BED, BANK & SHORE BED, BANK & SHORE PROTECTION - CHAPTER 8 pdf

In inland waterways, ships may cause wave: • Primary wave : starts with the front wave, followed by the depression and ending with the stern wave → severe attack on the banks narrow nav

BED, BANK & SHORE

Chapter 8

Ships – Loads, Stability and erosion

(3 class hours)

In inland waterways, ships may cause wave:

• Primary wave : starts with the front wave,

followed by the depression and ending with the stern wave → severe attack on the banks (narrow navigation

channels)

• Secondary wave : The much shorter waves that originate from the hull (plays an important role in larger navigation channels)

• Propeller wash : The currents caused by the

ship's propeller (it is particularly important when ships manoeuvre near

a berthing place or a jetty)

Introduction (cont)

Most damage on revetment cause by:

• large ships ~ sailing slowly

Æ erosion due to return current

• small service crafts, tugs… ~ sailing fast

Æ bank erosion due to secondary waves

Introduction (cont)

Flow around fixed object & moving object in stagnant water:

Introduction (cont)

Phenomena around a

moving ship in a waterway

Introduction (cont)

Propeller wash

Limit speed:

The speed of displacement-type ships can not exceed the celerity of their own generated waves, unless they are being towed by another (longer) ship.

Assuming that the maximum wave length caused by a ship is

equal to the ship’s length, the limit speed can be approximated:

Definition in 1-d approach

Loads (cont)

A s = B.D : cross-section area of the ship

A c = b.h : cross-section of the waterway

v s : water flows

ur : return flow

z : water-level depression

Combine this with Bernoulli:

Maximum speed is reached when return flow becomes critical,i.e when derivative of return flow to waterlevel becomes zero

Loads (cont)

limit speed a a function of blockage As/Ac

limit speed as a function of waterdepth and blockage

Loads (cont)

For the purpose of bank design, a speed

of 90% of V l is recommended

s c

v u

waterlevel depression as a function of blockage

Loads (cont)

return flow velocity as function of blockage

Loads (cont)

deviation from the 1-d case (Eccentric position ship):

origin of diverging and transverse waves

Secondary wave pattern

Loads (cont)

Secondary wave pattern

Loads (cont)

Fr < 0.75,

- the cusp locus line is at an angle of about 20o with the sailing line

- and the direction of propagation of the cusps is at an angle of

about 35o with the sailing line Æ hence the angle of approach for a bank parallel to the sailing line is 55o

Fr = 1, transverse and diverging waves coincide

Fr > 1 transverse waves can no longer exist

s

v Fr

gh

=

secondary wave height measurements

s : distance from ship’s sailing line

example (1)

Given: ship 10 m wide, draught 3 m, ship sails 10 m from bank (y= 5 m)

canal 40 m wide, 5 m deepCalculate: Maximum Wave height

Limit speed: As/Ac = (10*3) / (40*5) = 0.15 fig 8.4

standard values in the Netherlands

Wave heights (m) Currents (m/s) Wind waves Ship waves Natural current Return current Lakes

Canals

Rivers

Small waters

0.25 – 1.00 0.10 – 0.25 0.25 – 1.00 0.10 – 0.20

0.10 – 0.50 0.25 – 0.75 0.25 – 0.75 n.a

0.1 – 0.5 0.5 – 1.0 1.0 – 2.0 0.2 – 1.0

0.1 – 0.25 0.5 – 1.0 0.5 – 1.0 n.a

Data from CUR 197

“Breuksteen in de praktijk”

The Suez Canal

The Suez Canal, damage

Bank erosion along the Suez Canal

A normal ferry and a fast ferry

Propeller action

Loads (cont)

turbulence in propeller wash and

in free circular jet

Loads (cont)

equations for propeller jets

2

2

0

15.7 0

0.69

2.8 /

2.8 0.21

/

m

r x

r b m

u u

velocity distribution

in propeller wash and free jets

Loads (cont)

velocities behind propeller

measured flow in a propeller jet

data from thesis Schokkink, 2003

Loads (cont)

Turbulence in a propeller jet

data from thesis Schokkink, 2003

Loads (cont)

Flow caused by a propeller on an inclined slope

data from thesis Schokkink, 2003

Loads (cont)

model setup

Loads (cont)

stability of bed protection

2 sin

1 sin

r n

u d

2 sin

1sin

b n

u d

φ

Δ =

−

0.33 max

stern wave effect:

return flow effect:

2

2

1 1.2

1

sin

r n

u d

n

example (3)

¾ stern wave dominates problem

¾ action of stern wave only at waterline

¾ at deeper water return flow dominates

¾ at more spacious water bodies secondary waves become dominant

1 sin

b n

u d

2.5* 2.25

0.4 60 / 300 1.65*9.8

n

Bed erosion due to propeller wash

0

ln

5.6 15.7

• Three types of the loads:

- the primary wave, related to the length of the ship

- the secondary waves, related to the shape of the bow of the ship

- the propeller wash, related to the propulsion of the ship

- the length of the ship

- the water-depth

- the blockage of the waterway

90% of the limit speed

- small waterways (blockage of the cross-section by ships is not negligible )

- the stern wave usually gives the most severe attack on the banks of the

waterways

- blockage of the cross-section by ships does not play a role

- magnitude of the waves depends much more on the shape of the bow than

on the draught or length of the ship

- The angle of approach to the banks is more or less constant (55o) for

displacement ships

- plays an important role at locations where ships manoeuvre at low velocities

- The flow in can be approximated with the same kind of relation as used for

a circular jet

End of Chapter 8