Why Winnall Moors has flooded in recent storms

Winnall Moors nature reserve is upstream of Winchester. The water channels are managed carefully to allow as much water through as possible, however bridges, old mills and properties create a bottle neck and limit the amount of water that can flow through the city. If the volume of water becomes too great then extensive flooding could happen. Winnall Moors is upstream of the bottleneck, and by allowing the floodplain to function, flood risk downstream is reduced.

The southern section of Winnall Moors, owned by Winchester City Council and managed by Hampshire & Isle of Wight Wildlife Trust, usually open to the public, has been closed due to the high-water events we have been experiencing over the last month. The path network has become hazardous, with some areas washed away or damaged.

Managing Winnall Moors as a haven for wildlife and a defence against flooding

We work closely with Winchester City Council and the Environment Agency to ensure that Winnall Moors can withstand flooding and take on water before it reaches the city.

At the reserve, we have completed a floodplain restoration project to improve the reserve for wildlife and reconnect it to the River Itchen. We rebuilt sluices, opened carrier ditches and reinstating extensive grazing to the floodplain. We also removed flood embankments along the river and raised the riverbed in places.

This work benefited a variety of species, including salmon, sea trout, otter and water vole, as well as the various habitats that Winnall Moors hosts, such as the flower-rich fen grassland, reedbeds and alder woodland. It also reconnected the Itchen with its floodplain, taking some of the intensity out of flood events experienced by  the city.

How the flooding is expected to impact on wildlife

The recent flooding events at Winnall Moors are likely to have impacted the wildlife there. Water voles will have been displaced from their riverbank burrows, so will be dependent on thick vegetation such as tussock sedges and reedbeds to build aerial nests. They are also more vulnerable to predation if they can’t keep out of sight. Plant species will also be affected.

Species-rich floodplain meadows are adapted to cope with winter flooding, when plants are dormant. However, when the flooding is so prolonged that the oxygen in the soil is used up, many species will start to die. As the winter has been so mild, there is a risk that the flooded soils at Winnall Moors started to run out of oxygen in February, whereas in a normal, colder year they might have been safe until March.

Flood events that occur between March and September are much more damaging because when the soil is warm, it runs out of oxygen very quickly. Additionally, where a hay cut cannot be taken because a site is too wet, we see a decline in species richness because tall, coarse species tend to smother the finer ones beneath.

Within the waterways high flows can both carry silt away and bring it in, which affects species that like to burrow into these muddy areas. This includes species such as burrowing mayfly nymphs, fine lined pea mussel and brook lamprey.

The reserve has also seen increased numbers of wetland birds in the last few weeks, such as mallard and gadwall, a short eared owl has been seen on the northern meadows, probably brought in with the cold weather and hunting opportunities with the flooded ground. 

How we are protecting affected species

Flooding is a natural and expected feature of floodplains, nature is adapted to this.

To help wildlife cope with the flooding, we have ensured that a thick margin of bankside vegetation is in place to help animals avoid being swept away or picked off by predators. This same thick margin will also slow the flow of water, so it can gently flow over the side of the channel without causing damage to the edge of the riverbank.  

What can be problematic and damaging is increased impacts, such as pollution being carried into an already stressed waterway. Soil washed off farmland, hydrocarbons swept in from roads or sewage released from overloaded water works. This can all impact wildlife and have lasting effects.

Our highly fragmented nature rich areas may be some of the last areas where rare wildlife is clinging on. Winnall Moors holds the largest and best condition area for marsh marigold and crested dog’s tail grassland in Hampshire. Too much flooding, too often could threaten this special wildflower community.

“We want to see more high quality, better connected wildlife habitats, “ Says Rachel Remnant, Reserves Officer for Hampshire & Isle of Wight Wildlife Trust at Winnall Moors. “Our floodplains have multiple benefits for biodiversity, carbon storage, food production and natural flood management. Let’s look after them and restore the natural processes that help them function.”