Geoscience Reference
In-Depth Information
Geotextile bags must be designed to account for the stability parameter at the
edges (
1.5) due to the higher forces at the edges.
The Shields parameter depends on the type of element:
Φ
=
for small geotextile bags (
<
0.3 m
3
):
Ψ
=
0.035;
for larger geotextile bags:
Ψ
=
0.05.
The turbulence factor accounts for the extent of turbulence in the current.
In Table 3.1 several values are given.
Using the depth factor (
K
h
), the depth-average flow velocity is translated into a
flow velocity at any depth (
h
) along the structure:
2
(3.12)
K
=
h
2
⎛
⎜
⎛
⎜
⎞
⎟
⎞
⎟
h
12
log
⎛
⎝
⎞
⎠
⎛
⎝
⎞
⎠
k
r
where:
h
=
water depth [m];
r
=
equivalent roughness according to Nikuradse [48] [m].
For the equivalent roughness
k
r
, an initial estimate of the effective thickness (
D
k
)
of the geotextile bags can be used. It should be noted that formula (3.12) applies to a
fully developed current profile. If this is not the case, then the following relationship
should apply:
02
=
⎛
⎞
−
h
k
undeveloped current profile
(3.13)
K
⎛
⎝
⎜
h
r
K
h
≈
1.0
for a very rough current (
h/k
γ
<
5)
(3.14)
The slope factor
K
s
is a function of the influence of the angle of shearing resistance
between the geotextile bag and the subsoil:
2
sin
=−
⎛
⎛
⎝
⎞
sin
α
(3.15)
1
K
s
δ
Table 3.1
Turbulence factor
K
T
for various hydraulic conditions [22].
Condition
K
T
[
−
]
Normal turbulence in rivers
1.0
Higher turbulence: river bends
1.5
Turbulence at groynes
2.0
Strong turbulence: hydraulic jumps, sharp bends, local disruptions
2.0
Turbulence as the result of propeller jets and other water jets
3.0-4.0
