APPLICATION TO PAST DISASTERS OF A METHOD OF SETTING THE RANGE OF DEBRIS FLOW DAMAGE TO HOUSES

Transcription

1 APPLICATION TO PAST DISASTERS OF A METHOD OF SETTING THE RANGE OF DEBRIS FLOW DAMAGE TO HOUSES Hideaki MIZUNO Senior Researcher Erosion and Sediment Control National Institute for Land and Infrastructure Ministry of Land, Infrastructure and Transport, JAPAN

2 Today s topic! Background! Objectives! Method! Results of application! Conclusion

3 Sediment-related disasters Slope failure Landslide Debris flow Slope failure Landslide

4 Recent occurrence of sedimentrelated disasters Numbers of occurrence 2,000 1,750 1,500 1,250 1, Slope failure Landslide Debris flow Year

5 Physical and Non-physical measure Measure for preventing slope failure disaster Grating crab works Retaining wall Check dam Measure for preventing debris flow disasters Drainage tunnel work Siren Measure for preventing landslide disasters Measures with constructs are called physical measure. Measures without constructs are called non-physical measures.

6 Number of hazardous area! Numbers of hazardous area are increasing. 300, , Previous This time Numbers 200, , ,908 79,318 86, ,04211, Debris flow Slope failure Landslide

7 Number of houses in hazardous area! Numbers of houses in the hazardous area are also increasing. 2,000,000 1,500,000 1,611, ,295,210 1,262,792 1,776, Previous This time Houses 1,000, , , , Debris flow Slope failure Landslide

8 Establishment a new law! The number of hazardous area is increasing.! A lot of time and money will be needed to make the hazardous area safety with physical measures. To inform the risk of the hazardous area To install warning and evacuation systems To restrict new land developments for housing in the hazardous area To promote relocations of houses existing in the hazardous area! The Sediment-related Disaster Prevention Law was established in 2001.

9 Hazardous areas in the new law This study focuses on the sediment-related disasters due to debris flow. Special hazard area Torrent Hazard zone Area vulnerable to sediment disasters Land gradient of two degrees or more. Crest of alluvial fan Hazard area Special hazard zone Among hazard areas, special hazard areas are designated where is a fear of damages to buildings and serious human injuries.

10 Special hazard area! Special hazard area is the range of the land where the hydrodynamic force of a debris flow exceeds the strength of its buildings A house was here. Debris flow Photo Damaged house in Kamaishi 2002

11 Method for setting a special hazard area (1) Field survey (2) Set a reference point (3) Estimate sediment volume deposited on a torrent (4) Estimate the flow route (5) Set calculation points on the flow route (6) Estimate the hydrodynamic force and the strength of the buildings at the calculation points (7) Compare the hydrodynamic force (Fd) and the strength of the buildings (P) (8) Draw the range where Fd>P (1) Field survey (3) Volume of deposition (2) A reference point (8) Special hazard area (5) Calculation points (6) Estimate F and P (4) A flow route

12 Estimation of the hydrodynamic force and the strength of houses! Hydrodynamic force F d = ρ U 2 d U :flow speed of the debris flow (m/s)! Strength of houses P = h h ( ) h :flow depth of the debris flow (m) In order to estimate the hydrodynamic force and the strength of house, we have to estimate the velocity (U) and depth (h) of debris flow. However a method for estimation is not defined

13 Objects of this study! To develop a simplified method for estimating velocity and depth! To apply the method to the past disasters and verify the accuracy

14 Method for estimating velocity and depth of debris flow! Combining five equations Continuity equation QSP = BUh Momentum equation 1 U = h n 2 3 ( sinθ ) 1 2 Empirical formula for discharge 0.01VC* QSP = Cd C* Cd 0 C V = C C C * d d d 0 V 0 U n H θ Q sp B C * : h Formula for sediment volumetric concentration C d = ρ tanθ ( σ ρ)( tanφ tanθ ) :flow speed of the debris flow (m/s) :roughness coefficient :flow depth of the debris flow (m) : gradient of slope of the torrent bed (degrees) :peak flow volume of the debris flow (m 3 /s) :flow width of the debris flow (m) sediment concentration of deposition on the torrent bed B (Width at the surface of debris flow) A Surface of debris flow River bed Legend Flow section used to calculate the height of the debris flow (approximately rectangular) Flow section used to calculate the area of the flow section The shape of the flow section resembles a rectangle such as the one enclosed by the dotted lines.

15 Estimation of the flow width! If B is calculated by the above method, there are presumed to be cases where, on an alluvial fan, the value of B is identical to the full width of the alluvial fan or cases where it cannot be set.! So B is set as the upper limit value (B MAX ). Bmax [m] B = Q max 4 SP Q SP [m 3 /s] Fig. Relationship between peak discharge and maximum flow width

16 Application to past debris flow disasters! Two new indices are designated for use in evaluating the results of the application - the damaged house inclusion percentage and the undamaged house percentage. Damaged house inclusion percentage =6/8 =0.75 Undamaged house percentage =4/(6+4) =0.40 Debris flow spreading area Range of debris flow damage to house Damaged house 8 6 Undamaged house 4 4

17 Examples of the applications LEGEND House damage zone House without damage Damaged house Cross section for calculation Debris flow deposition Debris flow deposition Debris flow deposition 0 500m 0 500m 0 500m (a) 10m (b) 100m (c) 200m The damaged house inclusion percentage was 33% (10 m case) and 100% (100m case and 200 m case). Because there were two undamaged houses (10m case) and one undamaged house (100m case and 200m case), the undamaged house percentage was 66.7% (10 m case) and 25% (100m case and 200m case)

18 Results of the application Horizontal distance used to calculate gradient of the land (m) 10m 100m 200m Damaged House Inclusion Percentage Total completely or partly destroyed Completely destroyed 1/2 Breakdown Pertinent number 1 All 2 3/4 Breakdown Pertinent number 3 All 4 5/ Breakdown Pertinent number All 6 Partly destroyed Undamaged house percentage Table. Damaged House Inclusion Percentage and Undamaged House Percentage

19 Conclusion! Maximum value of debris flow width is estimated by Bmax = 4 QSP.! About 70% of the damaged houses in the past debris flow disasters were included in the range of debris flow damaged to houses set by the method.! Because the undamaged house percentage is high, ranging from 70% to 80%, a method of studying a way of lowering the undamaged home percentage should be studied.