Earth’s Future
RESEARCH ARTICLE
10.1002/2013EF000184
Key Points:• Reservoirs trap sediment, losing
storage capacity• Downstream reaches can become
sediment starved• Many dams can be
designed/operated to pass sediment

Corresponding author:
G. M. Kondolf, kondolf@berkeley.edu

Citation:
Kondolf, G. M. et al. (2014), Sustainablesediment management in reservoirs
and regulated rivers: Experiences fromﬁve continents, Earth’s Future, 2,
256–280, doi:10.1002/2013EF000184.

Received 30 SEP 2013
Accepted 28 APR 2014
Accepted article online 4 APR 2014
Published online 23 MAY 2014

Sustainable sediment management in reservoirs
and regulated rivers: Experiences from ﬁve continents
G. Mathias Kondolf1, Yongxuan Gao2, George W. Annandale3, Gregory L. Morris4, Enhui Jiang5,
Junhua Zhang5, Yongtao Cao5, Paul Carling6, Kaidao Fu7, Qingchao Guo8, Rollin Hotchkiss9,
Christophe Peteuil10, Tetsuya Sumi11, Hsiao-Wen Wang12, Zhongmei Wang5, Zhilin Wei13, Baosheng
Wu14, Caiping Wu5, and Chih Ted Yang15
1 University of California, Berkeley, California, USA, 2 Natural Heritage Institute, San Francisco, California, USA, 3 Golder

Associates, Lakewood, Colorado, USA, 4 GLM Engineering COOP, San Juan, Puerto Rico, USA, 5 Yellow River Institute ofHydraulic Research, Zhengzhou, Henan, China, 6 University of Southampton, Southampton, UK, 7 Yunnan University,
Kunming, Yunnan, China, 8 Institute of Water Resources and Hydropower Research, Beijing, China, 9 Brigham Young
University, Provo, Utah, USA, 10 Compagnie Nationale du Rhone, Lyon, France, 11 Kyoto University, Gokasho, Kyoto,
Japan, 12 National Cheng Kung University, Tainan, Taiwan, 13 Wuhan University, Wuhan, Hubei, China, 14 Tsinghua
University, Beijing, China, 15 Colorado State University, Fort Collins, Colorado, USA

Abstract By trapping sediment in reservoirs, dams interrupt the continuity of sediment transport
through rivers, resulting in loss of reservoir storage and reduced usable life, and depriving downstream
reaches of sediments essential for channel form and aquatic habitats. With the acceleration of new dam
construction globally, these impacts are increasingly widespread. There are proven techniques to passsediment through or around reservoirs, to preserve reservoir capacity and to minimize downstream
impacts, but they are not applied in many situations where they would be eﬀective. This paper summarizes collective experience from ﬁve continents in managing reservoir sediments and mitigating
downstream sediment starvation. Where geometry is favorable it is often possible to bypass sedimentaround the reservoir, which avoids reservoir sedimentation and supplies sediment to downstream reacheswith rates and timing similar to pre-dam conditions. Sluicing (or drawdown routing) permits sediment
to be transported through the reservoir rapidly to avoid sedimentation during high ﬂows; it requires
relatively large capacity outlets. Drawdown ﬂushing involves scouring and re-suspending sedimentdeposited in the reservoir and transporting it downstream through low-level gates in the dam; it works
best in narrow reservoirs with steep longitudinal gradients and with ﬂow velocities maintained above thethreshold to transport sediment. Turbidity currents can often be vented through the dam, with the advantage that the reservoir need not be drawn down to pass sediment. In planning dams, we recommend thatthese sediment management approaches be utilized where possible to sustain reservoir capacity andminimize environmental impacts of dams.
1. Introduction

This is an open access article under
the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and distribution in any medium, provided theoriginal work is properly cited, the use
is non-commercial and no modiﬁcations or adaptations are made.

KONDOLF ET AL.

1.1. Reduced Sediment Loads Downstream of Dams
Dams interrupt the continuity of sediment transport through rivers systems, causing sediment to accumulate within the reservoir itself (impairing reservoir operation and decreasing storage) and depriving
downstream reaches of sediments essential to maintain channel form and to support the riparian ecosystem. The most common discourse on sediment problems has been that of increased erosion and sedimentloads from poor land use and expansion of human impacts into previously undisturbed areas [Walling,
1999]. However, most river systems around the world actually show decreased sediment loads, because oftrapping by upstream dams [Walling and Fang, 2003]. Estimates of sediment that reached the ocean (orat least deltas) under pre-human-disturbance conditions have been in the range of roughly 15–20 billion
tons per year (Bty−1 ) [Walling, 2006]. Of this amount, Syvitski et al. [2005] estimated that catchment-level
human disturbances have increased the erosion of sediment from uplands and its delivery to rivers by
about 2.3 Bty−1 , but that the net eﬀect has been a reduction in sediment loads of rivers by an estimated 1.4
billion tons due to sediment trapping in reservoirs. Vorosmarty et al. [2003] extrapolated estimates from633 large reservoirs to over 44,000 smaller reservoirs and concluded that more than 53% of the global© 2014 The Authors.

