Geoscience Reference
In-Depth Information
4.5.3 Stability in waves
In general, geotextile mattresses should not be used where the significant wave height
H
s
is greater than 1.0 m. [22, paragraph 5.4.4] gives the following empirical stability
ratio:
H
45
23
for
H
s
<
1.0 m
(4.6)
s
≤
ξ
Δ⋅
t
D
t
D
where:
H
s
=
significant wave height [m];
Δ
t
=
relative density of the geotextile mattress [-];
D
k
=
effective thickness of the geotextile mattress [m];
ξ
=
breaker parameter [-].
The breaker parameter is the ratio between the slope angle and the wave steep-
ness, see formula (3.6). For ship induced waves, the wake of a ship has a general wave
steepness (
s
op
H
s
/
L
0
) of 0.035 to 0.050. The wave height is mostly less than or equal
to 0.5 m and the wave length is 10 to 15 m.
=
4.5.4 Stability in longitudinal currents
To determine the stability of geotextile mattresses in longitudinal currents, use can be
made of the Pilarczyk relationship [23], as is done for geotextile bags:
2
Φ
KK
T
u
⋅
K
⋅
Th
(4.7)
Δ
D
≥
0 035
⋅
cr
t
k
.
2
Ψ
Kg
s
⋅
K
s
where:
cr
=
critical flow velocity along the structure [m/s];
Φ
=
stability parameter [-];
Ψ
=
Shields parameter [-];
T
=
turbulence factor [-];
h
=
factor related to the depth [-];
s
=
factor related to the slope angle of the bank;
g
=
acceleration due to gravity [m/s
2
].
For the stability parameter the following values apply (as for geotextile bags):
for continuous top layer:
Φ
=
1.0;
for edges:
Φ
=
1.5.
0.07.
The turbulence factor describes the extent of turbulence in the water flow, see
Table 3.1. Using the depth factor (
K
h
) the depth-averaged flow velocity is translated
into a flow velocity just above the toe of the structure. For the determination of this,
see formulae (3.12) to (3.14).
For geotextile mattresses the Shields parameter (a guide value) is [23]:
Ψ
=
