River Embankment Failure due to Overtopping - In Case of Non-cohesive Sediment -

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Bernard Cameron
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Nov 4, 2014 2014 International Workshop on Typhoon and

Case of Non-cohesive Sediment - Prof. Hajime NAKAGAWA Prof.

1 Nov 4, International Workshop on Typhoon and Flood River Embankment Failure due to Overtopping - In Case of Non-cohesive Sediment - Prof. Hajime NAKAGAWA Prof. of Disaster Prevention Research Institute, Kyoto University, Japan

2 Contents 1. About river embankment failure 2. An example of recent flood disasters due to river embankment breach 3. Flood risk in Japan 4. Research on bank breach due to overtopping 5. Conclusions

3 3 1. About river embankment failure

Characteristics of river EMB Principally made of sediments from the several viewpoints (economic, easy to get the material, easy to repair, easy to stick to foundation ground, easy to modify, good

4 Characteristics of river EMB Principally made of sediments from the several viewpoints (economic, easy to get the material, easy to repair, easy to stick to foundation ground, easy to modify, good for environment, etc.) Cause of river EMB failure Phenomenon Cause of REB failure Form of failure Overtopping Sliding Erosion Leakage of water Abnormal flood Water level rise due to bridge pier, weir, etc. Short of river EMB height Seepage of rainwater into river EMB body Seepage of river water due to water level rise Water collide portion Steep slope river Insufficient protection of river EMB against seepage Existence of water channel in the river EMB body Erosion at the top and toe of the slope Slope siding Erosion at the toe of front and back side slope Piping failure boiling Illustration by Zina Deretsky/National Science Foundation h1834c228#h1834c228

5 Classification of failure form of REB (Muramoto,1986) Damaged form overtopping not overtopping total Breach Partial collapsed (riverside) Partially collapsed (residential side) (75%) (25%) Not damaged total

6 6 2. An example of recent flood disasters due to river embankment breach

7 EMB breach at the Maruyama River in 2004 Breach at Tachino, Maruyama River Torii Bridge River Bank Breach 11

8 8 3. Flood risk in Japan (Related to river EMB)

Cities are locating on low-lying area in Japan In Japan, 50% of population and 75% of property exist in the flood prone area that is 10% of the Japan territory.

9 Cities are locating on low-lying area in Japan In Japan, 50% of population and 75% of property exist in the flood prone area that is 10% of the Japan territory. Therefore, once big flood and bank breach happen, damage becomes very severe. Low lying areas are Protected by REB Forest Low lying areas are Protected by REB River & lakes Out of low lying area Population Low lying area Teritory (380,000km 2 ) Landuse in Japan Property

10 Delay of river improvement works Holland (storm surge) UK (Thames R.) USA (Mississippi R. ) France (Seine R. ) Japan Completed 89% in 2002 Completed in 1988 Completed in 1985 Completed in 1983 Completed 60% in /30-1/40 years (Big rivers) 1/5 1/10 (Small rivers) (Current target) 1/100 1/500 1/1,000 1/10,000 1/Return Period

11 No./year No./year 6. Extraordinary heavy rainfall Precipitation in one hour > 50mm (1,000 observatories) ~ events ~ (year) Precipitation in one hour > 80mm 31 (1,000 observatories) ~ ~ times events ~ 2004(year) 1.2 times ~ Global warming effect, vulnerability of the society, rainfall 14 prediction, runoff process, etc events events events events

12 Necessity of Research on River Embankments In such an environment and situation, basic research on the mechanism of river embankment and its countermeasures are essential.

13 13 4. Research on bank breach due to overtopping 4.1 Objectives

14 Objectives River EMB Research:Approach from the Hydroscience and Hydraulic Engg. The main objective of this research is to develop a new numerical model that can compute the river EMB erosion process in accordance with the infiltration calculation at the EMB surface. This study focuses on erosion due to overtopping flow non-cohesive homogeneous sediments River EMB of the Jamuna River, Bangladesh

dam body *Unsaturated soil Slope failure

15 Water level raising Overtopping flow Erosion *steep slope Seepage Embankment dam body *Unsaturated soil Slope failure Deposition Embankment dam body The process of embankment breach by overtopping is very complex because it involves mutually dependent interactions between fluvial hydraulics, sediment transport and embankment stability. 15

