Reservoir Sediment Management Using Replenishment:
A Numerical Study Of Nunome Dam
S. Cajot & A. Schleiss
Laboratory of Hydraulic Constructions, Swiss Federal Institute of Technology, Switzerland
sebastien.cajot@gmail.com

T. SumiDisaster Prevention Research Institute, Kyoto University, JapanS. Kantoush
The German University in Cairo, New Cairo City, Egypt

ABSTRACT:
Improving the integration of dams in the natural environment and recovering their storage capacity lost to sedimentation are two topics
of growing concern. To address these two issues, the accumulated sediments are being relocated down into the dam's tail water. This
process is called sediment replenishment (SR), and has been tested in Japan and other parts of the world. More study is required toenhance its effectiveness, and the present research thus offers a practical method to assess positive outcomes of SR on the downstream
ecosystems.
It is shown through a 2-D numerical model of Nunome River that SR positively influences the river's morphology by generating
riffle-pool structures and sand bars. These patterns increase the channel's global heterogeneity, and create hydraulically favorable
habitats for fish and spawning. The habitat quality was quantified by applying suitability indexes to the computed hydraulic variables,
and the SR-induced geomorphologies were designed according to field observations of past SR tests and validated by the 2-D model.
Keywords: reservoir sedimentation management, sediment replenishment, habitat modelling

1. INTRODUCTION
1.1. Context
Given the recent events that occurred in Japan,
Fukushima, and their significant impacts in international
politics, the future of nuclear energy is not bright.
Impacts do not stop at the nuclear energy debate
however, and –not even to mention the countless, direct
victims of the disaster– the impacts on the ocean near thedisaster area are uncertain, but may eventually cause
local fisheries to suffer from fishing restrictions, orworse, population boycott (Grossman, 2011).
Within this unstable nuclear situation, elaborating othersources of energy, in particular renewable ones, has thus
become more than ever the center of attention.
Hydro-power generated by dams could be part of theanswer. However, managing these dams with care for thedownstream environment is now an integrated part of thedeal. Furthermore, this management takes full meaning
when it comes to preventing river fish populations which
are unaffected by the nuclear spill from declining.
Dam functions are multiple and have already become anecessity to our modern societies. Disaster prevention,
flood mitigation, energy production, water storage, somany functions which we could probably not live

without. However, their side effects are consequent, asdams induce strong modifications in flow anddiscontinuity in sediment and river organisms. In the past
few years, even before any nuclear event camechallenging our relation to energy consumption, dam
related issues have become most preoccupying, withraising concerns about environment and increase of
sedimentation issues in reservoirs. Not only is thisproblem becoming critical, as most dams are reaching
several decades of age, but it is also accelerating due toclimate change (Takeuchi, 2004).
The two principle impacts modern research focuses on
are thus the loss of reservoir capacity, and the ecologicaldisturbance in dam tail-water systems. Hereafter, thesediment replenishment (SR) technique is discussed,
which offers solutions to both of these issues, andfocuses on the case of a specific dam from the JapaneseAlps, Nunome Dam.
1.2. Sediment Replenishment
Sediment replenishment basically consists of dredging or
excavating the accumulation of sediments in a dam's
reservoir and transporting them to the reach just below
the dam, where natural or artificial floods will distributethem along the riverbed. A sorting process occurs inmost reservoirs, during which coarse particles settle inthe upstream area and finer particles reach the hydraulicstructure (Schleiss, 2008; Knoblauch, 2006; Okano,2004). This distribution process can be used to selectoptimal material for SR, as coarser sediments are more
beneficial for river systems than silt, which may impact
ecosystems by clogging bed sediments, and causing highturbidity (Hartmann, 2009). Check-dams, locatedupstream of larger dams, may trap sediments before they
enter the functional reservoir and facilitate their removal
by land-based excavation, and do not require any waterlevel modification in the larger reservoir (Okano, 2004).
Japan is one of the current leaders in SR application, withnearly 25% of its dams resorting to sediment excavation,
and a growing use as downstream replenishment. Other
worldwide examples illustrate the great diversity of ways
to replenish sediments, each contributing to expand theknowledge of sediment replenishment effects.

