Curtain Drain

A curtain drain is a linear drainage element typically used to intercept groundwater flows on steep slopes. Curtain drains are often used where groundwater flows would cause slope instability or where groundwater flows would emerge on the side of the slope causing further problems.

Curtain drains are also often used to capture runoff on slopes. This prevents the runoff from travelling too far down the slope which can cause natural channels and gullies to form if left unchecked. This erosion of the slope surface can cause instability in some cases or simply cause aesthetic issues.

Curtain drains are also often used to prevent groundwater or runoff water before it reaches building foundations. In some cases a fin drain or other foundation drain will not be sufficient on its own to remove all the runoff and groundwater travelling down a slope towards the building foundations. In these conditions, a curtain drain around the house can assist by removing a certain portion of the runoff and groundwater flow before it reaches the house foundation. The curtain drain may only need to be installed to 1 or 2 sides of the house in some cases.

Curtain Drain Design

French drain systems are often used as curtain drains because they are linear elements and if they are installed to the correct depth they can intercept all the groundwater as it flows down the slope.

The curtain drain depth must be sufficient to capture the groundwater. The curtain drain trench can be taken down to an impermeable layer such as heavy clay or rock. This will prevent any large volumes of groundwater from escaping underneath the curtain drain and emerging further downslope.

The french drain design can be modified slightly to include surface water and groundwater flows. The CivilWeb French Drain Design spreadsheet includes a tool for the designer to override the surface water runoff values which can be used for curtain drain design. The spreadsheet can be used to calculate the surface water runoff, then the designer can modify this value upwards to include for the likely groundwater flow to produce a suitable curtain drain design.

The curtain drain pipe, filter material and trench width can be designed in a similar way to a traditional french drain. For further info on the design of the perforated pipe, see our Perforated Pipe Design post.

Curtain Drain System Details

French drains being used as curtain drains can be modified slightly with an impermeable geotextile layer installed to the downslope side of the french drain trench to prevent any water from travelling through the trench and into the surrounding soils on the other side. A small soil berm can also be installed at the downslope side of the trench to channel any surface water into the trench and prevent any large flows from travelling over the top of the french drain trench. The curtain drain depth must also be sufficient as discussed above.

A typical french drain detail is shown below including modifications to make the french drain suitable as a curtain drain. Other than these modifications, the curtain drain can be installed to the same details as a traditional french drain. For further information please see our French Drain Details post.