Back to the home page

Core Engineering, civil infrastructure

Drainage and sewers

A street section showing how rainwater and waste water travel out of a site and into the public sewer. Scroll to follow each route. Arrows show the direction of flow, and gravity does most of the work.

Whole street
Surface water Foul water Infiltration

This 3D view needs a browser with WebGL. The written steps on the right still explain each part of the drainage system.

Overview

A street section

Rainwater and waste water are kept apart and carried to the public sewers beneath the street. Most of the route is a gravity fall, with a pumping station where the ground is too flat or too deep for gravity to work.

The section is cut through the road, footway, a building and a sustainable drainage swale, so each part of the system sits where it would in the ground.

01 · Surface water

Rain from the road and roofs

Gullies at the kerb take rainwater from the road into a surface water sewer. Roof downpipes send rain from the building into the same sewer.

  • Gullies: grated inlets at the kerb edge, with a trap to stop smells rising.
  • Separation: keeping rain out of the foul sewer stops it being overloaded in heavy storms.
  • Downpipes: roof water is carried down the building and connected into the sewer.

02 · Foul water

Waste water from the building

Waste from toilets, sinks and showers leaves the building through a drain laid to a fall. It joins the public foul sewer under the street.

  • Building drain: laid at a set gradient so waste flows out without blocking.
  • Ventilation: pipes are vented so that the flow is not held back by trapped air.
  • Capacity: the public sewer must carry the peak flow from every property connected to it.

03 · Gravity falls

Pipes laid downhill

Both sewers are laid at a gradient, so the flow runs downhill by gravity towards the treatment works. The deeper the sewer, the further along the route it has to fall.

  • Gradient: the fall is set so the flow is strong enough to carry solids away.
  • Depth: sewers go deeper as they run towards the outfall, so they can keep falling.
  • Limits: where the ground is too flat or too deep, a pumping station has to lift the flow.

04 · Manholes

Access to the network

Manholes are shafts that give engineers access to the sewer for inspection, cleaning and repair. They are built at points where the sewer changes direction or joins another pipe, and at regular spacings along straight runs.

  • Position: at changes of direction, junctions and at set distances along straight runs.
  • Safety: entering a manhole counts as entering a confined space, so strict safety rules apply.
  • Records: the position and depth of each manhole are kept in the asset data.

05 · Sustainable drainage

Managing rain where it falls

Sustainable drainage (SuDS) systems hold, slow and soak up surface water before it reaches the sewer. The swale and permeable paving here do that work.

  • Swale: a shallow, vegetated channel that stores and slows the water.
  • Permeable paving: the surface lets water soak through into the ground below.
  • Benefits: less load on the sewer, lower flood risk and cleaner water.

06 · Pumping station

Lifting where gravity can't

Where the sewer would have to run too deep to keep falling, a chamber collects the flow and a pump lifts it into a rising main, a pressurised pipe that carries it to a higher sewer.

  • Wet well: stores the flow until the pump switches on.
  • Rising main: a pressurised pipe carries the water uphill to the next gravity section.
  • Maintenance: pumps need regular servicing and alarms that warn when they fail.