天然壩破壞型式中以滲流引發之驟然破壞最具威脅性，潰壩流量對下游保全對象所造成之災害衝擊遠遠超過其他破壞型式，若能於天然壩形成初期有效且快速評估天然壩可能破壞型式，將能有助於天然壩防救災應變能力之提升。影響天然壩破壞型式主要與以下有關：(1)流量；(2)壩體組成材料；(3)壩體幾何形狀；(4)滲流臨界安定坡度。本研究透過46組天然壩室內實驗，分析各因子對破壞型式之影響性並予以量化，建立溢流與滲流破壞之區別函數，其正確率為95.5%，並進一步將滲流破壞以滲流臨界安定坡度理論區別漸進式與驟然破壞，以8處天然壩案例予以驗証，預測結果與實際結果十分符合。本研究建立之天然壩破壞型式判別方法，於天然壩形成初期配合天然壩所在之地形、壩體形狀與河川水理等資料，可有效且快速研判天然壩可能之破壞型式，供天然壩防救災相關單位之參考。

Study on Seepage Failure of Landslide-Dam
Chin-Min Chen
Chjeng-Lun Shieh
Department of Hydraulic and Ocean Engineering, NCKU
SUMMARY
Landslide dams may fail due to overtopping, progressive or sudden failure of seepage. The peak discharge of flood produced by sudden failure of seepage is higher than other types. For practical application, a rapid and efficient assessment method for failure type of landslide dam is needed. By review of previous study, we conclude the factors be considered for evaluation. First, factors of overtopping and seepage failure include (a)Discharge; (b)Material properties of landslide dam; (c)Geometry of landslide dam. Second, factors of progressive and sudden failure of seepage include The Critical Stable Seepage Slope.
To analyze and quantify the relation of each factor of different failure type of landslide dam, we carry out the 46 laboratory experiments. At first, discriminant function of overtopping and seepage failure is analyzed by experiment results. The correcting rate of discriminant function is 95.5%. Then, The Critical Stable Seepage Slope can evaluate seepage failure is progressive or sudden.The result have good agreement of 8 field site case.
The study quantify the effect of geometry and hydrology parameters, the stability and failure type can be evaluated by quick evaluation of landslide dam. It could be helpful for emergency action of countermeasure of landslide dam.
Key words: Landslide Dam, Failure Type, The Critical Stable Seepage Slope.
INTRODUCTION
Extreme earthquake and rainfall events may result in landslide dams. Landslide dams will block the river flow and form the lake. Landslide dams usually fail soon after the lake formation (Peng and Zhang, 2012), and cause downstream flooding. Failure type of landslide dam include overtopping, progressive and sudden failure of seepage. Although sudden failure of seepage is not common case, but the peak discharge is higher than other types.(Takahashi and Kuang, 1988; Awal et al., 2007).
The main objective of this study is to evaluate failure type of landslide dam quickly by a rapid and efficient assessment method. We conclude the factors be considered for evaluation in this study, then to quantify the effect of geometry and hydrology parameters by 46 laboratory experiment results.
Failure type of landslide dam can be evaluated after formation of landslide dam, it could be helpful for emergency action of countermeasure of landslide dam.
MATERIALS AND METHODS
By review of previous study, we conclude the factors be considered for evaluation. First, factors of overtopping and seepage failure include (a)Discharge; (b)Material properties of landslide dam; (c)Geometry of landslide dam. Second, factors of progressive and sudden failure of seepage include The Critical Stable Seepage Slope.
At first, discriminant function of overtopping and seepage failure is analyzed by 46 experiment results. The correcting rate of discriminant function is 95.5%. Then, The Critical Stable Seepage Slope can evaluate seepage failure is progressive or sudden.The result have good agreement of 8 field site case.
RESULTS AND DISCUSSION
In figure 1, X axis and Y axis are dimensionless parameters. Overtopping and seepage failure separated well by ( )( ), the factor was combined with discharge and landslide dam shape. Equation of discriminant function is , the correcting rate is 95.5%. And seepage failure can be evaluated by , when the factor increase gradually, progressive failure of seepage was transformd into sudden failure of seepage.
Figure 2 is a rapid assessment method of landslide dam failure type. Collect Geometry and Hydrology parameters of landslide dam, and calculate relevant factors as ( )( ), . Then the result of landslide dam failure type can be provided after formation of landslide dam.