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sediment ﬂux in regulated basins is potentially trapped in reservoirs, or 28% of all river basins, for a total
trapping of 4–5 Bty−1 .
No matter what estimate is used, sediment trapping by reservoirs is now of primary global importance.
This has signiﬁcant consequences, both for the channels downstream, and for the sustainability of thereservoirs and thus future water supplies. There is increasing evidence of channel erosion and ecosystem
impacts resulting from sediment starvation downstream of dams, often termed hungry water [Kondolf ,1997; Draut et al., 2011; Grams et al., 2007; Ma et al., 2012; Schmidt and Wilcock, 2008; Singer, 2010]. Coastal
areas that rely on riverine sediment supply are especially vulnerable to impacts of reduced sedimentsupply [Vorosmarty et al., 2003], such as sand-starved beaches that have narrowed or disappeared, accelerating erosion of coastal cliﬀs [Inman, 1985; Gaillot and Piégay, 1999] and deltas [Syvitski et al., 2009]. The
Mississippi River Delta has lost over 4800 km2 [Coastal Protection and Restoration Authority of Louisiana(CPRA), 2012] due largely to reduced sediment supply from trapping in upstream reservoirs [Meade andParker, 1985; Meade and Moody, 2010]. Of the world’s 33 major deltas, 24 are sinking, largely from humancauses including reduced sediment supply; in combination with an assumed 0.46 m rise in sea level by
2100, this would lead to a 50% increase in ﬂooding, with profound consequences for coastal populations[Chen et al., 2012]. Actual sea level rise will not be uniform across the globe because of isostatic and gravity
eﬀects associated with melting ice caps [Milne, 2008].
1.2. Coarse vs. Fine Sediment
It is useful to distinguish between coarse and ﬁne sediments, both in their role in river systems and theirsusceptibility to being trapped by reservoirs. Coarse sediment (gravel and sand) can be viewed as forming the “architecture” of most riverbeds, as the material that constitutes the channel bed, bars, and oftenbanks. Moreover, many geomorphic features that serve as important habitats, such as riﬄes, are composed of coarse sediments (gravels, cobbles). Downstream of dams, reduced supply of coarse sedimenthas resulted in channel incision and consequent eﬀects on bridges and other infrastructure, and degradation of aquatic habitat quality, including loss of gravels needed by spawning salmon [Kondolf , 1995].
Fine-grained sediment (silt and clay) is important for the structure of some riverine forms, such as vertically accreted ﬂoodplains and estuarine mud ﬂats, but it also plays important roles distinct from coarse
sediment, such as a source of turbidity, and its role in transporting nutrients and contaminants adsorbed
onto clay particles. Anthropically increased loads of ﬁne sediment (e.g., from land disturbance) can cause
problems of increased turbidity in the water column and sedimentation in river channels, estuaries, andharbors [Owens et al., 2005], and deposition of ﬁne-grained sediment in streambed gravels can aﬀect
salmon spawning habitat [Kondolf , 2000] and aquatic habitats generally [Wood and Armitage, 1997]. Lossof a river’s natural ﬁne-grained sediment load can have a range of negative impacts, as the native species
in a river are, by deﬁnition, adapted to the natural conditions. Construction of Glenn Canyon Dam dramatically reduced turbidity and summer water temperatures in the downstream reaches, providing excellent
habitat for exotic rainbow trout but nearly extirpating the native ﬁsh species [Schmidt et al., 1998], andbecause outmigrating juvenile salmon can avoid predators in turbid water better than in clear waters[Gregory and Levings, 1998]. Artiﬁcially clear releases below New Don Pedro Dam on the Tuolumne River,
California, have likely contributed to observed high predation rates on juvenile salmon [EA Engineering,
Science, and Technology (EA), 1992].
Nutrients are often associated with ﬁne sediments, and along with trapping of ﬁne sediments, nutrient trapping by dams can have signiﬁcant ecosystem impacts downstream. After construction of Three
Gorges Dam (and other dams upstream), suspended sediment loads decreased in the Middle Yangtze
River by 91%, total phosphorous decreased by 77%, and particulate phosphorous by 83% annually [Zhouet al., 2013]. As phosphorous is the limiting nutrient for bioactivity in the river, its reduction implies a likely
reduction in primary productivity of the river and coastal region downstream.
Because gravel moves through river channels as bed load, it is virtually certain to be trapped by dams.
Dams typically have trap eﬃciencies of 100% for gravel, with only small dams on steep channel capable ofpassing bed load. (However, once any dam has completely ﬁlled with sediment, bed load can presumably pass over the structure.) Because sand can be transported as either bed load or suspended load,
depending on turbulence of the ﬂow, its trapping eﬃciencies are variable. Sand can pass many smaller
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dams on steep streams with turbulent ﬂow, but typically not large reservoirs. Silt and clay are always
transported as suspended load, and are often termed washload, i.e., suspended load ﬁner than sand thatcan pass through the natural river network (and small dams without signiﬁcant storage) mostly without
being deposited. Large reservoirs with protracted residence times can trap even washload [Morris andFan, 1998]. The percentage of suspended sediment trapped by a reservoir increases with residence time,and generally increases with the ratio of total reservoir storage to inﬂow, as estimated by the widely used
Brune curve [Morris and Fan, 1998].
1.3. Loss of Reservoir Capacity
Sediment trapped behind dams reduces reservoir capacity, and in some high-proﬁle cases, reservoirs havealready ﬁlled with sediment, not only impairing functions and/or rendering useless the dam infrastructure,
but posing safety hazards as well [e.g., US Bureau of Reclamation, 2006; California State Coastal Conservancy, 2007; Wang and Kondolf , 2014]. Sumi et al. [2004] reported that global gross storage capacity was
about 6000 km3 and annual reservoir sedimentation rates about 31 km3 (0.52%), such that (ignoring new
storage created after that date), global reservoir storage capacity would be half lost by 2100. Annandale[2013] estimated that global net reservoir storage has been declining from its peak of 4200 km3 in 1995
because rates of sedimentation exceed rates of new storage construction. With increasing demands forwater storage, and fewer feasible and economically justiﬁable sites available for new reservoirs, loss of
capacity in our existing reservoirs threatens the sustainability of water supply [Annandale, 2013]. Thus, we
can think of the sediments accumulating in reservoirs (with negative consequences there) as “resources
out of place,” because these same sediments are desperately needed by the downstream river system tomaintain its morphology and ecology, as well as replenishing vital land at the coast.
1.4. The Imperative to Sustainably Manage Sediment
The impetus to develop alternatives to carbon-based energy sources has renewed support for hydroelectric power projects globally [e.g., the World Bank, 2009], with new hydropower projects mostly in thedeveloping world, many ﬁnanced by outside investors. For example, in the Mekong River basin, largely
undeveloped before 1990, 140 dams are built, under construction, or planned [Grumbine and Xu, 2011].
An assessment of pre-dam sediment yields by geomorphic province and systematic analysis of sediment
trapping by planned dams (accounting for changes in trap eﬃciency over time and for multiple dams in agiven sub-catchment) indicates that full build of the 140 dams as planned would result in a 96% reduction
in sediment load to the Mekong Delta, i.e., once erodible sediment stored in channel bed and banks was
exhausted, the Delta would receive only 4% of its natural sediment load (G. M. Kondolf et al., Dams on the
Mekong: cumulative sediment starvation, submitted to Water Resources Research, 2013). With such profound change to the river’s sediment load, the ongoing subsidence and land loss in the Mekong Delta arelikely to accelerate.
Trapping of sediment by dams is not inevitable, at least not by all dams. Some dams can be designed
to pass sediment, either through the dam or around the reservoir, using a range of proven techniques,
each applicable to a range of conditions. However, sediment management approaches are not used inmany reservoirs where they could be. It may be that dam developers and operators are not aware of therange of potential management approaches, nor that they have been demonstrated to be eﬀective (andunder what conditions diﬀerent methods are appropriate). Thus, collectively we are missing opportunities
to sustain reservoir functions into the future, and to minimize downstream impacts of sediment starvation. With large numbers of new dams planned for Asia, Africa, and South America, it is timely that lessons
learned from successful reservoir sediment management be used to inform planning and design of newdams, and to establish policies and design standards for sustainable reservoir design and management.
At the Fifth International Yellow River Forum in September 2012, reservoir sediment management experts
from abroad joined Chinese experts for two fruitful days of presentations and discussion to exchange
collective experience in sustainably managing sediment in reservoirs and addressing problems of downstream sediment starvation. Because of the extremely high sediment loads of the Yellow River, China has
innovated more than most other countries in sediment management, and has a rich experiential baseupon which to draw. Likewise, good examples of successful sediment management can be studied fromother Asian countries, Europe, North and South America, Africa, and the Middle East. This document summarizes key points from the collective experience of the expert group and reﬂections on approaches
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to sustainably managing sediment in and through reservoirs. It builds upon prior work [e.g., Morris andFan, 1998] through its incorporation of more recent and diverse experience in managing sediments inreservoirs, and through its synthesis of the subject in a more concise form. We begin with a review ofapproaches to sustainable sediment management and examples of successful application drawn fromthe experience of our participants and the published literature. From this base of experience, we present
general principles for managing sediment, and conclude with implications for dam planning, design, andoperation.
There is a wide range of sediment management techniques to preserve reservoir capacity and pass sediment downstream, many of which represent ways to achieve the goals expressed by the Chinese expression, “Store the clear water and release the muddy.” Many of them have been successfully employed
in reservoirs in a range of settings, as described by Morris and Fan [1998], Annandale [2011], Sumi et al.
[2012], and Wang and Hu [2009]. Although terminology diﬀers somewhat, the reservoir sediment management classiﬁcations of Morris and Fan [1998] and Kantoush and Sumi [2010] both distinguish among threebroad categories: (1) methods to route sediment through or around the reservoir, (2) methods to remove
sediments accumulated in the reservoir to regain capacity, and (3) approaches to minimize the amountof sediment arriving to reservoirs from upstream (e.g., Figure 1). The ﬁrst two methods maintain reservoir
capacity and provide sediment to downstream reaches, but the third category (reducing sediment delivery from upstream) addresses only the reservoir capacity issue, not downstream sediment starvation. We
begin describing methods in the ﬁrst two categories (that pass sediment through or around the reservoir, minimizing sediment accumulation in the reservoir and supplying some sediment to downstream
reaches), then address strategies to reduce sediment supply from the upstream catchment (which do notaddress downstream sediment starvation), and ﬁnally strategies to mechanically add sediment to river
channels downstream of dams (which do not address reservoir capacity sustainability).

2. Reservoir Sediment Management Strategies
This section reviews reservoir sediment management strategies that both prolong reservoir life and beneﬁt downstream reaches by mitigating the sediment starvation that results from sediment trapping. Someof the terms for sediment management have been used in diﬀerent ways by various authors (e.g., “sluicing” and “ﬂushing” are often assigned confusing meanings), so we endeavor to clearly indicate how wedeﬁne the various terms.

Reduce Sediment Yield
From Watershed

Minimize Sediment
Deposition

Increase or
Recover Volume

Reduce Sediment Inflow
From Upstream

Route Sediments
Around or Through Storage

Recover, Increase, orReallocate Storage Volume

ReduceSediment
Production

Trap Sediment
Above
Reservoir

Soil
ErosionControl

Structures
on Main
Channel

Off-Channel
Reservoirs

TurbidDensity
Currents

Dry
Excavation

Drawdown
Flushing

Streambank
ErosionControl

Dispersed
Headwater
Structures

Flood
BypassDrawdown
Routing(Sluicing)Dredging

Pressure
Flushing

Sediment
BypassNonstructural
Measures

Sediment
Pass-ThroughMechanical
Excavation

Redistribute
Sediment

Hydraulic
Excavation

Enlarge
Storage

Reallocate
Pool

Figure 1. Classiﬁcation of strategies for sediment management from the perspective of sustaining reservoir capacity.

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2.1. Sediment Bypassing and Oﬀ-Channel Reservoir Storage
Sediment bypassing diverts part of the incoming sediment-laden waters around the reservoir, so that they
never enter the reservoir at all. Typically, the sediment-laden waters are diverted at a weir upstream of
the reservoir into a high-capacity tunnel or diversion channel, which conveys the sediment-laden waters
downstream of the dam, where they rejoin the river (Figure 2). Normally the weir diverts during high ﬂows,
when sediment loads are high, but once sediment concentrations fall, water is allowed into the reservoir.
(A variant of this approach may involve the use of a bypass that diverts sediment-laden waters already ina reservoir.)
The ideal geometry for sediment
bypass is one where the river makesa sharp turn between the point of
sediment collection and the pointof sediment reintroduction to minClear water diversion
Sediment bypass
imize the length of the conveyancedevice and take advantage of the relaFigure 2. (a) Conventional reservoir, which traps incoming sediment, contrasted
to alternative conﬁgurations for bypass of sediment-laden ﬂood ﬂows aroundtively steeper gradient for gravity ﬂow
the storage pool: (b) bypass oﬀ-stream storage, wherein a diversion dam in the(Figure 2c). Where that ideal condition
river diverts water to the oﬀ-channel reservoir during times of clear ﬂow but
does not exist, the technique is mostdoes not divert when suspended sediment concentrations are high, and (c) asediment bypass channel or tunnel, which during times of high water and highpractical where the reservoir is relasediment concentrations, diverts ﬂow from the river upstream of the reservoir,
tively short, as there must be suﬃcientpassing it around the reservoir and into the downstream channel.
gradient to drive the transport of sediment through the diversion tunnel ordiversion channel. At Nagle Dam in South Africa, the river takes a sharp bend at the reservoir, providing an
ideal “short cut” for the bypass, with steep slopes [Annandale, 1987].

