Hydrological Research Letters 7(3), 54–59 (2013)
Published online in J-STAGE (www.jstage.jst.go.jp/browse/hrl). doi: 10.3178/hrl.7.54

Sediment replenishment to downstream reaches below dams:
implementation perspectives
Giyoung Ock, Tetsuya Sumi and Yasuhiro Takemon

Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University, Japan
Abstract:
Sediment replenishment below dams is an increasingly
common practice in Japan to compensate sediment deficits
downstream and improve habitat quality and ecological
functions. We reviewed implementation methods in thecontext of sediment placement, and compared the design
and implementation activities undertaken in the Nunome
River of Japan and Trinity River of California. Nunome
River was supplied with a maximum of 500 m3 of mixed
sand-gravel to the stream channel from a single site througha high-flow stockpile method during a relatively small(80 m3/s) and short (less than 4 hours) peak flow. Trinity
River was supplied through an in-channel stockpile, highflow stockpile and high-flow direct injection in combination
with mechanical rehabilitation with the aim of re-creating
gravel bar features through fluvial processes. More (max
51,000 m3) and greater (gravel to boulder) sediment has been
added from more than four sites along the downstream
channels during a longer (max 5 days) and higher (up to
311 m3/s) peak flow regime that are designated dependingon the water year types which are determined by systematic
analyses in a given year. This comparative study provides
the present programs in Japan with some recommendations
that will inform proper methods corresponding to riverspecific high-flow and sediment regimes.
KEYWORDS sediment
replenishment;
sediment
management; Nunome River; Trinity River;
dam impacts

INTRODUCTION
The combination of flow regulation by dam operation(i.e. loss of flow volume, magnitude and variability) andsediment deficit downstream by reservoir sedimentation canalter downstream channel morphology, such as riverbed
degradation, channel incision, bank erosion and vegetation
encroachment (Tsujimoto, 1999). The geomorphologicalchanges can damage infrastructure such as bridges andembankments (Kondolf, 1997), but also simplify aquatic
habitat structures and subsequently degrade ecological
functions (Tanida and Takemon, 1999). In particular, lossof alluvial geomorphic features such as riffle-pool sequenceand several types of riverine bars was reported not only todegrade spawning gravels of anadromous fish, such as
salmonids (e.g. Oncorhynchus tshawytscha, O. kisutch andCorrespondence to: Giyoung Ock, Water Resources Research Center,
Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto
611-0011, Japan. Present address: Institute of Urban and Regional
Development, University of California Berkeley, CA 94720, USA. E-mail:giyoung.ock@gmail.com ©2013, Japan Society of Hydrology and Water
Resources.

O. mykiss) (Ligon et al., 1995) and Ayu (Plecoglossus
altivelis) (Takemon, 1997), but also to elongate transportdistance of reservoir plankton and disturb trophic balancein the riverine foodweb system (Ock and Takemon, 2010).
In order to address the sediment deficit downstream andto mitigate the morphological changes derived by the
sediment starved waters, comprehensive sediment management strategies have been developed in Japan andworldwide, such as ‘sediment replenishment’ (Wheaton
et al., 2004; Kantoush et al., 2010), ‘sediment bypassing
tunnel’ (Sumi et al., 2004), ‘flood mitigation dam’ (Sumi,
2008) and ‘dam removal’ (Stanley and Doyle, 2003). Amongthe mitigation measures, sediment replenishment, which is
the method whereby bedload-sized sediment is artificiallyadded to the river channel below dams, is an increasingly
common practice in Japanese rivers particularly on the east
coast connecting the Pacific Ocean.
Sediment replenishment, otherwise known as sedimentaugmentation, has been undertaken below at least 15 damsin Japan (Okano et al., 2004), including the Kizu River basin
system such as the Nabari River below Muro Dam (since2006), Hinachi Dam (since 2008) and Shorenji Dam (since2009), and Nunome River below Nunome Dam (since 2004)(Kantoush et al., 2010). However, these applications stillrequire a systematical development in the stages of planningand implementation, both of which are in concert withspecific objectives and local hydrophysical restrictions in
the basin, reservoir and river. In particular, predicting
flushing flows (magnitude, frequency and timing), determining quantity (amount added) and quality (grain size andsource materials) of coarse sediment, and selecting aneffective implementation technique for adding and transporting sediment in the fields are key factors in achieving
a successful result in ecological river management andrestoration.
In northern California, US, gravel augmentation has been
widely used in more than 18 regulated rivers with the aimof constructing riffles for salmon spawning (Kondolf andMatthews, 1993; Kondolf and Minear, 2004). Among them,
the Trinity River below Lewiston Dam was first undertaken
in 1976 and the augmentations have continued to this day.
The Trinity River Restoration Program (TRRP) has been at
the forefront of efforts to restore bed-load supply andtransport in the regulated reaches below the dam in the
United States (Gaeuman, 2012), developing systematic
sediment management techniques and accumulating empirical data that covers various aspects of physical andecological researches.
In the present study, we compare the sediment
replenishment activities undertaken in Nunome River, Japan—54—