CONCLUSION
The study quantify the effect of geometry and hydrology parameters in failure type by 46 laboratory experiments. And the discriminant function of overtopping and seepage failure is based on experiment result, the correcting rate is 95.5%. Then seepage failure is progressive or sudden can be evaluated by The Critical Stable Seepage Slope.
The failure type of landslide dam can be evaluated immediately by the rapid assesment method, then it could provide a plausible results after formation of landslide dam.

1.中華水土保持學會，水土保持手冊，2006。
2.行政院農委會林務局，國有林天然壩監測與防災通報系統建置計畫，成果報告，2011。
3.伍恆志，天然壩材料粒徑對壩體溢流破壞之影響，國立成功大學水利及海洋工程研究所碩士論文，2012。
4.林震岩，多變量分析SPSS的操作與應用，台北：智勝文化，2007。
5.林務局臺東林管處，龍泉溪上游天然壩土砂觀測規劃與建置，成果報告，2009。
6.林務局花蓮林區管理處，木瓜溪事業區19林班崩塌區及天然壩防災計畫，成果報告，2009。
7.林琮文，堰塞壩潰決型態對下游河床變化影響之探討，國立中興大學水土保持研究所碩士論文，2011。
8.林家興，謝正倫，曾志民，蔡元融，陳俞旭，天然壩潰壩水位模擬之研究－以小林村為例，中華防災學刊，第4卷，第1期，第5-15頁，2012。
9.林岱本，崩塌型堰塞湖形成條件之研究，國立成功大學水利及海洋工程研究所碩士論文，2013。
10.陳樹群，天然壩潰決機制與減災工法研究，中華水土保持學報，第30卷，第4期，第299-311頁，1999。
11.陳樹群、陳聯光，天然壩溯源潰決實驗分析，兩岸土石流與洪水災害防治研討會論文集，第315-324頁，2001。
12.陳天健、王香云、徐瑞麟、蘇群雅，台東知本溪天然壩，地工技術，第106期，第87-92頁，2005。
13. 陳天健、王束銘、陳樹群、蘇群雅、林潤榮，臺灣地區天然壩特性初步研究，臺灣公共工程學刊，第4卷，第2期，第1-8頁，2008。
14.陳樹群、吳俊鋐，莫拉克颱風引致小林村堰塞湖之形成與潰決歷程，中華水土保持學報，第40卷，第4期，第377-392頁，2009。
15.陳樹群、許中立，莫拉克颱風形成之堰塞湖及危險度評估，地工技術，第122期，第77-86頁，2009。
16.陳昆廷、臧運忠、郭玉樹、呂明鴻、謝正倫，莫拉克颱風引致天然壩之案例分析，中華防災學刊，第2卷，第1期，第43-50頁，2010。
17.許強，裴向軍，黃潤秋等，汶川地震大型滑坡研究，科學出版社，2009。
18.經濟部水利署水利規劃試驗所，天然壩引致災害防治對策之研究(1/3)，第一年研究成果報告，2002。
19.臧運忠，天然壩快速安全評估方法之研究，國立成功大學水利及海洋工程研究所博士論文，2013。
20.劉格非、江宏晟，土體破壞過程之實驗觀察，中華民國力學學會第三十五屆全國力學會議，2011。
21.賴柏蓉，影響天然壩天然壩壽命之因子探討，國立中央大學應用地質研究所碩士論文，2013。
22.簡桐煙，立霧溪托博濶天然壩的處理經過，台電月刊，第359期，第56-66頁，1978。
23.田畑茂清、水山高久、井上公夫，天然ダムと災害，古今書院，第187-196頁，2002。(日文)
24.建設省，災害情報システムの開発報告書，第Ⅲ巻，基幹施設編，p.353-404，1992。(日文)
25.高橋保、匡尚富，天然ダムの決壊による土石流の規模に関する研究，京都大學防災研究所年報，第31 B-2期，第601-615頁，1988。(日文)
26.Alden, W. C.,“Landslide and flood at Gros Ventre, Wyoming“, American Institute of Mining and Metallurgical Engineering Technical Publication Transaction, pp. 347-361, 1928
27.Awal. R., Nakagawa. H., Baba. Y., Sharma. R. H.“Numerical and experimental study on landslide dam failure by sliding.”Annual Journal of Hydraulic Engineering, JSCE, Vol.51, pp.7-12,2007.
28.Awal. R., Nakagawa. H, Fujita. M, Kawaike. K., Baba. Y. Zhang. H. “Study on Piping Failure of Natural Dam.”Annuals of Disas. Prev. Inst., Kyoto Univ., No.54 B,2011.
