Helping ourselves, how a miniature flood alleviation scheme helps protect a local community

You may well think to yourself, “What can I do to help”, and “The Government should sort this out”.

The answer to questions or statements like this is “a little contribution can amount to a lot if everybody did it” and of course the government has to prioritise.  Larger Metropolitan areas will get priority over rural communities as that makes economic sense as there is a limited pot of money for managing nationwide flood risk.

So what can you do that is cheap and easy?

The first thing you can do is disconnect your downspouts from your drains and allow them to discharge onto your garden or yard.  Better still, you can create an attenuation pond for the water that runs off your roof using a water butt, at each of your downspouts.  Leave the tap open, the water fills the butt and it drains out slowly from the tap onto the ground perhaps into a flower bed and not into a drain.

In this small way you have slowed the flow to the river by following ether of these two simple steps.  If you are paving your garden use a permeable membrane below it rather than using plastic, or leave several paving stones out and fill with plants or gravel.  Better still leave gaps between the paving stones and gaps in the mortar between them.

All of these measures increase the infiltration of water to the ground and prevent flow to the drains which eventually discharge your water to the rivers.

If you have got some land, even a small plot or garden take a look at your land when it is raining heavily how is it draining?  Where does all that water go?  If it is running quickly off your land downhill then it is going to give your downhill neighbours a problem possibly.

With this in mind here is an example of what can be done at small expense.

I own a smallholding above Pecket Well.  The land backs onto Wadsworth Moor and falls from here downslope to a bridleway also used to access smallholdings further along this same lane.  A field drain crosses my land which feeds into along with other field drains, a small brook which continues downhill to Far Shawcroft Hill, from here the brook crosses below Ackroyd Lane and then into Keighley Road (A6033) to join Hebden Water in Midgehole.

The field drain has been problematic for many years leaking water onto the lower field which is boggy with rushes growing.  An attempt ten years ago to fix the problem which cost a considerable sum appeared to have no effect.  The drain was excavated and reconstructed but with no improvement.  On Boxing Day 2015 the water rushed out of the ground onto the fields and then onto the bridleway causing damage to the bridleway surfacing.  Similar, less severe rainfall events than Boxing Day having similar effects.

In the 2012 summer floods a property at Far Shawcroft Hill narrowly escaped flooding from the adjacent brook only by intervention by the property owner who was home at the time.

Following the Boxing Day flooding I decided to investigate an idea I had regarding slowing the flow in brooks or ditches using letterbox slot plate weirs (see pictures) and I began installing a few experimental ones here on this very same brook.

I wrote about this in January 2016 in my “Pound for pound” article:

There are eight of them installed here now and they are working well attenuating water in small ponds within the brook channel during heavy flows (See Pictures).   I then turned my attention to the field drain and I employed my neighbouring farmer to do some initial digging to see if we could locate the problem.

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I was expecting to find the pipe I had replaced some ten years earlier with a 150 mm diameter pipe to be broken or blocked at the point from where the water was issuing.  In fact I discovered the pipe to be intact and running freely. However, further excavations revealed an old square stone drain running obliquely into the new drains which ten years previously I was not aware of.  This meant that the new 150 mm diameter pipe had combined flows at this point from another roughly 200 mm square stone drain.  Clearly in heavy flows there was insufficient capacity and this was the reason the water was breaking to surface flooding the lower field.

The obvious solution was to replace the 150 mm diameter pipe with a larger pipe of say 300 mm diameter, but over a distance of 75 metres that was going to prove expensive, bearing in mind this pipe was only ten years old and in good condition.

The solution was to employ my neighbour a while longer and excavate and construct an attenuation pond on the drain.  I calculated the size of the pond needed to be 200 m3, a pond roughly 14 x 14 x 1.0 metre deep would provide that volume and this would provide a 1 in 30 year return period level of protection.

The pond is now dug and grass re-sown (See pictures).  In normal circumstances it is a dry pond that can be used for grazing. It only begins to fill during heavy downpours when the 150 mm diameter pipe cannot take the combined flows.  Upon cessation of the storm the pond gradually empties.

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There are five positive outcomes from these small interventions, in order of importance these are:

  • Reduced flood risk for the wider community by slowing the flow to the valley floor
  • Reduced flood risk for the nearer community at Far Shawcroft Hill
  • Reduced risk of flood damage to the bridleway
  • A dryer lower field that can be returned to productive grazing
  • A cost saving over replacing the 150 mm diameter pipe with a larger pipe

All at a cost of under £500 for the pond and the eight plate weirs, such a process applied across the catchment makes good economic sense for slowing the flow into the Calder.

All we are doing is skimming the top off the flood peak. If enough of us can do what I have done here then we might just make a difference!!

Stuart Bradshaw C.Eng.

 

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