Fighting the tide

In recent weeks a new feature has been constructed around the outside of the cricket pitch in Poole Park that is trying to deal with high ground water levels, rising tides and a legacy of drainage systems that are impossible to maintain.

The regular flooding of the edge of the cycle track causes a lot of problems for people enjoying the third of a mile loop around the cricket pitch. In order to explain the new work that has taken place, it is worth understanding the whole system and why this area is so wet:

• surface water drains under the ground take rainwater from surrounding roads and move the water to the smaller freshwater lake. This means during heavy rainfall a lot of surface water is directed straight to the lake and lowest point in the park.
• the drainage within the park is really old and originally worked by taking the water from the cycle track and cricket pitch, putting it in to a drainage pipe and feeding it to the smaller freshwater lake. Over decades these pipes have become full of silt and no long work, meaning the water seeps in slowly; during heavy rainfall they back up, causing flooding.
• the small lake feeds in to the larger lake via a pipe beneath the pathway. The large lake level is controlled by boards that allow the water to weir over the top of them, into a pipe that then runs under Whitecliff Road, keyhole bridge and to the Baiter sluice channel. On a high tide and with strong south-westerly winds the harbour water pushes back up this pipe, blocking the water in the lakes and causing all the various water levels through this system to rise.
• at the cycle track this means that the water cannot drain anywhere and flooding increases

The new drainage feature, a swale (a version of an open ditch) has been created to take the flood waters and retain it away from the cycle track, effectively moving the water out of the way of anyone wanting to walk on the path. Over the next couple of weeks the height of the cycle track will be raised as a new surface is laid, lending more help to getting the water away from the hard surfaces.

This is not a complete solution as there will always, and increasingly, be severe storm events linked to high tides and strong winds that inundate this area with flood water. At these times the swale will become full and the surrounding area will flood, as experienced over the weekend of 3rd/4th October 2020 (see photo below). The good news is that once the flood waters started to recede and the tide changed the flood around the cycle track quite quickly dried up, leaving the swale full and the path dry. It is likely that the swale will hold water all year round, drying up to some degree in the summer.

Sandbags have been used to create a structure at each end of the swale; Purbeck stone boulders will be placed on them to make more them attractive. Native wildflower seed will be sown along the length of the swale to introduce species that are used to growing in these wet conditions and provide a new area of meadow for next spring.

Once works to the swale and cycle track is complete its effectiveness will be monitored over the winter to see how it copes with water levels. A longer-term aspiration is to remove the tidal link at the Baiter sluice channel, which would prevent so much water coming in to the drainage system. For now we hope that the swale and surface improvements to the cycle track help to improve this area of the park for all its users.