Design Standard of Control Structures Against Debris Flow in Japan · 2014-09-17 · Design...

Design Standard of Control Structures Against Debris Flow in Japan

Paper:

Design Standard of Control Structures Against Debris Flowin Japan

Nobutomo Osanai∗, Hideaki Mizuno∗, and Takahisa Mizuyama∗∗

∗Erosion and Sediment Control Division, Research Center for Disaster Risk Management,National Institute for Land and Infrastructure Management, Ministry of Land, Infrastructure, Transport and Tourism, Japan

Tsukuba, Ibaraki 305-0804, JapanE-mail: [email protected]

∗∗Graduate School of Agriculture, Kyoto UniversityOiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

E-mail: [email protected][Received January 6, 2010; accepted April 23, 2010]

Countermeasures to prevent or mitigate sediment-related hazards, debris flow in this paper are classi-fied as structural measures or nonstructural measures.The structural measures are Sabo dams, leading dikes,and channels for debris flow control. The nonstruc-tural measures are the designation of areas prone todebris flow, proper land use in the areas, the reinforce-ment of houses, the creation of warning systems andthe evacuation of the inhabitants in case of emergency.A technical standard on debris flow control structuresof Japan was revised in 2007. Major revision is thatcheck dams called Sabo dams constructed near housesare changed from closed type Sabo dams to open typeSabo dams because the occurrence frequency is lowas once for longer than one hundred years, naturaltorrent environment should be conserved in ordinarydays and sediment trap capacity should be kept untildebris flow occurs. Points changed are explained suchas the opening of open type dams is as equal as themaximum stone size.

Keywords: sediment hazard, countermeasures, structuralmeasures, nonstructural measures, technical standard fordebris flow control

1. Introduction

Countermeasures to prevent or to mitigate sedimentrelated hazards are classified as structural measures ornonstructural measures. In Japan, the SABO (Erosionand Sediment Control) Department of the Ministry ofConstruction (present Ministry of Land, Infrastructure,Transport and Tourism) has made some laws and techni-cal guidelines related to debris flows, shallow landslidescaused by heavy rainfall, and deep seated landslides. Aguideline for debris flow prone streams and areas wasmade in 1999; ‘The guideline for survey of debris – flow-prone streams and survey of debris flow hazard area.’‘Standard Procedures for Investigation of Potential Land-slide Sites’ has been used for deep seated landslides. For

shallow landslides or slope failures, ‘Inspection Proce-dure for Steep Slope Failure Hazard Areas’ is applied. Aswarning and evacuation, Sabo department has made thefollowing guidelines; ‘Sediment-related Disaster Warningand Evacuation Guidelines’ and ‘Guidelines for Develop-ment of Warning and Evacuation System Against Sedi-ment Disasters in Developing Countries.’

A law concerning the promotion of sediment-relateddisaster prevention in sediment related disaster hazard ar-eas (“Sediment-related Disaster Prevention Law”) is thefirst law aimed at proper land use in debris flow and land-slide areas in Japan. It was enacted in 2000 after sedi-ment disasters in Hiroshima, Japan, in 1999. It designatesdangerous areas, prohibits housing development in thoseareas, and moves existing houses in the areas to safer ar-eas. The law also orders that houses built be sufficientlystrong against debris flow or landslide. However, the lawwas enacted too late. It should have been enacted 20 or30 years before, before housing areas were expanded tothe foots of hills and on the debris flow fans in the ar-eas that were groves or paddy fields before development.It was impossible, however, to resist the boom in devel-opment. The law can be found at the following URL inEnglish: http://www.sabo-int.org/law/index.html

The design standard had not been established in 1982when Ministry of Construction had been starting thenew policy named “Promotion of Comprehensive Mea-sures Against Sediment-related Disaster caused by DebrisFlows.” For that reason, Ministry of Construction startedstudies on the phenomena and measures, and compiledthe results and issued the “Technical Guideline againstDebris Flow” in 1989. However the technical guidelinehad some theoretical assumptions such as the method forestimating the peak flow rate, velocity and depth of de-bris flow and so on because all features relating to debrisflow were not clarified completely at that time. Ministryof Land, Infrastructure, Transport and Tourism has beencollecting and analyzed the detailed reports on sediment-related disasters. By analyzing the details of the bothreports and the research results collected until 2007, wefound followings; 1) Not only boulders but also woody

