatment of sediment and measures for effective use of sedimentwere examined based on the amounts of passing
sediment and excavation for maintenance, whichwere obtained by the sedimentation balance calculation under the Sediment Management Scenario(Fig. 2), which is being examined by the “Yahagi
Dam River System Comprehensive SedimentManagement Committee.”
2.3

Issues after development of the sediment BP
and measures in sediment management

Development of the sediment BP will bringexpectation for long life of the dam (by securing2187

Figure 4.

Issues after sediment BP development and merits in sediment replenishment.

Figure 5.

Issues with the present state of sediment balance in Yahagi River.

the capacity for flood control/water utilization)and improvement in the downstream river environment (restoration of sandy dry river bed,etc.), while it will also give rise to concern about
decrease in the discharge capacity (flood control
function) due to sedimentation in river channel.
For these reasons, for the sediment generated by
excavation for river channel maintenance, which
will be required after development of the sediment BP, examination was made on the sedimentmanagement (proposal) for the optimization by
conducting economic evaluation with the sediment management approach considering the balance between flood control, water utilization,and environmental conservation measures. See
Figures 2, 4, and 5.

3

EXAMINATION FOR EFFECTIVE
USE OF SEDIMENT WITH SEDIMENTMANAGEMENT

For effective use of sediment, it will be advantageous
to be close to the place of demand in light of thecost of transporting excavated sediment. Since
supply and demand will be well balanced for thepresent in the upstream of the Asuri Dam after
development of the sediment BP, consideration wasmade about the plan to recycle sediment excavatedfor maintenance in the downstream of the Asuri
Dam and use the sediment for the downstream
section of the Yahagi River and Mikawa Bay.
Note that sediment whose transportation iscostly, i.e., sediment excavated for maintenance of2188

Figure 6. Area of excavation for maintenance and amount of sediment, and image of effective use under sedimentmanagement.

Table 1. Results of conjoint analysis concerning theenvironment of Mikawa Bay.
Items

MWTP
(yen/month per household)Tidal flat/shallow bottom
Water quality
Clam-digging
Beach
Aquatic life

139
105
33
37
140

the Sasado Dam and Dozuki Dam and sedimentexcavated from the Yahagi Dam reservoir, will bedisposed of in the disposal site, etc. in the upstream,and use of sediment for the construction of linear
motor train line was determined to be consideredafter effective use becomes available. See Figure 6.
u Method for Effective Use of Sediment Excavated for Maintenance (Proposal)
Utilization for restoration of tidal flat/shallow
bottom in Mikawa Bay
Utilization for restoration of sandy dry river
bed in downstream river zone
Supply to the market as aggregate
Supply to the market as value-added material, such as glass and building material (collar
sand)
3.1

Utilization for restoration of tidal flat/shallow
bottom in Mikawa Bay

The areas of tidal flat and shallow bottom in the
Mikawa Bay are said to have disappeared about
1,200 ha (See Fig. 2.) but were restored by thedevelopment with about 600 ha under the Blue Sea
Project using the sediment dredged for channel
development for the Nakayama Channel.
When considering the target value to restore
the tidal flat/shallow bottom that disappeared,
sediment for further development of about 600 ha(6 mil. m3 assuming the height of development tobe 1 m) will be required. Hence, utilization of thesediment dressed from the Yahagi River for thisdevelopment would be considered. According to
the conjoint analysis (Table 1) conducted in fiscal 2010 covering the residents in the Mikawa Bay
area, the Marginal Willingness to Pay (MWTP)
for “Creation of tidal flat/shallow bottom” was ashigh as “Preservation of aquatic life,” which shows
the high needs of local residents for the restorationof tidal flat/shallow bottom (Table 1).
Results of an interview survey involving port
managers also showed that there are high needs for
sediment to be used for the development of tidal
flat/shallow bottom in the Mikawa Bay and thatabout 30,000 to 50,000 m3 of such sediment is available in a year, which will require transportation ofsuch sediment to the Port area, screening according to grain sizes, and treatment of impurities inthe stockyard. To implement such scheme, the sediment management will require the developmentof management organization and framework.
Results of the interviews with the persons
concerned with the Port and fisheries about theutilization for restoration of tidal flat/shallow bottom in the Mikawa Bayu There is demand for using sediment in the country (Port Office)’s Ise Bay Regeneration Project
and the prefecture’s Satoumi (marine-coastal
ecocystems) Restoration Project.

