An earlier blog provided an overview on the work the South East Rivers Trust (SERT) is carrying out in the School Stream catchment to identify potential Natural Flood Management (NFM) projects alongside partners in the Medway Flood Partnership and EU Interreg North Sea FRAMES Project.
Our approach has combined initial desktop mapping exercises, catchment and site-level walkovers with landowners, and consultation with local individuals and groups, to understand the flooding processes in the School Stream. From this work we can determine how best to work with natural processes to provide a flood risk benefit to communities downstream at Headcorn and further downstream.
We’re very excited to tell you about a project which has been delivered in the School Stream catchment, which will help slow down and store floodwaters in the headwaters of the catchment, easing pressure on areas downstream in Headcorn. The project will also provide significant benefits to the local ecology, as well as create a beautiful spot for the landowners and passers-by to enjoy.
The site is something of a mystery, possibly an old quarry which had over time become overgrown, and the owners were at a loss as to what was best to do to improve the area. Due to the dense, closed canopy there was very little in the way of ground flora. A survey carried out by SERT’s ecologist determined that the area was of limited ecological value, with the ground surface composed almost entirely of dry and decaying leaf litter with occasional nettles and thistles.
Following a walk over the site with the landowners a plan was devised: by thinning out the trees and excavating to the clay base it would be possible to restore the pond which had been dry for many years. Permanent and semi-permanent areas of water would provide valuable habitat and immediately improve the ecological status of the site, whilst also providing additional flood storage. It was hoped the project would be a beautiful example of the multiple benefits of natural flood management, which the landowners could also enjoy.
How does it work?
The simplified plan below shows how the scheme is designed to store and slow down the flow of water during heavy rainfall events. Most of the time the flow within the School Stream will be left to continue along the watercourse as normal.
However, during heavy rainfall events, when the level in the School Stream can increase rapidly, a proportion of the flow will be diverted into the newly restored pond. The water level is allowed to build up in a controlled manner, storing up to approximately 600m3 of flood waters. Once the flood peak has passed the flood waters are slowly released and the water level falls leaving a smaller area of permanent/semi-permanent water.
Delivering the project on the ground
Work began in November 2019 to avoid bird nesting season, starting with clearance of a number of fallen willows and stacking of deadwood to create habitat piles. Care was taken to reserve any trees which could have potential as bat roosts.
Once the trees were cleared, excavation of the pond could begin.
The work was hard going with tricky weather and ground conditions, with the site becoming something of a mud-bath halfway through.
However once the thick layer of leaf litter was removed from the pond area, you could see everything start to take shape. The depth of excavation was varied in order to create different depths of water within the pond. This will lead to a mix of permanent and semi-permanent areas of water.
Finally, a low clay bund was constructed at one end of the pond to stop floodwaters from flowing rapidly through the pond and straight back into the School Stream. This causes the water level in the pond to temporarily increase as water flows in from the School Stream. A small pipe was installed to allow the pond to drain in a slow and controlled manner once the flood peak has passed.
The work was delivered by local contractors G&S Brown based in Headcorn and so this was a great opportunity to work with people who understand the devastating impact flooding can have on a community.
Work was completed in early January 2020, and the system was soon put to the test by heavy rainfall a few days later, and the exceptional February storms ‘Ciara’ and ‘Dennis’.
Excess flood waters diverted into the pond following heavy rainfall in January 2020:
The outlet from the pond during storm Denis:
Monitoring – Flood Attenuation
An important aspect of our NFM project is monitoring the work that we do so we can be confident that the features we create are working to provide effective flood attenuation. We include a level of flexibility within our design to allow tweaks to be made if monitoring suggests that it is needed to get the most out of the scheme. Outputs from monitoring can also be very useful to learn lessons in our work and improve it for future projects.
Two water level loggers were installed at the site to monitor the water level within the School Stream and the pond. As explained in a previous blog, in NFM we aim to flatten the flood hydrograph, reducing the peak flow and also delaying the time it takes for the river to reach its peak. In order to do this, we want to make sure that our storage area – in this case the restored pond – is not full up long before the School Stream peaks. If the pond is full when river flow is still relatively low, then it cannot store any more flood waters when the river level really starts to pick up.
The figure shows water levels at the site during Storm Ciara. The graph shows the pond beginning to fill up gradually as the water level within the School Stream reaches a certain point. As the water level within the School Stream continues to increase rapidly, so does the water level within the pond. The pond reaches capacity soon after the flood peak which suggests that the storage area is performing fairly well. We will continue monitoring the site to see if we need to make any small improvements.
Monitoring – Ecology
The combination of sunlight, a good water supply, fertile soils, and the seeds washed in during the winter floods has contributed to rapid colonisation by aquatic vegetation.
Species such as Water Plantain Alisma plantago-aqautica, Celery-leaved buttercup Ranunculus sceleratus and Water Starwort Callaitriche spp. are being noted for the first time. We’ll be monitoring the pond with interest this summer and in the following years, expecting its rapid colonisation by aquatic fauna such as dragonflies and amphibians, and its attraction for mammals, reptiles and birds.
The origin of the pond
We think we may also have solved the mystery of the creation of Kingsnoad pond. During our ecological surveys we found a piece of the very rare ‘Bethersden Marble’ – a highly prized decorative stone found only in this area and nowhere else in the world. Examples are found in the stone steps to the altar at Canterbury Cathedral and the stone’s value may explain the presence of an ancient quarry at this site. These thin bands of limestone embedded in the mud were formed by the accumulation of freshwater snails during the Cretaceous period, when the entire area was a giant delta, comparable to that of the Nile today. Effectively this means that at Kingsnoad, SERT may have just restored a 130 million year old pond, which we imagine is something of a record!
By its very nature, NFM will not function to strict design criteria as a hard-engineered solution would. However, NFM offers multiple benefits that hard-engineering schemes most often do not. This project is a perfect example of this; as well as flood attenuation, significant ecological benefits have been realised by restoring the pond. The landowner is very pleased with the end result, which would have been much less likely if we’d proposed pouring in lots of concrete!
In this case Natural Flood Management has allowed us to deliver a project which meets our project objectives, and works brilliantly for the landowner too – this is NFM working at its best.