Several researchers performed model tests on scour downstream of culvert
structures and the combined results suggest the following design guide-
lines (see Breusers and Raudkivi, 1991):
scour depth below bed level, ys0.65D(Uo/U*c)1/3 (10.1)
whereUoflow velocity at exit and U*cShields critical shear velocity
((c/ )1/2);
scour width, Bs7.5DF2/3r, (10.2)
scour length, Ls 15 DFr2/3 (10.3)
whereDculvert height, FrU 0 /(gD)1/2, 0.27Fr2.7 and 0.22d(mm)
7.3. Equation (10.1) may be modified in the case of graded material
as
ys3.18DF0.57r
d
D
50
0.114
g^ 0.4 (10.4)
whereg(d 84 /d 16 )1/2. In order to protect the channel bed against scouring,
a minimum stone size is recommended as
ds0.25DFr (10.5)
for low tail water levels. For high tail water levels (D/2) the recom-
mended stone size is reduced by 0.15D.
Fletcher and Grace (1974) recommended a lining of trapezoidal
cross-section downstream of the culvert exit extending to a length 5 D
with a bed slope of 1 in 10 followed by a curtain wall to a lengthDat a
slope of 1 in 2; the side slopes of the trapezoidal lining are recommended
to be 1 in 2. Alternatively, the design curves of Simons and Stevens (1972)
may be used for non-scouring and scouring bed protection in rock basins
(see Breusers and Raudkivi, 1991).
Blaisdell and Anderson (1988a, b) made a comprehensive study of
scour at cantilevered pipe outlets and suggested the ultimate maximum
scour hole depth, Zmax, below tailwater level (zero elevation) as
Z
D
max
7.5[1^ e^ 0.6(Frd^ 2)] (10.6)
forZp/D1 and
Z
D
max
10.5[1^ e^ 0.35(Frd^ 2)] (10.7)