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

Recovery of Riverbed Features and Invertebrate Community in Degraded Channels by Sediment Supply
through Bypass Tunnel
Sohei Kobayashi, Christian Auel, Tetsuya Sumi, Yasuhiro Takemon
Water Resources Research Group, Disaster Prevention Research Institute, Kyoto University
Kyoto, Japan

ABSTRACT
Sediment, which deposits and damages the function of reservoirs, is an
essential element of aquatic habitats in downstream ecosystems. We
reviewed ecosystem features of degraded channels associated withsediment deficiency below dams and ecosystem responses to changes
in sediment conditions after management practices in Japan. Sediment
bypass tunnel (SBT) is an effective way to transport sufficient amountof sediment to downstream ecosystems. Based on a concept of suitablemass and size of sediment for ecosystem, some effects and limitationsof SBT on downstream ecosystems were discussed.

KEY WORDS: dam; sediment; bypass tunnel; channel
degradation; riverbed; grain size; invertebrate community.
INTRODUCTION
Sediment deposition is a main issue in the sustainable management ofreservoirs as well as of river channel. Sediment accumulations that
increase after a flood event in reservoirs are critical for the longevity ofdams. Different measures have been proposed to reduce sediment in thereservoirs (Sumi and Kantoush 2011). Concerning to river channels,
sabo works that prevent erosion and sediment supply from hillslopes
and headwaters, large dam constructions in the main stem of drainage
systems, and direct excavations have reduced sediment deposition
especially downstream reaches and coastal areas (Fujita et al. 2008).
Such sediment reduction has caused substantial changes in riverbed
conditions including a loss of easily entrained bed materials (e.g., sand,gravels), bedrock exposure, bed stabilization, and channel incision.
Accordingly, many endemic species were replaced by exotic species
due to their requirements for certain bed and flow conditions. In
addition, overgrowth of particular plants were promoted due to the bed
stabilization. Sediment should be increased in the river channels if wewish aquatic community and ecosystems to recover to a state, in whichpeople can enjoy benefits from riverine production and biodiversity.
However, measures to increase sediment in the channel have been lessconducted probably because we concern more on flood risk reduction
and less on increasing the potential of ecosystem services.
Considering the future water demands and increasing importance ofreservoirs, sediment management measures of reservoirs are the limited
possibilities to increase sediment in downstream rivers. Replenishmentof sediment excavated from reservoir has been conducted in several
dams in Japan (Kantoush and Sumi 2011). However, due to limitations

in excavation, transportation, placement, and available discharge forflushing, as well as the cost of these works, the amount of sedimentsupply is usually low (<1,000–10,000 m3 per year). Flushing/sluicing
techniques of sediment through dams are effective for transporting
large amount of sediment to downstream reaches. Sediment bypass
tunnel (hereafter SBT) is intended to route sediment around a dam that
otherwise accumulates in reservoirs (Vischer et al. 1997). Sedimenttransport through SBT during flood is another effective way of
supplying large amount of sediment to downstream. The constructionof SBT in the world is still limited, and most of them are located inJapan and Switzerland so far (Sumi 2015).
We have conducted field monitoring and also collected environmental
and ecosystem data from different sources of several dams with SBT tounderstand the effect of SBT operation on ecosystem recovery of thedownstream reaches (Awazu et al. 2015). In this paper, we firstly
showed important habitat components for river organisms, and thechanges of their habitat by channel degradation in Japan. Secondary,
we showed some of the effects of SBT system on the recovery of
riverbed and ecosystem features based on the results of our study. We
also mentioned about approaches to evaluate the recovery of riverenvironment. Finally, based on a concept of the suitable amount andsize of sediment for ecosystems, we discussed the limitations of SBT
system on the recovery of downstream environment. Here, we focusedon benthic invertebrates (mainly insects) as a representative of riverine
ecosystem. Invertebrates are an important component in the food webof riverine system, because they control the primary production byalgae and are also the main food source for various fish and bird
species. Invertebrates are also rich in species so that community
composition can change clearly to environment of various aspects.

