A method for delineating restricted hazard areas due to debris flows

Transcription

1 A method for delneatng restrcted hazard areas due to debrs flows K. Takanash Sabo Fronter Foundaton, Tokyo, Japan T. Mzuyama Kyoto Unversty, Kyoto, Japan Y. Nakano Mnstry of Land Infrastructure and Transport, Tokyo, Japan Keywords: debrs flow, soft countermeasures, three-dmensonal numercal map ABSTRACT: To prevent debrs-flow dsasters n Japan, structural and non-structural measures for debrs-flow dsasters have been promoted effcently. In Aprl, 2001, Japan enacted a Sedment- Related Dsaster Preventon Law to protect lves and bodes from landsldes and debrs flows. The purpose of the law s to promote measures for preventng sedment-related dsasters, arrange warnng and evacuaton systems n hazardous areas, and regulate specfc types of development wthn areas lkely to be susceptble to landsldes and debrs flows. At the end of 2006, around 9300 restrcted areas at rsk from debrs flows were desgnated under the law. In ths report, we state the dea of delneatng hazardous areas through verfcaton of past debrs-flow events. We propose methods of dentfyng areas subject to debrs-flow nundaton and areas debrs flow wll not reach. Usng our methods, we dentfy areas where warnng and evacuaton systems should be establshed. We also dentfy areas where development should be restrcted by consderng the range of forces lkely to be exerted on buldngs wthn the nundaton zones and comparng those forces to the strengths of the affected buldngs (wooden structure). 1 INTRODUCTION In Japan 70% of the land conssts of mountan ranges and hlls, and n the remanng narrow lvable space the populaton exceeds one hundred mllon nhabtants. Populaton concentratons n urban areas contnue to ncrease, and many people resde n dangerous areas. Accordng to the results of natonwde nvestgatons n March 2003, the number of sedment-related hazard areas that are prone to the falure of land wth steep slopes, debrs flows or landsldes s as many as 525,307. In 2006, sedment-related dsasters occurred n both Kyushu and Honshu Dstrcts where debrs flows and slope falures ht a great number of houses and caused 24 fataltes. Under the crcumstances, the Japanese Government enforced n 2001 a law to promote soft countermeasures, such as the establshment of warnng and evacuaton systems (Sedment-related Dsaster Preventon Law), to reduce loss of lves. In ths law, Law (2001), dangerous land such as the foot of a clff, the downstream basn of a torrent, and areas n landslde terran can be desgnated as sedment-related hazard areas. Such desgnatons enact establshment of warnng and evacuaton systems, and at the same tme enable regulatons that restrct buldng usage and lmt development. Establshng such hazard zones requres accuracy and effcency. It also requres dentfyng the flud dynamc forces exerted by a debrs flow on structures that are nundated and an estmaton of the amount of sedment lkely to be delvered. Such computatons typcally are complcated and requre a lot of tme and effort.

2 In ths regard, we have been workng on development and mplementaton of a hazard-zonng system whereby the process of dentfyng and delneatng sedment-related hazard areas can be made accurately and effcently, Takanash et al. (2004). Ths system utlzes state-of-the-art GIS (Geographc Informaton System) technologes by means of three-dmensonal topographc nformaton and ortho-photos. 2 ADVANTAGES OF THE HAZARD-ZONING SYSTEM 2.1 Increased effcency and reduced cost As stated above, a huge amount of tme and cost s requred to dentfy sedment-related hazard areas. Hence, the hazard-zonng system we developed enabled the tme for work to be shortened and the cost to be reduced, by ncreasng the effcency of work and automatng the complcated work that requres tme and effort. At the same tme t allows for the vsualzaton of topographcal and geologcal condtons to provde support for the judgment made by techncal personnel. 2.2 Improved objectvty, accuracy, and reproducblty Once sedment-related hazard areas have been specfed, the law enacts regulatons on structures and lmtatons on development. Therefore, t s mportant to accurately, objectvely, and reproducbly desgnate such zones. Hence, upon dentfyng hazardous areas subject to restrctons, a mechansm has been devsed by whch the accuracy, objectvty, and reproducblty of such desgnatons can be secured. 3 DEVELOPMENT OF A SYSTEM FOR DELINEATING DEBRIS-FLOW DISASTER HAZARD ZONES The system for establshng sedment-related hazard areas, etc. was developed n terms of three phenomena: falures of land wth steep slopes, debrs flows, and landsldes. Of these phenomena, ths paper dscusses development of a method for delneatng debrs-flow hazard zones, a method that requred the most complcated and hgh-grade technologes. 3.1 Spatal nformaton The followng three types of spatal nformaton are requred for delneatng hazard zones: 1) Dgtal maps.

