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

4.1 Introduction 4.2 Scour without protection 4.3 Scour with bed protection 4.6 Summary... • Erosion and Scour is the excess removal of bed material by sediment transport.. • Scour - the

BED, BANK & SHORE

PROTECTION

Lecturer: PhamThu Huong

Faculty of Coastal Engineering

Chapter 4

(3 class hours)

4.1 Introduction

4.2 Scour without protection 4.3 Scour with bed protection 4.6 Summary

• Erosion and Scour is the excess removal of bed material by sediment transport

• Scour - the interaction between flow,

structure and sediment

• Scour may be caused by:

‰ change hydraulic conditions (e.g acceleration or increased turbulence)

‰ difference between sediment transport capacity and sediment transport

The Scour process

In which:

- Zb is the position of the bed

- S the total sediment transport per unit width

General picture local erosion

• S2 = S1 > 0 dynamic equilibrium situation

• S2 > S1 = 0 clear water scour (no sediment transport)

• S2 > S1 > 0 live-bed scour (active bed-load transport)

sediment transport is not always identical to sediment transport capacity

Experiment – erosion due to turbulence

Erosion downstream of a sill, due to turbulence

¾ Influence average velocity;

z v = 0.2 m/s bed position and scouring hole at: 0 min, 5 min, 10 min, 20 min, 40 min, 80 min;

z same for v = 0.3 m/s at: 2 min, 5 min, 10 min, 20 min,

40 min;

¾ Influence of Turbulence, by placing a sill on the rough bed, after 10, 20 and 40 min

Sediment transport formula

scour during construction of Eastern Scheldt storm surge barrier

Structures suffering scour

z Barrages, tidal inlets, navigation channels with groynes

z groynes, seawalls, breakwaters

z seabed pipelines, flowlines, electrical cables

z vertical pipes, piles, piers

z gravity based structures, platforms, offshore structures

z moorings and marinas

Scour process

types of scour

¾ scour without protection

z jets and culverts

z detached bodies (bridge piers)

z attached bodies and constrictions

• abutments

• groynes

¾ scour with bed protection

z scour development in time

z dustbin factor α

¾ flow slides

Scour without protection

¾ Scour in horizontal Jets and Culverts

¾ Scour in horizontal Jets and Culverts

2 0

¾ Scour around detached body

¾ Scour around a cylinder (from Breusers / Raudkivi, 1991)

Scour around 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

0.85 0.8 0.6

Ks = shape factor

Kα= angle of attack

Ku= velocity factor

Ku = 0 for u/uc < 0.5

Ku = 1 for u/uc > 1 and

Ku = (2u/uc - 1) for 0.5 < u/uc < 1

¾ Scour around abutments

Rectangular ("Blunt") Cylindrical

Streamlined

1.0 0.75 - 1.0 0.5 – 0.75

Flow velocities and scour in Zeebrugge

erosion in gradual constriction

Scour around groyne

Scour with bed protection

=

Δ

hs(t) maximum scour depth

h0 original water depth

u vertically averaged velocity at end of protection

uc critical velocity

t time in hours

α dust bin parameter

Scouring formula for clear-water scour behind a bed protection:

influence of α

use local value of α

αL = 1.5 + 5 r (for αL >1.8)

comparison model and prototype

De Grauw and Pilarczyk 1981

values of α for vertical and horizontal

constrictions (for L/h0=10)

relation between α , turbulence and length

steps to calculate α

2 1.08 0

6.67

D L D g r

B

Bs is original gap width

b is reduced gap width

equilibrium clear water scour

live bed scour

the slope angle β

2

4 0

0 50

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 an excess pressure on the pore water and thus forcing it out of the pores This excess pore pressure in loosely packed sand decreases the contact forces between the grains Æ reduction of the shear strength

Æ The soil becomes, temporarily, a thick fluid ⇒ liquefaction

schematic view of a flow-slide

when the flow forces can no longer take away sediment (clear-water scour) or when the inflow and outflow of

sediment become equal (live-bed scour).

faster than with protection.

is coupled as much as possible with turbulent flow

phenomena

the stability of a structure