K-3 Fourth International Conference on Scour and Erosion 2008
HOW IS THE GAP BETWEEN THE CONCEPT AND PRACTICE OF INTEGRATED SEDIMENT MANAGEMENT BRIDGED?
Koh-ichi FUJITA
Member of JSCE, Research Coordinator for Environmental Affairs, Environment Dept., National Institute for Land and Infrastructure Management (NILIM), Ministry of Land, Infrastructure, Transport and Tourism (MLIT) (Asahi 1, Tsukuba-shi, Ibaraki-ken, 305-0804, Japan) E-mail: [email protected]
In Japan, various and intensive modifications to river systems through projects for sediment/flood control, water resources development, electric power supply, river improvement and so on have played extremely important roles in mitigating flood/sediment related disasters and improving our lives and society. At the same time, they have changed river-basin-scale sediment transport systems, bringing new problems with their system soundness in terms of continuity, sustainability and ecological functions. Perceiving that this shows limitations of a locally optimized approach by the area and purpose, which had been taken because of its efficiency, the government has already laid out the concept of “integrated management of a sediment transport system” as a new national policy direction. However, there still appears to be the gap between the concept and its practice, which may retard sweeping development of the new policy. Instead of seeking for “magic technology” that alone can bridge the gap, this overview paper stresses three keys to firmly establishing integrated sediment management: (a) grasping and sharing an overall image of a sediment transport system by using a “common language” that macroscopically describes sediment transport phenomena, not being limited to excessively precise analysis; (b) appropriately performing a diagnosis to identify the structure of problems through a scenario-driven approach, not being preoccupied by stereotyped thoughts; (c) prioritizing the development of component technologies and linking them with policy setting & implementation processes. Specific methodologies essential to obtaining the keys are presented on the basis of characteristics of sediment transport systems in Japan. Finally, eleven issues on how to overcome obstacles in the implementation stage are discussed from perspective ranging from science and engineering to planning and social aspects, with the aim of suggesting strategy of research and development leading to the integrated sediment management.
Key Words : integrated sediment management, river-basin-scale sediment transport system, river bed variation, reservoir sedimentation, coastal erosion, ecological function, habitat
1. GEOGRAPHICAL CHARACTERISTICS of continental rivers, and riverbed gradients are OF RIVERS AND RIVER BASINS IN generally steep. Mountainous land, which yields JAPAN – A COUNTRY WHICH MUST most of the country’s sediment, occupies 70% of COEXIST WITH LIVELY MOVEMENT Japan, and boundaries between mountainous zones OF SEDIMENT and plains are clear (see Fig.1 & 2). There are many geologically fragile places in the mountains. In a
warm humid climatic zone influenced by monsoon In Japan, which is an island nation with steep Asia, rainfall is frequent throughout the year mountain ranges forming the backbone of its long, (national average of about 1,700mm/year), it is often narrow national land, the topography of its river struck by typhoons, and torrential rain can, as shown basins is complex and their scale smaller than those 1
41
Population by geomorphologic classification 山地Mountain 丘陵／台地Hill / terrace 沖積平野Alluvial plain 0510谷底平野Valley bottom lowland [million] 百 海岸沿いの低地Coastal lowland Ise Bay drainage basin
A 1000 A Elevation [m] 0 0km A Horizontal distance 100km s Bay Ise 200km A’ A’ A’ Population distribution: Geomorphologic classification darker red indicates higher Topographical section along population density A--A’ 1) Fig.1 Typical geographical characteristics of JapanP .P –Taking Ise Bay Drainage Basin as an example – Mountains are steep and clearly demarcated from plains, and cover a large percentage of the land. But the population of the mountains is very low, with about a half of the total population living on the rest consisting of alluvial plains, valley bottom lowlands and coastal lowlands, and with the remainder distributed on hills and terraces. This means that many people live near or at places where flood inundations or the transport and deposition of sediment can occur.
events in a decade Total number of observed Hourly precipitation >50mm Hourly precipitation >100mm
Alluvial plains Others Assets
Population
Area 1977- 1987- 1997-2006 1977- 1987- 1997-2006
2) 3) P P Fig.2 Distribution of assets, population and area in JapanP P . Fig.3 State of occurrence of torrential rainfalls in Japan . in Fig.3, fall from late spring to early autumn. important sediment production sources. Although As a result, specific sediment yields are large massive failures induced by extremely torrential compared with the world average, and in some river rains are rare, once they occur, they dramatically basins they exceed 2mm/year or even more, as is transform downstream river courses, supplying a shown in Fig.4. huge amount of sediment in short periods of time. On the other hand, the mountainous land in Japan The sediment produced has been transported is mostly covered with relatively good quality forests down steep rivers to small areas of shallow sea water which have the capacity to absorb heavy rainfall to a around the edges of the mountainous zones, forming great extent. Therefore, it should be also noted that alluvial plains. The alluvial plains occupy only about devastated streams and massive failures of 10% of the national land, but during the period of mountainsides play significant roles in sediment modernization and urbanization, the small precious yield (see Fig.5). Although the overall area of plains were occupied by half the population and devastated streams is small, they persist in some about 3/4 of the nation’s assets were concentrated on mountainous regions, continuously forming their land as is shown in Fig.2, because the 2
42
Legend for specific sediment yields [mm/year] ～
Below 0.1
Fig.4 Specific sediment yields for dam catchment areas 4) calculated from dam reservoir sedimentation data.P P mountainous land covering 70% of the national land is too steep for land use. Daily life and economic activities in Japan are conducted mainly on alluvial plains formed in only Fig.5 Examples of devastated streams and massive failures. the past approximately 10,000 years and which are Devastated streams in the Tedori River basin (top left) and still under the influence of their formation process, the Kurobe River basin (top right). The bottom photo shows a along rivers which transport large quantities of massive failure in the Naka River basin on Shikoku Island sediment with spatial and temporal variety, and near induced by the torrential rain in the excess of 1000 mm daily sediment production sources occasionally activated precipitation in 2004, inflicting a severe disaster including fatalities and destruction of homes, roads and bridges. The by sudden phenomena. And these conditions will collapsed sediment flowed down into a tributary stream of the remain unchanged in the future as a result of the basic river, raising its bed by several meters. geographical conditions of Japan. This means that the people of Japan will continue to live along with lively energetically implemented to reduce disasters caused movement of sediment. by sedimentation of narrow torrential river courses by excessive runoff of sediment, the collapse of slopes, landslides, and debris flows. 2. DEVELOPMENT OF THE COUNTRY Dam reservoirs, which have played a key role in ACCOMPLISHED THROUGH flood control (flood discharge regulation), water VARIOUS MODIFICATIONS TO resource development (securing water for urban and RIVER SYSTEMS irrigation use), and supplying power to support the reconstruction of the national land after World War II In Japan, artificial control of rivers has played and high speed economic growth, have been extremely important roles in supporting economic designed appropriately and constructed in growth and in preventing disasters and mitigating mountainous regions throughout Japan to achieve their effects so that people can live in comfort. Since these multiple purposes (see Fig.6). Dam reservoirs the late nineteenth century when the end of the feudal in Japan are now capable of storing a total of 25.2 3 period brought industrialization and urbanization, a billion mP :P but, it is equivalent to only 6% of all variety of projects have been undertaken to control theoretically available water resources. rivers and these were accelerated after World War II, Those who live in and use alluvial plains have transforming the state of Japan’s rivers. The faced on an urgent need to reduce the danger of following are typical examples. floods inundating their land by increasing flood In mountainous regions, sediment control discharge capacity of river courses. Expanding river projects, well-known as Sabo, have been cross-section areas by excavation has been, along
3
43
Legend for dam catchment area ratio ① Kushida River
⑤ Tenryu River
⑥ Ooi River
Dams managed by MLIT or the Japan Water Agency
Dams managed by prefectures or others
③ 佐久間 Sakuma Dam ② ④
⑥ ⑤
① Ise Bay
4) Fig.6 River system map of the Chubu Region for dam catchment area ratioP .P The left half covers Ise Bay Drainage Basin, geographical characteristics of which are shown in Fig.1. Dam catchment area ratio is defined as a percentage of area of dam catchment to the total catchment area with reference to a location on a river. Dam reservoirs have been constructed in mountainous regions throughout Japan, effectively controlling floods and supplying water. While there are river systems on which the ratio is large down to the river mouths, there are river systems where this value is high only in upstream areas. The degree of impact of dam reservoirs on downstream rivers can vary greatly, depending on the location of the dams on a river system. with the construction of levees, an effective and practical countermeasure, as is shown in Fig.7. Right-bank levee Left-bank左岸堤防 levee 右岸堤防 67 1934 65 2002 3. EFFICIENCY AND LIMITATIONS OF 63 OPTIMIZED APPROACHES BY THE 61 PURPOSE AND THE AREA 59 100m Elevation [m] 57 Efforts to improve river systems as described 55 above have, needless to say, made a great contribution to progress towards each of the goals: Fig.7 Cross-sectional change between 1934 and 2002 at the sediment-relate disaster countermeasures mainly in Tedori River 11km point from the river mouth located in its alluvial fan river reach. upstream and mountainous areas, flood-related This typically shows a great effect of channel excavation disaster countermeasures mainly along alluvial river on flood disaster prevention by increasing a flood flow reaches, water resource development and electric capacity and by lowering flood water stages. Unless a river power supply by dam reservoir construction, and so responds quickly to excavation, channel excavation is a on. At the same time, these have resulted in great simple, but also a strong and highly reliable method.
