are withstood without damage and geotextiles placed between the soil and
riprap are cheaper and give greater control during construction than a
graded aggregate filter, particularly in underwater applications (see, e.g.,
PIANC, 1987 and Pilarczyk, 1999). The use of geotextiles as a concrete
form has been developed for heavy duty (IFAI, 1992).
Unanchored inflatable tubes or geotubesare sleeves of high strength
geotextile fabric (nylon, polyester, polypropylene, polyethylene), thickness
from 0.8 to 4 mm, filled usually with sand (or with slurry mixture or even
concrete). Since the 1980s geotubes have been marketed to replace sand-
bags in flood protection and have found increasing use in coastal and river
engineering. Their height ranges from 0.7 to 2.5 m (in an elliptical cross-
section) and length from 15 to 30 m (exceptionally appreciably longer
tubes have been used in some coastal engineering projects).
There are many commercially available products used in bank protec-
tion, and the manufacturers should be able to provide information about the
best conditions for their use, their durability, and about equivalent rough-
ness sizes or coefficients of friction and maximum permissible velocities. For
natural materials these velocities would typically range from 0.5 m s^1 for
fine sand to 1.5 m s^1 for shingle, 1.85 m s^1 for hard clay and 0.8–2.1 m s^1
for various types of grass (and soil conditions). Stone-filled wire-mesh
mattresses and gabions can withstand velocities over 5 m s^1 if thicker than
0.30 m. Manning’s coefficient for this type of mattress can vary from 0.016
(corresponding to a roughness size of about 3.5 mm) for a channel lined
with mattresses, grouted, and sealed with sand asphalt mastic, with a
smooth finish, to 0.027 (k125 mm) for a channel lined with gabions and
filled with unselected quarry stones.
It is important to appreciate that any protective facing of banks must
be continued to the river bed and be provided with a good footing (founda-
tion). A good filter adapted to suit the subsoil is essential, as is drainage of
sufficient capacity underneath more or less impermeable revêtment (see
also Section 2.6.3 and Worked example 9.1). Where permeable revêtment
is used (the majority of cases in river training) it must provide sufficient
drainage from the slope without air being trapped. An example of the use
of fascines with a foundation of heavy brush rollers, used for the protection
of a bank, is shown in Fig. 8.18. The use of gabions and gabion mattresses
for bank strengthening is illustrated in Fig. 8.19, and bank protection with a
flexible mattress of prefabricated concrete elements (Armorflex) on a geo-
textile layer is shown in Fig. 8.20; this could also be covered by vegetation,
making the bank protection almost invisible. For further details of bank
protection design, materials and construction, see for example Brandon
(1987, 1989), Hemphill and Bramley (1989) and Escarameia (1998).
For further information on resistance of vegetation to flow and its
effect on bank stability see, e.g., Thorne (1990), Copin and Richards
(1990), Oplatka (1998) (which includes also an extensive bibliography on
plant biology and mechanics) and Armanini et al. (2005).
amelia
(Amelia)
#1