Open Channels and Drains

The most basic way to drain off rain and stormwater is via open channels. A more developed but also more expensive solution is a separate sewer system. Separately collected stormwater has high potential for increasing agricultural production in rural areas. In addition a more integrated approach to water management in urban areas can be achieved by replenishing freshwater resources after a natural pre-treatment of the collected stormwater (e.g. free-surface constructed wetlands) and a recharge of surface or groundwater bodies (see also stormwater management). In both cases the receiving water bodies may be sea, lake, river or groundwater (also see surface disposal of liquids, surface groundwater recharge, subsurface groundwater recharge).

An open channel or drain system generally consists of a secondary drainage system, with a network of small drains attached (micro-drainage). Each serves a small catchment area that ranges from a single property to several blocks of houses (WHO 1991). These small drains bring the water to a primary drainage system, composed of main drains (also called interceptor drains), which serve large areas of the city. Thereafter these drains are generally connected with natural drainage channels such as rivers or streams (WHO 1991).

Not all water precipitating as rain needs to be removed by the drainage system. Some will direct absorption by naturally infiltrating into the ground, while some may stand in puddles and other depressions and eventually evaporate. The part that drains off the ground surface (runoff) into the drainage system is known as the runoff coefficient. There is little chance for evaporation during a rainstorm, so that the runoff coefficient used to calculate the size of the drains required should be based on the infiltration capacity of the ground. The latter mainly depends on soil condition (if the drainage system is not lined, as is often the case in rural areas, chances of percolation are high); the slope of the terrain (steep slope or flat area); land use in the catchment area (roofs and pavement prevent infiltration) and the intensity of rainfall (e.g. design for a 5-year storm return period).

A return period of five years is widely used to design primary drainage systems in tropical cities, but shorter periods (three years or less) are more suitable for micro-drainage within residential areas, where an occasional overflow is less likely to cause serious damage. If it is designed too big, it may never be fully amortised within its lifetime. In this case the money for the big construction could have been used for other constructions (e.g. micro-drainage in areas without any canals).

On steep terrain, erosion could damage the drain. It is therefore important to prevent water from flooding down in uncontrolled flows that may wash out houses and turn passages such as pathways and streets into an impassable gulleys. As a rule of thumb, slopes of more than 5% can be considered steep slopes. There are several types of constructions that impede the water flow (WHO 1991).

In flat low-lying areas subject to flooding, a major problem often results from the relatively high level of the receiving water body. Because of the limited slope to which drains can be laid when water flows along them it is quite slow and inefficient. Together with the difficulty of digging deep drainage channels especially where the groundwater level is high, results in drains having to be proportionally wider in order to have sufficient water passage. There is also a risk of puddle building in which pests such as mosquitoes can breed. Building a drainage channel with sloping sides and a narrow bottom helps to maintain a steady flow speed whatever the water level in the channel. A refinement of this principle is to build a channel with a composite section (see picture below) (WHO 1991).

Transportation of wastewater in open drains

As mentioned in the executive summary, it could be a temporary solution to transport wastewater (greywater or even blackwater) in open drainage systems.Open drains are not a satisfactory technology for transporting sewage, even when the solids have been removed by some form of septic/interceptor tank (even sullage is likely to carry faecal contamination from laundry wastes, hand and body washing etc., albeit the concentrations being lower this should be considered). There are two reasons why open drains are unsatisfactory for sewage transport (CORCORAN et al. 2010):

1. People can easily come into contact with the wastewater, with its potentially high pathogen content.
2. Since it is almost impossible to keep stormwater out of drains, any flooding will be floodwater merged with diluted sewage (WORLDBANK 2011).  Very often open drains/channels are misused for depositing litter, excreta and household sewer lines are sometimes illegally connected to open drainage systems.

Compared to underground sewer systems (e.g. conventional sewer or separate sewer) open drains are a less expensive solution. The precise cost depends on local conditions. If terrain is very step, extra construction has to be fabricated to slow down the velocity. Also very flat areas could raise the budgeted cost (deep, or where groundwater table is high, wide excavation). Consideration of community participation could have a positive offset on the overall costs. For example voluntary labour can permit significant savings in cost, which can be an important consideration for most municipalities in developing countries, due to very limited availability of funds to invest in infrastructure. A typical value for the annual cost of maintenance would be about 8% of the construction cost of the system (WHO 1991).

Keeping drains free of refuse poses a problematical obstical. Unfortunately it is commonly believed that a drain is a convenient place for depositing solid waste, wastewater, greywater and even faecal sludge, especially where there is no adequate refuse or wastewater collection service (see also water pollution). Refuse and faecal matter in drains quickly becomes malodorous as it decomposes and poses as a suitable medium for flies and mosquitoes for egg-laying and as a hospitable site for many pathogenic bacteria and viruses. Removal of such material from the drains is not a popular task (WHO 1992), but crucial for protecting the public health (see also health and hygiene issues).

The main duties and responsibilities for operation and maintenance of an open channel drainage system are (WHO 1991):

• Routine drain cleaning
• Reporting of defects and blockages
• Semiannual inspection
• Repairs
• Payment for maintenance
• Passing of by-laws regarding the use of drains
• Enforcement of by-laws.

However, the most important issues to optimise existing open drain systems are: preventing overflow (see operation and maintenance above) and waste entering the system. To prevent faecal matter and solid waste from entering the system is to cover the drains and provide adequate wastewater treatment and collection systems as well as solid waste management.

Open drain bear a high health risk because they are very often used illegally for the discharge of domestic and/ or industrial wastewater, and solid waste. Moreover, ponding water forming pools encourage mosquito breeding, and children have a tendency to play in them. Ponding is likely to occur where: the terrain is flat and the drain slope small, where the drains are rough and unlined so that water collects in depressions, where solid waste is deposited in the drains leading to a clogging effect and where drains are filled to allow vehicles or pedestrians to cross (WHO 1992). This is a high risk for the public health (see also waterborne diseases - pathogens and contaminants). Also defecation into those drains is not an unusual habit and needs to be circumvented by the community.