Sustainable Drainage Systems (SuDS)

[benedmonds]

I have a client wanting to construct some overflow carparking near to some ash trees. The carpark is outside of the RPA's, however the TO is concerned about the increased runoff and possible pollution..

 

"As highlighted within the Design and Access Statement, this site is subject to impeded drainage which can be attributed to the soil type and the slowly permeable nature of clayey soils. Risks associated with this soil type include overland flow with pollutants moving in suspension or solution into the wider landscape. An influx of parked vehicles and associated spillages from these vehicles may have an impact in the wider landscape.

 

I have concern of the effect of extensive impermeable surfacing being placed at this site and how this change in soil hydrology and increased run-off will affect the roadside trees (GP1 in the Arb Report). The majority of trees which make up the species composition of GP1 are ash which is already a vulnerable species due to spread of Chalara. Any additional stress would further increase their vulnerability. I would currently consider the site at the upper limit of the soil moisture regime for this species and additional increases in water would push the site into the unsuitable range for ash species adding further stress and increased likelihood of loss.

 

I would like to see solutions for mitigating impeded drainage to protect these and the surrounding trees. Solutions could include more permeable surfaces, a dedicated SUDS to increase water capacity through a retention system and pre-treatment through filter trenches. Dedicated space to allow for SUDS would probably result in a reduced number of parking spaces, although the compromise should be considered necessary to safeguard the wider landscape. In addition to this, more planting should be considered to increase canopy cover and enhancing the green corridor. This can possibly be combined with the SUDS through wet-woodland creation."

 

Has anyone had any experience with SuDS?

[eggsarascal]

My Mrs knows a fair bit about SuDS, but she isn't prepared to type an essay. If you send a phone number by pm she will call you.

 

I think a more permeable surface would be a start, then, maybe finger drains into the football pitch. Clay, and drainage aren't a good combination, or I'd suggest soakaways, still could be a consideration.

[benedmonds]

23 minutes ago, eggsarascal said:

My Mrs knows a fair bit about SuDS, but she isn't prepared to type an essay. If you send a phone number by pm she will call you.

 

I think a more permeable surface would be a start, then, maybe finger drains into the football pitch. Clay, and drainage aren't a good combination, or I'd suggest soakaways, still could be a consideration.

Cheers, the football pitch is up hill...

My solution is to run an infiltration trench and vegetated buffer strip between the bank of trees and the (permeable tarmac)carpark, and direct any excess runoff around the sides of the group.

 

[eggsarascal]

15 minutes ago, benedmonds said:

Cheers, the football pitch is up hill...

My solution is to run an infiltration trench and vegetated buffer strip between the bank of trees and the (permeable tarmac)carpark, and direct any excess runoff around the sides of the group.

 

I'm not sure I follow. An infiltration trench/French drain is designed to work on permeable soil. I would think it will struggle on clay, ( depending on its size), I don't know what a vegetated barrier is, vegetation to slow down the flow?

Edited May 23, 2018 by eggsarascal

[benedmonds]

6 minutes ago, eggsarascal said:

I'm not sure I follow. An infiltration trench/French drain is designed to work on permeable soil. I would think it will struggle on clay, ( depending on its size), I don't know what a vegetated barrier is, vegetation to slow down the flow?

 

[donnk]

Just use block paving, it will look nicer and you wont have to faff about with any trenches or drains. Make sure you get a decent depth of type 1 under it. put a slope on it of course then if you experience pooling pipe it off to the nearest ditch to flow away.

Edited May 24, 2018 by donnk

[aspenarb]

Its hard to comment on this without more detail but would seem that interceptors and attenuation tanks would be the solution, that would give a degree of pollution control and slow release .  I cant see car park run off being allowed directly into a watercourse being an option.

 

Bob

[daltontrees]

The ideal self-contained system is to collect the surface run-off into gullys and then feed the gully-outflows into a network of perforated pipes in filtration trenches immediately beneath the car park. That way there is no net increase in  water anywhere, it is simply captured above ground and redistributed evenly below ground. The storage capacity if a large network of pipes is huge.

But you need to capture all the water, and have gullys, which means kerbs and careful management of levels. I was involved in doing this for a town centre car park, and as I recall there was no feed into the surface water drainage system offsite.

[donnk]

7 hours ago, daltontrees said:

The ideal self-contained system is to collect the surface run-off into gullys and then feed the gully-outflows into a network of perforated pipes in filtration trenches immediately beneath the car park. That way there is no net increase in  water anywhere, it is simply captured above ground and redistributed evenly below ground. The storage capacity if a large network of pipes is huge.

But you need to capture all the water, and have gullys, which means kerbs and careful management of levels. I was involved in doing this for a town centre car park, and as I recall there was no feed into the surface water drainage system offsite.

pointless its clay.

[daltontrees]

12 hours ago, donnk said:

pointless its clay.

Clearly you haven't understood. The same amount of rain falling on the car park will be absorbed by exactly the same area of land, just as with your block paviour solution. There can be no argument that it is increasing run-off. Or to put it another way, your block paviour solution is just as pointless as mine. Arguably more so, as you may require a pipe to the nearest ditch.