(a)

(b)

(c)

Overall, Japan and Switzerland are the leading countries for sediment bypass tunnels: in Japan, three
are in operation and two under construction; in Switzerland, six are in operation [Vischer et al., 1997;
Auel et al., 2010] (Table 1). The oldest sediment bypass tunnel in Japan was installed at the municipal
water supply reservoir Nunobiki dam near Kobe city 8 years after completion of the dam in 1900. This
bypass scheme has successfully diverted coarse sediment for more than 100 years as described by Sumiet al. [2004]. At the Miwa and Asahi dams in Japan, the rivers are suﬃciently steep that a straight tunnel
has adequate gradient to carry most sediment load downstream of the dam [Sumi et al., 2004; Suzuki,
2009; Sumi et al., 2012]. Miwa Dam (on the Mibu River, in the Tenryu River basin) was built in 1959 with30 million m3 (Mm3 ) storage capacity. Subsequent deposition of 20 Mm3 of sediment has prompted
expensive sediment removal eﬀorts. To prolong the reservoir’s life, a 4.3-km-long sediment bypass
tunnel and diversion weir at the upstream end of the reservoir were constructed in 2005 (Figure 3). The
dam and diversion tunnel operate such that during the rising limb of a ﬂood, sediment-laden ﬂows
are diverted into the bypass tunnel, but the tunnel inlet is closed on the falling limb of the ﬂood so
the clear waters can be stored (Figure 4) (assuming the commonly observed hysteresis curve of higher
sediment concentrations on the rising limb). The system is successfully routing sediment downstream,
the eﬃciency being a function of the magnitude of the ﬂood and the timing of the operation, and withno impacts detected on the downstream ecology in the 7 years after the scheme’s inception [Sumi et al.,2012]. Asahi Dam on the Shingu River was built in 1978; sedimentation problems motivated the 1998
construction of a sediment bypass with a 13.5-m high diversion weir and 2350-m long tunnel. By 2006,
sediment bypassing through the tunnel had avoided a cumulative 750,000 m3 of sediment deposition[Mitsuzumi et al., 2009].
In Taiwan, the sediment-plagued Shihmen Reservoir on the Dahan River, will be retroﬁt with a sediment bypass, taking advantage of the sharp river bend at the reservoir [Wang and Kondolf , 2014; Water
Resources Agency (WRA), 2010). Sediment bypasses are expensive because of the cost of the tunnel, buthave many advantages, in passing sediment without entering the reservoir, and without interfering withreservoir operation. In case of coarse sediment bypassing, an anti-abrasion design for tunnel bottom
surface is essential for minimizing long-term operation costs, as described by Visher et al. [1997] and Sumiet al. [2004].
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Table 1. Characteristics of Successful Sediment Bypass Tunnels in Japan and Switzerland
TunnelAnnualCross

TunnelGeneralDesignOperation

Date

TunnelSectionLengthSlope

Design Q

VelocityFrequency

Name of Dam

Country

ConstrShape(BxH) (m)

(m)

(%)

(m3 s−1 )(ms−1 )

(days/a)NunobikiJP

1908

Archway

2.9 × 2.9

258

1.3

39

7

—

Asahi

JP

1998

Archway

3.8 × 3.8

2350

2.9

140

12

13

Miwa

JP

2004

Horseshoe

2r = 7.8

4300

1

300

10

2–3
—

Matsukawa

JP

2015

Archway

5.2 × 5.2

1417

4

200

15

KoshibuJP

2016

Horseshoe

2r = 7.9

3982

2

370

9

—

Egshi

CH

1976

Circular

R = 2.8

360

2.6

74

10

10

Palagnedra

CH

1974

Circular

2r = 6.2

1800

2

110

13

2–5

Pﬀaﬀensprung

CH

1922

Horseshoe

A = 21 m2

280

3

220

14

ca. 200

RempenCH

1983

Horseshoe

3.5 × 3.3

450

4

80

12

1–5

Runcahez

CH

1961

Archway

3.8 × 4.5

572

1.4

110

9

4

Solis

CH

2012

Archway

4.4 × 4.68

968

1.8

170

11

1–10

Notes: CH = Switzerland, JP = Japan, Q = discharge.

Spillway

Figure 3. Diagram of sediment bypass system for Miwa Dam, Mibu River, Japan. A check dam traps coarse sediment, and a diversion
weir diverts ﬂows with high suspended sediment concentrations into a bypass tunnel.

An alternate approach to sediment bypass is to build oﬀ-channel reservoir storage, such that the diversions from the weir are clear-water diversions, while sediment-laden water is left in the river to pass downstream (Figure 2b) [Morris and Fan, 1998]. Similar to sediment bypass, there needs to be suﬃcient gradientto drive ﬂow through diversion channels or tunnels to the oﬀ-channel storage feature. One advantage ofthis approach is that all bed load can be excluded from the reservoir. Simulations using daily data fromstreamﬂow and sediment gages in Puerto Rico indicate that it is possible to exclude between 90% and
95% of the total sediment load from an oﬀ-stream reservoir, thereby prolonging reservoir life by a factor
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Figure 4. Sediment bypass at Miwa Dam June 19, 2010: (a) hydrographs for inﬂow into the reservoir and for ﬂow through the bypass
tunnel, and (b) suspended sediment concentrations (mg/l) at the diversion weir and in the bypass tunnel. [Modiﬁed from Kantoush
et al., 2011]of more than ten as compared with an on-channel reservoir on the same river [Morris, 2010]. The intake
structure can be designed to present a much smaller impediment to the migration of ﬁsh species thana dam, and downstream river morphology is maintained because sediment load and ﬂows capable oftransporting sediment are not impaired. The rate at which water can be diverted to the oﬀ-channel storage reservoir is limited to the capacity of the diversion channel, so this approach is less suited to ﬂashy
streams in semi-arid zones where water ﬂow is concentrated in ﬂoods. Under appropriate hydrologic conditions, even a diversion of relatively modest capacity may result in ﬁrm yields close to those achieved byan on-channel reservoir.
Oﬀ-channel storage could be more widely used than has been the case. In run-of-river hydropower
projects, turbines run at full capacity during the wet season when streamﬂow exceeds the plant’s
design capacity. During the dry season, an oﬀ-channel reservoir can provide a small live storage volume, to store inﬂow over a 24-h period for delivery to turbines during the hours of peak demand.
For example, the recently designed San José project in the Andes Mountains of Bolivia is fed by eight
intakes, has 125 MW capacity with 600 m of gross head, and requires a 0.35 Mm3 regulating reservoir
to provide 6 h of peak power (Figure 5). Oﬀ-channel storage was ideal to provide peaking power atthis site, because vertical canyon walls made site access diﬃcult for construction of a main stem dam,and the high load of large bed material (up to 1 m diameter) presented an unfavorable situation for
sediment management. Coarse sediment (>0.15 mm) will be removed by desanders prior to entering the regulating reservoir, and ﬁner sediment trapped in the pool will need to be excavated after
several years.
The Cameguadua and San Francisco oﬀ-stream reservoirs in the Cauca River basin near Manizales, Colombia, have operated successfully for many years. These two reservoirs have a total installed capacity of
197 MW at ﬁve power stations; they are fed by seven intakes, and accumulated ﬁne sediment is removedby dredging.
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Figure 5. Schematic layout of 125 MW San José hydroelectric project, Paractia River, Bolivia, incorporating an oﬀ-stream regulating
reservoir for pondage and desanders to remove the coarse sediment abundant in the streams.

2.2. Sediment Sluicing
Drawdown routing, or sluicing [ICOLD, 1999], involves discharging high ﬂows through the dam during
periods of high inﬂows to the reservoir, with the objective of permitting sediment to be transported
through the reservoir as rapidly as possible while minimizing sedimentation. Some previously deposited
sediment may be scoured and transported, but the principal objective is to reduce trapping of incoming sediment rather than to remove previously deposited sediment. One advantage of this approach isthat deposition in the reservoir is minimized and the sediment continues to be transported downstream
during the ﬂood season when sediment is naturally discharged by the river. Finer sediments are more
eﬀectively transported through the reservoir than coarse sediments.
Sluicing is performed by lowering the reservoir pool prior to high-discharge sediment-laden ﬂoods(Figure 6). This approach requires relatively large capacity outlets on the dam to discharge large ﬂows
while maintaining low water levels and the required velocities and transport capacity. These outlets neednot be at the very bottom of the dam, and at some sites with smaller storage volumes, tall crest gates canbe used for this purpose.
A drawdown and sluicing strategy may be employed at reservoirs of all sizes, but the duration of sluicing
depends on the watershed size and the time scale of ﬂood events.
For dams of small watersheds with rapidly rising ﬂoods, the reservoir may be drawn down only for a periodof hours. In other cases, such as dam sites with small storage volumes for daily regulation (pondage), thereservoir may be held at a low level during the entire ﬂood season to maximize sediment pass throughwhile continuing to produce power and using a desander to protect hydro-mechanical equipment from
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High Discharge

Water Level
Gate
Dam

Open Gate

High SedimentTransport Capacity
Dam

Increase Energy Slope

Normal Operation

Sluicing

Figure 6. Schematic representation of sluicing operations.

