A Numerical Method for Determine the Dredging Requirements for Channel Restoration Using Alishan Creek in Central Taiwan as an Example

Transcription

1 A Numerical Method for Determine the Dredging Requirements for Channel Restoration Using Alishan Creek in Central Taiwan as an Example Instructors : Dr. Jie-Dar Cheng Dr. Honglay Chen Dr. Chao-Yuan Lin Dr. Der-Guey Lin Speaker : Ming-Po Tsai Department of Soil and Water Conservation National Chung-Hsing University, Taichung, Taiwan

2 Outline 1. Introduction 2. Literature Review 3. Methods (a) Study Area (b) Study Flow Chart 4. Results and Discussions 5. Conclusions

3 Introduction In early August 2009, Typhoon Morakot hit Taiwan with extremely heavy rainfalls and triggered many landslides, debris flows and floods, resulting in more than 634 fatalities and 76 people missing. Many tributaries lost their channel capacities due to heavy accumulation of sediments and various sizes of rocks. It is desirable to have a method that will be able to determine the required dredging volumes to allow the channels to have hydraulically function capacities for different return periods. In this study, a numerical method is applied to achieve this objective for the Alishan Creek watershed in central Taiwan.

4 Literature Review Many researchers developed numerical models to simulate debris flows under different situations. Steady flow solutions with a yield stress were studied by Johnson[1]., Coussot and Proust [3]. studied roll waves for mudflows. Iverson et al. [4] used a two-phase flow model to simulate debris flows flowing from a large-scale flume to a wide deposition basin. O Brien and Julien [5] used the Julien and Lan [2] model to simulate hyperconcentrated sediment flow and developed the commercial software FLO-2D. Liu and Huang [6] also use the generalized Julien and Lan rheological model to simulate debris flows for field application and developed the program DEBRIS-2D.

5 Literature Review Rickenmann et al. [7] comparison of 2D debris-flow simulation models: DFEM (Debris flow Finite Element Model), HB (Herschel-Bulkley Model), and FLO-2D All three models are capable of reasonably reproducing the depositional pattern on the alluvial fan after the models have been calibrated using historical data from the torrent. Because simulation models often require calibration, a major drawback in view of engineering applications is that most of these models have not been rigorously tested against field events. Accurate representation of the channel and fan topography is especially important to achieve a good replication of the observed deposition pattern.

6 Background of Alishan Creek C h I n g s h u i Taipei Taiwan Location of Alishan Creek Watershed R I v e r A l I s h a n Shigupan Creek C r e e k Landslide Area (ha ) (yrs) 年 89 年 92 年 95 年 96 年 99 年時間 Historical landslide area. (source:swcb) Summary of historical disasters Event type Typhoon Herb ( ) 921 Earthquake ( ) Typhoon Toraji ( ) Typhoon Nari ( ) Typhoon Mindulle ( ) Typhoon Bilis ( ) Typhoon Sepat ( ) Typhoon Morakot ( ) Damages Destruction of primarily roads and agriculture Caused Tsaoling landslide dam Destruction of Laji Bridge Lajida Bridge and Samlong Bridge. Destruction of primarily roads and 400 people trapped Destruction of primarily roads and 412 people trapped Destruction of No.149 country road Destruction of No.Chia155 township road 4 people died and more than 500 people trapped. Destruction of 46 houses 11 Bridge primarily roads, No.149 No.162 and No.169 country road,

7 Accumulated Rainfall (mm )Rainfall Intensity (mm )Hourly Rainfalls during Typhoon Morakot Rainfall Intensity>120mm/hr Accumulated Rainfall > 3000 mm Return period > 200 yrs 8/7 8/8 8/9 8/10 8/11 Time (hr) August 7 - August 11, 2009.

