Worked Example 9.1
This example considers the design of a glacis-type weir. The following data
are used: maximum flood discharge1800 m^3 s^1 ; HFL before con-
struction300.00 m AOD; river bed level293.00 m AOD; normal
upstream pond level299.00 m AOD; allowable afflux1 m; permiss-
ible exit gradient1 in 6; silt factor f1; crest level of canal regu-
lator297.50 m AOD; FSL downstream of canal regulator296.00 m
AOD; canal bed level downstream of regulator293.50 m AOD.
Design the various elements of the weir foundations using Bligh’s
theory. Also determine the waterway required for the canal head regula-
tor in order to draw a flow of 100 m^3 s^1.
Solution
The régime width (equation (9.9)) of the upstream waterway, B
4.75 1
8
0
0
200 m. Adopting the gross length of the weir as 200 m and
assuming 20 spans of 10 m each centre-to-centre of piers, and a pier thick-
ness of 1.5 m,
clear waterway 200 19 1.5171.5 m.
Neglecting the pier and abutment contractions and assuming the weir to be
broad crested (Q1.7bH3/2 modular flow; Chapters 4 and 8),
total head over the crest, H(1800/(1.7171.5))2/33.36 m,
velocity of approach, V1800/(2007)1.3 m s^1 ,
approach velocity head0.08 m,
therefore
upstream total energy level (TEL)300.000.08300.08 m AOD
beffectivebclear 2(nkPka)H
wherenis the number of piers (19),kPis the pier coefficient (0.01 for
semicircular piers), and kais the abutment coefficient (0.1 for 45° wing
walls). Therefore,
be171.5 2(190.010.1)3.36169.55 m
and the actual energy head,
380 DIVERSION WORKS