the abrasive sediment that is mobilized by sediment sluicing. In storage reservoirs on large rivers the reservoir may be held at a low level for a period of many weeks at the beginning of the ﬂood season and ﬁlledwith late-season ﬂows.
By virtue of passing the rising limb of the ﬂood, which generally contains higher sediment concentration
than the falling limb of the ﬂood hydrograph, sluicing is consistent with the Chinese strategy to, “release
the muddy ﬂow and store the clear water” [Wang and Hu, 2009]. In China, sluicing has most-famously
been implemented at the Three Gorges dam where prolonged seasonal drawdown during the early partof the ﬂood season is designed to maximize ﬂow velocity and sustain sediment transport through thereservoir, and also mobilize some of the previously deposited sediment. The reservoir level is raised laterin the season to ﬁll storage for sustaining releases during the low-ﬂow season (Figure 7). The objective isto sustain the natural patterns of ﬂood and sediment discharge along the river, while producing power
and assisting navigation. This strategy to stabilize reservoir capacity is best suited to narrow reservoirs.
The Three Gorges Reservoir, e.g., is about 600-km long but does not exceed 1.5 km in width, and it has ahigh-discharge capacity at the dam.
Reservoirs trap less sediment when the ﬂood-detention period is reduced, and a change in the reservoir
operating rules to minimize ﬂood-detention time, especially on the rising limb, can reduce sediment trapping at a very low operational cost. While sluicing operates most eﬀectively in long narrow reservoirs,
beneﬁts can also be achieved in storage reservoirs having other conﬁgurations. For example, the John
Redmond reservoir in Kansas (USA) has a nearly circular conﬁguration, a large ﬂood control pool, anda small water conservation pool. Analysis of historical operations during 48 ﬂood events plus modelingshowed that a measurable increase in sediment throughput could be achieved by making relatively minorchanges to the operating rule, while still maintaining downstream ﬂood control targets [Lee and Foster,

180

Normal Pool Level (NPL)
175 m

NPL 175 m

Pool Level (m)170
Dry Season Control Level
155 m

160
150

Flood control level
145 m

140
130
Jan Feb Mar

Apr

May

Jun

Jul

Aug

Sep Oct Nov Dec

Month
Figure 7. Seasonal pool operation at Three Gorges Reservoir. [Redrawn from Zhou, 2007]

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2013]. Compared to the conventional reservoir operation, “the altered scenario purposefully minimized
reservoir elevation and residence time through larger, more rapid releases of water after periods of highinﬂows,” resulting in measurably decreased trap eﬃciency [Lee and Foster, 2013:1437]. This reduction in
sediment trapping eﬃciency is achieved without any structural modiﬁcations, by simply including a sediment management objective in the reservoir operating rule.
2.3. Drawdown Flushing
In contrast to sluicing, whose aim is to pass sediment without allowing it to deposit, drawdown ﬂushingfocuses on scouring and re-suspending deposited sediment and transporting it downstream. It involves
the complete emptying of the reservoir through low-level gates that are large enough to freely pass theﬂushing discharge through the dam without upstream impounding, so that the free surface of the wateris at or below the gate soﬃt (Figure 8). While ﬂushing can be undertaken in reservoirs having any conﬁguration, because the ﬂushing channel will typically not be wider than the original streambed, ﬂushing will
recover and maintain a substantial fraction of the original reservoir storage only in reservoirs that are longand narrow.
The best scenario for ﬂushing is toestablish river-like ﬂow conditions
through the reservoir upstream of thedam, which is favored by the following conditions: narrow valleys with
Erosion ofsteep sides; steep longitudinal slopes;
Deposited
No Obstruction, Free
Sediment
river discharge maintained above
Flowing Water(Remobilization)
the threshold to mobilize and transport sediment; and low-level gates
Figure 8. Schematic representation of drawdown ﬂushing.
installed in the dam [Morris and Fan,
1998]. Flushing is best adapted tosmall reservoirs, and on rivers with strongly seasonal ﬂow patterns [White, 2001].
Gate

Flushing diﬀers from sluicing in two key respects [Morris and Fan, 1998]. First, as discussed above, ﬂushingfocuses on the removal of previously deposited sediments, instead of passing incoming sediments
through the dam. Secondly, (and consequent to the ﬁrst) is that the timing of sediment release to thedownstream channel may be diﬀerent from that of the sediment inﬂow into the reservoir, and thediﬀerence is greatest if ﬂushing is conducted during the nonﬂood season. Flushing can release large
amounts of ﬁne sediment to the downstream channel during periods of relatively low ﬂow, when theriver is unlikely to have suﬃcient energy to transport the sediment downstream. The accumulation ofsand and ﬁner sediment on the bed can have substantial impacts on the river ecology, and if the deposits
are suﬃciently large it can also impact the channel’s capacity to convey ﬂoodwaters. Flushing during theﬂood season also has the advantage of having greater discharges available, with more erosive energy,
and incoming sediment can also be carried through the dam as well as the sediment being eroded andresuspended from reservoir deposits [Morris and Fan, 1998].
Flushing has been successfully implemented in many dams globally, such as: Unazuki and Dashidairadams in Japan [Kokubo et al., 1997; Liu et al., 2004; Sumi and Kanazawa, 2006], Sanmenxia dam in China[Wan, 1986; Wang et al., 2005], Cachi Dam in Costa Rica [Jansson and Erlingsson, 2000], and Genissiat Damon the Rhône River in France [Thareau et al., 2006], and recommended as the only sediment management
measure feasible in terms of public acceptance and cost for Gavins Point dam on the Missouri River [USArmy Corps of Engineers, 2002].
For ﬂushing to be successful, the ratio of reservoir storage to mean annual ﬂow should not exceed 4%,
because with larger storage the reservoir cannot be easily drawn down Sumi [2008] (Figure 9). Because
ﬂushing ﬂows need to pass through the low-level outlet without appreciable backwater, it may not be
feasible to use large ﬂoods which exceed low-level gate capacity as ﬂushing events.
Sediment deposited from ﬂushing can have signiﬁcant environmental impacts, especially if ﬂushing iscarried out during nonﬂood season and sediments remain on the bed of the downstream channel. Ecologically important pools can ﬁll with sediment, gravel, and cobble riﬄes can be buried in ﬁner sediment,
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Flushing

Figure 9. Plot of ﬂushing projects from diverse environments showing that successful cases are characterized by impoundment
ratios of 0.4 or less. That is, reservoir storage capacity divided by mean annual runoﬀ (inﬂow to the reservoir) should be less than 0.4.

and ﬁne sediment can clog the bed, thereby eliminating surface-groundwater exchanges, smothering
eggs, and clogging the void spaces between stones used as habitat by aquatic invertebrates and larvalﬁsh. Even a small release of sediment (i.e., a small fraction of the river’s natural annual sediment budget)
during the river’s base-ﬂow period can have large impacts because the sediment cannot be transported
downstream. On the Kern River, California, sand was ﬂushed from a small diversion dam during base ﬂow
in 1986 in anticipation it would be transported away the next winter. However, a series of dry years followed, and the ﬂushed sand remained on the bed for several years because the river did not experience a
suﬃciently large ﬂow to transport it away [Kondolf and Matthews, 1993].
As a general rule, ﬂushing sediment-laden water through the power house is not recommended because
it can cause abrasion of the turbines. Sand in particular will quickly destroy turbines. The Zhengzhou
workshop presentations included reports of some cases in which ﬁne sediments were successfully passed
through powerhouses, but any such ﬂushing scenario must be carefully monitored so that the penstocks
can be shut oﬀ before sand is mobilized. However, as experienced at Nathpa Jhakri, India, even silt witha high-quartz content (70%–80%) can destroy turbines within months. It is therefore important to assess
the mineral content of sediment and susceptibility of the hydro-mechanical equipment to damage, and tostop power production when the reservoir level drops to the point that abrasive sediment may be eroded
from the reservoir delta and carried into the power intake.
The main challenges are to sustain the largest possible reservoir storage volume over the long term
under drawdown ﬂushing operations, while minimizing adverse downstream environmental impacts as
described by Gerster and Rey [1994] and Staub [2000]. There is a trade-oﬀ between frequent ﬂushing with
its frequent power losses and less frequent ﬂushing operations. Generally, more frequent ﬂushing (e.g.,annually) has less downstream impacts because it delivers sediment to the downstream channel, where itis needed for river health, more often and in small pulses. This reduces the potential for sediment pulses
to overwhelm the river’s transport capacity and aggrade the channel. Opening of the gates gradually andat appropriate times such as high ﬂows (e.g., the rainy season or snowmelt season) will lessen the impactsof change in sediment concentration on the downstream environment [Sumi et al., 2009]. Another consideration is consolidation of cohesive sediments. With time, cohesive sediments can “set up” and develop ahardened surface that requires heavy equipment to break up and push into the ﬂushing current. Regular
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Figure 10. Deposition within Jen-San-Pei reservoir in Taiwan, 1938 to present. Prior to 1955, no sediment management was
conducted and the reservoir ﬁlled rapidly, but beginning 1955 seasonal drawdown and sediment passing maintained reservoir
capacity. From 1998 to 2012, the pass-through operations were stopped, allowing sediment to accumulate. Pass-through operation
was resumed in 2013. [Modiﬁed from Huang, 1994 and extended to present using data from Water Resources Agency of Taiwan]

