12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

Abrasion Damage in Sediment Bypass Tunnels
Christian Auel, Tetsuya Sumi
Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University
Kyoto, Japan

sediment bypass tunnel; wear

Boes 2011, Kondolf et al. 2014). Figure 1 shows an overview of thesetechniques and their corresponding strategies: (1) sediment yield reduction, (2) routing sediments around or through the reservoir, and (3)recover volume by sediment removal or dam heightening. Furthermore,
two more strategies may be added: (4) dam removal and (5) no action.
Sediment routing is ecological favorable compared to other strategies
as operation is conducted during high flows. River bed erosion downstream of the dam can be decelerated resulting in an increase of morphological variability (Fukuda et al. 2012, Facchini et al. 2015, Martín
et al. 2015). Moreover, only sediments provided from the upstream
river reach are conveyed, while hardly any removal of sediments thathave already accumulated in the reservoir occurs. The sediment concentration in the tailwater of the dam is therefore not affected by thereservoir itself and keeps its natural character (ICOLD 2009).

INTRODUCTION

SEDIMENT BYPASS TUNNELS

Dam-regulated rivers interrupt continuous sediment transport along ariver system and cause accumulation in the reservoir. Hence, a sustainable use of reservoirs implies the application of strategies to counteract
sedimentation. Mean annual sedimentation rates vary from 0.2 to some
2 to 3% with a global annual average rate of about 1% and worldwide,
increase in sedimentation volume exceeds increase in reservoir capacity
revealing a gross storage loss in the near future (Schleiss and Oehy
2002, ICOLD 2009). Reservoir sedimentation causes various problems.
Firstly, the volume decrease leads to a loss of energy production, waterused for water supply and irrigation, and retention volume (Annandale2013). Secondly, both an endangerment of operating safety due toblockage of outlet structures and an increased turbine abrasion due toincreasing specific suspended load concentrations may result. Finally, a
dam retains sediment causing downstream river incision and inhibiting
its ecologic connectivity (ICOLD 2009, Kondolf et al. 2014).
Sediment management to minimize aggradation in reservoirs isachieved with a variety of techniques categorized in three main strategies (ICOLD 2009, Morris and Fan 1998, Annandale 2013, Auel andSediment routing using a bypass tunnel (SBT) is a very effective strategy regarding both bed and suspended sediment load (Sumi et al. 2004,
Auel et al. 2016a).
In general, all sediments are guided into the tunnel intake using guiding
structures such as walls or weirs and the reservoir is kept free of sediments downstream of the intake. Only if the tunnel design discharge is
exceeded, a partial flow is entering the reservoir leading to suspended
load entrainment (Auel and Boes 2011).
The number of tunnels is still limited with about 30 facilities worldwide. Most SBT exist in Switzerland (Egschi, Hintersand, Palagnedra,
Pfaffensprung, Rempen, Runcahez, Sera, Solis, Ual da Mulin, Vald’Ambra) and in Japan (Asahi, Koshibu, Matsukawa, Miwa, Nunobiki),some others in China and South Africa. Two SBT are under construction in Taiwan (Nanhua, Shimen). Additionally, a number of flood
bypass tunnels exist showing similar flow characteristics to SBT. These
are i.e. in Switzerland the Rovana, the Grindelwald glacier and the
Matter Vispa downstream of Zermatt.

ABSTRACT
Sediment bypass tunnels are an effective and sustainable strategy
against reservoir sedimentation. Sediments are diverted into the downstream during floods without deposition in the reservoir, hence morphological and ecological variability increases. One major drawback ofthese tunnels is the severe invert abrasion due to a combination of highflow velocities and bedload sediment transport. The abrasion phenomena is briefly described, different abrasion prediction models are presented and their applicability for the estimation of concrete abrasion is
discussed.

KEY WORDS: Abrasion prediction model; reservoir sedimentation;

12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

Fig. 1 Classification of strategies against reservoir sedimentation (adopted from Auel et al. 2016a).
In general, SBTs are operated in supercritical free surface flow conditions (Fig. 2), although some are operated in pressurized flow for alimited time period. The intake is located either at the (1) reservoir heador (2) inside the reservoir. The first location results in long tunnels, but
operation is simple and the entire reservoir is kept free from sedimentdeposition. The second location allows for short tunnels, but both construction and operation are challenging. The reservoir has to be partially lowered prior a flood event to allow for sufficient transport capacity
towards the tunnel intake.

Fig. 2 SBT flow conditions sketch (adopted from Auel 2014).