8 1,050 Channel Deposition Estimation 0 1, , ,000 Bed Elevation (m) Volume of sediment deposition caused by Typhoon Morakot Bed elevation after Typhoon Morakot (2009) Bed elevation before Typhoon Morakot (2007) Sushin Bridge Total Channel Deposition Volume 13,000,000 m 3 Laigida Bridge Laigi Bridge Width of Alishan Creek after Typhoon Morakot 900,000 Deposition Volume (m 3 ) 600 Width of Alishan Creek before Typhoon Morakot Channel Width (m) Horizontal Distance (km)

9 Overview of Alishan Creek after Typhoon Morakot Laigi Village Neighborhood. 1 and 2 Laigi Bridge AlIshan Creek Laigi Village Neighborhood. 3 C h I n g s h u i R I v e r A l I s h a n Laigi Village Neighborhood. 4 Laigida Bridge AlIshan Creek C r e e k Air-photo of Alishan Creek, Chiyi, Taiwan, Aug

10 How to design the required dredging volumes to allow the channels to have hydraulically function capacities? Laigi Village Neighborhood. 1 and 2 Laigi Bridge Laigi Bridge ( ) Laigi Bridge ( ) C h I n g s h u i R I v e r Laigi Village Neighborhood. 4 C r e e k A l I s h a n AlIshan Creek Air-photo of Alishan Creek, Chiyi, Taiwan, Aug

11 Study Flow Chart DEBRIS-2D Input Data Simulation suitable for debris flows with granular material. All inputs are determined independently. Application to field case with error less than 5% Rheological data (yield stress) Topographical data Debris sources (volume) Rainfall pattern Mitigation Countermeasures is suggestion. Conclusion

12 Numerical Simulation Program Visco-Plastic Plastic-Collision model Julien and Lan (1991): yield τ = τ 0 + viscous collision du du μ d + μ c ( dz dz du = 0 dz ) 2 τ τ < τ τ 0 0 Strong Shear Weak shear (Plug flow) Intuitively, yield stress, viscosity and collision effects are all included.

13 Governing Equations Weak Shear Plug Flow B.L << Plug y Strong Shear Boundary layer

14 Results and Discussions With field samples being full of granular material, we use a rotating drum to measure. Field samples usually have large boulders. In order to use the present equipment, we removed the largest boulders and stones, then collected 1m 3 sample from the field.

15 Delineating the influence area Return Period 50 years Return Period 200 years Return Period 50 years After Channel Dreding

16 Structural Works Aimed at Debris-flow Mitigation dredging Banking Terrace A l I s h a n C r e e k 嘉 155 Laigi Village Neighborhood. 1&2 Laigi Village, Chiyi, Taiwan, Aug

17 Conclusions In this study, a numerical method is applied to achieve this objective for the Alishan Creek. The application of this numerical method to design provided satisfactory results that passed the test of heavy rainstorms brought by typhoons. However, structural works aren`t entirely reliable, using warning and evacuation system will make the mitigation economically and technically sustainable.

18 References [1] A.M. Johnson, Physical Processes in Geology (Freeman, New York, 1970). [2] P.Y. Julien and Y. Lan, Rheology of hyperconcentrations, J. Hydraul. Eng. ASCE 117 (1991) 346Y353. [3] P. Coussot and S. Proust, Slow, unconfined spreading of a mud flow. J. Geophys. Res. 101(B11) (1996) 25217Y [4] R.M. Iverson, T.J. Denlinger, R.G. LaHusen and M. Logan, Two-phase debris-flow across 3-D terrain: Model prediction and experimental tests, in: Proceedings of the 2nd International Conference on Debris Flow Hazards Mitigation, Taipei, Taiwan, Aug. 16Y18 (2000), pp [5] J.S. O Brien, and P.Y. Julien, On the importance of mudflow routing, in: Proceedings of the 2nd International Conference on Debris Flow Hazards Mitigation, Taipei, Taiwan, Aug (2000) [6] Ko-Fei Liu and Ming Chung Huang, Numerical simulation of debris flow with application on hazard area mapping, Computational Geosciences (2006) 10: [7] D. Rickenmanna,b, D. Laiglec, B. W. McArdella and J. Hubl, Comparison of 2D debris-flow simulation models with field events, Computational Geosciences (2006) 10: [8] G. Lorenzini and N.Mazza (2004) Debris Flow Phenomenology and Rheological Modelling WIT Press.

19 Thank You For Listening