C.2 Instrument Shelters for Stilling Wells
Protecting instruments against varied and severe climate and operating conditions is very important. In addition to shielding the recording equipment, an instrument shelter protects the contents of the stilling well. Shelters are built of metal, wood, concrete, or reinforced plastics. Some locations may require certain design standards and construction materials to conform with site surroundings.
Shelters should be built as securely as practicable and designed to discourage unauthorised entry. In extreme circumstances, shelters must be lined with steel or heavy wood, or be fenced for added protection from vandals or marauding animals. Posting a sign on the entry door that identifies the instrumentation, its owner, and its role in a water management program may help deter some visitors from trying to enter the shelter.
Safety of operators must also be considered. With some look-in shelters, one side can be levered up to provide a canopy and it must be well supported or blocked. Shelters that are elevated above ground level must have safe stairs and/or walkways to the entry door.
Shelters are generally classified by size and utility as look-in or walk-in. Which type to construct depends on expected station life, instrumentation, importance, and security concerns.
If the gauging station will be operated in winter, the shelter should be winterized to help control heat and condensation, and to protect the equipment.
Insulation may be needed; non-absorbent formaldehyde-free insulation is best. A properly insulated shelter also prevents the intrusion of troublesome insects.
Heating may also be needed, and is practical in insulated shelters. If a stilling well is heated the shelter must also be heated otherwise excessive frost accumulates inside the shelter and on its contents. Recorders tend to perform better in heated shelters.
Any shelter floor opening, below the float line pulley of the recorder, should have a bar installed across the opening in line with the axis of the pulley. This will prevent the float lines disappearing down the well after being lifted off the pulley.
All shelters must be watertight.
The look-in shelter (Figure C-4) is the most common, especially for smaller streams; it is mounted on top of a small-diameter stilling well. Although it can vary in size and shape, usually it measures about 0.8 x 0.8 x 0.6 m high, with a hinged front, and lid for entry. The lid is sloped to shed rain and snow.
The base, fastened flush with the well top, has a removable section, or a semi-circular opening to accommodate the float lines, which permits visual access into the well and access to the fasteners connecting the shelter to the stilling well pipe. Shelters, while secure in the closed position, must be easily removable in order to clear ice or install frost tubes.
Many shelters of this type utilize commercially available electrical junction box covers adapted to fit on the stilling well, or are of custom-made welded aluminium construction. These shelters must be ventilated near the top. The vent opening should be fitted with an insect barrier.
Walk-in shelters (Figure C-5) are the most convenient to work with. The average size is 1.6 x 1.6 x 2.2 m high with a standard-size entry door. A portion of the floor is removable for access to the stilling well. This trap door should be inspected for safety on each visit. Never leave it open, nor the well unprotected under any circumstances.
Walk-in shelters should be solidly positioned over top of the stilling well so that the floor forms a secure cover for the stilling well. Inside the shelter, a solid, level bench for mounting the recorder is positioned about 1.0 m above the stilling well. Positioning the recorder at this level makes it possible to access the well without interfering with the free movement of the float and float wire. Holes are drilled through the bench top and floor where necessary to accommodate the float wire. A trap door in the shelter floor provides safe, convenient access to the ladder within the stilling well. The flushing tank or portable pump connectors with connecting riser pipes and shut-off valve control rods are usually located above floor level and opposite the recorder mounting bench. Adequate ventilation is important. Plans are available.
