Flexible lining for flood channels 100509

HIP SharjahWater and Power Business Group

Flexible Lining for Flood Channels

14 May 2009Waqar Ahmed

FLEXIBLE LINING FOR FLOOD CHANNELS

•Types of channel lining

•Advantages of channel lining

•Design considerations

• Manufacturer’s Guidelines

•Introduction to ChannelPro

•Construction of Gabions

•Placing of Gabions

TYPES OF CHANNEL LINING

CHANNEL LINING

RIGID LINING FLEXIBLE LINING

GABIONS LOOSE RIP RAPCONCRETEGROUTED

STONE PITCHING

MATTRESS

BASKETS

BASKETS

MATTRESS

STONE PITCHING

TYPES OF CHANNEL LINING

ADVANTAGES OF LINING

The main advantages of providing the lining to any flood channel are:

• Stabilization of channel bed and bank slopes

• Prevention of soil erosion

• Pre determination of roughness coefficient value

• Increase in capacity of channel

• Promote vegetation / Improve aesthetic (Flexible lining)

• Safety of valuables

DESIGN CONSIDERATIONS

i Hydrology

Flood frequency analysis

ii Hydraulics

Slope of channel

Shape of channel

Velocity of flow

function of discharge, size & slope

Choice of lining

Velocity

Economics

iii Components of Design

Average stone size, D50

Thickness of lining, T

Filter Design


Wadi Hailu Flood Channel Design•Road Starting from mountain toe•Expected flood of the order of 122 m3/s (Flood Frequency Analysis)•Channel Width = 30m (Hydraulics)•Depth of flow = 1.5m•Velocity of flow = 3.5m/s

•Stone Pitching Design

USBR recommends the following formula for determining the size of stone that will not be dislodged under turbulent flow conditions

D50 = (V / 4.915)2

Using V=3.5m/s, D50 will be 0.5m.

The specific gravity of the stones was assumed to be 2.65 (i.e. density of 2,650kg/m3). If less dense stone is used, then the stone size should be increased correspondingly.


The grading of the stone pitching should be as follows:

•Maximum stone size = 1.5D50 (0.75m)

•Minimum stone size = 0.5D50 (0.25m)

•Not more than 40% of the stone should be smaller in size than D50

•The thickness of the pitching should be 1.5 times the stone D50 size. It is usual to place the pitching on a filter layer, to prevent fines being washed out from the embankment during the flood recession.


Scour DepthScour Depth

The Lacey empirical equation may be used to compute the depth of scour. The design scour depth below bed level (D) is given by:

Design scour depth (D) = XR – Y [metric units]

Where:X = scour factor dependent on type of reach

(1.25m)Y = design depth of flow [m] (1.5m)R = 1.35 (q2/f)1/3 (1.5m)q = the maximum discharge per unit width [m2/s]f = Lacey’s silt factor


Scour Factors

Type of Reach Mean Value of Scour Factor "X"

Straight 1.25

Moderate bend (most transitions) 1.50

Severe bend (also Shank protection at spurs) 1.75

Right angled bend (and pier noses and spur heads) 2.00

Nose of Guide Banks 2.25


Lacey’s Silt FactorSoil Type Lacey's Silt Factor

"f"

Large boulders and shingle 20.0

Boulders and shingle 15.0

Boulders and gravel 12.5

Medium boulders, shingle and sand 10.0

Gravel and bajri 9.0

Gravel 4.75

Coarse bajri and sand 2.75

Heavy sand 2.0

Fine bajri and sand 1.75

Coarse sand 1.5

Medium sand 1.25

Standard silt 1.0

Medium silt 0.85

Fine silt 0.6

Very fine silt 0.4

Clay 5.0

Design scour depth (D) = 2m



USE OF GABIONS AND STONE PITCHING


FILTER LAYERFILTER LAYER

•Stone protection placed on embankments should be laid on a filter layer to prevent piping. When one filter layer is sufficient it is called a “graded filter”. When more than one filter layer is used, the coarser filter is placed on top of a finer filter (i.e. the permeability increases outwards), and the filter is called an "inverted filter".

•The gradation of a graded filter should conform to the following guidelines established originally by Terzarghi:

d15 filter / d85 soil < 5;

d15 filter / d15 soil > 5; and

d50 filter / d50 soil < 25

Where d85 is the sieve size which will pass 85% of the material, and similar for other percentages (d15 and d50)..


