Single chamber settling basin design (15 m³/s)

Given: $Q=15$ m³/s, particles $\ge0.2$ mm, $w=0.02$ m/s, factor of safety for area $=1.5$, $L/B=8$, sediment concentration $C=1$ kg/m³, particle density 2500 kg/m³, flushing interval 8 h, packing factor 0.5, additional flushing discharge 10%.

Step 1: Design flow

$$Q'=1.10\times15=16.5\ \text{m}^3/\text{s}$$

Step 2: Surface area of the basin

For a particle to settle, the surface area must satisfy $A\ge Q/w$. With the factor of safety:

$$A=\frac{FS\times Q'}{w}=\frac{1.5\times16.5}{0.02}=1237.5\ \text{m}^2$$

Step 3: Width and length ($L/B=8$)

$$B=\sqrt{\frac{A}{8}}=\sqrt{\frac{1237.5}{8}}=12.44\ \text{m},\qquad L=8B=99.5\ \text{m}$$

Adopt $B=13$ m and $L=104$ m ($A=1352$ m²).

Step 4: Depth of flow

Limit the horizontal velocity to about 0.3 m/s. Take the flow depth $H=4.5$ m:

$$v=\frac{Q'}{BH}=\frac{16.5}{13\times4.5}=0.282\ \text{m/s}<0.3\ \text{m/s}\quad\text{(OK)}$$

Step 5: Sediment storage (flushing every 8 h)

$$\text{Mass of sediment}=C\,Q'\,t=1\times16.5\times(8\times3600)=475200\ \text{kg}$$ $$\text{Volume of deposit}=\frac{\text{mass}}{\rho_s\times p}=\frac{475200}{2500\times0.5}=380.2\ \text{m}^3$$ $$\text{Depth of deposit}=\frac{380.2}{1352}=0.28\ \text{m}\ \Rightarrow\ \text{provide }0.5\ \text{m storage}$$

(All the sediment entering is assumed to be trapped, which is conservative.)

Step 6: Overall dimensions

PLAN (single chamber, flushing gate at the downstream end)
          inlet           settling chamber          outlet
 canal   transition  +========================+   transition
 =====>  \  guide  / |                        | \  weir   /===> to
         /  vanes  \ |  13 m wide x 104 m long| /         \    headrace
                     +========================+
  trash-                          |  flush gate
  rack                            v  --> flushing channel --> river

Answer: a single chamber of about 13 m x 104 m, with a flow depth of 4.5 m ($v=0.282$ m/s) and 0.5 m of sediment storage, flushed every 8 hours (deposit of 380.2 m³ per cycle).