Spur design for concave bank, 5000 m3/s

A spur on the concave bank deflects the current away from the eroding bank. It is designed from the regime width, the scour depth and the severity of the bend. The design uses Lacey and standard spur practice (Garg).

Data

$Q = 5000$ m³/s, bed level 1000.00 m, HFL 1010.00 m (flood depth $h = 10$ m), $f = 1.0$.

Step 1. Regime width and scour depth

$$\begin{aligned} W &= 4.75\sqrt{Q} = 4.75\sqrt{5000} = 335.9\ \text{m} \\ R &= 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\times17.10 = 8.04\ \text{m} \\ t &= 0.06\,Q^{1/3} = 1.03\ \text{m} \end{aligned}$$

Step 2. Length and spacing

• Length of spur is limited to about 20 % of the regime width: $L_s = 0.2\times336 = 67$ m; adopt $L_s = 65$ m (measured from the bank line at HFL).
• Spacing on a concave bank $S = 2$ to $2.5\,L_s$; adopt $S = 2L_s = 130$ m.
• Spurs are set pointing slightly upstream (about $100^\circ$ to $110^\circ$ with the bank), with a T-head or round head at the nose.

Step 3. Height and side slopes

• Top level $=$ HFL $+ 1.0$ m $= 1011.0$ m (bank end); nose end is kept about 0.5 to 1.0 m lower, but not below HFL.
• Total height at the root above bed $= 1011.0 - 1000.0 = 11.0$ m.
• Top width 4 m (3 to 6 m).
• Side slopes: 2:1 on the shank (middle) and bank part, 3:1 at the nose; stone pitching thickness $t = 1.03$ m (adopt 1.1 m) on a 0.3 m filter.

Step 4. Launching apron at bank, middle and nose

Scour below HFL is taken as 1.5R at the bank (root), 1.75R in the middle and 2.25R at the nose. The apron slope is 2:1 and its width is $1.5D'$.

Thickness of apron laid $\approx \sqrt5\,t = 2.29$ m (launched thickness $1.5t = 1.54$ m).

Answer: $L_s \approx 65$ m, spacing $\approx 130$ m, total height 11 m, side slopes 2:1 (3:1 at the nose), apron widths 3.1 m (bank), 6.1 m (middle) and 12.1 m (nose).

 bank ====root====|========== shank ===========\ nose
 HFL+1 --------------------------------------    \
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 bed ____apron 3.1m___apron 6.1m_______apron 12.1m__