A New Wetland Opportunity

Diversion of the stream would leave a lowland area to be converted to biodiverse wetland

Last Tuesday I was approached by Dan and Emily to discuss what they might do with the long proposed large pond in the lower wood. The bridge across Wilderness Stream is pretty well finished, but what it will be crossing is still a mystery.

I think Dan’s original conception of this water body was largely aesthetic and recreational. The former perhaps demanding a large, deep pond held back by a substantial earth bank and tapering up the valley into the Wilderness Stream. One can imagine Lancelot ‘Capability’ Brown inspired plantings and vistas.

This grand landscape vision would perhaps have been compromised a little to allow for access by visitors indulging in a spot of wild swimming in its clear blue waters.

If course visions always have to address practicalities, if they are to fly. Lancelot Brown’s clients had enormous amounts of land, bottomless pockets and political influence. Wilderness Wood alas can boast none of these.

Planning constraints, cost of construction, water supply and public use of the wood are modern day realities. One other key consideration of course has to be the need to consider the wood's ecology and biodiversity.

So Dan and Emily’s revised plan is to take what has been learned from the 3 ponds excavated out of the silt of the former penstock pond just downstream of the new bridge and create a smaller, shallower pond with a margin of marsh loving plants to support wetland insects, amphibians, birds etc.

However, the term ‘silt’ raises one particular issue. Rivers erode, transport and deposit material. Even a small stream does. The centuries old penstock pond silted up because of these processes. Ideally we want to prevent silting of any ponds created on site. In fact pond experts suggest avoiding streams altogether, for this and other valid reasons, but this is not a luxury we have. Because of the Wilderness Wood terrain, if we do want any wetland on site, it seems it has to be here, or nowhere.

The penstock ponds restored in 2022 attempted to sidestep this silting problem by building a bypass channel to take most of the water and silt away, from the two lower ponds at least. This was achieved with some success. Granted, the area of open water is far less than it would have been three centuries ago, but the biodiversity benefits provided by having areas of marsh and marginal vegetation far outweigh the reduction in water volume.

Two examples of bypass channels: Left - Wilderness Wood penstock pond bypass channel can fill the ponds via the right hand branch, or continue its flow from top to bottom of the photo. Right - Sedlescombe, Battle - water can fill the pond (beyond the tree), but once it is full of water excess water and silt are taken away via the left hand bypass channel.

Can we do something similar with the area adjacent to the new bridge? Possibly, but the valley is much more constrained here.

The silting issue has been shown to be mitigated further upstream by building leaky dams. These porous structures, which have now become a fixed part of the Wilderness Wood landscape, hold back transported sediment which might otherwise fill any new pond. Obviously the sediment still accumulates behind the leaky dams but as I demonstrated last week, it is relatively simple to dig 3 years’ supply of silt out of a small settling pond with just a spade. A mechanical digger could probably scoop out any unwanted sediment upstream of the new pond in a day or two.

Five minutes digging removed silt and created a shallow pond behind this leaky dam

To suit colonisation by native plants and animals the pond ideally needs to have shallow margins and a long sinuous ‘shoreline’. Different species prefer specific water depths, so the water needs to shelve gently towards the maximum depth of perhaps 1-1.5m.

Then one has to consider the challenge of deterring unwanted people and dogs disturbing the wildlife and making the pond water turbid. Wetlands are delicate and easily suffer damage when disturbed. The problem of active ingredients in dog tick and flea powders is another issue once it gets into a pond. So some sort of fencing will be required around the pond and it's associated wetland.

Fortunately visitors will have the option to observe the wildlife from the great view afforded by the elevation of the new bridge already built across the valley. In time perhaps we will have the angry chatter of Reed Warbler from the tall marginal vegetation and be able to watch the aerial gymnastics of several species of dragonfly flitting around us, happy in the knowledge that both are being supplied with aquatic food busily breeding in the pond below.

Taking into consideration all of the above, I can see that we have the basis for optimism regarding this project. We have learned much from the ponds and leaky dams already constructed. Now all we need to do is put this acquired knowledge into practice. I'm looking forward to the prospect of having an increasingly large strip of wet woodland, marshland and ponds (seasonal and perhaps permanent) in the lower wood.

I have already seen an increase in birds (Woodcock, Grey Wagtail), amphibians (Common Frog) and invertebrates (dragonflies, diving beetles) over the last 18 months, so within 5 to 10 years time we can envisage much greater changes still. Bring it on!

26th March 2024
I rock-up this Tuesday morning ready for persistent light rain, but I'm in luck, Michael Fish and John Kettley have retired and the new guys have sent me fine weather instead.

I ask Dan if he has any plans of the topography of the lower end of Hemlock Valley where the pond is proposed, but alas it looks like I'll have to draw my own.

Within 15 minutes I'm down in the lower wood taking photos and measuring heights and distances in the hope of magicing-up some sort of working plans.

The bridge is an excellent level to work off and after an hour of tape measuring and dead reckoning I have the bones of it. There's a suitable area to build a bypass channel to take the stream and silt away from the pond.

I'm convinced we can build a modestly sized bank to hold back enough water to create an area of wetland and perhaps even a shallow pond before summer droughts evaporate it. I'm sure the frogs, dragonflies and visitors will love it.

Sketch of proposed wetland (top is due north). Wilderness Stream will be diverted down a channel (left of sketch), between bridge piers 2 and 3 and rejoin its natural course downstream.

David Horne - follow my travels around the UK coastline and other areas on www.leggingroundbritain.com

I have also published the whole of the blogs from March 2023 until March 2024. Copies available soon.