reservoir ﬂushing can reduce or interrupt consolidation of cohesive sediments and aid in ﬁne sedimentremoval. It is particularly important to be able to release a ﬂow of clear water after ﬂushing to mobilize
sediment and carry it further downstream. This may be in the form of a natural ﬂood hydrograph, or an
additional release from the dam with the reservoir at a higher level so that sediment is no longer being
scoured.
Flushing will not solve all sedimentation problems. Not only is there the limitation imposed by the limited
width of the ﬂushing channel with respect to the overall width of the reservoir, but there is also the problem posed by the limited hydraulic energy that can be generated with ﬂushing. Thus, ﬂushing discharges
may eﬃciently remove ﬁne sediments, but coarse sediments transported into the reservoir by large ﬂoods
will continue to accumulate without being removed by lower discharge ﬂushing ﬂows. In Cachi Reservoir(Costa Rica) and Hengshan Reservoir (China), coarse-grained deltas are prograding downstream toward
the dam despite regular sediment ﬂushing [Morris and Fan, 1998].
In some cases there is no clear-cut transition point between reservoir drawdown for sluicing and for ﬂushing, since drawdown for sluicing can scour and mobilize deposits, just a ﬂushing does. Flushing and sluicing may be combined in a seasonal reservoir operation, wherein the pool is emptied and outlet gates are
opened at the beginning of the rainy season to allow high ﬂows to pass through the empty reservoir, carrying their incoming sediment as well as eroding stored sediment. This approach is employed in some
Chinese reservoirs. For example, the Sanmenxia dam on China’s Yellow River remains empty for over 2
months during the ﬁrst part of each ﬂood season, allowing sediment-laden ﬂoods to ﬂush out sedimentdeposited during the previous year, and also allowing sediment-laden ﬂoods to pass through the reservoir[Wang et al., 2005].
Seasonal operation has also been used at the Jensanpei Reservoir in southern Taiwan, which was built
in 1938 to supply water to a sugar mill, which operated only part of the year [Huang, 1994]. Throughthe early 1950s, the reservoir was rapidly ﬁlling with sediment, and lost 4.3 Mm3 of its original 7 Mm3
capacity (Figure 10), but beginning in 1955, the dam was operated to pass sediment through a seasonal
drawdown approach. The reservoir would be drawn completely down and the outlets left open forthe ﬁrst 2.5 months of the rainy season (Figure 11). During this time, inﬂowing ﬂoods could transport
most of their sediment through the reservoir without depositing it, and they could also scour sediment
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85% of Annual rainfall

Precipitation pattern

1st part of rainfall
for flushing

2nd part of rainfall
for reservoir storage

NPL

closure timeof sluice-gatesstorage decreased for
Sugar Mills use

empty for flushing(2.5 months)
storage increased byimpounding runoff water
May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

Figure 11. Seasonal operation of Jen-San-Pei reservoir in Taiwan to pass sediment. The reservoir was drawn completely down andthe outlets left open for the ﬁrst 2.5 months of the rainy season, to allow inﬂowing high ﬂows to transport their sediment throughthe reservoir and scour sediment already deposited. Midway through the rainy season, the outlet gates were closed, so the reservoir
could impound water to process sugar cane, harvested from November to April. [Redrawn from Huang, 1994]

already deposited. Midway through the rainy season, the outlet gates were closed, and the reservoir
began impounding water for processing sugar cane, which is harvested between November and April.
However, by the late 1990s, because of economic changes, the sugar mill was no longer used, and the sitearound the reservoir was developed for tourism. For tourism, the drawn-down reservoir was considered
unattractive, and the seasonal drawdown operation was abandoned from 1998. As a result, sediment
began to accumulate in the reservoir until 2013, when the operators resumed seasonal drawdown andsediment pass through after ﬁnishing repairs to the sluice gate, which had become nonfunctional dueto the sedimentation and lack of maintenance for the years without drawdown (Figure 10). The dam wasalso raised twice (in 1942 and 1957) to increase reservoir capacity from the original 7 Mm3 to 8.1 Mm3 ,but the beneﬁt of this was minor compared to the beneﬁt of seasonal sediment pass through. Jensanpei
is an example of a combination of sluicing (allowing inﬂowing ﬂoods during the ﬁrst half of the rainyseason to pass through the reservoir without depositing their sediment loads) and ﬂushing (scouring
sediment deposited). It worked because the reservoir could be drawn down seasonally without aﬀecting
its functions.
2.4. Flushing Sediment for Dams in Series
In ﬂushing sediment through a series of dams, simultaneous ﬂushing can be accomplished by releasing
the ﬂushing pulse ﬁrst from the upstream reservoir. Just before that pulse reaches the next downstream
reservoir, its lower level gates are also opened to pass the sediment. After ﬁnishing the sediment ﬂush, thereservoirs are reﬁlled and clear water released from upper level gates to ﬂush the downstream channel ofdeposited sediment.
A notable example of management of dams in series is the operation of 19 dams on the Rhône from the
Swiss border to the Mediterranean Sea, whose operation is coordinated by the Compagnie Nationale
du Rhône with two dams upstream in Switzerland. Except for Genissiat Dam on the Upper Rhône, all
are run-of-the-river dams that operate by short-circuiting the “old river” with a straight canal, leaving
abandoned meander bends with greatly reduced ﬂows, some of which have been the loci of ecologicalrestoration eﬀorts [Stroﬀek et al., 1996]. With availability of storage in Lake Geneva, sediment is managed
in reservoirs and channels of the Upper Rhône by ﬂushing, such that the opening of gates is coordinatedfrom dam to dam as a pulse moves downstream. However, on the Lower Rhône, storage is lacking, andwhile it would theoretically be possible to coordinate ﬂushing with high tributary inﬂows, disruptions to
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navigation must be arranged a year in advance, so ﬂushing is not attempted, and instead, sediment isremoved mechanically [Compagnie National du Rhône, 2010].
“Environmentally friendly ﬂushing” from Genissiat Dam limits the potential impacts of ﬂushing on downstream aquatic life, water supply intakes, and restored side-channel habitats. This approach is of particularinterest because this ﬂushing is conducted under extremely strict restrictions on turbidity and suspended
sediment concentrations, not to exceed 5 g/l on average over the entire operation and not to exceed
15 g/l over any 15-min period [Thareau et al., 2006]. The dam is equipped with outlets at three levels: a bottom gate, an outlet halfway up the dam, and a surface spillway. Concentrations are controlled by mixing
waters with high sediment concentrations from the bottom of the water column with enough “cleaner”
water from higher in the water column (normally via the mid-level outlet) to stay within the required concentrations. Genissiat Dam receives high sediment loads from Verbois Dam upstream, which is ﬂushed to
avoid sedimentation and consequent backwater that could ﬂood parts of urban Geneva. In four decadesof ﬂushing every 3 years, an estimated 23 million tons of sediment could have deposited in the reservoir,
but only 4.5 million tons have actually deposited. The operation is costly to the Compagnie National duRhône, which engages a staﬀ of about 400 over approximately 10 days, at a cost of about €1.4 million(based on the 2003 ﬂushing, [Thareau et al., 2006]). Nevertheless, to remove an equivalent volume (1.8
million tons in 2003) by dredging would have been far more costly.
On the Kurobe River, Japan, Dashidaira and Unazuki dams are operated in coordination, with high runoﬀ
triggering ﬂushing of the upstream dam and sluicing through the downstream dam [Kokubo et al., 1997;
Liu et al., 2004; Sumi and Kanazawa, 2006] (Figure 12). The basic sequence of operations is to draw down
the reservoir water level, maintaining a free-ﬂow state over several hours (the duration being determinedby the amount of sediment to be ﬂushed), and then allowing the reservoir water level to recover. In July
2006, a free-ﬂow condition was continued for 12 h to ﬂush out an estimated 240,000 m3 of deposited sediment (Figure 13). The ﬂushing/sluicing operation is followed by release of a clear-water “rinsing” ﬂow to
remove accumulated sediment from the channel downstream.
2.5. Pressure FlushingThis technique is a variant on drawdown ﬂushing: rather than drawing the reservoir down so that it isacting like a river in carrying its sediment load, pressure ﬂushing works only to remove sediment directly
upstream of the dam to keep intakes operational. The reservoir level is not lowered, but outlets are
opened to remove sediments a short distance upstream of the outlet, creating a cone-shaped area ofm
1,600

Kurobe dam
1,448.000

1,400

1,200

Altitute

1,000

Sen-nin dam
851.000

800

Koyadaira dam
529.400

600

400
Unazuki dam
245.000
200
Shimokurobe

Dashidaira dam
343.000

Aimoto
0

10

20
30
40
The distance from the sea

50

60 km

Figure 12. Kurobe River, Japan. Map of drainage basin and location of major reservoirs (a), and longitudinal proﬁle showing all
reservoirs (b).