dimensions were ca. 1.5 m2 in the main channel, andlarger in the flood plain to reduce calculation time. The
topographic data was mapped to the grid following theDelaunay triangulation method (TIN).
2.1. Model Calibration
Model parameters were calibrated using field andnumerical reference values. A 1-D analysis of the reachwith HEC-RAS was first performed and calibrated using
field observations and water surfaces measured
downstream. The water elevations and velocities
computed by HEC-RAS were used to calibrate the 2-D
model. The grid accuracy, maximum time step andManning's roughness were adjusted in order for
Morpho2D to predict satisfying hydraulic behaviors.

1.3. Nunome Dam

3. SCOPE OF THE WORKThis 72 meter high concrete dam is located in the Kizu
River system, and is part of a group of five other dams.
Constructed in 1990, its two main purposes are flood
control and water utilization. It served since 2004 forsmall-scale SR testing, which were monitored tounderstand the evolution of bed morphology andecological effects (Kantoush, 2010). Each year, theselected material was deposited on a flat ledge located on
the bank, approximately 600 meters below the dam (Fig.
1). This is done when the discharge is low and the ledge
is dry and accessible. When a natural flood occurs, thewater elevation rises and submerges the sediments,
which are progressively eroded and transported into thechannel and downstream.

3.1. Objectives

Figure 2. Overview of the remaining goals to enhance SR.

Sediment replenishment is a technique under current
development, and two main questions still require further
research to improve the method's efficiency (Fig. 2).

Figure 1. Sediment replenishment concept at Nunome Dam.

2. NUMERICAL MODEL
The reach below Nunome was modeled in RIC-Nays(2009), a two-dimensional pre- and post-processing
application for computational flow models. Hydraulicparameters were computed with Morpho2D, a separate
numerical solver developed for RIC-Nays. Morpho2D
solves the shallow water equations using a
TVD-MacCormack scheme. The equations are written ina generalized curvilinear coordinate system and based on
the finite volume method, which offers flexibility indesigning the mesh. The final reach considered for thesimulations was cut down to 500 meters, thus improving
computational stability and grid accuracy. Grid cells

The first is how to relocate sediments in the riverbed
efficiently. This question needs specific research on theeffects of grain size, replenished volumes, frequency ofoperations and sediment transport. Answering the second
question will define when SR is actually effective. In
particular, it involves studying how ecological habitats
benefit from the contribution of large amounts ofmaterial. In term, this question should also balance theenvironmental benefits with social and economical
aspects, by pondering the urgency of reservoir capacity
loss. Indeed, these three pillars present somewhat
conflicting interests when it comes to planning SR
operations.
In the present study, focus will be set on the ecologicalassessment of dam tailwater systems. The main goal wasto evaluate habitat improvement after sediment
replenishment
operations.
Twodifferent
bed-morphologies, both induced by SR, were comparedto the reference case.

3.2. Scenario Design
Three different bed-morphologies were considered tomeasure the effects of SR on habitat quality. They wereconstructed using RIC-Nays' pre-processing module,
which allows manual modification of the topographic
data (Fig. 3).

Flowrevealed newly formed sand bars (Kantoush, 2010). Twolateral sand bars were implemented in the original meshbased on these observations. In the mesh, cross section
data was raised on the river sides to reproduce these bars.
Their height goes up to 1.40 m, and the dimensions of thelarger bar are 205 m long, and 10 m wide in the center.
The shorter bar is 95 m long, has a rougher tip facing theflow, and reaches 6 m into the channel. The equivalent
volume of sediments required to build up the bars can be
estimated based on their shape. The large bar represents
approximately 1000 m3 of sediments, and the small bar
400 m3. These amounts seem consistent with typical
replenished quantities, which vary from 100 to 700 m3 andmaximum transport capacities during floods calculatedwith the 1-D model.
3.3. Scenario Comparison

Figure 3. Aerial representation of the reference (0), riffle-pool(1) and sand bar (2) scenarios.