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u The appropriate grain size of sediment using
short-necked clam as an indicator is about 1 or
2 mm to 1 cm and variety in size is preferred.
u As a result of the test for utilization of thesediment deposited in the Yahagi Dam for thedevelopment project of the prefecture's fisheries
division, the quality of the sediment proved to begood, but not suited for the development of fishing grounds mainly because fishing equipment
cannot be used when large stones are mixed.
u For development projects, the prefecture’s fisheries division has demand for sediment of 30,000
to 50,000 m3 per year, and the prefecture’s port
division has demand for sediment amounting
to 40,000 m3 per year, so sediment utilization is
likely to be commercialized with stable supply.
u The prefecture’s port projects expect dredged
sediment to be transported to the Port for use, so
it is only necessary to transport the sediment tothe stockyard of the port division or the development site. Since the national standard requires“within 50 km” for transportation of sediment,
it is effective to discharge the sediment to be
used for restoration to the downstream area tothe extent possible.
3.2

Utilization for restoration of sandy dry river
bed/shallow bottom in the downstream zone

In and around 1965, there were many areas likesandy beach in the dry bed of the Yahagi River,which were used for boat landing, playing in thewater, and washing laundry. There was also tidal flatat the river mouth, providing valuable habitat forbirds, shellfish, etc. At present, however, the areas ofsandy river bed and tidal flat at the river mouth havedecreased, so restoration of such areas is expected.
As shown in Table 2, according to the results ofthe conjoint analyses conducted in fiscal 2009 (forupstream area (Yahagi Dam–Koshido Dam)) and in
fiscal 2010 (downstream of Koshido Dam) concerning the river environment, the Marginal Willingness
To Pay (MWTP) for restoration of “sandy dry bed”

Table 2. Results of conjoint analysis concerning theenvironment of Yahagi River.
MWTP (yen/month per household)
Items

Downstream of
Yahagi River

Upstream of
Yahagi River

Water quality
Reed field
Sandy river bed
Paddling pool
Ecosystem

111
3
110
234
187

163
106
22
162
194

was higher among the residents of downstream area
than those in upstream area. Accordingly, for utilization of sediment to restore sandy dry bed andtidal flat in the downstream area, it is expected todischarge the sediment from the BP into the downstream beyond the Koshido Dam to the extent
possible or excavate and transport the sedimentdeposited in the upstream and discharge it in thedownstream of the Koshido Dam.
3.3

Supply to the market as aggregate

In fiscal 2010, a pilot program was implemented tosurvey how the sediment provided to the public forfree was used by preparing a sediment stockyard(developed temporarily) in an area near the
Koshido Dam. This survey included a questionnaire to the sediment users (business operators).
As the result, (i) the sediment was used for concrete, aggregate for asphalt, golf course sand, andplasterer’s sand, and that (ii) about 770 yen was
spent for waste disposal and transportation. It
was found from these results that, assuming thesediment purchase price to be 300 yen/t (based on
the results of the 2009 interview survey), the priceof aggregate will be generally about 1,070 yen/t
including the costs of disposal and transportation,
which is about 800 yen/t lower than the estimatedcurrent acquisition cost of 1,860 yen/t.
This is expected to be used as an incentive forsediment management.
3.4