The flow velocity in free surface open channel flow depends on thewidth and slope. As the tunnel cross section should be compact in order
to confine construction costs, the slope is the governing parameter.
Two contrary requirements have to be fulfilled. The tunnel has to be
steep enough to transport all sediment, but the slope should be as mild
as possible to limit the flow velocity. Due to both high flow velocities
up to 15 m/s and bedload sediment transport with grain sizes in thedecimeter range, many SBT face severe invert abrasion (Jacobs et al.
2001, Auel and Boes 2011, Boes et al. 2014). Figure 3 shows abrasion
damages up to several meters at Palagnedra SBT in Switzerland andseveral decimeter at Asahi SBT, Japan. The up to 4 m deep damage in
Palagnedra was mainly caused by an extreme flood event shortly after
inauguration in 1978. In 2012 and 2013, about 20% of the invert wererefurbished. The invert of Asahi SBT was abraded by 1.3 m since 1998
leading to periodical repair works.
These tunnels, however, operate effectively despite the large abrasion.
These abrasion problems cause high periodical maintenance costs additionally inhibiting the successful implementation on site.

Pros & ConsThe first advantage of a SBT is the high efficiency keeping the reservoir free of sediments. Research on the Japanese SBTs Asahi andNunobiki showed that in average 77% and 94% of the incoming sediments are diverted to the downstream river reach, and the estimatedreservoir life enlarged to 450 and 1200 years, respectively (Auel et al.
2016a). The second advantage is its ecological function by connecting
the up- and downstream reaches. Only sediments that are transported
towards the reservoir are bypassed whereas already deposited sedimentis not mobilized. The natural morphological river characteristics are
thereby reestablished and microhabitat and invertebrate richness in thedownstream reaches is leveled to the upstream values (Kobayashi et al.
2016).
The main disadvantage is the high construction cost, hence SBT are
best applicable for small and medium-sized reservoirs (<107 m3) astunnel length plays a crucial role in terms of cost per meter length.
Secondly, the flood is directly diverted into the downstream and noretention occurs. Therefore, SBT are favorable in regions with highwater availability whereas in arid regions the priority is to retain theflood by all means.

Fig. 3 Abrasion patterns at a) Palagnedra SBT, Switzerland and b)
Asahi, Japan (courtesy of C. Auel).

ABRASION
Abrasion is a wear phenomenon involving progressive material loss
due to hard particles forced against and moving along a solid surface
controlled by kinetic energy due the vertical component of a saltating
particle impact (deformation wear), and friction due to grinding stress(cutting wear) caused by the horizontal component (Bitter 1963a, b). In

12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

general, abrasive damage can always be expected when particle bedload transport takes place. The governing process causing abrasion on
brittle materials such as bedrock and concrete is saltation, whereas
sliding and rolling do not cause significant wear (Whipple et al., 2000;
Sklar and Dietrich, 2004; Turowski, 2012; Beer and Turowski, 2015).
The extent of damages along the wetted perimeter, i.e. mainly on the
invert and the lower parts of the tunnel walls, typically increases withincreasing sediment load, particle size and distribution and flow velocities i.e. shear stress as well as quartz content in the mineralogical composition of the sediments (Boes et al. 2014).

dimensional abrasion coefficient encompassing both the particle andbed material characteristics, qs = specific gravimetric bed load rate[kg/(sm)], and qs* = specific gravimetric bed load transport capacity[kg/(sm)]. The last term on the right of Eq. (4) is related to the covereffect accounting for bed load partly covering the bed, resulting in decreasing impact energy (e.g., Sklar and Dietrich, 1998; Turowski,2009). Sklar and Dietrich (2004) applied correlations encompassing
analysis of particle motion characteristics (hop length, hop height andparticle velocity) for a wide data range to Eq. (4) and proposed thesaltation abrasion model for bedrock river abrasion as:
1.5

Abrasion prediction models
A number of mechanistic models exist to predict the abrasion rate.
While the models for prediction of bedrock incision (e.g., Sklar andDietrich 2004, Lamb et al. 2008, Chatanantavet and Parker 2009) focus
on typical flow conditions in river systems in the sub- and low supercritical flow regime, the others for prediction of abrasion on concretesurfaces (Ishibashi 1983, Helbig and Horlacher 2007, Auel et al. 2016b,c) have to account for highly supercritical flows. All models take thephysical process of particle impact into account and are derived fromexperimental research on particle motion characteristics, i.e. the analysis of particle impacts, velocities and saltation trajectories.
The first mechanistic model to determine concrete abrasion was proposed by Ishibashi (1983). The abraded invert volume Va is calculatedas:=
Va C1 Ek + C2W f(1)

2−0.5 

U  
Yq =
Ar 0.08 g ( s − 1) M 2 ⋅ qs ⋅ 1 − ∗s  (T ∗ ) 1 −  ∗    Vs  
kv f t qs 



[m/s]

(5)

where U∗ = (gRhS)0.5 = friction velocity, Rh = hydraulic radius, S = energy line slope for steady but gradually-varied flow, or bed slope foruniform flow, Vs = particle settling velocity, and the transport stage T*
follows as (e.g. Sklar and Dietrich, 2004):
2