SEDIMENT AND RIVER HABITAT
Habitat refers to a living place of organisms. In rivers, it usually means
by locations of different flow, depth, and substrate (i.e., bed material)conditions (Fig. 1). For example, riffles (shallow, fast flow, coarse bed)
and pools (deep, slow flow, fine bed) are different habitats. Habitats
can be also defined at smaller spatial scales within riffles and pools
based on flow and substrate (called microhabitats, Fig 1). The poolriffle geomorphic (bedform) structure usually occur in every reaches ofgravel-bed river, but topography, surface flow, and bed materials differ
according to the sediment condition. Some other habitats, such as
secondary channel flows and temporal ponds, are associated with adevelopment of bars (Fig. 1). Invertebrate biomass and species richness

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

Boulder

Cobbles(clogged)

Pond
Pool

TerraceMud/silt

Sand

GravelPond
Reach

Fig. 1 Example plan view of river reach with definitions of habitats

Riffle slope = 2%
0

0.4

0
-4
-8
-12
-16

0.8

1.2

Riffle slope = 5%

0.4

0.8

1.2

Distance from upstream-end (km)Clinger

GliderRelative value

Invertebrate biomass
a

1

ab

a

0.4

bc

abc

bc

0.4

a

ab

0.6

0.6

0.2

Species richness

0.8

0.8

ab
b

0.2

c

b

0

0

100m

representation of boulders and bedrock. Accordingly, flow and bed
materials are considered to be spatially more uniform within reaches.
The shape of pool-riffle structure also associates with the grade of thechannel. Individual riffles are often steepened as finer particles are
removed and coarser particles are remained (Kobayashi et al. 2012, Fig.
4). As a consequence, individual riffles shrink while pools expand in
degraded reaches. Sometimes swift flows (e.g., >1 m/s) are lost in
degraded reaches without any changes in the discharge.

Fig. 2 Some invertebrate species and their life type1

1.6

Fig. 4 Longitudinal bed profile of above and below dam reaches (left)
and schema of the change in riffle length with riffle steepening (right)(from Kobayashi et al. 2012)

BurrowerNet-spinner

1.6

Downstream

0

Case maker

RifflePool/Run1m

Cobbles(porous)

Upstream-2

1m

RiffleBackwater

Leaf patch

0
-4
-8
-12
-16

1m

Active channel

RiffleWood

Pool

Backwater

Pond

Vascularplant

Relative elevation (m)Forewater

Riffle slope = 1%

Terrace

Pond

Pond

Bed particle class

Fig. 3 Relationship between bed material and invertebrates (from a
review study of Kobayashi and Takemon 2012)
are often higher in riffles than pools within a given reach. Thus,
researchers sometimes focus on riffles to make comparisons between
reaches if the time and effort are limited. Potentially more different
species can live in riffles or reaches with various bed materials (i.e.,
more microhabitat types).
Bed material size is an important determinant of the occurrence of each
invertebrate species, and also of the potential (i.e., maximum) total
species and biomass that can live in a given bed area. Different species
have different preferences for bed materials according to their mode ofexistence (e.g., burrow into sand, cling on stable coarse material) andtheir flow preference (e.g., some species are abundant in cobble beds
because the flow is swift) (Fig. 2). Generally, when all invertebrates are
considered, species richness and biomass increase along representative
bed size from sand to cobbles, and decrease from cobbles to bedrock(Kobayashi and Takemon 2012, 2013) (Fig. 3). Not only the
representative grain size but also the combination of different sizes arealso important (Kobayashi and Takemon 2012). For example, cobbles
do not provide suitable interstices for invertebrates without sand andgravel, while cobbles embedded in too much fines are also unsuitable.

DEGRADED CHANNELS
Degraded channels are typically deeply incised with cross-sectionallyconcentrated flow, lack of developed sediment bars and related habitats(i.e., secondary channel flow, temporal ponds, Fig. 1), and over-According to the relationship between bed material size andinvertebrates, invertebrate community composition, species richness,and biomass change with degradation. For example, species that utilize
sand for their living (e.g., cased caddisfly) are less (Katano et al. 2010),
while species that prefer coarse and stable substrate (e.g., net-spin
caddisfly) are more below dams (Hatano et al. 2005). Invertebrates thatutilize underneath cobbles (e.g., stonefly) require certain amount ofsand and gravel for suitable interstices, and they are less in uniformly
cobble/boulder and bedrock exposed beds. Highly degraded channels
with immobile bed materials often promote overgrowth of attachedalgae on stones. Invertebrates that require clean surface cobbles (e.g.,
glider and swimmer mayfly, blackfly) are less below dams (Hatano etal. 2005). Some invertebrate species that require finer materials (e.g.,burrower mayfly, aquatic worms) are sometimes more in the degradedchannels. This is probably due to their preference of slow-flow
conditions. Invertebrate species that adapted to swift flows can also
decrease with riffles in degraded reaches. Invertebrate species richness
is expected to be low in degraded reaches if the spatial heterogeneity inflow and substrate is reduced. On the other hand, total invertebratebiomass may be higher below than above dam due partly to increased
bed stability and food supply.