16 Unsaturated Soils In some circumstances, the water in the soil will form menisci between particles. The internal stress is induced by the capillary tension of meniscus water clinging to the contact point of soil particles and acts so as to connect the soil particles tightly. Surface tension and interface curvature give rise to a difference between pore-water pressure (u w ) and pore-air pressure (u a ) that is generally equated to matric suction (=u a u w ). Meniscus water 16 Bulk water Meniscus water Particle F u a u w Fig. Scheme of Unsaturated Soils r 2 F T r 1 Particle Suction:u a -u w u a u w 1 T r2 1 r 1 u a : pore-air pressure u : pore-water pressure w r 1,r 2 : radius of curvature of meniscus T : surface tension of meniscus water

Apparent shear strength mh 2 O Water content Resistance shear stress due to suction is Moisture characteristic curve No.8(d=0.1mm) No.7(d=0.174mm) No.6(d=0.

17 Apparent shear strength mh 2 O Water content Resistance shear stress due to suction is Moisture characteristic curve No.8(d=0.1mm) No.7(d=0.174mm) No.6(d=0.334mm) Suction No.8(d=0.1mm) Suction No.7(d=0.174mm) In the unsaturated condition, sediment of smaller sized diameter is more difficult to be eroded due to suction. No.6(d=0.334mm)

18 Critical shear velocity of the sediment particles(u *c ) Density of sediment particle: 2.65 Kinematic viscosity of water: 0.01cm 2 /s Sediment diameter : d(cm) Iwagaki Formula (cm/s) 2 Shear stress is almost proportional to sediment diameter. In the saturated condition, smaller the sediment diameter, easier to move resulting in more erodible.

19 Numerical Model of Embankment Failure

20 Numerical Model of Embankment Failure 1. Seepage flow model Richards equation 20 2-D (vertical slice) van Genuchten equation(1980) 2. Flow model Depth-averaged shallow water flow Horizontal 2-D equations (momentum and continuity equations) 3. Erosion and Deposition Using framework of Non-Equilibrium sediment transport model: pick-up rate equation (Erosion rate) considering the effect of shear strength due to suction sediment movement is calculated using motion of equation sediment deposition (probability density function ) 4. Slope failure Slope stability analysis: 2-D (vertical slice) Simplified Janbu s Method considering the effect of shear strength due to suction

21 Seepage analysis in the EMB Seepage analysis model (1/2) Richards equation continuity equation in the unsaturated EMB K x x x K z z 1 z C t K, : water pressure head, : hydraulic conductivity in the x and y directions, C : specific moisture capacity, C w x K z w : volumetric water content of the soil 21

Overtopping flow analysis Flow model Depth averaged model u t v t Momentum equations u u H u v g x y x v v u v x y H g y bx h by h xx ( ) x xy ( ) x xy ( ) y yy ( ) y h t Continuity equation ( uh) (

22 Overtopping flow analysis Flow model Depth averaged model u t v t Momentum equations u u H u v g x y x v v u v x y H g y bx h by h xx ( ) x xy ( ) x xy ( ) y yy ( ) y h t Continuity equation ( uh) ( vh) x y h : flow depth 0, 22 u, v : depth averaged flow velocities g Gravitational acceleration H : water level bx, by : bottom shear stresses xx, yy xy : Reynolds stresses : density of water n : Manning s roughness coefficient xx u 2, x u * h 6 bx by gn 2 gn 2 u v yy v 2, y u u 2 2 v v 2 2 h h xy u y v x

Pick-up rate model(nakagawa Tsujimoto, 1985)+ introduction of effect of suction V p E r p s S p p *c *suc p s d 1 g F G 0 1 :non-dimensional shear stress k p* c :mesh area projected on the horizontal

