Design procedure of sloping glacis weir bay

Design of the weir bay of a sloping-glacis weir (Garg / Punmia style).

Step 1: Data. Maximum flood $Q$, HFL, pond level (PL), river bed levels, silt factor $f$, afflux, safe exit gradient.

Step 2: Waterway and discharge intensity. Total waterway $L$ (from Lacey's perimeter, $P = 4.75\sqrt Q$), less the width of the undersluices and piers, gives the weir bay length. Discharge per metre $q = Q/L$.

Step 3: Crest level. Fixed from the pond level and head required to pass the design flood over the crest: $Q = \frac{2}{3}C_d\sqrt{2g}\,L\,H^{3/2}$ (crest level $=$ HFL after afflux $-$ $H$); also crest $=$ canal FSL $+$ regulator losses.

Step 4: Regime (scour) depth. $R = 1.35\,(q^2/f)^{1/3}$.

Step 5: Downstream floor level and length.

• Total energy $E_L =$ (u/s TEL) $-$ (d/s bed or floor level); $H_L =$ (u/s TEL $-$ d/s TEL) and use Blench's curves (or Crump's) to find $E_2$ (the conjugate depth after the jump) for the given $q$ and $H_L$.
• d/s floor level $=$ d/s HFL $-$ (d/s depth required for the jump) $=$ TWL $- \,E_2$ (with a safety margin of 0.5-1 m).
• Length of the horizontal floor $= 5\,(E_2 - E_1)$ (about $5\times$ height of jump) – the glacis length is included with a slope of 3:1.

Step 6: Total length of floor and cutoffs.

• Total length from Bligh's/Khosla's criterion: Bligh $L = C\,H$ with $H$ = max static head; or by Khosla's exit gradient.
• U/s cutoff depth $= 1.0R$-$1.5R$ below u/s HFL; d/s cutoff depth $= 1.5R$-$2R$ below the d/s bed.

Step 7: Thickness of impervious floor. Check Khosla's pressure at key points and take the thickness

$$t = \frac{4}{3}\cdot\frac{h}{G-1}$$

at the toe of the glacis (max thickness), provide about 0.6 m at the ends. Taper thickness along the floor. The d/s floor under the jump should also resist the dynamic forces.

Step 8: Protection works. U/s and d/s aprons (blocks and inverted filter) with launching aprons of length $1.5$-$2\times$ scour depth; and check exit gradient $G_E \le$ safe value.