Received 28 June, 2013
Accepted 19 August, 2013

SEDIMENT REPLENISHMENT IMPLEMENTATION BELOW DAMS
and Trinity River, US, in the context of main objectives,
planning and implementation, with the aim of providing
practical recommendations for improving effectiveness of
sediment replenishment projects that are currently appliedor planned.

SEDIMENT REPLENISHMENT METHODS
DEPENDING ON IMPLEMENTATION TYPES
Sediment replenishment to rivers below dams is called‘Dosha-Kagen’ in Japan and simply ‘gravel augmentation’in United States when gravel- and cobble-sized sediments
are input directly or indirectly to stream channels. Comparedwith other sediment management strategies, this approach
is likely to be unsustainable owing to periodical
implementations to make up for the sediment deficit causedby the dam. However, this approach has been widely appliedin Japanese rivers as an important component of the efforts
because of its relative merits such as immediate benefits,particularly to restore the downstream habitat primarily withregards to improved fish spawning habitat (Wheaton et al.,2004).
Although a considerable amount of researches haveaddressed the sediment replenishment (mainly spawning
gravel quality and quantity), those focusing on implementation technique itself are scarce. Among increasing issues
in relation to sediment replenishment, a number ofrestoration projects described the difficulty of implementation activity (Harvey et al., 2005). Several implementation
techniques have evolved in response to available spaces,
release flow regimes, costs and accessibility as well as
primary objectives. Coarse sediments can be added tochannels by following different injection types, and Kondolf
and Minear (2010) proposed three different methods for
sediment placement with their advantages and weaknesses,
and McBain and Trush (2004) provided schematic conceptson the classification.
Firstly, In-channel bed stockpile is a relatively old
method which started from 1970, but even now it is widelyused for supplementation of riffles or pool tails. Thisapproach places directly spawning gravels within the low
flow channel to provide immediate usable habitat features.
This method, however, involves in-channel work at low
flows, which may increase turbidity downstream. Sedimentthat is largely immobile under low flow would start totransport downstream when the flow exceeds bed mobility
threshold. Secondly, the High-flow stockpile method places
gravel along the channel bank margin to be distributed
downstream by high flows. This method assumes that theriver itself will transport sediment and reshape the channel
during high flow events. Where peak flows in stream channel
have a short duration and high magnitude, it would beapplicable as an efficient method. It is a common method
currently applied in Japan. This approach can add relatively
large amounts of gravel at relatively low cost, but is limited
to volume piled and the number of suitable sites. Thirdly,
the Point bar stockpile method introduces coarse sediment
to augment or create a point bar. The augmentation isaccomplished using site-specific low flow and bankfull
channel dimensions of the reaches. The volume of coarse
sediment introduced can be exaggerated because some of it

Figure 1. Sediment replenishment methods according tosediment placement or injection types. (a) In-channel bed
stockpile, (b) High-flow stockpile, (c) Point bar stockpileand (d) High-flow direct injection

can be routed downstream during high-flow events.
However, the coarse sediment stockpiled at the inside bendin a meandering channel would be limited to erosion andtransport downstream during high flows. Additionally,
Gaeuman (2013) proposed the process of High-flow direct
injection, which directly introduces gravel to the river
channel during a high flow event using heavy equipment
such as a conveyor belt. While the high-flow stockpile canbe quickly exhausted in a large flow, this approach can allow
new sediment to replace the material already transported.
Also, an advantage of this method over high-flow stockpile
is that a larger volume and coarser gravel can be introduced
during the course of a flood. Figure 1 differentiates themechanisms of the methods in relation to flow regime andsediment placement, and Table I summarizes the advantages
and weaknesses of the four approaches.

