Design of River Banks and Bed Erosion Protection

Works

•River-stabilization structures are designed to protect the

riverbanks and prevent lateral migration of alluvial channels
through bank erosion

•River-stabilization methods can be classified according to two

different approaches:

(1) Strengthening the banks and

(2) Reducing hydrodynamic forces
 Flow-control structures offer an alternative
approach by reducing the hydrodynamic forces
applied against the riverbanks
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 Contd

•From both a geotechnical and geological viewpoint, it is

convenience to classify river banks as follows:

Cohesive banks in which there is a significant amount of

clay. Some peats can also be grouped under this heading

Non-cohesive banks which have little or no cohesion, i.e.

those with a small amount of clay, and generally comprising
sand or gravel

Composite banks which have layered structure, e.g.

cohesive soil overlying a non-cohesive soil
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 Contd

Cross section on meander bends in river with composite

bank
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 Contd

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 Contd
Bank-erosion processes
•Processes of bank erosion are directly linked to the lateral
migration of alluvial channels.

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 Contd
Typical types of river bank failure

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 Contd

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 Contd

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 Contd

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 Types of River Bank Protection Works
• There could be two broad ways of stabilizing banks

The direct methods of protecting the slope, and

The indirect way by providing structures

• Amongst the direct methods available for bank

stabilization, the following broad categories are as follows:

Self-adjusting armour made of stone or other

materials
Rigid armour
Flexible mattress
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 Contd
• As for the indirect methods for bank stabilization, these
may be classified into the following categories

 Dikes - Permeable or Impermeable

Retards - Permeable or Impermeable
Other flow deflectors, like Bendway weirs, Iowa
vanes, etc

The advantages of this type of protection are that little

or no bank preparation is involved. This reduces costs of
local environmental impacts, and simplifies land acquisition.

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 Contd
• Natural Bank Protection •Vertical Bank protection
Grass, including turf Steel sheet piling
Grass reinforced with Steel and asbestos
synthetic materials trench sheeting
Gabion structures
Reeds
Concrete, brick and
Willows and other masonry gravity walls
trees Precast Units
Timberwork Reinforced earth
Brushwood structures

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 Types of Armour Layer

1. Stone
a) Rip-rap or rock armour, occasionally grouted
b) Hand –pitched stone
c) Masonry, random or dressed
d) Gabion or wire mesh mattresses

2. Concrete
a) Plain precast blocks, open-jointed or grouted
interlocking blocks
b) Cable-tied or geotextile-bonded blocks
c) Cast in-situ slabs and monolithic structures
d) Fabric containersDaniel A. Unity University-2010A.Y
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 Contd

3. Geotextiles
a) Grassed composites-mats, fabrics and meshes
b) Three-dimensional retaining mats and grids
c) Two-dimensional fabrics

4. Asphalt
a) Open stone asphalt-filled geotextile mat
b) Open or dense stone asphalt

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 Rip-rap
•Riprap is the term used to describe loose quarry stone with a
wide grading (D85/D15 = 1.5 to 2.5 and W85/W15 = 3.4 to
16) that is used for the protection of beds and banks against
hydraulic forces.

•Riprap revetments are formed by randomly placing layers of

light grading stone and sizes are typically greater than
200mm or 250mm.

•Riprap is specified both by weight(most frequently) and by

size
•Riprap is one of the most versatile types of revetment as it
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be specified to suit aDanielvery range of flow and soil
A. Unity University-2010A.Y
conditions.
 Contd
•Usual engineering qualities of riprap include:

General ease of placing, can be placed underwater

Flexibility

High hydraulic roughness to attenuate waves and currents

Low maintenance requirements and convenience of repair

Durability

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 Contd

Example of rip rap

protection on a river
bank

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 Contd

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 Riprap Characteristics Contd

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 Design of Rip-rap
•The important factors to be considered in designing rock
riprap blanket protection are:
(1) the velocity (both magnitude and direction) of the
flow or shear stress in the vicinity of the rock
(2) the side slope of the bank line being protected
(3) the density of the rock

(4) the angle of repose for the rock

(5) the riprap blanket thickness

(6) the filter needed between the bank and the blanket
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 Contd
•Two methods to determine the size of rock riprap required
for stream bank stabilization are presented

(1) Shear-stress method

(2) Flow-velocity method

•Riprap stability on a side slope is a function of

(1) the magnitude and the direction of the flow velocity
or shear stress in the vicinity of the stream bank
(2) The side slope angle, and
(3) the properties of the rock including size, density, and
angularity.
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 Contd
•A simplified approach to the calculation of the rock size that
corresponds to the beginning of motion on a side slope θ1 is
possible when the following approximations are acceptable

Flow in the downstream direction

The downstream bed slope is negligible

The specific gravity of the rock is close to 2.65

The viscous drag is small compared with the lift force

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 Contd
•The effective rock size dm required for stabilizing a
riverbank under applied shear stress τ0 is estimated from
Lane’s relationship as

