A stilling well is a vertical pipe-shaped enclosure placed vertically in or near the streambank. It is watertight, except for restricted access to the outside body of water. The purpose of the stilling well is to dampen water level fluctuation and protect the float sensor components. Figures C-4 and C-6 show various stilling wells and shelters.
Figure C-6. In-bank stilling well.
Site conditions and related equipment dictate the stilling well's construction materials and positioning. Proper design is required to meet the site's physical and climatic conditions, and to ensure constant passage of water flow. Refer to Section B.1.3 on site selection.
The size, shape, and materials for constructing a stilling well vary, but the well must be large enough to allow free and unobstructed movement of floats and counterweights. Space must be allowed for other components and equipment, such as flushing lines.
The well is connected with the body of water by intake pipes made of steel or thick-walled plastic (in-bank installation), or simply by hole(s) drilled directly into the well. All stilling wells require screen-covered vent holes near the top to allow humidity to escape.
Intake pipes used for transferring the water level of the stream must be positioned level and 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 pileup 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; pileup has the opposite effect.
The drawdown or pileup of water in a stilling well can be reduced by attaching a static tube (Figure C-7) to the stream end of the intake pipe. A static tube is a short length of perforated pipe attached 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.
The general arrangements of stilling wells and intake pipes are shown in Figures C-8 and C-9.
Figure C-7. Static tube for intakes.
A stilling well must be positioned vertically and secured sufficiently to eliminate vertical and horizontal movement. The bottom of the stilling well must be placed at least 0.3 m below the minimum expected water level. The top must be above the maximum expected water level.
Where stilling wells are installed to record continuous flow at measuring structures, a wet stilling well is attached to the wall of a dry well and the intake pipes are fitted with rodding eyes, shut-off valves, and an open-end upstand pipe. The top of the upstand pipe is set at precisely the same elevation as the invert of the measuring structure control. This arrangement permits setting zero flow on the recorder. A second, clear acrylic upstand pipe is bracketed to the dry well wall in front of standard gauge plate(s). This is the inside gauge; it extends to the top of the flume side wall. Figure C-10 shows the viewing pipe at the Jamieson Creek Flume.