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Figure 13. Coordinated ﬂushing on reservoirs in the Kurobe River, Japan, 1–3 July 2006. Precipitation (a), inﬂow and outﬂow
hydrographs and reservoir stage for Dashidaira (b) and Unazuki (c) dams, and resultant suspended sediment concentrations (d).

scour just upstream of the outlet, the scour hole being created in a fraction of the time it would take to
reﬁll [Ullmann, 1970]. Shen et al. [1993] developed an empirical formula for the dimensions of the ﬂushing“cone” as a function of hydraulic and sediment variables, which could inform design of the dam outlets[Lai and Shen, 1996]. However, the scale of sediment removal by this technique is much smaller than withdrawdown ﬂushing. Rather, pressure ﬂushing serves to reduce sediment concentrations to the intake andthereby reduce abrasion of hydraulic structures by sediment [Lai and Shen, 1996]. To maintain or restore
reservoir capacity, pressure ﬂushing is not an eﬀective technique.
2.6. Turbidity Current Venting
Turbidity (or “density”) currents are important in the transport and deposition of sediment in reservoirs
worldwide. Turbidity currents form when inﬂowing water with high sediment concentrations forms a distinct, higher density current that ﬂows along the bottom of the reservoir toward the dam without mixing
with the overlying, lower density waters. If the bed of the reservoir is highly irregular, with protruding
features that would break up the ﬂows and cause turbulence, turbidity currents may not sustain themselves. However, turbidity currents occur in many reservoirs, and it is often possible to allow this dense,
sediment-laden water to pass through outlets in the dam, a practice referred to as “venting” of turbidity
currents (Figure 14). This can be undertaken as a sediment management technique, even at large reservoirs where other techniques, such as reservoir drawdown, are not feasible. Some dams have been able topass half of the inﬂowing sediment load by venting turbidity currents, but the technique is possible only
in cases where the turbidity current has suﬃcient velocity and turbulence to maintain particles in suspension and the current can travel all the way to the dam as a distinct ﬂow, where it can then be passed
downstream [Morris and Fan, 1998].
Facilities for the venting of turbidity currents should be provided at every project where turbidity currents are anticipated to convey substantial amounts of sediment to the dam. Advantages of turbidity

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Clear
Water

current venting are that it delivers
suspended sediment to downstream
reaches during the ﬂoods when thesediment would naturally be delivered, and that it does not require
reservoir drawdown or otherwise signiﬁcantly impact reservoir operations.

Both Sanmenxia and Xiaolangdi Reservoirs on the Yellow River vent turbidity
Low Level
currents, along with ﬂushing to disOutlet
charge sediments, and the Yellow
River Institute of Hydraulic Research
Figure 14. Schematic representation of density current venting.
has developed a new formula to predict the formation of plunge point for
density currents, which can help in selection of optimal dam sites for density current venting, and criteria for design and operation of reservoirs to create eﬀective density currents. With installation of a curtain(typically a sheet of geotextile hanging vertically from the water surface, suspended from ﬂotation tanks
and secured in place by a cable and anchor system, extending partway down the water column to force
ﬂow underneath), it may be possible to vent density currents at higher outlets on the dam, avoiding problems of clogging low-level outlets.
Density Current(moving current)

2.7. Dredging and Mechanical Removal of Accumulated Sediments
Accumulated sediments can be removed by suction using hydraulic pumps on barges with intakes. If
cohesive sediments have “set up,” cutter heads may be required to break up the cohesive sediments.
Dredging is expensive, so is most often used to remove sediment from speciﬁc areas near dam intakes.
If there is suﬃcient hydrostatic head over the dam, it can create suction at the upstream end of the discharge pipe to remove sediment and carry it over the dam as a siphon. This hydrosuction is typicallylimited to reservoirs less than 3 km in length, and to low elevations, where the greater atmospheric pressure facilitates the function of the siphon. In China, hydraulic suction machinery is commonly used to stir
the sediment within the reservoir with hydraulic and mechanical power, then to discharge the highly concentrated sediment-laden water out of the reservoir through siphons by the help of water head diﬀerence
between upstream and downstream of the dam.
If a reservoir is completely drawn down, mechanical removal can be employed using scrapers, dump
trucks, and other heavy equipment to remove accumulated sediments. While still costly, mechanicalremoval is commonly less expensive than hydraulic dredging, and can remove coarser sediments, but it
requires the reservoir to be drawn down far enough to expose coarse sediment. Mechanical removal isbest adapted to reservoirs that remain dry for parts of the year such as ﬂood control reservoirs. Cogswell
Reservoir on the San Gabriel River, California, was mechanically dredged in 1994–1996, with 2.4 Mm3
removed and taken to a nearby upland disposal site, at a cost of $5.60/m3 (or $6.47/m3 if planning andpermitting are included) [Morris and Fan, 1998]. Another 2.55 Mm3 has been identiﬁed as requiring excavation following a 2009 wildﬁre that increased erosion in the catchment [Los Angeles County Departmentof Public Works (LACDPW), 2012].

3. Upstream Sediment Management Approaches
Various approaches have been employed to reduce the amount of sediment entering the reservoir fromupstream. These methods do not mitigate downstream sediment starvation eﬀects, only sediment accumulation in reservoirs.
3.1. Catchment Erosion Control
Various attempts have been made to reduce sediment inﬂow into the reservoirs through changes in landuse, notably reforestation and altering agricultural practices to emphasize contour plowing and othererosion control approaches. While these methods oﬀer a number of beneﬁts, such as maintaining soil productivity for food security, increasing inﬁltration, and reducing storm runoﬀ, their beneﬁts in reducing
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sediment inﬂow to reservoirs have not been clearly demonstrated [Annandale, 2011]. San Francisco-based
Paciﬁc Gas & Electric Company invested in watershed restoration and erosion control projects in the catchment above their dams on the North Fork Feather River for some years until concluding that, other beneﬁts aside, they could not justify the cost in terms of reduced maintenance or greater generation at theirfacilities [Kondolf and Matthews, 1993].
3.2. Checkdams
Checkdams, often called by their Japanese name, sabo dams, can reduce sediment yield to a downstream
reservoir in two ways. The ﬁrst is by inducing deposition of debris ﬂows and reducing the rate of hillslope
erosion [Takahara and Matsumura, 2008; Mizuyama, 2008; Cheng et al., 2007]. Small checkdams locallyreduce the channel gradient and thereby induce deposition of debris ﬂows and ﬂuvially transported sediment, because stream energy is dissipated in the check dams, reducing the gradient in between. The
checkdams also direct the main ﬂow of water through the channel centerline, to reduce the tendency forthe channel to undercut the side slopes. Successful applications of this technology have been reported in
the Duozhao Ravine, Jiangjia River basin, southwestern China [Zeng et al., 2009] and the Loess Plateau of
China [Ministry of Water Resource of P.R. China (CMWR), 2003].
The second way checkdams reduce sediment yield to downstream reservoirs is by trapping sediment
before it reaches the downstream reservoir. The cumulative volume of sediment trapped in small
check dams is usually trivial, so larger check dams have also been built explicitly to store sediment
before it reaches a larger reservoir downstream. The obvious problem with this approach is that thecheck dams ﬁll with sediment, and in high sediment-yield river basins this can occur quickly, creatinga new set of problems, with multiple sediment-ﬁlled reservoirs, all potentially unstable and costly tomaintain.
Multiple checkdams to intercept sediment above the Saignon Dam in southern France were deemeda “failure” by Chanson [2004] because both checkdams and reservoir had ﬁlled within 2 years of construction. Likewise Ran et al. [2004] found that nearly all checkdams built in four Yellow River tributaries
had ﬁlled with sediment within 25 years, but concluded they had been successful in reducing sedimentdelivery to the downstream reservoir, if only temporarily. Sukatja and Soewarno [2011] reported that the
Sengguruh Reservoir (East Java) lost 93% of its initial 21 Mm3 capacity in 16 years, despite annual dredging and ﬁve checkdams built upstream, all of which were completely full within a decade. Eight additional
checkdams were then proposed for the upstream channel, with a combined capacity of 6.9 Mm3 , equivalent to about 2 years average sediment inﬂow into Sengguruh Reservoir.
The construction of Shihmen Reservoir in 1963 on the Dahan River, Taiwan, was accompanied by construction of over 120 checkdams upstream to reduce sediment delivery to the reservoir. By 2007, 38% of
Shihmen Reservoir’s initial capacity of 290 Mm3 had been lost to sedimentation, and virtually all of thecheckdams’ cumulative capacity of 35.7 Mm3 (equivalent to about 12% of the reservoir’s initial capacity)
had ﬁlled with sediment [Wang and Kondolf , 2014]. Three large checkdams accounted for 86% of the total
checkdam capacity, and one of these checkdams, Barlin Dam (capacity 10.5 Mm3 ), failed in 2007, releasing
most of its sediment in a massive downstream wave of water and sediment (Figure 15). The Taiwan Water
Resources Agency is now conducting modeling studies to design a sediment bypass around Shihmen
Reservoir, which promises to provide a more sustainable way to manage the high sediment loads [Wangand Kondolf , 2014]. Despite the extent of (and large investment in) checkdams in the Dahan River basin
and elsewhere, the experiences reported in the literature illustrate that the beneﬁts from checkdam storage upstream of larger reservoirs are temporary at best, and the sediment-ﬁlled checkdams become newhazards throughout the landscape.
3.3. Sediment Traps
Low dams located just upstream of reservoirs can function as traps for (mostly coarse) sediment. Theseshould be designed for easy access by heavy equipment, so the trapped material can be easily excavated and either used for commercial aggregate or trucked to the downstream river channel for sedimentaugmentation, as done in Japan [Kantoush and Sumi, 2010]. The Alameda County Public Works Agency(California) built a sediment trap upstream of Cull Canyon reservoir on San Lorenzo Creek in 1981. It eﬀectively trapped sediment during high ﬂows in the 1990s, and 4500 m3 was excavated and stockpiled as
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(a)

(b)

2002

July 2007(c)

Sep 2007
Figure 15. Sequential photographs looking at Barlin Dam, showing the dam (a) prior to ﬁlling with sediment (October 2002), (b)after ﬁlling with sediment (September 2005), and (c) after failure (September 2007). (Photographs courtesy of Taiwan Water
Resources Agency, reprinted by permission.)a source of aggregate for county projects [T. Hinderlie, Alameda County Public Works Agency, personalcommunication 2013].
3.4. Warping
Warping involves diverting sediment-laden water onto agricultural land to permit deposition of suspended sediments, to improve soil fertility. A traditional English term, warping was conducted in English
lowlands through the nineteenth century [Creyke, 1845; Williams, 1970], but the technique has also been
implemented for two millennia on highly sediment-laden rivers of the Loess Plateau of China (with suspended sediment concentrations typically exceeding 200g/l), yielding not only the traditional beneﬁts toagricultural land, but now also serving the function of reducing sediment loads to reservoirs downstream,such as Heisonglin [Morris and Fan, 1998; Zhang et al., 1976].