Journal of Disaster Research Vol.5 No.3, 2010 307

Osanai, N., Mizuno, H., and Mizuyama, T.

debris triggered floods by blocked stream, 2) the observeddata of peak flow rate of debris flow was often greater thanthe estimated value by using the methods in the technicalguideline, 3) a debris flow flowed to downstream, becom-ing wider in gullies, and 4) the narrower the open spacewas made the larger amount of sediment could be trappedby the open type dam. Therefore, Ministry of Land, In-frastructure, Transport and Tourism decided to revise thetechnical guideline in order to take into account of theabove-mentioned findings. The main points of the revi-sion were reported. This report mainly aims to introducethe technical standard for planning and designing struc-tural measures and points out the technical problems thathave been still under studies.

2. History of Technical Standards on DebrisFlow Control and Woody Debris Control inJapan

The first technical standards for planning facilitiesagainst debris flows were drafted by the Overall Techni-cal Study Committee on Countermeasures against DebrisFlows from 1982 to 1983. Then, the third subcommit-tee of the Overall Study Committee on Enhanced Debris-Flow Countermeasures from 1984 to 1987 was estab-lished in 1985 and modified the above-mentioned tech-nical standards for planning facilities against debris flowson the basis of the results of its practical application todebris-flow-prone torrents. On October 9, 1989 the di-rector of the Erosion and Sediment Control Departmentof the River Bureau in the Ministry of Construction, thatis now the Ministry of Land, Infrastructure, Transportand Tourism, issued the findings of the study commit-tee nationally as technical guidelines for countermeasuresagainst debris flows.

The draft technical guidelines were then amended onJuly 13, 2000 and issued nationally. In March, 2004the planning part of the draft technical standards forthe erosion-and-sediment control and river engineeringwas revised and published. It stipulates that the basicplan for erosion-and-sediment control should consist ofthe following five areas: erosion-and-sediment controlfor river systems; countermeasures against debris flows;countermeasures against woody debris; measures for vol-canic erosion-and-sediment control; and countermeasuresagainst extraordinary landslide disasters caused by naturallandslide dams.

As for countermeasures against debris flows and woodydebris, the guidelines for the basic planning of erosion-and-sediment control, that is the part on countermeasuresagainst debris flows and woody debris and set out standardmethods for the basic planning of erosion-and-sedimentcontrol, and the design technology guidelines for coun-termeasures against debris flows and woody debris wereissued on March 13, 2007. They were issued under thejoint names of the Erosion-and-Sediment-Control Plan-ning Section director and the Maintenance Section direc-

Fig. 1. Gradation of mountain stream bed in sediment dis-placement morphology (rough standard).

tor of the Erosion-and-Sediment-Control Department ofthe River Bureau of the Ministry of Land, Infrastructure,Transport and Tourism to supersede the draft technicalguidelines for countermeasures against debris flows andthe draft guidelines for countermeasures against woodydebris. Later, guidance on these two guidelines was com-piled primarily by the National Institute for Land and In-frastructure Management and published as the Institute’sMaterials No. 364 and 365 in March, 2007.

Below is intended to describe the modifications and ad-ditions to the draft technical guidelines for countermea-sures against debris flows, modifications and additionswhich were made in the course of compiling the guid-ance on the guidelines for the basic planning of counter-measures against debris flows and woody debris and thedesign technology guidelines for countermeasures againstdebris flows and woody debris.

3. Main Points in the Guidance on the Guide-lines for Basic Planning of Countermea-sures Against Debris Flows and Woody De-bris

3.1. Sediment Transport ModesFigure 1 shows the sediment transport mode in the ba-

sic plan. Basic planning of erosion-and-sediment controlshall be based on sediment transport modes. Sedimenttransport modes are largely divided into the debris-flowand the bed-load sections, depending on the gradient ofthe mountain stream beds. In the current revision, it isso stipulated that the boundary between the two sectionsshould be the point where the gradient of the mountainstream bed is 2 degrees.