3.2.1.
Scenario 0 – Reference channel
The topographical data used for this scenario is based oncross section measurements from 2005. It thus represents
the degraded bed affected by 15 years of dam sediment
retention. The overall reach in consideration is 530
meters long, and the upstream end of the model is locatedapproximately 600 meters below the dam's outlet, wherereplenished material was deposited in past operations.
3.2.2.
Scenario 1 – Riffle-pool
Riffles and pools are typical features which develop in
natural gravel-bed rivers with slopes of 0.001 to 0.02.
They are defined as topographic highs and lows along theriver's thalweg, and through this alternating pattern, they
produce contrasting hydraulic and morphologicalenvironments, which can serve a wide range of aquatic
species at different life stages (Brown, 2008; Emery,
2003; Ock, 2010). More specifically, they induce water
downwelling and promote oxygenation of bed substrate,
and are often used by salmonids (Kondolf, 2000). Ock(2010) established the riffle-pool characteristics induced
by SR, as well as the key parameters which influenced
positively the river's ecology. Following these guidelines,
the main channel elevation was either decreased by
0.60-0.65 m to create pools, or increased by 0.30-0.40 m
to create riffles. This was done in a fairly asymmetricalway, in order to mimic natural structures. Riffles weresystematically designed longer than pools so that theriffle length fraction was of 63%. It appears valid toconsider this structure as an effect of SR, as the 2009
field monitoring revealed riffles and pools which wereabsent before the SR operations.
3.2.3. Scenario 2 – Sand bar
After the 2009 SR operations below the Nunome Dam,
field monitoring, and aerial photographs of the river

The habitat quality of each bed configuration wascompared by applying habitat suitability indexes to thecomputed hydraulic variables. The first index describes
the preferential depths and velocities of salmonids (basedon Chinook Salmon observations; USFWS, 1999), andthe second describes those most suitable for the spawningof Ayu Plecoglossus altivelis (Nagaya, 2008), a fish
which was historically found in Nunome River (Fig. 4).
Because of its physiological resemblance with salmonids,
the first index was applied as well to this popular andcommercially interesting fish, however local suitability
observations would be necessary to improve the quality of
results. The depth habitat suitability index (DHSI) for the
Chinook Salmon at a juvenile stage is optimal at depths
between 0.6 and 1.0 m, and optimal VHSI occurs at
velocities between 0.1 and 0.2 m/s. For the spawningsuitability, depths are not limiting above 3 cm (theapproximate body height of Ayu fish), whereas optimal
velocities are between 0.6 and 1.0 m/s. The global habitat
suitability index was calculated as GHSI = DHSI0.5 x
VHSI0.5 and GSHSI = DSHSI0.5 x VSHSI0.5 for each life
stage respectively. Final GHSI values were classified aspoor (0-0.1), low (0.1-0.4), medium (0.4-0.7) and high(0.7-1.0) (Brown, 2008; Pasternack, 2004). The predictionof sediment transport was done by resorting to theanalytical equation of shear stress, given by the Einstein
log-velocity equation, and the entrainment criteria for the
replenished particles was evaluated through thedimensionless Shield's stress τ *.

Figure 4. Depth and velocity habitat suitability indexes(DHSI, VHSI) for the juvenile Chinook Salmon.

4.

RESULTSThe basic hydraulic parameters were computed with
Morpho2D for each scenario grid, and for four different
discharges (Q1, Q20, Q50, Q81, discharge units of Q are
given in m3/s). The spatial hydraulic values wereconverted to the variables of interest, Shield's stress (τ*),
global habitat and spawning habitat indexes (GHSI andGSHI), as well as the intermediate depth and velocity
indexes (DHSI and VHSI).

become generally too high, and GHSI values remain poor
in most areas of the channel. This tends only to worsen forQ50 and Q81, as the effects of morphological features
tend to diminish, and depths over riffles, pools, and bars
become overall too high.