29.Budhu, M. and Gobin, R.. “Slope Instability From Ground-Water Seepage.” Journal of Hydraulic Engineering, ASCE, Vol. 122, No. 7, pp. 415-417, 1996.
30.Casagli, N. and Ermini,L.. “Geomorphic analysis of landslide dams in the Northern Apennine.” Japanese Geomorphological Union, Vol. 20, No. 3, pp. 219-249, 1999.
31.Ermini, L.and Casagli, N..“Prediction of the behaviour of landslide dams using a geomorphological dimensionless index.” Earth Surface Processes and Landforms , 28 ,pp.31-47, 2003.
32.Chapuis, R. P. “Predicting the saturated hydraulic conductivity of sand and gravel using effective diameter and void ratio.” Canadian Geotechnical Journal, Vol.41, Issue.5,pp 787-795, 2004.
33.Chapuis, R. P. “Predicting the saturated hydraulic conductivity of soils: a review.” Bulletin of Engineering Geology and the Environment, Vol.71, Issue.3, pp 401-434, 2012.
34.Chen, K. T., Kuo, Y. S., Shieh, C. L.. “Rapid Geometry Analysis for Earthquake-induced and Rainfall-induced Landslide Dams in Taiwan.” Journal of Mountain Science, Vol.11, Issue.2, pp. 360-370, 2014.
35.Costa, J. E. and Schuster, R. L., “The formation and failure of natural dams”, Geological Society of America Bulletin, Vol. 100, pp.1054-1068, 1988.
36.Dong, J. J., Tung, Y. H., Chen, C. C., Liao, J. J. and Pan, Y. W.. “Discriminant analysis of the geomorphic characteristics and stability of landslide dams.” Geomorphology,Vol. 110, pp. 162-171, 2009.
37.Dong, J. J., Tung, Y. H., Chen, C. C., Liao, J. J. and Pan, Y. W.. “Logistic regression model for predicting the failure probability of a landslide dam.” Engineering Geology,Vol. 117, pp. 52-61, 2011.
38.Geertsema, M., and Clague, J.J., “1,000-year record of landslide dams at Halden Creek, northeastern British Columbia”, Landslides, Vol. 3, pp. 217–227, 2006.
39.Gregoretti, C., Maltauro, A. and Lanzoni, S.. “Laboratory Experiments on the Failure of CoarseHomogeneous Sediment Natural Dams on a Sloping Bed.”Journal of Hydraulic Engineering, Vol. 136, No. 11, pp. 868-879, 2010.
40.Harr, M. E. “Groundwater and seepage.”McGraw-HillBook Co., Inc., New York, N.Y., 1962.
41.Hossein,G., Soheil, G.“Stability of sandy slopes under seepage conditions.”Landslides, 5, pp. 397–406,2008.
42.Iverson, R. M., Major J. J.“Groundwater seepage vectorsand the potential for hillside failure and debris flow mobilization.”Water Resources Research, Vol.22, No11, pp.1543-1548, 1986.
43.Kuo, Y. S., Tsang, Y. C., Chen, K. T. and Shieh, C. L.. “Analysis of landslide dam geometries.” Journal of Mountain Science, Vol. 8, No. 4, pp. 544-550, 2011.
44.Liu, N., Zhang, J. X., Lin, W., Cheng, W. Y., Chen, Z. Y.. “Draining Tangjiashan Barrier Lake after Wenchuan Earthquake and the flood propagation after the dam break. ” Science in China Series E: Technological Sciences, Vol. 52, Issue 4, pp.801-809.,2009.
45.Meyer, W., Schuster, R. L. and Sabol, M. A.. “Potential for seepage erosion of landslide dam.”Journal of Geotechnical Engineering, Vol. 120, No. 7, pp. 1211-1228, 1994.
46.Peng, M. and Zhang, L. M.. “Breaching parameters of landslide dams.” Landslides, Vol. 9, pp. 13-31, 2012.
47.Xu, Q., Fan, X. M., Huang, R. Q.. “Landslide dams triggered by the Wenchuan Earthquake, Sichua Province, south west China.” Bulletin of Engineering Geology and the Environment, Vol.68 ,pp 373-386, 2009.
48.Zhan, L.-J., Qin, C.-Z. and Zhu, A.-X. 2012. “Which type of slope gradient should be used to determine flow-partition proportion in multiple-flow-direction algorithms–tangent or sine?” Hydrology and Earth Syst. Sci. Discuss., 9, 6409–6418,2012.