SEDIMENT REPLENISHMENT IN THE
NUNOME RIVER
The Nunome Dam which was built in 1994 in the
Nunome River, a tributary of the Kizu River in Naraprefecture, Japan (34°42'N, 135°58'E), was planned to set
sedimentation capacity to 100 years (1,900,000 m3), but
sedimentation in the reservoir progressed faster than that of
the original plan, and the sedimentation rate in 2006 alreadyreached 13 percent (243,000 m3) of the planned sedimentation (Kantoush et al., 2010). The rate would have accelerated
unless appropriate sediment management in the reservoir
and upstream basin had been employed. In the Nunome
Dam, sediment replenishment began in 2004, and then atotal of 2,585 m3 of coarse sediment was replenished almost
annually to Nunome River through to 2009.
The sediment for replenishment was mechanically
excavated at the check dam located at the upstream end of
the reservoir, moved to the downstream river below the damby dump truck, and then deposited on the right bank 300 m
below the Nunome Dam. The placed sediment consists—55—

G. OCK ET AL.
Table I. Classification of sediment replenishment according to augmentation techniques
Types

Advantages

Weaknesses

In-channel bed stockpile– immediately usable habitat features such as
spawning riffles are expected– in-channel riffles can be supplemented orreconstructed in a given channel– potential for transport is expected using a relatively
small release flow

– in-channel construction is required– turbidity during addition would be an issue– constructed features may remain temporally only
until the next flood event– risk that existing habitats may be degraded byerosion during the next flood– sediment transport is dependent on the next flood

High-flow stockpile– coarse sediment can be routed when channeltransport capacity is high– large volumes and sizes of sediment can be added
at a relatively low cost
– little in-channel construction is required– more functional channel morphology is expected
than the in-channel method– peak flows with a short duration and high magnitude
would be applicable– number of addition sites are limited– timing for implementation and amount of sedimentadded are limited to high-flow events– coarse sediment supply may be quickly exhausted
during a short high flow

Point bar stockpile– in-channel features can be created within long pool – a large volume of coarse sediment is required– it is difficult to locate project site
reaches where spawning habitats are currently– coarse sediment that is placed at inside channel
limitedmeander would be limited to erosion and transport– alternate bar morphology that potentially has a highdownstream during high flow
habitat quality can be induced at low-flow channels

High-flow direct injection – coarser sediment can be introduced at thalweg line
during high transport capacity– larger volumes of gravel can be added during longer
peak flows– longer transport and deposition of added sedimentare expected by downstream fluvial processes– working conditions in field would be hazardous– amount of sediment is dependent on high flow
releases and peak duration– sediment should be immediately available forplacement

Data were partially derived from Kondolf and Minear (2004), McBain and Trush (2004) and Harvey et al. (2005)

Figure 2. High-flow stockpile process in the Nunome River, Japan

mainly of sand and gravel with 0.38 mm median size (d50)ranging from 0.075 to 19 mm. The volume of replenishedsediment in a single implementation varied from 80 to
500 m3, which depended on magnitude and duration of
flushing flow. Based on the event in October 2009, 500 m3
of coarse sediment was transported downstream by an
81 m3/s peak flow for 4 hours as shown in Figure 2.
Monitoring researches on geomorphological changes
such as sand bar formation below implementation site andbed material variation along channel were described in Sumi
and Kantoush (2010) and Kantoush et al. (2010).
Longitudinal variation of particulate organic matter
dynamics and benthos communities was discussed in Ock(2010).

GRAVEL AUGMENTATION IN THE TRINITY
RIVER
The Trinity River located in northwest California is thelargest tributary to the Klamath River. The TRRP has its
primary management reach between Lewiston Dam(40°43'N, 122°47'W) and the North Fork Trinity River(Figure 3), as this reach of approximately 64 km has had its
aquatic and terrestrial habitats severely changed by the
Trinity River Division Project which diverted about 90
percent of the river’s water to the Sacramento River basinin the Central Valley. Subsequent reduction of both flow
and sediment downstream resulted in the loss or degradation—56—

