Working with water
Fascinating hydrology and geology abound at High Fen Wildland, Nattergal’s second site, where our baselining soon challenged conventional thinking.
Humans can have a tumultuous relationship with water. For centuries we have drained our land to create farmable soils, getting water off the land and into the sea as fast as possible. Now, however, we must recognise the catastrophic ecosystem collapse this distorted hydrology has contributed to, as well as the fact that climate change is leading to a cycle of extremes between drought and flood. With all this turbulence in our environment, we have to find new ways to keep water on the land and reinstate natural hydrological function.
Our Fenland site, High Fen Wildland, is a brilliant case study for understanding the complexity of hydrology in an engineered landscape. The Fens’ history is long and complicated: from landscape-shaping glaciers, retreating icecaps, sea level rises and retreats, the geology of the Fens is wonderfully complex. On top of this, the Fens have been intensively drained by humans over the past few millennia, taking one of the richest ecosystems in Europe and turning it into one of the most productive agricultural landscapes.
The ‘black gold’ – peat, formed by waterlogged vegetation – has the capacity to hold vast quantities and diversity of life, and equally the capacity to grow huge quantities of food. The problem is, it’s a limited resource. Lowland peat, when dry and drained, becomes farmable and hugely productive, but it also begins to oxidise when exposed to the air, decomposes and emits huge volumes of carbon emissions. The peat may well be feeding us, but it’s disappearing fast and is a hugely significant contributor to anthropogenic climate change.
A vision for High Fen Wildland
It's certainly a complex issue. So when High Fen became available, we knew it was an opportunity too great to pass by. Already too difficult to farm for arable (it stopped daffodil and arable production 20 years ago), High Fen is seasonally wet. Our challenge, to deliver for biodiversity and carbon emissions, is to get it as wet as possible all year round.
High Fen Wildland has not only changed our original thinking about how we manage it, but it has challenged the British Geological Society (BGS) maps and taught us to think in bathroom analogies!
The plan is that this 292 ha grassland becomes a wetland biodiversity hotspot, an ecosystem driven by water buffalo, red deer, cattle and Konik ponies. Rather uniquely in the Fens, High Fen Wildland is a hydrologically isolated unit. It is surrounded by water on all three sides: a cut-off channel (an enormous drain built in the 1960s), the River Wissey and the Methwold (local village) lode (a short stretch of navigable water). Further rewetting the landscape and seeing the diversity that it will attract is the primary objective (as is actively stopping the peat drying out in times of low rainfall to stop it emitting huge amounts of carbon).
Understanding the High Fen landscape
Nattergal bought High Fen in December 2022 and knew the location was special. Very flat, sunken and through the winter, very wet – it could be the perfect place for migrating birds, a beautiful habitat mosaic, and a functioning wetland system. Initial understanding of the site geology was that it had about a metre of peat, which sits on top of 70% of the site being chalk and 30% clay (according to the BGS). Knowing what is beneath our feet is always crucial for how we welcome nature back to our sites. Clay is impermeable and chalk is (mostly) permeable: if you can seal away the chalk with clay then you can keep the site wet. That was our thinking.
However, 19 boreholes later (at a depth of 8m), we found out there was no clay! It was a substantial shock – as well as the first time in our consultant hydrologist Nick Haycock’s experience that the BGS map wasn’t completely correct.
Yet all was not lost. Our actual geology profile is peat, on sand, on chalk marl which sits on top of chalk. We found out that the chalk marl is very similar to clay in consistency and therefore still highly impermeable. However, when the water travels down through the peat, it is held above the marl and drains laterally through the sand. This is where, when it stops raining so much at the end of winter, we lose all of our water. How then do we stop this happening?
Plugging the holes in our bathtub!
Imagine a profile of (on average) 1m peat, 15cm sand, 2.5m of impermeable chalk ‘clayey’ marl, followed by deep permeable chalk. At certain points the chalk marl layer is either very thick or very thin. The net result is that in winter, when the water table is high and there is lots of rain, then High Fen Wildland gets wet (as in the photo above). The minute it stops raining and the water table gets low, High Fen dries out very quickly through these thin areas of marl.
Our challenge then, is to identify the weak points and seal them off with a clay-lined trench. The way we will do this is by placing digital water loggers in our boreholes, in a series of dipwells and in the ditches to trace the subterranean direction of water flow. This will tell us how and where water is moving (and draining out of the site) – and with this information, we can make a restoration plan.
High Fen Wildland’s whole hydrological understanding is best described (in its most simplistic form) by a bathtub analogy. It’s surrounded on all three sides by water, the edges of the bath – so the High Fen itself is the bathtub. This bath has taps (an abstraction point where we syphon water into the site from the River Wissey); it has an overhead shower (the rain); it has an overflow point (which is a drain into the cut off channel if High Fen ever got too full); and it has a rather loose plug and a few rusty holes in the bottom of the bath (these are the thin areas in the chalk marl, our chalk hill, the sandy bank of the cut off channel).
When the shower and taps are on together (in winter), the bath fills quicker than the holes and the plug are draining, and the site gets wet. However when the shower stops, and the aquifer goes down, the plug and holes drain much faster than it fills, meaning the bath ultimately empties – the peat and the site dry out completely. If we can find the holes and the plug, and seal them off, then we can keep the water level high (even through the summer and despite any lack of rain).
There is still much to do at High Fen Wildland and the baselining work continues apace. But now we understand exactly what geology is under our feet, we can start to plan a future that sees biodiversity increase, habitat restoration undertaken, and the securing of wetted peat. The future is starting now.