3 2) Three-dmensonal topographcal models. TIN (Trangulated Irregular Network) and DEMs (dgtal elevaton models). 3) Orthophoto and three-dmensonal vew. 3.2 Establshng the course of debrs flow and the startng pont of nundaton The course of a debrs flow s ndcated on a screen by desgnatng a pont 200 m upstream of the startng pont of nundaton that s estmated on a map and n an ortho-photo, whereupon the system automatcally searches the route of flow from a three-dmensonal topographcal model. By adoptng ths technque, the dfference n the result of route settng accordng to the person who made the settng was elmnated, thus securng reproducblty. Regardng overflow n the bend of a channel, the system has been made to be able to handle t by ncorporatng a tool that enables the set watercourse to be corrected and changed. Snce the settng of the startng pont of nundaton needs to be determned by makng comprehensve judgment based on the topographcal stuaton of the whole basn as well as local condtons (such as houses and local topography), a functon has been developed n whch the settng of the startng pont of nundaton s made by dsplayng a longtudnal profle and plan of the flow course and desgnatng a threshold gradent on the longtudnal profle. Fg. 1 shows a longtudnal profle of a stream course n whch each gradent secton s ndcated by a dfferent color. (In the fgure, the vertcal dotted lne marked wth 0 denotes the startng pont of nundaton.)

4 Fgure 1. A longtudnal profle of a stream course. 3.3 Desgnatng hazard-zone lmts A "zone wth danger of harm" (Yellow Zone) means the land s recognzed as posng a rsk of causng harm to lfe and property n the event of a debrs flow. Such a zone s defned as those areas downstream of the dentfed nundaton startng pont that have a gradent of 2 or more and a catchment area of 5 km 2 or less. In ths system, a mechansm has been made n whch, as shown n Fg. 2, by desgnatng the ponts on both sdes of the channel havng a relatve cross-secton heght of 10 m above the nundaton startng pont, the zones of whch gradent n the lower reach s 2 or more are automatcally delneated. In the algorthm, the steepest gradent vector s drawn from the startng pont of calculaton toward the drecton of a crcle of 40 m (horzontal dstance) n radus. Next, a 30 open angle vector s drawn, whch s 30 open toward the outer angle from the startng pont of ths steepest gradent vector. If the dfference n relatve heght between the ground heght ponted by the steepest gradent vector and the ground heght ponted by the 30 open angle vector s 5 m or more, the angle of the 30 open vector wll be reduced so that the dfference n relatve heght wth the outer angle wll become 5 m. Ths calculaton s repeated untl the gradent of the land wth the steepest gradent vector becomes less than 2. Note that the steepest gradent vector s bascally drawn wth the radus beng set to 40 m, but t s possble to use a radus of 20 m or 60 m as well dependng on the topography.

5 Fgure 2. Method of settng a land wth danger of harm 3.4 Delneatng a "zone wth a consderable danger of harm " A "zone wth a consderable danger of harm" (Red Zone) s a zone of land n whch the magntude of a force that s estmated to be appled to a buldng as a result of a debrs flow surpasses the force that an ordnary buldng (wooden structure) can wthstand wthout sustanng damage. In ths zone there s a hgh probablty for consderable loss of lfe. Steps for desgnatng such a zone nclude: 1) Determnng the cross-sectonal shape and longtudnal gradent perpendcular to the flow drecton of a debrs flow. The measurement lnes are set at 20 m ntervals from the startng pont of nundaton to establsh closely spaced channel-area cross sectons (Fg. 3). The longtudnal gradent at each cross secton represents the average gradent n a secton 200 m upstream and downstream of the cross secton. Fgure 3. Cross-sectonal vew (of a part 100 m downstream of the startng pont of nundaton).

6 2) For each cross secton, the peak flow rate of a debrs flow s calculated by usng Eq. 1. Eq. 1 s an expresson of the relaton between the volume sedment concentraton of a debrs flow and ts peak flow rate, where Qsp s the peak flow rate at an arbtrary calculaton pont, Cd s the volume sedment concentraton of the movng debrs flow, θ s the channel gradent, Qsp 0 s the peak flow rate of the debrs flow at the nundaton startng pont, Cd 0 s the volume sedment concentraton of the movng debrs flow at that pont, V 0 s the sedment dscharge, and C * s the volume concentraton of deposted debrs. Qsp = ( C *-Cd 0 ) ( C * - Cd ) Qsp 0, Qsp 0 = 0.01 C * Cd V 0 (1) 0 Computatons of Cd and Cd 0 are gven by Eq. 2. If the calculated values, Cd and Cd 0, are greater than 0.9C *, then they wll be set to be 0.9C *, but the lower lmt wll not be set. However, f the gradent of the land at each calculaton pont forms an adverse slope (θ > θ -1 ), then Cd wll be set to Cd =Cd -1 on condton that the amount of debrs wll not ncrease downstream. ρtanθ Cd = (σ- ρ)(tanφ-tanθ ) (2) In Eq. 2, each of σ, ρ, φ, and θ represent the followng parameters: σ = densty of pebbles contaned n the debrs flow, t/m 3 ; ρ = densty of flowng water contaned n the debrs flow, t/m 3 ; φ=nternal frcton angle of debrs, degrees; and θ=surface slope, degrees. 3) The wdth of the nundaton zone s set by usng Eq. 3, a type of Mannng's formula, f the cross-secton can be clearly delneated, or otherwse by usng Eq. 4: U = Q A = 1 R I n b where U =average cross-sectonal velocty, m/s; Q = dscharge, m 3 /s; A = cross-sectonal area of the flow, m 2 ; n= roughness coeffcent; R = hydraulc radus, R=A/S (S s the length of the wetted permeter), m; I b = longtudnal gradent of the waterway, degrees. (3) B β sp = α Q (4) where B =flow wdth, m; α and β are coeffcents (set to α =4.0; β =0.5). On the bass of the prevous dsaster nvestgaton date ganed n the feld, values of α and β are set from the flow wdth that roughly contans a house that was completely destroyed. The heghts of the debrs flow at the startng pont of nundaton and at each of the cross sectons are calculated wth Eq. 5 by means of the flow wdth, the peak flow rate of the debrs flow, and the gradent of the land. h n Qsp = B 0.5 ( snθ ) 3 5 4) Delneatng a "zone wth a consderable danger of harm" s done by obtanng a force (F d, kn/m 2 ) that s estmated to be appled to a buldng by the debrs flow at the startng pont of nundaton and each calculaton pont by usng Eq. 6, calculatng the proof stress of the buldng at ordnary tmes (P, kn/m 2 ) by usng Eq. 7, and by comparng these two values. F = ρ U (6) 2 d d (5)

