9.1.6 Barrage width and scour depth
(a) Barrage width
The barrage width must be sufficient to pass the design flood safely. The
present trend is to design the barrage for a 100–150 year frequency flood
and provide a breaching section along the main earth bund, located at a
safe distance from the barrage itself. The breach section acts as a fuse
plug, and its provision is a more economical solution than providing a
large spillway capacity in the barrage.
The minimum stable width of an alluvial channel is given by the
régime equation (Chapter 8)
B4.75Q1/2 (9.9)
whereBis the waterway width (measured along the water surface and at
right angles to the banks) in metres, and Qis the maximum flood dis-
charge in m^3 s^1.
(b) Regime scour depths
The river bed is scoured during flood flows and large scour holes (not to be
confused with local scour, Section 5.3.3) may develop progressively adja-
cent to the concrete aprons which may cause undermining of the weir
structure. Such flood scour depth below HFL corresponding to a regime
width (equation (9.9)) is called régime scour depth (or more precisely
régime hydraulic radius), Rs, estimated by the following (Lacey’s) formula
Rs0.475(Q/f)1/3 (9.10)
if the actual waterway provided is greater or equal to the régime width
(equation (9.9)) and
Rs1.35(q^2 /f)1/3 (9.11)
if the waterway provided is less than the régime width, where Rsis measured
from the high flood level (HFL) and fis Lacey’s silt factor (Singh, 1975):
f1.75d1/2 (9.12)
wheredis the mean diameter of the bed material (in mm) and qis the dis-
charge per unit width of channel (see also Section 8.3).
Weir failure due to scour can be prevented by extending the sheet-
pile cut-offs to a level sufficiently below the régime scour depth across the
full width of the river (Fig. 9.9).
376 DIVERSION WORKS