EVALUATION OF RECOVERY AFTER SBT OPERATION
Methods of Evaluation
One of the recommended approaches for evaluating effects of SBT
operation on downstream ecosystem is to monitor downstream sitefrom before the SBT operation. It is better to monitor also for areference site and analyze by the Before–After Control–Impact design.
Another recommendation is the space-for-time substitution approach,in which downstream change is evaluated by comparing present state ofvarious reaches (dams) with different years of SBT operation. We
mainly adopted the latter approach to grasp a rough but whole image of12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

Japan

Proportion

Koshibu
Solis
Pfaffensprung

Years of SBT operation

SwimmerNet-spin
Glider

0.8
0.6

0.4
0.2

Clinger

0.4
0.2

1

UP

17 yrsReservoir
SBT

DD>90 yrs

DS

BC

Sites offield survey

DT

Tributary

Fig. 5 Surveyed dams with SBT (upper), the space-for-time approach(middle), survey sites each dam (lower)
the recovery, by field surveys at 4 dams in Japan (Koshibu, Asahi) and
Switzerland (Solis, Pfaffensprung) (Fig. 5). SBT is new for Koshibu(no operation) and Solis (operation for 2 years), while old for Asahi (17years) and Pfaffensprung (>90 years). In addition, we examined a longterm change of a downstream of Asahi after the SBT operation. At eachdam, we set up 4 sites with different sediment conditions (UP:upstream, DD: below dam but upstream of SBT, DS: downstream of
SBT, DT: downstream of tributary confluence, Fig. 5). Comparisons
between DD and DS also give us insights on the effect of SBT.
At each site, we measured grain size of bed materials, microhabitats,and invertebrates. Grain size distribution was evaluated by photos takenon the ground and 30-m up from the ground using a drone. We defined
18 types of microhabitats such as bedrock, boulders, cobbles,
embedded cobbles, gravel, sand, moss-mat, algal-mat, wood, leaves,and mud. The abundance of each type was recorded by four levels (0:no, 1: minor, 2: frequent, 3: major). Benthic invertebrate were sampled
in riffle and pool using a D-framed net for 10 to 20 minutes by two
persons. Most of the invertebrates were mayfly, stonefly, or caddisfly,
and the detail species were identified using a microscope. We separated
the abundance (number of individuals) of each species into 4 levels (0:no, 1: 1-10, 2: 11-100, 3: 101 or more). Each species was classified into
groups by life type or habitat preference. Life types are swimmers,
gliders, clingers, burrowers, net-spinners and case makers. Habitat
preferences are riffle dweller, pool dweller, and no preference.
Certain indices are needed to evaluate environmental recovery.
Common indices are richness (number of species or types) or diversity(evaluate both richness and evenness). However, we found these
indices do not always response clearly to the sediment recovery. Indeed,
invertebrate species richness was higher in DS of Koshibu with no SBT
operation. Instead, we consider a pristine state as a goal of recovery,
and evaluate how much the state of a given time is close to the pristine
one. Because no data of the pristine state (i.e., before construction ofdam) was available, we used UP site as a reference. We evaluatedBray-Curtis Similarity Index (BC) between UP and DS asnn 
BC = ∑ min iUP , iDs i N UP N Ds 

DD

DS

0

DT

No prefer
Pool dweller
UP

DD

DS

DT

Fig. 6 Difference in invertebrate community among sites in Asahi

Pfaffensprung
2 yrs

UP

Riffle dweller

Asahi

Koshibu
0 yrs

Habitat preference composition
1

0.8
0.6

0

Asahi

Solis

Life type composition

1

Switzerland(1)

Microhabitat composition

1

0.8

0.8

0.6

0.6

0.4

0.4

Invertebrate community

Dam (years of SBT operation)

Fig. 7 Relationship between years of SBT operation and BC (similarity
between UP and DS)
where ni is abundance of each type or species, and N is sum of theabundance of all types or species. BC ranges from 0 when no common
type or species between the two sites to 1 when exactly the same
composition between the two sites.