U 
θ
−1
T * =  ∗  − 1=Uθc
 *c 

(6)

where θ = Shields parameter calculated as θ = U∗2/[(s−1)gD], s = ρs/ρwith ρs = particle density and ρ = fluid density, g = gravitational acceleration, D = particle diameter, θc = critical Shields parameter and U∗c =critical friction velocity at the onset of motion. The last term in Eq. (5)accounts for the mode shift from saltation to suspension using a nonlinear function additionally increasing the hop length.

where Ek = total particle kinetic energy by saltating particles, Wf = total
friction work by grinding particles, and C1 and C2 = invert material
property constants for either concrete or steel. The total kinetic energy
Ek is given by:

Auel et al. (2016b, c) found new parameter correlations (e.g. particle
impact velocity, saltation trajectories) and proposed a revised versionof Eq. (4):Ek = 1.5Vts ∑ Ei N i ni

(2)

YM ( s − 1) g 
q 
⋅
Ar =
qs 1 − ∗s 
kv f t 2
q
230
s 


(3)

with kv = 105 due to the new parameter correlations. Eq. (7) reveals thatthe abrasion rate Ar is linearly dependent on the sediment rate qs supported by findings of Chatanantavet and Parker (2009), Johnson andWhipple (2010), and Inoue et al. (2014). In contrast, in Eq. (5), Ar isadditionally dependent on (T*)-0.5.

[kgf m]and the total friction work Wf by:

 Up 
W f = 5.513µ sVts ∑  Ei N i ni
 Wim [kgf m]where Vts = amount of transported sediment [m3], μs = dynamic friction
coefficient, Ei = kinetic energy of a single particle, Up = horizontal
particle velocity, Wim = vertical particle impact velocity, Ni = L/Lp =impact frequency, with L = total invert length and Lp = particle saltation
length, and ni = amount of particles per sediment volume. Further details regarding calculation and verification of Eq. (1) are given in Auelet al. (2016d).
A widely applied model for bedrock incision was proposed by Sklar
and Dietrich (2004) and follows in its general form:

A=
r


YM W
q 
⋅
⋅ qs ⋅ 1 − ∗s 
kv f t 2 L p
q
s 

2
im

[m/s]

(4)

where YM = Young’s Modulus of elasticity of the bed material [Pa], ft =splitting tensile strength of the bed material [Pa], kv = 106 = non-

[m/s]

(7)

For abrasion of concrete, Auel et al. (2016c) used correlations for thecompression strength fc and Young’s modulus by Arioglu et al. (2006)
and Noguchi et al. (2009) and proposed:

Ar ≈ ca( s − 1) g
kv f

0.93
c

 q 
qs 1 − ∗s  qs 

(8)

with fc in [Pa], ca = 94.4 = abrasion coefficient in [Pa-0.07], and as in Eq.
(7), kv = 105.

Recommendations
The model of Ishibashi (1983) is an outstanding pioneering work based

12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

on laboratory experiments in the 1960ies (Ishibashi and Isobe 1968).
Ishibashi introduced two terms, the particle impact and grinding stress,
to determine the abrasion. Later works (e.g. Whipple et al., 2000; Sklar
and Dietrich, 2004; Turowski, 2012; Beer and Turowski, 2015) suggestthat the latter term plays a minor role which is supported by recent
results of field data in Asahi SBT (Auel et al. 2016d). The model bySklar and Dietrich (2004) is widely applied in river engineering andgeomorphology and regarded as state-of-the-art. Auel et al. (2016b,c)revised this model and proposed a new version covering various bed
configurations, i.e. planar and alluvial beds in a wide range of flow
conditions (sub- to highly supercritical). Additionally, a version for
concrete abrasion was proposed.
We suggest to estimate the abrasion rate for concrete using all threemodels to obtain a sound data base. However, it should be kept in mind
that in Eq. (1) only the first term (particle impact) should be used, Eq.
(5) was developed for moderate flow conditions, whereas Eqs. (7) and(8) are based on the largest data sets covering all flow and bed conditions.
Abrasion may be minimized by (1) optimizing the hydraulic design and(2) the choice of an appropriate invert material as high performance
concrete, cast basalt, granite blocks or steel linings. Recommendations
for the former are given in Boes et al. (2014), and for the latter in e.g.
Jacobs et al. (2001) and Hagmann et al. (2015).

CONCLUSIONSIn this paper, we discuss sediment bypass tunnels as an effective andecologically sustainable strategy against reservoir sedimentation. Pros
and cons are listed and the main disadvantage, invert abrasion, is described in detail. Furthermore, three different models to determine
abrasion are described emphasizing their specific advantages and shortcomings.

ACKNOWLEDGEMENTS
The first author acknowledges the financial support of the JapaneseSociety for the Promotion of Science (JSPS).

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November 6-10, 2016, Tainan, Taiwan.

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