SEDIMENT REPLENISHMENT IMPLEMENTATION BELOW DAMS
of salmon habitats and caused naturally produced salmon
populations to decline by up to 60 to 96 percent dependingon the species. Although considerable restoration efforts
were made through to the year 2000 focused on ‘mechanicalactivities’ such as dredging, channelization, artificial
spawning beds and vegetation removal, these efforts havebeen evaluated to have been ineffective in restoring thenaturally produced salmon population levels. Thus, therestoration strategies were completely changed in the year
2000, to induce ‘natural processes’ morphologically leading
to building and maintaining required salmon habitat (ROD,
2000). ‘Coarse sediment replenishment’ was a focus of the
management actions in TRRP.
TRRP determines five water year types based solely on
Trinity Reservoir inflow. The probability of these water year
types occurring is based on historical data from the previous
eight decades. Total allocation ranges from 1,005 mcm(million cubic meters) of the ‘extremely wet year’ to
455 mcm in the ‘critically dry year’. Based on average inflow
to Trinity Reservoir, allocation to Trinity River represents
43 percent of total inflow into Trinity Reservoir (Table IIa).
Once the water year type is designated and the allocation
downstream is established, the discharge pattern proposed
is outlined in the Trinity River Flow Evaluation (USFWSand HVT, 1999). Peak discharge is to range from 311 cms(cubic meter per second) to 42 cms corresponding to thewater year type (Table IIb). Release patterns for each water
year class were developed to accomplish a variety of fisheryand geomorphological objectives such as water temperature
requirements to provide optimal holding/spawning/survival
temperature for adults, smolts and fry stages, ramping to

Figure 3. Location of the Trinity River below the Lewistonand the Trinity Dams in northwest California, US

peak flow and reducing travel time of outmigrating smolts
and sediment transport to create or maintain bar morphology.
In order to address the sediment deficit downstream
caused by both upstream dams (Trinity and Lewiston Dam),
TRRP has added gravel-size sediments (over 8 mm) that is
appropriate for a spawning bed. The amount of gravel
introduced into the river is based on scientific analyses andcalculation of a gravel budget for the river (USFWS and
HVT, 1999; Gaeuman, 2008; Gaeuman and Krause, 2011),
and is designated up to 51,000 tons depending on the fivewater year types as described in Table IIb. Augmentation
has been implemented by the High-flow direct injection andthe High-flow stockpile by placement. The added gravels
are generally delivered from ‘on-site’ sources in thefloodplain where terraces are mechanically lowered andriparian berms are removed in order to improve channel
dynamics in combination with gravel augmentation.
Also, the grain size of sediment added as well as thevolume should be estimated from comparison of pre- andpost-dam topography. The sediment transport capacity of
the river below dams is reduced after dam construction, thus
it would rarely be appropriate to add the pre-dam sedimentload to the channel below the dam. Because dams typicallyreduce not only the volume of sediment transported but also
the gravel sizes able to be transported, the size distributionof augmented gravel should typically be smaller than the
pre-dam load. In addition, as many riverbeds have coarsened
below dams, the added gravel is significantly smaller thanthe existing bed material size to increase availability of
spawning-sized gravel and sediment mobile under thecurrent flow regime. In TRRP, the gravel added to river isgreater than 15 mm in size after sieving to remove finesediments. It consists mainly of large cobbles ranging insize between 25 to 102 mm and able to become mobile
under anticipated flow regimes and be suitable to support
spawning (Gaeuman, 2008).
In selecting the number of sites and locations where
gravel augmentations are implemented, the sedimentdelivery from tributaries and sediment transport capacity
below injection site were considered as well as accessibility
and effectiveness for field work. Within the TRRP project
area of 40 miles in length, a total of four locations were
designated for high flow coarse sediment augmentation,
because the downstream channel below the region shows
the natural geomorphological process by sediment introduction and flow variation from several tributaries.

Table II. Systematic criteria of the five water year types and corresponding peak flow and sediment amount in a given year
type in the Trinity River Restoration Program(a) Downstream release depending on reservoir inflow(b) Peak flow and sediment augmented

Water year type

Reservoir
inflow(mcm)

Downstream
allocation(mcm)

Ratio inflow
to allocation

Probabilityof occurrence

Peak flow
released(cms)

Peak flow
duration(days)

Volume ofsediment
augmented (m3)Extremely wet
Wet
NormalDry
Critically dry
average> 2,467
1,665–2,466
1,264–1,664
801–1,263
< 801

1,005
865
798
559
455
733

35%
44%
58%
56%
81%
43%

0.12
0.28
0.20
0.28
0.12

311
241
170
127
42

5
5
5
5
36

23,701–51,224
7,645–13,761
1,376–1,682
115–1910‘mcm’ and ‘cms’ indicate million cubic meters and cubic meters per second, respectively.

