Why settling basins are essential in Nepal

Why a settling basin is essential in Nepal

A settling basin (desander) is an enlarged chamber after the intake where flow velocity is reduced so that suspended sediment settles and is flushed back to the river. In Nepal it is essential because:

1. Young, fragile Himalaya: rivers carry very high sediment loads, especially in the monsoon (often several thousand ppm).
2. Hard, sharp quartz particles: quartz and feldspar content is high, so turbines (especially high-head Pelton and Francis) suffer severe abrasion/erosion of runners, needles, guide vanes and seals.
3. Most plants are run-of-river: there is no large reservoir to trap sediment.
4. Sediment fills headrace canals and tunnels, reducing capacity and increasing head loss.
5. Wear causes loss of efficiency, frequent shutdowns and costly repairs (e.g. Jhimruk, Khimti, Kaligandaki A).

Typical layout

 PLAN
        inlet          settling zone       outlet
 from  +-----+--------------------------+-----+ to
 intake| /   |  ====   chamber 1   ==== |   \ |headrace
 ----> |<    |--------------------------|    >|---->
       | \   |  ====   chamber 2   ==== |   / |
       +-----+--------------------------+-----+
        transition    |flushing| spill   transition
                      v channel v weir

 SECTION (longitudinal)
  WL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   \ inlet                          outlet|
    \___                        ____weir__|
        \______________________/
         bed slope 1:20-1:50 --> flushing
                               gate/pipe

Design criteria

• Particle size to remove: 0.15–0.2 mm for high-head plants (> 100 m) and 0.2–0.5 mm for low/medium heads.
• Trap efficiency: usually ≥ 90% of the design particle size.
• Flow velocity: less than critical (scour) velocity, by Camp: $V = a\sqrt{d}$ cm/s (d in mm, a = 44 for 0.1–1 mm), normally 0.2–0.4 m/s.
• Size: settling length $L = \dfrac{hV}{w}$ (w = fall velocity), increased for turbulence (Vetter/Camp efficiency); $L/B$ about 4–10.
• Inlet and outlet transitions: gradual (about 1:5 to 1:8) to give uniform flow; baffles or flow-straightening walls.
• Number of chambers: at least two, so one can be flushed or repaired while the other works.
• Flushing: continuous or intermittent (hydraulic flushing gates, Serpent/SSSS systems, pipes); bed slope 2–5% towards the flushing outlet.
• Spillway: to pass excess water during floods or load rejection.
• Provision for sediment monitoring and bypass of flood flows.