Wadi Hailu Flood Channel Design

(Scnario-2)

Flood Discharge = 122 m3/s

Channel Width = 7m

Flow depth = 2m

Velocity = 5.7m/s

According to USBR formula , D50 = 1.32m

Corresponding Max stone size is 2m.

A stone size of 2m for a channel bed width of 7m is not suitable. For such conditions we refer to Gabion Mattress.


For design of Gabions we can refer to different Manufacturer’s Guidelines available for the selection of suitable mattress. Terra Aqua is one of them. Which gives a range of mattresses to be selected depending upon the flow velocity.

From the table below it is evident that Gabions of thickness 450mm is suitable for taking up velocity of 5.6m/s

Product Thickness(mm)

Stone Size(mm)

D50

(mm)Critical Velocity

(m/s)Limit

Velocity(m/s)

Reno 150 75-150 85-108 3.5-4.21 4.21-4.51

225 75-150 85-118 3.5-4.51 5.4-6.09

300 75-150 98-123 4.2-5 5.4-6.4

Gabions 450 100-200 148-188 5.8-6.4 7.6-8


Critical VelocityThe velocity at which Gabions will remain stable without movement of stone fill.

Limit VelocityThe velocity which is still acceptable although there is some deformation of Gabions due to the movement of stones within the internal components or cells.

GABIONS-DESIGN CONSIDERATIONS

SHEAR STRESS ANALYSIS

Sywb ..

The revetment is considered stable when there is no movement of individual stones.

The shear stress acting on revetment is calculated as below:

Where

bw

= Shear stress on inverts

= Unit weight of water

y = Depth of water

S = Slope

Also, shear stress on banks or side slope is given as follows:

Sywm ...75.0

m = Shear stress on banks or side slopes


CRITICAL SHEAR STRESS

The Critical shear stress acting on invert is calculated as below:

And Critical shear stress on banks or is computed as:

c

s

= Critical shear stress on inverts

C

mwsc dC )..(

2

2

sin

sin1. cs

s = Critical shear stress on banks

= Shield's parameter (0.1 for Gabion)

= Unit weight of stone

md = Mean size of stone rocks

= Bank side slope angle

= Internal friction angle of stone fill

Where:


STABLE LINING:

1. The revetment is stable when:

cb 2.12. The lining on the banks or side slopes of channel is stable if:

sm 3. With limited deformation it may be acceptable if:

sm 1.2ττ


ESTIMATION OF POTENTIAL DEFORMATION

When the shear stress reaches the critical value of the condition of “initial movement”, the stone fill in the Gabion moves downstream inside each compartment. With the shear stress increasing and more stone movement development, the deformation of Gabion mattress occurs.

To evaluate degree of deformation, a parameter Dz/dm is used. Calculated using the relation below.


LINING DESIGN WITH WAVE ACTION

Banks of channels, canals, irrigation basins etc. are subject to wave action generated by wind; and the banks of navigation canals and rivers are subject to wave action caused by vessels. The major parameter in designing the revetment to resist the wave action are the wave height and bank slope angle.

3cot for

cot.2S

m

H

2cot for 3/1).(cot4 Sm

H

The equations are only valid for wind induced waves less than 0.9m and frequent waves 1.4m high.


GABIONS Vs RIP RAP

In accordance with the shear stress analysis, we conclude that “For any given hydraulic condition the average size of stones needed in Gabions is half that required for a dumped loose rip rap revetment”


i Overall stability of Gabion Mattress lining depends on:

Strength of double twisted mesh

Thickness of lining

Grading of the stone fill material used

ii Thickness of Gabion Mattress depends on:

Flow Velocity

Sediments and Bed loads

Gradient of channel

Curvature of channel trace

INTRODUCTION TO CHANNELPRO

FLOW VELOCITY

V = R2/3 . S1/2 (Manning Formula)n

Where

V = Velocity (m/s)

n = Manning Roughness coefficient

R = Hydraulic Radius (m)

S = Channel Slope

AVERAGE STONE SIZE, D50

INTRODUCTION TO CHANNELPRO

CONSTRUCTION OF GABIONS

PLACEMENT OF GABIONS

Laying at Channel Bed

•Length dimension of unit is laid parallel to the water flow

Laying at Channel Side Slopes•Length dimension of unit is laid perpendicular to the water flow

PLACEMENT OF GABIONS


•QUESTIONS


THANK YOU

Flexible lining for flood channels 100509

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