4
44 45 changes to the basic characteristics of the movement 4. NATIONAL POLICY DIRECTION: of sediment in Japan’s rivers. INTEGRATED SEDIMENT Fig.8 is called a sediment transport system census MANAGEMENT OF A SEDIMENT map prepared by aggregating, combining and TRANSPORT SYSTEM processing all measured data related to sediment transport and geomorphic change in the sediment The basic directions of sediment management in transport systems of the Class A rivers. A sediment line with such basic awareness have already been transport system is defined as, “a continuous range of formulated at the government level. First, a clear sediment movement from the upstream source of a awareness of the challenges was presented in the river basin to its coastline”. From this figure, it is policy proposal “Basic directions of future river possible to know the total dam reservoir improvement policy looking ahead to the society of sedimentation volume and rate in a river basin. It the twenty-first century”, presented by the River shows the distribution of devastated mountain land, Council to the Ministry of Construction (MOC), river course variation trends (riverbed aggradation or predecessor of the Ministry of Land, Infrastructure, degradation), state of shorelines, total quantity of Transport and Tourism (MLIT) in 1996. sediment transported artificially outside the river “Problems concerning sediment occur in various course (based on sand & gravel mining and channel ways in each area: mountains, plains, river mouths, excavation data recorded since 1945) and so on for coastlines, and so on. Problems seen in the mountains each sediment transport system. include the occurrence of disasters caused by This map provides fundamental information that sedimentation of torrent streams by sediment run- off should be the starting point to gain a comprehensive from devastated mountain land, massive failures of overview of the overall state of sediment transport mountainsides, landslides, and debris flows, plus systems in Japan. And this figure shows that if the problems such as the reduction of dam functions by total quantity of sediment deposited in dam dam reservoir sedimentation in high sediment yield reservoirs is obtained, it is approximately 1.18 billion 3 regions. Problems on alluvial plains, at river mouths, mP .P And the total quantity of sediment transported and along coastlines include riverbed degradation, artificially outside the river course is obtained as 1.13 3 plugged river mouths, and retreating coastlines. Past billion mP .P If this combined quantity of sediment was countermeasures were planned individually spread over the entire land of Japan, it would raise its according to the purpose in each area, but such surface by 6mm. individual responses cannot achieve sweeping Throughout Japan, anxiety spread that various resolution of these problems.” river improvements and sediment control projects Basic policy directions to resolve such problems undertaken energetically on river courses in this way were laid out as follows in a report by the to achieve each of these goals would result in Sub-committee of the River Council on Integrated significant disruption of sediment transport systems Sediment Management under the title “Towards which span entire river systems, and that this would integrated sediment management of a sediment undoubtedly cause the following new problems. Fig. transport system” in 1998. 8 certainly shows a dominant trend for riverbed 1) As a new perspective to the overall resolution of degradation throughout Japan and for retreating sediment-related problems, the concept of shorelines along many coasts, providing fundamental “sediment transport system” defined as a support for such concerns. In fact, as explained later, continuous range of sediment movement from the it is essential to take a more cautious scientific upstream source of a river basin to its coastline attitude to the issue of the causal relationships must be introduced. And integrated sediment between phenomena on each river system. However, management should be performed under this at least, on the assumption that the above concerns concept. To do so, “spatial continuity”, “temporal are generally justified, a common awareness of the continuity”, “quantity and quality of sediment need to create appropriate technological (grain size and habitat formation)” and countermeasures has been established. We are also “relationship with river flow” need to be aware that at the same time as approaches by the considered. purpose and by the area have been extremely 2) The goal of sediment management is the creation effective, they are accompanied by specific of a rich and vigorous society by preventing side-effects and that integrated management is disasters caused by sediment movement, definitely required to overcome these side-effects. conserving the ecosystems, landscape, etc. in river
and coastline environments and appropriately
using rivers and coastlines based on the
6
46
Implementation of countermeasures Monitoring
Sediment transport measurement Sediment yield at a check dam survey
Regulation of gravel mining
馬込川 River-bed 中田島砂丘 variation Micro-habitat survey survey on grave bars 海岸侵食が進行
離岸堤（３基）と 養浜を計画 Topographic survey on coasts
Sediment flushing Permeable type check dam at dams Sand bypass in coastlines
Fig.9 Various initiatives conscious of integrated sediment management of a sediment transport system.
characteristics of each river and coastline. above approaches. 3) Choosing some sediment transport systems as pioneer cases, river managers should conduct The above appears to show that the basic policy monitoring of the state of sediment, test direction has been placed on a firm foundation and countermeasures such as supplying sediment, and the process is steadily reaching the implementation observe their effectiveness and impacts. Quantities stage. In fact, in some sediment transport systems, and the quality of sediments should be monitored activities have been undertaken energetically in and research conducted to improve prediction recent years, as is shown in Fig.9. Examples of these methods, in order to prepare plans to undertake include large-scale measurements of sediment integrated sediment management and to take transport using weirs, measurements of riverbed countermeasures. Specifically, the following are variation during floods, the construction of sediment important. check dams designed with slits or openings so that
MonitoringU of sediment movement:U On sediment harmless sediment flows downstream and of dam transport systems where problems with sediment reservoirs with sediment bypass or removal management have occurred, the quantity and functions, the resupply of sediment deposited in quality of sediment should be systematically and dams to rivers downstream, the restoration of bare consistently monitored throughout the system. gravel riverbeds by reactivating gravel transport, and
WiseU control of sediment runoff in mountains and artificial beach nourishment or sand bypasses on
foothills:U Disasters caused by unexpected and coastlines. excessive runoff of sediment should be prevented These have been undertaken with greater while sediment supply should be maintained as consciousness of integrated sediment management appropriate for downstream rivers and along throughout a sediment transport system than in the coastlines. past. But it cannot be stated that a unified
EstablishmentU of new sediment management implementation system has been introduced with
systems at dams:U Effective and safe sediment clear common targets and under integrated sediment management systems around dam reservoirs such management plans for the said sediment transport as sediment bypass, pass-through and removal system, so sediment management is still partly should be established. dependent on enthusiastic efforts to undertake
AchievementU of appropriate and consistent individual projects, which are not always sediment management throughout sediment well-coordinated. So here, a gap remains between the
transport systems:U Integrated sediment concept and practice of integrated sediment management including sediment supply should be management in a sediment transport system. implemented while maximizing the use of the 7
47
the sediment transport system. Cases where 5. THREE IMPORTANT POINTS TO phenomena related to a sediment transport system FIRMLY ESTABLISH INTEGRATED constitute only a part of a problem can occur. It is SEDIMENT MANAGEMENT important that, instead of thinking that dealing appropriately with a sediment transport system will To bridge this gap in order to bring integrated resolve all problems, you specify what kinds of sediment management closer to the stage of full-scale phenomena other than sediment transport are related application, I wish to stress the following three to the occurrence and resolution of problems. 