Road drainage

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Street drainage has the task of keeping water away from the street or draining it away quickly, since the structure of the street and traffic safety can be endangered by water. The planning of the drainage facilities on roads in Germany takes place today on the basis of the guidelines for the construction of roads - drainage , RAS-Ew (Ew for drainage).

Types of water

In road drainage, a distinction is made between surface water and ground water . The collected water is fed into natural or artificial bodies of water, the receiving waters .

The water that accumulates during rain and snow is called surface water. Since the road is paved, this water cannot seep into the ground and thus endangers road traffic. Surface water can e.g. B. lead to aquaplaning and black ice. In addition, the water can penetrate the road surface and cause damage, e.g. B. frost breakout, can arise.

The water present in the soil, for example seepage water or groundwater, is called soil water.

Surface drainage outside built-up areas

In the open location (outside built-up areas), the surface water is first collected in the drainage basin or in the drainage ditch . It is then routed into pipes via drainage shafts and led to the receiving water.

Drainage trough

A drainage trough is a green drainage facility for collecting, seeping away or conveying surface water. In contrast to the ditch or infiltration basin, it is flat.

There are three different hollows:

  • Lawn hollows
  • Hollows with very slight gradients;
  • Holes with large longitudinal slopes.

Lawn hollows

Lawn troughs are used to collect and seep (drain into the subsoil) of surface water. They are used outside built-up areas as they take up a lot of space.


  • Width: 1–2.5 m
  • Depth: 20–50 cm (usually 1/5 of the width)
  • Longitudinal slope: min 1%, max. 4% 10 cm topsoil with lawn seeds

Holes with a very low gradient

The gradient is less than 1%. You will receive a smooth base fastening made of precast concrete parts on a 10 cm thick sand bed.

Holes with large longitudinal slopes

The gradient is over 4%. Troughs with large longitudinal slopes are equipped with rough fastenings to reduce the flow rate and drag force.

The type of attachment depends on the longitudinal slope.

with 3–5% longitudinal gradient:

  • Trough with a rough base: coarse gravel on 10 cm gravel sand or grit

with 4–10% longitudinal gradient:

  • Trough with a rough base: natural stone quarry, paving stones or concrete turf stones in 10 cm gravel sand or grit

over 10% longitudinal gradient:

  • Robbery bed recess: 18–36 cm stone set on 15 cm gravel sand, with additional protection if necessary
  • Embankment cascades.

Drainage ditch

The drainage ditch (also street ditch ) is mainly created for the drainage of agricultural and forestry roads or as a collector for conveying water to the receiving water. It is seldom laid out next to traffic areas because its depth poses a danger to road traffic.

There are four different types of trenches:

  • Digging without a slope
  • Digging with a very slight gradient
  • Digging with great longitudinal slopes
  • Catchment ditch

Trenches with very small and large slopes differ from the corresponding hollows only in the shape of their cross-section. The type of attachment is the same for both.

A special form of the drainage ditch is the Abfanggraben (rarely Abfangegraben called). It is created on slopes above road embankments and catches water that could damage the stability of the slope or embankment.

Making trenches and hollows

Holes and trenches are systematically dug out of the existing, mostly overgrown soil. In the case of smaller measures, this is done by hand, in the case of larger dimensions it is done mechanically with an excavator. Backhoe buckets are used for this, and special trench buckets if necessary for trenches. In the case of mechanical excavation, the profile must be reworked by hand. Holes and trenches are always excavated in the opposite direction to the direction of flow.

Surface drainage within built-up areas

In the closed location (within built-up areas) the surface water cannot be discharged openly because of the adjacent buildings. It is caught in the drainage channel at the edge of the road and discharged into road gullies that are connected to the public street canal or separate drainage canal.

Drainage channel

A drainage channel is a component for the linear drainage of surfaces. Drainage channels fulfill the function of absorbing surface water from adjacent paved areas, transporting it and diverting it to the sewer system. They are placed directly next to or between paved traffic areas such as lanes, sidewalks, parking lots and the like. Drainage channels are fixed and mainly occur in the locality.

You need them on open stretches

  • on the median of two-lane streets
  • on roads and paths if there is no hollow at the edge of the road
  • on roads in water protection areas, where surface water from the carriageway has to be collected and discharged

There are different constructions:

  • Edge gutter made of concrete gutter plates
  • Edge gutter with a continuous roadway
  • Pointed gutter made of paving stones
  • Trough gutter made of paving stones
  • Box gutter
  • Slotted channel made of concrete slotted pipes
  • Slot channel made of modular sheet steel units that are concreted on site
  • Pendulum channel (occurs on traffic areas with a longitudinal gradient of less than 0.5%)

Manufacture of drainage channels

The curb gutter (combination of high curb and gutter) with a continuous lane is most common on streets and parking lots.

It is installed as follows:

  • first the subgrade is made, then the frost protection layer is applied
  • A drainpipe is placed in the frost protection layer to drain off soil water
  • Next, the concrete foundation (C20 / 25) is made
  • the high kerbstone and the drainage channel will be built on it
  • last of all, the actual road superstructure is built

Soil drainage

If soil water occurs in the area of ​​a road structure , it must be caught and drained off with seepage systems in order to prevent damage to the road. The drainage takes place via seepage layers , such as B. over the frost protection layer, as well as over seepage strands .

