CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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Figure 5.3 A typical settling basin and its components
Concentration (C) = kg of suspended matter / m3 of water
Unfortunately, there have not been any studies regarding
the concentration of sediment in small mountain rivers of
Nepal that are appropriate for micro-hydro installation.
Therefore, in the Nepalese context, the designer has to rely
on data available from large hydropower projects. The
recommended practice in Nepal is to use C = 2 kg/m3 for the
design of settling basins for micro-hydro schemes .
The sediment storage requirement in a settling basin is
calculated as follows:
1. Calculate the sediment load using the following equation:
Sload = Q T C
where:
Sload = sediment load in kg stored in the basin
Q = discharge in m3/s
T = sediment emptying frequency in seconds
C = sediment concentration of the incoming flow
in kg/m3 .
A reasonable emptying frequency (T) in the Nepalese context
could be about once to twice daily during high flow, which
results in less than once a week during the low flow season
when the sediment concentration is low.
2. The next step is to calculate the volume of the sediment.
using the following equation:
V = S / S Ssediment
load density * factor
where:
Vsediment = volume of sediment stored in the basin in
m3.
Sdensity = density of sediment in kg/m3, about 2600
kg/m3. Unless other data are available this
Pfactor
value should be used for Sdensity .
= packing factor of sediment submerged in
water.
When submerged in water, particles occupy more space than
when dry. This is measured in terms of packing factor, which
is the ratio of unit volume of dry sediment to unit volume of
wet sediment (i.e. volume of 1 m3 of dry sediment divided by
the volume of this sediment when submerged). Packing factor
for submerged sediment is about 0.5 (i.e. the volume of dry
sediment is doubled when submerged).
Below the settling zone must be the capacity to store the
calculated volume of sediment, Vsediment . This storage space is
achieved by increasing the depth of the basin as follows: