C.3.3 Operation and Maintenance
The operation of stilling wells is affected by seasonal changes, runoff patterns, and weather. Work performed on or around stilling wells is subject to safe working conditions. Operating staff should be aware of safety requirements and look for and correct any unsafe conditions before beginning hydrometric work.
Open water season is that part of the year when no ice is present on the watercourse. Each time the technician visits the station, the shelter and site must be inspected for any potential problems such as damaged flooring, broken or bent intake pipes, inoperative shutoff valves, and insecure ladders. Any observed faults should be repaired immediately if possible, or scheduled for repair at the earliest opportunity. Check for obstructions in the well that may impede the float, float line, or clock weight travel.
Check the well for sit deposition by sounding with a weighted tape; compare this measurement to the original clean depth of the well.
Note: Remember, the lower intake pipe may be only 0.15 m above the well bottom.
Promptly remove excessive silt deposits by using a pump and discharge hose to place the accumulated silt into a suspension and to build the water head inside the well. Discharge the water head out through the intake pipes to remove the suspended silt. Continue flushing until the plume of silt being discharged through the intakes is no longer visible. Stilling well system accuracy and reliability depend upon the free flow of water through the intakes.
Flush all intakes immediately before, during and immediately following seasonal high stage. For the rest of the season, flush the intakes during each station visit, or as required. Frequent flushing may be necessary where silting is an obvious problem.
Flush all intakes prior to obtaining a discharge measurement. Make sure the intake valves are open after flushing and before you leave the site. Check to verify that the float, float lines and clock-weight line are not tangled together and see that they are free from obstructions. Also ensure that the float tape or line has not been knocked out of position on the float pulley.
The open water season described above excludes ice conditions normally associated with spring start-up and fall shut-down.
The operational `open water' period for many hydrometric stations is April 1st to October 31st, whether ice is present on the watercourse or not. However, before and after this period the technician must undertake both pre-season and post-season activities.
Pre-season activities involve clearing ice from the stilling well, repairing winter damage, and activating recorders.
Many stilling wells are constructed with 12-inch-diameter (305-mm) ABS water pipe, sealed at the bottom with a glued-on pipe cap. Ice removal from these relatively small diameter wells can prove quite a challenge even when the ice is no longer attached to the wall of the pipe. Preparations for ice removal must take place before freeze-up in the fall (see below).
If ice is frozen to the pipe wall, start-up can be delayed, or water added to the top to speed melting. The judicious application of heat to warm the pipe sufficiently to detach the ice block is suggested, if the stream ice is beginning to break up.
If the ice block is freed from the sides of the stilling well, it may be removed by lifting, although it may weigh up to 40 kg. The shelter must be removed from the stilling well first.
In all location subject to winter freeze-up, ice may present problems if it occurs before the end of the operating season. Regardless of the operational period, all recording stations require some degree of winterization if they are to be easily reactivated on time in the spring. Some preventive measures taken at the end of the season will make the following season start-up, much easier. The usual problem found on arrival at station start-up is a large block of ice remaining in the stilling well. If the float has been left activated, it will almost certainly be stuck in the ice and will have to be freed before the ice can be removed. The following shut-down procedures are recommended at the end of the season.
Remove the float, counterweight and float lines (carefully coil and secure lines to avoid kinking) from the well.
Through a 15-25 cm diameter metal or thick plastic disc with a 12 mm hole drilled in the center, pass a 10 mm polypropylene rope through the center of the disc, tie a figure of eight knot, and lower disc to the bottom of the well. Align the rope to hang down the center of the well and tie it off at the top.
Note: Painting the shelter and exposed well exterior black, will promote earlier ice detachment from the well wall. Also, if the recorder is to be left on site over the winter, detach the chart roll from the take up roller, rewind on supply tube and tape. Wind negator spring and leave the clock running.
Water which freezes in the stilling well and/or intakes during the winter period, presents serious problems. Continuous operations in cases where there is no ice free zone in the stream are extremely difficult.
Ice Formation: If the intake pipes are below frost level and the end of the lowest intake does not become encased in ice, the stilling well can continue to operate. Formation of ice within the stilling well can be prevented or controlled with an insulated frost floor, heaters or an oil cylinder.
Fully exposed metal wells present the greatest problem. At freezing temperatures, ice forms quickly around the inside wall of the well and on the inside water surface which traps the float. Further freezing produces a cylindrical plug of ice.
Ice Effects on Floats: Warming temperatures may free the ice plug, but if the float is trapped within it, records of the response to water level changes will be inaccurate. For an accurate record, the technician must free the float and remove the ice from the well.
Severe damage to the water level recorder can occur if both the float and counterweight become trapped in an ice plug. If the water level below the ice plug recedes, the weight of the ice will place extreme stress on the float pulley shaft and float wire above.