23 Pick-up rate model(nakagawa Tsujimoto, 1985)+ introduction of effect of suction V p E r p s S p p *c *suc p s d 1 g F G 0 1 :non-dimensional shear stress k p* c :mesh area projected on the horizontal plane :specific weight of sediment particle (=2.65) : non-dimensional critical shear stress suc Effect of suction is considered as a increase of non-dimensional critical shear stress :sediment volume picked up in a unit time A3d Vp pss p A2 Vp :erosion rate Er 1 p S p :pick-up rate :porosity : increse of non-dimensional critical shear stress due to suction G *, m p G * cos kls 1 k L s s cos b sinb cos 1 kls cos k L : coefficients of pick up rate and nondimensional critical shear stress considering the local slope and flow direction k L :ratio of drag force to lift force (=0.85) s In saturated condition, * suc 0.0 :friction coefficient of sediment(=0.7) :angle with near bed flow direction and direction of sediment particle motion :angle with direction of sediment particle A, A motion and max. slope of bed 2 3 : shape coefficients of sediment A 4, A3 particle F 0, k p, m p : experimental coefficients s 2 s 6

24 Shear strength due to suction, Vanapalli et al. (1996) suc Drag force R Friction force T 2 In the critical shear stress condition, R T F u 2 b u a u w r r tan tan g s r s r CDub A2d F A3d gf suca2d 1gd 3C C 1gd D tan D 2 tan 1.0, suc CD 0.4 u u, u * u* c a u w Log low : matric suction u : pore-air and pore-water pressure a w : Water pressure head of surface mesh : moisture content w : internal frictional angle In order to take into account the role of shear strength due to suction, we consider critical condition for sediment movement. (Egiazaroff, 1965) u b 30. ad 5.75log 2 10 u * dm a 0.63 By addition of the shear strength to friction force, Egiazaroff formula can be express by following equation. 2 For a uniform sediment, d u* 4 1 m d c * c 2 2 suc 1gd 3C D 5.75log1019d dm * suc C 1gd5.75log 2 D suc C log 2 D gd 1019 d dm * c * c * suc 24 u b d f A A tan : flow velocity affects the sediment : diameter of sediment

25 Sediment transport analysis (transportation and deposition) Sediment transport process after detachment is calculated by the following motion of equation, Motion of equation of sediment particle C m k 3 sed j, k 3 D A d du dt j, k W j, k F j, k j 1,2 direction 25 D j, k:drag force W j, k :weight in the water F j, k :friction force m sed k :additional mass u sed k :velocity of sediment C m :additional mass coefficient :ratio of sediment particle density to that of water (= 2.65) 3 msed k Cm A3d k A 6 3

Erosion process of embankment surface Flow Flow Saturated layer Unsaturated

layer infiltration rate: Depth of saturated layer in one-step erosion

layer s E r z t E E r r t s t u K z t E t D : Hydraulic conductivity in z

26 Erosion process of embankment surface Flow Flow Saturated layer Unsaturated soil Saturated layer t Unsaturated soil D s t s Erosion within Saturated layer infiltration rate: Depth of saturated layer in one-step erosion Erosion rate: if if Flow U D D D s s s s K : U E E rs rs t t t Saturated layer s E r z t E E r r t s t u K z t E t D : Hydraulic conductivity in z direction : water pressure head rs s t t E s ru t D u t s u E t t rs s t u 0 26 Unsaturated soil

27 Modeling of local sliding Slope stability analysis As a slip surface calculation, simple Junbu method is adopted. Flow Safety factor considering the effect of suction is a Δx Safety factor N i F s n i1 Wi cli 1 F s N i c u u wi l wi i tan u n i1 tan W i i u ai sin ai u i u wi wi r tan l s r i cos 1 tan tan F i r s s r tan Slip surface l i sin i X X i+1 i E E i+1 i W T N i i b Dynamic programming method is used for the calculation of slip surface with minimum safety factor 27

28 Numerical simulation for experiments (comparisons with experimental results)

29 Experiments on river EMB erosion by over topping Experimental flume Ujigawa Open Laboratory, DPRI, Kyoto Univ. Water supply 50cm 500cm EMB Fixed bed Drop Pump Channel width = 30cm Video Camera Water tank

30 Embankment Shape Type-A Flow 40cm Fixed bed Dam body 30 80cm Flat bed (bottom of the flume) 35cm 80cm Type-B Flow Dam body 15cm Fixed bed Erodible bed 15cm 20cm 30cm 10cm 30cm 30cm 10cm Flat bed (bottom of the flume) Fig. Side view of embankment shape