5) [C] Prioritizing the development of component points, which were presented by Tsujimoto and IP .P [A] Grasping an overall image of the sediment technologies and linking them with policy setting & transport system or not being limited to precise implementation processes analysis of details At the stage where necessary knowledge related The state of a sediment transport system is, of to A and B above is obtained to study integrated course, clarified by surveying, observations, and countermeasures used to manage a sediment monitoring, phenomena analysis, and by modeling, transport system, it is essential to first prepare but it is vital that these activities be matched to the applicable component technologies. It is important to purpose: understanding of overall phenomena in a hold discussions to decide what component sediment transport system. technologies are necessary and under what priority It is not easy to grasp an overall image of a they should be developed, and it is also necessary to sediment transport system extending from the clarify how the application of each component drainage basin to the seacoast. There is an inevitable technology will contribute to the total management tendency for phenomena analysis and precise of the sediment transport system. modeling of separate areas to take precedence. When Sediment transport system management handles a scientific analysis advances, there is a tendency for temporally and spatially wide range of phenomena technologies that express more details with greater deeply linked to society, so it is important to hold precision to follow. If such methodologies become discussions to determine the best methods of too familiar, people probably adapt too much to using proposing, coordinating, and deciding different “languages” for each area concerned. In countermeasures. Specifically, topics to be discussed order to avoid becoming preoccupied with include management planning, the concept of setting phenomena in each area, it is essential to establish a goals, ways to achieve a consensus, balancing a “common language” as a way of gaining a common variety of goals, and implementation strategies. understanding of a sediment transport system Another important point is the scientific and targeted. This is so because even if detailed technological information which should be provided phenomena have been minutely collected, overall for this purpose. phenomena are not necessarily obvious, and this Items A, B, and C defined above do not exist means that it is vital to discover resolution which is independently, but they are strongly interdependent, appropriate to viewing overall phenomena. so it is vital that all three be discussed in parallel. [B] Appropriately performing a diagnosis to identify problems and how the sediment transport system are linked to them 6. HOW TO OBTAIN A COMMON Managing a sediment transport system is a LANGUAGE: VITAL FOR method of achieving goals, and its heart is OVERCOMING POINT [A] “management for what purpose?”. In other words a consciousness of the goal of resolving problems is (1) Grain size distribution supplied from essential. Without it, a strange conversation could mountains to alluvial plains in Japan occur as follows. “Yes, integrated sediment Sediment supplied by production sources in the management is very important. Let’s begin! By the mountains includes a wide range of material ranging way, what are we going to do to implement it?” This from clay to boulders. Although it cannot be means it is essential to understand the structure of generalized that the importance of the abrasion and problems in the sediment transport system and then crushing in the production of fine-grain sediment is find how to link handling of the sediment transport ignored, when discussing a sediment transport system to resolving problems. And phenomena related to the structure of problems are not limited to
8
48
] 2 600 Estimation range /y/km 3 500 400 300 nt supply [m 200 100 0 Kiso Shinano Yodo Oota Hii Specific sedime Names of the river basins Silt and clay deposited in sea areas Silt and clay deposited in alluvia Legend Sand deposited in alluvia Gravel and larger sediment deposited in alluvia Calculated from dam reservoir sedimentation data for all grain sizes Fig.10 Long-term mean quantity of sediment supplied to the alluvial rivers for each grain size range investigated based on the 6) volume and grain size distribution of their alluviaP :P the quantities for all grain sizes can be compared with dam reservoir sedimentation data indicated by the adjoining grey bars. An overwhelming percentage of all sediment supplied from mountains is sediment with a grain size of sand and finer. The transport of gravel and larger sediment causes vigorous sediment transport and great riverbed variation which cause sediment disasters, mainly on mountain river courses. However, in terms of overall quantity, most of that supplied is fine grain size material. Interestingly, long-term (about 10,000 years) mean specific sediment supplies estimated correspond to those based on dam reservoir sedimentation data which indicate far short-term (a couple of decades) values.
Clay Silt Sand
Gravel and larger i j i j aw g Ha esawa awa awa Ikari Ikari Tase Sono- hara Grain size distribution [%] distribution size Grain miwa g g Yu da iwara Oodo Kawa Ooishi Futase Futase j Tedori- gawa Gosho Gosho Nomura Nomura Naruko Naruko Aimata Tsuruta Tsuruta Koshibu Koshibu Yabakei Su Ishite Taisetsu Fu Ishibuchi Kamafusa Kamafusa Izari The names of the dams Midorikawa Kasturazawa 7) Fig.11 Grain size composition of sediment deposited in the dam reservoirsP .P
礫Gravel 砂Sand
r
シルトSilt & 粘土Clayl Percentage fine Percentage
0.001 0.01 Grain size [mm] Fig.12 Grain size distribution of sediment deposited at the Sakuma Dam on the Tenryu River (thick red line) and the grain size 8) distribution in the downstream river (orange) and on the seacoast near the river mouth (blue, black)P P : the location of the dam is shown in Fig.6. Although the main river-bed material is gravel, sand that will be beach material is already supplied to the dam in large quantities.
9
49 system in Japan, overall, it should be assumed that at cases finer sand), sand, and gravel & larger. the stage where it has flowed through the mountains a 3) To control the phenomenon which is the focus of certain distance, the percentage of sediment flux attention, it is necessary to track movement of the between gravel & larger sediment, sand, and silt & grain size group that governs this phenomenon. clay have become generally constant. Inversely, it is not necessarily reasonable to track A lot of borings for various construction works all grain size groups. have been made on alluvial plains in Japan, which A good example for explaining the movement of has made it possible to more precisely draw 3-D each of the above grain size groups is a river course strata structure of alluvia, which have been deposited that is seen typically for large alluvial rivers in Japan: for years. By calculating the volume of alluvia for a steep slope gravel-bed reach (segment G) each grain size group from strata structure obtained connecting with a mild slope sand-bed reach in this way, we can estimate long-term mean (segment S) (see Fig.13). Grain size groups which sediment supply from mountainous land (about are gravel & larger, sand, and silt & clay, in the above 10,000 years for major alluvial plains in Japan) if we percentages, are supplied from mountains to alluvial choose the case where sediment transported away to rivers. the bottom of the deep sea can be neglected. From upstream to downstream, gravel is Based on the volume and grain size of alluvia for transported mixed with riverbed material (gravel) in example, the grain size distribution of sediment segment G, then stops at the downstream end of supplied to alluvial plains from mountains during the segment G. Sand almost entirely passes through past 10,000 years has been estimated to be 50 to 60% segment G without its transport flux varying very silt & clay, 30 to 40% sand, and less than 10% gravel much, and after it reaches segment S, it is transported and larger, as is shown in Fig.10. Grain-size mixed with riverbed material (sand) until it reaches distribution of sediment deposited in a dam reservoir the river mouth where it is finally discharged into the generally corresponds well to these percentages, as is sea. Accordingly, in segment S, sand that is riverbed shown in Fig.11. Sediment with a grain size of sand material is actually supplied directly from the and finer which account for most of the sediment sediment production source. Some silt & clay can be transport in downstream rivers already exists at deposited on floodplains (natural levees), on gravel positions in dam reservoirs in the mountains (see beds as top fine sediment layers in alluvial fan rivers, Fig.12). and on river-beds near the river mouth as temporary For such reasons, as a primary approximation, it is riverbed material. But most passes through both possible to grasp sediment transport systems within segments G and S to flow into the sea. the framework: How does sediment with the above It is possible to explain the basics of the bed grain size distribution supplied from mountains to profile evolution with sharp longitudinal sorting as is alluvial plains behave in rivers and on coastlines? shown in Fig.13, by macroscopically grasping the movement of each grain size group. (2) Effectiveness of the approach: grasping In the above, the movement of gravel governs sediment movement for each grain size group riverbed variation in the gravel bed reach, and the The behavior of sediment with such a wide range movement of sand governs riverbed variation in the of grain sizes can be grasped effectively by tracking sand bed reach. The movement of fine-grain movement for each of the grain size groups as shown sediment governs the formation of top fine sediment below. layers attributable to the rapid expansion of stable 1) The manner in which sediment in a sediment vegetation areas on gravel bars (see Fig.14), transport system is transported, is exchanged with floodplain accretion forming natural levees (see riverbed material and influences river topography Fig.15), its deposition in super low velocity areas all vary greatly according to the grain size group. near river mouths, and the material transport of Consequently, treating sediment as a uniform mass nutrient salts or pollutants etc. (including supply to is meaningless. On the other hand, separating grain the sea). sizes into too many fractions from the beginning, It is highly likely that main bed materials in the as is often the case with sediment transport furthest downstream river course segment play a modeling, is too complex and even hinders primary role in supplying material forming coastline essential study of overall trends. topography, but there are cases where, according to 2) In many cases, it is better to begin by considering the grain size of the material which governs the sediment movement for each of the three grain size formation of coastline topography, even finer groups: fine-grain sediment (silt, clay and in some
10
50 山からのSediment 山からの Segment G of an alluvial fan river 土砂供給土砂供給supply Silt & clay Sand Gravel 砂 礫 シルト・粘土シルト・粘土 砂 礫 (G) (FS) 海 Sea 海 Segment G or S of a natural levee river Some can become material-s or –t.
