Introduction
In mountain areas, where debris torrents might have catastrophic and dramatic impacts, "check dams" (also called debris dams or sabo dams) can be used to reduce the impact of debris torrents. Debris dams are common features in Europe, North America and Far East Asia. Illustrations are shown in NAKAO (1993) and CHANSON (2001).
The term "debris dam" is used to describe both consolidation dams and sediment retention structures. The former is generally a wall-type structure (e.g. Photo No. 1). It is designed to elevate the torrent bed, to fix and to stabilise the bottom profile. The latter type of structure is commonly an open structure (i.e. grid dam, beam dam and split dam) designed for the trapping of medium- to large-size debris (rocks, boulders, logs) (e.g. Photo No. 2).  ARMANINI et al. (1991) described several examples of open structure check dams.
The dam construction decision process is a very important step in the design. The choice of debris dam(s) building and their location must be sound and optimal to prevent debris flow catastrophes.

Sabo works in Japan
The Japanese islands is characterised by a steep unstable topography with frequent volcanic activities and earthquakes. Debris flows are frequent and numerous disasters occurred. The original purpose of the sabo structures was to reduce the excess sediment discharge to prevent river degradation further downstream and to enable ship navigation in the downstream streams. More recently the emphasis of Sabo works shifted to the control of debris flows.
In Japanese, the direct translation of Sabo (sa-bo) is "sand protection". Generally the term "sabo works" refers to mountain protection system. Early sabo works were undertaken during the 17th and 18th centuries. For example, a 10-m high masonry soil-retention structure in Fukuyama (Hiroshima prefecture) is still standing. During the second half of the 19th century and early 20th century, numerous sabo works were constructed under the guidance of foreign engineers and of Japanese engineers educated in Europe. For example, Dutch engineers were invited to come to Japan in 1873, among which Johannis de RIJKE had a very significant great influence. The Austrian engineer Hoffman designed sabo works near the town of Seto, 20 km NE of Nagoya, in 1909. Photo No. 31 shows an artificial stepped channel designed by a Japanese engineer, modeled on Durance catchment works (construction : 1916-18) near Matsumoto, Nagano Prefecture. The first concrete sabo check dam was completed in 1916 : i.e., the Ashiyasu dam, Yamanashi Prefecture which is still standing.
A major debris structure is the 63-m high Shiraiwa Sabo dam (also called Siraiwa dam). The main dam is equipped with a 12-step overflow spillway (NAKAO 1993) and it is designed to trap up to 1 Mm3 of sediment. Another large sabo structure is the Mount Tokachidake Sabo works in the Furano river catchment. After completion, the sabo system include 72 check dams, 11 slit dams and 71 consolidation dams. The total cost was 0.8 Billion of yens in 1993. An interesting example of sabo works system is the Kakurajima Volcano Sabo works. The Kakurajima island is 12 km wide by 19 km long. The volcano is active and large scale volcano eruption took place in 1471-1476, 1779-1785, 1914 and 1946. All 18 rivers are equipped with Sabo works and debris flow occurred each year. During debris runoff, the velocity of debris flow were observed to reach 40 to 70 km/h (NAKAO 1993).

