Car-Park Drainage

Car-parks as with school playgrounds are large open areas that can at times be required to deal with great volumes of storm water in a short period of time, because of the nature of these sites they are also prone to litter, silt and the polluted run off from car exhausts, all of which affect and dictate the type of drainage required.

Traditionally road gullies and lengths of linear channels were used to direct the water into storm drainage systems with the run off often passing through an interceptor of some description in an attempt to separate oil and petrol from the water, these systems are prone to silting up after many years of neglect and if the drainage at surface level is not maintained on a regular basis localised flooding occurs because the rain water run off can not enter the below ground drainage fast enough.

Sustainable Urban Drainage Systems (Suds)

Through urban development storm water is collected and delivered rapidly into rivers and brooks which can lead to flooding further down the system, less water is allowed to filter naturally through the ground at the point that is falls and the Suds legislation aims to deal with this.

Permeable and porous pavements and hard standings are encouraged on modern car-park areas with storm water retention encouraged below ground so that the run off can filter into the sub-soil (where applicable), or at the very least it can be filtered and dispersed into a natural water system in a controlled manner.

Road Gullies

These are large and deep trapped gullies that as with traditional gullies allow water to pass through the trap whilst retaining any silt and debris, they come in several shapes and sizes and can be constructed from vitrified clay, concrete, plastic and some of the older systems are brick built chambers with an un-trapped outlet. The gratings that sit on top of the gullies are as varied as the gully pots themselves but they tend to be hinged on one side presumably to stop the pikeys from having a good weigh in at the scrap yard, dependant on the position of the gully these gratings can be slightly dished or flush with the surrounding hard standing.

Many of these gullies have a rodding access over the top of the trap but over time the rodding cap will go missing which means that when the silt and debris reaches a certain level it flows directly into the main line system, there are also road gullies that have silt buckets fitted internally to make the emptying
process a bit easier.

Cleaning Road Gullies

As the gullies prevent silt from entering the main line drainage it is necessary to empty them occasionally, due to the depth of these units specialist equipment would normally be required such as a gully or post shovel and grafter it is also good practice to jet or flush test the outlet after you have cleaned out the trap.

If you are emptying a good number of gullies then you can call in a specialist firm with a gully emptying unit, the kind that the local authority send out to empty the gullies at the side of an A Road during the rush hour on a Monday morning.

Linear & Channel Drainage

Channel drains are the long stretches of ground level drainage you see at the super market, they are popular as the flush grating prevents the slipping and trip hazards that a road gully in the middle of a car-park can present.

There will be a silt trap prior to the channel entering the below ground drainage system and again they are prone to silting up so require maintaining on a regular basis

Cleaning Channel Drainage

The gratings on these channels are bolted down and can be removed so that the channel can be cleaned out by hand, however this is not the way you want to spend a wet Wednesday afternoon.

It is much easier to lay a line of planks or timber over the channel possibly over a strip of visqueen and high pressure water jet the lines clean, you should however make sure the working area is isolated as it is inevitable that the odd jet of water will escape at ground level and cover somebody`s car in silt.

There are other alternatives to draining car-parks such as the kerbing shown here, every so often there is an access plate so that the internal bore of the kerb can be jetted clean.

Car-Park Petrol & Oil Interceptors

Petrol interceptors are installed at commercial and industrial sites and on car-parks to intercept oil and petrol contained in the storm water run off from hard standings, the image on the right shows a traditional brick built trap where the water passes through three chambers before discharging into a storm water system.

Each chamber outlet has a dip pipe which is extended towards the base of the chamber, the idea being that the oil and petrol will rise to the top of the water with the deeper, cleaner water moving through to the next chamber. An air vent system runs above ground to release the build up of vapour within the tanks.

The main defects found on these systems are usually due to poor maintenance, if not emptied on a regular basis the oil and petrol build up will pass through the chamber and into the main storm system, silt and debris can also build up in the base of the chambers until it seals the dip pipes and prevents water from moving through the system.

Modern petrol interceptors are prefabricated in plastic or fibre glass and as with the older systems prevent silt and contaminated water from passing into the main line systems, they are also fitted with alarms that activate when the unit requires emptying.

There are several types of forecourt interceptors including units for vehicle and wash down areas where detergents may enter a storm drain system.