シルトSilt & 材料s，材料tになることも シルト (G or S) (FS and Silt) 材料s，材料tになることも clay 粘土粘土 Segment S of a deltaic river Pass-through 通過型 材料s，材料tに 通過型type 材料s，材料なることもtに Sand砂 なることも 砂 混合型Exchange混合型 type ｾｸﾞﾒﾝﾄ M（山地部河 材料mを構成 材料mを構成 材料mを構成 (S) (Silt and clay) To become海岸 材料mを構成 礫 礫Gravel 海岸 の材料 岩盤 coastalの材料 material Symbols in () indicate bed material. in many cases. Transported as material-m (G): gravel or larger. (S): sand. (FS): fine sand. Fig.13 Explanation of macroscopic movement of grain size groups (silt/clay, sand, and gravel/larger) and their roles in longitudinal river-bed profile evolution for a river course composed of a steep slope gravel-bed reach (segment G) connecting with a mild 6) slope sand-bed reach (segment S)P .P On the left, the classification of river bed material into m, s and t is presented. Material-m plays a primary role in governing the formation of longitudinal bed profiles and river-bed configuration, roughness coefficients of a low water channel, scouring depth essential for design of structures, the habitat structure composed of riffles and pools, and so on. When the words, “riverbed material” are said, the speaker is often referring automatically to material-m. A riverbed made of material-s is formed by the deposition of formerly suspended fine-grain size sediment beside the main flow. Material-s even forms floodplains themselves in natural levee zones. The deposition of material-s in alluvial fan is generally thin wherever it occurs. However, vegetation flourishes far more readily than at a location where gravel (material-m) is exposed. The existence of material-t is often temporary and unstable, as it is flushed out or sharply reduced by even the smallest floods. The covering of material-m by material-t, even partially or temporarily, is important as a change in micro-habitat structure, and it could also impact the material cycle when low flow condition is prolonged. CPOM can be included in this category.
Flood Right after the 1974 flood
C
c
r
h
o
a
s
n
s
-
g
s Pioneer plants
i Pioneer plants
e Right before the
n
c
g
t
i
t 1981 flood
o
o
n
c
s
o
m Top fine-sediment layer
p
o
u
n
d
-
t
y
p
e Top fine sediment Gravel bed Right after the Top fine-sediment layer 1981-1983 floods layer
Fig.14 Role of the top fine sediment layer in the riparian No flood onto the woodland expansion observed in the Nagata reach of the higher riverbed 9) Tama RiverP .P The top fine sediment layer which was carried on gravel beds by the floods of 1981 to 1983 triggered the wood land and Present dense herbaceous plant area. The top fine sediment layer formed on top of the gravel bed of an alluvial fan river is only a few tens of centimeters thick at the most, and does not raise the riverbed very much, but it has an important impact on the Higher riverbed prevalence of vegetation. Lower riverbed
11
51
68.8km from68.8k the river mouth 71.8km from the river mouth 178 178 1979 1979 1983 1983 174 174 C 1987 1987 1990 1990 170 170 1996 1996 標高 [m] 2000 166 2000
Elevation T.P. [m] T.P. Elevation 166 2006 2006 Elevation T.P. [m] Elevation 162 80 120 160 200 240 162 80 120 160 200 240 Transverse distance横断距離 [m] : looking [m] downstream Transverse distance [m] : looking downstream
] m [ Elevation T.P. Elevation T.P.
Coarse sand
Longitudinal distance [m]
10) Fig.15 Channel narrowing caused by the floodplain accretion observed in the Sendai RiverP :P an example of contribution of fine sediment. Tops: Cross-sectional changes. Bottom: Soil stratification observed on the face of the trench excavated longitudinally with about 2m deep and about 40m long, lateral location of which is point C shown in the top left figure. The excavation and observation was made in 1993-1994. Most of the newly deposited sediment is fine sand or finer, while material-m is gravel. Normally, since fine grain-size sediment is of the pass-through type, it does not cause deposition of such scale, but if external impacts force the channel shape to deviate from quasi-equilibrium, significant deposition of fine grain-size sediment (otherwise pass-through type) can occur.
important to categorize macroscopic movement of sediment is effective. such series of grain size groups as either exchange 6) 11)
If we extend the concept of grasping sediment type or pass-through typeP .P “Throughput load”P P and 12) movement for each grain size group, it is not difficult “over-passing load”P P are similar to the concept of to reach another concept —that of the “effective grain pass-through type. Please refer to Fig.13 again to size group”, which is defined as a grain size group grasp the following explanation on these two types. that affects certain specified river geomorphic Exchange type is defined as a macroscopic change and phenomena concerned. transport manner of a grain size group in which the group is transported with significant exchange with (3) Exchange type and pass-through type: a key to the main bed materials. The flux of sediment understanding macroscopic sediment transport belonging to this type varies longitudinally, movement and the transport of main riverbed materials is of the In addition to the approach of grasping sediment exchange type, which is explained by its definition. movement for each grain size group, it is also very Sand transport in Segment S and gravel & larger
12
52 sediment transport in Segment G in Fig.13 are situation before and after dam reservoir construction categorized into exchange type. were created. These are being used as powerful tools Pass-through type is defined as the other to macroscopically clarify the change of sediment macroscopic transport manner in which a grain size movement over time, and to assess the possible group concerned exchanges little with the main bed effects of river modifications on sediment movement. materials (at the most, to a degree which fills gaps in Typical maps are shown in Fig.16-18. the main riverbed material on the surface). Sediment These maps, for instance, clarify which grain-size transport of this type can not contribute to riverbed groups are supplied primarily by which sub-drainage variation unlike exchange type. Sand transport in basins, and specify the grain-size groups that segment G and silt & clay transport in segment G and compose the river mouth terrace and the grain-size S in Fig.13 are categorized into pass-through type. groups that compose the bottom sediment of lakes The flux of sediment transport of this type varies beyond the terrace. As for the impacts of constructing longitudinally only a little. As was mentioned above, a dam reservoir on each grain-size group, it is some silt & clay becomes floodplain material (natural possible to give appropriate primary diagnoses that levee) and top fine sediment layer, but its quantity is answer questions like: Does this or does it not appear much smaller than the total flux, or its deposition is in the form of riverbed variation or in coastline temporary. That is why the categorization of silt & erosion? clay transport into pass-through type in Fig.13 is justified. (5) More strategic observation and improvement of sediment transport modeling (4) Efforts to prepare river-basin-wide sediment Initiatives such as the above are counted on to movement maps as a practical tool for play a role in refining surveying, observation, and obtaining a common language also monitoring strategies that not only contribute to A river-basin-wide sediment movement map point [A], but also clarify the weak points of present presents the movement of sediment in a river system sediment movement observations for our based on unified guidelines and the method shown understanding of both phenomena at individual below, in line with the approaches described in the locations and overall sediment transport systems. For previous sections. example, map preparation initiatives have created 1) The thickness of a river course usually drawn awareness of problems that include the following. schematically in the map is indicated by the flux of 1) A practical and reliable method which can be used a grain size group of sediment. to stably observe bed load as quantitative data has 2) The sediment production sources are displayed by not been established. dividing the river basin into sub-river basins as 2) Wash load can be observed relatively easily by river basin management units with scale combining bucket sampling, automatic samplers appropriate to the clarification of the river basin and turbidity meters: however suspended load characteristics. (specifically, for sand) must be assessed by 3) It displays sediment movement for each grain size clarifying its vertical distribution. Existing group. methods still need to be improved. 4) It displays sediment movement classified into 3) Using dam sedimentation data is an extremely exchange type or pass-through type. useful way to supplement the weak points of 5) Based on measurements, it is supplemented by sediment flux observations. It is emphasized to calculation as necessary, and parts not understood promote the clarification of dam sedimentation by should be clearly indicated, for instance by grain size, as well. labeling them “unknown”. 4) It is necessary to perform mutually supplementary MLIT (MOC before 2001) carried out a project of observations and calculations to clarify the systematically observing sediment in 21 river movement of sediment in a sediment transport systems throughout Japan from 1999 to 2002 and system, because sediment transport measurement summarized the results as river-basin-wide sediment during floods, in particular, requires considerable 14) movement mapsP .P In addition, maps were prepared effort. An important point of debate is the ideal separately for other river systems as an advanced way to divide the roles of observation and initiative. For some river systems, maps for a number calculation. of periods from the past to the present or maps of the
13
53
Iida dam and Grain size group: its catchment area 0.075mm – 0.4mm Sediment observation station Inada Kataniwa Upper-Hinuma
Name and area Sakura of the sub-basin Grain size group:
Longitudinal 0.015mm – 0.075mm distance Shiore
Hinuma-mae
Kansei
Terrace at the river mouth
Lake Hinuma
Grain size group: finer than 0.015mm
Fig.16 River-basin-scale sediment movement map for the Hinuma River sediment transport system (total movement by a flood on 13) September 16, 1998) P P The river reach sandwiched by broken lines indicates exchange type. The reach without them indicates pass-through type. The numbers 3 2 with mP ,P with kmP P and with km indicate sediment volume transported, catchment area and longitudinal distance from the rive mouth respectively.
