BED, BANK & SHORE BED, BANK & SHORE PROTECTION - CHAPTER 4 potx

BED, BANK & SHORE BED, BANK & SHORE PROTECTION PROTECTION Lecturer: Lecturer: PhamThu PhamThu Huong Huong Faculty of Coastal Engineering Faculty of Coastal Engineering Chapter 4 Chapter 4 Flow Flow - - Erosion Erosion (3 class hours) Content Content 4.1 Introduction 4.2 Scour without protection 4.3 Scour with bed protection 4.6 Summary Introduction Introduction • • Erosion and Scour is the excess removal of Erosion and Scour is the excess removal of bed material by sediment transport. bed material by sediment transport. • • Scour Scour - - the interaction between flow, the interaction between flow, structure and sediment structure and sediment • • Scour may be caused by: Scour may be caused by:   change hydraulic conditions (e.g. change hydraulic conditions (e.g. acceleration or acceleration or increased turbulence) increased turbulence)   difference between sediment transport capacity difference between sediment transport capacity and sediment transport and sediment transport The Scour process The Scour process In which: In which: - - Z Z b b is the position of the bed is the position of the bed - - S S the total sediment transport per unit width the total sediment transport per unit width 0 b z S tx ∂ ∂ + = ∂∂ General picture local erosion General picture local erosion • • S S 2 2 = S = S 1 1 > 0 > 0 dynamic equilibrium situation dynamic equilibrium situation • • S2 > S1 = 0 S2 > S1 = 0 clear water scour clear water scour (no sediment transport) (no sediment transport) • • S2 > S1 > 0 S2 > S1 > 0 live live - - bed scour bed scour (active bed (active bed - - load transport) load transport) sediment transport is not always sediment transport is not always identical to sediment transport capacity identical to sediment transport capacity Experiment Experiment – – erosion due to turbulence erosion due to turbulence Erosion downstream of a sill, due to turbulence Erosion downstream of a sill, due to turbulence ¾ ¾ Influence average velocity; Influence average velocity; z z v = 0.2 m/s bed position and scouring hole at: v = 0.2 m/s bed position and scouring hole at: 0 min, 5 0 min, 5 min, 10 min, 20 min, 40 min, 80 min; min, 10 min, 20 min, 40 min, 80 min; z z same for v = 0.3 same for v = 0.3 m/s m/s at: at: 2 min, 5 min, 10 min, 20 min, 2 min, 5 min, 10 min, 20 min, 40 min; 40 min; ¾ ¾ Influence of Turbulence, by placing a sill on the Influence of Turbulence, by placing a sill on the rough bed, after 10, 20 and 40 min rough bed, after 10, 20 and 40 min Sediment transport formula Sediment transport formula ()or () c Sf Sf ψ ψψ =− = 0 ss c wc z υ ∂ + = ∂ dynamic equilibrium threshold value Settling sed. Stiring up sed. scour during construction of scour during construction of Eastern Scheldt storm surge barrier Eastern Scheldt storm surge barrier Structures suffering scour Structures suffering scour z z Barrages, tidal inlets, navigation channels with Barrages, tidal inlets, navigation channels with groynes groynes z z groynes groynes , seawalls, breakwaters , seawalls, breakwaters z z seabed pipelines, seabed pipelines, flowlines flowlines , electrical cables , electrical cables z z vertical pipes, piles, piers vertical pipes, piles, piers z z gravity based structures, platforms, offshore gravity based structures, platforms, offshore structures structures z z moorings and marinas moorings and marinas [...]