Intermittent vs continuous flushing basin

Given: $Q=5$ m³/s, depth $H=3.2$ m, $w=0.025$ m/s, $\lambda=1.5$ (this coefficient gives the required trap efficiency of about 90% with turbulence), flushing discharge 15% in the continuous case. Assume a horizontal velocity of $v=0.25$ m/s (within 0.2-0.4 m/s). Simple settling theory: $A=\lambda Q/w$.

(i) Intermittent flushing ($Q=5$ m³/s)

$$A=\frac{\lambda Q}{w}=\frac{1.5\times5}{0.025}=300\ \text{m}^2,\qquad B=\frac{Q}{vH}=\frac{5}{0.25\times3.2}=6.25\ \text{m},\qquad L=\frac{A}{B}=48.0\ \text{m}$$

Provide two chambers of 3.5 m x 48 m so that one works while the other is being flushed. Sediment storage is also added (about 1 m) because the sediment stays in the basin between flushes.

(ii) Continuous flushing ($Q'=1.15\times5=5.75$ m³/s)

$$A=\frac{1.5\times5.75}{0.025}=345\ \text{m}^2,\qquad B=\frac{5.75}{0.25\times3.2}=7.19\ \text{m},\qquad L=\frac{A}{B}=48.0\ \text{m}$$

Two chambers of 3.6 m x 48 m. The settled sediment drops continuously into a hopper and is removed by the flushing water of 0.75 m³/s.

Comparison and justification

The length is the same ($L=\lambda vH/w$) while the continuous basin is 15% wider because it carries 15% more water. The continuous basin needs no storage volume, no gate operation and no interruption, but it consumes 0.75 m³/s water all the time; the intermittent basin needs no flushing water except when flushing, but it needs two chambers and storage. For a small plant with little spare water, intermittent flushing is justified; for a plant with silt-laden water and automatic operation, continuous flushing is better.

Answer: (i) 2 chambers of about 3.5 m x 48 m; (ii) 2 chambers of about 3.6 m x 48 m, at a depth of 3.2 m.