31 Experimental Conditions for Type-A, B Case Dam Type Table Experimental cases Discharge (cm 3 /sec) A 7840 Sediment type Sediment -No.6 Sediment -No.7 Sediment -No Sediment B 5 - -No.7 Initial water content (%) Porosity of dam body Percent finer by weight, % (Non cohesive sediment) Sediment-8 Sediment-7 Sediment-6 No.8 No.7 No Diameter (μm) Grain-side distribution of EMB Each sediment size (Type-A,B) Parameters Sediment- No.6 Sediment- No.7 Sediment- No.8 K s (m/s) 2.15* * *10-5 d m (mm) d50(mm)

32 Simulated Results Failure of River embankment due to overtopping (Sediment-No.7) Simulated failure process of embankment due to overtopping with volumetric water content variation (Sediment-No.7) Experimental results (Video data) 40s 60s 80s 20s 32 Comparisons between simulated and experimental results of failure process

33 Simulated Results (Sediment-No.6) d m =0.334mm Embankment shape (Dam type-a) 40s 30s 10s 20s Simulated and experimental results of embankment shape (Sediment-No.7) 40s 60s 80s d m =0.174mm 20s (Sediment-No.8) d m =0.100mm 100s 80s 40s 60s 20s 33

34 Simulated Results Shields Diagram Embankment shape Trial calculation * c 1.7 (Sediment-No.8) d m =0.100mm 40s 20s 100s 60s Simulated and experimental results of embankment shape (Dam type-a)

35 Simulated Results Comparisons of with and without suction (Sediment-No.8) } with suction without }suction Comparisons of simulated results with and without suction big erosion rate Sensitivity of hydraulic conductivity (Sediment-No.8) K1= K2 * 0.5 K3= K2 * sec. 40 sec. K bigger then E faster Sensitivity of hydraulic conductivity (Case-3: sediment-no.8)

36 Volumetric Water Content Volumetric Water Content Volumetric Water Content Simulated Results Seepage process inside the EMB (Type-B) 5cm 5cm 5cm 5cm 15cm m1 m5 m4 m3 m2 WCR 30cm m8 m7 m6 15cm 25cm m9 15cm 20cm 30cm 10cm 30cm 30cm 10cm Fig. Positions of WCRs (m1 to m9) in Case-5 experiment 15cm Volumetric Water Content Time (sec) Exp (m1) Sim (m1) Volumetric Water Content Exp (m2) Sim (m2) Time (sec) Volumetric Water Content Time (sec) Exp (m4) Sim (m4) Time (sec) Exp (m5) Sim (m5) Exp (m6) Sim (m6) Time (sec) 0.2 Exp (m8) 0.1 Sim (m8) Fig. Comparisons of simulated and experimental results of temporal moisture variations inside the embankment Time (sec)

37 Simulated Results Elevation (cm) Embankment shape (Dam type-b) 35.0 (Case-4: sediment-no.7) d m =0.174mm Distance (cm) Initial sim-10s sim-20s sim-30s sim-40s exp-10s exp-20s exp-30s exp-40s Simulated and experimental results of embankment shape (Dam type-b) 37

38 38 5. Conclusions

39 5. Conclusions (1/2) An embankment erosion model is developed using combined depth average flow, seepage flow, sediment transport model based on nonequilibrium model, and slope stability model. In this study, a new expression for resisting shear strength due to suction was derived in pick-up formula to compute the erosion of unsaturated river embankment by flow overtopping. We used pick-up formulas based on both saturated and unsaturated conditions to simulate the erosion process of embankment surface that occurs simultaneously with infiltration. The proposed model was tested for erosion of embankment under different sediment sizes (Sediment No.6, 7, and 8) and embankment shapes (Type-A and B). Generally, the proposed numerical model well reproduced the experimental results on processes of river embankment surface erosion due to overtopping flow. 39

40 Conclusions (2/2) In small-sized sediments condition, the great shear strength and the low infiltration rate cause a lag of erosion and delay the failure time, and these phenomena were able to be reproduced using the proposed model. A numerical model that can reproduce the temporal change of embankment shape during the erosion process due to overtopping flow is important and useful to consider a countermeasure against embankment failure. There remains many problems to be solved, such as effects of cohesiveness, mixture of sediment size, applicability of the model to the real river EMB, etc. The model refinement is necessary in very near future. 40

41 Thank you very much for your kind attention!

EXPERIMENTAL STUDY ON RIVER DYKE BREACH CHARACTERISTICS DUE TO OVERTOPPING

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