τ o is the applied shear stress

θ 1 is the side slope
φ is the angle of repose of the rock riprap
τ ∗c is the critical value of the Shields number
•When the flow is fully turbulent over a hydraulically rough
horizontal surface, incipient motion approximately
corresponds
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to τ ∗c = 0.047
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 Example
#Determine the rock riprap size required for stabilizing the
banks of a straight river given the river width W = 300 m, a
flow depth of h = 7 m, and a channel slope of S = 60 cm/km.
The bank slope is θ1 = 30◦, the rock density G is 2.7, and the
angle of repose is φ = 40

Solution

Step 1: The shear stress applied on the particle is

τ 0 = γ w hS = 9810 N m 3 × 7m × 0.006 = 41Pa

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 Contd
Step 2: The term in brackets from the equation of recommended
Particle size

Step 3: The effective riprap size is obtained from τ ∗c = 0.047

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 Riprap gradation

•The size of rock representative of the stability of the riprap

is determined by the larger sizes of rock

•Riprap consisting of angular stones is more suitable than that

consisting of rounded stones.

•The representative grain size dm for riprap is larger than the

median rock size d50

•The effective size dm approximately corresponds to d65, and

we can use dm ∼=1.25 d50
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 Contd
•The computations of the representative grain size dm for the
recommended gradation are illustrated in Fig. in terms of d50.

Suggested riprap gradation curve.

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 Contd
Recommended gradations from the U.S. Army Corps of
Engineers are presented in Tables

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 Filters
•Filters are used under the riprap revetment to allow water to
drain easily from the bank without carrying out soil particles

•Filters are required when the d15 of the riprap gradation

exceeds five times the d85 of the bank material

•Filter blankets must meet two basic requirements: stability and

permeability.

•Two types of filters are commonly used: gravel filters and

synthetic filter cloths.
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 Contd
•Gravel filters consist of a layer, or blanket, of well-graded
gravel placed over the embankment or riverbank before
riprap placement

•Sizes of gravel in the filter blanket should be from 3/16 in.(5

mm) to an upper limit, depending on the gradation of the riprap,
with maximum sizes of approximately 3–31/2 in.
(90 mm).

•The filter thickness should not be less than 6–9 in. (20 cm).
Filters that are one-half the thickness of the riprap are
quite satisfactory
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 Contd
•Suggested specifications for gradation are as follows:

•Multiple filters may be used when the base material is

very fine
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 Contd
•Synthetic filter cloths (plastic cloth and woven plastic
materials) are also used as filters, replacing a component of a
graded filter

•Numerous plastic filter fabrics exist with a wide variation in

size of number of openings and in strength and durability of
material.

•Opening areas of 25%–30% appear desirable to minimize

the possibility of clogging and to reduce head loss for synthetic
filter cloths
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 Example-Application to filter design
•The following filter design example involves the properties of
the base material and the riprap given

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 Contd
•The riprap does not contain sufficient fines to act as the
filter because

which is much greater than 5, the

recommended upper limit

•Also

which is much greater than 40,

the recommendedDaniel
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upper limit
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 Contd
•The properties of the filter to be placed adjacent to the base,
from requirement are as follows:

Thus, with respect to the base

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 Contd
•The properties of the filter to be placed adjacent to the
riprap are as follows

•Therefore, with respect to the riprap, the filter must satisfy

these requirements:
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 Gabions and mattresses

•Gabions are patented rectangular wire boxes (or baskets)

filled with relatively small-sized stone, usually less than 8 in.
(20 cm) in diameter

•When flow velocities are such that small stones would not
be stable if used in a riprap blanket, the wire boxes provide
an effective restraint.

•Limiting recommended maximum velocity for use of gabions

ranges from 8 to 15 ft/s (2 to 5 m/s), depending on the
manufacturer
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 Contd
•Box gabions are normally stacked on relatively steep slopes
to form a massive structure capable of resisting the forces of
both river flows and also unstable bank line materials.

•Box gabion structures generally are aligned either along the

stream bank toe to form a retaining wall for the bank
materials or out from the bank to form dikes for diverting
flows away from the bank

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 Contd

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 Contd
•Mattress gabions are shaped into shallow, broad baskets and
are tied together side by side to form a continuous blanket
of protection. They are normally placed on a smoothly graded
riverbank slope.

Example
of bank
protection
with
gabion
mattress

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 Contd

Box Gabion

Gabion Mattress

Sack Gabion
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Examples of gabion types (adopted from Hemphill and Bramley, 1989)
 Contd
Suitability

•Box gabions are suited for the protection of river beds and
banks subjected to heavy current attack. They can be stable at
flow velocities of the order of 5 to 6m/s(or more, depending
on quality of construction and assembly)

•Since they require smaller stone sizes than riprap for the same
flow conditions, they are a good alternative for regions where
large stone sizes are difficult(and expensive) to obtain

•They also have a long design life of several decades,

particularly if the mesh is PVC coated or made of polymers
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Comparison of the various river bank protection measures

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 Contd

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 Contd

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