Results from Different Data
We found substantial differences in bed materials, microhabitat, andinvertebrate composition between DD and DS sites of Asahi Dam with
17 years of SBT operation. Because the two sites had similar channel
conditions and water discharge of non-flood period, most of thedifferences were caused by with/without sediment through SBT. The
bed of DD was dominated by boulders, while that of DS was dominatedby cobbles with sand and gravels as matrix, which was similar to thatof UP. Invertebrates of DD were much dominated by net-spinners interms of life type, and pool dweller and no-preference in terms ofhabitat preference (Fig. 6), which are typical of degraded channel asmentioned. Invertebrates of DS were much dominated by swimmers,
gliders, clingers, and by riffle dweller, which was similar to UP. On theother hand, differences in microhabitat and invertebrate composition
between DD and DS of Solis Dam with new SBT were not as large as
Asahi.
The recovery of downstream environment was also evident from thespace-for-time substitution approach. Representative grain size, D60,
was larger in DS than UP for Koshibu and Solis with new SBT, while
similar between DS or DT and UP for Asahi and Pfaffensprung with old
SBT. BC, the similarity index, for microhabitat composition between
UP and DS tended to increase with years of SBT operation (Fig. 7).
This is explained by the change in size of dominant bed materials andin the abundance of moss- and algal-mat, which are typical of degradedchannels with stable bed. BC for invertebrate community between UP
and DS also tended to increase with years of SBT operation (Fig. 7).
This is explained by the changes in life type composition and habitat
specialty type as we observed between UP and DS of Asahi (Fig. 6).
Increase of gliders that prefer clean stone surface while decrease of netspinners that prefer stable substrates suggest that bed mobility anddisturbance increase with years of SBT operation. Dimensionless shear

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

stress, which was greater in DS of older SBT and UP of all dams,
supported this idea. Increase of riffle dweller while decrease of pooldweller suggest an increase of fast flow and decrease of slow flow withyears of SBT operation. This process was supported by a following
study that surveyed micro-topography using aerial photos (Kobayashi
et al. 2016). Protruded stones that act as obstacles against flow andboulder steps that dissipate flow energy were abundant in DS and DT of
Koshibu Dam, while these structures were less abundant in DS and DT
of Asahi Dam and UP of these dams.

Years since SBT operation

Stable and rough bed with stagnantsFilling of micro-concaves

Full cover by finer materials
increase mobility

Fig. 8 Schema of the environmental recovery after SBT operation

Yearly data of invertebrates for 16 years since the SBT operation in the
Asahi Dam also showed an increase in BC of invertebrate community
between above and below dam sites with years. Changes in community
composition in terms of life type and habitat specialty, which we
showed in the previous paragraphs, were evident. The yearly data also
showed that most obvious changes in invertebrate community below
dam occurred within a few years after the operation.

CONCLUSIONKEY GRAIN SIZE FOR DOWNSTREAM RECOVERY

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Results of our study suggest that riverbed conditions and invertebrate
community in degraded channel below dams will recover with years
after the operation of SBT (Fig. 8). Sediment transported by SBT,
which is finer than the bed materials of degraded channel, firstly fills
micro-concave topography, makes longitudinally smoother bed, andincreases swift flows. Covering of bed surface by these materials willfurther increase the chances of bed material mobility. These changes
contribute to establish invertebrate community typical for the naturalrivers. The change in bed conditions and invertebrate community may
then affect fish community. Further increase of sediment in the channel
may also develop gravel bars, generate habitats of secondary channels,
and elevate water-filtration capacity of bed.
Although we showed a positive effect of SBT on environment recovery,
the effect may differ according to the amount and size of sedimentthrough SBT. Only sand or finer sediment are expected to betransported through SBT in certain dams to reduce maintenance cost oftunnel abrasion. Such supply of fine sediment may have no effect oreven negative effect on downstream ecosystem. First, sand and finermaterials are easily entrained by small floods and unlikely remain in
the reach without gravel and small cobbles. Even if they remain,
cobbles or boulders embedded by such fines are unsuitable for benthic
invertebrates as mentioned previously. For Koshibu Dam, the shortagein the downstream reaches was much severe for gravel than sand
(Kobayashi et al. 2016). Gravel and small cobbles are likely to be thekey for the ecosystem recovery of the reaches.
Although bed and invertebrate community below Asahi Dam appears
to have been recovered by SBT operation, we also noticed that large
cobbles and boulders were less represented below than above the dam(Kobayashi et al. 2016). This evidence may suggest that not all sizes of
bedload materials but smaller size materials are selectively transported
through SBT. Although the reduction of large cobbles and boulders are
essential for the recovery of the degraded channel, over-reduction ofthese materials can deter the recovery as these materials are also
important elements of invertebrate habitats. SBT of most of the dams
are considered to have some limitations for transporting large sediment.
Additional ideas for transporting large materials, which deposit in
upstream reaches of SBT inlet, may be required for a full recovery ofthe downstream environment.

We showed that sediment transport through SBT can promote theenvironmental recovery of degraded channels below dam. However,
SBT seems to have limitation in transporting some of large materials.

REFERENCES