Drivesett - run-off problems - global warming...! - Improve drainage

[mrlong]

My driveway was installed nearly ten years ago using Marshalls 'Drivesett 50' blocks according to the invoice. The surface, which slopes very gently towards the garage, appears to be in good condition. However with each of the recent sudden downpours - 3 so far over the last 18 months - the rainwater has ended up running in a torrent into the garage. (Now everything vulnerable is up on blocks and palletts!) The drive edges are sealed with a wall on one side and the usual kerbing on the other. There is a Keydrain across the front of the garage which normally copes perfectly. I investigated to see whether the Keydrain draining into a soakway was blocked in any way: seemed generally OK but I cleaned it out anyway as best I could.
My initial thoughts are could I make the drive more porous? I didn't fancy the idea of replacing the whole drive using Priora blocks, the only solutiuon that Marshalls could offer. The drive cost over £5725 in 1997: I hate to think what it would cost now (especially as there's nothing wrong with the drive except in tropical downpours)! So is there another solution? For example could some of the blocks - just in the areas where the run-off occurs - be replaced with some other type of porous or perforated blocks?
Any help would be most welcome.

[Tony McC]

Have you observed the surafce water cascading across the linear channel? It would be interesting to learn just how it is managing to 'jump' a 100mm wide chasm.

Is the grating clean? Is the channel itself clean?

If you can't identify just why and how the water is getting across that single channel, then maybe istalling a seocnd, parallel linear channel would be the answer, rather than replacing all of the blockwork.

Just a thought: you do mean Keydrain, don't you, as in a linear channel with a grating, and not KeyChannel, which is the shallow dished channel affair?

[mrlong]

Not only did we observe it - during the first storm we were at the time trying to brush the water out of the garage - unsuccessfully in the end: the quantity of water was just too much - it had nowhere else to go.

The water was flowing across the linear channel (it has a grating) because it was full! It was also clean. At the other end of the 10m long garage a greenhouse is situated. That greenhouse floor level is some 15cm lower than the garage. During those rain storms the greenhouse became flooded. That had never happened in previous years. I deduced that the water table had risen drastically. Hence the keydrain couldn't carry the water away. About 15 minutes after each storm passed, the keydrain started to take water away again, and the flood water soaked away from the greenhouse.

So I reason that if much of the water could be diverted before it reached the garage, e.g. by soaking into the ground at the higher end of the drive (the paved drive is about 20m long, averaging 3.5m wide) less water would get to the lower levels of the garage and greenhouse.

I should add that none of the water was run-of from the road. We live on a hill side and the water cascading down the road didn't/couldn't enter our drive (fortunately!).

I hope that makes more sense...? Any suggestion leading to a solution would be very welcome.

[Dave_L]

My suggestion is to clear the outfall from the linear drain! Get it to drain freely.

Does the linear drain have a fall to it? As a rough guide, slap a spirit level on the top grate and observe the bubble - does the channel fall to the outlet?

[Tony McC]

I, too, would be most concerned that the lin channel was surcharging. Properly installed and functioning, such a channel should easily be able to cope with anything the skies over these islands can throw at it.

To where does it drain? Can you determine whether it is to a soakaway or to the SW system? Is that where the problem lies? Can you get hold of the original contractor and get them to recollect where they connected the new lin channel?

[mrlong]

I agree that the linear drain should cope. It does, and has done quite successfully over the last 10 years.
But with the three recent extreme rainstorms - each estimated at a rate of over 300mm/hour during some 15 to 30 minutes - I'm not surprised that it failed to cope.
Perhaps I didn't make the situation clear: it does drain/fall correctly to a soakaway, which is not obstructed. Kindly re-read my messages...
The bottom line is, what porous blocks could I use to swap for some of my existing blocks; or is that not a good idea? Or is there a better solution (apart from a major exercise to augment the linear drainage, which probably won't work because the local water table becomes too high...)?
Your further help would be most welcome...please!!

[Tony McC]

There's no way rainfall reached 300mm per hour, at least not on this planet, anyway! A once in 50 year storm event gives 75mm per hour: are you really sure you had three events in less than a year, each of which was four times as heavy as an "official" deluge?

You can't simply swap permeable blocks for conventional blocks. The whole structure of a permeable pavement is different, from sub-grade level upwards. Kindly read the poages on the main website that deal with permable paving systems.

If the existing soakaway can't cope, and you state that the local water tablle is high, then there's your answer - you're buggered! Your only viable option is to massively increase the storage capacity of the existing soakaway, and if you really are experiencing 300mm per hour storm events, it's unlikely that you have enough land, unless you happen to own the whole of Surreycestershire.

[lutonlagerlout]

at a rate of over 300mm/hour

you better ring the met office mr.long you broke the british record 3 times,look here
for the records
as for your channel ,think what time of year it is?
winter,winter follows autumn when all the leaves fall from the trees
chances are your soakaway pipe is either blocked or silted up
your path of least resistance is to clean/unblock this pipe then hey presto you got another 10 years of dry garage
good luck
regards LLL

[mrlong]

Sorry, I only repeated (300mm/hr) what the local paper reported! I will give the met office a call to see what the real figure was/is.
Thanks Tony McC; you told me what I thought. But I was hoping that by replacing just a small number, spaced well apart, along the drive mainly towards the top/highest end (away from the bottom end water table problem), I'd get away with the existing subbase which by now must be well compacted and stable. Otherwise, as you say, I'm buggered!
Thanks again Tony.

[mrlong]

Actually I've just found your "SUDS Drive & Patio Infiltration" section in your most excellent and comprehensive website. Since the over 3/4 of my driveway from the top end is on Bagshot sand (i.e. free draining) I'm wondering if that could be the basis of a solution to my problem. Could such a system, simplified by using the existing kerb with spacer blocks alternating with pebbles/cobbles be used? (I should have added that the majority of the run-off does occur on one side of the drive.) Any thoughts?

May I add that with the climate changing, perhaps more consideration could be given (e.g. on your site) to the application of SUDS principles to existing domestic installations. My thought/suggestion, for what it's worth!