0～ 0.075mm
5100
190 1350 Terrace at the segmentSegment S 3 河口テラスriver mouth
450 Fig.17 River-basin-scale sediment movement map for the Hinuma 250 River sediment transport system: Detailed map focusing on its 550 13) 涸沼 river mouth area P .P The numbers inside the rectangles indicate 1990-1998 mean annual 0.075 ～ 0.4mm 3 quantity of transported sediment to be deposited or to pass through [mP ]P Hinuma River Lake Hinuma
14
54
Sediment yield region Sediment yield region 土砂生産域 鳴子 土砂生産域 鳴子 with Naruko Damダム ダム 8.7万 4 2.5万4 8.7× 10 セグメント１ 2.5 × 10 セグメント１ 0.2 × 104 0.2× 104 31km 0.2万 31km 0.2万 4 Segmentセグメント2-1 G Segmentセグメント2-1 G 1.4× 10 4.7× 104 1.4万 0 0 26km 0 26km 0
4.7万 4.7万 Segment S ★図中の数字は Segment S ★図中の数字は Number セグメント2-2 土砂移動量 セグメント2-2 土砂移動量 [m 3/年] [m 3/年] Mean annual 1973～1993年の平均 1973～1993年の平均 万 4 quantity of 0km 4 0km 6 0km : 3.6× 103.6 万 0km : 3.6× . 10 transported confluence(旧北上川 confluence(旧北上川 3 sediment (1973 との合流点） 粒径集団Finer 粒径集団0.1mm 粒径集団1.5mm との合流点） 粒径集団Finer 粒径集団0.1mm 粒径集団1.5mm with the ～0.1mm 0.1～1.5mm 1.5～50mm with the ～0.1mm 0.1～1.5mm 1.5～50mm – 1993) Kitakami than – – Kitakami than – – River 0.1mm 1.5mm 50mm River 0.1mm 1.5mm 50mm Grain size groups Grain size groups
13) Fig.18 River-basin-scale sediment movement map for the Eai River sediment transport system P P Left: Mean annual quantity of sediment transported under the present situation (with the dam). Right: Mean annual quantity of sediment transported when the condition of no sediment deposited at the Naruko Dam is given. The river reach sandwiched by broken lines indicates exchange type. The reach without them indicates pass-through type. By comparing the two maps, we can make a primary examination on possible patterns of channel response to sediment supply reduction. For example, it can be judged from the comparison that while the supply reduction of the 0.1-1.5mm group causes little degradation in the reach of Segment G where it is transported as pass-through type, degradation is likely to occur in the reach of Segment S where the group is transported as exchange type and the supply reduction can bring longitudinal decrease in sediment transport flux. Although precise prediction of the degradation should be made by river-bed variation calculation, such a primary examination enables us to identify the way precise calculation can work to diagnose and resolve problems.
5) When an attempt is made to reproduce the phenomena to reproduce phenomena may become a transport of a pass-through-type grain-size group round-about approach, simply because the sediment by conventional methods of riverbed variation transport system is governed by wide-ranging, calculation, it may be calculated that sand that is complex and varied phenomena. Which combination supplied in larger quantity than gravel is gradually of such methods is good depends on the scenario, so mixed with the so-called exchange layer of a it is more important to collect good examples that gravel bed surface, and that the main riverbed have clarified the structural outline of problems than material contains a large quantity of sand. Such it is to rush to generalize the method. calculation results in a balance of sand in the sediment transport system that differs from the (2) Importance of abandoning stereotyped actual situation. It is necessary to conduct further diagnoses studies of conditions under which this awareness is One more key to clarifying the structural outline appropriate, along with more rational handling of a problem through an appropriate diagnosis is to 5)12)15)16)
methodsP .P avoid preconceptions. For example, there have been reports suggesting the conspicuous degradation of downstream riverbeds attributable to a dam reservoir. 7. PROBLEM RESOLUTION-ORIENTED Fig.8 can definitely be interpreted to show that a MANAGEMENT: FOR OVERCOMING clear tendency for riverbeds to fall is dominant POINT [B] throughout Japan, and as stated above, if the total sediment deposited in dams is aggregated, an amount 3
of 1.18 billion mP P is obtained. For these two reasons, (1) Importance of setting hypotheses to diagnose there is a tendency to argue for a simple link between the problem blocking of sediment by dams and the resulting The key approach applied in order to clarify the riverbed degradation. But this is not correct. structural outline of problems is to first establish a That is because to consider the causes of riverbed hypothesis (scenario for occurrence of phenomena degradation, at the least, channel excavation and that cause and foster problems), then select and apply gravel/sand mining must be added to candidate a method needed to judge whether or not the factors. The total quantity of both amounts to 1.13 hypothesis is correct. Inversely, adopting a set of 3
billion mP .P Needless to say, a direct effect of channel methods related generally to sediment transport 15
55 excavation and gravel/sand mining is the degradation (3) Framework for interpreting geomorphic of riverbeds, through which channel excavation for effects of a dam reservoir on downstream river improvement achieves the increase of flood reaches without falling into stereotyped flow capacity. diagnoses It is, therefore, necessary to, at the least, consider a) Case focused on a sand group the degree of effectiveness of phenomena occurring A sand group is generally transported faster than a at the location of dams and the direct action of gravel/larger group along a river system, and as excavating river courses and mining gravel/sand on shown by Fig.10 to Fig.12, the quantity supplied is rivers in order to analyze the causes of riverbed far higher. If a dam cuts off or greatly lowers its degradation. As shown by Fig.10, 11 and 12, much supply, therefore, its impact reaches the downstream of the sediment accumulated in dams is of a grain size river relatively rapidly, and there is a high possibility of sand and finer. Therefore, the quantity of riverbed that the physical environment of the river degradation in gravel bed reaches (segment G) is downstream from the dam will be in a state governed completely out of balance with the quantity of gravel by the sand supply from rest of the river basin where trapped by dams, and the excavation and mining of no dams stop sand supply to the location. At the in-stream sediment is clearly a major cause of the location reached by the sand supply reduction, there degradation. The quantity of sand stopped by dams, is a high possibility of the following occurring on the other hand, is in the same order as the quantity qualitatively.
removed, on the assumption that about half of the GravelU bed reach (segment G) sediment removed from the river course is sand. For In this section, the sand group is the pass-through this reason, regarding sand, it can be pointed out that type, so the presumed pattern is little change in the the direct action of the removal of sediment from the riverbed form, only with a reduction of sand river course and the halting of sand by dams may temporarily deposited in gaps between the surface of both impact on the lowering of riverbeds (plus, land main bed materials, in areas behind obstacles such as subsidence has been another major factor for some boulders, and on the bottom of a pool formed by rivers in Japan). scouring.