Flow

4.1. Habitat Suitability
At low discharge, the reference scenario yielded low
depths, up to 0.35 m, and fairly low velocities ranging
typically between 0.4 and 0.7 m/s. The corresponding
velocity and depth indexes were rather low (Fig. 5). Evenfor the lowest flow Q1, velocities computed in thereference channel are too high to represent a suitable
habitat for the river's maturing fish. Therefore, the GHSI
is ultimately poor or low, and thus the reference scenario
is, as expected, ecologically unfavorable. This result
occurs in fact most of the time, as in the Nunome River,Q1 is the residual flow. The habitat conditions even tend
to worsen with increasing discharges.
Conversely, the model predicted a clear improvementwith the riffle-pool scenario, in which appeared threelarge high quality zones, directly correlated to the riffle
and pool pattern (Fig. 6). Indeed, the expected effects of
the riffle-pool units are accurately represented by
Morpho2D, and distinct velocity accelerations can beobserved over riffles and slower flows appeared over the
deeper sections. This scenario, at Q1 flow, in fact
cumulates optimal depths, which now reach 0.7-1 m, andoptimal velocities below 0.2 m/s. Moreover, both of these
optimal sets of values occur over pools, and thisoverlapping thus leads to the excellent GHSI reported on
Fig. 6.

Figure 5. Construction of the global habitat suitability index
for the reference scenario at Q1.

Flow

Figure 6. Construction of the global habitat suitability index
for the riffle-pool scenario at Q1.

4.2. Spawning Habitat Suitability

In the light of these results, the gain of SR is obvious when
it is supposed to induce riffles and pools. However, results
were not as positive for the second scenario. Indeed, theeffects of sand bars are somewhat ambivalent. The firstbar did improve slightly the velocities just upstream of it,
as it acts like a local obstruction to flow. However, it also
acted like a constrictor which reduces the width of themain channel and accelerates the flow to unsuitablevelocities. The final GHSI for scenario 2, Q1, wastherefore generally low, and locally poor while passing
the large bar, due to excessive velocities and shallow
water.

Juvenile habitat analysis previously showed that the ideal
living conditions occurred at low flow, and that these weresystematically improved by SR induced morphology.
Spawning habitat on the other hand benefits from anincrease in discharge, and optimal conditions were yieldedat Q20. Therefore, an adapted dam outlet management,
established according to these facts, could simply increase
the fish living conditions throughout the year. In winter,spring and summer, maintaining the natural Q1 flow
would be suitable for juvenile and maturing fish. Then,
during the Ayu's spawning season in autumn, a slightincrease in the outflow would increase velocities andallow optimal spawning conditions.

For larger discharges, habitat quality was generally lower
for all scenarios, despite an increase in terms of DHSI at
Q20. Indeed, velocities yielded for large discharges
systematically reduced the VHSI. Though for all scenarios
velocities offer poor habitat quality, scenario 1 offered thebest conditions, because the two downstream pools still
managed to reduce velocities towards more favorable
values. Ultimately, starting at 20 m3/s, the velocities

Another interesting result which appeared by comparing
the spawning and non-spawning habitat preferences inscenario 1 was the delimitation of the high quality zones.
Indeed, it was shown that mature fish had their highquality habitats coinciding with pools along the reach(Fig. 6). By analyzing more accurately this result with thedelimitation of the high quality spawning areas, it appears

that the latter systematically stretch a bit further towards
emerging riffles. This was previously observed by many
researchers (Hunter, 1991; Brown, 2008; Wheaton, 2004),
which pointed out that salmonids preferably spawn ingravel beds located at riffle heads or at pool downstream
edges. On Fig. 7, it is noticeable that spawningpreferences begin inside the pools, and end near riffle
heads, which corroborates both the consistency of thenumerical model, and the suitability indexes.

during low flows, which may be beneficial for salmonid
spawning, without preventing normal transport process of
the replenished volumes.

Flow

Flow

Figure 7. Streamwise delimitations of habitat (left, GHSI)
and spawning (right, GSHSI) habitat preferences. Maturing fish
find comfort in pools, whereas spawning is best if located at poolends and riffle heads.

Figure 8. Shields stress is represented for two discharges, Q1(top row) and Q81 (bottom row), and for two substratedimensions, fine (left column) and coarse (right column).

4.3. Sediment Mobility

5.