4. Sediment Augmentation Downstream of Dams
To compensate for the lack of sediment supply downstream of dams, sediment may be added to the channel, an approach termed sediment augmentation or sediment replenishment (Figure 16). Most commonly,
the sediment added is gravel and sand, and in rivers with important ﬁsh species and other sensitive ecological resources, there are often prohibitions on increasing turbidity, which restrict the addition of ﬁner
sediments. Most examples to date are from the US, Japan, and Europe, and the majority have been for
habitat restoration. In most cases, the sediments added are obtained from gravel quarries in the ﬂoodplain or other such sources, but in some cases (such as the Middle American River at Ralston Afterbay andseveral dams in Japan), the sediments are taken from the reservoir delta deposits or sediment traps built
at the upstream end of the reservoir [Jones and Stokes, 2003; Kantoush and Sumi, 2010].
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The largest ongoing sediment augmentation project is on the Rhine
below the Barrage Iﬀezheim, where an
average of 170,000 m3 of coarse sediment is added to the channel annually,
to prevent channel incision and consequent damage to infrastructuresuch as bridges, pipeline crossings,
and embankments [Kuhl, 1992]. However, most projects are undertaken
to restore aquatic habitat, especiallyfor ﬁsh. Sediment augmentation canbe implemented by placing sedimentdirectly in the ﬂowing river (high-ﬂow
direct injection), but is more comFigure 16. Gravel augmentation via high-ﬂow stockpile on the Sacramento
monly done as a gravel deposit along
River, below Keswick dam (photo by Kondolf , 1990).
the margins of the river, where it canbe mobilized by high ﬂows (e.g., in-channel or point-bar stockpiles) [Ock et al., 2013]. Past sediment augmentation for habitat has often involved building forms from gravel (e.g. riﬄe construction). Where theupstream dam controls ﬂows so much that scouring is rare, these projects can be successful in creating habitat [e.g., Wheaton et al., 2004; Merz and Chan, 2005], but such projects have promptly washed
out in high ﬂows on other rivers [e.g., Kondolf et al., 1996]. Increasingly, gravel augmentation projects aredesigned to restore some part of the river’s gravel load below the dam, so that the river itself can transportand redistribute the gravel in natural bars and riﬄes, which have high habitat value [Gaeuman, 2013].

5. General Principles Drawn From Collective Experience in Reservoir SedimentManagement
With reference to the Brundtland Commision’s [United Nations (UN), 1987] deﬁnition of sustainable development, that is, to meet the needs of the present without compromising the ability to meet the needsof future generations, we recommend that all dams be designed and operated so that they continue
to provide beneﬁts to future generations. The geologic and hydrologic settings (as well as operational
requirements) of reservoirs vary widely, and thus no one approach is suitable for all sites. Nonetheless, theexpert group agreed on a set of principles that have broad applicability. We ﬁrst present a set of generalprinciples, followed by more speciﬁc guidance for siting, design, and operation.
5.1. All Dam Proposals Should Address Sedimentation
Sediment trapping by dams creates problems for dam operation, reduces dam and reservoir lifetime, andcauses downstream sediment starvation. We recommend that sedimentation be explicitly addressed inplanning and design documents for all proposed dams, including quantiﬁcation of upstream sedimentyield to the reservoir, and with projections of reservoir sedimentation rates into the future for the conventional design approach as well as management based on more sustainable principles. Rivers vary widely intheir sediment loads, and the load carried by the river should be explicitly acknowledged in planning documents. Planning and design documents should indicate how reservoir sedimentation will be managed in
the long term to contribute to sustainable development.
While it is sometimes possible to retroﬁt concrete dams with additional low-level outlets for sedimentﬂushing, as done for Sanmenxia reservoir, it is generally not possible to retroﬁt compacted earthen dams,
and in any event, retroﬁtting is much more expensive than incorporating such outlets in the initial designand construction, and may impair the stability of the dam. Thus, it is always better to take sediment into
account in the initial planning phases, and to incorporate appropriate features at the outset.
5.2. Plan Over Suﬃciently Large Temporal and Spatial Scales
In planning dams, reservoir sustainability and downstream impacts should be analyzed over a suﬃciently
long temporal scale (300 years or more) to capture long-term impacts, and a spatial scale much larger than
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the reservoir and its immediate environs should be adopted. The upstream river basin should be analyzed
for its sediment production, with respect to additional dams, and other changes. Downstream impacts to
the river sediment balance should be an integral part of the analysis of proposed dams, and extending
downstream far enough to incorporate the limit of impacts, including the coastal zone where appropriate.
As many large rivers cross international boundaries, there can be important transboundary dimensions
to sediment management. As discussed in Section 2.4, ﬂushing of Verbois Dam on the Rhône River in
Switzerland is coordinated with ﬂushing of the Genissiat Dam downstream in France, in a successful
example of cross-border coordination. In the Mekong River basin, the Lancang River, the upper reachesof the Mekong in China, formerly supplied 50% of the river’s total sediment load, but dams being constructed on this upper reach will trap 83% of the natural sediment load from the upper basin, withpotentially signiﬁcant consequences for lower basin countries (Kondolf et al., submitted manuscript,2013). The Douro River ﬂows from Spain into Portugal, and multiple dams in both Spain and Portugal
have cut oﬀ sediment supply to downstream reaches, contributing (along with instream sand mining)to the sediment starvation that led to collapse of the Entre Os Rios bridge in 2001, killing 59 [Sousa andBastos, 2013]. Such transboundary impacts are typically not anticipated when dams are planned, nor
mitigated.
5.3. Adopt a Life-Cycle Management Approach to Design and Operation
For purposes of dam design and operation we recommend adoption of a life-cycle management
approach in lieu of a design life approach. Planning and economic studies for reservoirs are commonly
based on a design life of only 50 years [Morris and Fan, 1998], which eﬀectively makes it diﬃcult tomanage sedimentation problems during and after that period. A 50-year design life is the economic
norm, because all costs and beneﬁts are usually calculated to represent present values. The costs arethen compared to the beneﬁts using a market-based discount rate. Because any beneﬁts farther than
50 years in the future, when reduced to a present value, are extremely low, additional capital costs tomanage sedimentation well into future generations are not “economically justiﬁed.” This means, e.g., thatmost dams do not have large, low-level outlets that could be used to manage sediment both during a
traditional design life and well beyond. To the extent that sedimentation has been considered, it has mostcommonly been addressed by provision of a sediment storage pool within the reservoir’s dead storage,commonly designed to accommodate 100 years worth of sedimentation [Morris and Fan, 1998]. However,with adequate maintenance and management of sedimentation, the usable life of a reservoir can beextended for a much longer period [Palmieri et al., 2003].
5.4. The Dual Nature of Reservoir Storage Space
Renewable resources are used at a rate that is smaller than their rate of regeneration, thus they are“renewed.” Exhaustible resources are used at a rate greater than the rate of their regeneration, and areoften considered to have ﬁxed quantities, which can be “exhausted” by use. How should we classify
reservoir storage space, as exhaustible or renewable? In reservoirs that are (by design) allowed to ﬁll withsediment, reservoir capacity is properly classiﬁed as an exhaustible resource. Alternatively, in reservoirs
managed to prevent or minimize storage loss from sedimentation, reservoir capacity can be viewed as arenewable resource [Annandale, 2013]. This fact means that the nature of reservoir storage space depends
on a developer’s decision to either implement reservoir sedimentation management approaches or not.
A decision not to implement reservoir sedimentation management approaches means that reservoir
storage space, once lost to sedimentation, is no longer available for use by future generations.
If traditional cost–beneﬁt analysis practice is to be continued, assigning the correct value to implementation of reservoir sedimentation management approaches to preserve reservoir storage space requires
application of the Hotelling Rule, which says that for the maximum good of current and future generations, the price of exhaustible resources should increase at the rate of interest, to maximize the value ofthe resource stock over time [Solow, 1974]. Hotelling was responding to the problem of natural resourcesthat were priced “too cheap for the good of future generations, that … are being selﬁshly exploited at
too rapid a rate” [Hotelling, 1931]. Given that good reservoir sites are limited and many already used, reservoir storage space should be viewed as an exhaustible resource in cases where reservoir sedimentation
management is not implemented. However, if reservoir sedimentation management is incorporated as
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an integral part of the design, operation, and management of a dam and reservoir, the reservoir storage space can be viewed as a renewable resource. The decision as to whether reservoir sedimentation
management should be implemented or not, i.e., whether the reservoir is viewed as an exhaustible or arenewable resource, has signiﬁcant implications for the economic analysis of dam and reservoir projects[Annandale, 2013]. Thus, sustainable development of dams and their reservoirs requires close attention toeither preventing sediment deposition or removing deposited sediment from reservoirs.
There are reasons to consider alternatives to the traditional cost-beneﬁt approach when considering reservoir sedimentation. Issues such as climate change, reforestation, and the safe storage of nuclear waste all
have very long time horizons. Published procedures exist that oﬀer diﬀerent approaches to discounting,such as hyperbolic and exponential discounting, declining discount rates, and intergenerational discounting [Johnson and Hope, 2012], all of which deal with very long time horizons.
5.5. Distinguish Between Behavior of Fine and Coarse Sediment
Both suspended and bed load sediments are important to river systems. Not only do reservoirs trap diﬀerent grain sizes with diﬀerent eﬃciencies. It is important to understand downstream sediment impacts andto plan for them. As discussed in Section 1.2, the transport characteristics, trapping potential, and downstream impacts of ﬁne and coarse sediment are quite distinct, and should be considered separately. For
example, gravels are trapped with 100% eﬃciency in most reservoirs, commonly leading to gravel deﬁcits
downstream, and it is rare that gravels can be sluiced or ﬂushed except in small reservoirs. Sluicing andﬂushing work best with ﬁner grained sediments, which in any event, are usually the vast majority of sediment. In all cases, it is essential that the caliber of sediment coming into a reservoir be known to eﬀectively
design for it.

