Drainage is the disposal of excess water on land (either used or in form of storm water). It must be distinguished from flood control which is the prevention of damage as a result of overflow from river. There are two type of system adopted for waste water collection. Separate sanitary and combined system. In separate sanitary system, there is a separate sewer that collects the household, commercial and industrial waste water and disposes them while a separate sewer collects the storm water and disposes it separately. In the combined system, both the storm water and domestic water are conveyed through the same pipe network.
Sanitary sewers should have a self cleansing velocity of (.6-1
m/s). This self cleansing velocity is achieved by laying the drain on a steep slope. Manholes should be placed at interval of 100-120m for the purpose of maintaining and servicing of the sewer. It should be noted that waste water in the sewer are usually transported by gravitational force rather than mechanical means (pumping) for convenient sake. This is why in laying sewer pipes; the topography of the area should be well understood.
The storm water flow can be estimated by .278CIA where C is the coefficient of runoff, I is the rainfall intensity and A is the catchment area. C is usually got from table which is a function of factors that affect infiltration. From rainfall data I can be estimated.
A can be estimated by using planimeter or superimposing scaled grid paper on the map and note the grid that fall within the boundary of the map . The waste water from industry, commercial centers and domestic uses are usually estimated by knowing the quantity of waste water the users’ discharges through the use of questionnaire and personal interview. The population of people and number of industries should be noted for this estimation.
The overall steps in layout sewers include:
- Establish comprehensive map of the area including road contour, topography and utilities
- Drains are shown with a single line with arrows for direction
- Manholes are shown with a dot at all its necessary locations as the junctions and its intermediate point of 100-120m interval
- Sewers are design to follow natural topography
- Sewers are mostly branch network
- Sewers or drains are usually located along the road
- Design approach for sewers is as follows;
- Estimate manning constant and select slope
- Compute the section factor
- Get the best hydraulic section either rectangle or trapezoid
- Check the satisfaction of minimum velocity
- Addition of adequate freeboard to avoid overflowing
The disposal of the waste water could be treated or not treated. However, the treatment is usually by biological method. The disposal of waste water is of immerse important for economic growth. The treated or untreated waste could be used as a source of irrigation, supplement stream or river flow and could be used as a source of recharge for ground water.
It is obvious in the cities of the developing countries that the most of construction industries do not take note of all the above mentioned process. As a result storm water follows drains that are not specified for them or overflows the drains there by causing erosions. On the other hand, some of the storm water are stagnant in the drains there by becoming a breeding place for mosquitoes and toads. This could result in increase of malaria and water related disease in the vicinity. All this is because; most government in such cities have not yet understand the important of drainage systems and the role it plays in social- economic development.