Background In many situations it is desirable to remove under water sediment deposited without having to drain off the overlying water or to lower the water level. The sediment can be particles like gravel, sand, silt, organic particles or other particles. Water flowing in rivers, canals, tunnels or pipes may carry sediment. If the flow velocity decreases, sediment heavier than water will sink to the bottom and be deposited. This may take place in detention basins, intake basins or sedimentation basins in connection with power plants or irrigation plants, sand traps, in tunnels or tanks or basins in connection with process plants. It is often desirable to empty storage tanks of particulate materials by means of water or air. In the following description all such sediment basins are for simplicity denoted sand traps, independent of their intended purpose.

Sediment may cause high wear if permitted to flow with water through the turbine (s) in a power plant. It is a well known fact that turbines in some plants need to be repaired or replaced more than once a year due to such wear from sediment. Furthermore, canals and tunnels may be filled with sediment. Thus sand traps are often arranged in connection with hydroelectric power plants, particularly in areas where the rivers tend to carry large amounts of sediment. Still further, it is commonly desirable in connection with irrigation plants, to be able to separate at least the coarser fraction of particles at the water intake to such plants, so that the irrigation canals do not become clogged or plants sprinkled with arenaceous water.

Sediment that deposits in sand traps may range from rocks and gravel to very fine- grained particles. In hydroelectric power plants a normal criteria for particle sizes to be separated, will be from 0.15 to 0.5 mm.

Known techniques for removal of sediment comprise draining and mechanical excavating or flushing by lowering the water level so that the flow velocity over the sediment increases. Both these methods have the disadvantage that the water supply to e. g. a power plant will be interrupted. Production interruptions occur and in addition there are costs in connection with the run-down and the start-up of a power plant.

Furthermore the conflict between desired production and removal of sediment will often lead to a deferment of the latter, thereby resulting in increased turbine wear.

Other techniques comprise flushing through holes in the bottom of the sand trap down to a flushing channel. These holes may optionally be provided with closing mechanisms (Bieri patented system). The disadvantage with this system is that it requires motors, power transmission and movable parts, that may be subject to shutdown.

A further system, known as"Serpent Sediment sluicing system"has been developed by SINTEF, Trondheim. This system is characterized by a longitudinal slot over a flushing channel, temporarily shut by a flexible hose. The hose may be emptied or filled with water, and will, as it floats up or sinks down, respectively open or shut the flushing channel. Disadvantages of this method include the costs of the flexible hose, its vulnerability to extraneous objects and a comparatively complicated operation.

It has previously been described that a pipe with a longitudinal and at least partly continuous slot at its underside, is suited for sucking up sediment in such a way that a highest possible sediment concentration is obtained, without risk of sediment blocking the outgoing pipe, hose or channel. Sediment which is sucked up, will be the sediment deposited over the slotted pipe. This slotted pipe was described in Gemini, No 3, Dec.

1994, p 20-21, published by SINTEF/NTNU. The disadvantage with this known slotted pipe is that if the layer thickness of sediment over the pipe is low compared to the pipe length, sediment will only be sucked up at one distinct point, not uncovering the entire pipe as desired.

Objectives It is an object of the present invention to provide a method for the removal of sediment from sand traps and other locations where sediment becomes deposited. It is a further object that such sediment may be removed before the sand trap is filled completely with sediment, in a manner such that the water supply need not be suspended and such that the consumption of flushing water is as low as possible.

It is a further object to be able to perform the method according to the invention in a manner that is inexpensive and such that personnel operating the method do not need technical education or lengthy training.

It is a still further object to be able to adjust sediment concentration in order to optimize the concentration according to the water level in the sand trap and the nature of the sediment.

It is a further object that the method be reliable and that the need for maintenance be low.

The invention The objects are achieved by the method according to the invention, which is defined by claim 1.

Preferred embodiments of the invention are disclosed in the dependent claims.

The invention is described in the following in more detail with reference to the accompanying drawings.

Figure 1 is a longitudinal section of a sand trap where the method according to the invention is implemented, Figure 2 is a cross section of the sand trap of Figure 1, Figure 3 is a longitudinal section of a sand trap where an alternative embodiment of the method according to the invention is implemented, and Figure 4 is a longitudinal section of a sand trap with still another embodiment of the invention.

