CHAPTER ONE
1.0              INTRODUCTION
Some aspects of Civil Engineering is devoted to the planning and construction of facilities to drain the high ways or streets and carry run-offs.
Removal of surface water from the road or street is known as surface drainage; drainage as distinct from plumbing is a system which collects waste products from baths. Wash – basins, water closets and sinks together with rain and discharges into public sewers, septic tanks, soakaways or cesspools.
The construction of drain is therefore principally concerned of the Manson. It is accomplished by constructing the road so that it has a crown and by sloping the shoulders and adjacent areas so as to control the flow of water either towards existing natural drainage, such as open ditches, or into a storm drainage system of catch basin and underground pipes. If a storm drainage system is used as it would be with city streets, the design Engineer must give consideration to the total area drainage unto the street, the maximum rate of run-off expected, the duration of the design storm, the amount of ponding allowable at each catchments area and the proposed spacing of catch basin along the street. From this information the desired capacity of the individual catch basin and the size of the underground piping network are calculated.
In designing facilities to carry run-offs from pavements, the Engineers must determine the area to be drained, the maximum probable precipitation over the drainage basin, the highest expected run-off rate, and then using this information must calculate the required capacity of the drainage structure.
Generally designs are made adequate to accommodate not only the largest flow ever recorded for that may be expected under the most adverse conditions for a given number of years.
Factors considered in calculating the expected flow through a culvert opening include the size, length and shape of the entrance and down stream end of the conduit, the maximum allowable height of water at the entrance, and the water level at the outlet.
There is a trend to use designs that permit drainage structure yard. Such procedures permit better control of the work. For example with recast concrete pipe section, it is often possible to avoid building small box culverts in the field. Circular culverts of large diameter are now also constructed in the same way.