Civil Engineering Reference
In-Depth Information
to the submerged substructures. The depth of the degradation scour,
d
d
, can then be
estimated based on the width of the channel bed. Accurate assessment methods for
general scour,
d
g
, are available (TAC, 2004; Richardson and Davis, 1995).
3.2.3.2.2.1 Contraction Scour
During live-bed scour the contraction scour
depths are affected by deposits of sediment from upstream. Scour will cease when
the rate of sediment deposit equals the rate of loss by contraction scour. During clear-
water conditions sediment is not transported into the contraction scour depth increase
(creating channel bed depressions or holes). Scour will equilibrate and cease when
the velocity reduction caused by the increased area becomes less than that required
for contraction scour. Equation 3.12 provides an estimate of the approach streambed
velocity at which live-bed scour will initiate,
V
s
, as (Laursen, 1963)
D
1
/
3
6
y
1
/
6
u
V
s
=
50
,
(3.12)
where
y
u
is the depth of channel upstream of bridge crossing,
D
50
is the streambed
material median diameter at which 50% by weight are smaller than that specified
(the size that governs the beginning of erosion in well-graded materials).
Contraction scour depth,
d
c
, for cohesionless materials under live-bed conditions
can be estimated as (Laursen, 1962)
Q
c
Q
1
,
6
/
7
B
b
k
1
n
c
n
u
k
2
d
c
=
y
u
−
(3.13)
where
Q
c
is the discharge at contracted channel (at bridge crossing);
b
is the net or
effective width of bridge opening;
B
is the width of the channel without obstruction or
constriction;
n
c
is Manning's surface roughness coefficient at the contracted channel;
n
u
is Manning's surface roughn
ess coeffici
ent at the upstream channel;
k
1 and
k
2
are exponents that depend on
gy
u
S
u
/V
D
50
, where
S
u
is the upstream energy slope
(often taken as streambed slope);
V
D
50
is the median fall velocity of flow based on
D
50
median particle size.
Contraction scour depth,
d
c
, for cohesionless materials under clear-water condi-
tions can be estimated as (Laursen, 1962; Richardson and Davis, 1995)
⎡
⎤
6
/
7
3
/
7
B
b
V
u
42
( y
u
)
1
/
3
(D
50
)
2
/
3
⎣
⎦
.
d
c
=
y
u
−
1
(3.14)
3.2.3.2.2.2 Local Scour
Local scour occurs at substructures as a result of vortex
flows induced by the localized disturbance to flow caused by the obstruction. The
determination of local scour depths is complex but there are published values relat-
ing local and general scour depths that are useful for preliminary scour evaluations.
Procedures for establishing local scour relationships for abutments and piers are avail-
able (TAC, 2004; Richardson and Davis, 1995). For most of the modern bridges,
local scour can generally be precluded, particularly at abutments, by the use of prop-
erly designed revetments and scour protection. Local scour depth for cohesionless
