Fluvial Design Guide - Chapter 11

River and canal structures

11.5 Mechanical and electrical design

Mechanical plant for river and canal structures is usually associated with one of the following applications:

  • locks for accommodating changes in the height of navigable waters;
  • gated flow control structures;
  • gravity inlets and outlets connecting to pipes, culverts or channels;
  • pumped inlets and outlets connecting to pipes, culverts or channels;
  • gated flood control barriers.

Details of the types of mechanical plant generally used in these applications are given below. For specific design considerations, please refer to the publications highlighted amongst the references at the end of the chapter.
 


11.5.1 Gates and similar

Vertical lift gates can be wheeled or sliding – of steel or composite construction – and vary in size from small sluice gates or penstocks for controlling water into pipes up to major steel constructions weighing in excess of 100 tonnes (see Figure 11.6).

Other variants of vertical lift gates include double leaf hook gates as installed on the Cardiff Bay barrage.

Figure 11.6 Examples of gates of varying sizes

Set of small bottom pivoted flapgates for river regulation. The flaps are raised when river flows are low, and lowered in flood conditions.

A large vertical lift gate which is lowered to prevent high tidal levels from flooding the city of Hull.

Mitre gates are the traditional installation for river and canal locks. Traditionally they are made of wood and hand operated (see Figure 11.7), but on larger and more frequently used locks, these have developed into steel construction and powered operation (see Figure 11.8). They are fitted with sluices within the gate or in the abutments for lock filling and emptying.

Figure 11.7 Traditional wooden lock gates

Figure 11.8 Modern steel lock gates

Sector and radial gates are of superficially of similar design, except for the orientation of the axis of rotation – vertical for sector gates (see Figure 11.9) and horizontal for radial gates (also known as Tainter gates) (see Figure 11.10).

Two pairs of sector gates can be used to form a lock, and sector gates are a more modern solution to dock gate design. They have advantages including low operating forces and the ability to withstand bidirectional head, but require more extensive civil works.

The drum gates on the Thames barrier are a variant of radial gates.

Figure 11.9 A pair of vertical axis sector gates used for a lock entrance

Figure 11.10 Automatic horizontal axis radial gate for automatic control of upstream river level

Other types of gate-like equipment include stoplogs, rymer weirs and inflatable weirs.

Stoplogs were traditionally made of wood as indicated by the name and are lowered into an opening registering in grooves in the walls to isolate a gate or structure to allow working in the dry. Larger examples are now made of steel.

Rymer weirs are an historic form of adjustable weir which still exist in some places despite serious health and safety disadvantages. A number exist on the River Thames, but are being phased out; they are mentioned here for completeness. An example at Northmoor Lock on the River Thames is described on http://www.the-river-thames.co.uk/locks.htm.

Inflatable weirs are of relatively modern design and made of a composite rubberised material in the shape of a long tube, which is inflated and deflated with air or water in order to control the crest height. They are common in Japan but have yet to be adopted in the UK.
 


11.5.2 Operating equipment

Operating equipment comes in many forms to suit the wide range of equipment being operated. It can be grouped essentially into:

  • manual operation by handwheels, cranks, mitre gate balance beams and other means;
  • electrical motors driving mechanical equipment such as winch drums;
  • electric motors producing hydraulic power, utilised via cylinders for linear motion or hydraulic motors for rotational motion.

The final movement of the gate leaf can be achieved by direct mechanical linkage, hydraulic cylinder, rope or chain.

11.5.3 Trash handling facilities

When trash is a problem it either has to be repelled so as to pass further downstream by such methods as booms, bubble barriers and other techniques, or it has to be collected. Collection normally involves a static screen cleaned by manual or mechanical means, or a moving screen that is cleaned automatically.

11.5.4 Fish provisions

In a similar way, fish either have to be deterred – for example by preventing them entering intakes – or facilitated in their movement upstream or downstream.

Deterrence can be by screen or more elaborate systems involving ultrasonics, lights, acoustic effects, drum, electrified, etc.

Facilitation of movement can be by fish ladders or even a fish lift. For example, the Cardiff Bay barrage has a large fishpass aimed at facilitating the movement of migratory fish (http://www.cardiffharbour.com/harbour/barrage/fish%20pass.html).
 


11.5.5 Pumping plant

Pumping equipment for drainage purposes is normally either a centrifugal type or using an Archimedean screw. Individual pump flow rates may vary between a few litres per second for minor drainage works to pumps with capacities of several m3/s.

Centrifugal pumps

Centrifugal pumps are generally considered to be any pump with an impeller producing radial or mixed flow, although axial flow pumps are also often referred to as centrifugal type.

