Attenuation and Permeable paving for Stormwater attenuation

[Davybeano]

Hi all

I am currently trying to design an attenuation tank with a permeable paving area for a 100 year storm with a max volumn of 501m3 as required by the client on the job. All drainage initially runs to the attenuation tank. I have sized my tank at 144m3. The plan area of the pavement area is 3400m2 and can take a capacity of 357m3 of storm water in the granular layer underneath. The tank will reach capacity flooding the granular area via fin drains. My tank dimensions are 2.5dx8mx8m

My question is how reliable is such a solution.

Also there is an adjoining site. Will a suitable impermeable layer prevent any passage of water into this site???

Grateful for any advice that may be offered.

Regards

Davy

 

[TerryScan]

What is the conductivity rate of the base soil? or..will this tank have an outfall? What type of permeable paving? How will one maintain the paving to insure it from clogging? What happens to the runoff if the paving does clog? And since this is the 100yr storm event, what is the flow rate that can pass into the paving vs. peak on the storm (vs. just overall volume)?

 

[Davybeano]

I have been told to conservatively to ignore infiltration under the permeable pavement. Yes the tank has an outflow into a main sewer with flow limited to 5 l/sec using a hydrobrake. The pavement is a precast slab 1m * 3m with slots between to allow for drainage. Underneath is 50mm of clean stone on 350mm of heavier stone - which is the drainage material. The paving is subject to traffic from pedestrians so we would expect no heavy debris for clogging. Approx 250m3 of storm water can pass directly into the pavement - which is then drained to the tank. Should the tank flood. It backs up through the fin drains being relieved once again by the granular material.

 

[cvg]

I would say the system you propose is somewhat unreliable based on the information you have given.

ignoring the infiltration under the pavement is not that conservative as it is likely that you will have significant runoff during a 100-year storm event such that the water will not soak into the pavement (fast enough) and be retained. This water must be captured and directed into your underground tank.

The clogging will occur because of leaves and sediment, not heavy debris. For instance, I have a drain in my back yard which has a small slotted grate. It is set in a graveled area. During even a small storm, leaves, grass and other debris tend to clog it so that I still get flooding in the yard.

 

[Davybeano]

Yes cvg. I definitely take your argument about the runoff during the 100 year storm. I think myself that the Tank should be sized to allow a min amount of backup - ie the storage in the granular fill. Thanks for the help

 

[HvZ]

Davy,

I working in Ireland for number of years and stormwater attenuation is a planning requirement. The problem i had most was low lying flat sites and as a result it was difficult to introdution a tank type attenuation system, because we simply did not have the depth between the discharge of the system and the receiving water course, to introduce a tank or alternative. and the cost of such an underground tank was huge.

To get around the problem, there are two excelent options.

Both option use permiable concrete paving block pavement with a specially designed structural formation layer below the entire carpark/road area for storage or infiltration.

Tobermore in the UK and IRL manufactures a permiable concrete paving block, which we used as the surfacing for the entire carpark. This is then layed on a specifically design base layer with approx 30 to 35% voids. the depth of the layer is detemined by void ration and the volume of water that you need to remove from the surface during.

since the entire surface of the carpark is permiable, you do not need any gullies or gravity sewer pipes and your carpark can be absolutly flat. No need to spend many hours trying to design falls to gullies.

Now that you have passed the surface water into the base layer, you have two option.

1. The base layer is placed on a permiable geotextile membrane that allows the water to drain into the ground below. Provided that the soil allow it and you do not have to high a water table.

2. The second option is to substitute the permiable geotextile with an impermible plastic membrane. the membrane it turned up all around the perimeter of the carpark. This will give you an attenuation "tank" with the same surface area as your carpark and the depth of your base layer. All you need to do now is to introduce an outlet pipe anyware along the perimeter you need it and it is sized to discharge no more that you allowable discharge.

Because the area of permible paving is so large, clogging is not a problem because the water will just flow the over and around it. It was also tested and proven that you do not need a hydrocarbon interceptor in a typical carpark, since spills are smalll and infrequent and any spill that passes through the pavement is trapped by the bases layer.

another advantage is that you building rainwater downpipes are just run through a trapped gully and discharge straight into the carpark base layer.

go to

http://www.tobermore.ie

who manufacture the permiable concrete pavement blocks. They work together with Dr. John Knapton form the UK who is expert on this concept. His website

http://www.john-knapton.com

has a link to paper he published on this subject.

cheer

hvZ