Desilting basin 32 m³/s, 0.15 mm

Given: $Q=32$ m³/s, particles coarser than 0.15 mm, $w=0.025$ m/s, $\lambda=1.5$ (Velikanov's coefficient for turbulence and the required efficiency). For a high-head plant, two chambers are provided so that one can be flushed while the other works.

Step 1: Horizontal velocity (Camp)

$$v=44\sqrt{0.15}=17.04\ \text{cm/s}=0.1704\ \text{m/s}$$

Step 2: Depth and width

Assume a flow depth $H=5$ m:

$$B=\frac{Q}{vH}=\frac{32}{0.1704\times5}=37.56\ \text{m}\ (\text{two chambers of }18.78\ \text{m})$$

Adopt a chamber width of 19.0 m.

Step 3: Length by simple settling theory

Without turbulence: $L_0=\dfrac{vH}{w}=\dfrac{0.1704\times5}{0.025}=34.1$ m.

Using $\lambda$ as a factor on the surface area:

$$A=\frac{\lambda Q}{w}=\frac{1.5\times32}{0.025}=1920\ \text{m}^2,\qquad L=\frac{A}{B}=\frac{1920}{37.56}=51.1\ \text{m}$$

Step 4: Check by Velikanov's method

$$L_V=\frac{\lambda^2v^2(\sqrt H-0.2)^2}{7.51\,w^2}=\frac{1.5^2\times0.1704^2\times(\sqrt{5}-0.2)^2}{7.51\times0.025^2}=57.7\ \text{m}$$

The larger length governs. Adopt $L=60$ m, giving $L/b=60/19=3.2$.

PLAN (flushing gates at the downstream end)
          inlet        settling chambers      outlet
 canal   transition  +=================+   transition
 =====>  \         / |  chamber 1      | \         /===> to
         /  guide  \ +-----------------+ /  weir   \    headrace
 =====>  \  vanes  / |  chamber 2      | \         /
         /         \ +=================+ /         \
  trash-                  |  |  flush gates
  rack                    v  v  --> flushing channel --> river

LONGITUDINAL SECTION
 inlet          settling zone (depth H)      outlet
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  WL
 \    --->   --->   --->   --->   --->   --->  |
  \  ______________________________________   | weir
   \/   sediment storage / hopper slope 1:30 \__|__ to headrace
          flush gate -> flushing channel -> river

Flushing arrangement: each chamber has a hopper floor sloping (about 1:30) to a central flushing channel, with a flushing gate at the downstream end. To flush a chamber, its inlet gate is closed and the deposit is flushed out while the other chamber carries the flow. The flushing water runs through the flushing tunnel to the river.

Answer: 2 chambers, each 19.0 m wide x 60 m long x 5 m deep (plus storage).