Khosla's method for barrage floor design

Khosla's method for the design of a barrage floor

Khosla's theory gives the pressure and exit gradient from the flow of seepage water below the floor. The floor is designed in the following steps.

Step 1: Data. Maximum flood discharge, HFL, pond level (PL), d/s tail water level, bed level, silt factor $f$, safe exit gradient $G_E$ (1/5 to 1/7 for sand), specific gravity of floor material $G = 2.4$.

Step 2: Scour depth. Lacey's $R = 1.35\,(q^2/f)^{1/3}$, with $q$ = discharge per metre width. Use it for the cutoff depths: u/s cutoff $\ge 1.0R$-$1.5R$ below HFL and d/s cutoff $\ge 1.5R$-$2R$ below the d/s HFL.

Step 3: Maximum static head $H$ = PL $-$ d/s bed (or floor) level with no flow (usually nil tail water); and under flow with HFL.

Step 4: Exit gradient and length of floor.

$$G_E = \frac{H}{d}\cdot\frac{1}{\pi\sqrt{\lambda}},\qquad \lambda = \frac{1+\sqrt{1+\alpha^2}}{2},\ \alpha = \frac{b}{d}$$

Taking $d$ as the d/s cutoff depth, find $\alpha$ (hence floor length $b$) such that $G_E \le$ safe value. Provide the total floor length with some margin.

Step 5: Pressures at key points. Compute $\phi_E,\phi_D,\phi_C$ for each pile line:

• u/s pile: $\phi_{E_1} = 100$, $\phi_{D_1} = 100-\phi_D$, $\phi_{C_1} = 100-\phi_E$.
• d/s pile: $\phi_{E_2} = \phi_E$, $\phi_{D_2}=\phi_D$, $\phi_{C_2}=0$.

Apply corrections: (a) mutual interference of piles $C = 19\sqrt{D/b'}\,(d+D)/b$; (b) floor thickness; (c) slope of the glacis.

Step 6: Floor thickness. Uplift head at a point $h$ (from corrected $\phi\times H$). Required thickness

$$t = \frac{4}{3}\cdot\frac{h}{G-1} \quad(\text{33\% extra for safety})$$

The thickness is greatest near the toe of the glacis and is decreased in steps (min 0.5-0.6 m).

Step 7: Floor details. The d/s floor length is fixed by the hydraulic jump, and the thickness of the u/s floor is provided with nominal values. Provide the u/s and d/s aprons (blocks with inverted filter) of length 1.5-2 times the scour depth, with launching apron.

Step 8: Check pressure line (HGL), exit gradient, and thickness for both conditions (no flow and flow).

Typical section of a barrage

  Pond level ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        gates (piers)             HFL ~~~~~
 u/s floor |   crest  |\ glacis | jump floor | d/s apron
 ==========|__________|  \______|============|== blocks
   u/s block apron    |         |      filter, launching
 u/s pile (cutoff)  intermediate pile     d/s pile (cutoff)

Components: gates and piers, crest, u/s floor, glacis, horizontal d/s floor (jump), u/s and d/s cutoffs, intermediate piles, aprons with filters and launching aprons, u/s and d/s wing walls.