6. Speciﬁc Recommendations
The expert group oﬀered a number of points that often arise in sediment management. These can beviewed as speciﬁc examples under the broader recommendation that dam planning, siting, design, andoperation be undertaken in the context of a longer term and larger spatial scale perspective.
6.1. Avoid Sediment Problems Through Dam Siting
Decisions about siting reservoirs largely determine future reservoir performance. To date, dams have typically been sited based principally on engineering, economic, and often political considerations, commonlyon land already owned by the dam proponent or otherwise convenient for the purpose. For sustainability, siting decisions should account for the larger spatial and temporal context. For example, sedimentproblems can be minimized by giving preference to river channels with lower sediment loads (e.g., in lesserodible areas, and perhaps higher in the catchment) and to sites where sediment passing is more feasible (e.g., steep gorges instead of low-gradient reaches). For a given level of hydroelectric generation
within a river basin, it may be possible to minimize impacts by concentrating dams in a smaller number ofrivers (preferably with naturally low sediment yields), allowing other rivers to ﬂow freely, contributing sediment to downstream reaches and supporting habitat. This sort of “triage” is often diﬃcult to implement
politically.
The eﬀect of reservoir siting on the severity of reservoir sedimentation problems is illustrated by diﬀerence between the two principal water supply reservoirs for Taipei, Taiwan: Shihmen and Fetsui Reservoirs.
As noted above, Shihmen Reservoir has been plagued by sedimentation, losing 38% of its capacity since
its construction in 1963. WRA (2011) estimated an average of 3.53 Mm3 of sediment ﬂows into Shihmen
Reservoir annually, compared to 0.95 Mm3 estimated to ﬂow into Fetsui Reservoir, implying that the Fetsui
site is better suited from the perspective of reservoir sustainability.
6.2. Anticipating Future Sediment Management in Dam Design
The long-term equilibrium proﬁle, i.e., the river bed proﬁle through the reservoir after anticipated sedimentation has occurred to reach equilibrium between incoming sediment and outgoing sedimentthrough the dam or bypass, should be calculated in advance for every project, drawing upon hydraulicand sediment transport models. Gates should be placed and sized with respect to the requirement toachieve the desired long-term proﬁle. There is no standard location for the proper placement of gates,
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because this will depend on the situation at each dam, but in general gates should be set low enoughand with suﬃcient hydraulic capacity to establish the desired equilibrium proﬁle and support the type of
sediment management operation identiﬁed for long-term use. For example, if ﬂushing is to be performed
during a low-ﬂow period, the gates may be smaller and placed very low in the dam section, while gatesfor drawdown sluicing will have much greater hydraulic capacity and will probably be set at a higher level.
In many cases, an array of large radial gates at the bottom of the dam may be the best option. Their highinitial capital costs are likely to be oﬀset by the longer economic lifetime of the reservoir.
Although the need for a new outlet tunnel or new gates may not be manifest until some future point
when sedimentation has advanced to the point that a new operational rule is required, such future needs
should be anticipated during initial design and the dam designed to accommodate such future requirements. It is preferable to install the needed gates at the outset for the integrity of the dam and so that they
are more likely to be operated when needed to pass sediment.
The location and conﬁguration of intakes should take into consideration the long-term equilibrium sediment proﬁle and the ability to naturally scour sediment away from the area of the intake to sustain water
deliveries despite sedimentation.
6.3. The Larger Context
Dams in a series should be operated in concert to achieve management of sediment transport along
the river system, even where the river cross territorial boundaries. Poor results and conﬂicts between
upstream and downstream dams will result if dams are operated independently. Therefore, when a seriesof dams are developed along any river, particular attention should be given to establishing the appropriate coordination and data-sharing among the parties, including both the historical and real-time monitoring data required to determine the eﬃciency of the operation and to identify means to improve theoperation and pass sediment.
The availability of long-term, accurate hydrologic and sediment data are essential for the purpose ofdesign and for analyzing impacts. This should include, at a minimum, reservoir sedimentation surveys,
where possible suspended sediment sampling, and monitoring of reaches downstream likely to beaﬀected by sediment starvation, ﬂushed sediments, etc. Ironically, a glance at the dates of reservoir
sedimentation surveys [Ackerman et al., 2009] suggests that reservoir sedimentation surveys have becomeless frequent in the US since the 1940s, despite improved technology, and in many river basins, such as
the Sado River basin in Portugal, no sediment surveys have ever been conducted, despite the presenceof dams on all the river’s tributaries. Sediment management designs and impact analysis cannot be any
better than the data on which they are based. Thus, data collection eﬀorts should be emphasized, as well
as data-sharing among agencies and political jurisdictions.
Operational ﬂexibility to ﬂush sediment while minimizing impacts on power system costs or reliability iseasier to achieve in larger grid systems where the fraction of power generated by the reservoir relative tothe total mix of generators is relatively modest. Therefore regional integration of power grids may enableimproved sediment management operations.

7. Conclusions
Sediment trapping by dams has consequences for the reservoir and for the downstream channels and, insome cases, coastal zones. As sediment accumulates behind dams, it can impair reservoir functions andultimately reduce or eliminate storage capacity, threatening the sustainability of water supply and hydroelectric generation. Good dam sites are limited, so reservoir storage capacity should be viewed under the
Hotelling Rule as an exhaustible resource if a conscious design decision is made to allow a reservoir toﬁll with sediment, and, consequently, reservoir storage capacity should be valued more highly than it ispresently in dam planning. Today many dams are planned and built without any consideration of sedimentation, or at best, the reservoir is designed to store anticipated sediment loads for 50–100 years
before its functions are impaired. Yet in many cases, dams can be designed to pass much of their sediment load, and in this case, reservoir storage capacity can be viewed as a renewal resources, with much
more positive implications for the future sustainability of water supply and hydroelectric generation.
Based on the broad experience of our assembled expert group, we recommend that dams be planned
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and designed for sediment management, where possible by passing sediment through or around thereservoirs.
Choices in the siting, design, and operation of dams determine their ability to pass sediment. Siting decisions are irreversible, and to retroﬁt dams after they are built with sediment passage facilities such asdischarge gates is expensive at best and impossible at worst. Therefore, the most important consideration in sediment management through reservoirs is getting it right from the start. This principle implies
that existing plans for dams not yet built should be urgently and fundamentally revisited to consider a fullrange of sediment passage options. Even for existing dams, an assessment of options to improve sedimentmanagement is desirable and recommended.
Acknowledgments
This paper presents a review of experience in reservoir sediment management, based on distillation ofpresentations and consensus of discussion at the Reservoir SedimentManagement Workshop in Zhengzhou,
China, in September 2012, with therecommendations based on the collective experience of the workshop
participants as well as supplemental
literature review. The workshop wassponsored by the Yellow River Conservancy Commission and the Natural
Heritage Institute (NHI), with funding
from the U.S. Agency for International Development through a projectcalled “A Climate Resilient Mekong:
Maintaining the Flows that Nourish
Life.” Participants in the workshop
included George Annandale, Yongtao
Cao, Paul Carling, Kaidao Fu, Chuanchang Gao, Yongxuan Gao, Qingchao
Guo, Rollin Hotchkiss, Enhui Jiang,
Siqi Jiang, Martijn Karelse, G. Mathias
Kondolf, Gregory Morris, Christophe
Peteuil, Tetsuya Sumi, Gregory Thomas,
Hsiao-Wen Wang, Zhongmei Wang,
Zhilin Wei, Baosheng Wu, Caiping
Wu, Junqiang Xia, Jianhua Xu, Chih
Ted Yang, Baishan Zhang, Junhua
Zhang, and Deyu Zhong. We thankSteve Darby, Giyoung Ock, Timothy
Randle, Desirée Tullos, Philip Williams,
Albert Kettner, editor Mike Ellis, and an
anonymous reviewer for their helpful comments on the manuscript.
Sameh Kantoush kindly shared data
and allowed use of a ﬁgure. We thankJihn-Sung Lai for his insights on ﬂushing considerations, and Greg Thomas
for insights from a policy perspective
and for making the workshop a reality.

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