Our allotment is situated on the crest of a hill. It basically slopes down from NW to SE, although most plots are orientated either NS or EW.
Three plots at the bottom end of the site tend to get waterlogged in winter, as does the entire site to a lesser extent and the surrounding hillside. Indeed, the wood even further down the hill, far from absorbing water, tends to send it gushing to the children's playground at the very bottom, where it forms the most outrageous swamp for a couple of days following heavy rain.
The hill is made of heavy clay resting on top of impacted grey/blue clay.
We are a new site, so the dynamics of what will happen with rain are not entirely known.
However, last night, there was extremely heavy rain and certain areas on plots became seriously waterlogged. After studying them, it became apparent that if somebody had dug land the soil became wet but not waterlogged. But an adjacent piece of land on the same plot, even though it was one foot away, but hadn't been properly dug, became excessively waterlogged.
In other words, what appeared to be happening was that water was flowing downhill underground, penetrating ground where this had been dug, but if it met a solid edge - the edge of a path or the hardened edge of a piece of land not yet dug - it began to rise to the surface and essentially back up, looking for somewhere to drain to. Hence a 3m strip on the downward part of a plot would be waterlogged, but the upper 3 metres would not.
Now, I'm wondering if our three "bog" plots at the bottom of the site, which we haven't yet been able to use, would become OK for cultivation were they to be dug, since this would allow the rain water to move beyond, possibly flowing further down the hill.
A couple of decades ago, this was a farmer's field and had been thus cultivated for many years. It was abandoned when a new A road was built about 20 metres away and some 10 metres below the bottom of the site. So food has been successfully grown here in the past.
My question is this: were the three bog plots to be cultivated, do you think we could radically improve the drainage there?
I'm thinking that digging some narrow drainage channels and filling them with stones might also take "trapped" water even further down hill. What do you think?
It might make a short-term difference, but would be unlikely to do more than that, as by digging you create voids within the soil which will fill up, but the water's not going anywhere. I'm on waterlogged ground next to a stream, and raised beds have sorted my situation out.
Not much feedback on this, but doing lots of studying suggests that, being on a hill, water clearly moves down it if the surface is compacted.
Once you dig the soil, water penetrates it, but hits the hard pan of clay beneath and simply rolls down that.
I reckon - but please contradict me if you think I am wrong - either Robert's suggestion of going up (into raised beds) should work, or trying to loosen the pan beneath with a fork should too.
We are also experimenting with sharp sand or grit on top of the disturbed pan.
But as Robert says, the water has to go somewhere.
In the height of winter, I put my wellington into a puddle (sic) on my plot and saw it sink down nearly a foot (excuse the pun), so saturated was the soil, given a water table that had risen to just below the surface - scarey stuff!
Based upon your description of the topography of your site, does the road built below have engineering inserted to support the bank, such as sheet steel piling? Does this have visible drainage? It may be holding back water, causing the problem. Highways dept may be able to explain/reassure. A saturated clay bank on a slope should give reason for investigation.
Are there any vegetative clues along the contour , such as rushes, or bog plants
The successive years ploughing may have caused a "hardpan" at the plough depth which may also be the cause.
This winter has been above average in many parts of the country for rainfall, and groundwater is high in many water tables. See Environment Agency website for data in your area.
https://www.gov.uk/government/collections/groundwater-current-status-and-flood-risk