The output flow from any centrifugal pump depends on the head against which the pump is discharging. The pump therefore has to be selected to suit the particular head and flow requirements. In most cases a centrifugal pump delivers its maximum head at zero flow. As the flow increases so the head reduces, and the pump operating efficiency increases until the peak efficiency is achieved; this is known as the best efficiency point (BEP). If the pump flow continues to increase, then the efficiency begins to reduce. If the flow is allowed to increase further as a result of falling head, the net positive suction head (NPSH) required by the pump begins to increase to the point where the NPSH required exceeds the NPSH available. At this point cavitation occurs, resulting in damage to the pump impeller.

The selection of a centrifugal pump must therefore consider the hydraulic conditions.

Figure 11.11 Typical pump performance curve

An example of a typical pump performance curve for a centrifugal pump showing how the head varies with flow.

The efficiency curves are also shown on the H v Q curve.

The four lines represent different impeller diameters.

The other curves show the NPSH and power absorbed by the pump.

If the pump is required to work over a very large range of heads, consideration must be given to throttling the discharge flow either using a control valve or resorting to a variable speed drive. Throttling wastes energy and is generally used only in exceptional cases. Where there is a wide variation in pump head, a variable speed drive is often used. This is achieved by changing the supply frequency of the driving motor using an electronic inverter. The pump flow varies proportionally to the change in speed, and the pump head varies as the square of the change in speed.

Figure 11.11 shows a typical pump performance curve.

Pumps can be deployed in either a dry well or a wet well. Wet-well pumps, as their name implies, are installed directly in a pump chamber. Dry well installations still require a ‘wet’ inlet chamber, but the pumps are installed in a dry chamber and have piped inlets from the adjacent inlet chamber. Figures 11.12 and 11.13 show examples of dry well pumping installations.

Figure 11.12 Dry well pumping installation

In this example, there are vertical shaft pumps driven through drive shafting from vertical axis motors at ground level.

Figure 11.13 Dry well pumping installation

In this example, horizontal split-casing pumps are directly coupled to their driving motors.

Over the last 20 years, submersible pumps – where both the pump and the driving motor are submerged in a wet well – have been widely used, particularly for the smaller flow rates.

For larger flow rates (generally in excess of 1 m3/s), vertical wet well type pumps are normally used. In such installations, the pump impeller and volute are submerged and supported by a pipe which acts as the discharge pipe through which the pump drive shaft passes (see Figure 11.14).

The pump motor is located at the top of the discharge pipe and bend, normally at ground level.

Although wet well installations usually have a lower capital cost, they are more difficult to maintain. Dry well installations have the advantage that the pumps are always accessible and, after closing isolating valves on their suction and discharge mains, they can be dismantled insitu without having to remove the entire pump.

Figure 11.14 Wet well pumping station

Typical cross section through a wet well pumping station

Centrifugal pumps are not self-priming and require the pump volute to be filled with water before starting. Having the pump submerged ensures it remains fully primed and avoids having to provide a separate system for priming the pump. Failure to prime a centrifugal pump properly is one of the most common problems associated with pumping plant. A centrifugal pump is not capable of operating dry – it must always have water available to pump.

Other potential problems with centrifugal pumps can be as a result of debris in the water. It is essential to provide adequate protection to prevent the ingress of weed and other material that can get carried into pump intakes such as wood and plastics. Smaller pumps are more susceptible to debris, as the water passages through the impeller are smaller, so are more likely to get blocked – although any pump is at risk. Normally, as a minimum, a bar screen with 25mm spacing should be provided, with the facility to rake it periodically to remove any build-up of debris. Where weed or other debris is a known problem, then finer screening should be employed – possibly with automatic cleaning such as a rotating band screen, clog-resistant ‘vee-wire’ screen or similar.

In most instances, pumps are driven directly from electric motors. Where a mains power supply is not available, one option is to provide a diesel generator to supply electrical power. Alternatively the pump could be driven directly from a diesel engine, rather than having a diesel generator and an electrically driven pump. In special situations, other methods of pump drive such as hydraulic motors can be used, although these are not common on drainage applications.

Archimedean screw pumps

The Archimedean screw pump is probably one of the oldest type of pump and consists of a large screw located in an inclined channel (see Figure 11.15). The channel is usually cast concrete but, for smaller pumps, can be steel. The main advantage of the screw pump is that the flow remains almost constant, irrespective of the change in head. The flow is still proportional to the speed of rotation.

Figure 11.15 Archimedean screw pumping station

 

A screw pump can also operate even when there is little or even no water to pump. For drainage applications this makes the Archimedean screw pump an ideal choice. The main drawback of this type of pump is that their discharge head is limited. A head in excess of 10m is unusual due to the span of the screw required for high-head applications. Where higher heads are required two-stage lifts using two pumps in series can be used.

The main disadvantage of screw pumps is the size of the installation and the cost, which is usually significantly higher than the cost of a centrifugal pump of the same head and flow.
 

 

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