In order to assess the relevance of the designed
morphologies, and also to evaluate bed stability, which ispartly required for optimal spawning conditions,dimensionless shear stresses were calculated at Q1 andQ81 for two different grain sizes (Fig. 8).

In the opening of this paper, the need to evaluate theeffects of sediment replenishment (SR) on theenvironment was emphasized, and reinforced by thecurrent energy crisis. Indeed, dams may become essential
actors in tomorrow's energy policies and all efforts must
thus be focused on reducing their ecological side effects.
In this context, the present study contributed in increasing
knowledge of this underrated reservoir sedimentmanagement method.

Predicted results were fairly consistent with those from
HEC-RAS. During low flow, an alternating pattern ofhigh and low shear stresses appears. Interestingly, it
matches the riffle-pool structure adopted in scenario 1.
This result illustrates how riffle-pool structures may arise
from erosion and deposition fluctuations such as thesecomputed by Morpho2D. This pattern is not visible
anymore for Q81, however the stress distribution in thebend similarly illustrates how the monitored sand bar,
represented in scenario 2, could arise.
Because Shields stress values were systematically larger
than the critical entrainment value for the replenishedgrain size (τ*cr = 0.0369 [-]), a second substrate wastested, with larger characteristic diameters. A net decreasein τ* values was thereby obtained. At Q1, transport is only
partial in most of the reach, and complete stability occurs
in the central part of the reach. For Q81, despite a decreasein overall shear stress, full mobility is still achieved,
allowing transport of such material during high floods.
This result may be of interest to future SR operations, as
selecting slightly coarser sediments may increase stability

CONCLUSION

Put into a sustainable development context, SR faces thenecessity of identifying predominant issues andsite-specific goals of a reservoir before considering any
remedy. Indeed, sediment reservoir management raises
conflicting social, economical and environmental
interests, which need to be balanced in order to apply SR
with optimal efficiency. To help balance objectively theinterests of reservoir sedimentation and downstream
health, there is a need for effective ecological evaluation
tools. Two habitat suitability indexes were proposed, andapplied to a test reach in the Nunome River. These
allowed evaluating the positive impact of rivergeomorphology on ecological health. The indexes
quantified fish habitat and spawning quality, as well as
sediment mobility. The latter could be employed toevaluate habitat quality, but also to predict SR-induced
morphologies.

The results established with the numerical model weresatisfying on two different levels. First, they showed that
Morpho2D was able to reproduce successfully the change
in flow patterns expected by the modified bed
morphologies. Indeed, the way riffles and pools modify
water depths and velocities is well documented, and themodel calculated these modifications efficiently.
Furthermore, the suitability indexes were able to highlightaccurately specific fish habitat preferences. These positive
observations appreciate favorably Morpho2D, which tothis day remained fairly unpopular outside of Japan.
Second, results almost systematically validated theSR-induced morphologies. At low flow, it was shownhow the riffle-pool morphology offered high quality
habitats, whereas the reference scenario, lacking suchchannel diversity, presented only medium to low habitat.
At higher flows, despite a generalized loss of habitat
quality, the sand bars and riffle-pool structures tended toenhance higher quality areas. Regarding the spawning
habitat assessment, results pointed out optimal flows,which could be managed at the dam's outlet during the
reproduction season.
The sediment mobility analysis was able to validate theriffle-pool and sand bar developments. Furthermore, theconsistency observed between 2-D and 1-D shear stresspredictions tends to acknowledge Morpho2D 's reliability.
The assessment method applied in this report is howeverneither exhaustive, nor flawless. It could be improved forexample with a higher computation capacity, allowing
finer mesh accuracy. Additionally, the model could
include aspects which were disregarded in this study, e.g.
water temperature and quality. The indexes applied shouldbe improved with additional local fish monitoring, rather
thanextrapolating
fromgeographicallyandphysiologically distant measurements. Moreover, in order
to discover optimal morphologies, it would be interesting
to test habitat quality for different riffle-pool
configurations, or sand bar locations and elevations.
Nevertheless, in the light of the methodology applied inthis thesis, the next step may now be approached withconfidence. That is, trying to answer the following
practical questions. How should sediments bereplenished? Where should they be deposited in thechannel? What volume is necessary to produce proper bed
morphologies? Which grain-size is most suited for fish,
invertebrates and other organisms? The interaction ofreplenished material with flow should also be studied.
How far will sediments be transported during floods, andwhere will they settle? How long does it take to recoverfrom the bed degradation? When will significant riffles
and pools appear, and how persistent are they? Whichbrings up the question: what is the required frequency of
SR operations to conserve suitable habitats?
As this point, much work is still required to improve SR
techniques, however all of the above questions may only
be answered by considering the effects on the downstream