3.2. Control Plan for Debris Flows and Woody De-bris

Control plan for debris flows and woody debris can beexpressed by Eq. (1):

V −W − (X +Y +Z) = 0 . . . . . . . . (1)

308 Journal of Disaster Research Vol.5 No.3, 2010


Fig. 2. Sediments to be calculated in control plan for debrisflows and woody debris for open-type.

where V denotes the total volume of sediment and woodydebris carried by a plan size debris flows; W , denotesthe planned allowable volume of sediment and woody de-bris; X , planned captures; Y , planned sediments; and Z,planned retentions of generated flows. V,W,X ,Y and Z,each representing a sum of debris flows and woody de-bris, are expressed in the following expressions (2)–(6):

V = Vd +VW . . . . . . . . . . . . (2)W = Wd +WW . . . . . . . . . . . . (3)X = Xd +XW . . . . . . . . . . . . (4)Y = Yd +YW . . . . . . . . . . . . (5)Z = Zd +ZW . . . . . . . . . . . . (6)

where Vd denotes planned debris flow volume; VW ,planned woody debris volume; Wd, planned allowabledebris flow volume; WW , planned allowable woody de-bris volume; Xd , planned detentions of debris flow vol-ume; XW , planned detentions of woody debris volume; Yd ,planned sediment debris flow volume; YW , planned woodydebris volume; and Zd , planned detentions of generateddebris flow volume; ZW , planned detentions of generatedwoody debris volume.

In order to incorporate planned captures and plannedsediments in the control plan for debris flows and woodydebris, it is necessary to estimate the volume of X , Y and Zin the Eqs. (4), (5) and (6). Figs. 2, 3 and 4 shows the vol-umes of X ,Y , and Z. The slope of the normal sediment-gradation is half of the tangential value of the original bedslope. And the slope of the planned sediment-gradationis two thirds of the tangential value of the original bedslope. These values are derived from the past disasters. Itis necessary to establish and implement a debris-removalplan, comprised of periodic removals of debris includingwoody debris and contingent removals of debris includingwoody debris.

3.3. Peak Debris Flows Discharge and Depth of De-bris Flows

Peak debris flow discharge in all cases are to be calcu-lated by Eq. (7) for peak debris flows based on outflow-sediment volume.

Qsp =0.01C∗Vdqp

Cd. . . . . . . . . . . (7)

Fig. 3. Sediments to be calculated in control plan for debrisflows and woody debris for closed-type.

Fig. 4. Sediment to be calculated in control plan for debrisflows and woody debris for partially open-type.

where Qsp denotes peak debris-flows; Vdqp, outflow-sediments including voids expected from one wave of de-bris flows; Cd , concentrations of debris flows; C∗, volu-metric concentrations of mountain stream bed sedimentsand the value is usually around 0.6.

Vdqp is calculated as follows. First of all, we have todetermine the section where debris outflows are expected.

Eq. (7) is derived from the past records of debris flow.As shown in the Eq. (7), it is important and difficult to setthe value of Vdqp because the computed value of Qsp isproportional to the value of it. At this moment, it is diffi-cult to find out a fact that two or more debris flows startflowing down at the same time according to our experi-ences in Japan. Therefore we assume that the sedimentvolume transported by one debris flow affects on the peakdebris-flows. Fig. 5 shows a schematic figure of a courseof one debris flow. As shown in Fig. 5, the expected de-bris outflow area is determined to be a section rangingfrom the upstream point circled in Fig. 5 between thepoint where Qsp is calculated and the most downstreampoint in the flow area of debris flows to the farthest water-shed point along the mountain stream, including mountainsides called zero-order basins. If there are branch streams



Fig. 5. Schematic drawing of expected debris-outflow sections.

Fig. 6. Cross section of peak debris flows.

in addition to the main mountain stream in the watershed,more than one area where debris outflows are expectedhave to be set up.

In the next place, we calculate “erodible sediments”for each such section and “transportable sediments by theflow due to rainfalls of a magnitude that will exceed theprobabilities assumed in the annual planning” that is sameas “erodible sediments” described in Paragraph 2.7.3 ofthe Guidance on the Basic Planning Guidelines for Coun-termeasures against Debris Flows and Woody Debris, andwe choose the smaller value of these two calculations as“expected sediment outflow volume.”

Lastly, we choose the greatest value of the “expectedsediment outflows” in these sections as Vdqp.