It should be possible to avoid the kind of SandU bed reach (segment S) elementary interpretational errors mentioned above In this reach, the sand group is the exchange type, simply by performing analysis according to the so the riverbed may be lowered and the longitudinal actual state of the sediment transport system, by slope may decline. When such a change occurs, the studying the balance of each grain-size group (in the change leaps over the gravel reach and begins from above case, sand group and gravel group) instead of the upstream end of the sand bed reach. This means only comparing entire quantities of sediment, by that change of the river course does not always occur separating phenomena occurring at dam locations sequentially from upstream. The reason for this has and direct actions on the river course, and by been already explained by the movement of each remaining aware of the basic characteristics of the grain-size group described in Fig.13. movement of the grain-size groups as shown in In a case where the gravel segment is stretched to Fig.13. the river mouth (no sand bed segment exists), it can Other stereotyped and hackneyed diagnoses be hypothesized that the flux reduction of the sand concerning dams are “armoring by dams” and group transported through the gravel section as “reduction of the supply of coastline materials by pass-through type rapidly propagates downstream dams”. Of course, according to conditions, there are and easily causes reduction of sand supply to the cause-and-effect relationships between these, but seacoast. Inversely, on a river with a sand bed inversely, there are many cases where other factors segment such as that shown in Fig.13, the impact of have greater effects, or where such phenomena do not the halting of the sand group by the dam will, first, occur. Placing simple and naive faith in a hypothesis appear near the upstream end of the sand bed or a preconception is not a good way to handle this segment; then, after a little while, the impact of the matter. So what is a good way to grasp the reduction of the sand supply will be absorbed by the downstream effect of a dam in terms of sediment riverbed degradation from that point. Thus, it is movement, geomorphic change, habitat formation presumed that the construction of a dam does not and so on? immediately reduce the sand supply to the seacoast. A variety of processes related to the transport of the sand group can occur according to river characteristics. Among them, cases where the blocking of sediment by a dam does not lower the
16
56 sand supplied to the seacoast are not rare. Therefore, excavation in a gravel bed river reach (including Instead of attempting to analyze such a situation as it gravel mining), installation of a transverse structure is, trying to directly link the total quantity of (in particular, a fixed structure), or combinations of sediment deposited in a dam reservoir to the these. In the case of large rivers in Japan in particular, reduction of the sand supplied from the river to the gravel mining, which had been actively done in the seacoast, for example, is far from scientifically past, lowered the overall longitudinal river bed rational. profile, as is discussed in (2). Its consequences still b) Case focused on a gravel group significantly remain in present river courses. This case deals exclusively with a gravel bed c) Does armoring always occur downstream from segment (segment G). A gravel group moves far a dam? Discussion based on field survey results 17) more slowly than a sand group. Therefore, even if a Two dams were surveyedP :P Dam A, which had dam etc. located upstream blocks gravel supply, it is been in service for more than 30 years, and Dam B, not likely that effects of the blockage will quickly which had been in service less than 10 years at the propagate far downstream (several tens of kilometers time of the survey (February to April 2006). for example). Hydraulic and river course conditions and riverbed Within a certain range directly downstream from materials upstream and downstream from each dam the dam, various changes may appear to be caused by were investigated, and results common to both dams blockage of the gravel supply. The ways these are summarized as follows in 1)-4) below. Fig.19 changes appear, however, are not limited to armoring shows longitudinal changes of each river in mean or riverbed degradation. Instead, a variety of changes riverbed elevation, water surface width, velocity, can occur caused by factors such as river discharges, mean grain size of material-m, bed surface covering a convergence pattern with tributaries, characteristics rate of material-t in the aquatic area and material-s on of the original river course, and so on. For example, the terrestrial area on both sides. Please refer to both in some cases, a dam is constructed on a river course rivers in the figure. Spatial distribution of where the mobility of gravel & larger sediment as the materials-m, -s and -t in the surveyed sections are main bed material has already been low (such river schematically shown in Fig.19, as well. The courses are not unusual in mountainous regions). In definition of material-m, -s, -t is explained in Fig.13. this case, the slight decline of flood discharges due to 1) The grain size of material-m (gravel and larger) is dam operation sharply lowers the mobility further, almost identical upstream and downstream from which does not coarsen the gravel or lower the the dam. riverbed. 2) The bed surface covering rate of material-t (sand) It must be assumed that downstream effects of in the aquatic areas is clearly smaller directly dams on gravel beds have a variety of patterns in this downstream from the dam: these reaches are way. It is possible to broadly categorize the response identified by the thick arrows in Fig.19. patterns by combining the degree of the decline of 3) The fall of the bed surface covering rate of gravel-carrying capacity downstream from a dam material-t (sand) directly downstream from the accompanying flood control [large or small decline] dam is rapidly restored between 4 and 10 km with the degree of gravel supplied from tributaries downstream from the dam. downstream from the dam [large or small supply]. It 4) The bed surface covering rate of material-s (sand) can be hypothesized that, of these, armoring of the on the terrestrial areas is almost identical upstream main bed material or degradation is likely to occur in and downstream from the dam. And these areas are cases of the [small vs. small] combination; while in covered with herbaceous vegetation. the [large vs. small] combination, the river bed is It can be seen that material-m is unchanged by the likely to lose mobility (the above example); and in dam or changed only slightly, while material-t in the the [large vs. large] combination, even sedimentation aquatic areas is significantly reduced by the dam in can occur downstream of tributary convergence. specific sections. And based on these results, it can At a certain distance from the point where the be stated that downstream effects of dams on river gravel supply is blocked, the blockage of the gravel bed materials can be described appropriately only itself may have only a small impact on the riverbed. when material type (m or s or t?) and areas (aquatic or At such locations, rather than the blockage of gravel terrestrial?) are clearly explained. supply by the dam etc., the direct action on the river Only two examples have been introduced here, so course, and in particular its past history, is likely to it is, of course, impossible to say the results represent be more attributable to experienced change in the riverbed. Examples of direct action include riverbed
17
57
支川 Dam B 支川 Dam site Tributaries ＡダムDam A Damダダムム site位位置置 Ｂダム ダム位置 Mean bedダダダ ムムム位位位置置置 300 Mean bed elevation elevation平均河床高 400 平均河床高 Tributaries 350 RReesseervvooiir 250 Reserrvoirr 300 1／90 1／270 （m） （m） 250 200 Reservoir RReesseerrvvooiirr 1／400 200 1／170 150 150 150 水面幅 Water 160 水面幅Water surface width surface 100 120 width (m) 80 50 (m) 40 0 0 4 流速Velocity 5 流速Velocity 3 4 3 2 (m/s)
(m/s) 2 1 1 0 0 600 代表粒径Mean grain size of material-m 1000 代表粒径Mean grain size of material-m 500 800 ) ) 400 600 300 材料ｍ
（mm 400 （mm
200 材料ｍ 100 200
0 0 表層2mm以下の割合 表層2mm以下の割合□: material-s, in terrestrial areas 100 100 ●: material-t, in aquatic areas （％） 80 （％） 80 60 ●水域 60 ●水域
（％） 40 □陸域 （％） 40 □陸域 材料s,ｔ
材料s,ｔ 20 20
材料sとtの 材料sとtの 表面存在割合 0
0 Bed surface covering ratio [%] 材料sとtの 表面存在割合 010203010 0 10 20 010 55 10 0 155 2010 （km） Bed surface covering ratio [%] Downstre ←upstreamUpstream Distanceダムからの距離（km） from the dam sites Distance fromダムからの距離（km） the dam sites Downstream→
Spatial distribution of materials-m, -s and -t in the surveyed sections schematically illustrated.
Aquatic Terrestrial陸域 area 水域 Terrestrial陸域 area area 砂・シルト 表面の一部に砂・シルト Material-s (sand,(材料ｓ) silt) Material-t(材料ｔ) (sand)
平水時の水位 砂礫(材料 ｍ) Material-m (Gravel and larger)
Fig.19.Example of an upstream vs. downstream comparison of riverbed material around a dam reservoir for river reaches whose 17) hydraulic conditions are similar upstream and downstreamP .P Downstream is oriented to the right. Velocity and water surface width are those at the time of mean annual peak discharge. Material-m is expressed by its mean grain size. Material-s and –t are expressed by the bed surface covering ration; i.e. the percentage of area where material-s or –t covers the bed surface of material-m. In these surveys, the grain size of material-s and –t is 2mm or less. a normal response. There are also examples of rationality that is suited to the level of analysis showing that material-m is significantly sediment-hydraulic knowledge. Examples include changed; coarsened or washed away, for example. “Before and After (BA) Design”, which involves What is important is not to adopt the premise that if it tracking change of a river course along with is downstream from a dam, the same kinds of hydraulic forces in the same reaches downstream phenomena will occur. Instead, it is important to from a dam from before it was constructed until after clarify whether or not the river course is changed by it began operating; “Control and Impact (CI) Design the dam on a factual basis and, if so, how this change (example introduced above), which involves occurs, and to analyze the overall relationship of the comparing the upstream and downstream state of a dam with the downstream river course. river course where sections with identical hydraulic In order to make such analyses, it is vital to plan a and river course conditions exist directly upstream survey which will permit the application of scientific and downstream from the said dam; and a
18
58 combination of the two (BACI design).