Figure 1 shows a longitudinal section of a sand trap 1 with a narrow ditch 2 in the bottom 3. Arranged in the ditch 2 is a slotted pipe 4 with an outlet 5 at one end and an elevated opposite end 6. A valve 8 close to the outlet may be opened and closed to adjust the operation of the slotted pipe 4. The drawing also shows a supply conduit 12 through which water may be added to the pipe 4 downstream of the sediment, by opening a valve 13 to a greater or lesser degree.

Figure 2 shows a cross section of the sand trap shown in Figure 1. Over the narrow ditch 2 is arranged a grating 7 or the like to prevent larger objects, like branches from falling into the narrow ditch.

Figure 2 shows two slotted pipes 4,4'arranged in the ditch 2 one over the other. These pipes may either be used simultaneously to increase the capacity, or the second one may serve as a fallback in case the first one should cease to function. Figure 2 furthermore shows that the bottom 3 of the sand trap slants downwards to the ditch 2, which constitutes a preferred embodiment of the invention.

Figure 3 shows a slotted pipe 4 with outlets 5,5'at the ends thereof, so the sand trap may be emptied alternatingly both ways. At each end 6,6'the slotted pipe is elevated above the upper level of sediment, and it may be closed by valves 11, 11'.

Figure 4 illustrates several partly separated sand traps with separate slotted pipes. This represents a relevant solution if a sand trap has large dimensions and particularly if it is desired that the sand trap not be made very deep. Furthermore, by utilizing several slotted pipes operational reliability will increase.

The design ensures that the slotted pipe will become covered by sediment along its entire length. This is required to obtain hydraulic conditions such that the pipe sucks up sediment along its entire length and not only at distinct points. For the same reason it is convenient to perform the operation at certain intervals. The slotted pipe will typically be operated by opening the valve close to its outlet and possibly by simultaneous operation of one or more pumps (not shown) to increase suction force in the pipe.

The dimensions of the ditch may vary within wide limits depending on the area of use and the dimension of the sand trap. A common width can be from 0.25 to 1 metre and the depth typically from 1 to 5 metres. The dimension of the slotted pipe is adapted to the need.

It is preferred that the upstream end of the slotted pipe is elevated to a level beyond the level of the sediment. Consequently the existence of a starting point above all deposited sediment is ensured.

It will often be convenient to utilize the height difference between the water level in the sand trap and the outlet 5 as a driving force for the suction of sediment into and through the slotted pipe 4. By utilizing this height-or pressure differential the operational reliability will increase as the operation may continue even if the supply of electric

power is disrupted. In addition the outlet pipe may be designed completely smooth which reduces the risk of clogging. One or more pumps may be used to obtain a higher driving force. The pressure difference required to suck sand out from the sand trap is typically 1-10 metres.

The slotted pipe is suited to transport particles up to a certain size. It is thus not convenient for particles beyond this size to be collected in the ditch. It is therefore preferred to cover the top of the ditch in substantial parts of or all of its length with a grating that holds particles of sizes over a certain limit..

To obtain protection against failure of the system, it is convenient to arrange two or more slotted pipes in one and the same ditch. Should one of the pipes become clogged operation may continue by means of another slotted pipe. In a situation where it is required, more than one pipe may be operated simultaneously to increase the capacity, e. g. during flooding or other extreme situations.

In order to be able to control the sediment concentration within the capacity of the outlet 5, it is convenient to provide the slotted pipe 4 with a water supply 12 at its downstream end. The sediment concentration at the outlet is controlled by the amount of water added.

While normally all sediment will be led in one direction only, there may also be situations where it is convenient to lead the sediment in alternating directions of the slotted pipe. To obtain such functionality a system as depicted in Figure 3 must be established, where both ends of the slotted pipe are connected to elevated pipe sections 6'and 6 respectively, and a lowered outlet 5 and 5'respectively. By means of valves and in cooperation with an appropriate"driving source", the direction of the sediment suction is determined.

While the description above refers to sand traps, which represents the most common situation, it is understood that the invention may also be utilized where the surrounding fluid is not water, but e. g. oil or gas.