Figure 6 shows a cross sectional view of a debris flow.Originally we assumed that a cross-section of a debrisflow was a rectangle shape with the river bed width inthe former guideline. However we found that debris flowsmoved to downstream, becoming wider in gullies in the2002 debris flow disaster in Mie Prefecture, Japan. There-fore we assume that cross-section of a debris flow islike Fig. 6. (Flow discharge rates and depths of the de-bris flows are obtained from simultaneous equations) Thecross-mean velocity and flow depth corresponding to thepeak flow discharge are obtained by Eqs. (8)-(10):

U =1

KmD2/3

r (sinθ)1/2 . . . . . . . (8)

Qsp = UAd . . . . . . . . . . . . . (9)

Dd =Ad

Bda. . . . . . . . . . . . (10)

where, as in Fig. 6, Dr denotes hydraulic radius of debrisflow, and the value is approximated to the depth of debrisflow denoted as Dd ; θ , mountain stream bed gradient ; Kn,roughness coefficient; and Ad , flow sectional area of peakdebris flows; Bda, breadth of debris flow.

4. Main Points of Guidance on Design Tech-nology Guidelines for Countermeasuresagainst Debris Flows and Woody Debris

4.1. Openings of Open and Partially Open TypeDams

Mizuyama and Mizuno (1997) reported that one of thekey factors that affects whether or not a Sabo dam of theopen type and the partially open type was the dimension



Table 1. Dam openings.

Fig. 7. Openings.

of the openings shown in Fig. 7 and Table 1. Fig. 7 showsterminologies of the parts of Sabo dam of the open typeand the partially open type. The openings described inFig. 7 should be as broad as possible, as broad as the widthof the valley in principle, so that the functions of the opentype can be fully exploited and so that no backwater becreated on the upstream side of the Sabo dam before thearrival of debris flows.

4.2. Cross-Section of Opening of Open type andPartially Open Type Dams

Mizuyama and Mizuno (1997) reported that the Sabodam could capture debris flows if the (ration) ratio ofheight or width of an opening to a maximum size of boul-ders, that were transported by debris flows, was less thantwo. On the other hand, it is very difficult to estimatethe maximum boulder size in real cases. Therefore, in or-der to take consideration of uncertainty, the dimension ofopening sets to half of the size that Mizuyama and Mizunoreported as shown Table 1. In principle, the openingsshould have a cross sectional shape as indicated in Ta-ble 1 to ensure the capture of debris flows and to preventthe captured debris flows from escaping and continuingdownstream. However, the opening may have a shapeother than the one in Table 1 on the condition that theopening has the function of allowing normal sedimentsto flow downstream and that those sediments fully meetconditions (a) and (b) below.

(a) The opening shall be closed off, without fail, belowthe water level of debris flows by large rocks in thedebris flows, and this closure must hold during debrisflows.

(b) The opening shall be closed off, without fail, abovethe water level of debris flows by subsequent debrisflows, and this closure must hold during these subse-quent debris flows.

Table 2. Combination of design external forces for open-type excluding self weight.

4.3. Design of Spillway Section of Open and Par-tially Open Dams

The opening shall be designed so as to secure stabilitywhen conditions such as the design external forces in Ta-ble 2 are acting on the dam, in addition to the self-weightsof the Sabo dam in Fig. 8. The openings of the partiallyopen-type dams shall be designed so that stability is se-cured when conditions such as the design external forcesin Table 3 are acting in the way shown in Fig. 9, in addi-tion to self-weights of the Sabo dam.

However, the breadth of the top or crown of the dam inthe spillway section shall be designed so as to be able towithstand the impact of large rocks and woody debris and,in principle to be able to withstand the impact of boulderswith diameters twice or more the design maximum.

As for structural calculations, structural members shallbe designed so as to be safe against the design externalforces in Table 4. Sabo dams shall have a structure withas high a level of fail-safe redundancy as possible, so thatany partial breakage of the members will not lead to thecollapse of the entire structure.

Out of the members that are provided to block debrisflows, members that do not constitute structural membersthat maintain the shape of the structure may be allowedplastic deformation, because such members can be con-sidered as having served their purpose provided that theyhave successfully captured the sediments in the flows.And these members are called “functional members.”

4.4. Design of Wing Section in case of Open, Closed,and Partially Open Types

Designs for the non-overflow section shall be based onstability calculations in which the following design exter-nal forces act on the wing section in addition to the self-weights: for the open type, design external forces shownin Table 5; for the closed and partially-open types, designexternal forces of the overflow section of the closed type.