(4) How to handle cases where the problem is not limited to the sediment transport system For example, if cases where reduction of sediment supply from a river or the construction of port and harbor facilities causes coastline erosion are concerned, the problem is entirely limited to a matter Fig.20 State of the river bed surface observed by hydraulic of a sediment transport system. But in cases where a experiments designed to simulate a steep slope river problem includes both phenomena in a sediment with gravel or cobble as material-m. transport system and phenomena unrelated to the The left photo shows the state when little sand is transported, and the right photo shows the state when an sediment transport system, it is impossible to extremely large quantity is transported, while the rest of the examine the structural outline of problems and ways hydraulic conditions are set the same. In normal mountain to manage the sediment transport system unless their river courses, the usual situation is like the left photo. The relationships are clarified. A typical case is one quantity or height of sand between the gravel varies according to the quantity of sand transported. Even when the where the target is conserving or restoring the natural quantity of sand supply increases greatly, gravel can hardly environment of a river. be completely covered by sand, as the right photo shows. There is no question that the sediment transport system is one fundamental condition which the stage where it completely covers the gravel bed determines the river natural environment. Therefore, (see Fig.20). Therefore, the perception that to discuss sediment transport system management pass-through type of sediment transport brings little that is intended to conserve and restore the natural to river beds is correct as long as river course environment, it is essential to answer the question, management to control flooding is concerned. “What kind of sediment transport system can However, when the objects of control are conserve or restore the natural environment of the expanded to include micro-habitats on the riverbed river?” or more directly, “What kinds of sediment surface, it is impossible to ignore possible changes should be supplied (and what discharge should carry caused by differences in the transport of them) and how should they be supplied to benefit the pass-through-type grain-size groups. As for the natural environment of the river?” example above, the environmental conditions for Recently, extensive research has been carried out benthic fauna, periphyton and so on can be guided by an awareness of the importance of significantly different between when there is almost collaborative efforts between river engineering and no sand in the interstices of the gravel bed surface ecology in order to conserve ecosystems in Japan. and when sand partially or fully fills it. Therefore, For example, collaborative research intended to once it is necessary to examine the state of a river improve the environments of river courses course from the perspective of living organisms or downstream from dams has been conducted, ecosystems, it is also necessary to employ a new accumulating knowledge which appears to be related appropriate descriptive method or calculation method 18) to the above questionsP .P The relationship between for sediment transport phenomena. In this context, dynamic systems which form the physical for instance, how to specify “sediment maintenance environments of rivers with living organisms is being flow” for gravel-bed river reaches downstream of a clarified through research on cases in individual 21) dam reservoirP P has been one of the important 19)20) fields and experimental streamsP .P But research research targets. has not begun full systematization to clarify the On a number of sediment transport systems in overall relationships between sediment transport Japan, sedimentation of dam reservoirs has advanced systems and natural environments or study on the to the degree that there is an urgent need for most effective way to manage a sediment transport countermeasures. As keys to tackling this challenge, system to conserve or restore its natural environment. the study and implementation of flushing and bypass It is, therefore, necessary to expand the capacity of sediment from dam reservoirs have already been of methods for describing phenomena in sediment 22) launchedP P in addition to mechanical removal, where transport systems. For example, above it was stated target grain size groups are usually sand, silt and that in a steep slope gravel bed reach, the transport of clay. This is because a large amount of sedimentation sand can be viewed as pass-through type that does is occupied by those grain size groups as is shown not contribute very much to riverbed variation. The Fig.10-12. In this case, the targets are the additional sand is quite likely to be only deposited in the quantities of sand, silt, or clay with high mobility interstices of the gravel bed surface without reaching accompanying the implementation. Significant
19
59
Inflow to the dam reservoir (natural conditions?) In the Kurobe River, after a flushing process is Outflow from the dam reservoir (conditions modified completed, clean water is released for a certain by regulation with the dam) amount of time as “cleansing” to wash fine sediment Increase in sediment concentration of the outflow accompanying countermeasures against dam reservoir on gravel bed surfaces which is deposited possibly sedimentation during the flushing. This is the important germ of Is there any “environmental specifications” for the Q vs. Qs s preferable region? C relation, where its time sequence is considered as h ic h e w ? l well. e w ab in lo ri l e a y b v In cases where the problem cannot be limited to t e an p n e li ? ? re k d the category of a sediment transport system as e e e o h b th o Cs = Qs/Q t l s st is f I u , a explained above, it is necessary to further expand the o g ? m s n If ri u way of defining a sediment transport system to d ? ? consider its relationships with other phenomena (above, living organisms and ecosystems). This Q presents an opportunity to expand the horizon of Fig.21 Idea on the control of the Q vs. Cs (= Qs/Q) sediment transport system technologies. relationship for sand or finer sediment to maintain or restore the soundness of instream micro-habitat. On the assumption that the black thick line has been 8. ISSUES ON THE IMPLEMENTATION changed to the red broken line by flow regulation with the dam upstream, countermeasures against dam reservoir STAGE OF INTEGRATED SEDIMENT sedimentation are supposed to raise the red line toward the MANAGEMENT POLICIES: FOR black line, as is shown by the blue arrows. However, OVERCOMING POINT [C] according to methods adopted for flushing, bypass and so on, increased Cs may exceed the black line. For conserving instream micro-habitat, should we set a cap for the Q vs. Cs Based on the previous discussions, points of (= Qs/Q) relationship? Or should we control the relationship contention concerning integrated sediment so that it will be kept within a “preferable region”? management policies that consider the impact that modifications to sediment movement have had on riverbed variation is not likely to occur in steep slope river basin environments, including coastlines, are gravel bed reaches, including mountainous streams, listed below. even if large-scale sediment flushing is done. 1) Is “making comprehensive and integrated efforts” Moreover, since sediment flux from dam reservoirs a tool for advancing matters or an ultimate has been reduced, it can be said that the additional “golden” goal? quantities play a role in restoring sediment transport 2) What should be a desirable balance between as it used to be. However, as stated above, when the survey research and practice? How do we reach a goal is to precisely control the state of micro-habitats consensus concerning ways of obtaining this on riverbeds to a specified range, more advanced balance? How long will the problems wait for us? sediment flushing (bypass, pass-through) 3) Is it absolutely good for rivers and sea coasts that technologies are needed. the sediment which had been stopped begins to Specifically, the key is to find a way to control the move again? And, is such situation positioned as relationship of the flux Qs of sand or finer grain-size environmental restoration? If so, can the resolution sediment with the discharge Q [the Q vs. Qs relation] to achieve the restoration of natural environment or the relationship of Cs (= Qs/Q) with Q, where Cs be the driving force for implementing an corresponds to sediment concentration [the Q vs. Cs integrated sediment management policy? Or, relation] (see Fig.21). And, it is necessary to clarify rather, is reviving sediment movement a target of the degree of deviation from the band width of the environmental impact assessment for avoiding Q-Qs or the Q-Cs relationship of natural (or present?) environmental deterioration possibly caused by conditions that can be allowed from the perspective newly brought conditions of sediment movement? of its impact on living organisms in the riverbed. Or is the direction forward found somewhere Discussion and clarification on this “allowable between these attitudes? And, if so, how are they deviation” will lead to the “specifications for natural balanced? environment conservation” required for sediment 4) Is “returning to the past” a banner? Is it always flushing and so on. When such discussion is right? How influential has the accumulation of advanced, it will be inevitable to consider that the several decades since modifications to a river allowable deviation may change in time during a system started been? In circumstances where flood. surroundings (social or natural) have been
20
60