Supply to the market as value-added material
other than aggregate

When glass material is assumed to be value-added
material that may be available from sediment, thepercentage of SiO2 contained in silica sand (pitsand) is generally more than 90%, while as the resultof analyzing the sediment taken from the YahagiRiver, the percentage of SiO2 was 72.8%, lower thanthat of general glass material to some extent.
Accordingly, to use sediment from the YahagiRiver as glass material, such treatment as classification according to the percentage of SiO2 would
be required. Meanwhile, there are some glass workshops along the Hirose River (in Sendai City) etc.,
where river sand is directly processed to manufacture glass products. Hence, it would be possible
to use the river sand in the Yahagi River as it is
to manufacture local products (glass cups, flower
vases, etc.). In addition, sediment taken from the
Yahagi River was dried and colored to examine thepossibility of use with added value as color sandfor construction material after baking sedimentin the hearth like earthenware and taking color
treatment. As the result, it proved to be difficult
to use the sediment for color sand products since2190

Table 3. Results of cost-benefit ratio of sedimentmanagement (transport 50,000 m3 to the river mouth andthe port, respectively).
Total cost

*Up to stockyard*From stockyard
Figure 7.

Samples produced by coloring sediment.

the lower percentage of SiO2 in the sediment prevents generation of a single stable color (Fig. 7).
However, the glittering of mica is characteristic
and the color has a natural tone unlike singlecolored material. If applications using such characteristics are devised, it would be possible to use
the sediment for value-added material (e.g., sandpicture material, wall material for Japanese-stylebuildings).
Develop a site near Toyota Stadium as a sediment museum (also aiming to promote the effective use of good quality sediment).
Develop a site near the Yahagi River Toyota
Disaster Prevention Station, also aiming to savethe sediment for disaster prevention.
Develop a site in the river channel (downstreamof Benten Bridge), also aiming to use as sediment augmented for restoration of nature in thedownstream (restoration of sandy dry bed).
4

EXAMINATION FOR EFFECTIVE
USE OF SEDIMENT WITH SEDIMENTMANAGEMENT

4.1

Examination of potential Stockyard (SY)sites for effective use of sedimentTo use excavated sediment effectively, it would
be necessary to develop a stockyard where sediment excavated for maintenance is temporarily
stocked and supply-demand of sediment quantity is adjusted. Hence, the following potentialsites were examined mainly from the viewpointsof securing site and reducing the cost of sedimenttransportation.
4.2

Cost-benefit analysis of sedimentmanagement with optimizationof the amount of effective use

For the selected potential SY sites, the arrangement of SY expected to minimize total cost was

Excavation
cost (C (1))
Transportation
cost (C (2))
Transportation
cost (C (3))
Total (C)Cost
(mil yen/year)
112
25
17
154

Distribution benefit

Benefit (hundred
mil yen/year)

B/C

Aggregate utilization
Glass material
Color sand material

24
194
104

0.16
1.26
0.67

examined according to the two patterns of “singlepoint concentration” and “multi-point distribution” using the cost of transportation calculated bymultiplying the distances from the place of excavation to SY and from SY to the place of effective
use by truck mileage based on the amount of excavation for maintenance the estimated amount of
sediment for effective use.
According to the results of examination, thecost of SY management was the minimum in“multi-point distribution.” In addition, in order toreduce the cost of transportation by truck, there isa measure of increasing effective use of sedimentin SY (i.e., cost of transportation from SY will be
zero when aggregates are marketed in SY). For
example, assuming to reduce by half the amountof sediment to be transported to the river mouth/
port and to market all the sediment excavated fromthe Asuri Dam, 62,000 m3/year, the total cost of
sediment management was minimized due to thedecreased transportation cost.
Further, for each of the two patterns above, thebenefit from marketing in SY and the cost-benefit
ratio based on excavation/transportation cost were
calculated. As the result, assuming that 50,000 m3of sediment was transported to the river mouth
and the port, respectively, the ratio of cost to benefit (B/C) will be no more than 1 unless the sediment is supplied as a high-value-added material
like glass material (Table 3). On the other hand,
if all the sediment excavated from the Asuri Dam,
62,000 m3/year, can be fully distributed in the market, the ratio of B/C will be 0.99 even in the caseof “use for aggregate,” whose unit distribution costis low, so the costs of excavation for maintenanceand transportation can be covered by the benefit

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Figure 8.