In road construction, infiltration systems consist of a filter body:

  • linear as a seepage strand
  • planar as a percolation layer that should extend into the depths.


Cross section through a drainage ditch

Drainage strands run alongside the roadway and absorb the seepage water from the frost protection layer and the ground water and direct it into the terrain or into a receiving water, e.g. B. a channel. They consist of a filter body and mostly have a drainage pipe. The filter bodies of the seepage strands are made of water-permeable, weather-resistant and filter-stable building materials, such as. B. filter gravel. The filter body can be constructed in one or more stages. Single-stage filter bodies have the same grain composition, while multi-stage filters have minerals of different grain compositions. Plastic, concrete and stoneware pipes are used as drainage pipes . They have openings, usually slots, through which the water can enter the pipe. Drainage lines can either be created as a drainage ditch , as a wedge-shaped drainage slot or together with the collecting line for the surface water. A drainage ditch is particularly suitable if, in addition to surface water, mainly groundwater is to be collected. The construction is quite complex and has to be renewed regularly over the years due to the rotable components.

Seepage layers

Soakaway layers are necessary when soil water occurs over a large area.

This is possible:

  • in the area of ​​the superstructure on the subgrade
  • on the incision slopes
  • under embankments on the embankment

There are five different seepage layers:

  • Frost protection layer
  • Load seepage
  • Embankment seepage layer
  • Deep seepage layer
  • Drainage support disc

The frost protection layer is the best known seepage layer. It is part of the superstructure and the horizontally applied seepage layer. It prevents water from the subsoil from penetrating the superstructure. It ensures that water penetrating from above can run off on the subgrade. A minimum gradient of 2.5% must be obtained for this. The draining water is collected in the drainage line lying parallel to the road or drained into the side ditch on paths.

The load seepage layer can be necessary in cuttings or dams if the subgrade or the dam support is temporarily or permanently in the groundwater. The load seepage layer has the task of lowering the groundwater level under the superstructure. For this purpose, drainage lines must be created on both sides of the seepage layer to drain off the groundwater.

The embankment seepage layer is used to catch the stratified water that escapes at various points in the embankments. It is discharged into the seepage line created at the base of the embankment. This avoids landslides on the embankment and protects the road.

Cross flowing to the road axis soil water in the lower range is compared with the Tiefensickerschicht collected. It is arranged parallel to the road at the base of the embankment, secures it and prevents water from penetrating the road.

The seepage support disc is a deep seepage layer that is arranged perpendicular to the slope. With it, water channels lying deeper in the embankment are recorded, which could endanger the stability of the embankment. The water is collected in a drainage pipe at the bottom and fed into the drainage line at the base of the embankment. In order to achieve a supporting effect, several panes are built at a distance of 10–20 m. They are made from single-grain concrete or crushed stone.

Discharge of the collected water to the receiving water

The surface water and ground water that was caught in the previous facilities (e.g. troughs and gutters) is routed through shafts in pipelines, collected and directed to the receiving water . These pipelines are also called collecting lines .

Manifolds are built when

  • hilly terrain requires
  • the open channels cannot absorb the amount of water that occurs,
  • in addition to surface water, ground water is to be absorbed and
  • Surface water has been collected in channels and can be drained away.

The water reaches the collecting line through drainage shafts. A drainage shaft consists of a shaft cover with an inlet grate that is 3 to 5 cm lower than the connected paved channel bottom. The manhole neck and the manhole base with channel are located below the top.

Drainage lines are laid outside the street on the open stretches (outside built-up areas) , for example under the drainage trough. The slope is aligned with the hollow or the street and the nominal pipe width must be at least DN 250. The shafts under the drainage trough are called trough drainage shafts and they can drain up to 400 m² of traffic area.

On built-up stretches (within built-up areas ) , the shafts are arranged on streets and parking lots and drain the water from the channels. These shafts can drain up to 500 m² of traffic area.

In the course of the collecting lines, test shafts (visual inspection shaft ) are built in order to be able to monitor and maintain the line. The distances between the shafts must not exceed 80 m. The pipeline must be straight between the test shafts.

If an additional seepage pipe (full seepage pipe) is arranged above the collecting pipe, it is referred to as a piggyback pipe . The seepage pipe is surrounded by filter material and absorbs the seepage water from the frost protection layer.

As a rule, the water from the road surface is to be temporarily stored in rainwater retention basins in the event of heavy rain and treated in rainwater treatment basins in the event of traffic loads over 5000 vehicles per 24 hours .

Subgrade drainage

The water that accumulates on the subgrade is to be drained off through subgrade drainage. For this purpose, a drainage pipe is laid on the lower edge of the subgrade. Alternatively, a multipurpose line for collecting the seepage water and transporting the road surface water is also possible.

The resulting water is drained off together with the road surface water.


  • Structural engineering Specialized in construction, Verlag Europa-Lehrmittel, ISBN 3-8085-4029-X
  • Specialist level civil engineering, Verlag Handwerk und Technik, ISBN 3-582-03575-1

See also