ecosystems, and being able to quantify them. This thesis
therefore strived to develop, discuss and apply the tools
able to assess environmental quality, which can now be
used to evaluate future SR outcomes in Nunome, and inother dams worldwide.
REFERENCES
Brown, R.A., Pasternack, G.B. (2008): Engineered channel
controls limiting spawning habitat rehabilitation success onregulated gravel-bed rivers. Geomorphology 97: 631-654.
Emery, J., Gurnell, A. M., Clifford, N. J., Petts, G.E., Orrissey,
I. P., Soar, P. J. (2003): Classifying the hydraulicperformance of riffle–pool bedforms for habitat assessment
and river rehabilitation design. River Research andApplications 19: 533–549.
Grossman, E., 2011. Radioactivity in the ocean: Diluted, but far
from harmless. Yale Environment 360.
Hartmann, S. (2009): Sustainable Sediment management ofalpine reservoirs considering ecological and economical
aspects, Volume 7, Alpreserv.
Hunter, C. J. (1991): Better trout habitat: a guide to stream
restoration and management. Montana Land Reliance, 320 p.
Kantoush, S.A., Sumi, T., Kuboata, A. (2010): Geomorphic
response of rivers below dams by sediment replenishment
technique. River Flow 2010: 1155-1163.
Knoblauch, H. (2006): Sustainable Sediment management ofalpine reservoirs considering ecological and economical
aspects, Volume 4, Alpreserv.
Kondolf, G.M. (2000): Some suggested guidelines forgeomorphic aspects of anadromous salmonid habitat
restoration proposals. Restoration Ecology 8 (1): 48–56.
Nagaya T., Shiraishi Y., Onitsukab K., Higashinoc M., TakamiT., Otsukad N., Akiyamab J., Ozeki H. (2008): Evaluation ofsuitable hydraulic conditions for spawning of ayu withhorizontal 2D numerical simulation and PHABSIM.
Ecological Modelling 215 : 133–143.
Ock, G. (2010): Particulate organic matter dynamics in thedownstream of dam reservoirs: roles of channel
geomorphology and responses of benthos communities.
School of Engineering, Kyoto University, Japan, 233p.
Okano, M., Kikui, M., Ishida, H., Sumi, T. (2004): Reservoir
sedimentation management by coarse sediment
replenishment below dams. Proceedings of the ninth
international symposium on river sedimentation, Yichang
China, 2004.
Pasternack, G.B., Lau Wang, C., Merz, J.E. (2004): Applicationof a 2D hydrodynamic model to design of reach-scale
spawning gravel replenishment on the Mokelumne river,
California. River research and applications 20: 205–225.
RIC-Nays, (2009): http://i-ric.org/nays/en/index.html.
Schleiss, A.J., De Cesare, G., Jenzer Althaus, J. (2008):
Reservoir sedimentation and sustainable development. CHR
Workshop Erosion, Transport and Deposition of Sediments,
Berne.
Takeuchi, K. (2004): Importance of Sediment Research in
Global Water System Science. University of Yamanashi,
Japan.
United States Fish and Wildlife Service, (1999): Trinity River
Flow Evaluation. U.S. Department of the Interior
Washington D.C.
Wheaton, J. M., Pasternack, G. B., Merz, J. E. (2004):
Spawning habitat rehabilitation – II. Using hypothesis
development and testing in design, Mokelumne River,
California, U.S.A. Intl. J. River Basin Management 2 (1):
21–37.