Thomas and Dedo, 2002 stated that “The basic underlying purpose of any drainage system is to keep people from water, to keep water from the people and to protect and enhance the environment while doing so”.
Patil, 2011says that during the rains, part of the rain water flows on surface and part of it percolates through the soil mass as gravitational water until it reaches to the ground water. Some water is retained in the pores of the soil mass and on the surface of the soil particles which cannot be drained by normal gravitational methods and this water is termed as held water. it is required that the surface water from the carriage way and shoulder should effectively be drained off without allowing it percolate to subgrade. The surface water from adjourning land should be prevented from entering the roadway. The side drains should have sufficient capacity and longitudinal slopes to carry away all the surface water collected. Also in water logged areas special precaution should be taken. There are many such roads which are not having the proper drainage system. This causes the failure of the roads due to many reasons like increase in moisture content, decrease in strength, mud pumping, formation of waves and corrugations, stripping of bitumen, cutting of edges of pavement, frost action etc.
Mwangi, 2013 stated that drainage is a must component in the road construction. In lay man language we know that tarmac and water are never “best friends” for this reason in most designs of the road, the first thing to be put in place is drainage system. The presence of water in the pavement layer will tend to reduce the bearing capacity of the road and thereby its lifetime. It is required that the surface water from carriage ways and the shoulders should be efficiently drained off without allowing it to the subgrade of the road.
Mwangi notes further that, it is essential that adequate provision is made for road drainage to ensure that a road pavement performs satisfactorily. The main function of a road drainage system is to prevent flooding of the road and ponding on the road surface, to protect the bearing capacity of the pavement and the sub-grade materials to avoid the erosion of side slopes.
Water and road construction do not make for a harmonious couple [Dawson 2008]. From this statement, Mwangi (2013) states “for this reason it can be seen that there is a very serious effect of poor drainage on maintenance on the condition of the road. Therefore surface water from adjoining land drain should be built in such a way that it will have a sufficient capacity and longitudinal slope to carry away all surface water collected.
Paul and Norman,1989 says that road structure has to be provided with an adequate drainage to remove precipitation from its surface as expeditiously as possible, failure to which early pavement distortions and deterioration eventually occur [Gichaga 1989]. Furthermore, flood waters are a deterrent to free traffic movement and create unnecessary perils for the uses of the facility. For the reason, road designers provide pavement crown and shoulder slopes to expedite the removal of surface water.