Image of stockyard arrangement for effective use of sediment.

Table 4. Results of cost-benefit ratio of sedimentmanagement (optimize the amount of effective use).
Total cost

*Up to stockyard*From stockyard

Excavation cost(C (1))
Transportation
cost (C (2))
Transportation
cost (C (3))
Total (C)Cost (milyen/year)
112
6
8
126

Distribution benefit

Benefit (hundred
mil yen/year)

B/C

Aggregate utilization
Glass material
Color sand material

125
1004
536

0.99
7.97
4.25

6

of distribution. If it is possible to use the sedimentfor high-value-added material, such as glass material and color sand, the profit necessary for sediment management will be produced (Table 4).
5

in the reservoir and returning part of sales profits
for local development.
For the sediment BP being considered for the
Yahagi River, it is required to manage effectively
the sediment discharged from the river considering
for flood control, water utilization, and environmental function, while such effective management
is also very significant for nature restoration interms of returning to the conventional sedimentbalance. Therefore, it would be required to develop
the scheme, management/operation organization,and system that lead to proper environmental restoration and local development, including cooperation of each facility manager and collaboration
with private sector (finance, corporate CSR, etc.).

ORGANIZATION OF SEDIMENTMANAGEMENT ORGANIZATION ANDUTILIZATION OF PRIVATE-SECTORVITALITY

To realize the effective use of sediment as describedabove, it is necessary to establish organization andsystem for management and operation. For example, the sediment management system of a certain dam in the Kanto Region has developed thescheme and system for selling sediment deposited

CONCLUSION

When development of the sediment BP is officiallydecided, adjustment with various stakeholders
concerned with the Yahagi River will be required,including those to be affected by discharge ofanother sediment, those benefiting from environmental improvement, facility managers, and sediment users. In consideration of merits and demeritsof such stakeholders, this study on sediment management has provided the possibility of discharged
sediment as material for restoring the original natureand sediment balance of the Yahagi River and forcontributing to the revitalization of local economy
and local development by considering that the discharged sediment can be used effectively.
ACKNOWLEDGEMENT
We express deep appreciation to Professor Shinichi
Aoki at Osaka University (formerly at Toyohashi

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University of Technology), who have instructed us,
as well as those who cooperated with us by providing material on effective use of sediment in the
Mikawa Bay, including those at Aichi Prefectural
Fisheries Experimental Station, Fisheries Divisionof Agriculture, Forestry and Fishery Department,
Port Division of Construction Department, andExecutive Office of Life of Water Chubu Forum.
REFERENCES
Mikawa Port Office, Chubu Regional Bureau, MLIT,
2005. Verification of Effects of Development of Tidal
Flat/Shallow Bottom in Mikawa Bay using Dredged
Sediment.

Tomita, K., Sumi, T. and Watanabe, M. 2010. Economic
Evaluation of Comprehensive Sediment Managementin River, Proc. of River Engineering, JSCE, Vol. 16,
529–534 (in Japanese).
Tomita, K., Sumi, T. and Watanabe, M. 2011. Evaluationof River Management by Sediment Bypassing from
Dam and Sediment Management—Optimization andCost Benefit Analysis of Comprehensive SedimentManagement in Yahagi River from Dam to the Coast,
Proc. of River Engineering, JSCE, Vol. 17, 53–58 (inJapanese).
Yahagi Dam Management Office, Chubu Regional
Bureau, MLIT, 2010. Technical Report on the Yahagi
Dam Sediment Bypass.

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