SUDS Trials led to “Twitchers'” Event

Reed Warbler

A successful trial in Redditch using vortex flow control devices instead of penstocks to control discharge from balancing ponds proved so successful that rare nesting birds moved in – causing an influx of local ‘twitchers’!


*This is one of a series of case study examples shared by Local Authorities on Engineering Nature’s Way to outline the experiences of local authorities in introducing SUDS principles in their areas.

Project Outline

The trials clearly demonstrated that using vortex technology reduced flood risk in the ponds and offered an opportunity for less land take.  Most importantly there were major costs savings due to significant decreases in maintenance intervention.  Left undisturbed, the ponds attracted an influx of wildlife and biodiversity.

Local Authority Leadership

Clive Wilson is Redditch Borough Council’s Land Drainage Officer. Working in the  Land Drainage Division since the early 1990s, both in capital works and with the maintenance teams out on the ground, he has gained an all round perspective of how the council’s surface water control demands constant updating. And he also advocates the need to look beyond authority boundaries.


“Redditch is a mostly urban area with some interesting features which must be appreciated. The sub-soil is almost entirely heavy clay, except for small isolated areas. For Redditch this means that we have to concentrate on storage and retention as primary flood protection measures. The Borough has around 100 km of water courses and actually owns 41 km of them.  So we feel a keen responsibility for the quality of water in them; there is no divided ownership and good practice is felt, from where the rain falls, and all the way through our ponds, streams and rivers.

The Role of the Local Authority

Outlining the problem:

The basis of our surface water control policy is balancing ponds, as we have almost no opportunity to achieve infiltration to ground water; we have, at the moment, around 30 ponds large and small as the key to our flood protection policies.

Up to 2008, discharge from these was all controlled by penstocks, with a screw thread controlled gate. These required a very high amount of maintenance as the narrow orifice required for flow restriction was susceptible to blocking by natural and man-derived detritus.

The maintenance teams were constantly checking and unblocking on a routine basis, as well as needing to be available where non-routine blockages occur, especially at times of high rainfall.  There were also occasions when a blockage was inevitable and flooding occurred; it was our responsibility to sort it out.  In bad weather, there were of course calls from nervous householders anticipating a flood.

All of this was costly and the service was getting too expensive. Additionally, whilst we avoided major flooding in Summer 2007, we started to monitor the ponds more closely up to the end of 2008.  It was clear our control measures were eventually going to be overtopped more often.

The solution

We decided to look at vortex flow control measures as a serious alternative. They are really a fascinating and elegantly simple design.  They provide good control while reducing the chance of blockages because the inside diameter of the control device is several times larger than that of the equivalent penstock arrangement. While the water flow is restricted by the self-pressurised air core created by the vortex, leaves, plastic bags and so on just go straight through the middle of the device and out the other side.

In trials on two closely monitored ponds, the vortex control devices were substituted directly for penstocks, and we quickly noticed several major improvements.

  • The average water level was something like 30% less in the pond. Less water reduces flood risk, but also means you can reduce the volume of the pond if required, so taking less land; as we get more crowded, less land take is increasingly important – or as in our case, make existing ponds effectively larger without any increases in physical size.
  • We found that we needed far less maintenance intervention, and could rapidly reduce the monitoring schedule. We had no call outs, either; maintenance costs were dramatically reduced, within a few months. There was no need to grease the penstock mounts to ensure they worked in an emergency, in fact there is nothing moving, and no power requirement.

Natural response

It was an unexpected ‘twitcher’ event, which confirmed the extent of the low maintenance success.  One of the converted ponds attracted a pair of reed warblers which nested on the edge of the pond. The species hadn’t been seen in our region for a long time – normally, nesting reed warblers are only seen in East Anglia.  So we had twitchers arriving from all over.

Key Lessons

  • Because the pond was significantly less disturbed (except by birdwatchers!), wild life was returning and diversifying.  This was an unplanned, but important outcome of the trials.
  • As a result of the success of the trial, we are committed to reviewing the performance of all penstocks and other control mechanisms on our ponds, and replacing them in rotation, reducing our maintenance budget and call out coverage systematically to offset the capital expenditure. The devices are extremely easy to insert using the same pipelines as the penstocks.
  • The success of the programme, using a SUDS-friendly device, has spurred us to look at other SUDS techniques to assess whether they could also have similar benefits and build these up as best practice. An example is assessing the existing field drainage in the rural parts of our Borough.  It serves a definite function and should be kept in proper condition, not just allowed to silt up because someone builds across the discharge point.
  • Another consequence, and following the Flood and Water Management legislation, is that we and our neighbouring local authorities are more aware that we are not isolated. Our approach to surface water also affects our neighbours. So we are talking and aware of each other’s policies; such increased awareness can only be beneficial to surface water control and flood prevention in the country as a whole.