On behalf of Halcrow Pakistan, I would like to thank SEPA for organizing this public hearing. I would also like to thank the honorable Secretary, DG SEPA, Conservator Wildlife and other respected guests for their participation. Your views and comments would be valuable for the consultant’s and the proponent.
Transcript
1.
HIP Sharjah
Water and Power Business Group
Flexible Lining for Flood Channels
14 May 2009
Waqar Ahmed
2.
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
3.
TYPES OF CHANNEL LINING
CHANNEL LINING
RIGID LINING FLEXIBLE LINING
GABIONS LOOSE RIP RAPCONCRETE
GROUTED
STONE PITCHING
MATTRESS
BASKETS
4.
BASKETS
MATTRESS
STONE PITCHING
TYPES OF CHANNEL LINING
5.
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
6.
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
7.
DESIGN CONSIDERATIONS
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.
8.
DESIGN CONSIDERATIONS
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.
9.
DESIGN CONSIDERATIONS
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
10.
DESIGN CONSIDERATIONS
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
11.
DESIGN CONSIDERATIONS
Lacey’s Silt Factor
Soil 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
13.
DESIGN CONSIDERATIONS
USE OF GABIONS AND STONE PITCHING
14.
DESIGN CONSIDERATIONS
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)..
16.
DESIGN CONSIDERATIONS
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.
17.
DESIGN CONSIDERATIONS
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
19.
DESIGN CONSIDERATIONS
Critical Velocity
The velocity at which Gabions will remain stable without movement of stone fill.
Limit Velocity
The velocity which is still acceptable although there is some deformation of Gabions due
to the movement of stones within the internal components or cells.
20.
GABIONS-DESIGN CONSIDERATIONS
SHEAR STRESS ANALYSIS
Sywb ..γτ =
The revetment is considered stable when there is no movement of individual stones.
The shear stress acting on revetment is calculated as below:
Where
bτ
wγ
= 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 γτ =
mτ = Shear stress on banks or side slopes
21.
GABIONS-DESIGN CONSIDERATIONS
CRITICAL SHEAR STRESS
The Critical shear stress acting on invert is calculated as below:
And Critical shear stress on banks or is computed as:
cτ
sγ
= Critical shear stress on inverts
C
mwsc dC )..( γγτ =
ϕ
θ
ττ 2
2
sin
sin
1. −= cs
sτ = Critical shear stress on banks
= Shield's parameter (0.1 for Gabion)
= Unit weight of stone
md = Mean size of stone rocks
θ = Bank side slope angle
ϕ = Internal friction angle of stone fill
Where:
22.
GABIONS-DESIGN CONSIDERATIONS
STABLE LINING:
1. The revetment is stable when:
cb ττ 2.1≤
2. The lining on the banks or side slopes of channel is stable if:
sm ττ ≤
3. With limited deformation it may be acceptable if:
sm 1.2ττ ≤
23.
GABIONS-DESIGN CONSIDERATIONS
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.
24.
GABIONS-DESIGN CONSIDERATIONS
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 ≤θfor
θ
τ
cot.2
S
m
H
=
2cot ≥θfor 3/1
).(cot4 θ
τ S
m
H
=
The equations are only valid for wind induced waves less than 0.9m and frequent waves 1.4m high.
25.
GABIONS-DESIGN CONSIDERATIONS
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”
26.
GABIONS-DESIGN CONSIDERATIONS
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
27.
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
34.
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
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