Breusers, Nicollet and Shen (1977), Clark and Novak (1983), Richardson
and Richardson (1994), Melville and Chiu (1999), Melville and Coleman
(2000), Oliveto and Hager (2002), Coleman, Lauchlan and Melville (2003),
Sheppard, Odeh and Glasser (2004) and Dey and Barbhuiya (2005).
(e) Scour protection works around bridge piers
Although the presence of scour tends to reduce the backwater levels
upstream of the bridge, the damage to the foundations of the structure
may far outweigh the possible benefit. Hence protective measures, both to
minimize the scour and to prevent undermining of the foundations, have
to be taken. Piers with base diaphragms (horizontal rings) and multiple
cylinder type piers have been found to minimize the scour considerably.
The normal practice for protection of the foundation is to provide thick
protective layers of stone or concrete aprons around the piers.
A riprap protection (Bonasoundas, 1973) in the shape of a longitudi-
nal section of an egg with its broader end facing the flow is recommended
for a cylindrical pier. The recommended overall width is 6band length
7 bof which 2.5bis upstream of the pier. The thickness of riprap is 1/3b
with a maximum stone size, d, given by
d0.06 0.033U0.04U^2 (10.25)withUin metres per second and din metres.
The mean critical flow velocity Uc(m s^1 ) with a flow depth y 0 (m) is
given by
Uc≈ 6 d1/3y 0 1/6 (10.26)dbeing the armour stone size in metres (with (^) s2600 kg m^3 ).
For horizontal beds (US Army Coastal Engineering Research
Center (1984); Chapter 14) the simplified empirical relationship is
Uc≈4.92d1/2. (10.27)
The riprap should be placed on a suitable inverted filter or a geotextile
fabric (Fig. 9.11).
For further discussion of scour protection works refer to Zarrati et al.
(2006) and Unger and Hager (2006). For a comprehensive treatment of
bridge hydraulics (including hydraulic aspects of bridge construction and
maintenance) refer to Neill (2004).