Figure 10 shows an example of how design externalforces act on a dam of less than 15 m in height. The wingsection shall meet all of the following four conditionsagainst the three design external forces of “self-weight ofthe wing,” “fluid forces of debris flows,” and “greater im-pact force between rock and woody debris.”



Fig. 8. Acting positions of design external forces for open-type.

Table 3. Combination of design external forces for partially open-type excluding self weights.

Fig. 9. Acting positions of design external forces for partially open-type: Dam heightless than 15 m: Upper figure for debris flows, lower figure for flooding.

Table 4. Combination of design external forces for structural calculations.



Table 5. Combination of design external forces for open-type excluding self-weights.

Fig. 10. Acting positions of design external forces for dams of less than 15 m in height:Upper figure for debris flows and lower figure for flooding.

(a) The angle of inclination of the wing on the upstreamside shall be in principle vertical.

(b) Slope inclination of the wing on the downstream sideshall be either vertical or identical to that of the mainbody on the downstream side.

(c) If the inclination of the wing on the downstream sideis made identical to that of the main body on thedownstream side, the breadth crown of the dam atthe wing shall be 1.5 m at the minimum.

(d) The safety factor for shear sliding against design ex-ternal forces shall not be less than four on the bound-ary surface between the wing and the main body.

After that, tensile stress is to be calculated for the casesin which the above-mentioned three design external forcesare acting. In case the calculations should indicate anytensile stress higher than that allowable, such tensile stressshall be undertaken by reinforcing bars or steel frames,which shall be so arranged as to pass the boundary surfacebetween the wing and the main body.

5. Conclusions

The revised technical guideline has still assumptions.For example, the control plan mentioned in 3.2 is estab-lished under an assumption that debris flow and woodydebris will be transported from the all gullies in the basin.However it is reported that traces of debris flows andwoody debris were found in not all gullies in a basin, sothat we are planning to find out the tendency and revisethe technical guideline in the future.

Recent Sabo facilities are increasingly being expectedto take into consideration not only disaster prevention butalso environmental or landscape issues, as practiced incomprehensive sediment control activities. Accordingly,more and more attention has come to be paid to openor partially open Sabo dams, which can efficiently con-trol sediment discharges and capture debris-flows/woody-debris, while not affecting the continuity of river-bed lev-els or water currents or unduly obstructing the passage ofanimals or fish. As a result, a great number of such damshave been constructed.



We have also come to recognize more and more thetrue benefits of such dams. Considering the current cir-cumstances, it is reasonably expected that an increasingnumber of sabo dams will continue to be aggressivelyplanned and constructed in the future, and we expect thiswill require us to seek further advancements in technicalstandards and guidelines. Accordingly, we intend to in-vestigate at sites what kinds of effects may be expectedof open and partially open dams, so that we may incorpo-rate accumulated or advanced knowledge into subsequentrevisions of the technical standards and guidelines.

References:[1] Erosion and Sediment Control Division, Research Center for Dis-

aster Risk Management, “Manual of Technical Standard for estab-lishing Sabo plan for debris flow and driftwood,” Technical Note ofNational Institute for Land and Infrastructure Management, No.364,2007 (in Japanese).

[2] Erosion and Sediment Control Division, Research Center for Disas-ter Risk Management, “Manual of Technical Standard for designingSabo facilities against debris flow and driftwood,” Technical Note ofNational Institute for Land and Infrastructure Management, No.365,2007 (in Japanese).

[3] T. Mizuyama and H. Mizuno, “Prediction of debris flow hydrographpassing through grid type control structure,” ASCE, Proceedingsof first international conference on debris-flow hazard mitigation,pp. 74-82, 1997.

