2
Simple Steps to Siphoning
Introduction
and implementation of siphons β specifically applicable
to small-dam owners and operators. Many older dams
were not constructed with an outlet or other means of
draining the reservoir. Lowering the reservoir may be
needed for temporary construction or for emergency
response. Siphons can be a low-cost means of
providing a reservoir outlet if one does not exist.
Operational Theory
Siphons used in reservoir drawdown operate by
atmospheric pressure pushing water over an obstacle
(i.e., reservoir water over an embankment dam) and
discharging on the other side at a lower elevation than
the reservoir. In the same way a barometer works,
atmospheric pressure pushes liquid up a siphon into
the region of reduced pressure at the top/apex of the
siphon. The region of reduced pressure at the top of
the siphon is caused by liquid (water) falling on the exit
side, creating a pressure differential. The maximum
height, or lift, of a siphon is limited by the atmospheric
pressure at the site. The height a siphon can lift water
will, therefore, be lower for dams at higher elevations
(for instance the western United States).
There are several parameters that must be evaluated
when establishing the feasibility and design of a
siphon. Bernoulli's equation can be applied to
estimate a siphonβs maximum lift, discharge capacity,
diameter, and pressure.
Maximum Siphon Lift
The most critical parameter for a siphon at a given site
is to determine whether it is hydraulically possible to
βpushβ the water the desired height over the dam or
spillway crest. The required lift height can be
determined by comparing the dam crest elevation
(DCE) to the lowest desired reservoir water surface
elevation (RWS); see Figure 1. At sea level,
atmospheric pressure is generally 14.7 psi which is
equivalent to a column of water about 34-ft high. Thus,
34 ft is the maximum theoretical height for a siphon.
However, the maximum achievable lift is reduced by
friction and other minor losses in the system due to
velocity head. Therefore, it is good practice to assume
that the maximum lift achievable by atmospheric
pressure at sea level is equal to only about 20 to 25 ft
of water. Atmospheric pressure can be assumed to
decrease by about 4 percent (or 1 ft) for every 1,000-ft
increase in elevation. Therefore, the maximum lift
height of a siphon can be conservatively taken as: π»πππ₯=20β²βπππ1,000
Where π»πππ₯ = Maximum achievable siphon lift.
RWS = Lowest desired reservoir water
surface in feet of elevation
DCE = Dam crest in feet of elevation.
The maximum achievable siphon lift, Hmax, must be
less than the value of (DCE β RWS). If the dam crest
is too high compared to the desired reservoir-
drawdown elevation, consider routing the siphon
through a spillway or a temporary notch in the dam
crest to reduce the required lift.
Predicted Siphon Discharge
Estimating the discharge capacity will help the designer
Figure 1: Typical Syphon Schematic