10.2.4 Dips
The dip is a shallow structure without excessive approach gradients. In
arid regions, streams with infrequent flash floods and shallow depths
(0.3 m) may be allowed to flow through the dipped area. The upstream
road edge should not be discontinuous with the stream bed in order to
avoid scour, and at the downstream edge protection works such as a cut-
off wall, concrete, or riprap paving must be provided. Also, the profile of
the dip should, as far as possible, conform to the profile of the stream to
minimize local disturbances to the flow.
The road surface has to withstand the expected flow velocities and
debris. Bitumen-bound macadam may withstand velocities of up to 6 m s^1 ,
whereas up to 7 m s^1 may be permitted on asphalted road surfaces. Low-
level embankments, where occasional overtopping is permitted, must be
protected against scour and bank-slope stability. The permissible mean
velocities for a range of protective materials are suggested in Table 10.5
(Watkins and Fiddes, 1984).
The overflow discharge over an embankment may be predicted by
using the weir formula of the type
QCbH3/2 (9.29)
whereCis the sill coefficient (m1/2s^1 ),bis the length of the flow section
andHis the total head upstream of the sill.
The coefficient Cis a function of h/L(his the head over a sill of
widthL) for free flow conditions. For non-modular flow conditions a cor-
rection factor, f, as a function of hd/s/H, may be incorporated in equation
(9.29) (Tables 10.6 and 10.7).
442 CROSS-DRAINAGE AND DROP STRUCTURES
Table 10.5 Permissible velocities to withstand erosion
Type of protection Velocity (m s^1 )
Grass turfing:
Bermuda grass 2.0
buffalo grass 1.5
Cobbles:
100 mm 3.5
40 mm 2.5
Coarse gravel and cobbles (25 mm) 2.0
Gravel (10 mm) 1.5
