Run-off from driveway

clonagh

Hi All,
As part of a Further Information request from the council I have to show how I'm going to deal with run-off from my proposed driveway. The driveway is 650m long but the bit I'm most concerned with is a 300m long stretch along a watercourse/drain. The council are concerned with possible contamination of the drain. The driveway will be 3m wide and the nearest edge of it will be about 6m from the edge of the drain. My plan is to put a French drain along one side of the driveway but I need to work out the volume of water that will be coming off it so I can design a soakpit that will cater for it. I did a bit of Googling but didn't have much luck. I'm hopeful some of the good folks on here could give some pointers on the best way to go about this and how to calculate the amount of runoff.
Sorry for the long winded post but I thought some detail might be useful.
Thanks.

ban resistant recalcitrant debutant

Why not drain that part of the driveway entirely to the side that doesn't contain the drain? Then you're 9m away from the drain if it's 6m gap plus 3m roadway.

Schorpio

Hi clonagh,

Calculating run-off rates and storage volumes can be a complicated process. Ideally for a soakway design, the rainfall/run-off restrictions would be modelled in software to work out the size of soakway required.

However, there are some rough calculations that can be undertaken - just to get a broad idea - but these would involve a lot of assumptions to be made. I'd use the simplified rational myself - an online tool for it can be found at https://www.lmnoeng.com/Hydrology/rational.php. I'd assume a rainfall intensity of 50 mm/hr. With the peak run-off (in m3/s), I'd work out the volume you need to store (based on your outflow restrictions) for a 15 min storm, and that should give you an estimation of your soakway size.

Based on the contamination issues you've mentioned though - are you planning a permeable soakway (which will infiltrate over time), or an impermeable one with a point of outfall?

Rackstar

They'll want you to dispose of the water within your site.

My engineer calculated the amount of run off and sized adequate soak pits to deal with the run off.

clonagh

I was thinking of possibly doing that but I still need to work out the run-off and size some soakpits.

clonagh

Thanks Schorpio,
It'll probably be semi-permeable with an overflow to the drain it they accept that.

BryanF

650m long..

Would you consider gravel grid system or similar

I like the idea of grass Crete for a driveway.

clonagh

BryanF wrote: »

650m long..

Would you consider gravel grid system or similar

I like the idea of grass Crete for a driveway.

Hi Bryan,

Any idea how much that system costs? Would it be able to support block/concrete lorries etc?

BryanF

you'd just lay hardcore for construction stage. And install system afterwards, I'd expect it to be slightly less than tarmac. But the point is it's a 'sustainable drainage system' so you'll have an easier time complying with drainage requirements.

ban resistant recalcitrant debutant

This issue is fairly confusing to me.

Of course I understand that drainage is important.

My issue is this. What percentage of the existing drainage area is being covered by the new road?

For example, if a strip of self draining land, like grass, was 20m wide, and you put in a 3m wide driveway, you've covered approx 15% of the draining area. In that case, surely the 15% of the rain which falls on the road will simply drain into the remaining 85% of the draining area. Does it have to be shown in all cases exactly how the rain will flow?
What proportion of self draining land do you need to cover before you have to address drainage?
The fact you have put in a driveway hasn't increased the rainfall. The road would have to be slightly raised from the surrounding ground.



Is the issue the council have the fact that cars leave residue or dirt on a road, and that residue or dirt can be washed away into a drain and contaminate the drain?
If that was the case then the fact it's a driveway with presumably low levels of vehicle use would mean the contamination would be very low.


Is it not sufficient to simply drain the driveway to the other side of the roadway from the existing drain? The water can then disperse under the new roadway.

T-Maxx

Circumvent the problem by making the whole thing permeable.

Compacted 804. Done.

clonagh

My initial cross section drawing of the roadway showed a stone layer with a top layer of 804. They've come back asking about how it's going to be drained. I rang the engineer but she was vague at best and wouldn't commit to a design solution. In my mind if the water was drained to the opposite side of the road there should be sufficient distance between it and the drain to avoid any contamination but this won't satisfy the council so I have to calculate the volume of run off and put in some sort of filtration system hence my idea of a French drain and semipermiable soak pits.

T-Maxx

That council engineer is thick. 804 is permeable and therefor satisfies the Suds requirement. Tell her to cop herself on and talk to her supervisor.

Schorpio

T-Maxx wrote: »

That council engineer is thick. 804 is permeable and therefor satisfies the Suds requirement. Tell her to cop herself on and talk to her supervisor.

804 is permeable, but by itself I don't think it satisfies the SuDS requirement (if that's what OP is being held to).

It sounds like contamination risk is the main issue, and whist 804 does provide a treatment stage, the run-off still isn't being managed. Do we have any idea of infiltration rates and or existing ground water levels?

Presumably this is an ordinary driveway, which only cars will use and very occasional HGV use? If so, it does sound a bit overkill considering the contamination risk would be very low.

clonagh

Schorpio wrote: »

804 is permeable, but by itself I don't think it satisfies the SuDS requirement (if that's what OP is being held to).

It sounds like contamination risk is the main issue, and whist 804 does provide a treatment stage, the run-off still isn't being managed. Do we have any idea of infiltration rates and or existing ground water levels?

Presumably this is an ordinary driveway, which only cars will use and very occasional HGV use? If so, it does sound a bit overkill considering the contamination risk would be very low.

Yeah, contamination seems to be the main issue.
It's an ordinary domestic driveway and will only have lorries on it for the construction stage.
The drain is dry during the summer and has about 6 inches of water in it during the winter. Standing water in the surrounding field isn't an issue.

Lumen

clonagh wrote: »

Any idea how much that system costs? Would it be able to support block/concrete lorries etc?

I priced a plastic grid system for a 300sqm gravel driveway resurfacing and it was expensive. It might even have doubled the price, I can't quite remember.

That grasscrete looks interesting though. The problem with any even slightly permeable surface is weeds. Presumably with grasscrete you can just mow it.