Water Management at Wilderness Wood.
A shallow trench dug to transfer rainwater to the young Bat Park hedge shrubs
Today I'm going first to Bat Park to extend the side drain which connects to the French drains dug several years ago. If my cunning plan works it will irrigate the beleagured hedge plants on the downhill side of the grassland area. I'll achieve this by the simple expedient of digging out a shallow line of turf divots and just wait for the next heavy rains to do the watering for me. Pretty rough and ready perhaps but there is clear evidence that water after heavy rainfall does indeed following my new drain. Time will tell.
Then it is down to the three (restored) ponds in the lower wood to check on improvements made there. Two of the ponds are already bypassed by the stream, which reduces silt deposition in them, but the uppermost one is not and is quickly silting up. I did have a go at digging a simple channel in the accumulated silt deposit to bypass this one too. So far it seems to be functioning well, with the stream water now doing an excellent job of taking silt away rather than depositing it in the pond.
The upper pond silting up. A bypass channel has been excavated to take Wilderness Stream water around it
My last job is to insert an overflow pipe in the upper of the two dams built in the former drainage ditch which once carried the Wilderness Stream. This is a pretty filthy job since the clay is very sticky and ends up covering me from head to toe. But I love it! Without this overflow pipe any water gathering here will quickly erode the dam material away.
All I have to do now is check on the progress of each project after any heavy rainfall event to see what impact my tinkering has had. If it doesn't go according to plan then it will be back to square one, but with a good lesson learned.
Clay dam built across the former channel of Wilderness Stream, with overflow pipes inserted
If you want to find out more about water management at Wilderness Wood, read on:
The management of water and its movement at Wilderness Wood has been a concern of mine for some years. Having worked at the wood for 10 years now I can recall I first became interested in water after a couple of years here because of my extensive field studies teaching experience. As an experienced environmental educator I realised that water and life in it was an important part of the school curriculum. Various environmental education centres I have worked at benefited a great deal from providing water related courses, including the age old 'pond dipping' activity to learn about the ecology of freshwater and also studies of rivers and the wider hydrological cycle.
The biggest problem developing this idea at Wilderness Wood was that for much of the summer there is no water in the wood. Initially I thought that we should maybe dig some ponds for school and other groups to benefit from. However, there are plenty of better placed environmental education providers who do have more suitable water resources to call on, so I've since abandoned this idea.
At about the same time I took on responsibility for managing Bat Park and I quickly became aware of how challenging it was going to be to develop the acid lowland grassland in this very dry location. We planted perhaps 1,000 hedge shrubs and those that are still alive continue to struggle to grow some 5 or 6 years later. Ironically Bat Park has the reverse problem in the winter months, when it is often covered in surface water. We successfully addressed this issue by inserting French drains, but summer droughts continue to be an issue. Hence my recent attempts at diverting the French drain waters towards the drought challenged hedge plants. Hopefully this will alleviate some of the drying out of the soil during the summer months. Further, it is hoped that bracken (which does not like damp/water-logged soils) will no longer swamp the hedge plants here.
In about 2019 I started thinking about creating in-line ponds in the Wilderness Stream so that we might retain some water throughout the summer months. This quickly morphed into a more hybrid approach with the construction of leaky dams. We now have 106 of these scattered around the wood, serving the dual purpose of limiting flash flooding further downstream and subsequent erosion in the winter months, as well as retaining water either as shallow ponds, or as marshy areas. Both wetland types benefit invertebrates as places to breed and for larger vertebrate species to access drinking water.
Eventually money was provided to restore a former pond in the lower wood by digging out silt to form three scrapes. These have been very successful, especially for the frog population in the wood. However, they still dry out for a few months in the summer.
The latest attempts at retaining water in the woodland involved redirecting the Wilderness Stream back to its original course. The original pond (before becoming the three ponds mentioned above) acted as a penstock, or header pond supplying extra water to iron smelting sites further downstream for several centuries. When this water was required it would have been released down a much straighter channel, bypassing its natural course. Release of this water eroded a 1.5 metre deep ditch which still remains, now that the Wilderness Stream's natural course has been restored.
This rearrangement of water movement has given rise to two distinct water retention projects. The stream now follows an anastomosing course through the wet woodland downstream of the three ponds. This saturates the soil of the woodland, hopefully favouring wet woodland species such as alder, willow, black poplar, ferns, mosses and wetland angiosperms. The deep ditch that once carried the stream will hopefully serve a different function. By collapsing the sides of the ditch at intervals, the clay material has been used to create in-line dams in this former stream channel. Last year I was delighted to find that the first of these held water right through the summer months. Our first permanent pond in the wood since I've worked there. The second one will doubtless achieve the same, with still more to follow.
More ponds may well follow elsewhere in the wood, especially in the lower wood where water seeps out of springs even in the summer time. The biggest of these is at Streamside, being created by the building of a large leaky dam, which has since morphed into a more permanent one and now holds back a body of water some 20 metres in length, 5 metres in width and 30-45 centimetres deep. It still dries out in the summer, but only perhaps for a couple of months and even then the mud in it remains wet.
David Horne