Sabo check dams
The most common type of sabo dams is the vertical concrete wall (e.g. Photo No. 6, Photo No. 7, Photo No. 10, Photo No. 34,). The structure has the initial purpose to trap sediment material (Photo No. 18) and to reduced the slope of the upstream catchment when the reservoir is filled (Photo No. 35). The downstream face of the dam is nearly vertical, followed by a short stilling structure. In steep topography, the downstream channel may be stepped to contribute to further energy dissipation, in a fashion somehow similar to stepped spillways (Photo No. 6, Photo No. 19) (1). Dam heights range from 3 to 15 m typically.
Other types of sabo check dams include permeable check dams, tubular grid dams, slit dams and overflow stepped weirs. Permeable check dams are designed to trap small to medium size debris. They do not hold water. In forest areas, permeable dam may be made of steel grids. Figure 8 shows a permeable structure near Matsumoto, Nagano prefecture.  Others may be made of concrete elements commonly used for coastal protection. Photo No. 2 shows a permeable Sabo work off Takatoyo beach, Enshu coast while Photo No. 23 presents debris material and concrete blocks on the Osawa-gawa, Western slope of Mount Fuji.
Tubular grid dams are made of large-size steel tubes (diameters between 0.5 to 1 m) anchored in reinforced concrete foundations. They are designed to hold heavy sediment blocks (e.g. boulders, huge rocks) weighting over 10 tons. Figure 15 shows a tubular grid structure, 9 m high, 60 m long. The steel tubular elements are 7 m high and the tube diameter is 0.7 m. The design technique was developed in the late 1960s, but it became more common since the late 1980s.
Slit check dams are a form of permeable debris dams for medium-size debris. They are designed with one or several vertical opening(s) to allow small to medium flow while large flow will overtop the structure. Photo No. 4 shows the Inokubo-kawa Kikan Sabo system (Mt Fuji, Japan). The Inokubo stream is located on the Western slope of Mt Fuji, close to Osawa-gawa and Urui river. A major debris retention system, called Inokubo-kawa Kikan, was in construction in Nov. 2001. The system includes a flat, wide flood plain area to store large material and a slit check dam downstream. ARMANINI and LARCHER (2001) presented recently a detailed model study of slit check dams (single opening).
Another form of check dams is the series of overflow stepped weirs. Each structure is about 1 to 4 m high. Stepped overflow weirs are designed to reduce the upstream slope while the steps contribute to some energy dissipation of the overflow at small to medium flow rates. For large flows, the weir acts as a large roughness element. Examples include Photo No. 9 near Mitomi town, Yamanashi prefecture and Photo No. 43  in Wales (UK).

Discussion

During a debris flow motion, what controls the height and propagation speed of the wall of mud ? If the river bed is initially dry, the problem is the classical dam brek wave (on dry bed). This has been developed thouroughly in several textbooks for clear-water (HENDERSON 1966, MONTES 1998, CHANSON 2004a,b). If the river bed is filled with water, the leading edge of the surging waters becomes a positive surge (i.e. a hydraulic jump in translation). The problem is drastically different from the dam break wave on dry bed. (See for example, HENDERSON 1966, MONTES 1998, CHANSON 2004a,b for a thourough treatment.)
The propagation of a mud flow is however a more complicated problem in real situations, particularly steep catchments, when sediment-laden flows may exhibit some non-Newtonian properties. Relevant references include WAN and WANG (1994), COUSSOT (1997) and CHANSON et al. (2006). Recent studies demonstrated completely opposite trends depending upon the initial fluid properties (i.e. rheological properties of the mud). CHANSON et al. (2004) developed a kinematic wave approximation of the Saint-Venant equations for a thixotropic fluid flow down a prismatic sloping channel. A simple thixotropic fluid model was used which is based upon a minimum number of parameters, and described the instantaneous state of fluid structure by a single parameter. Analytical solutions of the basic flow motion and rheology equations predicted three basic flow regimes depending upon the fluid properties and flow conditions, including the initial degree of jamming of the fluid : (1) a short motion with relatively-rapid flow stoppage for relatively small mass of fluid, (2) a fast flow motion for a large mass of fluid, or (3) an intermediate motion initially rapid before final fluid stoppage for intermediate fluid properties. This behaviour, unknown to turbulent or laminar fluid motion, is typical of well-known thixotropic fluid flows, such as pasty cement flows, some mud flows, and subaerial or submarine landslides. It was validated with physical experiments investigating systematically dam break wave of thixotropic fluid down a 15-deg channel (CHANSON et al. 2004).

Footnotes

Detailed photographs

Early sabo works
Photo No. 31 : Ancient Sabo works (1895-1920) near Matsumoto, Nagano Prefecture. Artificial stepped channel designed by a Japanese engineer, modeled on Durance catchment works (construction : 1916-18). Photograph taken in Nov. 1998.