... sediment transport and velocity: S =kum, in which m = 4 ÷ 5) Scour around groyne ⎛ Q ⎞ h0 + hse = 2.2 ⎜ ⎟ ⎝ B −b⎠ 2/3 Scour with bed protection Scouring formula for clear-water scour behind a bed protection: (α u − u c ) 1.7 hs (t ) = hs(t) h0 u uc t α 10 Δ 0.7 h 0.2 0 t 0 .4 maximum scour depth original water depth vertically averaged velocity at end of protection critical velocity time in hours dust bin... 1.5 + 5 r0 f c with fc = C 40 ( f c = 1 for C ≤ 40 ) From Hoffmans (1993) The r0 comes from Hoffmans and Hinze steps to calculate α Hinze (1975): D −2 L − 6 D 1 .45 g −1.08 r0 = 0.0225(1 − ) ( + 1) + 6.67 h h C eq 2.13 D = step height h = downstream waterdepth Hoffmans (1992, 1993) αL = 1.5 + 5 r Hoffmans and Booij (1993) α = 1.5 + 5 r0 f c with fc = C ( f c = 1 for C ≤ 40 ) 40 Trinh (1993) α = (1.5 +... β1 + cot β 2 ) hs2 2 3 .4 0 .4 = ⎡ 005 ( cot β1 + cot β 2 ) Δ −1 .4 h0 (α u − uc ) ⎤ t 0.8 = K t 0.8 ⎣ ⎦ I= I red = K t 0.8 − S ⋅ t → hs red = I red 0.5 ⋅ ( cot β1 + cot β 2 ) 5 dI ⎛ 0.8 K ⎞ = 0 → 0.8 K t −0.2 = S → te = ⎜ ⎟ → hse = dt ⎝ S ⎠ K te0.8 − S ⋅ te 1 ( cot β1 + cot β 2 ) 2 stability of protection β This distance has to be large enough L = f(β) the slope angle β 2 ⎡ ⎤ u0 4 + ( 0.11 + 0.75 r0 )... ⎡ ⎤ u0 4 + ( 0.11 + 0.75 r0 ) f c ⎥ β = arcsin ⎢3 ⋅10 Δgd50 ⎣ ⎦ C ( f c = , f c = 1 for C ≤ 40 ) 40 stability and slides - Sliding occurs after a slope has lost its stability The final slope will be gentler than the angle of repose φ 1:6 can serve as an indication of an average slope for density packed sand - When a shear stress is exerted on loose sand, the grains tend to a denser packing, producing...Scour process types of scour scour without protection jets and culverts detached bodies (bridge piers) attached bodies and constrictions • abutments • groynes scour with bed protection scour development in time dustbin factor α flow slides Scour without protection Scour in horizontal Jets and Culverts Scour in horizontal Jets and Culverts ⎛ u0 ⎞ hse... cylinder as function of water-depth and diameter (Experiment results given by Breusers et al, 1977) Bridge failure due to scour Bridge which failed due to scour at the base of piers caused by a turbulent horseshoe vortex system scour in case of other forms hs ⎛ h0 ⎞ = 2 K S Kα K u tanh ⎜ ⎟ D ⎝D⎠ Ks = shape factor Kα= angle of attack Ku= velocity factor Pier shape l/b KS Cylinder - 1.0 Rectangular 1 3 5... Cylinder - 1.0 Rectangular 1 3 5 1.2 1.1 1.0 Elliptic 2 3 5 0.85 0.8 0.6 Ku = 0 for u/uc < 0.5 Ku = 1 for u/uc > 1 and Ku = (2u/uc - 1) for 0.5 < u/uc < 1 Scour around abutments Abutment shape KS (to be used in equation Rectangular ("Blunt") Cylindrical Streamlined 1.0 0.75 - 1.0 0.5 – 0.75 Flow velocities and scour in Zeebrugge erosion in gradual constriction m −1 B1 h1 ⎫ Q = B1 u1 h1 = B2 u2 h2 → u2... forcing it out of the pores This excess pore pressure in loosely packed sand decreases the contact forces between the grains reduction of the shear strength -The soil becomes, temporarily, a thick fluid ⇒ liquefaction schematic view of a flow-slide . BED, BANK & SHORE BED, BANK & SHORE PROTECTION PROTECTION Lecturer: Lecturer: PhamThu PhamThu Huong Huong Faculty of Coastal Engineering Faculty of Coastal Engineering Chapter 4 Chapter. Engineering Chapter 4 Chapter 4 Flow Flow - - Erosion Erosion (3 class hours) Content Content 4. 1 Introduction 4. 2 Scour without protection 4. 3 Scour with bed protection 4. 6 Summary Introduction Introduction • • Erosion. sediment transport) (no sediment transport) • • S2 > S1 > 0 S2 > S1 > 0 live live - - bed scour bed scour (active bed (active bed - - load transport) load transport) sediment transport