C.3.2 Intake Pipe Design and Placement
Water from the outside the well in the water body is transferred to and from the stilling well through intakes. Depending on the type of stilling well, the intakes can be a series of holes in the wall of the well, a single hole in the well bottom, or connecting pipes. The intakes must be large enough to maintain the water level within the well at the same depth as the outside water at all times. At the same time, the intakes must be sufficiently constricted to dampen unwanted oscillation of the water level.
Note: Rule of thumb for sizing the intake--the total area of the intake(s) or the holes in the well should equal one thousandth the area of the stilling well.
To prevent plugging by deposited silt, the intakes should be positioned at least 0.2 m lower than the lowest expected water level, but at least 0.15 m above the well bottom.
Pipe intakes should be positioned horizontally, below the frost line. The open ends of the intakes should protrude into the outside body of water to a point clear of obstruction. Two or more pipe intakes are normally installed at elevations throughout the range of water level variation. During high water, if silt deposits obstruct the lower intake the upper intakes will continue to operate. Record the elevation of the intakes in the field data book on the Description of Hydrometric Station Form (Form AQU-01).
The intake pipes must be positioned at right angles to the direction of flow. The direction of the flow past the intake pipe may vary at different stream levels. If so, the effect of the flow velocity past the end of the intake pipe may cause drawdown or pile-up of the water level in the stilling well. Drawdown causes the water level in the stilling well to be lower than that of the stream; pile-up has the opposite effect.
The drawdown or pile-up of water in a stilling well can be reduced by attaching a static tube to the stream end of the intake pipe. A static tube is a short length of perforated pipe that is attach to an elbow on the end of the intake pipe and extended horizontally downstream. The end of the static tube is capped. Water enters or leaves through the perforations. A properly designed static tube eliminates the pressure due to the stream velocity, but it still reacts to water pressure according to the depth of water over the intake.
To provide low water levels, the outer end of an intake pipe may have to be located some distance from the stilling well and may be subject to damage due to bedload movement. In such circumstances, the intake pipe should be buried in a trench leading out to a perforated chamber embedded in the streambed.
The most suitable form of chamber is a length of concrete or plastic pipe one metre long and 0.3 m in diameter placed vertically with the top at least 0.2 m below the stream bed. The end of the intake pipe enters the chamber through a hole punched or drilled through the wall of the pipe at least 0.3 m below the top. If a socket pipe cutting is used, a circular piece of steel plate drilled with a number of 10-mm holes can be fitted to rest in the bellmouth opening. The bottom of the chamber is left open, while the cover plate, fitted with a lifting handle, is covered with gravel level with the stream bed.
The position of the intake chamber should be carefully referenced to two or more points on shore so that the chamber may be located easily for cleaning. The reference data should be entered on the Description of Hydrometric Station Form (AQU-01) and on the Water Stage Recorder - Station Record Form (AQU-06).
When seepage or channeled rainfall gets into the stilling well, or when water leaks out of the stilling well through the wall or bottom, the accuracy of water level trace decreases. These problems should be rectified.