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What is Road Gully Spacing Thumb Rule? 6 Important Points

road gully

Road Gully Spacing plays a very important role while developing drainage design and modelling. Surface water flow width, longitudinal slopes and many other factors are considered to determine gully spacing, please read this blog for further detail:

What is Road Drainage?

Road Drainage is the normal or non-natural amputation of a surface’s water and sub-surface water from a road surface. If the road surface water is not removed, it would severely weaken road pavement particularly asphalt pavement.

In-case the surface water trickles down to the sub-base, the foundation of road pavement would lose its bearing strength and the road pavement may fail under heavy traffic conditions. Absence of adequate drainage system leads to having a poor-quality road pavement such potholes, uneven road surface, etc.

What is Road Gully?

Road Gully is a small pot placed along road edge or kerb edge and covered from the top by placing gully grating which is made of the steel frame. The main purpose of Road Gully is to collate surface water of the road surface and then transmit it to the downstream career pipe and then ultimately discharge it to existing drainage network or to an attenuation pond.

How does Road Gully Work?

As mentioned above road gully collates surface water, so the principle of gully design works by placing several gullies at regular interval and at low points along the road edges. Following are the thumb rules of gully spacing:

  • Gully Spacing to be designed as per UK DMRB, HA-102.
  • Surface Water Flow Width B = 0.75m.
  • The surface area covered by one gully should not exceed 200 square meter area.
  • The spacing between gullies should be in the range of 40 meters to 50 meters but in case of sag curve or low points or lower side of the super-elevated road, it can be placed in the range of 25 meters to 30 meters.

The primary factor that affects the road gully spacing depends upon the surface water flow width. This means higher the flow width, greater the Gully spacing. It is recommended that the flow width doesn’t spill-over to wheel track zone.

Also, the road edge treatment plays a very important role in collating surface water. For example, if the road edge treatment is kerbed edge then water flow would higher than the flush road edge with verge or permeable hard shoulders.

What is the Road Gully Types?

There are three types of Road Gullies commonly used for Road drainage system.

Standard size Trapped Gully (or Flat Top Gully)

Standard size Trapped Gully is the most common form of Gully. The gully pot comes with steel frame grating and flushes with the road surface.

Side Entry Gully

Side entry gully is preferred to minimise obstruction to on-road cyclist due to gully grating. Side Trapped gullies are generally flushed vertically with Kerb edges. The efficiency of side trap gullies is sometimes less than the standard flat top gullies.

Gully Inset into Verges

Inset Gullies are preferred in highspeed motorway to minimise the obstruction to high-speed vehicles especially the hard shoulder or hard strip is very narrow.

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What is the Difference between Road Gully and Catchpit?

Catchpit is like a pot having an outlet and inlet connection slightly above the floor level that collects debris and needs maintenance at the infrequent interval. Catchpits are designed to placed at steep slopes or non-highway location. Also, they can be formed of various shapes, but Gullies are placed at road surface levels or footway surface or urban realm surfaces and collect surface water.

What is Road Gully Cleaning?

Road Gully Blockage is a common problem due to excessive debris collection or poor maintenance or bad weather condition or due to damage by heavy vehicles. If the blockages in the gully pot are not removed on time, it would lead to local flooding or leakage to sub-base or sub-grade levels or generate bad odours or may damage career pipe downstream.

Road Gully cleaning is carried out with advanced technologies along with vacuuming and high-pressure jetting through the water. CCTV surveys are carried before performing high pressure jetting. The survey records the status of existing gully and pipe network. It shows all the blockage within the network and the jetting help to clean out the blockages.

I hope this blog will help you understand the surface water collection through road gullies and thumb rules will help you to determine the road gully spacing. Please feel free to like, share and comment.

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