—57—

G. OCK ET AL.

SUMMARY AND RECOMMENDATIONS
Table III summarized the sediment replenishment
differences between Nunome River and Trinity River
including the purposes, implementation methods used andflushing flow properties and augmented sediment properties.
Based on the findings from the gravel augmentation projectin the Trinity River, some recommendations are presented
for improvement of the Nunome River restoration project.
There is a strong requirement to use an appropriate
implementation method to respond to project purpose,channel transport capacity, cost effectiveness and accessibility. TRRP has developed the methods based on local field
restrictions, and has adopted the high-flow direct injection
and high-flow stockpile. In some vegetated riparian areas,
riparian berms were removed in combination with gravelaugmentation. The Nunome River replenishment project has
mainly used the high-flow stockpile method during only alimited peak duration caused by intensive rainfall andtyphoon. Although the method is effective for large amountof sediment stockpiled to be transported downstream in asingle event, the downstream length for added sediment tobe transported and deposited is likely not greater than thehigh-flow direct injection method. Major deposition was
actually found at just 300 m below the implementation site
below Nunome Dam (Kantoush et al., 2010). If we expecteda much longer transport distance of replenished sediment,
the high-flow injection would be effective. The method isable to add sediment directly to the thalweg line and enables
a greater amount of gravel supplied downstream than highflow stockpile when a longer peak duration is allowed. In
addition, in the case where the riparian berms occupied bydense bamboo in the Kizu River system were designedly
restored into a point bar, the bank rehabilitation in
combination with removal of the riparian berm would
promote the fluvial geomorphologic processes derived by
sediment replenishment.
In the Nunome River, the largest amount of coarse
sediment replenished in a single event was recorded as 500
tons on August and October of 2009. The grain size consisted
mainly of mixed sand and gravels of size less than 19 mm.
The frequency and amount of the sediment replenishment
has been dependent on flushing flow released downstream
during the flooding season in July to October. To date, themagnitude of the flushing flow was likely determined forthe purpose of mainly flood control to drawdown thereservoir water level. On the one side, the replenishment
activities might be effective in the aspects of riverbed
degradation and detachment of attached algae, and sand bar
formation at the meandering point. However, on the otherside, the sand deposited along the channel could be
problematic in terms of habitat condition by burying
spawning gravel and filling rearing pools. Also, the amountand grain size of sediment replenished is likely to be
insufficient to create an in-channel bar system. Thus, if wedesign to restore the bar features that are available forspawning beds, and to transport the coarse sediment much
farther downstream, the much higher magnitude of theflushing flow and longer duration time of the peak flow will
be required.
In the planning stage for determining the spatial
boundary of the research project, the possible sedimentcompensation capacity from tributaries should be considered. As described before, TRRP sets up the limit boundary
for gravel augmentation projects from just the downstream

Table III. Comparative analyses of sediment replenishment activities between Nunome River and Trinity River
Nunome River

Trinity River

Year started

2004

1972

Organization

Japan Water Agency

US Bureau of Reclamation

Physical objectives

to protect riverbed degradation

to induce natural fluvial process for gravelbar formation

Ecological objectives

to detach attached algae

to restore salmon habitat structureHigh-flow stockpile

In-channel stockpile, High-flow stockpile,
High-flow injection

Peak discharge

by peak control by the damby the water year type determined basedon reservoir inflow within a given year

Peak magnitude

approx. 80 m3/s

by the water year type ranging from 42 to
311 m3/s

Frequency

once or twice a year in summer

once a year in spring

Peak duration

2–4 hours

over 5 days

Source of excavatedsediment

check dam located at end of reservoir

on the channel derived from mechanicalfloodplain rehabilitation

Number of sites

a single site just below dam

four sites along downstream channels

Sediment volume
added per yearby the peak discharge (max 500 m3)

by the water year type (max 51,224 m3)

Grain size

mixed sand and gravel (0.075–19 mm)

mixed gravel and boulder (15–102 mm)

Purposes

Implementation method

High flow regime

Sediment added—58—

SEDIMENT REPLENISHMENT IMPLEMENTATION BELOW DAMS
site to the upper half river below the dam, because from thelower half, sediment deficit seems to be sufficiently
compensated through tributary introduction. The mainstream of Kizu River, fortunately, has no large dam built.
The Nunome Dam and the other four dams were constructedin the tributary basins. Therefore, the coarse sediment loadfrom tributaries below the confluence of Nunome Rivershould be one of the determinants for planning the amountof sediment added and implementation sites.

ACKNOWLEDGEMENT
We are grateful to Trinity River Restoration Programand Dr. Matt Kondolf for providing TRRP research reports.
Two anonymous reviewers provided careful reading andvaluable comments on this paper. This research was partially
supported by the institutional program for young researchers
overseas study in Disaster Prevention Research Institute of
Kyoto University and by a research project titled“Development of river-bed management techniques for riverenvironments” financed by the Japanese Ministry of Land,
Infrastructure, Transport and Tourism.

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