More information can be obtained from following web sites;http://www.sabo-int.org/law/index.htmlhttp://www.sabo-int.org/guideline/index.html

Name:Nobutomo Osanai

Affiliation:Head, Erosion and Sediment Control Division,Research Center for Disaster Risk Management,National Institute for Land and InfrastructureManagement, Ministry of Land, Infrastructure,Transport and Tourism

Address:1-Asahi, Tsukuba City, Ibaraki 305-0804, JapanBrief Career:1999-2001 Shikoku Mountainous Region Sabo Work Office, Ministry ofLand, Infrastructure and Transport2001-2004 Erosion and Sediment Control Research Group, Public WorksResearch Institute2004- Erosion and Sediment Control Division, National Institute for Landand Infrastructure ManagementSelected Publications:• “Temporal change in the sediment discharge from the gigantic landslideoccurred on Mt. Bawakaraeng, Indonesia,” Jour. of the Japan LandslideSociety, Vol.45-2, pp. 95-105, 2008.• “Experiments on impulsive load of soil collapse to the wall : Effects ofdisplacement of wall on impulsive load,” Jour. of the Japan Society ofErosion Control Engineering, Vol.59, pp. 3-12, 2006.• “An experimental study on the way to induce riparian trees at channelworks,” Jour. of the Japan Society of Erosion Control Engineering, Vol.53,pp. 4-15, 2000.Academic Societies & Scientific Organizations:• Japan Society of Erosion Control Engineering (JSECE)• Japan Landslide Society• Japan Geoscience Union

Name:Hideaki Mizuno

Affiliation:Senior Researcher, Erosion and Sediment Con-trol Division, Research Center For Disaster RiskManagement, National Institute for Land and In-frastructure Management, Ministry of Land, In-frastructure, Transport and Tourism, Japan

Address:1-Asahi, Tsukuba City, Ibaraki 305-0804, JapanBrief Career:1995-1997 Ohsumi Work Office, Kyushu Regional Bureau, Ministry ofConstruction1997-2001 Erosion and Sediment Control Division, Public WorksResearch Institute, Ministry of Construction2001- Erosion and Sediment Control Division, National Institute for Landand Infrastructure ManagementSelected Publications:• “Temporal change in the sediment discharge from the gigantic landslideoccurred on Mt. Bawakaraeng, Indonesia,” Jour. of the Japan LandslideSociety, Vol.45-2, pp. 95-105, 2008.• “Experiments on impulsive load of soil collapse to the wall : Effects ofdisplacement of wall on impulsive load,” Jour. of the Japan Society ofErosion Control Engineering, Vol.59, pp. 3-12, 2006.• “An experimental study on the way to induce riparian trees at channelworks,” Jour. of the Japan Society of Erosion Control Engineering, Vol.53,pp. 4-15, 2000.• H. Mizuno, T. Mizuyama, N. Minami, and S. Kuraoka, “Analysis ofSimulating Debris Flow Captured by Permeable Type Dam using DistinctElement Method,” Journal of the Japan Society of Erosion ControlEngineering, Vol.52, No.6, p. 4-11, 2000.• H. Mizuno, N. Osanai, T. Shimizu, and T. Inamura, “Development ofSupport System for Establishment of Comprehensive SedimentManagement Plan,” Civil Engineering Journal Vol.49 No.7, pp.26-31, 2007.• H. Mizuno, “Sediment-Related Disaster Forecasting Warning System Pro-ject,” World Meteorological Organization, WMO/TD-No.1520, 49pp., 2010.

Academic Societies & Scientific Organizations:• Japan Society of Erosion Control Engineering (JSECE)

Name:Takahisa Mizuyama

Affiliation:Professor, Laboratory of Erosion Control, Divi-sion of Forest Science, Graduate School of Agri-culture, Kyoto University

Address:Oiwake-Cho, Kitashiraka, Sakyo-ku, Kyoto 606-8502, JapanBrief Career:1978-1990 Sabo Division, Public Works Research Institute, Ministry ofConstruction1981-1982 Visiting researcher, Colorado State University1990- Department of Agriculture, Kyoto UniversitySelected Publications:• “Structural countermeasures for debris flow disasters,” International Jour.of Erosion Control Eng., 1-2, pp. 38-43, 2008.• “Measurement of bedload with the use of hydrophone in mountaintorrents,” IAHS Publ., 283, pp. 222-227, 2003.• “Sediment control with slit sabo dams,” INTERPRAEVENT 2000,Villach vol.1, pp. 251-258, 2000.Academic Societies & Scientific Organizations:• Japan Society of Erosion Control Engineering (JSECE)• Japan Society for Civil Engineers (JSCE)• International Association for Hydro-Environment Engineering andResearch (IAHR)• International Association of Hydrological Sciences (IAHS)


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