Drainage is the removal of excess water from land (either used or in the form of rainwater). It should be distinguished from flood control, which is the prevention of damage as a result of river overflow. There are two types of systems adopted for the collection of wastewater. Separate and combined sanitary system. In the separate sanitary system, there is a separate sewer that collects domestic, commercial, and industrial wastewater and disposes of it, while a separate sewer collects stormwater and disposes of it separately. In the combined system, both rainwater and domestic water are conducted through the same network of pipes.

Sanitary sewers should have a self-cleaning rate of (.6-1

MS). This self-cleaning rate is achieved by placing the drain on a steep slope. Manholes should be placed at intervals of 100 to 120 m in order to maintain and repair the sewerage. It should be noted that sewage in the sewer is generally transported by gravitational force rather than mechanical means (pumping) for convenience. This is why in the placement of sewer pipes; the topography of the area must be well understood.

Stormwater flow can be estimated by .278CIA where C is the runoff coefficient, I is the rainfall intensity, and A is the catchment area. C is usually obtained from the table, which is a function of the factors that affect infiltration. From the rain data it can be estimated.

A can be estimated using a planimeter or by overlaying scaled graph paper on the map and note the grid that lies within the boundaries of the map. Wastewater from industry, shopping centers and domestic uses is usually estimated by knowing the amount of wastewater discharged by users through the use of a questionnaire and personal interview. The population of people and the number of industries must be taken into account for this estimate.

General steps in culvert design include:

• Establish a comprehensive map of the area including road outline, topography and utilities
• Drains are shown as a single line with arrows for direction
• Manholes are shown with a dot at all their required locations such as junctions and their 100-120m interval midpoint.
• Culverts are designed to follow the natural topography
• Sewers are mostly branch network
• Culverts or drains are usually located along the road.
• The design approach for culverts is as follows;
• Estimate the maning constant and select the slope
• Calculate the section factor
• Get the best hydraulic section either rectangular or trapezoidal
• Check compliance with the minimum speed
• Addition of adequate freeboard to prevent overflow

Wastewater disposal can be treated or untreated. However, treatment is usually by biological method. Wastewater disposal is of great importance for economic growth. Treated or untreated waste could be used as a source of irrigation, to supplement the flow of streams or rivers, and could be used as a source of groundwater recharge.

It is obvious in the cities of the developing countries that most of the construction industries do not take note of the whole process mentioned above. As a result, stormwater follows drains that are not specified for them or overflows them causing erosion. On the other hand, some of the rainwater stagnates in the drains, becoming a breeding ground for mosquitoes and toads. This could result in an increase in malaria and water related diseases in the surrounding area. All this is because; Most of the governments of these cities have not yet understood the importance of drainage systems and the role they play in socio-economic development.