Related designs in the world
Photo No. 41 : Stepped storm water way (Hong Kong) under Hatton road, below Hong Kong University (photograph in Sept. 1994).
Photo No. 42 : Loyalty Road Flood Retarding dam spillway (Sydney NSW, Australia, 1996) View from downstream (Courtesy of D.Patrick JAMES). Dam height : 30 m. RCC construction. Spillway capacity : 1,040 m3/s. Chute width : 30 m.
Photo No. 43 : Rhyd-y-Car Land Reclamation cascade (Wales). Design flow : 10 m3/s. Located at Merthyr Tydfil town centre (approx. 50 km North of Cardiff, UK) (Courtesy of Steve BRIGHT).
Photo No. 44 & 45 : La Motte-du-Caire, Durance catchment (France). Photographs taken in June 1998.  Debris dams on the road to La Motte-du-Caire. Concrete check dam upstream of the fully-silted Saignon dam, La Motte-du-Caire (CHANSON 1999). The Saignon dam reservoir (1961, H=17 m, volum:1.8E+5 m3) became fully-silted in less than 2 years despite upstream check dams (Photo No. 45). View from the right bank of the dam, looking upstream. The reservoir is located in a black marl catchment (3.5 km2 area).
Photo 46 & 47 : Check dams and river training, Ruisseau Ravin de St Julien, St-Julien-Mont-Denis (France). Photo No. 1 : river training in St-Julien-Mont-Denis on 11/2/2004; note the slit check dam in background. Photo No. 2 : slit check dam looking downstream.
Photo 48 : Check dam and sediment retention basin, Ruisseau St Bernard, Saint-Martin-de-la-Porte (France). Photo No 1:  looking upstream on 11/2/04.
Photo 49 : Sediment load in an artificial channel beneath the Autoroute de Maurienne, France on 11/2/2004.
 

Related links

References

Bibliography

Acknowledgments

Hubert CHANSON is a Professor in Civil Engineering, Hydraulic Engineering and Environmental Fluid Mechanics at the University of Queeensland, Australia. His research interests include design of hydraulic structures, experimental investigations of two-phase flows, applied hydrodynamics, hydraulic engineering, water quality modelling, environmental fluid mechanics, estuarine processes and natural resources. He has been an active consultant for both governmental agencies and private organisations. His publication record includes over 620 international refereed papers and his work was cited over 3,700 times (WoS) to 6,300 times (Google Scholar) since 1990. Hubert Chanson is the author of several books : "Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways" (Pergamon, 1995), "Air Bubble Entrainment in Free-Surface Turbulent Shear Flows" (Academic Press, 1997), "The Hydraulics of Open Channel Flow : An Introduction" (Butterworth-Heinemann, 1st edition 1999, 2nd editon 2004), "The Hydraulics of Stepped Chutes and Spillways" (Balkema, 2001), "Environmental Hydraulics of Open Channel Flows" (Butterworth-Heinemann, 2004), "Applied Hydrodynamics: an Introduction of Ideal and Real Fluid Flows" (CRC Press, 2009), and "Tidal Bores, Aegir, Eagre, Mascaret, Pororoca: Theory And Observations" (World Scientific, 2011). He co-authored two further books "Fluid Mechanics for Ecologists" (IPC Press, 2002) and "Fluid Mechanics for Ecologists. Student Edition" (IPC, 2006). His textbook "The Hydraulics of Open Channel Flows : An Introduction" has already been translated into Spanish (McGraw-Hill Interamericana) and Chinese (Hydrology Bureau of Yellow River Conservancy Committee), and the second edition was published in 2004. In 2003, the IAHR presented him with the 13th Arthur Ippen Award for outstanding achievements in hydraulic engineering. The American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE-EWRI) presented him with the 2004 award for the Best Practice paper in the Journal of Irrigation and Drainage Engineering ("Energy Dissipation and Air Entrainment in Stepped Storm Waterway" by Chanson and Toombes 2002). Hubert Chanson edited further several books : "Fluvial, Environmental and Coastal Developments in Hydraulic Engineering" (Mossa, Yasuda & Chanson 2004, Balkema), "Hydraulics. The Next Wave" (Chanson & Macintosh 2004, Engineers Australia), "Hydraulic Structures: a Challenge to Engineers and Researchers" (Matos & Chanson 2006, The University of Queensland), "Experiences and Challenges in Sewers: Measurements and Hydrodynamics" (Larrate & Chanson 2008, The University of Queensland), "Hydraulic Structures: Useful Water Harvesting Systems or Relics?" (Janssen & Chanson 2010, The University of Queensland), "Balance and Uncertainty: Water in a Changing World" (Valentine et al. 2011, Engineers Australia). He chaired the Organisation of the 34th IAHR World Congress held in Brisbane, Australia between 26 June and 1 July 2011.
 His Internet home page is http://www.uq.edu.au/~e2hchans. He also developed a gallery of photographs website {http://www.uq.edu.au/~e2hchans/photo.html} that received more than 2,000 hits per month since inception.

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