transformed, if only part of the system is assumed understood and methods for predicting them are to have returned to the past, can challenges appear firmly established, B: scenarios and prediction elsewhere? If so, what sort of challenges? methods have been proposed but not verified It is probably certain that the sediment scientifically, and C: only rough scenarios have transport system has changed sharply from its past been discussed. When we examine integrated state due to the modifications; on the other hand, sediment management for a sediment transport however, it is also true that many people may system, it is likely that A, B and C are all believe “the present situation is normal” after a concerned. In this situation, do we have strategies certain amount of time has passed. In such a to coordinate them to reach the constructive situation, there may be cases where the concept conclusion? And are we rational enough not to “return the sediment transport system to its past retard overall decision making just because not all state” alone will not be adequate as a means for handled phenomena are supported by level-A promoting policy implementation. prediction? This leads to another important challenge; that 9) For policies which will take an extremely long is, how to find the best implementation process time to become clearly effective, how will the where gaining a consensus is crucial. In order to do effectiveness of the policies be positioned or so, it is necessary to explain the structural outline explained? How will it be possible for people to of the problem described under point [B]: how the savor a feeling of achievement? sediment transport system management that is 10) Is “adaptive management” effective and being attempted will change (or will not change) attractive as the core of such policies? On the the river system extending to the coastline, and assumption that it is so, when solidly constructed how this will be beneficial to the residents of the structures are necessary to perform trial efforts river basin. and monitoring, how will they be matched with 5) How specifically and definitely can we indicate the framework of “adaptive management”? the quantity and grain size of sediment which must 11) Have we already obtained techniques and flow downstream in a river? Is such indication foundations whereby we can build social pin-point or is it presented as a range? Are there consensus regarding the creation of a new only conditions to avoid? Or will quantity and sediment movement order and the adoption of grain size be examined adaptively while setting various ways to do so? hypotheses? 6) In order to advance integrated management, should we find a primary conductor who has 9. CONCLUDING REMARKS strong incentives and motives to practice it, who is ready to dictate it, and whom the rest can count on? Needless to say, it is valuable and important as a Possibilities include a dam reservoir manager who core concept for sediment management policy to has been urged to cope with severe sedimentation, restore sound sediment transport systems that have or a manager who administrates coasts and has had been deteriorated due to various modifications to great difficulties in handling severe erosion. Or is river systems, while maintaining the benefits these it preferable for everyone concerned to evenly modifications bring to our lives and society, and to participate in promoting integrated management achieve restoration through integrated sediment cooperatively toward shared goals on the state of management. sediment transport system to be achieved. At the same time, as was discussed in the previous Probably, the latter is more ideals, but that section, it can probably be said that it is excessively approach may have a tendency to let the discussion naïve to implement this by relying on the virtue of the wander. The former may weaken the concept of basic policy concept alone, without being prepared to “integrated”, but it can be more practical and carefully examine complexly interrelated realistic. phenomena, including social and historical aspects as 7) Two situations may emerge simultaneously in a well as scientific and engineering aspects. I would sediment transport system; one is where like to emphasize that it is necessary to tackle this management methods are readily and definitely challenge by adopting a strategy of wisely selecting determined; the other is where a lot of discussion and combining varied ways to deal with complex is needed to determine them. When this is the case, realities and by making steady improvements— not how can we coordinate such a mixed situation? jumping to a beautiful outcome— while keeping 8) The technological level of prediction of sediment naivety as a fundamental driving force to nurse our transport-related phenomena can be organized into resolution to improve the sediment transport system. three groups: A: phenomena are largely 21
61
REFERENCES 11) Cui, Y. and G. Parker : The arrested gravel front: stable 1) Wakamatsu, K., Kubo, S., Matsuoka, M., Hasegawa, K. and gravel-sand transitions in rivers, Part 2: General numerical Sugiura, M. : Japan engineering geomorphologic map, solution, J. Hydraulic Research, Vol.36, No.2, pp.159-182, University of Tokyo Press, 2005. (in Japanese) 1998. 2) Infrastructure Development Institute-Japan, Japan River 12) Egashira, S., Jin, H., Takebayashi, H., Ikeda, A. and Nagata, Association and River Bureau, Ministry of Land, T. : Bed variation and sediment budget in the downstream Infrastructure and Transport : Rivers in Japan, pp.8, 2006. reach of Kizu River, Ann. J. Hydraulic Engineering, JSCE, (http://www.mlit.go.jp/river/basic_info/english/pdf/riversinj Vol.44, pp.777-782，2000．(In Japanese) apan.pdf) 13) Fujita, K., Hiratate, O., Hattori, A., Yamanouchi, Y. and 3) Cabinet Office, Government of Japan : White Paper on Kato, N. : River basin map of sources, discharges and storage Disaster Management 2007, Fig.1-1 and Fig.1-2. (in of sediment for supporting integrated sediment management, Japanese) Civil Engineering Journal, Vol.41, No.7, pp.42-47, 1999. (In 4) Fujita, K., Tomita, Y., Oonuma, K., Oro, T., Ito, K. and Japanese) Yamahara, Y. : Facts and introductory knowledge about 14) River Bureau, the Regional Development Bureaus and the downstream effects of dams on the physical environment of National Institute of Land and Infrastructure Management of rivers in Japan, Technical Note of National Institute for Land the Ministry of Land, Infrastructure and Transport : Research and Infrastructure Management, No.445, pp.22, pp.28, 2008. project (FY1999-2001) on the development of sediment (in Japanese) measurement methods, the preparation of river-basin-scale 5) Tsujimoto, T. and Fujita, K. : On scenario-driven research sediment movement maps and the strategic plan of sediment and development toward the integrated management of the monitoring toward integrated sediment management in river river-basin-scale sediment transport system, Advances in systems, FY2001 Report of Ministry’s research and River Engineering, JSCE, Vol.10, pp.13-18, 2004. (In development projects, 2002. (In Japanese) Japanese) 15) Sekine, M. and Hayashi, M. : Semi-two dimensional 6) Fujita, K., Yamamoto, K. and Akabori, Y. : Evolution numerical analysis of bed evolution using a gravel-silt mechanisms of the longitudinal bed profiles of major alluvial packed bed model, Ann. J. Hydraulic Engineering, JSCE, rivers in Japan and their implications for profile change Vol.51, pp.973-978，2007．(In Japanese) prediction, J. Hydraulic, Coastal and Environmental 16) Fujita, M., Sulaiman, M., Ikhsan, J. And Tsutsumi, D. : A Engineering, JSCE., No.600/2-44, pp.37-50, 1998. (In bed-porosity variation model and its application, Advances Japanese) in River Engineering, JSCE, Vol.14, pp.13-18, 2008. (In 7) Sakurai, T., Kashiwai, J. and Ooguro, M. : Dam reservoir Japanese) sedimentation form and its properties, Civil Engineering 17) Yamahara, Y., Fujita, K., Oro, T., Tomita, Y., Oonuma, K., Journal, Vol.45, No.3, pp.56-61, 2003. (In Japanese) Fukuda, S. and Inoue, Y. : Comparison of bed materials 8) Committee on the integrated sediment management to cope between the upstream and downstream of dams almost with coastal erosion and dam reservoir sedimentation on a similar in hydraulic conditions, Advances in River sediment transport system (Chairman: Tsujimoto, T.): Engineering, JSCE, Vol.13, pp.147-152, 2004. (In Japanese) Technological framework for implementing integrated 18) Nakamura, F. (feature editor) : Feature: influence of dam sediment management of a river system and adjoining coasts structure, Ecology and Civil Engineering, Vol.2, No.2, on the basis of the case study of the Tenryu River system and pp.101-177, 1999. (In Japanese) the Enshu-nada Coast, Report of the Committee edited by the 19) River Ecology Research Group of Japan : Understanding the Japan Institute of Construction Engineering, pp.3/20, 2004. natural environment of rivers: perspectives of river ecology rd (In Japanese) research, 3P P edition, 2002. 9) Fujita, K., Lee, S., Watanabe, S., Tsukahara, T., Yamamoto, 20) Aqua Restoration Research Center, Public Works Research K. and Mochizuki, T. : Mechanism and simulation of the Institute: expansion and extinction of stable vegetation areas in a http://www.pwri.go.jp/team/kyousei/eng/about/m2_01.htm. gravel-bed alluvial fan river, J. Hydraulic, Coastal and 21) Wilcock, P. R., Kondolf, G. M., Mathews, W. V. G. and Environmental Engineering, JSCE., No.747/2-65, pp.41-60, Barta, A. F. : Specification of sediment maintenance flows 2003. (In Japanese) for a large gravel-bed river, Water Resources Research, 10) Fujita, K., MOODY, J. A., Uda, T. and Fujii, M. : Accretion Vol.32, No. 9, pp.2911-2921, 1996. 22) Sumi, T. : Reservoir sedimentation management in Japan, of flood-plains by wash load deposit and resulting channel rd narrowing, J. Hydraulic, Coastal and Environmental the 3P P World Water Forum Session Report on “Challenges to Engineering, JSCE., No.551/2-37, pp.47-62, 1996. (In the sedimentation management for reservoir sustainability”, Japanese) pp.207-227, 2003.
22
62