7 P = 35.3 H (5.6 H ) (7) where H = heght of debrs flow, m. If the condton of F d > P s satsfed at the startng pont of nundaton, the zone from ths pont up to the calculaton pont at whch the condton of F d P has been met for the frst tme wll be set as the "zone wth a consderable danger of harm". Note that ths system employs a method n whch the end of the "zone wth a consderable danger of harm" s obtaned by means of repeated calculatons carred out for each of the sectons of 1 m. It has been decded to clearly dstngush those areas where the heght of the debrs flow exceeds 1 m and the force estmated to be appled to a buldng exceeds 50 kn/m 2 from those areas where the heght of the debrs flow exceeds 1 m but the force estmated to be appled to a buldng s less than 50 kn/m 2. Fg. 4 shows the results of delneatng "zones wth danger of harm" (Yellow Zone) and "zones wth a consderable danger of harm" (Red Zone). In ths system, vsual verfcaton has been made easy by dsplayng the result of hazard-zone desgnatons on a dgtal elevaton model (Fg. 5). Yellow Zone Red Zone Fgure 4. Result of settng zones by means of the system.

8 Yellow Zone Yellow Zone Red Zone Red Zone Fgure 5. Hazard zones depcted on a dgtal elevaton model. 4 OPERATION OF THE SYSTEM FOR DELINEATING DEBRIS-FLOW HAZARD AREAS Snce the system for delneatng sedment-related hazard areas stores nformaton on each zone as GIS data t can be used not only for sedment-related hazard areas but also for other work as well, such as cty plannng and depcton of zones subject to regulatons by other laws related to dsaster preventon and land use. Also, snce a mechansm has been developed to store n a database the three-dmensonal topographcal nformaton that have been used for calculaton, varous types of parameters, and the results of hazard desgnatons, those data can be referred to at any tme and n the XML format. Therefore, even when they have been made avalable to the publc, they can be fully utlzed as explanatory materals when makng a response to resdents and clamants to the land. As an evaluaton of the operaton of the zone-delneaton system, a comparson of the accuracy of work and workng tme has been made wth the case n whch the same work has been done manually. Comparson shows that common mstakes made manually have been elmnated and the workng tme has been shortened by 90 percent. Also, snce ths system operates on a standard personal computer a large number of hazards assessment can be performed wth accuracy and effcency. 5 CONCLUSIONS In ths study we have descrbed a methodology for delneatng varous debrs-flow hazard zones effcently and objectvely. Our methodology makes use of three-dmensonal topographcal nformaton and GIS technologes. An evaluaton of the system has verfed that t s more effcent, objectve, and reproducble than manually determnng hazard zones. However, a hgh degree of accuracy n zonng s demanded for nhabtants n debrs flow hazard zones. It s very mportant contnuous on-the-spot nspectons n the actual debrs flow dsaster areas. Dependng on the flow-pattern of debrs flows, the startng pont of nundaton must be changed. We must make every effort to get many data n the actual dsaster areas.

9 ACKNOWLEDGEMENTS Specal thanks go to Sabo Fronter Foundaton staff members for ther expermental support. REFERENCES Law Law related to Promoton of Measures for Sedment-related Dsaster Preventon n a Restrcted Area etc. Due to Sedment- related Dsaster (8 May 2000, Law No. 57) Takanash, K., Shmoozak, Y., Suzuk, A. & Mzuyama, T Consderaton on delneatng restrcted areas due to debrs flows. Proceedngs JSECE Workshop. Myazak, Japan, May 18-20, 2004: