What are they?
An uncovered system of channels that picks up wastewater flows from individual households and conveys them to a disposal/reuse point. Drains may be unlined but perform better and are more easily maintained if lined with either brick, mortar, or concrete. In addition to wastewater, most open drainage systems carry stormwater run-off although they may not be designed to allow for peak run-off flows.
When to use them?
To convey sullage/grey water. Open drains are not a satisfactory technology for transporting sewage, even when the solids have been removed in some form of septic/interceptor tank (even sullage is likely to carry fecal contamination from laundry wastes, hand and body washing etc, but the concentrations are lower.) There are two reasons why open drains are unsatisfactory for sewage transport. First, people can easily come into contact with the wastewater, with its potentially high pathogen content. Second, since it is almost impossible to keep stormwater out of drains, any flooding will be flooding with diluted sewage.
Open drains are often the most common means of transporting wastewater in informal urban areas. As such, they are well understood by the various stakeholders. They collect more silt and rubbish than sewers and covered drains but are much easier to clean. Indeed, existing maintenance systems may be geared to cleaning open drains rather than sewers.
Open drains designed to carry wastewater invariably also carry stormwater, which is likely to cause problems for downstream treatment facilities. They may smell and the presence of the drain reduces the usable width of the street. Silting and frequent blockages with rubbish and construction debris are likely in collector sewers and covered drains to which open drains connect. Children may play with the raw sewage.
The drain should be large enough to carry the design flow, including at least moderate stormwater flows. However, it should not be too large. A common design fault is to provide large drain sections, even on branches carrying little flow. Over-sized sections are likely to become receptacles for rubbish and are more difficult to clean than smaller sections.