Frost Floor: A frost floor is an insulated sub-floor within the stifling well positioned below the frost line of the surrounding ground. The cold air spilling into the well is partially confined to the area above the frost floor, while the warmer air from below can not escape easily. This warmth retards the formation of ice within the well. Holes cut in the frost floor allow the float and counterweight line to pass through.
Wells in the stream bank are normally encased with back-filled earth, which provides some insulation. However, ice plugs can still form in the well if cold air spills into it. A frost floor may be enough to stop ice from forming as long as the water level remains below the frost level of the surrounding soil.
Frost Tubes: Frost tubes have proven quite effective in overcoming ice in both exposed metal and earth-protected stilling wells. A frost tube is an open-ended cylinder, usually 25 cm in diameter, fastened vertically inside the stilling well. The bottom end is elevated slightly above the well bottom; the top end extends to the well top, or to an elevation above the maximum expected winter water level.
At stations where 305 mm ABS stilling wells have been installed, the well becomes the frost tube and a quantity of oil sufficient to extend below the frost line or ice thickness is poured into it. A 0.1-m column of oil in this diameter of pipe will require 7.3 litres of oil.
To prepare a frost tube, a non-toxic oil is poured into the top of the cylinder. The column of oil that forms above the water forces the water surface in the cylinder below the surrounding water level in the stilling well. The recorder float is then positioned in the cylinder where it will respond to the variation in the oil surface level.
Since the oil surface level stands higher than the true water level, a compensating correction must be made to the record. The degree of correction will be proportional to the amount and specific gravity of the added oil.
To quantify this initial correction, reinstall the float assembly and circle the position of the pen. Read the graduated float tape or mark the beaded wire opposite the recorder base, and read the outside gauge. Remove the float from the tube before pouring the oil. Reinstall the float, circle pen and repeat readings as above. Differences in the relationship between pen/floatline and outside gauge is the correction for the quantity of oil and its specific gravity. Set the recorder pen to the outside gauge for this and subsequent station visits.
Oil Specification: An acceptable oil is `Esso Bayol 35M' with a specific gravity of 0.815. This oil is tasteless, colourless, odourless, and is used in food processing. Do not let the oil spill from the frost tube into the stilling well, because it causes bacteria to grow on the water surface within the well. Non-toxic oil must be used. Kerosene and other petroleum oils must not be used for environmental reasons.
Discontinue using the frost tube at spring break-up time. If the cylinder is not designed and installed as a permanent fixture, the oil and cylinder must be removed from the stilling well before the rising water level forces the oil over top of the tube. Frost tubes do not produce heat nor do they require any heat to be effective.
Prior to spring break-up, stilling wells must be cleared of winter operations fixtures and any ice remaining in the well. Where the intakes are blocked with ice, the use of a steam generator may be needed to remove ice from the intakes and the well.
Inspect the well for possible structural damage that may have occurred during the winter season. Check especially for vertical movement caused by frost heave. If signs of frost heave are evident, give special attention to the intake pipes, as they may be damaged at the point of entry to the well. Inspect the outer ends of the intakes and remove any attached ice from them.
Clean deposited silt from the stilling well bottom. Remember, in the next few months water will carry most of the annual silt deposit to the well. Flush all intake pipes and make sure they are free and clear of obstruction. When flushing the intakes, observe the discharge from the upper intakes. Finally be sure the intake valves are open before leaving the site.
In general, it is essential that the stilling well and the intakes are in good working order if the well is to provide accurate records through the upcoming spring runoff season. Corrective action for potential problems is far more difficult during spring when the well is full and the intakes are all under water. Under these conditions, repairs or maintenance work may not be possible. Many water level records are lost as a result of improper preparations prior to spring.
Ensure that the float and the suspension lines move freely and that they are clear of any possible obstruction.
During high flows the water surface in the well appears to be excessively turbulent. To dampen the oscillation, close one of the upper intake valves and look for improvement. Slowly open the valve until you achieve a tolerable level of disturbance. For in-stream installations, when the water level exceeds the elevation of the top vent holes, block the holes to avoid further velocity turbulence. Note these actions on the station record or log.
If the water level should exceed the upper limit of float travel in the well, cut a hole in the floor to allow passage of the float. This measure will provide additional distance for float travel which will in turn add to record collection. Manually pull the float through the hole in the floor to its upper limit of travel. Check to see if the counterweight hits the well bottom. If it does, shorten the float line proportionately. Remove all buoyant material from the floor to prevent it from obstructing the float line or restricting the float's return passage through the hole when the water level recedes.
Inspect the streambank for possible cave-in or excessive erosion. If the stilling well is endangered, arrange for emergency corrective action. Until proper corrective actions can be taken, the stilling well should be secured with a strong cable which can be anchored to inshore trees or other stable objects. This will prevent tilting or loss of the well.
Figure C-13. Stevens A-71 recorder with beaded float wire.
