Drives in Flood Defence Systems

From New Orleans to the Netherlands, the world is full of places threatened by flood. And the age-old weapon in the fight against disaster - the water pump - continues to get more efficient thanks to the use of variable speed motors and the drives that control them.

The devastating effect on human life and infrastructure

The World Health Organisation estimates that, between 1998 and 2017, floods affected the lives of over two billion people worldwide. Besides the immediate danger to human life, flooding can devastate agricultural land (frequently located in floodplain territory) and wreak havoc on both the structural and infrastructural assets of built-up areas

And with increasingly intense downpours and rising sea levels both observed consequences of climate change, severe flooding is expected to become more common in at least the short and  medium terms.

Pre-pump technology was largely confined to the construction of earthworks, embankments, dams and diversionary canals. Then the steam engine allowed the Victorians in the East of England to operate powerful pumps so that below-sea-level land could be drained well enough for farming. Steam power gave way in the twentieth century to the might of diesel and then - from the 1970s - to electricity.

Efficient Floodwater Pumping Systems: Strategies for Unpredictable Water Management

Floodwater is by definition unpredictable. If its arrival - on a potentially overwhelming scale - follows a long dry period, pumps that have stood for some time at a standstill need to be activated instantly, powerfully and efficiently in order to start transferring large volumes of water (the world’s largest pumping station, outside New Orleans, can move 150,000 gallons of floodwater per second) at continuously fluctuating flow rates.

Different strengths of response to the flood event are achieved in the first instance by varying the number of pumps in action.

Multiple pumps are a must in flood defence systems (even in the smallest designs a duty pump must have a backup). To distribute work over a greater number of smaller pumps rather than fewer larger is the first step towards a safer and more controllable system. The smaller the pump, moreover, the less vulnerable it is to stress from problems with floating debris, vortices and trapped air.

After this, variable speed motors are a prerequisite if pumps are to engage with the distinctive dynamics of floodwater behaviour with optimum efficiency.

The direct starting (or stopping) of a floodwater pump is likely to cause water hammer, following the abrupt change in flowrate through the pipe. These hydraulic shocks shorten the lifespan of equipment and lead to leaks at joints and burst pipes. Using soft starters or variable frequency drives (VFDs) ensures a gentle acceleration of the pump’s rotation up to rated speed; with VFDs offering the added advantage of fully regulated motor speed thereafter.

Though desirable in principle, effecting an efficient speed regulation of floodwater pump motors can be a complex affair.

For example, while it makes sense, from an energy-consumption point of view, for a variable speed motor to run no faster than is strictly called for, pumps and pipes that deal with large volumes of floodwater may, depending on design, be more or less vulnerable to a build-up of sediment. Systems where this is a problem may find that, by running at a consistently high speed, sediment build-up is kept to a minimum - as, then, is the corresponding mechanical stress on the motors.
It is important, too, that pumps don’t overdo it. In an area like the Fens around the Great Ouse river, for example, water levels have to be managed to allow for navigation (leisure boats today where it was once commercial waterway traffic). A balancing act, in other words, must be struck between neither underdraining nor overdraining.

So, what’s the challenge for Water Pump System Designers?