Settling basin design principle and components

A settling basin (desander) is an enlarged chamber in the water conductor system where the flow velocity is reduced so that suspended sediment particles larger than a chosen size settle out before the water reaches the turbines. In Nepalese Himalayan rivers the silt load is very high and quartz-rich, so desanders are essential to prevent turbine abrasion.

Design principle

The basin works on the settling theory: a particle entering at the water surface at depth $h$ moves forward with the flow velocity $v$ and falls with its fall (settling) velocity $w$. It is trapped if it reaches the bed before the end of the basin.

$$\begin{aligned} \text{Time to settle } t &= \frac{h}{w}, \quad \text{time to pass } = \frac{L}{v} \\ \text{Condition: } \frac{L}{v} &\ge \frac{h}{w} \;\Rightarrow\; L \ge \frac{h\,v}{w} \end{aligned}$$

Width $B = \dfrac{Q}{v\,h}$. In practice turbulence reduces the effective fall velocity, so the length is increased using Velikanov's or Sakhmin's methods, e.g. Velikanov:

$$L = \frac{\lambda^2 v^2 (\sqrt{h} - 0.2)^2}{7.51\,w^2}$$

where $\lambda$ depends on the required trap efficiency.

Design criteria

1. Particle size to be removed: usually 0.2–0.3 mm for medium heads; 0.1–0.15 mm for high-head Pelton plants (Nepal practice: 0.2 mm, or 0.15 mm for heads above about 250 m).
2. Trap efficiency: generally 90% or more for the design particle size.
3. Horizontal velocity: low enough to avoid resuspension; by Camp, $v = a\sqrt{d}$ cm/s ($a = 44$ for $0.1 < d < 1$ mm), usually 0.2–0.4 m/s.
4. Uniform flow distribution: gradual inlet transition (expansion about 1:5 to 1:8) and baffles/guide walls.
5. Depth: usually 3–6 m plus extra depth for sediment storage.
6. Flushing: sediment must be removed periodically or continuously (hydraulic flushing, Bieri system, Serpent system); flushing discharge must be available.
7. Number of chambers: at least two chambers so that one can be flushed while the other runs.
8. L/B ratio: generally 8–10 for good flow; a large width is split into several chambers.

Components of a settling basin

 PLAN
        inlet        settling zone        outlet
 from  ________   ___________________   ________
 intake  \     | |                   | |     /  to
 ------>  > ---| |   (two chambers)  | |--- <   headrace
 ______  /_____| |___________________| |_____\
        transition                  weir/transition

 L-SECTION
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ WL
   \  inlet  |   settling zone        |outlet
    \________|________________________|
     sediment storage  ->  flushing canal/gate

1. Inlet zone (transition): gradually expanding section with guide walls or baffles that slows the water and spreads it uniformly over the basin width.
2. Settling zone: the main chamber of required length, width and depth where particles settle.
3. Sediment storage zone: extra depth below the settling zone, with sloping or hopper-shaped bed, where deposits collect.
4. Flushing system: flushing gates, conduits or channels with sufficient slope to return deposits to the river.
5. Outlet zone: converging transition and overflow weir that collect clear surface water into the headrace without disturbing the settled sediment.
6. Spillway/escape: a side spillway to release excess water and protect the basin.